Author Topic: That dreaded old 6 k RPM dead spot SOLVED  (Read 638 times)

Offline herennow

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That dreaded old 6 k RPM dead spot SOLVED
« on: August 30, 2018, 09:17:38 PM »
Hi Folks,

I’ve been playing around with my carbs and jetting for quite some time now and have kicked around lots of thoughts on different posts for example here, here and here.

I remained fairly conservative in 20/60/132.5 jets and all was well except for the dreaded flat spot at 6 to 7 k RPM when running WOT.

I wasted a lot of energy on trying to eradicate this flat spot (I know, I know I could just drive around this but I’m stubborn that way, and a couple of times I had some hairy overtakes when the dead spot dropped me in an unpleasant situation...).  So, I tried larger main, tried shimming needle, tried larger mid mains. Nothing.  The only real clue I had was that when closing the throttle slightly from WOT there was no stumble and bike seemed to have more power generally just off WOT. Was this richer or leaner – who knew? My AFR probe told me rich but it was hard to get a good fix in the short time that the drop out occurred under WOT so I was not sure.

So in desperation, I did some old school tests:
-Firstly I placed a bit of tape over the filter mouth, covering maybe 5% of the opening. Bike ran terribly, the flat spot was huge and would struggle to get over about 70 MPH. I was truly amazed by how much this affected the running of the bike! I deduced that bike was running too rich before as this test (less air = richer) exacerbated the problem.
- Second test was to remove the air filter (more air = leaner) to see the effect. FANTASTIC!!! Takes a bit getting used to the very different sound of the bike but now the flat spot was completely gone! Bike was running perfectly.
I found this really interesting as it indicates that at mid RPM this almost stock bike runs RICH with only a 132.5 main jet difference (even with a K&N which I had originally and changed to OEM to rule out the free flow filter as a problem).

Some of the observations I’ve made are:
-   The “Mid-Main” jet only seems to work in the Choke/Enrichener circuit.  No effect on normal running of the bike in the mid range.
-   If closing the throttle slightly off WOT gives more power than bike is too rich (slightly closing drops the diaphragm and leans the mixture out)
Seeing as my problem happens at mid RPM where the Needle/Needle jet orifice is controlling fuel flow, I think that the only options to remedy the problem are :
1.   Get a smaller needle jet,
2.   Lower the needle a bit,
3.   Get a slightly stronger diaphragm spring
4.     Try a 130 Main jet but this should not help as the bike is nowhere near the main jet when at 7k RPM.

Obviously, number 2 is easiest and I will start there.

Hope this is of any help/interest to any other folks out there.

Cheers

HnN
« Last Edit: October 11, 2018, 07:27:57 AM by herennow »

Offline cbrfxr67

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #1 on: August 30, 2018, 11:22:29 PM »
Great stuff learned by experience. Thanks for sharing.
"Its something you take apart in 2-3 days and takes 10 years to go back together."
-buddha

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #2 on: August 31, 2018, 12:05:59 AM »
Cant lower needle in a US spec bike. But they run perfectly fine a lot of the time.
AKA - float level is suspect. Yea I know I have had it happen too.
The 01+ carb doesn't drift its floats high as much as the 89-00 but it still screws up O rings and causes the same problem.
Cool.
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Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #3 on: August 31, 2018, 01:20:16 PM »
I remained fairly conservative in 20/60/132.5 jets and all was well except for the dreaded flat spot at 6 to 7 k RPM when running WOT.
...
4.     Try a 130 Main jet but this should not help as the bike is nowhere near the main jet when at 7k RPM.

Obviously, number 2 is easiest and I will start there.


RPM alone is not what determines which jet is in use. Throttle position (and load) are also involved.

If you are at WOT and under load (accelerating, up hill, etc.), 6-7K will use the main jet. There's no way around it. You can get your mind past the myth that this or that jet is in use at XYZ RPM by observing the slides with the bike on the center stand and running. Rev to 7K and hold it. If the slides are ALL THE WAY DOWN then you are not drawing (much) fuel from the main jet. If the slides come up AT ALL then you are drawing fuel from the main jet. Of course the under-load behavior is different and hard to observe since most of us are riding the bike when it's under load. Stuff an action camera and a bright LED light in the air box and go for a ride? Either that or a dyno.

Case in point: I had a stumble at 3-4K rpm and about 1/8 throttle that was a result of it being too rich, and this condition was the initiation of the main jet. The internet will tell you this is "pilot jet" but it's basically impossible to be true because the pilot jet does not have an air supply increase via throttle. And I proved this by repairing the problem through a needle swap, AND reducing the main jet by one size.

So you've proven the flat spot is due to it being rich, and this is probably the overlap of the main and mid jets. The solution is smaller main jet. If you have stock needles then you won't fix it by changing the needle. The slide is probably up high enough to make the needle taper meaningless to begin with; the jet is effectively wide open and fuel metering is delivered per volume of air as fast as it can.

If the slides are not coming up then your problem is something else entirely.

I would not play with the needle at all. If it's stock, leave it. Drop one size in main jet. I bet the problem goes away. Ensure the slides are actually working correctly by observing them with the bike running on the center stand. Make sure a vacuum leak is not preventing the slide from operating correctly, but that would make it lean so I don't suspect that is related.

Here's where someone on the internet says "bigger jet is better" and we tend to just believe it because we assume that's a result of consensus. Real world is your actual bike.

[BTW after riding a lot since my own needle/jet swap to get rid of the overrich stumble I think my bike did have more power with the 127.5 main jets... I might just try to swap back, but it's not urgent. Current theory is the stumble was 90% needles, 10% jet, but the combination was making it not work].

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #4 on: August 31, 2018, 08:55:32 PM »
Thanks for the input folks.

Buddha, you are right, I need to adjust float height every two years. I did this a couple of months ago and, interestingly, fuel level was high and dropping it made no  discernable difference.
Regarding o-ring problems that can make it richer at midrange rpm, I don't quite follow you. Can you elaborate?

Josh, I respectfully differ from your opinion on 2 items. On a CV carb, diapragm height is only indirectly affected by throttle position. It is primarily affected by rpm and the associated vacuum created. No matter the load, a given rpm = a given air velocity snd hence, vacuum.
Secondly, the needle jet/needle create an annular orifice that is SMALLER than the main jet orifice at all but the highest diapragm height.  I have tested this in a few bikes by removing the main jet completely. Bike runs fine until it's past 3/4 rpm/throttle (CV/slide carbs) at which time it suddenly runs so rich it 8 strokes. (So rich it can only fire every second normal cycle hence "8 stroking)" this can be calculated from the needle and jet dimensions. This is also presented as fact in carb tech books.
However, I should indicate that as clear cut as this seems to me, cleverer folk than I argue about this.
I love this discussion. Detailed, civil and intelligent. Which side are you on after reading it ;-) both are compelling. But remember these are slide carbs, different to ours.
Vincent owners club carb tuning
« Last Edit: September 14, 2018, 05:17:42 PM by herennow »

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #5 on: September 01, 2018, 10:19:05 AM »
The O rings that hold the float spigot in are junk and leaking.
Now if that's fine, and not leaking try this test.
If lowering float didn't make a discernible difference - and be sure to test everywhere thoroughly - lower it more, like 2mm below the gasket and try it. Maybe you need 1mm lower, or 2, I really cant explain it, but I seem to get the best results 1-2mm lower. Of course they drift, so who knows how long they stay there cos I don't open it till its next problem shows up.
Cool.
Buddha.
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Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #6 on: September 14, 2018, 05:16:59 PM »
Buddha, I'm a bit thick - what do you mean by "float spigot". Is that the float valve?

I played with dropping the needle 1 mm and the flat spot is now *almost* gone.

I will play with fuel height next time I'm in there.
« Last Edit: September 15, 2018, 06:25:40 AM by herennow »

Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #7 on: September 14, 2018, 08:37:23 PM »
by "dropping the needle" you mean lowering it WRT the jet, or making the bike more lean in those conditions?

And BTW I agree with the above, you are right, physics says, barring irregularities like leaks and the like, slide position must be dictated by RPM. Unfortunately I can't square this reality with my observations and experience. I'm going to have to think about that for a while. What I can't settle in my mind is that to make power, you must do work at a certain rate. And the work of turning the crank freely (bike in neutral, for example) is only a small fraction of the work of pushing the bike up a hill at the same rpm. So at 7K rpm on the center stand, my GS MUST be doing less work than it is going 7K rpm in 4th gear at WOT on a hill. If it is doing less work at the same rate, then it is making less power. If it is making less power, it must be using less fuel. So either the slide position is purely dictated by RPM but there is another factor I am not accounting for that affects fueling according to load, or the slide position fully meters fuel (along with the jet size which is fixed) and there is a factor besides RPM that affects slide position.

FWIW I have not researched this at all so I'm just brainstorming here. It's a mental puzzle for me to work out.


EDIT: This thread had fascinating info for me:
https://www.triumphrat.net/hinckley-classic-triples/239762-vacuum-slide-operation-mikuni.html

The thing that gets me confused so easily is that fuel through the jet is dependent on air velocity and not air VOLUME, even transiently.
« Last Edit: September 14, 2018, 09:00:42 PM by mr72 »

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #8 on: September 15, 2018, 06:51:30 AM »
Hi, yes I dropped the needle down a mm in its seat so it will lead to a leaner mixture at a given throttle opening. (which should have a similar result of dropping fuel level, except dropping float fuel level will affect pilot and main as well)

Re: the link, Interesting. I have wondered if the stumbles are the slides overshooting upwards and the bike getting a shot of main jet fuel which bogs it down. I will at some point temporarily block one of the holes in the slides with a silicone plug and see what happens.

I get your conceptual difficulty around the carbs, they confuse me. It seems to me that on the centre stand at 1/4 throttle, the bike would probably be doing 8000 rpm, while going up a hill it would take full throttle to go 8000 RPM. So it appears that throttle variation is what is required to counter the extra work demand. 

However, this is at odds with the idea that fuel and air supply are only linked to RPM (and hence directly related to the velocity of the air), as its unlikely that the slide will be in the same place while doing 8000 RPM on the centre stand and 8000 RPM going up a hill. Maybe the engine has different suction / vacuum while under differing loads. Maybe opening the throttle allows more air to flow AND more of the engines vacuum to get to the slides and lift them. More to think about!!!   :dunno_black: :technical:
« Last Edit: September 15, 2018, 07:20:10 AM by herennow »

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #9 on: September 15, 2018, 11:30:57 AM »
I get your conceptual difficulty around the carbs, they confuse me. It seems to me that on the centre stand at 1/4 throttle, the bike would probably be doing 8000 rpm, while going up a hill it would take full throttle to go 8000 RPM. So it appears that throttle variation is what is required to counter the extra work demand. 

However, this is at odds with the idea that fuel and air supply are only linked to RPM (and hence directly related to the velocity of the air), as its unlikely that the slide will be in the same place while doing 8000 RPM on the centre stand and 8000 RPM going up a hill. Maybe the engine has different suction / vacuum while under differing loads. Maybe opening the throttle allows more air to flow AND more of the engines vacuum to get to the slides and lift them. More to think about!!!   :dunno_black: :technical:

Dude … I've been screaming for a looooooooooooong time - maybe 1997 that jetting depends on throttle position. Used to bug me endlessly when people talk about it with respect to rpm … well now I take bourbon or scotch for that.
Jetting depends on throttle position, and you're talking about the slide overshooting … Likely not but it could raise too fast, and raise too high and the newer carbs may have a biiger aspect of that happening. The 89-00 carbs were proven looooong before they ended up in the GS. 86 GSXR's and then the katana's etc etc, then it went in the GS. The 01 and the 04 and later carbs were first in the GS, cos no other new bike was running carbs by then. So not a proven product IMHO.

Cool.
Buddha.
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Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #10 on: September 15, 2018, 01:39:27 PM »
Re: the link, Interesting. I have wondered if the stumbles are the slides overshooting upwards and the bike getting a shot of main jet fuel which bogs it down. I will at some point temporarily block one of the holes in the slides with a silicone plug and see what happens.

I think that's the opposite of the point. If the slides rise too quickly it causes the air velocity to decrease, which reduces the rate at which fuel is picked up from the jet and makes it lean. The actual rate of air (volume over time) is controlled by the throttle, but the velocity of that air as it comes across the jet is controlled by slide position; lower slide = faster air = more fuel per unit air. When the throttle opens more then more (volume) of air is allowed in, and the slide can rise to keep the velocity of air the same by opening the throat and simultaneously allow more (volume) of fuel (at the same velocity) to be drawn into the air stream as the needle is moved out of the jet.

Quote
I get your conceptual difficulty around the carbs, they confuse me. It seems to me that on the centre stand at 1/4 throttle, the bike would probably be doing 8000 rpm, while going up a hill it would take full throttle to go 8000 RPM. So it appears that throttle variation is what is required to counter the extra work demand. 

The throttle meters the total volume of air available to the cylinder, which subsequently requires a greater volume of fuel and as the slide rises the velocity of the air is allowed to stay the same at varying load. So this tells me that in fact the assertion must be wrong. RPM alone does not control slide height but slide position is controlled one way or another by load or demand.

Now, I'm still trying to wrap my head around "one way or another" here but the objective reality is clear:
- slide position maintains a constant air velocity by restricting the air inlet size
- slide position controls the AFR by metering fuel quantity from the main jet via the needle, which depends upon the constant velocity of the air for it to work correctly

so:
On the center stand, you should only see the slides rise to the top while the engine is accelerating. When holding a constant rpm, ANY constant RPM, the slides will remain in some middle position wherein the air velocity is maintained to be the same as at any other RPM and the total volume over time of fuel is allowed sufficient to produce enough power to hold the engine at that speed under the static/passive engine/drivetrain load of the bike on the center stand (very small load). At any real-world load condition where the resistance is greater than the bike on the center stand, the slides will be higher for the same engine RPM because to produce more power requires bigger throttle opening, thus the slide must move up to maintain constant air velocity and by doing so allows more fuel.

So slide position is metered based on demand or load, not RPM, but under constant load, power output happens to be nearly proportional to RPM so on a dyno under any constant throttle the slide position will happen to vary proportional to RPM, but this is an effect of keeping the load constant. If the load were varying and instead the engine RPM kept constant and the throttle adjusted to track with the load (like a speed governor), then I presume we'd see the slide move up and down in accordance with load, even though RPM was constant. Or in other words, engine RPM at fixed throttle position (e.g. WOT) is an effect and slide position is an effect, but load is the cause in both cases.

This squares with my real-world observations. What I am trying to wrap my head around is exactly how cylinder pressure is affected by load.
« Last Edit: September 15, 2018, 02:01:26 PM by mr72 »

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #11 on: September 15, 2018, 03:07:42 PM »
Quote
lower slide = faster air = more fuel per unit air.

This pretty much sums up the chicken and egg conundrum I hit with carbs.  The alternative view is " Lower slide = smaller annular hole in needle/needle jet orifice = less fuel per unit of air"

Which one wins at any given time - I don't know.

Also in a CV carb, does the "Constant Velocity" not exclude the concept of "faster air"


Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #12 on: September 15, 2018, 09:58:55 PM »
Quote
lower slide = faster air = more fuel per unit air.

This pretty much sums up the chicken and egg conundrum I hit with carbs.  The alternative view is " Lower slide = smaller annular hole in needle/needle jet orifice = less fuel per unit of air"

Except that ignores the point of the CV carb which is to keep the velocity of air constant regardless of volume.

Quote
Which one wins at any given time - I don't know.

If it's working correctly, they there is no winner or loser. Air velocity stays constant, as air volume increases the slide is raised to keep velocity constant while also increasing available fuel volume.

Quote
Also in a CV carb, does the "Constant Velocity" not exclude the concept of "faster air"

It does. That's the whole point. I meant "faster air" compared with the same volume of air over time with the slide higher.

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #13 on: September 16, 2018, 07:22:45 PM »
Quote
jetting depends on throttle position.
  Scotch is good for many things including that, although I prefer the Irish variety  :cheers:

Buddha, I get what you are saying, however, I think that there is a subtle caveat that might need to be added. I suspect that "jetting depends on throttle position" at any given steady state. By this, I mean that if you are running on a flat road holding a given speed then slide position/jetting will always be directly in relation to the throttle opening. However, if one is accelerating, then the direct link between the throttle and slide position is broken and is not a direct relationship. The slide will find its own level based on what the engine needs at that transient point in time (up until it reaches a steady state). This might seem rather esoteric but the fact that my dead spot occurred only while accelerating made this highly relevant to me and what had me chasing my tail. Mr 72, I think this is also what you are saying in your more eloquent post.

Mr 72, I was not clear when I said: "Which one wins at any given time - I don't know." I was talking about my ex-flat spot. The fact that there was a problem indicated that there was a "winner", which in my case was shown to be gas, as in too much gas i.e. more fuel per volume of air, rather than less. (so I guess in my case I did know the answer...  ;))  What the exact mechanism is that leads to this rich condition I don't know, except that dropping the needle made it better.


Thanks for the thoughtful input folks.



Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #14 on: September 16, 2018, 11:08:12 PM »
herennow this has all helped me to improve my understanding of this, or at least my ability to enumerate that which I understand and expose what I don't. That's step 1 in learning.

So now I am (for the time being) convinced that slide position is affected by load. Somehow the engine provides feedback to the carburetor in accordance with load, or energy demand. I don't know the mechanism by which this happens. I'm having a hard time researching it. This is outside the normal descriptive theory of operation of an internal combustion engine. It's always assumed that load will be matched to input air/fuel in a direct, linear fashion. I presume practically speaking this happens to be true. I need more hp to accelerate than to maintain a steady speed so I provide more throttle which "allows" more air and that more air picks up more fuel but the question I have is how 487cc of displacement can be stuffed with different quantities of air and fuel depending on whether the engine needs to make 40 hp or only 4 hp.

BTW the fact that lowering the needle made a positive difference is a good indicator. Did you lower it to the stock position? Are these stock needles?

What I found was my stumble, which had a similar root cause to your problem (rich under certain throttle/rpm/load conditions) was cured by switching back to stock needles and stock needle position. Turns out Suzuki & Mikuni knew what they were doing from the beginning. I also gained about 15mpg with seemingly no negative effect on performance; however, if I could actually lower the needle (I can't... well maybe I can change that spacer to a smaller one) then I might put a bigger main jet and lower the needle a mm or two so I'd have the bike richer at max load but the same (or close) at all other conditions. I have a gut feeling it could be a bit quicker at WOT and high revs and it probably was before with the bigger main jet and wrong needle. But the truth is, the bike runs so good, it's hard to consider changing anything.


Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #15 on: September 20, 2018, 10:47:15 PM »
So I just replaced my air filter (for the first time. I've put ~25000kms on it and I don't know when the PO changed it last. Please don't judge me) and now what used to be a small hesitation at 6k at WOT is now a complete bog down. The bike will pretty much refuse to increase in revs unless I back off the throttle a bit at which point it pulls again fine.

I'm on a 2006 GS with #20pilot jet and stock mid and main jets. Originally I had the needle at the stock middle position, so I first changed it down to 2nd lowest, and then lowest (making it richer). I then saw this thread and so went the other way and currently have the needle at the highest position (leanest position). I think it improved things, but there is still won't go past 6k at WOT.

I'm also currently adjusting the pilot mixture screw (or whatever it is called) as I needed to muck with it a lot getting it to idle a few months ago (which turned out to be because I mucked up a valve clearance change and so exhaust valves never closed fully. The old air filter probably didn't help either) so right now the screws are at 2 turns out. It seems to idle fine, but I'm not sure if this adjustment will affect the 6k spot at all or not.

Without changing jets I can't really do much with the needle. Any advice from the great learned ones here?

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #16 on: September 21, 2018, 07:17:32 AM »
Hi, What air filter was in there and to what type/make did you change it to?

Did you do anything else at the same time? I wonder if your diaphragm got pinched or holed while changing needle position or the little o-ring that seals the diaphragm has been lost/forgotten. All these will prevent slide rising and limit your RPM.

How do your plugs look? Black or white?

In my case, while testing, I covered about 15% of the mouth of the filter as an experiment and the bike would not rev past 5 or 6k. I was quite surprised at how big an effect this was This might be your problem but I doubt it, unless there is a problem with the filter. I heard about someone who kept taking his new bike back to the dealer for losing power at high revs. They eventually found the front page of the owners manual would blow up and block the intake. And then lie back down when they tried to look for a problem.

Offline Kilted1

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #17 on: September 21, 2018, 11:10:23 PM »
Like herenow, I also wondered if you replaced the air filter with the same type.  I believe the stock filter has a partial inner sleeve that adds just a bit of a restriction.  Checking the plugs is a good idea too

Just for the record, I was out for a space cruise this morning in more rural parts of the county.  I'm generally a pretty conservative driver and don't see much above 5Krpm.  But thinking about this thread, I decided to hold it open and see what happens.  Nothing but smooth, even power up through 8K.  By which time I was well over the posted speed (in 3rd gear) so backed off.  Can't say the throttle was wide open against the peg but certainly close to it.  No flat spot that I noticed and the bike is pretty much all stock.

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #18 on: September 22, 2018, 04:30:16 AM »
The air filter is a HiFlo brand, but a direct replacement for the stock one. It shouldn't flow any better or worse but who really knows. I figure it should at least flow better than a stock-but-full-of-grot filter.
Stupid me didn't think of checking the plugs. Had a look now and the both were snow white, which to me says lean.
So I've now changed the needles back to stock (middle position) and put the air screws back to 2.5 turns out. We'll see how it goes.

Offline mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #19 on: September 22, 2018, 01:04:37 PM »
You need to observe the slides and make sure they are coming up and together. The problem is not pilot jet. Vacuum leak, sticking slide, holes in diaphragms, etc maybe. Rule that out first.

It's not the air filter.