Author Topic: That dreaded old 6 k RPM dead spot SOLVED  (Read 1745 times)

Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #20 on: September 22, 2018, 10:04:50 PM »
I just checked the slides, they look to be going up fine (and wow that induction noise without an air filter). I also made sure to check the diaghragms when I had them out and I can't see any holes/tears/pinching.

I don't think there is a problem with the new filter, I just think I had everything adjusted for an old clogged filter and now I have a clean one everything needs to be readjusted. I'll put everything back to stock adjustments and see how it goes and report back.

Offline Kilted1

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #21 on: September 22, 2018, 10:25:07 PM »

I don't think there is a problem with the new filter, I just think I had everything adjusted for an old clogged filter and now I have a clean one everything needs to be readjusted.

I was thinking the same thing after you said how icky the old filter was.   :thumb:

Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #22 on: September 25, 2018, 04:03:36 AM »
Well I now have the pilot screws set to 2.25 turns out, and the bike idles fine and is smooth and strong when in the low range of revs. Still have the horrible stumble at 6k though.
I'm fairly confident I don't have any leaks, but I still don't know if I'm running rich or lean at that 6k WOT spot. I guess it's time to play with the needle again.

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #23 on: September 25, 2018, 09:04:03 AM »
One trick is to take the air filter out. If it runs better you are rich. If stumble is worse you are lean. Is the stumble at any throttle opening?

Online mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #24 on: September 25, 2018, 12:23:20 PM »
Well I now have the pilot screws set to 2.25 turns out, and the bike idles fine and is smooth and strong when in the low range of revs. Still have the horrible stumble at 6k though.

The pilot mixture doesn't affect that 6K/WOT condition. That's a main jet issue.


Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #25 on: September 26, 2018, 02:14:36 AM »
I understand the pilot doesn't directly affect the 6K/WOT issue. I just had 2 seperate issues, so was clarifying that one seems to be under control now (the idle/low load problem).
I'm going to try running without the air filter this arvo and see how it goes.

Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #26 on: September 26, 2018, 06:39:05 AM »
Well I just took it for a quick run with the air filter removed. Learned 2 things:
1. God Damn it sounds good a full throttle without an air filter
2. No 6k dead spot at all.

This now sounds exactly what Herenow was seeing as well in the original post. It would then make sense that I am normally running too rich at WOT with the air filter in, and so should lean out the mixture/needle.
The next problem is I've already had the needle at the leanest setting, and still had a big 6k stumble.
I'm wondering if this HiFlo air filter is more restrictive than a stock one would be, so it can't suck enough air with this filter. Has anyone used these OEM replacement filters?

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #27 on: September 26, 2018, 07:51:57 AM »
Hi TGTwin,

Interesting stuff. I'm really interested that you could replicate my experience!

First, can you confirm that all is stock on your bike? (just to make sure no different variables)

Secondly, I've compared the Hiflo air filter with stock and in my opinion, the stock will be the same or more restrictive, I doubt very much it will be more restrictive.

Thirdly, when I dropped my needle 1 mm (made new plastic doughnut) the dead spot was reduced, BUT, the bike behaved a bit weirdly when starting, would not take load for a couple of minutes and reacted a bit strangely to the choke. This makes me think that the original needle position was correct when at idle. I.E the bike is possibly a little to lean at idle. (I know I know, the needle is not supposed to have any effect at idle but it seems there is a slight effect)

Fourth, (and this is where I go out on a limb) to share some ideas and other research I have done (I know I know, a gs500 is not worth all this time but I like solving problems) - I have been told by talking to some experienced mechanics that the reason one gets WOT problems can be that the slide goes up into the  carb body and you get a hole where previously there was a protrusion. This messes with the air flow and can cause various problems. Is this our problem, I do not know. But this chimes with what Buddha said to manage the problem with the wrist (i.e. avoid WOT).

Sooo, where does this leave us? It seems to me that the std needle position is good (in my case at least) but bike is getting too rich at certain times. The only other factor that I can control is the response to vacuum. By that, I mean either the diaphragm spring or the vacuum hole.

-I understand that the spring has a double duty, it helps to control the height of the slide and also tries to damp out oscillation.
-The vacuum channel transfers the vacuum to the slide a larger hole will increase the speed of the repsonse, while a smaller hole will reduce the response rate. Considering how much, and how fast, the slide moves - I don't know if this has any real effect but it is conceivable that a smaller hole may dampen the movement of the slide. If this affects the final position I don't know (by "final" positon I mean that, with a smaller hole,  does  the total amplitude of the slide movement get reduced because the slide no longer has the speed to move up and down as it did before) but my thinking is that this might not be important. Especially as others have reported quite stable slides at higher RPMs so even with one hole the final height of the slide will be the same as with two holes in a "stable" situation.

A back of the envelope calculation indicates that my diaphragm springs have cycled between 3 and 50 million times. Does that make them a bit tired and allow the slide to side too high or oscillate too much?  Who knows. My next test is to maybe order a couple of new springs as a reference (or maybe just to stretch my springs 10% or so, I know this does not affect spring rate but it will affect the "ride height"  ;) of the slide and should keep the slide a little lower=leaner at higher openings).  Based on that result I will temporarily block a vacuum hole in each slide with some silicone to see the effect if needed.

Time to bump my favourite dead thread re spring length survey  ;) here 

cheers

« Last Edit: September 26, 2018, 08:08:58 AM by herennow »

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #28 on: September 26, 2018, 08:06:23 AM »
double post deleted
« Last Edit: September 26, 2018, 08:07:45 AM by herennow »

Offline TGTwin

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #29 on: September 26, 2018, 08:30:15 AM »
My bike is pretty much stock, except for a one-size-larger pilot jet (#20? something like that).
Can't really say if the previous owner has done anything, but it doesn't look like it (all the parts looks stock, and I haven't got the impression of custom stuff on the bike).
Thinking about it, I did remove the extra spaghetti hoses and vacuum relay that connects to the top of the carbs (Not the PAIR system, the other type that has an "ecu" activated vacuum relay). Everything I could find online and in manuals seemed to indicate this was just some emissions thing (holding slide open on decel maybe?) but they were connected to the top side of the slide diaphragm, so maybe it did contribute to this whole mess.
Is your carb ad associated cruft completely stock?

Offline The Buddha

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #30 on: September 26, 2018, 09:36:01 AM »
Is this our problem, I do not know. But this chimes with what Buddha said to manage the problem with the wrist (i.e. avoid WOT).



Whaaaa …. No, The slide lifts too fast. The WOT is not to be avoided, you don't snap it open, gradual opening as hard as it can accelerate is fine.
To prevent it from opening too fast, we close 1 hole in the bottom of the slide with a 4-40 plastic screw and cut it flush etc. But - I prefer training my wrist.
I also think yours has a problem in float level, the bike is very sensitive to float level BTW, and you may have caused a nw problem lowering the needle below stock by sanding the donut, if your problem is just in the pilot/start up circuit - try 1 size larger pilots or may be a bit more mix screw.

Cool.
Buddha.
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Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #31 on: September 28, 2018, 02:59:31 PM »
All stock on my side. I had a K&N free flow filter, and changed it to a hiflo filter, but that made no difference to anything.

Offline Kilted1

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #32 on: October 01, 2018, 02:57:57 AM »
Don't know why I didn't think of this earlier but instead of trying to lower the needle, why not just go with a smaller jet?  That needle is going to go down only so far before it bottoms out.  But you can reduce the jet size, raise the needle, and end up in effectively the same place. 

Maybe the suggestion has already been made and I missed it. 

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #33 on: October 02, 2018, 05:17:07 PM »
Your question reminded me of a little work I was doing on understanding the factory jetting of the carbs, I got it finished today. Hope it is legible. It’s a table of all the jetting worldwide of the GS500's since 2001. It is from the factory service manual and errors are therefore possible...
Few things jump out:
1. the carbs are always "BSR34" but note that in 2004 they changed from BSR34SS to simply BSR34. Is this important or just a printing error in the annual updates, who knows?
2. Notice that the needle Jet never changes (P5-M), just needle position.
3. As can be seen from the key below the table (but I might be wrong on this and hope you folks can confirm/deny) is that ALL needles have the same 1st and 2nd taper, meaning that they are functionally identical. The only difference between all the years and countries is the 4th number which indicates "manufacturer’s code". I understand that this is primarily related to the model number and the fact that some have multiple needle grooves. However, the base needle diameter (the last numbers in the naming convention) is never indicated. At the moment I am assuming they are the same. Europe always has a clip adjustable needle while the US and Canada have fixed ones.
4. Interestingly the main jets for the same engine and carb vary from 115 to 135. A huge difference!  Also where the smaller main jet is specified (115), a leaner needle is also specified (2nd position) while when a larger main jet (130/135) is specified a corresponding richer needle position is specified (3rd/middle position)
After the first couple of years the jetting settles down to 130 mains with fixed needle or needle with middle position (probably the same effect?) with a couple of 135s specified. Did something change with the “SS” carb change?
I find it really interesting that specs change so for the same engine and carb. The engine must be running REALLY lean in some of the cases. Makes you think that, if the most relaxed emission rules applied, a 135 main jet would always be chosen, and should be the starting point for our bikes?

What the hell the Canadians (me😉) and the US in 2001/2002 were doing off at the 127.5 shallow end is anyone's guess...



I realise I’ve more questions than answers but was an interesting exercise nonetheless.
« Last Edit: October 03, 2018, 08:39:02 AM by herennow »

Online mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #34 on: October 02, 2018, 05:31:26 PM »
While I don't have any documentation to point to that would prove it, I suspect the 115 main jet is used on restricted bikes for EU and AUS.

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #35 on: October 03, 2018, 08:37:53 AM »
While I don't have any documentation to point to that would prove it, I suspect the 115 main jet is used on restricted bikes for EU and AUS.

Hi Josh. I also wondered about that but could find no information on the different models that run with the 115 jetting. However, by process of illumination (geddit?  :D) it seems this might not be the case. For example, in 2003 the 115 jet was supplied in the "GS500" and "GS500-U" models to UK, EU, OZ and Spain; and the 130/135 jets were supplied to EU only in the "GS500-H" and "GS500-HU" models. The idea of 115 low power models could only be true if that is the only model that was supplied to UK, OZ and Spain that year. Might be the case but not so sure. Sledge had stated previously that he had heard that the "GS500-U" was the restricted one.  Someone on this forum also stated that in OZ bikes were never restricted. More checking to be done...
« Last Edit: October 03, 2018, 09:44:11 AM by herennow »

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #36 on: October 05, 2018, 10:18:40 AM »
Had another interesting discussion this week. It appears as though the statement underlined in the quote below might be false. I was told that a slightly lower slide (due to a stronger spring) will lead to a higher air velocity that will suck more gas, especially at larger throttle openings where air speed provides more fuel signal than the vacuum. So I will tie two coils of my spring together with thin wire to shorten it and see what effect that has.

I know I know, I'm theorising too much and testing too little but my kid has been sick....

My next test is to maybe order a couple of new springs as a reference (or maybe just to stretch my springs 10% or so, I know this does not affect spring rate but it will affect the "ride height"  ;) of the slide and should keep the slide a little lower=leaner at higher openings).  Based on that result I will temporarily block a vacuum hole in each slide with some silicone to see the effect if needed.

Online mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED (probably....)
« Reply #37 on: October 05, 2018, 11:33:42 AM »
binding two coils of the springs will indeed make the spring stiffer, which will potentially have the effect you want to test.

Offline herennow

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Re: That dreaded old 6 k RPM dead spot SOLVED
« Reply #38 on: October 11, 2018, 07:47:11 AM »
Been busy and have not had a chance to get look at this further. Had 2 hours free and was too lazy to explore different springs etc. So I decided to lean the bike out the easy way. Dropped the float level to 3 mm BELOW the seam line, too much I know, but felt like pushing the issue to get some clear feedback.

Flat spot gone completely. Bike was PERFECT....

....until 5 minutes into "spirited" riding where the float bowl was not filling fast enough and I started to get fuel starvation.If I rode slowly for a minute I could then do another 5 minutes of caning...

So, next time I have time, I'll raise the fuel level by a bit at a time and hopefully find a happy medium.

So, if you have the dreaded 6k flat spot, try lower your fuel level!! Could it really be that easy? It seems so.

The way I do it is to measure fuel level with the clear hose. Drop the float bowl. Measure the float "height" that equates to that fuel level. This takes a little practice as you are doing it upside down but as long as you do it the same way each time it's ok. Then remove float (carb stays on the bike, need a very short Phillips/JIS), adjust tang and reinstall. Check float level is now where required, button it up and off you go. Obviously, you need to have the cap screws in the bowl to be able to remove it in situ.

Kudos to Buddha, who again called it some time ago!  I digressed badly but hell did I learn a lot!!!

Online mr72

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Re: That dreaded old 6 k RPM dead spot SOLVED
« Reply #39 on: October 11, 2018, 12:01:23 PM »
Replacing the float bowl screws with allen-head screws is a good idea, BTW. Facilitates in-situ float level modification.

I don't remember the whole history of this thread and I'm too lazy to go re-read it, but this sounds and feels to me like a needle issue. I wonder whether a new pair of stock aluminum needles wouldn't totally fix this.