Author Topic: hesitation pulling away  (Read 1557 times)

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
hesitation pulling away
« on: October 12, 2019, 04:34:04 PM »
I'm a little stumped by this one.

When riding off from a standstill, just normal clutch out / throttle on my GS hesitates. It starts to move but the revs don't really rise for a second or two then it gets it together and away it goes. If I bring the revs up then let the clutch out the problem doesn't occur but isn't particularly smooth. If I coast to nearly a standstill, clutch in but then then carry on the problem doesn't occur - moving away with a bit of momentum prevents the problem happening. Behaves perfectly the rest of the time but it makes moving off from the lights a pain.

I'm at clip position 4 (1 richer than middle), about 2.25 turns out (this is the best position for my bike, 3 turns bogs down, 2 turns and below hangs the idle). Plugs are relatively new, air filter is fairly new float position is at the bowl join point, carbs are synced. 125 mains (tried 120s, 125s are a little better) and 40 pilots. charging system works ok, battery is past its best but will still start the bike. The bike starts very easily, like on the button easy, barely needs any cranking. hestitates both on choke and off choke. idle is somewhere between 1100 and 1500 but the needle bounces so its hard to be precise.

Any ideas? I can only think the ignition is slightly advanced (I snapped the rotor pin on the crank and jb welded it back together so the rotor could be slightly out - the rotor itself is a replacement but looks about the same) - last time I checked the timing marks on the cams and the rotor were about perfect though.
« Last Edit: October 12, 2019, 04:55:18 PM by tobyd »

Offline herennow

  • Trade Count: (0)
  • Formula Extreme Racer
  • ***
  • Posts: 219
  • Karma: +3/-0
Re: hesitation pulling away
« Reply #1 on: October 14, 2019, 06:13:47 AM »
Hi, is this new behaviour? Has anything changed recently? You probably know this but , The GS is not very powerful and needs revs to get going.

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #2 on: October 14, 2019, 06:57:00 PM »
its been around longer than I've had an SV which doesn't do it. My XJ600 didn't do it either.

The marks on the plugs is about right with the discolouration starting around the mid-point of the ground. The plugs are even a fair tan colour. I suppose I should compression test it really, it seems alright otherwise.

or maybe you are right and I just need to give it more beans off the line.

Offline Bluesmudge

  • Trade Count: (+4)
  • World Superbike Racer
  • *****
  • Posts: 1920
  • Karma: +32/-0
Re: hesitation pulling away
« Reply #3 on: October 14, 2019, 10:35:35 PM »
Have you tried adjusting the clutch cable? Almost sounds like you are not matching your throttle position at takeoff with your clutch disengagement. Could be caused by too much free play at the clutch lever. I.e. your clutch is fully disengaged but because you haven't fully released the lever you haven't started giving much throttle yet.

But yeah, GS needs to be revved pretty good to get going. I tried going +2 on the rear sprocket just because of how much I dont like the GS500's lack of torque off the line. SV650 probably has double the torque of the GS at low rpms.
« Last Edit: October 14, 2019, 10:38:27 PM by Bluesmudge »

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #4 on: October 15, 2019, 02:30:33 PM »
I'm going to go the other way and suggest it's a position-dependent vacuum leak, like at the intake boot o-rings. Tiny air leak there will result in the slide not lifting as it should and that's what this sounds like.

Also, I had similar odd behaviors when my carbs had what I suspect are dynojet needles, which were completely and utterly fixed when I went back to the stock aluminum needles. No, I didn't put those DJ needles in the carbs, but I happened to have a spare set of carbs to rob OEM needles from. For me, the problem was an undiagnosable stumble under certain rpm/speed/throttle conditions and a hesitation similar to what you describe. Turned out it was just too rich at tip-in. Also I got way better mpg (like >10mpg better) with the alu stock needles.

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #5 on: October 15, 2019, 06:57:03 PM »
I think the bike should be able to get moving without revving it up first - I hadn't really considered a tiny air leak, the boots are new with oring but maybe I screwed up fitting them or trap the oring or something - their replacement might just about coincide with this starting and we attributing it to otherstuff I was doing at the time? Good idea man! will investigate, does make sense that the slides are slow to rise under those conditions and its got nothing to go on until the vacuum vacuums (or whatever it does). Certainly the bike runs badly with the needles at mid-point, its much happier at 4/5 (euro-5-clip-needles) rather than shimmed.

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #6 on: October 15, 2019, 07:34:29 PM »
You know, it might be the opposite: it might be rich under these conditions. Maybe the answer is return the needle to middle and jet up a notch.

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #7 on: October 19, 2019, 07:27:26 PM »
As in, bring the pilots up to 42.5 or 45 whilst bringing the needle down or bump the main jet up a bit?

Had an investigate earlier. the carb to petcock vac hose is ok, one of the sliders is a bit loose in its housing. They both fall at about the same rate but one has a reasonable amount more play than the other. the intakes are ok, they are fairly new anyway and no signs of leaking.

Offline herennow

  • Trade Count: (0)
  • Formula Extreme Racer
  • ***
  • Posts: 219
  • Karma: +3/-0
Re: hesitation pulling away
« Reply #8 on: October 19, 2019, 10:33:54 PM »
Easy to test. Take out your air filter. Does it ride better?  It's then too rich.
Put back filter, cover 15% of filter mouth with duct tape , if it runs better, it's too lean.

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #9 on: October 20, 2019, 12:49:53 PM »
As in, bring the pilots up to 42.5 or 45 whilst bringing the needle down or bump the main jet up a bit?

No, the pilot jet has nothing to do with the main needle. Try just moving the needle. Main jet should be sized for wot high rpm running. This off idle middle throttle will be about the transition from pilot, where it is probably fine, to main, where I am guessing it may be rich.

Quote
one of the sliders is a bit loose in its housing. They both fall at about the same rate but one has a reasonable amount more play than the other.

This could mean something serious is wrong. I'd pull that carb apart and see why it seems loose. If it doesn't fit in the carb correctly then it will let a lot of air through. If it has too !uch drag coming up then it will really screw up throttle response off idle.

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #10 on: October 20, 2019, 03:46:56 PM »
put the original 115 jets on (uk spec) and it run badly (flat, no pull at 5/6krpm) at WoT, so i'll put the 120s in and see what thats like, 125s felt a bit farty at WoT so maybe 120 is the sweet spot. on all of them it hesitates though. It pulls quite well low down and with maybe 1/3 throttle just keeping up with traffic but something isn't quite right at either end. something for next weekend now though.

The slides aren't sticking or obstructed, one is just a bit clattery in its beige housing thing (runner?). play as in pushing it into its housing or pushing/pulling it when the slide is halfway up.

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #11 on: October 21, 2019, 05:26:45 PM »
I guess this was a restricted bike? without the restrictor, whatever all that is, it should work best with 125 main jets and the needle in the normal position. My guess is you have too small jets and you are making up for it with needle position.

Offline herennow

  • Trade Count: (0)
  • Formula Extreme Racer
  • ***
  • Posts: 219
  • Karma: +3/-0
Re: hesitation pulling away
« Reply #12 on: October 21, 2019, 10:57:29 PM »
Remember that the main jet will have nothing to do with putting away from idle.

Question? Is your bike restricted?

Have you removed the restrictor!

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #13 on: October 22, 2019, 06:45:45 PM »
The UK spec ones for '98 are115. It went up gradually over the years. In the US I understand you got larger mains out-the-box but smaller pilots. We got 40s from stock whereas i think you slummed it with 37.5s?

I had 125s in it and its been the best so far, 120s were less good but not just not as good, 115s really struggled. i'm tempted to try some 130s just to see how that reacts.

The idle hesitation is a stange one. a colourtune says my idle is nice and blue burning, the plug colour is about right, the plug gap is right, the plug colour change is on the bend in the ground electrode so my timing is probably about right. if i richen out the idle mixture it gets worse and if i lean it out it gets worse. If i set the needle position to the middle i lose the midrange completely (where its pulling the hardest). Float height is on the bowl join mark and the carbs are sync'd reasonable well.

there is no restrictor.

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #14 on: October 22, 2019, 07:11:43 PM »
125/40 is also what I am running. I wonder if you have stock needles. ?

In any case, my problem was similar to yours with 125/40 jets and dialed in everything but when I had the stainless steel multi-clip needles in it, which now I wonder if they were DJ needles or just euro-spec or canada needles. In any case it was absolutely cured by putting the ordinary alloy needles in.

I still think needle position is the root cause, but needle taper might be what is causing you to choose your current needle position. Perhaps a 127.5 jet will work with those needles dropped down a notch or two.

Offline herennow

  • Trade Count: (0)
  • Formula Extreme Racer
  • ***
  • Posts: 219
  • Karma: +3/-0
Re: hesitation pulling away
« Reply #15 on: October 24, 2019, 12:40:18 AM »
Hi Josh, I know that you know this but I just want to clarify your last phrase just incase.

"Perhaps a 127.5 jet will work with those needles dropped down a notch or two." Changing the main jet will have no effect on pulling away from Idle.

Regarding the question of needles and jets, there has been a lot of discussion on this forum, and it's really weird that some bikes had 115 and other bikes head 130s. what's even weirder is that the restricted bikes had 115 on the left and 125 on the right for those years.  There were ideas that maybe the jets were different for environmental purposes and the way that the emissions were measured. (Measured at idle versus midget at full throttle).

I'm not sure if I buy that though, as a carb thats designed to run at full throttle with 130 would probably hole a piston running at full throttle on 115s for 5 minutes. I suspect there is some difference that we do not know about.

Offline mr72

  • Trade Count: (0)
  • MotoGP Racer
  • ******
  • Posts: 2272
  • Karma: +38/-0
  • Musician, Engineer, Sci-Fi Author
    • Josh's Books
Re: hesitation pulling away
« Reply #16 on: October 24, 2019, 12:39:18 PM »
Hi Josh, I know that you know this but I just want to clarify your last phrase just incase.

Alright... but just for clarification, I wrote this, which is true:
Quote
"Perhaps a 127.5 jet will work with those needles dropped down a notch or two."

But not this, which is a myth:
Quote
Changing the main jet will have no effect on pulling away from Idle.

If the slides come up, the main jet is in play. Period. It's physics. The bike idles using the idle air orifice to get air across the pilot jet. All of the fuel from the pilot jet comes via this circuit.

Now it might be true that due to needle profile and position etc. changing the main jet will have no meaningful effect on low throttle or pulling away from idle, but it's not true that the main jet has nothing to do with operation in this case. Believing this myth causes a lot of folks to chase their tail or to make their bikes extremely rich at idle and then struggle with other running issues.

Quote
Regarding the question of needles and jets, there has been a lot of discussion on this forum, and it's really weird that some bikes had 115 and other bikes head 130s.

This is the first I have heard of any bikes with the mk1 mikuni carbs coming stock with 130 main jets. That'd be quite big. I my own GS, 127.5 is too big, 125 is nearly perfect and 122.5 (stock) is completely usable even with my exhaust changes.

Quote
what's even weirder is that the restricted bikes had 115 on the left and 125 on the right for those years.  There were ideas that maybe the jets were different for environmental purposes and the way that the emissions were measured. (Measured at idle versus midget at full throttle).

I'm not sure if I buy that though, as a carb thats designed to run at full throttle with 130 would probably hole a piston running at full throttle on 115s for 5 minutes. I suspect there is some difference that we do not know about.

That all sounds quite weird. I am no expert on any of this. I only know about MY carbs, of which I have two sets, both had 122.5/37.5 jets stock, but each had different needles. And I have been told that the Euro bikes have needles with multiple circlip positions while the USA bikes have alloy needles with only one clip position. IDK if profiles are different, etc. Also I suspect that the restricted bikes with 110 or 115 or whatever jets likely also have different needles and perhaps different slide return springs, but I have no way to know this.

What I do know is that there are a lot of variables that come into play when talking about jets, needles, and airflow. The jet is sized according to the needle taper and the velocity of airflow. This is the most commonly misunderstood thing about CV carbs. It seems natural to assume that more air needs a bigger jet. But it's SLOWER air that needs a bigger jet. To have more air would require more displacement or higher rpms. Doing things like restrictors or air filter changes etc. affects the velocity of the air, not the volume.

Anyway, in tobyd's case, I think the following applies:

1. the pilot jet is fine, stop looking there, it's a fool's errand
2. the main jet should be selected for WOT/high-rpm max power and with stock exhaust and air filter 125 is probably as good as you can get, stop looking there, it's a waste of time.
3. anything between idle and max/WOT is determined by needle position (in the jet!! not the clip position!!) and profile. So if it's too rich, then the needle is either too high or the profile is too narrow at the position where it sits in the jet under those throttle/rpm conditions. If it's too lean, the needle is too low or the profile too fat. I put a micrometer on my two different needles and found the profile was quite different between the two and the stainless/multi-clip ones were too narrow and causing a very rich condition at like 1/8 throttle and 2500 rpm. My suspicion was that these are aftermarket, likely dynojet, needles which were coupled with a smaller main jet and provide "better midrange" at the expense of max WOT power, but I had no way to confirm or know this. Could be that they are Canada or Euro needles and that there is a different profile for emissions or fuel economy or to match different fuel type.

So tobyd, do you have a micrometer? Maybe measure the needle at 10mm, 20mm, 30mm from the end and compare? Somewhere I measured this and posted it on these forums. But if you say dropping the needles down a notch "kills the midrange" then I'd say go ahead and drop them and jet up one size as an interim.

Offline herennow

  • Trade Count: (0)
  • Formula Extreme Racer
  • ***
  • Posts: 219
  • Karma: +3/-0
Re: hesitation pulling away
« Reply #17 on: October 24, 2019, 01:13:05 PM »
I'll respectfully disagree regarding the effect of main jets on idle and mid throttle operation. The issue of air velocity does not come into it. The controlling factor is that the annular area between the her and the needle jet is the limiting factor. I.e it's smaller than the main jet all the way till the needle moves out the jet.
The ultimate test is to take out the main jet and go for a ride. I've done this on CV and on standard carbs. It's amazing , bike runs perfectly until throttle is at 80% and then the bike starts 8-stroking.  I've got a few online references for the maths I've posted previously.

I've also posted the key to reading needle numbers to assist in identifying them. I can dig it out when I get back from korea next week if anyone needs them.

Re the 130, you are right, I always forget France got the later 34 mm carb (with 130s, and interestingly , 135s on the restrictedss) 3 years before the rest of the world (98/99/00).  Those years the biggest standard carbs used 125 left and 125.5 right!!!

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #18 on: October 25, 2019, 08:35:52 PM »
Sure, i'll take it out since I need to put the 125 mains back in to get any kind or performance out of it and i'll measure the needles whilst i'm there. I suspect the needles are stock but I also thing they have a marking on them so maybe they are identifiable and.or worn out and need replacing. the houses they live in (I can't remember, whichever arrangement of needle jet, jet needle, net jeedle it is) aren't worn.

Whats possible muddier here in my bike is actually a French market one brought over here (UK) new so what was spec and what it came with are probably questionable, that and i'm owner 15 so who knows whats its had. I'd guess the 115s that came out when I first took the carbs apart were standard but the bike did run ok. It was also winter when I bought it so it didn't get a lot of WoT what with the cold and all so it might have run like crap.

The bike has a freer flowing exhaust and headers now too just to complicate things. I doubt its much freer though, it came with a nexen end came which was full of holes... I doubt the downpipes are much different from stock.

I suspect the 125/40 arrangement is ok but you raise an interesting point about the needle taper mr72 perhaps its wrong and messing up the mixture as I bring the throttle on to accelerate away. I'll investigate over the weekend, its going to piss down tomorrow so maybe sunday.

Its all further complicated that the SV650 I ride to work and back is that bit more powerful so riding the GS after a week on the SV always feels a bit slower.

Offline tobyd

  • Trade Count: (+1)
  • Formula Extreme Racer
  • ***
  • Posts: 265
  • Karma: +4/-0
Re: hesitation pulling away
« Reply #19 on: October 27, 2019, 05:17:17 PM »
Going to run a compression test on it just to make sure i'm not chasing my tail.