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1/4 throttle stumble

  • Thread starter Thread starter Kapoor
  • Start date Start date
K

Kapoor

Guest
Hello there! Not only am I new but I also have some questions to ask anyone who might be able to help!

Not even a few months ago after finishing my engine rebuild on my '80 Gs750Lx I rebuilt the carbs on my bike and replaced all of the jets, and o-rings, and plugs with new OEM parts. (I pretty much replaced everything but the needle jet, the jet needle, the slide, and the diaphragm which was in very good condition). Got her running again and synced the carbs. Everything was running very strong with a deep rumbly idle, but when i roll the throttle to almost exactly 1/4 throttle the engine stumbles, and coughs. This only happens when the engine is under load however, when in neutral it revs like there's no problems at all. Everything up to 1/4 is fine and almost immediately after 1/4 (maybe 3/8) it just takes off like expected. This is the same problem in every gear so i know it's a carb problem, but when looking around the internet all i keep getting is that the slide is what does the most work at 1/4 throttle with help from the needle jet and jet needle.

So my questions are how, or can i even adjust the slide and has anyone else had the same problem?

PS. when i had the carbs apart the slides were very smooth with no apparent wear that i can remember.
 
Try shimming the needle.
Remove the thick nylon spacer on top of the needle clip, replace it with two small washers which are slightly thinner than the thick spacer. You can get washers at radio shack, or some hardware stores.
You can do it with the carbs in place, just remove the top covers and pull the slides out.
 
Hey thanks for the quick reply!

@tkent02: I'm gonna go and try that. So by dropping the needle I'd be cutting the amount of gas that is allowed into the engine correct? or is it cutting the air thus increasing the air:fuel ratio?

@chef1366: Also sorry I meant to mention that, I currently have stock pipes, and the stock air box with a cylindrical K&N filter on it. I recently replaced the rubber connectors that go between the airbox and the carbs, but i haven't replaced the other set of rubber boots.

Does this sound anything like an air leak? I figured if it was it's run all sporadic at any throttle.
 
Actually you're raising the needle hence introducing fuel quicker and richening the needle circuit.
 
Kapoor said:
Hey thanks for the quick reply!

@tkent02: I'm gonna go and try that. So by dropping the needle I'd be cutting the amount of gas that is allowed into the engine correct? or is it cutting the air thus increasing the air:fuel ratio?
Click to expand...

No, you will be raising the needle in relation to the slide, the washers you put in will be thinner than the thick spacer you take out, the spring under the clip; will push the needle up higher in the slide. Then it will be letting in a tad more fuel, at a given airflow, especially around the 1/4 throttle area, that's where the needle has the most effect. The slides go up and down in response to throttle position and airflow, the needle rides up and down with the slide, in and out of the needle jet, controlling fuel flow... So raising the needle in the slide gives more fuel.
Go easy, small changes can have a big effect.

Don't lose the spacers, if it runs worse after you will want to go back to stock.

A lot of these bikes are very lean from the factory in the low throttle openings, at low power. They run OK wide open, as the were no EPA tests at full throttle, just the low speed stuff was set too lean.
 
Hey thanks again. I appearently didn't have the right image in mind earlier. I opened it up and installed a few washers instead. The problem is almost resolved at this point; just a little jittery still. I'm going to try to find some slightly thinner washers than the ones i had around the garage and install those instead and keep testing it out till it's right. Thanks again for the quick help. This problem has been killing me for the last few days. :mad:
 
Instead of scrambling for thinner washers, simply remove one from the stack you have put in there. :-\\\

.
 
Yeah, what matters is the thickness, if it got better you are going in the right direction, just go a little farther.

After you have the 1/4 throttle stumble fixed, readjust your mixture screws for the highest idle, the needle change may effect it a little.
 
Your advise is priceless! Thanks again. I'm going to go ahead and remove one of those washers once I get offwork tonight and give it a go. Unfortunatly I have to work till midnight, so t-minus 6hours till test time! Really excited to take my the bike for a longer ride than 150miles finally without getting annoyed with the throttle issues.
 
Alright, so after taking out the slides again, removing one of the washers, and losing a snap ring in the process. I got it all back together and took her out for a ride. It was way better, but there's is still a hesitation/stumble at about 1/4 throttle. Is it wise to remove the last washer? Is there anyother settings that I should make sure I hit while doing these changes? Thanks again for all of your help.
 
Now you're compensating a lean main jet by richening the needle too much.
You need to bump up the main jet a couple of sizes.
 
Last edited:
Ah makes sense. Well left the last washer in there for now. If I was to get a jet kit where would you suggest I begin my search? On the same note this probably means i need to pull the carbs out and take a quick peak at what exactly i have in there, so i know by how much i'd be upgrading.
 
I'd call around and check if you can find some 117.5 Mikuni large round main jets.
THEN jet the needle with the washers.
 
The accepted procedure for jetting CV carbs is to get the main jet right first, then work your way down.

Have you done any plug chops?
 
chef1366 said:
I'd call around and check if you can find some 117.5 Mikuni large round main jets.
THEN jet the needle with the washers.
Click to expand...

I'll put in some calls tomorrow before work and see if i can pick some up from one of the local shops over here.

tkent02 said:
The accepted procedure for jetting CV carbs is to get the main jet right first, then work your way down.

Have you done any plug chops?
Click to expand...

Once again as a total carburetor novice These are things that are go a long way! Thanks. :D
As for plug chops: A. I don't know what that is/does exactly:confused:, B. Other than clean them and jet the needle I haven't done anything

Edit: so I looked up plug chops.. yeah should have done that before reposting. Anyways Nope haven't run the engine at a specific speed/rpm,
then kill it, and check the plugs. Would you suggest doing this before getting the larger main jets or after? I'd assume after to better assess
the upgrade however it might be interesting to see how it is performing before the step-up and then after.
 
Last edited:
With the airbox/K&N and stock pipe you will be good to go with the 117.5s. You probably will lose the stumble with two washers.
 
Kapoor said:
... Nope haven't run the engine at a specific speed/rpm, then kill it, and check the plugs. ...
Click to expand...
It is not speed/rpm that determines when to chop the throttle to read the plugs, it is THROTTLE OPENING.

Use some white tape (masking tape will work, too), place a strip on the throttle grip and a strip on the housing. Place a mark across both of them, near the top. Hold the throttle all the way open, make a mark across from its mark, on the housing's strip. Measure between the two marks (idle and full throttle), make marks for 1/2 and 1/4 throttle. When you are done, it might look like this:

IMG_3646.jpg


Now you are ready to hold the throttle at specific THROTTLE OPENINGS to do your "plug chops".

.
 
Steve said:
It is not speed/rpm that determines when to chop the throttle to read the plugs, it is THROTTLE OPENING.
Click to expand...

Haha Thanks for the correction, essentially that's what I was trying to say. Also quick question do we have a pdf on proper procedure when doing plug chops on?
 
Kapoor said:
Also quick question do we have a pdf on proper procedure when doing plug chops on?
Click to expand...


Not a PDF, per-say, but I think you'll find the following links rather useful:

http://members.dslextreme.com/users/bikecliff/
http://members.dslextreme.com/users/bikecliff/images/carb_sync_notes.html
http://www.iwt.com.au/mikunicarb.htm
http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html

That's a pretty good primer in carb theory and tuning. Also, when you rebuilt them, you rebuilt them using this guide, right? :D

http://www.mtsac.edu/~cliff/storage/gs/mc_maint/GS_CV_Carb_Cleaning_Series.pdf

I know it's a no brainer, but I'd also wager that you replaced all those thirty year old o-rings with pieces from Mr. Robert Barr, correct? :D

http://www.cycleorings.com/#BS_Series_O-ring_Kit

Buy a bunch of spark plugs for your chops. Use a new set for each circuit, if you can swing it. That's a lot of plugs to buy, but you should be able to reuse the plugs after you get the tuning dialed, thus lowering your maintenance cost. Nice new plugs make it real easy to read the A/F ratio. A Colortune is also an option. Also, make sure you sync the carbs before trying to tune them.
 
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