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750E Hesitation ??

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Hi Guys, I have a 1979 gs750E all original and done 22000 miles, it starts with one kick every time or of course a push of the button, ticks over smoothly and pulls away with determination,,but,,when it is up to temperature it idles a little erratic and when I blip the throttle the revs dip instead of inceasing,,if idling at 1500 on blipping she'll dip to perhaps 900 and then gently but not immediately return to more or less 1500 though still slightly erratic going from 1450/1550 and anywhere in between.
I stripped the carbs and found absolutely nothing,,spotlessly clean,, I still blew out the jets and checked the float levels and valves,,,all ok and within spec.
Plugs are medium brown, put new points and condensers in, air filter is clean and oiled, and I can see no apparent air leaks or obvious cracks in the inlet manifold rubbers or whatever you call them.

So please what do you think as I'm running out of ideas !!!

Regards,
Mike.
 
get a spray bottle and fill it with water then give your intake boots on both sides of the carbs a misting the RPM's will drop if you have a vacuum leak.
 
An erratic idle especially when the engine is warmed up, makes me think you have an intake leak or you need to give the carbs a vacuum tool synch. Whenever the carbs are off, you should check the manifold o-rings. If it's been several years since they've been checked, I can almost guarantee you they need replacing. The manifolds themselves need checking too.
Poorly synched carbs will cause your problem too. Using a mercury tool, the vacuum levels need to be set so the difference between the highest and lowest level is no more than 1 cm to avoid vacuum/mixture problems.
A 1,500 rpm idle is too high, if you've deliberately set it that way. It should be about 1,100. A high idle will increase deposits on the plugs and cut into your gas mileage. You also run hotter in stop and go traffic and your brakes pads and clutch wear out just a little sooner.
If you're sure the carbs are seated in their boots/manifolds correctly and the clamps on both sides are tight, then I would go back and inspect for intake leaks and check out the o-rings regardless. Then I would do the carb synch with a tool.
One last thought...did you move the side air screws or the pilot screws underneath?
 
750E Hesitation ??

Thanks Don and Keith for your speedy response.
I did set the idle speed high purely for fear of it stalling as I sit at the head of the queue at traffic lights etc, the embarrassment of holding everyone up with a stalled machine would be too much to bare.
I have not dared touch the side air screws or the pilot screws as the manual is so emphatic that they should be left alone.
I was going to do a carb balance first thing today but now feel following your comments that I should do Don's test first and then pull the carbs so I can do a full inspection of the manifolds/rings etc, anyway I will let you know how I get on.
Thanks again,
Mike.
 
If this is in time before you do further work, allow me to back up just a bit.
If you deliberately set the idle that high, then I'm not so sure about an intake leak. It's still a good idea to inspect the o-rings and because they're cheap, you might as well replace them. Of course, you may have to order them, so you just have to check them first. If there are any cracks and you don't plan to have the carbs off again for a long time, you should replace them.
If you replace them or not, apply some hi-temp' bearing grease to them and torque to about 6 ft/lb.
The carb synch should be done as normal maintenance.
Also, has the bike just started doing this or did you just buy it like this?
I'm not sure what your idea of "blipping" the throttle is. These VM carbs can be "gagged" for a moment if you open the throttle too quickly. That does'nt mean there's a problem. When my bike was only a few days old I discovered this and took it to the dealer. The lead mechanic there showed me on two other customers bikes the same thing.
Like I said, I don't know exactly how quickly you're blipping the throttle but if it's a fairly quick twist of the wrist, then I'm just saying there's a good chance the carbs are reacting normally. It's not an easy thing to describe or diagnose. When you say the rpm's do not come back up immediately has me a bit concerned though. Again, hard to diagnose without being there.
 
Hi Keith,
Thanks for the update. with regard to the blipping of the throttle I guess it's like when you are about to pull away and you crank the grip that few milliseconds before letting the clutch out, this is when it the revs have tended to dip as apposed to rise, so I've become a bit wary of it now and try to bring the revs up well before letting the clutch out.

I have done a little work on it today, I did Dons test with water misting and there was no change, I also did a very detailed visual on the rubbers and found no cracks.
Before I go any further I ought to say that I bought the bike last year but did very little riding, then it stood for the winter in the garage, and only now do I find I have this problem. That said it could have been there when I bought it as I found two other things which were not right, firstly the timing,,, the points were miles out, in fact I rotated the stator plate a good 3/8 of an inch before the left set were on time and correctly gapped, I then gapped and reset the right-hand set.
To add to this, I decided this morning to count the turns in of the air screws and found them somewhat adrift, anything from one turn out to three turns out, so I set them all to 1 1/4 turns out followed by small amounts of adjusting so she sounded a bit more steady.
I then did a balance of the carbs and found them adrift though not by lot, I guess the most was about half a turn of the adjuster screw.
As yet I haven't tried it out on the road because as is usual in the UK just as I was about to take it out it started lashing with rain.
Anyway I will take it out shortly and let you know the result.

In the past I've had a gs550, gs650, a gsx 750et ( horrible truly horrible) an xj 900 and a fj1200 and none of these fours had the problem I have with the gs750, but if I am right all these bikes had constant velocity carbs which perhaps do not suffer from the gagging that you mention.

Thanks again Keith for you interest and valuable comments,
Mike.
 
OK. The way you describe it now, it does sound like a problem, not something you're doing to it.
It sounds like you did a manual carb synch? Let me assure you, a 1/2 turn of the adjuster screw is a HUGE adjustment. If you could see the results on a mercury gauge, you would see what I mean. I have manually synched these VM carbs many times and I've gotten pretty good at it. I always follow this with a vacuum tool synch. Every time I found at least one cylinder drawing significantly more vacuum than the others, but I've never had to adjust any screw a 1/2 turn to get the level right. On a gauge, you have to get the levels so there's no more than 1 cm difference between the highest and lowest level. If the difference is greater, you'll have mixture problems. The greater the difference, the greater the mixture problem. Too much vacuum results in a richer mixture and vice-versa. These carbs demand a gauge synch.
As for the side air screws, these need to be adjusted before doing a carb synch with a gauge. They are to be adjusted to achieve the highest possible rpm, then you re-set the idle with the idle screwknob.
Put the bike on the centerstand and warm it up completely. Make sure the throttle cables have the correct slack. In your case, lower the rpm's to 1,100 with the idle screwknob first before adjusting the screws. Now adjust "two screws at a time" in 1/4 to 1/2 turn increments to more easily hear the rpm's rise. Fine tune these two screws. Now do the other two and fine tune. Now re-set the idle with the idle screwknob to 1,100 rpm. You should end up with them about 1 1/2 to 1 3/4 turns out. Yours sound like they were off. Three turns out on any screw is too much. They may have been moved by the previous owner but they can move from vibration if they are missing their spring. Make sure they have their spring. Don't lose the little washer if you pull things out. The spring usually stays in the hole when you remove the screw. If these screws are stiff use a good fitting tool to avoid stripping the heads. If they are stiff, then it's doubtful they could have moved themselves and the check for a spring is probably pointless.
Adjusting these screws correctly may stop your problem. That's why I questioned them before. They will definitely effect the throttle response.
And just to be sure, I'm talking about the air screws mounted on the sides. Not the pilot screws which are reached from underneath. (There's been a lot of confusion in the past.)
 
Hi Keith,
For the record I used vacuum dial gauges to sync the carbs, though I think perhaps the mercury ones may be better and more accurate.
Anyway I took the bike for a spin and think I've got a result, she definitely feels smoother right through the range and did not hesitate once. The acid test will be tomorrow when I will do a journey that has not failed so far to produce the problem, if it passes that test then I know for sure I've got it beat or at least seem to be heading in the right direction.
Assuming the worst is over I do intend to follow your latest instructions to the letter, so that the carbs are really tuned to their best. Perhaps it would be a good idea to pull the carbs once more, give them another clean and blow out, change the rubber manifolds and O rings and finally reassemble, followed by your tuning suggestions, this would leave the carbs at their best, and not risk the need for future stripping to put new rubbers on.
Out of interest I noted that two of the top carb covers had no gasket on them, so on reassembly I put some blue hylomar on till I get the proper gaskets. Is there any real significance to these gaskets, can air be pulled down the sides of the slides this way?

I know I keep saying it but thanks again for your support and interest,
it is most appreciated.
Mike, in the sunny then rainy then sunny then rainy UK.
 
Hi Jake,
I did fit new points and condensers and used a timing light to set them up but as you will see by my emails the problem continued.

Thanks none the less for your suggestions, and a big thanks to GS resources for allowing me access to you guys in the know.

Warm regards,
Mike. UK
 
I would think the paper gasket is intended to stop vacuum leaking along with stopping dirt. The rubber grommets that the throttle shaft slides through are used for the same purpose. I think some air does get past the grommets and then past the slides. They don't seal perfectly. It's not enough to cause a vacuum problem though.
Good luck with it and let us know how she runs.
 
Hi Keith,
I'm pleased to say the ride today was as smooth as a baby's bum, absolutely no hesitation and a nice even tick over of 1100rpm at full working temperature, also a very smooth response throughout the range. So we've got it cracked!!
I still intend to replace the rubber manifolds and fine tune the carbs as per your instructions. When I get time I'll post a picture of my bike which is pretty near mint and so much nicer now that it goes.

Thanks Keith and all you other guys who gave me such valuable and encouraging advice.
:D
 
jazzman said:
Hi Keith,
I'm pleased to say the ride today was as smooth as a baby's bum, absolutely no hesitation and a nice even tick over of 1100rpm at full working temperature, also a very smooth response throughout the range. So we've got it cracked!!
I still intend to replace the rubber manifolds and fine tune the carbs as per your instructions. When I get time I'll post a picture of my bike which is pretty near mint and so much nicer now that it goes.

Thanks Keith and all you other guys who gave me such valuable and encouraging advice.
:D
Click to expand...
Glad to hear your bike is running good! :) Post some pic's of your bike.
If you want to see my '79 GS1000E, just click the WWW symbol below.
She's mint too, with several upgrades. :)
 
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