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'79 GS1000S Hesitation between 4-5,000 RPM

  • Thread starter Thread starter ianralph
  • Start date Start date
I

ianralph

Guest
1979 GS1000S, standard air filter, Tranzac 4 into 1, standard cams (as far as I know). I have read lots of the posts to this forum (what a fantastic resource), but none seems to describe my problem.

I bought this bike after it had been sitting for 4 years. The carbs needed a serious clean to get it running. I did not replace any of the jets, needles or pilot screws, I just cleaned them up and re-assembled the carbs. I have replaced all 8 induction hose clamps hoping to eliminate air leaks.

The pilot fuel screws had been given a hard time: http://www.pittwatertuition.com.au/GS/needles.jpg and I am trying to source new ones from here http://www.motorcyclecarbs.com/

This bike has reasonably new electronic ignition - although I have not checked the timing.

It now runs very cleanly up to about 4-5,000 RPM in 5th gear. If I sit at this speed it starts to hesitate slightly. It feels like it is about to run onto reserve. I have not done a chop test when I am having this problem.

If I then give it more throttle it pulls very cleanly with lots of power.

Any suggestions about the best place to start troubleshooting the cause of this behaviour would be appreciated.

Thanks
Ian
 
If you can find a quiet road and get the problem to happen, kill the ignition, let it cool and check the spark plugs. You want to see if one or more looks fouled up.

Did you check the operation of the petcock?
 
when you cleaned the carbs did you dip them or spray them with cleaner. Did you replace the rubber o-rings with new fresh ones that are gasoline friendly?
did you replace the intake boots and the big o-rings that live behind them?

your last fuel mixture screw on the right, needs to be replaced, also check to make sure none of the little holes the mixture screw block are clogged with broken off bits of screw. you should be able to see light shine through those holes if you look up from the bottom where the screw screw in.

are you running stock air box with stock air filter? have you check the carb sticky post above to verify you have all the the stock carb internals?

i have a link to where you can get the mix screws separately at home i'll post that later today
 
DimitriT said:
If you can find a quiet road and get the problem to happen, kill the ignition, let it cool and check the spark plugs. You want to see if one or more looks fouled up.

Did you check the operation of the petcock?
Click to expand...

Thanks Dimitri - next time I'm on the open road I'll do this test and have a look at the plugs.

I haven't checked the operation of the petcock because when I open the throttle wide it pulls aways with lots of power. It only hesitates when on constant throttle between 4-5,00 RPM in 5th gear.
 
first timer said:
when you cleaned the carbs did you dip them or spray them with cleaner. Did you replace the rubber o-rings with new fresh ones that are gasoline friendly?
did you replace the intake boots and the big o-rings that live behind them?

your last fuel mixture screw on the right, needs to be replaced, also check to make sure none of the little holes the mixture screw block are clogged with broken off bits of screw. you should be able to see light shine through those holes if you look up from the bottom where the screw screw in.

are you running stock air box with stock air filter? have you check the carb sticky post above to verify you have all the the stock carb internals?

i have a link to where you can get the mix screws separately at home i'll post that later today
Click to expand...

Yes when I cleaned up the carbs I soaked them in carby cleaner.

Yes I replaced the O rings with fuel friendly O rings - in fact I had to do it twice, because the first time I did it I didn't lubricate the O rings and broke most of them when I re-installed the screws.


No I have not replaced the big O rings behind the big intake boots or the intake boots themselves. I guess if nothing else works I'll have to try that.

Yes - standard airbox and filter.

I'm pretty sure the hole for the pilot fuel screw is clear - I'd expect much worse running if it was not. Thanks for getting the link for the pilot fuel screws - the kits are $29 each - a lot to pay for a screw!

Thanks for your time.

Ian
 
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If you haven't popped fresh spark plugs in, it might be a good idea to do that. They are often the cause of misfiring problems and rough running at higher rpm, in my experience.

But by all means keep checking out the carbs, jetting, fuel etc. But check your plugs. And trust me on the sunscreen. :)
 
tfb said:
If you haven't popped fresh spark plugs in, it might be a good idea to do that. They are often the cause of misfiring problems and rough running at higher rpm, in my experience.

But by all means keep checking out the carbs, jetting, fuel etc. But check your plugs. And trust me on the sunscreen. :)
Click to expand...

Thanks - I'll try that too - a lot easier than taking those carbys off again:???:!
 
I vote for the intake boots and o-rings allowing air in--I'm pretty sure that was my problem, but my 750 wouldn't rev past 5k at all. It could even be something as minor as using a 1/4 inch ID fuel line instead of a 5/16".

And if you can't solve the problem, I'll take the old thing off your hands. (I still regret not buying one of those when I had the chance...)
 
ExterminatorBike said:
I vote for the intake boots and o-rings allowing air in--I'm pretty sure that was my problem, but my 750 wouldn't rev past 5k at all. It could even be something as minor as using a 1/4 inch ID fuel line instead of a 5/16".

And if you can't solve the problem, I'll take the old thing off your hands. (I still regret not buying one of those when I had the chance...)
Click to expand...

The thing that puzzles me is that it pulls so strongly above and below that narrow range and throttle opening. I'm thinking that I'd see bigger problems than this if it was sucking air.

Interestingly I took it out for s short ride tonight and number 3 plug fouled and would not fire at all. It may be the plugs after all. I'll keep you posted.

Thanks for offering to take it off my hands - that's what solidarity amongst bikers is all about!:smile:
 
If the plug has fouled, you'll need to figure out why.

1. Filthy air filter?
2. Is the fuel tap leaking fuel down the vacuum line and into the manifold of #3 carbie? (Although from memory the vacuum line on the '79 models goes to #2)?
3. Filthy carbies, jets etc.?
4. Excessive oil burning (ie. worn rings or pistons or valve guides or valve seals... or the whole rotten lot)?

... and so on. Happy sleuthing!
 
tfb said:
If the plug has fouled, you'll need to figure out why.

1. Filthy air filter?
2. Is the fuel tap leaking fuel down the vacuum line and into the manifold of #3 carbie? (Although from memory the vacuum line on the '79 models goes to #2)?
3. Filthy carbies, jets etc.?
4. Excessive oil burning (ie. worn rings or pistons or valve guides or valve seals... or the whole rotten lot)?

... and so on. Happy sleuthing!
Click to expand...
It's been a while and I have made some progress so I thought I'd post this info to add some closure to this thread.

The problem was not the tuning of the carbys at all - it was, as tfb suggested, worn rings. I tested the compression and found #1 to be a lowish 130psi, #2 was OK at 150psi, #3 was way down at 95psi and #4 was OK at 150psi. When I added a few squirts of engine oil to #3 the pressure went up to 125psi, suggesting rings.

So I pulled the head and barrels off and they are away at the machine shop as I write. I think I may get away with a new set of rings and one of the inlet valves needs replacing.

Thanks to everyone who has taken the time to respond to my request for help. I'll let you know how it goes when it's back together. It was bloody fast with the compression as described, I'm looking forward to the performance when compression is closer to factory specs in all 4!

Ian
 
i still think your carbs are giving you issues, did you ever get those pilot screws replaced (don't order the one's i suggest previously, they are wrong) You will need to order a rebuild kit for each carb that needs those pilot screws, you can't get them separately, i know i tried.

Why i think your carbs are still the issue is my 1000 didn't have the best compression had numbers similar to you, but would still pull cleanly through the full rpm range. Hopefully your rebuild will help you out.
 
The needles you need are called Pilot Fuel Screws. You can purchase these screws as part of a complete rebuild kit from places like Z1 for about $17/kit USD. You may have to go for a 750 kit but don't worry, all the Pilot Fuel Screws are the same (I have the 750 screws in my 550 with VM carbs). While you are at it you should make sure the passage is open - it's very easy for a broken tip to stick in the carb body.
 
ianralph said:
the kits are $29 each - a lot to pay for a screw!
Click to expand...

Hmm.....better not touch that one.......\\:D/

To save you a bit of cash, you might post in the parts wanted section. Someone's bound to have a scrap set of carbs they can scavenge for parts. I've got an old set of 34CV's laying around, but the screws were frozen in them, part of the reason I got a new set. Well, that and the PO broke off two of the float mounting shafts.....
 
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