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GS750 stalling at full throttle

  • Thread starter Thread starter gs750e
  • Start date Start date
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gs750e

Guest
Hello friends,

My '78 GS750 has the following problem. When I open up the throttle to 100% in neutral, or when riding at low revs in 5th gear the engine just dies, I can shut the engine off like that. It also makes popping noises from the exhaust when riding and sometimes when stationary, when it's cold.

What I checked so far: intake boots, ignition timing, cam clearances, the carbs are cleaned, new airfilter, float heights are set to 26mm. The ignition coils and condensers are new, as are the spark plugs and wires. I also tried running it with the carbs hooked up to a different fuel reservoir, to rule out a problem with the fuel tank.

My bike has a marshall endurance exhaust, and stock jets. I asked around at my local bike shops and they told me that with these old 750 and 850's there is no need for bigger main jets so I should look elsewhere for the cause of the problem.

The exhaust is not 100% airtight at the cilinder port gaskets. At least I think so, because sometimes I can see some white smoke coming from there when it's cold. I don't know if this can cause problems as severe as this?

My next guess is to check the mechanical ignition advancer, can someone tell how to check if it's working properly?
 
gs750e said:
My bike has a marshall endurance exhaust, and stock jets. I asked around at my local bike shops and they told me that with these old 750 and 850's there is no need for bigger main jets so I should look elsewhere for the cause of the problem.
Click to expand...
Well, of course they told you that. :-k

As long as you have that old clunker, they can't sell you a new bike.
icon_shrug.gif


.
 
Hahah they are real nice guys with the best intentions, really. So you think I should go up one size problably? I have the stock size 100's in it now.

Thanks,
Jim
 
Of course they have the best intentions, they intend to sell you a new bike.
Most VM 750s used 107.5 main jet, but there were exceptions in the early ones.
Try about a 110 or 112.5 main jet and see what it does. Not familiar with the Marshall pipe, but I bet it's in that ball park if you have the stock intake system in good shape.
 
gs750e said:
... When I open up the throttle to 100% in neutral, or when riding at low revs in 5th gear the engine just dies, I can shut the engine off like that. It also makes popping noises from the exhaust when riding and sometimes when stationary, when it's cold...
Click to expand...

sounds like a fuel issue to me. as in... you're using fuel faster than the carb bowls can re-fill.

Is your petcock flow rate within spec ? Dirty tank ?
Do you have an aftermarket fuel filter stuffed in there?
 
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exdirtbiker said:
sounds like a fuel issue to me. as in... you're using fuel faster than the carb bowls can re-fill.

Is your petcock flow rate within spec ? Dirty tank ?
Do you have an aftermarket fuel filter stuffed in there?
Click to expand...

I do use and extra fuel filter between the tank and the carbs, yes. But yesterday I tried running the bike with a little container filled with fuel, hooked up directly to the carbs, and it did the same thing.
 
tkent02 said:
Of course they have the best intentions, they intend to sell you a new bike.
Most VM 750s used 107.5 main jet, but there were exceptions in the early ones.
Try about a 110 or 112.5 main jet and see what it does. Not familiar with the Marshall pipe, but I bet it's in that ball park if you have the stock intake system in good shape.
Click to expand...

Alright I'll try some 110 size jets, thanks man.
 
sounds like you need to sync the carbs using gauges, if you haven't already done so after the carb rebuild
 
Paul said:
sounds like you need to sync the carbs using gauges, if you haven't already done so after the carb rebuild
Click to expand...

Yes I already did that, but even if I wouldn't have, it would not create problems as stalling under full throttle right.
 
Today I found out that the stalling occurs when going full throttle from 1500rpm on. When I use 3/4 throttle to get to 4000rpm I can give it full power from there and it will rev further instead of stalling.

Also today I tried moving the gas needle from the middle position to the richest, and putting ducttape over the airbox snorkel, it's still stalling. Also moving the gas needle made it run worse when riding it or when going above 4000 revs.

This made me think that the problem lies somewhere else then the carburetors, so I checked the ignition timing again, and I replaced the mechanical ignition advancer for one that's as good as new, still no results.

Who knows what it could be?
 
Just out of curiosity, what are the settings on your pilot air and pilot fuel screws?

They should not be in the picture at full throttle, but they might be affecting the transition to it.

.
 
Your float height is wrong and you're starving the engine. Should be 23mm.
 
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Steve said:
Just out of curiosity, what are the settings on your pilot air and pilot fuel screws?

They should not be in the picture at full throttle, but they might be affecting the transition to it.

.
Click to expand...

The air screw is at 1 turn open, I haven't touched the fuel screws yet.
 
allojohn said:
You float height is wrong and you're starving the engine. Should be 23mm.
Click to expand...

I have the Haynes manual, do you think the information in this book is wrong? I could always try putting them at 23mm, I tried almost everything now.
 
gs750e said:
I have the Haynes manual, do you think the information in this book is wrong? I could always try putting them at 23mm, I tried almost everything now.
Click to expand...

The Haynes manual covers '77-'79 models generically. Trust me the '78 should be 23mm.
Here's some backup from BassCliff.
 
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Where are the bottom and side screws set?? bottom should be 3/4 to 7/8 out and the side 1 1/2 to 2 out. Needle clip on center groove.

Since you dont know what the problem is, I would suggest getting the floats set as was stated and setting everything back to the stock default positions.

The bottom ones wont need any more adjustment..so set and forget them. NOW, verify the valves are adjusted within spec and vacuum sync the carbs to the engine AFTER the valves are inspected.point is that youre chasing a gut off tail till you are positive the valves are right, carbs are synced, and your sure there are no air leaks like the orings in the intake manifolds ( carb holders ). Also stretched clamps wont tighten well and theres another source of contention.

Check the timing too...while your doing all the required stuff.

Moral of the story is that until and unless all the engine maintenance is done your just wasting time and energy.
 
Damn, I set the floats to exactly 23mm, but the problem stays. Any more ideas?
 
chuck hahn said:
Where are the bottom and side screws set?? bottom should be 3/4 to 7/8 out and the side 1 1/2 to 2 out. Needle clip on center groove.

Since you dont know what the problem is, I would suggest getting the floats set as was stated and setting everything back to the stock default positions.

The bottom ones wont need any more adjustment..so set and forget them. NOW, verify the valves are adjusted within spec and vacuum sync the carbs to the engine AFTER the valves are inspected.point is that youre chasing a gut off tail till you are positive the valves are right, carbs are synced, and your sure there are no air leaks like the orings in the intake manifolds ( carb holders ). Also stretched clamps wont tighten well and theres another source of contention.

Check the timing too...while your doing all the required stuff.

Moral of the story is that until and unless all the engine maintenance is done your just wasting time and energy.
Click to expand...

Hello,

I set the valve clearances 5,000 km's ago, and I set them on the loose side (when there was a choice between 0,04 and 0,09 I chose 0,09) The last time I did synching was last week. The intake boots and clamps are in very good condition. But as I don't have any other leads I'll check the valves tomorrow. Thanks,

Jim
 
No need..if they were on the loose side just 5,000 K ago theres no reason to think they are tight. So the intake manifolds and clamps are good, new orings in them and all that..ok.

So what are the screws set at?
 
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