GS750 stalling at full throttle

[gs750e]

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?

[Steve]

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.

Well, of course they told you that.

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

.

[gs750e]

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

[tkent02]

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.

[exdirtbiker]

... 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...

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?

[gs750e]

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?

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.

[gs750e]

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.

Alright I'll try some 110 size jets, thanks man.

[Paul]

sounds like you need to sync the carbs using gauges, if you haven't already done so after the carb rebuild

[gs750e]

sounds like you need to sync the carbs using gauges, if you haven't already done so after the carb rebuild

Yes I already did that, but even if I wouldn't have, it would not create problems as stalling under full throttle right.

[gs750e]

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?

[Steve]

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.

.

[allojohn]

Your float height is wrong and you're starving the engine. Should be 23mm.

[gs750e]

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.

.

The air screw is at 1 turn open, I haven't touched the fuel screws yet.

[gs750e]

You float height is wrong and you're starving the engine. Should be 23mm.

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.

[allojohn]

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.

The Haynes manual covers '77-'79 models generically. Trust me the '78 should be 23mm.
Here's some backup from BassCliff.