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1977 Suzuki gs750 cold cylinder

2162 Views 37 Replies 6 Participants Last post by  TrialsRider
The bike has one cold cylinder on the far right of the four. It gets spark but I’m not sure how it’s not getting gas or air. The bike is having a loss of power and I believe this it, I’ve been debating on getting new carbs but I don’t think that would fix this.
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If it is only running on 3, It will perform very poorly, stall lots, eat fuel and sound like crap.
It surprisingly doesn’t sound bad, it starts up quick but is sluggish with the throttle. It used to be perfectly normal and ride perfect, I’m just surprised it has spark though
Probably the first thing to do would be adjust valve clearances.
Suzuki only use 0.001" to 0.003" clearance and tight or burned exhaust valves are a possibility.
Being 'shim over bucket' they were rarely adjusted at correct interval after first year or two.
It's a real good idea if you find any where bucket can't be rotated with cold engine to set things at least 0.001" larger than stock as it will close up within a few hundred miles. (don't go too large though as if you rev it too hard the shims can get 'spat out' and pretty much destroy head)
Seeing base circle of cam scuffed is also a dead givaway that clearances got too tight
I ran a compression test and hit 127. Is that good or bad
On how many cylinders? More telling is how close they are together. Test hot and cold. If one cylinder drops , or shows a really big difference when hot, that’s a pretty good indication you have tight valve clearance on that cylinder
The cold one read 127 and the others were around 135-140
So there is your answer, you have a compression problem on that cylinder and you should hope that it is just a valve clearance adjustment required.
Before doing all this work, how is changing the valve clearance going to magically make the cylinder work again, could the carbs possibly be dirt and just not letting gas through?
Little quiz to make you think about the magic of how engine things work:
What powers a carburetor?
1) - electricity
2) - gravity
3) - fuel
4) - air movement created by engine vacuum pressure
... correct answer is #4) air movement created by engine vacuum pressure. Carburetors operate off engine vacuum pressure and without that controlled movement of air they just sit there and do nothing.

If your engines intake vacuum pressure is low, your carburetor will?
1) - do nothing
2) - perform poorly or not at all
3) - not operate
... pick any one, they are all the same difference. Compression in the gasoline powered internal combustion engine is every bit as important as air, fuel and spark. Because your engines compression and vacuum pressures are directly related, a problem contributing to reduced cylinder compression will similarly contribute to reduced intake vacuum pressure.

Why do valves need a set clearance?
1) - to accomodate dimensional changes in the valve train components when they are operating at normal operating temperature.
2) - to frustrate the operator and make less mechanically inclined owners take their motorcycle in for shop servicing.
3) - because metal expands with heat.
... best answer is #1) & #3) with #2) being an unfortunate side effect.

What is the valve clearance at normal engine operating temperature?
1) - same as when it's cold
2) - as close to zero as possible while still allowing the valve to completely close and seal.
3) - large enough for burning gasses to leak past the valve seat and burn metal parts.
... hopefull #2) because #1) is physically impossible and #3) is what will happen if you don't set your valves correctly.

Can you crack an exhaust valve seat by running the engine with no headers or exhaust system in place?
1) - well Ya! sure you can, the various metal parts shrink at different rates so when you shut the hot engine down, cold air contacting a hot valve seat can destroy it.
... if you don't believe that to be true, you're welcome to test the theory.
So I ran a compression test and they all seem to run about 126 psi to 127 but the far right/ cylinder number 4 still has a completely clean spark plug after running it, could the carbs just be dirty or something
If it was only one carb you should have trouble-tested that by now by swapping them around.
You can’t swap around them since they sit on a bracket the connects them all together
You can’t swap around them since they sit on a bracket the connects them all together
Yeah it already has a 4 into 1 exhaust on it, I plan on getting a set of Murray’s carbs at some point but I’m gonna try and get these ones to work in the meantime
How many parts on #4 carb are Not interchangeable with the parts on one of the other 3 carbs? Suzuki doesn't build the carbs, they buy them and it would become costly to order them all significantly different, they generally avoid that. I suspect you just need to try harder, or you might better start shopping for replacements, but sets of 4 carburetors at a time is going to get expensive real fast, new or used. New carbs are very costly and used carbs will need just as much repair service as your old ones did.
You're still lucky the originals are not CV carbs, that would have made it even more complicated, the ones you are working with are relatively simple. (y) best luck getting it firing on all cylinders
Yeah all the parts are interchangeable but there’s this bracket type thing on the back that holds all four together so can’t exactly swap carbs but I can swap parts around
While you can’t swap the carb bodies around, you absolutely should take them off the bracket to clean them thoroughly. You have to clean ALL the ports and passages
View attachment 106991
View attachment 106992
That’s exactly what I plan on doing once I have the time to do it
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