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1977 Suzuki gs750 cold cylinder

2164 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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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.
If that cylinder lacks compression, that would do it.
The carbs can be switched around to trouble-shoot them.

I'd start with a compression test and if compression is low on that cylinder, look at the valves for clearance problems or cracked valve seat or ...
If it is only running on 3, It will perform very poorly, stall lots, eat fuel and sound like crap.
Only advantage to a 4 cylinder bike is that you can switch things around to trouble-shoot many of the components.
If you switch carb 1 with 4 and the problem goes to the other side you found the problem and if not then you can forget about the carbs being the problem. Compression test is a lot easier to do if you have a tester and that will tell you if your valves are all good and opening and closing as they should, when they should.
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.
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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?
Because your engine is a great big air pump, if you make that air pump work better, then the carburetors will work better. It's not really magic, every engine you ever own with poppet valves will eventually need the valve clearances adjusted, it's a function of normal engine wear. If you really think it's a carb problem switch the carbs around and try to make the problem go to a different cylinder, but that won't fix a low compression problem and you can't balance the intake if you don't balance the compression.
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.
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If it was only one carb you should have trouble-tested that by now by swapping them around.
and yes sometimes a carburetor simply needs to be replaced, but that is usually because they were bad from day one. I had a snowmobile like that, bought 2 identical machines and one had a faulty carb body from the start, never performed as well.
You can’t swap around them since they sit on a bracket the connects them all together
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
They are different. The passages are drilled so the adjustments face outward. And 2 & 3 have the fill pipe drilled all the way through, where as 1 & 4 have the fill pipe on one side facing inward. #1 is the only one that has the mount for the choke lever, #3 has the port for the fuel tap, etc. There are other differences, too. You can’t just swap carb bodies around
Must be why they came up with those strange double barrel setups on the later models.
Maybe why I stopped riding Suzuki when they stopped building 2-strokes too.
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Hondas, sometimes #2 has a fuel pump added for deceleration control and so the idle doesn't hang lol

You guys are going to love fuel injection some day :cool:
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TrialsRider, the accelerator pump (usually on #2) is for acceleration, the air cut valve is to prevent after-fire on deceleration and is 'automatically' operated by intake vacuum
Usually (y)
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