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1977 Suzuki GS750
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Discussion Starter · #1 ·
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 ...
 

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Discussion Starter · #4 ·
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
 

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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.
 

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Check compression and valve clearance first like TR said. That bike has a stupid tight valve clearance spec ( like .003”) and the check interval is 3000 miles. Yes. Every oil change basically.

if the clearance has gotten tight, it’ll hold the valve open slightly and compression will drop.
 

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do what the above are telling you. i just checked my cmr and was getting 90 psi on cyl #1. took out a 2.55 shim ( valve wasnt closing) for a 2.35. now i get 125 plus on a cold cylinder
 

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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
 

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Discussion Starter · #9 ·
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
 

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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
 
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Discussion Starter · #11 ·
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
 

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Discussion Starter · #13 ·
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?
 

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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.
 

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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?
I answered that in my first post (post#6)

And BTW, you NEVER start carburetor work on these bikes before you sort out the valve clearances. Tight valve clearances will throw off everything in the carbs.
 

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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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I didn't know that last part about cracking a valve seat. Makes sense.
Thanks
 

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Discussion Starter · #19 ·
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
 
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