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GS850 back pressure through intake?

  • Thread starter Thread starter onchiman
  • Start date Start date
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onchiman

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I've got the carbs off my 79 GS850 for rebuilding. While they were off, I did a cold compression test (just out of curiosity). Compressioin doesn't seem abnormally low on any cylinder (all around 120-130psi). I did notice that when cranking the engine with the spark plugs in but coils grounded I could feel some back pressure or blow/back from all of the intake ports? Is this normal? I have the intake ports stuffed with rags to keep any dirt out while I'm rebuilding the carbs and when I crank the engine, the rags all blow out! Is a certain amount of this normal or do I need to look at my cam timing now? Thanks a lot. I've not had the bike running yet by the way.
 
Definitely check your cam timing. You should have suction at the intake, trying to suck the rags in, not blowing them out.
 
I noticed when I lightly rev my 850- the carb boots will balloon slightly. My piston port 2 stroke motors do this also.
I understand the piston port motors doing that.
Must be that the cams are not efficient at the low engine speed??
 
GS850_Coug said:
Definitely check your cam timing. You should have suction at the intake, trying to suck the rags in, not blowing them out.
Click to expand...
Not really. On the intake stroke there is some vacuum to pull them in, but it's a lot easier to push them out on the compression stroke than to pull them all the way through a small hole on the intake stroke.
 
I don't think you would get that kind of compression readings if your valve timing was off. You can get a manual and line up the various marks at TDC to verify.

Why the rags come out is kind of mysterious. Cylinder vacuum on the intake stroke would certainly try to pull them in. But that's only on one of the 4 strokes. Maybe with unrestricted air flow (and resulting lower vacuum), the ports actually have a positive pressure from the compression and exhaust strokes. Is it possible that valve timing overlap is causing this? Or maybe worn valve seats pushing back on the exhaust stroke. Just shootin in the dark.
 
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flyingace said:
I don't think you would get that kind of compression readings if your valve timing was off. You can get a manual and line up the various marks at TDC to verify.

Why the rags come out is kind of mysterious. Cylinder vacuum on the intake stroke would certainly try to pull them in. But that's only on one of the 4 strokes. Maybe with unrestricted air flow (and resulting lower vacuum), the ports actually have a positive pressure from the compression and exhaust strokes. Is it possible that valve timing overlap is causing this? Or maybe worn valve seats pushing back on the exhaust stroke. Just shootin in the dark.
Click to expand...
Yes, valve overlap causes this. Pull a spark plug and put your finger in the hole while cranking over the engine (preferably with the ignition disabled). You will feel significant pressure building in the port.
 
So basically what I'm seeing here is the overlap effect and is no cause for concern? There does seem to be plenty of suction at the intakes (my left palm will attest to that) and only this split second "pop" of pressure out the intakes. Maybe I'll pull the cam cover off and look the valve timing over just to be sure and check the shim clearances too.
 
Speaking for myself, I'm just theorizing. I don't know for a fact that overlap causes this. However, I honestly don't think anything is wrong just because rags blow out of the ports.

Just reassemble everything and see how it runs. If you did anything that might change valve timing, be sure you crank the engine by hand before using the starter. If something is wrong, you'll know it a hurry.
 
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