Strange compression test results..

[fasteddie313]

My #3 cyl isn't hitting at idle for some reason. The low speed air screw on carb #3 has no effect on the idle. Pulling the spark plug wire off #3 has no effect on the idle.. Have good spark on 3.. I think cyl 3 is hitting when riding but not at idle.

Compression test..
Throttle closed : 3=60psi 4=100psi
Throttle open: 3=175psi 4=170psi

Now why would 3 have such low comp with the throttle closed but higher comp with the throttle open?
Would a leaking valve or bad rings show up more on the compression test with the throttle closed?
Could it just be a carb sync issue? Like cyl 3 slide is closed further than the others? I'm pretty sure all of the sync adjusters still have that orange proof paint on them that they have not been moved.. I guess I should check the slide heights, idk.. I'm a bit confused..

Hmmm...
Now that throttle rod that goes through all 4 of the carbs that the slide raising levers attach to, can go in either way 180 degrees out.. If I put that rod back in the opposite way it came out that could throw the sync off, no?

And, if I choke cyl3 with my palm over the carb to richen it, it does not effect idle.. Idle screw in or out, palm choked or not, it doesn't care..

But man the sync would have to be wayyyy off for a cyl not to hit at idle at all wouldn't it? I didn't think about a sync issue until just now while posting this..

Thoughts?

 

[standswithabeer]

your symptoms sound like a blocked idle circuit on #3. good spark, good compression when tested correctly, idle screw has no affect, #3 possibly running at speed.

am guessing the #3 pipe is cold on first starting, while the others get hot, and firing when on the gas makes sense as you are then on the idle circuit and needle jet.

not sure how a "carb sync" error would cause your conditions. PO's are a nasty group tho, no telling what's been done. i'd start w/a valve clearance check, and checking the idle circuit. below may be handy if you have a vm26 or similar. or at least it will provide some insight.

edit. of course this assumes clean, filtered fuel. the little petcock tube/screen on my bike was missing, allowing crud from the bottom of the tank to flow into the carbs. i installed the stuff that was supposed to be there, added an inline filter, changed the fuel lines and rinsed them out before install, etc. to ensure a supply of clean fuel, i always start from tank to fuel bowl. otherwise, you may need to clean the carbs again.

 

[hampshirehog]

I think you may have answered your own question and it is carb synch. Certainly worth checking.

 

[Nessism]

Have you performed a vacuum carb sync? If #4 is more open at idle than #3 the closed throttle compression would be higher.

You might want to read the Newbie Mistakes thread linked in my signature and pay attention to first item that mentions that all the bikes maintenance needs to be up to date before attempting to troubleshoot running problems.

 

[Agemax]

have you got the petcock vacuum pipe connected on #3 carb? if not block it off on the carb end and see how #3 behaves then. or, if connected make sure it is on properly and not split anywhere.

 

[fasteddie313]

Yes, I do have the hose from carb 3 to the petcock, what exactly does that do? I should take it off the petkock and block the line to the carb as a test?

I also have a nice clear inline filter that looks clean, and new main fuel line.. Tank is clean..
I do not have a fancy vacuum sync tool but I'm going to check them with a drillbit or something..

Edit: The sync was wayy off so I gave that throttle bar a 180 and whaddaya know it put them back in sync and it idles better now..
So I guess be careful which way up that bar goes.. I don't think it has a mark but I'll make one if I take it off again..

 

[Steve]

Thoughts?

Thought #1: Bike is not mentioned, have to assume it's not a twin because cylinders 3 and 4 were mentioned.
Thought #2: Because bike is not mentioned, there is no way to know which carbs are in use.
Thought #3: Knowing which carbs are in use might lead to other questions to ensure that numbering is correct.
Thought #4: Having the stock orange paint on the sync screws is a sign of neglect.
Thought #5: Using an in-line fuel filter can lead to problems if it is not a proper type for a gravity-fed fuel system.
Thought #6: Apparently it's fixed, so none of this really matters.
Thought #7: It would be nice to create a signature that mentions your bike, so you don't have to with every question. :encouragement:

.

 

[fasteddie313]

Thought #1: Bike is not mentioned, have to assume it's not a twin because cylinders 3 and 4 were mentioned.
Thought #2: Because bike is not mentioned, there is no way to know which carbs are in use.
Thought #3: Knowing which carbs are in use might lead to other questions to ensure that numbering is correct.
Thought #4: Having the stock orange paint on the sync screws is a sign of neglect.
Thought #5: Using an in-line fuel filter can lead to problems if it is not a proper type for a gravity-fed fuel system.
Thought #6: Apparently it's fixed, so none of this really matters.
Thought #7: It would be nice to create a signature that mentions your bike, so you don't have to with every question. :encouragement:

.

You are so right and I completely over looked that.. Sorry.. It is a 77 gs750..
Yes, it still has the orange paintmarker on the sync screws.. I think they are VM26 carbs.

Funny you mention the fuel filter because I just went for a WOT run and it seemed to drop a cylinder for a little while just after it, I have no idea why, but the cyl came back, so maybe it drained the bowls and the filter didn't let the fuel supply keep up.. The filter is a clear one with the airstone looking thing in it..

 

[Steve]

The problem with filters is that most of them are designed for systems with a fuel pump that will push fuel through the rather restrictive filter media. Put one of them in a bike, there is a good chance you won't get enough fuel through it. If you really insist on having a filter, get one for a lawn tractor. Most of them also have gravity-fed fuel systems.

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