Topic: Lower cylinder compression... jet lean...

[mr.brandon]

So, to those familiar with the process... I have out of the 4, one cyl. With a lower compression. I don't have the time or money rite now to pull and bore and buy a piston set so I am going to try and lean the carb on that cyl. With a lower main. What I am wondering is, if I should increase or decrease the spark plugs (on just that one cyl.) Heat rating?

[ekpent]

Is that cylinder chronically fouling plugs ? I haven't kept up on all your questions before but how many miles have you driven it so far, for years or a new project ?

[madmtnmotors]

I don't see any "magic pill" here. Tune it to spec and ride it until it won't run anymore. No matter what you do that is where you will end up with this anyway. I ran one cb750 to the point that the compression across all cylinders dropped far enough that the bike would not go any faster than 70mph in any gear. The motor started, idled, and ran fine, it just lacked power from the low compression numbers and there is no way to recover from that. That particular motor is eagerly awaiting new pistons and top end. The only potential short term fix would be to pull it down, hone the cylinders, install new rings and hope for the best. That would require new gaskets, new rings, and a lot of work for a marginal fix that would likely result in piston slap before too long.

Lowering the main on one carb? Sounds like a recipe for disaster to me. If you go too lean you risk holing that piston, especially if the low compression number is a result of a stuck ring that could eventually free itself, raising the compression and contributing to an even leaner condition than the one you already created with the smaller main jet. 

[ekpent]

That's why I was asking how much it had been ridden so far. It may heal with some run time if the ring is stuck a little.

[Rosinante]

When people have it, we would like to see actual compression numbers.  I just tested mine today and got these numbers.  Dry.

1 - 120
2 - 75
3 - 100
4 - 100

I might have messed up on #2, and was worried since I am thinking of getting the carb synch tool and #2 is the cylinder everything ls measured against.  So then I added a bit of oil to the cylinders and got these numbers:

1 - 141
2 - 150
3 - 138
4 - 113

Not sure how to explain these, but this engine seems to have at least good enough compression for a stab at carb synchronizing.

Sorry for the hijack.

[HondaMan]

When people have it, we would like to see actual compression numbers.  I just tested mine today and got these numbers.  Dry.

1 - 120
2 - 75
3 - 100
4 - 100

I might have messed up on #2, and was worried since I am thinking of getting the carb synch tool and #2 is the cylinder everything ls measured against.  So then I added a bit of oil to the cylinders and got these numbers:

1 - 141
2 - 150
3 - 138
4 - 113

Not sure how to explain these, but this engine seems to have at least good enough compression for a stab at carb synchronizing.

Sorry for the hijack.

Those number changes with oil indicate a dry engine that hasn't been run very much in a while. Usually simply running it for 50-100 miles will straighten them back up: I have seen numbers half as big as these come fully back in 100 miles.

[Rosinante]

I've been riding this thin, but not fifty miles.  It looks like I need to get on it!  Literally. 

[mr.brandon]

My numbers are, #1@ 143psi, #2@ 130, #3@ 115 and 4@125. I have not tried the oil trick yet and have been meaning too. In all honesty,  I have not even performed a leak-down, I've been pretty 24-7 with work, the bike had an electrical issue and finally got that what I believe now, is straight. But tomorrow is play-day and will be able to wrench a bit. With any luck, I'll be back with some good news. Thanks,  b

[Rosinante]

FWIW, I am going to re-test #4 compression.  Doing the wet test is informative. 

BTW, your cylinder compression is whatever is the highest number you get.  That is.....you can't really get an extra high number by doing something wrong.  If the cylinder made 140 PSI, then it made 140 psi.  RE-doing the test is even better information.  I plan to ride this thing like Hondaman says, and then re-do.  The wet test basically isolates the rings as a culprit.