Cool running cylinders

The bike: 1976 KZ900a
The problem: Numbers one and two cylinders are running alot cooler than 3 and 4. After a few miles they will start loading up and backfiring.
I though it would be an ignition problem but after several days, $300.00, and alot of grief, that is not the case.
The carbs are clean and rebuilt, so this leaves what?
The valve clearances?
I have no idea where to go from here.
Biketoberfest starts friday and I'm stressing.

have you run a compression check

Cool running cylinders are often missing cylinders. If the plugs are carboned up they will not self clean, unless you're willing to go for a hundred miler with a crappy running bike. Sounds like you have a fuel drainage problem. At least that's what I can think of as common to cyl's 1 & 2.

How about this: Turn off the fuel. Remove the fuel line to the carbs. Is the petcock leaking?

Clean the plugs. Even, leave the plugs out for an hour or two. Now put in the plugs and go for a ride. About a half a mile from home shut off the fuel tap. When you get home put your bike on the center stand till your next ride.

If this procedure helps or eliminates the problem you need to go back into the carbs and pay particular attention to the float needle valves.

What I've learned is that enough residual fuel can load up in the intake tracts to foul a plug even though you may have corrected the problem.

Hey You said 1&2.

KZCSI

If the spark plugs don't fire the as in the combustion chambers, the cylinder will be cool. FIRST thing to do is check compression in all four cylinders. Do this with the throttle held open. Once you know if you have compression, you can move on. At this point, you need to have gas and spark and the state of tune must be correct. Troubleshoot the issue logically and it isn't hard to find a problem. First step is compression. If you have low or no compression, there is no point in looking for a problem in the carburetors or ignition and doing things in the proper sequence will also save you a lot of money. When you find the compression readings for all cylinders, we will know how to progress with tracking down the problem.

Compression first, yes. When you're moving to the next thought, have the carbs been synced since they were rebuilt and reinstalled? (or am I way off base)

Whether the carbs are in sync or not will not affect the cylinders firing. Off base a little...

No the carbs were not synch'd since the rebuild.
I'll be doing a compression check shortly and will report back. My buddy seems to think it might be the valve lash/shims so I'll be popping the cover too.
Anything in particular I should be looking for?

Added:
#1 115 psi
#2 125 psi
#3 115 psi
#4 130 psi

Post edited by: Kersten, at: 2007/10/16 15:22

Kersten wrote:

No the carbs were not synch'd since the rebuild.
I'll be doing a compression check shortly and will report back. My buddy seems to think it might be the valve lash/shims so I'll be popping the cover too.
Anything in particular I should be looking for?

Added:
#1 115 psi
#2 125 psi
#3 115 psi
#4 130 psi<br><br>Post edited by: Kersten, at: 2007/10/16 15:22

Wow! Are they really that close/consistent?
I guess I might fall short on my expections from these old engines, but it seems like you do not have much of a compression issue. Did you do the test with engine hot, or at least warm, with all the plugs out and the throttle open?

Actually, those numbers are not all that great, there should be no more than a 10% difference between the lowest and the highest, and the difference is larger than 10% in this case.

In an engine in good condition, I expect to see numbers with no more than 5 psi difference, and less is better.

A valve adjustment might even those out.

The consistency will have a lot to do with the cylinder head... older heads that just get cleaned up and the valves lapped often don't yield quite as consistent numbers as those with new guides and a valve job but that extra work comes at a pretty steep cost. My 79 shafty has compression from 140-150 psi across the board and the numbers listed were on the low side, meaning the engine will need to be freshened eventually but are not too bad.

The NEXT step is to check valve clearances; see the VALVE CLEARANCES tech article on my website under "wg's Tech Stuff Index". It might be useful.

Seems to me one of the most commonly neglected maintenance items is checking and adjusting valve clearances.

It's important to understand why merely checking the compression alone is not enough.

Am thinking the vast majority of compression checks are performed with a cold engine or at least on an engine well below operating temperature. Acceptable cold compression figures do not necessarily reflect the figures at operating temperature. Because it is entirely possible for acceptable cold compression figures to become unacceptable as the engine reaches operating temperature.

An insufficient valve clearance (gap) may allow the valve closing just enough in a cold engine to permit an acceptable compression reading when cold. But valve clearances lessen as the engine warms (due to expansion of metal parts). So a too small gap when cold readily becomes a no-gap at operating temperature, and actually prevents the valve from seating (closing). Lack of fully seating (leaking valve) results in loss of compression (and eventually burning the valve). This is why the specified gap clearances are supposed to be measured on a cold engine (preferably after sitting overnight).

It's easy to access the gaps for measurements. Just remove the fuel tank, plug wires, and valve cover. When checking the gaps, remember to note and save the numbers for future reference. Often the same valve cover gasket may be used again, unless it needs replacing anyhow because of leakage.

My 2 cents for the one person out there who might not already know this.

Ok

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vales are all in spec! 31 years old and still perfect, amazing.
Compression is good, valves are in spec, ignition is good----back to the carbs I guess.
The bike idles fine, right at 900 rpm, but then the 1 and 2 start backfiring pretty badly, but they are set up the same way as 3 and 4, airscrew, jet and needle settings, etc.