Low Compression and Backfiring...

Hello fellow KZRiders!

I recently tested the compression of my 81 KZ1000 and found that cylinder #1 has lower compression (~95psi) than the other three (~120psi). I then ran a 'wet' compression test on cyl #1 and the pressure ran way up to 150 psi seeming to indicate that the rings are worn. I'm wondering: if I don't correct the ring leak immediately will I be doing any permanent or extensive damage to the engine that could be prevented by performing the ring job immediately? The bike is running pretty strong now since I've rebuilt the carburetors. Enough performance for my use anyway, and I'm on a tight budget. But I would hate for spring to roll around and find out I'm going to be out of commission for a couple of weeks when I could have fixed it over the cold months...

I don't know if this is related to the low compression on cyl 1 or not, but the bike has been backfiring alot lately. It happens at a pretty regular interval at idle... There's a pop-pop-pop-pop coming out of the exhaust. Sometimes the backfire flame can even be seen coming out of the silencer, which although impressive, I would like to correct if possible. It has also backfired through the carb intermittently (prior to the carb rebuild) which is pretty interesting seeing a cloud of exhaust gas come out of the pod filter and then get sucked back in. I've only seen this out of cylinder one which leads me to believe that the problem may be related to the compression... Perhaps I need to re-shim the intake valve on cyl #1 too - but the wet compression test seems to indicate that the valves are seating well... Hmmm...

Thanks in advance for your advice! This site is very helpful! Hope everybody has a Merry Christmas!!!

Maverick

the valves could have some carbon on the seats causing them to stay open,or like you say it may be time to reshim.

at any rate you have valves that are not sealing/burnt
to cause backfiring IMHO.

I would check for an intake leak.Make sure your holding your throttle wide open when your checking compression.

What did a wet compression test on the other cylinders do? Backfire sounds like a valve or ignition issue. On a simple hand held compression tester big discrepencies between cylinders is possible. A leakdown test is the best way to check.

I ran a wet compression test on cylinder # 3 and it similarly went up to about 150psi as well. I'm going to reshim the valves next week.

Could a weak battery be contributing to the backfiring??? Maybe I need to do some electrical work suggested by WiredGeorge in the filebase.

Cheers!

Maverick

Maverick... I work on carburetors for a living. About 75 percent of the carburetor problems I see are weak ignitions. Backfiring and such can be caused by MANY things; not just weak spark but weak spark is a FIRST CHECK thing as it is easy to do and easy to correct. I would FIRST swap the 1 & 4 plug wires to see if the popping changed. I would check voltage to the 1&4 coil (heck, go nuts and check both of them)... if you find a weak voltage to the coil(s), then do the coil repower mod. The next step to look for is to check for air leaks... cracked intake manifolds, poorly sealing choke plungers, air leaks around the mixture screws due to bad o-rings, cracked vacuum caps... that sort of thing. If you don't find any leaks in those areas, perhaps you have poor sealing around your valves. Since you have poor compression due to bad rings on #1 based on your wet compresson test, it is time to remove the head and take the valves out and clean them and lap them and put some new seals in while you replace the rings anyway... That will likely freshen your bike right up! It won't hurt anything to have low compression but it WILL cause problems if you have air leaks in the pilot system as, depending on where they are, you can suck in cold air and burn a piston or valve.

sounds like broken rings in cyl one. Low comp, valves not sealing, = lean misfire.

Thanks for the tips WG... I swapped the plug leads around to no effect. How do you go about testing the voltage to the coils? Just set up a voltmeter on the primary leads? But the voltage is intermittent. Will my little analog multimeter be able to pick it up? What should the voltage be reading? I pulled the coils off and tested the resistance and it came back fine. One thing I noticed is that the 1/4 coil mounting plate was cracked. Don't think that would negatively affect anything though. I've checked for leaks by spraying WD-40 around and it doesn't appear to have any leaks. Thanks again!

Once your coils are installed, you will note that one has a yellow/red wire and a black wire connected and the other a yellow/red wire and a green wire. The black and green wires are coming from your ignition. The yellow/red are power wires. The power comes from your fuse area, through your ignition switch, then over to your switchgear (right) then through your kill switch and back out the switchgear where it goes back along the main harness to where it comes out above your valve cover in a dual out connector. From there the two yellow/red wires plug in and run to your coils. If you switch your key on and check voltage at the coils on the lugs where the yellow/red wire is connected, you SHOULD have the same voltage as is present at your battery terminals... about ~ 12.5 VDC. Since the power takes a circuitious path to get to this point, it well may be less. If less, you will get less spark even though the voltage will be up once the bike is rolling but the loss through all the switches and connectors will be similar. See the Filebase articles... there should be one on coil powering that is one of the most downloaded... The voltage coming from your points or ignition on the green and black wires would be tougher to measure and I am not sure every type of multimeter can measure it. I am not big on electrical theory... but I don't think this will be your problem.