Cylinder leaking

Did you check your compression with the engine at full operating temp? I checked mine with the engine cold and they were between 100 and 110psi. At full temp, which is what the shop manual calls for, they were at 130 and above.

KYKZ1100 wrote:

Before I put everything back together I did another compression test. Cylinder 4 (the one that was at 70 psi before) was right at 100 with the others 105, 115 and 120 (#1). I put a teaspoon of oil in #4 and it came up to 145 and did the same with #1 and it came up to 150. From what I read 100 psi is marginal and the difference between 100 and 120 is pretty big, but I decided to see how it runs and to ride it this summer if it runs OK and have the cylinders done this winter.

The service manual for my KZ-750 is as follows:

Test compression when engine is at FULL OPERATING TEMP.

max compression is 155 psi, service limit is 115 psi (hot)

Less than 10 psi variance from max to min reading across all cylinders HOT

The manual doesn't even give a cold limit. Having just rebuilt mine, I can tell you a tight engine has less than 10 psi variance from stone cold dry to hot. A worn engine may have 30 or more psi variance from cold to hot.

The manual does not spec a limit for that variance range, obviously less is best. If it runs OK and doesn't burn too much oil, I'd ride the hell out of it. I only rebuilt mine when it wouldn't run any more.




KYKZ1100 wrote:

I rode it today after I got everything back together and it seems to run well but is backfiring a lot. The bike is stock except the mufflers are pretty unrestricted, the air suction tubes are blocked off and I did Wired George's mod to increase the voltage at the coils (which worked well).

Likely makes it run lean.


KYKZ1100 wrote:

Any advice as to what to look at first to fix the backfiring? .

Timing


KYKZ1100 wrote:

Could the mufflers have anything to do with it? .

Yes, if it is running lean.


KYKZ1100 wrote:

Should I start by taking the carbs apart and cleaning them again? I cleaned everything really well last summer but the FSM says not to dip them so it's hard to be sure everything is clean. I haven't synced the carbs yet.

For the fuel system:

Take the tank off and drain it, flush it.

Clean the petcock assembly and screens.

Replace the inline filters.

Remount the tank and replace the inline filters.

Run clean gas down and into the carbs, open the drain screws and run clean gas into them.

Run some cleaner like seafoam.

Fordtruck and Bounty hunter,

I checked the compression again today with the engine hot. It was 105 on #4, which was 100 cold, and also 105 on #1, which was 120 cold. I don't know what to make of that, but I checked it with two different compression testers on #1 and got the same reading. I also noticed that the #1 piston is clean but 2-4 have large patches of really thick black crud on them. This is with about 50 miles since the valves were replaced and the pistons cleaned.

After I rode it, the pipes on cylinders 2, 3 and 4 were sizzling but #1 wasn't, although it was hot. When I took the plugs out, those in 2-4 were tan/brown but #1 was black although not wet.

I sealed the gas tank last summer and it's only been filled twice since then, so I don't think there is a lot of crud in the tank. I'll try the Seafoam. Do I just put it in the tank or is there a way to put it directly in the carburetors?

I'll try the stock mufflers tomorrow and see if that affects the backfiring. Thanks for your help.

KYKZ1100 wrote:

Fordtruck and Bounty hunter,

I checked the compression again today with the engine hot. It was 105 on #4, which was 100 cold, and also 105 on #1, which was 120 cold. I don't know what to make of that, but I checked it with two different compression testers on #1 and got the same reading. I also noticed that the #1 piston is clean but 2-4 have large patches of really thick black crud on them. This is with about 50 miles since the valves were replaced and the pistons cleaned.

Sorry it's still sick. I have not heard of comp readings dropping going hot(?) Oil does get thinner when hot, so maybe if the rings were not sealing the oil thinning out might not seal as well if the oil was helping the seal.

2-4 have large patches of really thick black crud on them. usually, thick black crud is cooked on oil gunk and carbon. Oil can usually only get onto a piston from either:

1) Leaking past the valve stem seal and running down into the cylinder through the valve opening

2) Blowing by the rings

3) Head gasket not sealing where the oil goes up into the head

I am really familiar with #1 because my old 750 had it so bad it was fouling spark plugs every other day from it.

One thing you might try: take out the spark plugs and rotate the engine and look into the plug openings. On mine, you can see the edges of the valves as they open that way. If oil is coming down by the seals, you might see gunk on the valves.

Good luck.

EDIT TO ADD: one other thing that chages when hot, metal expands. When the engine was apart, did you straight edge the head and cylinder top to make sure they were flat? If not, the head gasket may not be sealing.

KYKZ1100 wrote:

...After I rode it, the pipes on cylinders 2, 3 and 4 were sizzling but #1 wasn't, although it was hot. When I took the plugs out, those in 2-4 were tan/brown but #1 was black although not wet....

If not already done, would check service fuel level in #1 carb float bowl. It might be too high and causing an over-rich mixture. Use the clear plastic tube test.

Good Luck!