Slightly high cylinder compression numbers - do I sweat it?

My 82 XJ550 Seca has 20k on the clock, and I finally did my first compression test yesterday. I did the valves clearances less than 1000 miles ago last summer.

After warming it up for a few minutes, the numbers I got for all 4 cylinders were within 2 PSI of 140. The manual indicates a max of 135. Should I sweat the difference? If so, I'm guessing I should pull the cam cover and recheck my valve clearances?

 

140 isn't high - some people show 150's

Gauges vary alot, by design and "hose volume"

The fact that they are consistent to within 2 PSI is a great sign.

 

I would be OK with that, some gauges are 2-3% off and I had a buddy who had 150 and rode it 10K miles per year for 20 years.

 

140 IS slightly high for a 550; max is supposed to be 135.

That being said, the fact that they are so even is a good sign; you might want to borrow a different gauge and re-test. If the valves were in spec 1000 miles back, they should still be fine.

I wouldn't sweat it.

 

Thank you everyone!! My carbs are now off this bike as I try to rejet it to stock, because my plugs are BLACK! Since this is my first time, I might pay someone to oversee me doing it...

Stay tuned, ladies and gentlemen, for the next episode of "As the Seca Turns"....

 

Black plugs could indicate oil getting into the cylinders (which would also account for slightly high compression if you're effectively doing a "wet" test.) Since the compression test came back so even, I'd tend to rule out rings but you could be in need of valve stem seals. Are they ALL black, and evenly?

Why has it been re-jetted? Intake or exhaust mods?

 

Well, it's a flat black color, so I assumed it was not oil. Isn't oil on plugs somewhat shiny? I see what you are saying about burning oil and a 'wet test'. There can be a blue tinge to my exhaust smoke, so that's possible. It's mild, and was thinking it has to do with burning stabil or Sea Foam. And yes, the plugs are all evenly black.

When I bought the bike, it had a straight pipe on it, and I think a hi-flow air filter as well (not pods; the original airbox is on there). So, I'm assuming that the previous owner rejetted the carbs to make it run. Anyways, it's only tonight that I'm opening the carbs up to see what's in there and can tell you what jets are currently installed. So if they ARE the original jets, that means my valve stems seals might be in need of service?

 

"Carbon is free compression" says the oval track racer (stock class)

While you're in the carbs, check ALL jets, including air jets, and spring length, and float level - never know where someone was "playing around" in there.

(also - read "Nine Yards" and "Church of Clean")

 

It'd be surprising you'd have bad valve stem seals on all 4 cylinders at the same time. It looks to me like too rich mixture, spark plugs too cold-ranged for your riding habits, dirty air filter, choke not totally disengaging or simply too big fuel jets.

 

Good guess! So here's the follow-up: I went to a motorcycle carb guy yesterday. A nice, friendly young man. I showed him the plugs and he also thought they were sooty, but not oily. Upon opening my carbs, he said my floats were all set wrong - they were sitting against the bottom of the bowls, and the float's metal tangs were all bend upwards. This, he reasoned, is why I'm running so rich. He didn't have that nipple to attach to the bowl for the clear tube to adjust the fuel height, but he felt confident in doing it using a caliper and they are now all within 1mm of each other. He did also look at my pilot screws, because someone previously had touched them, so he checked and they were all at 3.5 turns. He brought them all down to 2.5 turns. One was stuck so we left that one where it was.

So, the plan is for me to reinstall the carbs at this point and see how my plugs look. I have ordered a couple of drain screws and that drain nipple from xjforever. It was disheartening a bit to see that some of the floatbowl screws were a larger size, and probably the result of stripping the smaller ones, but hey, I guess that's how it goes sometimes.

It would be nice to get a little confirmation from the group that poorly set carb floats can indeed make me run so rich and my plugs black.

Cheers!

 

Sure can. BIG time.

But you need to WET-SET the floats. The little "factory tool" (brass nipple) for the carb drains is about useless, I have one and it's a PIA.

Make yourself a plastic press-in fitting for the drain holes; I used a plastic hose fitting and "turned it down" (drill and a file) to fit. It came from either the auto parts or from my Mity-Vac kit, I don't remember.

Accurate float levels are critical to the Mikunis; the spec is only +/-1MM.

 

Sorry - I should of also mentioned that once we broke open the carbs, the stock jets were still in there, so the cause of the dark plugs had to lie elsewhere.

 

I'm throwing a red flag here -
Floats set low make you lean - floats stuck on the bottom might be too rich to run, and giving up on a Pilot Screw that is "stuck" is not an option - - especially for a paid mechanic.

There is no reason to re-install these carbs right now - they need more fixin' !!

 

Quick follow up: the carbs are back in the bike, and the plugs are burning much better. Hurray! Also looks like my gas mileage is significantly improved!

Once I was up at operating temp, it took me a while to adjust the idle with that central knob on carbs - could they make it any tighter to stick your hand in that tiny place against a hot, running motor?!

TIMEtoRIDE is correct - it would of been best to deal with the stuck pilot screw when they were off, although it's only 1 turn. I guess I don't mind waiting until I winterize her this fall and I'll remove them then and deal with the issue in a leisurely manner.

Thanks again for everyone who helped me with this!!