'81 XJ750rh (SECA) compression test

Finally got around to doing a compression test, and I think the results are pretty bad.

Left cyl was a bit under 120, right cyl a bit under 135, middle both a bit over 120.

How bad is that, and what's the next step? I'm guessing it means a top end rebuild, but can I avoid pulling the valve head apart, and just do the rings?

 

Did you hold the throttle wide open when testing.
Motor hot or cold.
What are the dry vs wet readings like ?

 

Trottle 100% closed, but sitting on shelf across room. As you can see from the picture below, the intake was not obstructed.

It was totally cold, for obvious reasons. Only turned over long enough to get stable compression values, so I guess that counts as dry.

Guess I won't worry about a re-build until after I get it running again; it ran OK when I pulled it from the bike, although there's a lot of carbon on the plugs and exhaust ports.

 

Those aren't horrible numbers; and until you do a "dry - vs - wet" test you don't know if it's rings or just valves needing to be adjusted.

 

What would a "wet" test be? And how would that info tell me if I need rings vs valves?

I do in fact know (from taking the valve cover off and checking with feeler gauges) that the left two cylinders (the ones with the worst readings) are out of spec on the intake side, while the right side one was spot on spec. I guess having the shims out of spec WOULD throw off the compression, eh? Or not the compression so much, as not having the correct intake to begin with. Good news then, guess it might be in better shape than expected. Replacing the valve shims is well within my capability, top end rebuild would be a stretch.

 

A "wet" test involves adding 1 ~ 2 TEAspoons of oil to the cylinders and re-testing. If any of the "soft" cylinders then show a markable increase in compression, it indicates worn rings. If, on the other hand, there is little or no change then it's a matter of valves (either out of adjustment or not sealing well.)

Out-of-spec valves will throw off compression readings; to varying degrees but they will have an effect.

 

" it ran OK when I pulled it from the bike" well then it should run ok when you put it back in. you going to race it ?

 

I think you are confusing 'ran OK' with 'was known to be a reliable engine'. The guy I bought it from and I retrieved it from a storage unit where a friend had left it, and we were able to start it with minimal trouble and ride it for 10 minutes or so.... and that's the whole sum of the info that goes into my saying it "ran OK".

Racing? No. Egregiously violating highway speed limits on a daily basis because I'm the only person using the HOV lane? Probably.

 

If you had posted that picture to begin with, it might alleviate the need for sarcasm.

 

If I saw those numbers on a dry/cold engine with out of spec valves, that previously had run & didn't smoke (I assume it didn't smoke anyway?...)

My personal course of action would be, put valves in spec, get running & warm up, change oil & filter while warm, re-test compressions with engine hot & throttle wide open. I'd expect to see the numbers even out somewhat given the warmth & correct clearances.

 

Wizard- no offense intended. Was just an attempt at dry wit, sorry if it seemed at your expense. I did not know all those things would make a substantial difference, so didn't bother with the photo. When asked, the photo seemed easier than explaining bits and pieces.

750E-II_29Rbloke - I hope your right. I don't recall any smoking. I probably wouldn't be looking for problems, except I was a bit freaked by the amount of carbon build up, and the seller letting it go so cheap. Seems likely the problems his friend had can be accounted for by the dirty / out-of-tune carbs and out of spec valves, plus electrical gremlins.

I'm also once bitten twice shy when it comes to potentially questionable motors because I blew out a gl1000 motor over the summer. That one did smoke (on start) and had a rod knock, so yeah, different beast.

 

Yeah I hope so too, I had similar numbers on my E-II before I got it running & the valves were OK on that... The carbon buildup you saw, was it dry powdery stuff or wet & oily? Dry powder is a symptom of rich running (float levels off/leaking needle, clogged air filter, damaged/loose jets etc) so take the carbs to church would be my advice if it was dry, and ensure the coils/leads/caps & plugs are all tip top shape 8)

 

Yeah, was a dry residue, although far to solid and thick to fit what I'd call 'powdery'. Probably normal for an (out of tune) exhaust track. And yeah, no idea how the spark was, but there were other electrical problems.

Carb work is already in progress. There was a lot of junk in one of them; given all the info I've gathered here, I'm suspecting the previous user attempted to compensate for partial blockage by fiddling with the choke / mix screws.

 

I suspect you'll be fine engine-wise if you get the valves in spec and do a proper carb service and adjustment. Your compression numbers weren't horrible to begin with, and the motor obviously hasn't run in a while. Get it tuned up and run it a bit; then do another compression test. BEFORE you rip into it for either a valve job or rings.

I'm still not sure why you think doing pistons/rings/cylinders is somehow easier than a valve job; it's the other way around. Especially if you take the head and a bag of new valve stem seals to a qualified professional shop and for about $100 they hand it back all sparkling clean, seats touched up/valves lapped, seals installed and ready to go. Or you can save the $C-note and do it yourself, lapping valves is easy and the tool needed costs all of $10 or less (essentially a stick with a suction cup on the end.)

 

100$ ? it costs 100$ to walk in the door where i come from

 

Fitz is short a zero.