Compression check.

So I picked up an 83 Spectre and figured I'd run a compression check on it, I was slightly surprised by the results:
150,150,140,170.

The 150's seem pretty good, the 140 is within the 10% tolerance of the 150's, but that 170 seems a tad high. I kind of expected to have lowish compression, just because it's older, so I'm wondering if there's some condition that would give higher than normal compression?

Only things I can think of are if the exhaust valve is closing much earlier than it should (not sure what would cause that) or if there was something accumulated in the cylinder, looking in through the spark plug it looks fine though.

I will say, the bike probably could have been warmed up a little bit more, but the studs for the headers are all jacked up, so they don't seal right and I felt bad for my neighbors keeping it running, and also didn't want to burn valves since it was halfway just venting (don't know if that's as big a problem on bikes as it is cars, seems like the exhausts are much less restrictive anyways on them).

could just be that the others have worn more. or a bad reading. compresion could be leaking at valves as well as rings. add some oil and retest the compresion.
wouldnt hurt to check the valve clearance.

I was actually more worried about the 170 being so high. I'm still waiting on my service manual to arrive, but isn't 170 well above what's normal? I thought 130-150 was more normal for these things.

FSM troubleshooting says too high compression (overheating and/or knocking) may result from "Carbon built up in compression chamber."

Perhaps the carbon build-up is due to extended running with an over-rich fuel mixture or clogged exhaust, maybe too large jets, and maybe too low clip position on the jet needle. And perhaps pussy-foot riding.

The FSM provides instructions about manually removing the excess carbon build-up.

Would suggest installing a new air filter and logging a few hundred miles of brisk riding, and at least one daily blast through the gears.

Would also install brand new NGKB8ES spark plugs, and run through a few tankfuls with Sea-Foam or similar product such as Kreen (made by Kroil) added to the gasoline.

There are products designed to loosen the carbon and allow it to be blown out. Am thinking for example that PBBlaster touts such remedy.

Good Fortune!

I thought the 750 fours had nominal compression close to 180? The 750 twins run about 155.

Compression figures are shown on page 169 in the KZ750 manual downloadable via this link:
kz.bike-night.com/media/GPz750-full.pdf



But am uncertain whether applicable to the particular bike at hand.

Good Fortune!

well, the link indicates it's for a GPz750, so it should be the same. Spectres were the same engine, but didn't get the porting the GPz's got, and had different carbs. At least that's what I've read. Same engine for sure. So it's good to see that my compression is at least in their range, and just the one cylinder outside of the 14 psi difference.

Carbon buildup is definitely a possibility, hadn't thought of that. I'm pretty ham-fisted with the throttle. But the previous owner was much more of a cruiser type, so I imagine he was not, and the carbs could do with a sync as well, so running rich is certainly possible. Though it does run at least run.

I ran a bit of seafoam in my last fuel tank, haven't heard of running kroil in the fuel before, but it does wonders elsewhere, so why not I suppose. How much would you recommend adding in to an otherwise full tank? I know I've had issues in other vehicles from being too liberal with seafoam in the tank (couldn't start).

Thanks for the input guys.

Del_Herring wrote: ...ran a bit of seafoam in my last fuel tank, haven't heard of running kroil in the fuel before, but it does wonders elsewhere, so why not I suppose....

Have edited earlier post to suggest Sea Foam, or the similar product Kreem (made by Kroil) as additive to the gasoline.
www.kanolabs.com/engCle.html

As I recall, PBBlaster is sprayed into the spark plug holes, and allowed to soak into the carbon, and then the engine is run to "blast" -- what else? -- the carbon from the combustion chamber.

There are many other products by different suppliers (Mopar and Yamaha come to mind) intended for spraying into the combustion chamber, while other products and magic chemicals are added to the gasoline, or introduced directly into the carb while the engine's running.

I believe some folks still run the engine while spraying water mist into the carb air intake, which supposedly loosens and removes the carbon build-up.

Good Fortune!

Best way to get carbon out of the cylinders:

Use the vacuum lines into the intake manifolds to suck Seafoam straight in.

Use small vise-grip pliers clamped on the vacuum line to adjust amount of seafoam sucked in and draw in a small (steady) amount for several minutes then SUDDENLY open the pliers and let the engine suck up a large amount of Seafoam which will stall the engine.

Let it sit in there for ten minutes to soften carbon.

Reattach vacuum lines.

Restart bike and watch a cloud of black smoke come out that is bigger than your house as the carbon blows out.

I've seen a similar method used on cars. I'm guessing the vacuum ports used for syncing would serve the same purpose as the line to the intake manifold on a bike. Now I just need to get my exhaust fixed so I can run it that long. There's a stud sheared off on one of them, and the others are in poor condition. The one that's sheared is right behind one of the frame spots, so I'm not sure how easy it's going to be to extract. We'll see can I get a drill in there head on.

Depending on the circumstances, it's sometimes possible to tack-weld a nut onto the stud (weld inside of the nut to end of the stud) , and then use the nut to twist out the stud.

Good Fortune!

So, the stud sheared off inside the head, so there's nowhere to get a nut or anything welded to it, and since it's the stud located right behind the frame, I can't get a drill in there. I now understand why the previous owner jury rigged that exhaust clamp, but I tossed that piece already, so no jury rigging for me.

It's looking like I'm going to have to remove, or at least partially remove, so I can tilt, the engine. I've removed engines before, but not on a shaft drive bike. My manual was supposed to arrive today at the latest, and still isn't here, any tips on how to get the shaft out from the engine so I can remove the engine to get to this stud?

Alternately, my neighbors will hate me, and I know it shouldn't be done for a long time, but would it hurt to get the bike up to temp with one pipe basically not attached? Mostly just so I can do a proper compression test. It was only partially warmed up before, and the carbs aren't synced, so they don't quite all warm up the same, and it idles pretty rough until it's warm. So I'm hoping if I can get it up to temp all around, I'll get better results (certainly more accurate).

If I can verify that the compression is good, I'll go ahead and get cracking on syncing the carbs, and cleaning if needed. It's been rideable up until now, but the long warm up time, combined with terrible idling until it's warm is distracting. If the compression's bad, then I'll know to start looking at actually working on the engine.

So far I like the bike, I imagine if I can get her running properly, it'll just get better.