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Post rebuild compression numbers

  • Thread starter Thread starter Lunchbox88
  • Start date Start date
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Lunchbox88

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Hi guys, I'm nearing completion on my 79' 850g project and got the engine running last weekend after rebuilding it. Everything got cleaned, I reused the pistons as they were in good shape and the motor only had 34k on it. But it has new rings, a good hone, lapped valves, new stem seals, and new gaskets all around. Then I re-adjusted valve clearance to get it into spec and she fired right up. Now the motor probably only has 5-7 mins of run time on it, but I wanted to check compression to make sure everything was fine. According to my compression gauge I am only getting about 120psi (plus or minus a couple) in each cylinder. This is pretty worrying because I was expecting at least 150psi. Are these numbers something that will go up with a proper break in, or do you have any ideas on what could cause such low compression on freshly rebuild motor?

Also here is a teaser picture:
055.JPG


Thanks,
Lunchbox
 
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I did the same things to my engine 82 750. Pre-rebuild 85-110psi now 150psi straight across with less than 100 miles. I'm doing valve guides for the fun of it.
My question to anyone-
Do out of spec.valve guides affect compression?

I see your paint gun in the pic. What's your setup? Gun type, compressor CFM. I want to get setup to spray.
Thanks
 
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I wouldn't worry too much. The compression will increase as the rings seat. When you get the bike going ride it aggressively so the rings will be forced out into the cylinder wall. Worst thing you can do is baby the engine early on.

Good luck
 
It takes about 50 miles to seat the rings.

Because all four jugs are similar, I wouldnt be too concerned about breaking a ring or having fair to stagger the ring gaps.

Don't worry.
 
Thanks for the info guys, it definitely put my mind at ease a little it just seemed like 30psi is a lot to get from a break in. Guess I will find out when my petcock gets back from Pingel, and I can finally get on and ride it.

BigRock said:
I see your paint gun in the pic. What's your setup? Gun type, compressor CFM. I want to get setup to spray.
Thanks
Click to expand...

Its my buddies setup, but the gun is a Sharpe FX1000 HVLP (http://www.sharpe1.com/sharpe/sharpe.nsf/Page/Finex+FX1000), and he has a big 60 gallon compressor rated at just under 10scfm. We ran 2 inline dryers, and it seemed to keep all of the water out. He did mention that because non of the lines are special hvlp lines and he wasn't using big bore connectors, that we had to run the gun at a little higher pressure to get the proper flow. Overall, the paint turned out really well for a garage job, and I just need to do a little clear coat sanding on two of the peices to get it were I want it. He really likes the gun for small stuff like this, and says its one of the best bangs for the buck he could find. The color I went with is a a charcoal grey by Ford, paint code : CX (Dark shadow grey).
 
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