Just rebuilt engine.........

[Guest]

'79 GS850G

Just finished buttoning up the top end of the motor last night. New pistons, new cam chain, valves etc. Turned the crank a few times, everything seems lined up good. Tensioner put back together and re-installed.

Question: Can I hit the starter button and do a compression test, just for the heck of it? Or do I need to get everything else (carbs, coils, boots) on first?

[Steve]

Ideally, the engine needs to be at operating temperature for a good compression test, but you should be able to get a good idea if your rebuild worked by checking it now. Check all of them once, then add a teaspoon or so of oil through the spark plug hole and check again.

Since you want to hold the throttle wide open when doing the check, you can eliminate that pesky little detail since the carbs won't be mounted. 8-[

Not having the coils is no problem, as you normally disconnect the plug wires, anyway.


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[brveagle]

Do that compression test. As long as you've hand turned it over to make sure clearance is good, you're good to go

[mark]

Just did it. This looks promising so far. Thanks.

[bwringer]

There's oil in there, right? :-D

I usually crank them on the starter at this stage until the oil light goes out, just to make sure there's a good supply of oil up top.

I've never bothered to test the compression before running the engine enough to seat the rings -- not sure if you'd learn anything useful, other than a confirmation that you remembered to put the rings in.

[don_gibb6512]

Just did it. This looks promising so far. Thanks.

So if you would please, what were your numbers? Just curious as I "refreshed" my engine (not new pistons but new rings, etc) and I took a compression test somewhere around the 50 mile mark and the numbers weren't great. Now at about 400 miles I have 150 psi equally across all four, dry test.

Thanks.

[mark]

So if you would please, what were your numbers? Just curious as I "refreshed" my engine (not new pistons but new rings, etc) and I took a compression test somewhere around the 50 mile mark and the numbers weren't great. Now at about 400 miles I have 150 psi equally across all four, dry test.

Thanks.

I didn't do a very good job and just used an old "push in" type of tester that I've had for about 30 years that doesn't hold pressure very well, my battery was low and only did a couple of pumps on each cylinder. Maybe 90 psi. I just wanted to check that I'd put the cams in correctly and that something was drastically wrong.

[trippivot]

numbers might be a little low until after break in period. rings and valves need a little time to really seal up tight.

[chef1366]

Start that puppy up and get the rings set in. The most crucial time for getting the rings to set in are the first few minutes of running.
Good luck!

[mark]

Start that puppy up and get the rings set in. The most crucial time for getting the rings to set in are the first few minutes of running.
Good luck!

Thanks for the encouragement. I can't work on it anymore for a few weeks, I'm heading out of town. I've been very patient for over a year. But I gotta tell you......I'm gettin itchy to get this sucker running.