The old Good Comp/Bad Comp

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Aug 23, 2004
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Hello all,

I think I killed my commando the other week. I ran it a bit too long when it was running a too thin mixture and noticeably overheating (it was 3am and i was in the middle of nowhere trying to get home...)

Eventually it just stopped, and refused to start. Before that the bike was always a one kick starter, and when kicking I always had to jump on the kick starter. Now I can stand and kick it around without using all my weight. I did a compression test and got 100psi on both sides, which i think strange since i would expect one valve to burn more than the others. Any thoughts on 100psi?

I am hoping that it is just a "little bit bad" and that I can get new plugs and it'll start and that maybe the real culprit is the worn out amal and leaky carb/intake mnifold. I have a new carb now. I would really like to ride untill it gets too cold, and do a rebuild in the winter.

so is this good comp or bad comp? :)
You haven't killed her, she's just hurt.
Do the easy checks first.
Check the valve clearances, hopefully they've closed up. If not check the condition of the plugs, look down the plug hole with a torch and inspect the piston top and the valves when they open. Remove the headers and check the exhaust valves again when open. Should give you an idea of what's gone wrong.
If you have to pull the head, it's not a difficult job. And don't forget to fix the weak running.
Best of luck,
Thanks Cash,
I felt the clearences the next day, and they felt very tight, i wondered if that was the problem so i opened them right up (didn't have any feeler gauges handy) and it did in fact feel like compression was better - but still not good.

i set clearences yesterday and comp was still in the "better - but still not good" department.

good tip on the visual inspection of pistons and valves! Why didn't i think of that last night?

I absolutely have to have the bike on September 1st so i really hope i don't have to pull the head....

So I guess if it starts, and i fix the lean running prob, it should be ok to ride, right :?
Fire her up she'll tell if she's got a major problem. You might find the compression will come back once the motor has ran a while. If so, check the valve clearances again. You just might get away without the need to pull the head.

Hi again,

yep she started with new plugs (pretty much first kick as usual) but wouldn't idle and spit and couged. Using a little start gas i found that the 1 into 2 intake manifold leaks on both sides. I guess the parts book "heat shield" has a sealing function. So I reckon that is the cause of the problem. Too bad I am going to have to order that part.....

Thanks for the help!

On a healthy 850, the compression should be around 150psi in each cylinder, measured with cold engine, both plugs out and fully open throttle when kicking over.

Thats assuming everything is standard - which is not always the case with an old bike which can have had many things altered over the years :wink:
Well I finally got around to replacing the carb, and that lead on to a few scary things!

I have a 1 in to 2 intake adapter that has two mounting screws inside the intake itself. One of these screws had come loose, and actually went into the intake port on the #1 cylinder. I am pretty lucky i guess because it doesn't appear to have done any damage, just wedged itself above and to the side of the valve pocket! In addition one of the heat shields had ripped in two so there was a pretty good air leak there.

Also, one of the carb mounting studs pushed trough the intake casting so it was sucking air through the carb side of the intake as well.

I have ordered new heat shields and will try to seal the threads on the carb mounting stud and see if it'll get me through the season.
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