JC_1992

I recently purchased a 1987 650sx that has been bone stock since it was new. I purchased it from a guy who had it sitting in his garage for 1 1/2 years without being started or anything. I cranked it and after about 10 seconds of cranking it caught and would consistently fire the back cylinder but the front was intermittent.

Now I have..
-removed all of the old gas
-put new gas and a good additive in
-removed the oil injection system and mixed a high quality TCW3 oil in the gas
-put two new NGK B7ES spark plugs in (gapped to spec).

Taking the front spark plug out it will run on the back cylinder like a champ. Both spark plugs are parking in the same manner and color when grounded on the head. I did cut the spark plug wire and re-thread it into the boot to see if that was the culprit. I also cleaned inside both of the boots with electrical contact cleaner and Q-tips.

First I was thinking it was a carb issue, but then why would one cylinder be running good? Could it be the reeds are damaged to that cylinder? Or could it be that something in the CDI box is worn out and not providing a good spark under compression?

Thanks for reading and I look forward to some advice.
-James

Josh.O

well your first mistake was trying to crank it without checking all of that first. i would go ahead and check everything you can check until you find out whats causing it. most of the time when you do that, you can find other stuff that needs to be fixed as well.

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"A simple plan for life is change"

JC_1992

Checking what first? I sprayed some penetrating oil in each cylinder and cranked it over with the plugs out. Then put them back in and fired it up.

jonesy54

I belive that the ignition on these is a "wasted spark" (both plugs fire at the same time) so if 1 spark was weak you'd think the other would be weak also unless there is a bad/loose connection somewhere in the box. the plug wires aren long enough to reverse which one goes on which plug are they? That may tell you if its in the CDI box or reed valve.

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-1984 JS550 clone - restored, 440 pump, Skat Trak, primer, thumb throttle
-1987 JS550 - bone stock and staying that way for now
-1990 X-2 - the weird one

JC_1992

Yes I was able to switch the plug wires around and still only the back cylinder fired/ran.

I checked compression today. The rear cylinder is 145psi and the front is 135-138psi. So I am ruling out compression as the problem.

What should I check next? The front reed? Or....?