so I am trying to help out a buddy and get his 87 running after sitting a LONG time. there was one of the carbs rasted up to the point I could not even get the throttle plate to open. so we grabbed some carbs off ebay, PERFECT shape. it is getting spark. fresh fuel, I used my tank on it to be sure. I even tried the cdi on my bike and it ran fine. now when we crank it over it seems to spin very quick. eventually it backfires through the carbs. it just WONT catch, not even a sputter. I am thinking that the intake valves on the cylinder that had the rusty carb must not be sealing??? this make sense? what else can I check?? starting to think it is a parts bike

 

do a compression check

 

thats what I figured, just do not have an adapter for my compression tester that will make it fit the bike

 

anyone know what size I need??

 

I know you said you were getting spark but check your plugs anyway, its possible they are getting spark but they are worn to much to ignite the fuel. seen it happen before on my old F3

 

will give that a shot, thanks!

 

i believe the spark plug thread size is 14mm by 1.25mm thread pitch. i'm 90% sure thats the size, it could be 12mm. call a local motorcycle dealer, they should be able to tell you for sure.

most compression gauge tools come with multiple adapters. normaly, if the plug is sparking, its gonna be good. i would do a compression check before i replace the plugs.

you want similar readings across the cylinders. below 100 psi is gonna be bad.

 

done a lot of comp testing before, but always on my snowmobiles. guess I will see soon!

 

It ran with your CDI? If so, there is the problem. If not, the compression check would definatly be next.

 

if it ran with your CDI, give me a PM, i got one i could sell you