After a year working on it, finished up today. Motor is an 82 shovelhead. Fired right up but backfiring and sucking on the rear cylinder. Has good compression. Plugs show real lean for the rear. Only thing done new were new pushrods/lifters.

 

Check the rubber boots from the intake to the head. Sounds like the rear is sucking air....

 

I checked that. Didn't seem like they were loose. Didn't start it after though. Try it again tomorrow, enough for one day. Big temperature difference between the front and rear when first started, but the rear does heat up. Don't have a temp gun at home but it does get hot, just a lot slower. Tried backing off the exhaust to see if that made any difference.

 

What motor work was done? Also are you sure the compression is good? Have you tested it?

 

At first no compression. Hooked shop air up to it and was blowing out the exhaust. Completely removed intake and exhaust push rods. Put air to it, no leak out the exhaust. Leakdown was the same front and rear(less than 2% difference). Put exhaust push rod back in, left loose enough to move maybe 1/8". Roll motor over and put some air to it and leaks. Put 120lbs to it and it stops leaking. Rocker shaft turns easily.


Had new rocker boxes, rockers and shafts. Push rods and lifters. Ran fine when I took it out of the 82 frame.

 

Well sounds like you found your problem... Little to tight on the valves.... I suggest you get a Factory Service Manual and set up the valves properly.....

 

Not adjustment, leaks until you put 120lbs of air to it. I think it's a busted/weak spring.

 

Oh ok, i got you..... When you gonna take the head off? If you don't want to remove the head you can get a tool to replace the springs with the heads still on....Glidein Wide just replace his springs..... Here's his link.....

https://www.hdforums.com/forum/evo-c...l#post16283104

 

Had to be the rear, can't get the rocker off without taking motor out. Frame is stretched enough to get the front off with the motor in. I have the tools to do it, just was hoping that there was something else that could cause the problem.

 

Looks like the bike is setup for single fire dual plug.. It's possible that you have cylinder wiring crossed but timing set correctly on front cylinder. If you do so rear cylinder spark will be about 90 degrees late. IIRC I managed to do this on one evo I owned when installing a new ignition (dyna 2ki).