backfire
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6th Gear
Joined: May 2017
Posts: 12
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From: wisconsin
backfire
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.
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6th Gear
Joined: May 2017
Posts: 12
Likes: 1
From: wisconsin
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.
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6th Gear
Joined: May 2017
Posts: 12
Likes: 1
From: wisconsin
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.
Had new rocker boxes, rockers and shafts. Push rods and lifters. Ran fine when I took it out of the 82 frame.
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6th Gear
Joined: May 2017
Posts: 12
Likes: 1
From: wisconsin
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Thread Starter
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6th Gear
Joined: May 2017
Posts: 12
Likes: 1
From: wisconsin
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.
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Joined: Dec 2005
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From: poway
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).