Compression question
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Joined: Mar 2007
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From: San Tan Valley, Arizona
Compression question
I have a modified 88 inch in my 99 Road King. I'm not sure what was done to it because it was done before I owned it and the previous owner did not know either. It has Screaming Eagle Jugs on it and an Upgraded carb. They seemed to do it right. I opened the cam chest and it is all gear driven so obviously they took some care to it. It drives me nuts not knowing what has been done and what size the thing actually is at this point. it runs pretty good. It easily beats my brothers M8 Heritage.
So out of curiosity I did a compression check on it. The front is at 170 and the rear is at 150. I thought there should only be at most 10 psi difference between the two. Does this sound like there is something going bad on the rear cylinder?
So out of curiosity I did a compression check on it. The front is at 170 and the rear is at 150. I thought there should only be at most 10 psi difference between the two. Does this sound like there is something going bad on the rear cylinder?
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Joined: Dec 2012
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From: river
I have a modified 88 inch in my 99 Road King. I'm not sure what was done to it because it was done before I owned it and the previous owner did not know either. It has Screaming Eagle Jugs on it and an Upgraded carb. They seemed to do it right. I opened the cam chest and it is all gear driven so obviously they took some care to it. It drives me nuts not knowing what has been done and what size the thing actually is at this point. it runs pretty good. It easily beats my brothers M8 Heritage.
So out of curiosity I did a compression check on it. The front is at 170 and the rear is at 150. I thought there should only be at most 10 psi difference between the two. Does this sound like there is something going bad on the rear cylinder?
So out of curiosity I did a compression check on it. The front is at 170 and the rear is at 150. I thought there should only be at most 10 psi difference between the two. Does this sound like there is something going bad on the rear cylinder?
10% is what I go by. Im sure someone will Crucify me for saying 10%. Idk 🤷🏽♂️. If youre concerned do a leak down test.
im assuming compression test done with bike hot, with throttle wfo and both plugs out?
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,023
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From: Poolville
Yes you have to pull both plugs and hold throttle wide open. If it's a CV carb you need to figure out a way to lift the slide cause it's vacuum operated... And as bustert said 10% is the variance...
HDF Community Team
Joined: Jun 2006
Posts: 12,763
Likes: 2,597
From: san antonio
Leak down test is a good idea. However, a second compression test would also be good idea; test with motor hot. Put the battery on a charger to make sure it is fully charged for the test; both plugs out and grounded, throttle wide open and cycle till the gauge stops registering. The Mikuni is cable operated so check to make sure the slide is coming all the way up before the test and air filter removed. Stock 88 should show, on average, about 160psi. The gear driven cams indicate aftermarket and it would be helpful to know what cams have been installed since the timing of the intake close will affect the CCP. If the cam chest is open, look at the end of the exposed front cam and check for any identification marks. If the readings continue to show a 20psi variance, do the leak down test. I would even go so far as to check with another gauge as well.
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Seasoned HDF Member
Joined: Dec 2005
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From: poway
Before you get too worried. Run some seafoam through the intake, take it out for a few WOT runs then do a hot compression test.
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