I'm working with a 90 Evo with 76,000 miles. I don't know the history of the motor but I know for sure that nothing has been done to it since the 50,000 mile mark. I just did a compression and leakdown test. Here are the results.

Front cylinder had 130 psi compression with a 35 percent leakdown and a little air comming out the carb and the timing plug.

Rear cyclinder also had 130 psi compression with a 30 percent leakdown and a little more air comming out the carb and a little air out the timing plug.

Seems to run fine. Doesn't use any oil but will soot up the plugs a little under 35 mph (the rear plug more than the front)

What are your thought on these tests results. Thanks

 

seems like 130 is pretty close to new. i have a 90 and im pretty sure that 145 is 8.5 compression ratio therefore 135 would be about 8 to 1. i have 165 but im sure the heads have been milled. 165 is about 9.5 ratio. id say if it doesn't smoke run it until it does.

 

In medicine, it is important to treat the patient and not treat a number. Run it until it gives you problems, then do something.

 

You have great results for the miles but you failed to say how long the leak down pressure held and at what rate did it lose pressure?

As far as the rear plug getting sooted up at 35 mph. The rear cylinder will always run hotter than the front in summer temps. What you may want to look into is turning the idle mixture screw but I can't rember which way is which?

 

Hey guys, thanks for the comments. Doc, you made a great point. I need to stop overanalyizing, get on it and enjoy the ride. If it ain't broke don't fix it. Thanks again, Todd

 

I've got a 95 Electraglide Evo with 71,000 miles. Started hearing a knock at about 65,000 and kept getting worse. Thought I had a wrist pin or skirt knocking. Decided it was time for a topend rebuild. Tore it down, only thing was carbon buildup on pistons hitting head. Could have cleaned up, new rings & gskt and been fine. But I wanted a little more grunt so I went 9.5to1 on comp, bigger intake valves,and 569/236 vthunder cam. Great for 2up pulling in the hills. But like I said, other than carbon buildup, it would have ran on. The bottom ends will run forever if you treat it right(CHANGE OIL REGULAR) Anyway, I'd run it till you have a reason to go in. Except for the lifters, early Evo's had a problem at about 50,000 mile, brgs would come out of rollers, if you don't catch it in time you can loose the whole engine.

 

Hotrod351 were did you get the info on psi to compression ratio . I would be interested to see a scale . What would 210 psi translate to ?

 

just noticed a question to my response here, im not getting notified by email when there is a answer or question to me, dont know why. any ways, i bought a 75 chevy truck new and it had, i believe, been a long time, 8.5 to 1 compression, each cylinder had 145 psi compression. i have a ****** jeep with a 350 sbc and it has 9.5 to 1, it has 160 psi. i have a 70 mustang with 10 to 1 and it has better than 180 psi. i know that certain cams can fool a person on cranking compression, more duration will, suppose, to give lower numbers. i havent ever seen that. now the harley manual that i have says 90 psi and above is good. all i would be concerned with is if i had 210 psi or 65 psi, one being to high and the other to low.

 

As far as the compression, general rule of thumb is 20% between the two so if you had 130psi on one cyl. the other cyl. could be as low as 104psi, but the compression is the same and the leak down #'s don't sound to bad either. If it ain't don't fix it.

 

Todd-

'90 would have the good inner cam bearing. So no worries there.

Ride that baby, & enjoy it..