compression

[cheifrider]

ok, this one is for you motor gurus out there ,i have a 1941 u model that used to run great, all of the sudden it WILL NOT START for love nor money.has spark ,is getting fuel, and timing is right on the money . i want to do a compression test but i cannot find the proper compression in any of my manuals .can someone give me the range i should be looking for?, thanks in advance ,tom

[cheifrider]

compression tested at 48 lbs front cylinder 24lbs rear cylinder...dont know if i like those numbers as there are only 490 miles on the top end of this junk heap

[KNUCK]

I am no expert, but the fact that they are uneven is not what one would want on a new or old motor. Did you flood it real bad? The one that is 24lbs is barely enough to fire. I would put a little squirt of oil in each cylinder and try to start or test again. I did this to an FLH that sat 10 years and it fired up quick. Smoked for about 5 min after, runs great now. Always have good clean plugs. Good luck and keep us posted.

[AdminGuy]

I'm thinking that your comp should be above 60.

So maybe your tollerances for pistons and valve guides were made too big upon build?

Just a thought. I've seen first hand that a little tighter tollerances can be used.

pull the heads and clean it up. Clean valve seats. Assem and retest. Running too rich, or blow by? Something to figure out. Let us know.

[VPH-D]

I would agree the compression should above 60, possibly closer to 100. IIRC compression figures should be not more than a 10%
variance. Is it possible the valves are too tight?
VPH-D

[cheifrider]

thanks for the input guys , checked all tappet clearances and they were right on the money , as for the fuel mixture it was a tad lean so i richened it to that dark tan color on the plugs,this was before it stopped running .also when i checked these plugs i killed the motor while going 45 mph to get a better and truer reading on the plugs.have to talk to the guy who did my topend to see if indeed he made tolerances slightly tighter (i believe he did)he just got back from fla yesterday so ill see him and see what he has to say

[cheifrider]

ps, do you guys think a "wet" compression test with a little marvel oil might help?ive found it real hard to saturate a flathead motor sparkplug ,but .....could raw fuel wash the cylinders to much?

[VPH-D]

IIRC, squirting oil into the plug hole will indicate excessive piston or ring clearance if the compression rises after application. Is the throttle wide open during your compression tests?

You might want to go easy on the tight piston clearance, as UL models were known to seize the rear piston. Some builders use increased clearance over stock to avoid seizing. Do you have copper head gaskets? If not, you will need to install them. The composite gaskets prevent proper heat transfer to the heads.
One of these days I'll get around to finishing my 45 UL!
VPH-D

[Kojack]

Make sure you got the basics right on your test. The engine should be warmed up before testing. Remove both plugs and make sure the throttle is wide open during testing. The engine will need to be run through compression about 4-6 times (not one pump). The highest reading reached is the compression for that cylinder. If I rember right, you should see at least 50% of the total reading on the first pump and readings shouldn't vary more than 15-20% between cylinders. If you add a little oil after testing and the reading goes up significantly on the next test, you have worn rings. If there's little difference it's bad valve seating, burnt, warped, out of adjustment, etc.

[T. Cotten]

I smell a vacuum leak.

....Cotten

[cheifrider]

ok......... i warmed the engine (as best i could) and the front cyl went to 50lbs the rear went from 24 to 50lbs.did a "wet" test after that and there was no change ,valves?.my local guru said to loosen em up a little ..........?this is a "u" motor so is 50 lbs acceptable? ps just for giggles i did a test on my 48 chief that sat for 36 years and it tested 78 lbs rear and 85 lbs front ..............just a thought here but .........why does not everyone have an INDIAN CHIEF?