Low compression, but runs fine?

[mikeyBskunk]

Soo i got my bike running about a week ago and it starts fine and idles fine but the right cylinder was fireing harder then the left. soo i just ran a compression test and bolth sides are reading 60-65psi. thats low! but i have read on here that the bike usually wouldent start with that low of compression....what should be my next step?? I dont want to fry my pistons...or make matters worse

 

[Downeaster]

Not knowing your level of experience, the FIRST thing I'd do is make sure my gauge was reliable and that I was using proper procedure to check the compression. Those numbers make me wonder if your throttle was fully open.

 

[mikeyBskunk]

i will try it again with carbs off then i let you know

 

[mikeyBskunk]

carbs off, wide open air, same readings 60-65psi 4 kicks on each side

 

[mrriggs]

Does it continue to climb if you keep kicking it? If you have one of those gauges with the long hose then it can take 10-12 kicks before it levels off.

 

[mikeyBskunk]

Nope tried it again and nothing kept kicking and just stayed a 60ish

 

[Downeaster]

Well then, you have issues. Try squirting a little oil in the plug holes and see if it comes up. If it does, that indicates worn rings.

 

[mrriggs]

I'd try a different gauge first. It's odd that BOTH cylinders would be THAT low and the thing would run well.

 

[KentMoney]

no, you want the carbs to be on. and you want to have your throttle twisted to WOT (wide open throttle) as you start up the bike.

will help to have someone hold the compression gauge while you hold the throttle and hit the e-starter.

 

[Downeaster]

no, you want the carbs to be on. ...<snip>....

Why? Am I missing something?

 

[KentMoney]

you stated in your first comment that you wonder if he did the compression check procedure correctly, and that it seemed like he didn't have the throttle open.

the throttle directly actuates the carbs, if you remove the carbs would holding the throttle have any effect at all? no.

i guess removing the carbs could simulate WOT, but then you're not getting any gas into the chamber. My understanding is that you want WOT when testing for compression because that will give you a strong stroke which will translate to a more accurate compression reading.

thats why you get like 60psi without the throttle open, but then it should jump to 125 with the throttle open.

 

[jayel]

opening the throttle opens the butterfly plates but that doesn't move the vacuum operated slides, they either need propped open or remove the carbs to eliminate any blockage, you don't want gas in the chamber if you're doing a compression check... won't hurt to check valve clearances make sure they are closing all the way

 

[mikeyBskunk]

As Mr.Riggs says ''Id try a diffrent gauge first", thats my next step i rented the one i have from O'rilley's auto parts, i agree on the thought that the bike is running on 60psi is a little strange... soo i will return the one i currentaly have tomorrow and try someplace else for another

 

[scabber]

A leak down test will tell you more then a compression test and where the problem is.

 

[pamcopete]

Well, another reason to have the carbs on when you do the compression check is so you can run the engine and warm it up. That gets the rings to seat better and also oils the cylinder walls.

Also, use the electric starter instead of the kick starter. That will bring up the compressing as well. With the engine turning over very slowly using the kick starter, some of the compressed air in the cylinders has time to escape, even with a "normal" engine. A leak down test takes all of this into consideration.

 

[gggGary]

I see no reason for carbs on, EXCEPT a warm engine. I am usually checking engines that last ran before Bill got the cigar from Monica. I normally get the same compression reading electric or kicking, with it taking a few kicks for the highest pressure. There are a few issues with compression gauges, most use a shrader valve and it can be dirty or dry and not seat properly. If it has couplings they also may leak. Usually if the valve holds the pressure reading those parts are fine. Also a little anti seize on the threads and a good o ring help. Tell us how you do with a different tester.

 

[pamcopete]

gggGary,

Then you must be testing engines with good compression. An engine with good compression will not bleed down as much as an engine with low compression. This engine has low compression as tested, so I think the exercise is to try and get a pressure check that is closer to what it is with the engine running, IE with higher revs than you would get with a kick start test on a cold engine.

A good engine will read about the same hot or cold, kick or electric start. If you are trying to determine if a low compression engine will make it through the riding season, you need all the help you can get.

 

[mikeyBskunk]

Wow! thanks for all the replys, let me add a few things, when i did the first test i had warmed up the engine, throttle slides proped open. I then performed the test again about fifteen min later with the carbs removed...same results. Like i had stated it just dosnt make sence to me that the bike would start and idle fine with such low compression so i am going first thing in the morning to rent another tester and we will see

Thanks again
Mikey

 

[mikeyBskunk]

ok good news! with the new tester here are the results, warmed engine right side is at 135-137psi and left is at 140 dead on. soo i am asueming that timing and a carb sync should straighten and even everything out?

 

[5twins]

Well yes, that along with correct timing chain and valve settings. Do those 1st in that order, then timing, then carbs. These bikes like their timing set exactly. If you still have points, it's possible but very tedious. With the dual points set-up, you're actually setting two separate single cylinder engines. For it to run best, you want the timing and points gap from one to match the other. Like I said, tedious but possible. The best thing you can do if you have points is upgrade to the Pamco (thank you Pete !!).