Compression Check

[jeffnew29]

Thanks for all the great help on the engine noiise. Ran a compression check and got consistent 85-90 psi across the cylinders. Noticed in the manual that the reading should be much higher but I am in Denver and was wondering if this is ok. Anybody know the correct reading for the mile high city?

[TopCatGr58]

Okay Jeff,

NOW...OPEN the throttle ALL THE WAY, have all 4 plugs out, and possibly use a car booster battery, and then repeat the test, and you'll probably find that they jump up to 130 or so...stock is 142 but that's at sea level, so a bit less would be expected, but not that much less!
T.C.

[jeffnew29]

All the plugs were out..cranked until got highest reading..no throttle.
Why do you need throttle if the engine is not running?

[DiverRay]

With the throttle closed, you can't move any air into the engine! You need WOT so there is no restriction. The readings should then all be about the same.

[skids]

90psi is low, but you have to realize that in the mile high city, air density is about 80% of seal level. You would get zero compression in outerspace. So I am thinking that .8 x 140 is about 112psi. Aslo, there are a lot if differences between gages. Even compression is a good sign. If she doesn't go through a lot of oil or foul plugs, I say run the thing!

[jeffnew29]

ok, about how many cranks to reach the highest reafing would you guess/

[skids]

Run the bike to normal temp, then let her cool off a bit, then do the readings. Crank it until the gage stops increasing.

ok, about how many cranks to reach the highest reafing would you guess/

[DiverRay]

Usually three or four POWER strokes should do it. If it will go up, but takes a little time, you may have problems. I try to watch for he first, just to see what it is. After that, I look at the end results.

[jeffnew29]

yes got the high reading ont the fourth crank..then quit increasing. manual said if you add oil to the cylinder and the reading increases you got bad rings.

[tw1980]

Wot

Wouldn't it be better to remove the carbs before making a compression check?It seems to me even at WOT while cranking The slides are still in the down position,right?

Terry

[jeffnew29]

If all the readings are even, am I ok? Really worried about this bike.

[MeatTooth]

I got 110 on mine and was getting worried. Then I opened the throttle and it shot to 160 by the fourth turn with all the plugs in. You really have to go at WOT to get a true reading.

[Ken Talbot]

Wouldn't it be better to remove the carbs before making a compression check?It seems to me even at WOT while cranking The slides are still in the down position,right?

Terry

Yes, the slides are in the down position even at WOT, but look how much room there is under them to allow air to pass:



When the throttle is closed, so are the butterflies, and this is where the air gets choked off:

[Incubus]

Do the test at WOT, but don't get all freaked out if you don't match the readings from the manual. If they all are about the same you are probably good. I got all unhappy when I couldn't match the compression readings the manual called for on one of my other bikes, even after a fresh top end rebuild and break in. I built it as a high-compression high-performance engine and while I was getting equal compression across all four cylinders I wasn't even getting the stock readings. Turns out the stock Honda compression tester the manual expected attaches directly to the plug hole and the one I was using had about a 12 inch hose. The amount of air trapped in the hose lowered the reading considerably. I tested it again with a gauge with a much shorter hose and the readings shot up way past stock.

Look for equal, not necessarily a target number.

Patrick

[fredintoon]

Hi Jeff,
how long had the bike sat around not running before you got it?
A bike that's been parked for a year or so will typically have low compression numbers because the rings may be partially stuck in their grooves.
So long as it runs nice, don't worry. Most likely the rings will free up as the miles add up.