Hi Everyone,
Was curious as to how my engine was doing and decided to run another compression check. Last time I did one a few years ago, it was great all across the board with a 5 - 6% variation. Everything still seems to be good, but oddly...one of the cylinders actually went up in compression. Way up.

Cyl:

1 165
2 163
3 158
4 161
5 180
6 165


I ran the test several times and came up with roughly the same numbers each time. Cylinder number 5 has greatly increased in psi since the last test I did (in which it was right in line with everything else.) Any ideas as to what would make a cylinders compression increase?

Not sure on this one...

 

Carbon build-up?

 

How did #5 plug look? Was there an oily spot on the end of the plug threads? Are you losing/burning any oil?

Could it be a bad valve seal? Had it been sitting for a long while when you checked it?
If you squirt a couple shots of oil in the others, do they come up to the same compression as #5?

 

Do a "leak down" compression check.

 

That's a distinct possibility. Also, if possible, check the cam lobes for this cylinder compared to the others. A change in overall duration due to wear could theoretically cause higher cylinder pressures. Not really likely, but worth a look if you are concerned about it.

 

I would guess carbon buildup what with the carburetor issues you've been tackling

 

yup, and there you have it. plus the fact, how long did the truck sit before you started using it agin?

 

Thanks for the info everyone. The plugs looked perfectly fine and #5 wasn't the slightest bit different than the rest of the bunch.

Overall, the truck's never sat longer than a week in the last five or six years and I drive it every day. It sees a lot of long distance trips as well (just got back from a 300 mile round trip).

I did have a temporary carb on there that was very rich and fouled all my plugs up to the point of them needing replaced until I got my 390 put back on. Those were soot black.

What could I do to clean the carbon out? How do I check for a bad valve seal? I'll have to see about getting a leak down test done.

As far as the cam lobes being worn down. Quite possible. It's 30 years old and never been replaced.

 

well......... I am about to throw the spoon in the pot here, and all the .02's will come out, BUT, we used to hold the throttle open slightly, and pour a small stream of water down through the carb. keep it running while you do this, and it will smoke, butthe carbon is gone when done.

 

What I want to know is how a carb could cause carbon build up IN ONE cylinder?

 

Dusty's steam cleaning service! We used to do the same thing, only with ATF. Let's talk about smoke....

 

My .02 is "been there, done that - works like a charm". You could try this and see if the pressures equalize after.

 

"Dusty's steam cleaning service!" Does he do carpets too?

 

Not exactly for one cylinder, but there is discussion on the proper orientation in mounting the carb. With the primary throttle plates parallel to the head or not. On mine I mounted it with the primary fuel bowl facing the firewall. To mount parallel I would have had to cut off the bottom of linkage to clear the intake. It's all about flow distribution to the cylinders. Jury is still out on the subject. I haven't had a problem, but I've only got about 1,000 miles on it.

 

I'll have to try to clean out any carbon that may be built up. I also know I'm due for a cam replacement as I have no idea how many miles are on it (165k, 265k, or 365k?).

The Offenhauser DP intake makes it so you can only orient the carb one way.