Runnin' in gear!

[barishiman]

I got up this morning and started to work on the mustang. Adjusted my valves again and moved my dizzy. After that it fired right up and ran the smoothiest I've ever heard in a long long time. I fiddled with the timing and lowered the RPMs a bit and decided, what the heck let my put it in gear. I dropped it down in reverse and it struggled but it was idling. Messed with the idling again and bam I was on the street in my neighborhood. It ran smooth and it was crazy loud with open headers. I didn't want to push the engine any, I got it up to 30 mph just fine. That 2 brl carb really gives the engine a kick.

After letting the engine run for awhile, I went ahead and did a compression test. The results were like yesterday:

1.) 160
2.) 160
3.) 100
4.) 150
5.) 160
6.) 100

This really baffles me. How can 2 pistons being two pistons apart have low compression. I adjusted the valves correctly as best as I could. I don't think my head gasket is blown. The last time I took off the head gasket four months ago the gasket wasn't blown. So I'm really lost here. The car runs great yet have compression low on two pistons. Plus the spark plugs were all clean except for six, it was alittle black.

Anybody got ideas why two pistons are low on compression like they are?

BTW thanks to Adam and Joe!

 

[Anonymous]

rings?
bad lifter?

wild guessing by me...I dont know shit

 

[MustangSix]

Do another compression check, but this time add a tablespoon or two of motor oil to the two low compression pistons. If it brings the compression way up, it is rings. If it stays low, it is gasket or valves.

If it stays low, then try backing way off the valve adjustment and to the compression test again. If it comes up, then you have a valve adjustment issue. If it still stays low, you should pull the head and inspect for a blown gasket or burnt valve.

BTW, if you run an engine with too little lash or too much preload for too long, you could burn an exhaust valve, and it only takes a few miles. Be careful on those valve adjustments.

 

[Lazy JW]

Yup, do what Jack said. If it still acts like it's the rings, give her the ol' BonAmi treatment.
Joe

 

[barishiman]

Jack, I went ahead this morning and did the wet compression test on 3 & 6. The compression raised alitte from 100/110 - 120/125. I noticed this the other day, that the wet test boosted the compression rating about 10 or so but nothing considerably high like 150. So I am lingering on that the piston rings are still good. But I did notice one thing, whenever I did the compression test on 3 & 6, 2 & 5 would puff smoke out the spark plug hole. Does this mean anything?

I then went ahead and loosened the valves on 3 & 6 and the results didn't change. I was still floating around 100/110 on both pistons. The valves are all new, I had four new ones that were bent replaced awhile ago. I really hope its not the valves. If it is, i'm going to a different machine shop.

It looks like I'm coming down to pulling the head off again. I really don't want to, but if that's the case then oh well.

what should a burnt valve look like?
How do I know I have too little lash or too much preload?

Plus the plugs are all starting to turn black. I guess to much of a rich mixture. On the Holley/Weber 5200 carb, don't you adjust the mixture screw out for less gas or is it more? I forgot. I guess the exhaust is getting to me.

This is frustrating, but I'm sticking with it. BTW if anyone lives near Douglasville, GA and would give me a hand one weekend I would appreciate it. I'm off to buy some head gaskets and a shop manual. Oh, lunch sounds good too.

And whats the ol' BonAmi treatment?

Thanks

 

[addo]

It does still sound like bore wear - up to a point. If the engine block is remanufactured, it's possible that resized rods were used, or an unmatched set, leading to differing deck heights between cylinders. Stranger things have happened. If there is no problem with the valves closing (you verified this), I suggest driving it a little to see whether the rings bed in more.

Adam.

 

[Lazy JW]

If the rings haven't seated, you can run a few teaspoons of BonAmi cleaning powder through the engine while it is running at a high idle. Then take it out and drive it hard for 20-100 miles, then change the oil. This only really works if the rings are good but haven't seated yet. Obviously, it won't cure worn out engine. It really does work though.
Joe

 

[Anonymous]

I'm not familiar with the BonAmi brand name, I don't think anyone sells it here. Is it anything like Ajax? Just wondering

 

[addo]

Basically the same. But be sure there's no really aggressive other ingredients in the mix. The finest pumice powder might work, too.

 

[MustangSix]

A burnt valve will be melted at the edge. I think it may not be this, however.

Don't pull the head just yet. I forget if this is a new engine or one with new rings, but it may take a few hundred miles to seat the rings. drive it a week and then check again.

 

[barishiman]

Well, the block and pistons aren't that new. The engine underwent a rebuild 6+ years ago by the previous owner. I didn't do anything to the block, I just cleaned the top of the piston and removed all the crap off the deck of the block.

Another thing that I noticed with the compression test on 3 & 6 is that it sounds like its loosing compression somewhere. Yes that is obvious by now, but just the way I hear it. Whenever I do it on the other pistons, it feels like a perfect seal. The first strokes bring the compression to 120 immediately. However on 3 & 6 it sounds like there isn't a perfect seal. The first stroke brings the compression up to 90. Another thing I noticed is that sometimes after I screw in the hose into the spark plug hole and crank the motor I don't get a reading. I'll take it out and screw it back in and then I'll get a reading. Weird.

Oh, do you think an exhaust leak would cause faulty compression tests? The top lobe on 6 is broken off and I had to rig something to tighten down the bolt. I know that bolt isn't torqued correcty. Maybe I should check my header bolts. I had a hell of a time torquing those with so little room. Big hands don't help either.

 

[Anonymous]

you know what they say...big hands=frustrated mechanic...


sorry, I saw an opportunity to make a joke...and I dropped the ball....


Chaz

 

[barishiman]

I'll be out tomorrow changing my brakes and oil on my truck and looking for some BonAmi. What do guys think about the low compression... exhaust leak or something else?

 

[Lazy JW]

Usually you can locate the leakage by listening. An exhaust valve will leak out the exhaust, the intake will leak out the carburetor, and the rings/piston area can be heard coming out of the oil filler cap. Sometimes compressed air plumbed into the spark plug hole is helpful (be careful with this one, it can rotate the engine). If you have broken rings or broken pistons the BonAmi won't do any good, but it also won't hurt.
Joe

 

[Anonymous]

i know some dealers (car) sell something called "top engine clean" and this has worked for me in the past