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Compression test

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M

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In another thread, there was a long discussion about an apparent problem I was having with #1 spark plug carbon fouling. There were many suggestions as to what to do to check this out, but the leading suggestion was to first run a compression check.

I ran one yesterday after warming up the engine and pulling all the plugs. This is what I got:

#1 = 165 psi
#2 = 165 psi
#3 = 170 psi
#4 = 165 psi

The consistency was good, but the psi seemed high, so I repeated the test. Got the same results.

Is this good news or otherwise?
 
I see this as good news. The other item would be a leak down test to see if your getting a leak in the 1 valves.

The bad news is it doesn't answer the rest of your question. What is fouling the #1 plug.

I would first put on a new wire for the number 1 plug or see if you can swap it with one of the others to see if the fouling goes with the wire.
 
How many "hits" per reading?
Seen a lot of folks that just keep cranking until the gauge quits moving, which doesn't give you good data.
I always use 5 hits per bore.
Dave
 
Hi, Tom,

Earlier, in the other thread, I mentioned that after rebalancing my carbs and resetting the mixture, the fouling on #1 went away. I think it is working properly now. The only mystery is why #1 was fouled and the other plugs were not fouled... and I don't know the answer to that. My mechanic says that #1 cylinder always run a bit cooler than the others.... and that could lead to fouling.

At any rate, the problem seems to be gone.

I intend to do a leak down test, just to learn how to do it, but I thought I'd start with a compression test.

As for wires and plugs, I did put a brand new wire in #1; even put a new distributer cap in place. Also put new plug (several) before getting the thing to work.

I am going to drive it for 1000 miles, more or less, and re-do the compression test to see if there is any change. In fact, I will do a compression test every 1000 miles to check this out.

Dave: I turned the engine over 9 times for each test.
 
Thanks, Andy. Now if the weather would only cooperate a bit. Some relatively mild early spring days around here have been replaced with cold rain and snow!
 
Yeah...we're getting forecasts up here in Albany (NY) for 6-12" possible late Thursday night and into Friday, depending on the storm track (and/or if the National Weather Service is willing to admit to their joke before April Fools Day)! :madder:
 
Nine is too many.
You have poor sealing, you will get an erroneous reading.
Some mechanics use 2, 3 4, or 5.
I settled on 5 (and count each one!) to do mine probably 35 years ago.
 
Strange... some authorities call for 12.

I think 5 is sufficient, and will use that next time.
 
What I've been told over the years is that the number of "hits" isn't all that important, but that how many "hits" it takes to get to the highest reading is. For some reason, I seem to remember that it's that much better if that highest reading comes within about three "hits"?
 
I did auto repair professionally for 35 years....even ran my own shop.
I don't know as I have ever seen data on what number is best......but you watch it, and if you've got a worn engine, yeah, you can get really good compression readings with 12 hits....and sell a car with "good" readings.
I've watched guys in shops crank engines until the reading came up.
What is MOST important is the exact same number of "hits" in each hole.
 
I'll go with five. It's been good enuff for th' same 35~40 years.

Bump-bump-bump-bump-bump... Read th' gauge. Spinning it more is a mug's game.

...JMHO.
 
I took my TR3 out for the first time this year and am getting black dry carbon fouling in the #3 cylnder again! I'm in the process of rebuilding a spare set of SU's and am going to install those eventually. I'll try a new plug wire in the meantime to see if that has any affect. What gets me is...... if it's a carb issue then why wouldn't the #4 cylinder foul as well?
 
What are you guys talking about, for starters a compression gauge is at most is a no-go gauge, good for telling you if you have a bad problem, not much else, as far as how to do a compression check, you run it until the needle quits moving and there ya go, a cylinder can not pump more compression than it has. You also hear on the forums how guys put a little oil in the cylinder to get a higher reading, well that fine and dandy, but if you got a higher reading all it has told you was your piston ring condition, or lack of, is the only reason for the higher reading, if the reading is still low the leakage is in the valves, a leak down tester gives you the abilty to pin point the leakage.
 
karls59tr,

Yes... that's exactly what was happening to me with my #1 cylinder. I couldn't figure out -- and never did figure out -- why #1 would foul while the other plugs were textbook normal, including #2 (on the same carb). I inspected the valve gap; I tried new plugs; I tried new leads; I event tried a new distributor cap... all to no avail.

What finally seemed to have an effect -- and it may have been just circumstantial -- was to rebalance my carbs and readjust the mixture. After that, the plug cleared up.

I do find that if I idle for some time for some reason, all the plugs get dry carbon fouling, and when I hit the road and take it up to 55 mph for a decent distance and then pull the plugs, they don't show the fouling. A friend has suggested: (a) that #1 cylinder runs a bit cooler than the others because it is close to the aluminum radiator (???), and (b) because my driveway is steep and I drive the car up in 2nd gear, that that last few yards slows things down enough to cause carbon to build up.

It's all a mystery to me and I wouldn't worry about it except that I want to make sure that (a) something doesn't have to be fixed, (b) I am not making some problem worse.

I've come to the conclusion to leave it alone and just drive the car.

Hap: You have echoed what many others have said. A compression check is no substitute for a leak-down test. But it is a good start.
 
I here you. When I put the rebuilt carbs on and adjust them properly it may make the condition go away as it did yours.I suspect sticking jet or dome issue. However in my case the plug is fouling so bad that it sounds like the the engine is running on only three cylinders so I cant really ignore the problem.
 
karls59tr,

If it sounds like the engine is running on only three cylinders, I agree that you cannot ignore the problem. It sounds like something other than a misadjusted carb.

I like to start with the absolute most simple posible fix first and then go on to increasingly complex ones. The simplest fix, and one that I found worked with another problem I was having, was to recheck the spark plug gap. Somehow, one of my spark plugs had gotten slightly mashed in replacing it, and it produced a lot of fouling and the feel of running on three cylinders. This is probably too simple to explain your problem, but if possibly you haven't checked the gap, you might start there.

It's all uphill -- or is it "downhill" -- from there.
 
Hap.
If it take 12 hits to get 125 on cylinder #3, and only 2 on 1,2 and 4, that DOES tell you something.
I do 5, THEN I do a wet, and then I know.
Leakdown? Yeah, but not first thing.
Leakdowns in shops generally are to find where the big hole is.
Apply air, open butterfly, if no "hiss" it ain't intake....listen at the exhaust, if no "hiss", it ain't exhaust, pull the oil fill cal, if no "hiss", it ain't rings.

Seen a whole lot of "mechanics" who can never find an issue because they crank it until the needle stops, no matter how many cranks, and, geez, lookit that, all the readings are the same.
Throws your logical troubleshooting ability right out the window.
 
TOC,

Three questions about your instructions in applying air in leakdown:

(1) Where would I listen for the "hiss" when I open the butterfly? (BTW... I assume that I open the butterfly by pressing down on the link rod assembly.)

(2) You say listen at the exhaust. I assume you mean near the exhaust valve (valve cover off) and not the exhaust pipe.

(3) What is an "oil fill cal." Did you mean "cap"?
 
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