TR6 Single Spark Plug is Burned

[SherpaPilot]

I have a 72 TR6 with the original Stromberg carburetors. Recently I experienced a popping noise during acceleration from the forward carburetor. The first thing I did was to pull the plugs. All the spark plugs looked like normal clean burning with the exception of #1 which was burned and had lots of carbon deposits. I installed fresh plugs, new distributor cap and new plug wires just to be certain the ignition system was working properly. The distributor was retrofitted with the Petronix electronic module a few years ago. I set the timing IAW the manual (4 degrees ATDC) at 850 RPM. The test drive showed no improvement thus the ignition system was ruled out. I cleaned both (K and N) carburetor air filters. The piston lift test showed me both carburetors were too lean as the engine wanted to quit as the piston was slightly lifted on each carb. Both diaphrams were checked and found to be without air leaks. I attempted to adjust the carbs to acquire a normal piston lift test without success. Both carbs would not get any richer no matter what I did. This makes no sense to me. Why would only #1 plug be abnormal and the other 5 normal. Why would the carburetors be unable to provide a suitable test as outlined in several manuals? I checked for manifold leaks and found nothing out of place. Could I have an engine valve problem with both Strombergs failing all at the same time? Am I the only guy this has ever happened to? Is there life on other planets? Ahhhh, questions and more questions keep popping up.

 

[trrdster2000]

On the carb test, did you make sure the needles were engaged with the adjuster. Sometimes the adjuster is turned so far it will disengage from the needle. You will have to go clockwise, I think, to get the screw to go to a stop and the piston should lift a little when you pull the tool out, then you know it's turning the needle in and out of the piston.

Wayne

 

[TR3driver]

Did you run a compression check? Problems with valves or rings are a common source of varying plug readings, especially if #1 is burning oil.

 

[SherpaPilot]

Did you run a compression check? Problems with valves or rings are a common source of varying plug readings, especially if #1 is burning oil.

I just accomplished several compression checks on my TR6 engine. Here are the results:

Cold vs Warm
1-100- 130
2-095- 120
3-110- 130
4-110- 130
5-110- 130
6-110- 130

Can anyone tell me their opinion on these numbers? I am thinking these cylinders and valves are normal but lack the experience to be positive.

 

[TR3driver]

#2 is a bit low, but not enough to worry about now, IMO. I'd be looking for a vacuum leak next (since it seems you've got ignition covered).

 

[SherpaPilot]

Thanks for the response. I had read on the net that normal compression for a Triumph engine should be 160-190. I really hate to pull this engine out at this point since I was planning on installing a complete new interior during the winter. I'll check for a vacuum leak within the next couple of days. I also performed an oil compression check by following the procedures outlined in another internet tech article. Shot a couple of squirts of engine oil into each cylinder and re-did the compression checks. Here is what I found:

Warm/Dry

 

[trrdster2000]

Give that another try, we didn't get any figures.

Wayne

 

[poolboy]

A freshly rebuilt engine might be in the 160 to 170 or higher and yours would probably be a little peppier with an overhaul, but I wouldn't do it just based on your compression in post #4.
You didn't mention oil consumption. Are you driving the car enough to know ?
Sometimes just using the car more and doing some spirited driving with a good brand of gasoline, (not necessarily a high octane) will spring those piston rings free and raise the compression

BTW, if you haven't had your carbs rebuilt within the last 10 years..it's time.
There are some rubbery and flexible components that don't age that well regardless of usage.

 

[SherpaPilot]

Thanks for the response. I had read on the net that normal compression for a Triumph engine should be 160-190. I really hate to pull this engine out at this point since I was planning on installing a complete new interior during the winter. I'll check for a vacuum leak within the next couple of days. I also performed an oil compression check by following the procedures outlined in another internet tech article. Shot a couple of squirts of engine oil into each cylinder and re-did the compression checks. Here is what I found:

Warm/Dry

Guess I accidently hit the post button before I was finished. Sorry.... Today I warmed up the engine and shut it down, removed all plugs and shot two squirts of oil into each cylinder in sequence. After connecting the compression gauge, I opened the throttle all the way and cranked the engine at 10 rotations each time tested. These are the results

1 155
2 145
3 170
4 150
5 145
6 150

As you can see, each reading was considerably higher than the dry test performed yesterday. I read in a tech article that if the new (wet) readings increase over the (dry) readings, I could have worn rings, leaky valve or bad head gasket. If this is normal, I can turn my attention to the carburetors but this is not consistent with the info I read. (or did I misunderstand)? Thanks again for your input.

 

[SherpaPilot]

Thanks for the response to my question. I have owned this TR6 for 26 years. As far as I know, this is the original engine which has never been overhauled. Although I shy away from sloppy weather or extreme heavy traffic, I drive this car frequently and have autocrossed it on occasion. I agree with you about the carburetor rebuild. I have rebuilt these twice (the last time was about 2 years ago). They are next on my hit list. I am hoping these compression results are not a bad report card for this engine and I can continue with my trouble-shooting and move on to the carb rebuild.

 

[poolboy]

I think it would have been better to have done a complete dry compression test with a warm engine and open throttle immediately followed by going back and squirting oil into the cylinders, one at a time, with a compression test in that wet cylinder, moving down the line.. squirting oil, checking compression, and so on one by one.
That would make for a more valid comparison between wet and dry results.

 

[pdplot]

Are you burning oil? Are there any signs of head gasket leakage like water in the oil & vice versa? My compression used to be from 160 to 175, steadily downward and now 145 on #1 down to 85 on #3 - all dry. Most are now in the 130 range. Have you had a valve job? I did and that's when I started burning some oil.

 

[svtmikey]

What you really need to do is a leak test.
That allows you to figure out if there is leakage "anywhere" from the cylinder including(but not limited to) intake/ exhaust valves, head gaskets, and rings. A compression test really only tests rings.
With a leak tester you pressurize the cylinder with compressed air, then listen for the hissing of air.
For example if you hear the air sound out of your exhaust, then it will be your exhaust valves....out of the radiator.....head gasket, out of the carbs....maybe intake valves....out of the oil filler cap.....you get the idea.
One other thing I was thinking is it could be "pre-ignition"....where bits of carbon in the cylinder are staying hot enough to ignite the fuel burn before the spark.....this is different than "detonation" which is caused by compression.
That's a long shot though