Cylinder Compression Checks

[Patrick67BJ8]

My BJ8 has developed a miss. The exhaust for 4,5,6 cylinders doesn’t have a smooth rhythm like the 1,2,3 cylinders.
I decided to do a compression check and my 1,2,3 cylinders are 170 lbs and my 4,5,6 cylinders are 160 lbs. i spun the engine for five compression cycles for my readings. I used a new compression tester for the tests. Is there a special procedure for doing a compression check on the Healey because of a difference between the front and rear carbs?

I use a Pertronix in my dizzy, red rotor from Advanced Distributers. Top loader cap, Bosch blue coil. Advanced Distributors rebuilt my dizzy and a spare I had which is for points.

I’ve been told that my plugs look like my engine mixture is too

 

[red57]

If you blocked the throttles fully open for the test, the carbs should have nothing to do with the results - if you didn't, do that and re-test. Beyond that, a 5% variance is usually considered acceptable.

If it's electrical, I'd suspect plugs, plug wires, cap & rotor - I would not worry about the Pertronix or coil, they usually work or don't.

Could be fuel but your post left off the last word, do the plugs look too - lean or rich?

When faced with a miss, sometimes it's easiest to just start swapping things for known good ones and see what make a difference.

BTW, if you are concerned about the compression test, I always urge a leak-down test to help identify what is the cause

 

[gonzo]

Try performing a vacuum test as well to help determine state of valve timing. Also consider spark plug HT leads and check for possible arcing at idle (move them around in low light condition). I discovered this once accidentally when one of the HT Leads arched against a small pry bar I was using to hold something in place.

 

[Patrick67BJ8]

If you blocked the throttles fully open for the test, the carbs should have nothing to do with the results - if you didn't, do that and re-test. Beyond that, a 5% variance is usually considered acceptable.

If it's electrical, I'd suspect plugs, plug wires, cap & rotor - I would not worry about the Pertronix or coil, they usually work or don't.

Could be fuel but your post left off the last word, do the plugs look too - lean or rich?

When faced with a miss, sometimes it's easiest to just start swapping things for known good ones and see what make a difference.

BTW, if you are concerned about the compression test, I always urge a leak-down test to help identify what is the cause

I didn’t do anything to the carbs. Throttles were closed. I’ll do another test with the carbs wide open.

My plug wires and top-load cap were purchased from Advance Dist. I do not like how loose the plug wires are on both spark plug and dizzy connections. I do have a spare side loader cap and wires that I can swap with.

The engine has probably less than 10k on full overhaul. All stock except bored 60 over. Carbs have UN needles which are supposed to be “rich." I agree on swapping parts for testing.

The plugs were gapped at 25k and actually varied a little from that. I purchased a spark plug gapping tool and I recapped them to 30k. I read that 30k was recommended on some other posts.

I’ll post photos of the plugs as soon as I resize t

 

[Bob_Spidell]

Assuming L-R = 1-6; plugs 2&5 look a little lean; usually it's 1&6 because of less manifold/carb overlap. Your problem is almost certainly ignition; with the engine running, pull one plug lead at a time--with insulated pliers--the missing cyl. will show less or no reduction in RPM (and/or no change in exhaust note). IIRC, 25thou is stock plug gap, you can run a little more with a Pertronix.

With a 'cranking' style compression test, pressure can drop as the battery gets discharged.

 

[nevets]

Not sure if it's relevant to your situation, but my BN6 had a persistent miss that turned out to be incorrect ignition timing. When I adjusted the timing to the correct setting with the aid of a strobe, the miss disappeared and performance improved.

 

[Dandare]

The timing appears to be a bit advanced going by those plugs?

Danny

 

[WHT]

Hi Patrick,

The leak-down test as red57 recommended will provide a lot of good information.

Also, you might do both a "dry" and "wet" compression check:

1. Do the dry check without adding any oil to the cylinders.

2. Then, repeat the check on each cylinder immediately after squirting in a few drops of oil. Gives a little more information on ring seal.

Regards, Bill

 

[Patrick67BJ8]

The timing appears to be a bit advanced going by those plugs?

Danny

It could be, but I used a strobe timing light on it. The kind you set on the dial timing you want and then with the engine running you line up the timing marks.
The timing light is a Sears unit and appears to be in good condition(dropped only once).

 

[Patrick67BJ8]

Hi Patrick,

The leak-down test as red57 recommended will provide a lot of good information.

Also, you might do both a "dry" and "wet" compression check:

1. Do the dry check without adding any oil to the cylinders.

2. Then, repeat the check on each cylinder immediately after squirting in a few drops of oil. Gives a little more information on ring seal.

Regards, Bill

Hi Bill,
Power just came back on. Where on a tornado watch until six pm.
I never did a leak down test, is there test equipment Ava to do that with?
Moneys burning a hole in my pocket today!
Cheers,
Patrick

 

[Patrick67BJ8]

Inside of my dizzy Top Loader cap.

 

[WHT]

Hi Patrick,

Amazon has several pages of leak-down testers. You can go from $30 to over $100 (OTC and Lang) depending on the quality you want and how much money is burning a hole in your pocket.

Not much need to go higher than that in a home shop where it is only used every now and then. Very simple equipment and they all work fairly well. The main difference is the quality of the two pressure gauges and hoses.

I would loan you mine, but you would spend more than that on gas!

Regards, Bill

 

[BobHaskell]

May-be stating the obvious, but you'll need an air compressor to do the leak-down check.

 

[Patrick67BJ8]

May-be stating the obvious, but you'll need an air compressor to do the leak-down check.

I have one.
Thanks

 

[Patrick67BJ8]

Hi Patrick,

Amazon has several pages of leak-down testers. You can go from $30 to over $100 (OTC and Lang) depending on the quality you want and how much money is burning a hole in your pocket.

Not much need to go higher than that in a home shop where it is only used every now and then. Very simple equipment and they all work fairly well. The main difference is the quality of the two pressure gauges and hoses.

I would loan you mine, but you would spend more than that on gas!

Regards, Bill

Thanks Bill, I just ordered one.

 

[roscoe]

Try and check to see what spark plug hole adapters come with your leak down tester. If they just give you one it won't likley fit your healey engine...see below. During leak down/ differential checks, If you use a wrench to turn you engine via the crank nut, do remove it before pressurizing a cylinder. Aircraft mechanics have been killed by a propeller swinging because the cylinder wasnt at top dead center when pressurized.
Also, you can do a poor man's leakdown check by pressurizing the cylinder to 60 or70 psi by making an adapter out of an old spark plug. Normally the check is done at 80psi but the exact pressure isnt critical. They are hard to gut but it can be done and just weld or braze a male air fitting to the outside end, thread it in and attach an air hose with a regulator. You don't need gages to hear excessive leakage through the rings or valves. A good cylinder should have only a very slight or better yet no hissing indicating leakage. The gages will tell you how far off from perfect your cylinder is, not where the leakage is. It's kind of like using a rubber tube to listen to your carburettor hiss to match airflow. It isn't quantitative but you'll know if you have a bad cylinder and where the leak is by listening at the oil filler cap or at the carb air inlets and exhaust pipe. Be sure your piston has not gone past TDC as you want the rings to be pushed towards the skirt as they are on a compression stroke.

 

[AN6-TX]

What type of gas are you using (corn %)? Do you run any additives?

 

[Patrick67BJ8]

Try and check to see what spark plug hole adapters come with your leak down tester. If they just give you one it won't likley fit your healey engine...see below. During leak down/ differential checks, If you use a wrench to turn you engine via the crank nut, do remove it before pressurizing a cylinder. Aircraft mechanics have been killed by a propeller swinging because the cylinder wasnt at top dead center when pressurized.
Also, you can do a poor man's leakdown check by pressurizing the cylinder to 60 or70 psi by making an adapter out of an old spark plug. Normally the check is done at 80psi but the exact pressure isnt critical. They are hard to gut but it can be done and just weld or braze a male air fitting to the outside end, thread it in and attach an air hose with a regulator. You don't need gages to hear excessive leakage through the rings or valves. A good cylinder should have only a very slight or better yet no hissing indicating leakage. The gages will tell you how far off from perfect your cylinder is, not where the leakage is. It's kind of like using a rubber tube to listen to your carburettor hiss to match airflow. It isn't quantitative but you'll know if you have a bad cylinder and where the leak is by listening at the oil filler cap or at the carb air inlets and exhaust pipe. Be sure your piston has not gone past TDC as you want the rings to be pushed towards the skirt as they are on a compression stroke.

I ordered this Tester.delivers tomorrow so I should have testing done over the weekend. It comes with 14mm adapter and will be okay because I have this sparkplug gapping tool andI used it to gap my plugs at 30k. First time I used it!

 

[Patrick67BJ8]

What type of gas are you using (corn %)? Do you run any additives?

Premium gas and no additives.

 

[AN6-TX]

@Patrick67BJ8

I asked because I had a similar issue - although it felt more like a hesitation than a miss if that makes sense. Wound up replace the coil, HT leads, plugs, dizzy cap (Pertronix was "new"). These were swapped out over the course of 6 weeks or so - plugs were last to be changed. Regardless, the issue got better and appeared to have been fully resolved when using a gas treatment. We can only get 91 Octane here in CO (and corn-free is hard to find) but the combination of things seemed to work. Good luck sorting it out!