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PostPosted: July 20, 2023, 12:08 pm 
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A friend has a new-to-him MGA that runs rough across all RPMs. It seems that it is not fining on #2. This is what we know:

1) Verified by pulling plugs wires on each consecutive cylinder. 1,3 &4 all decrease RPMs. No change when pulling #2.
2) Good compression, all around 120#
3) Swapped +1 and #2 plugs, still #2 not firing
4) Swapped plug wires #1 and #2, #2 still not firing
5) Found a possible arc trace inside dist cap around #2 wire. Put in new cap, spark seemed a bit stronger but #2 still not firing
6) MGB engine has a single intake port for #1 and #2. The front SU carb feeds them both. It it was bad, it should affect both, not just cyl #2.

We are assuming the problem to be ignition. The compression tells us that the cylinders all are good in the valve department. If not perfect, at least they should be working the same. What would be your next test/guess?

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PostPosted: July 20, 2023, 5:19 pm 
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What did the number 2 plug look like? wet? black? white? That'll tell you much of what's happening in the cylinder.


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PostPosted: July 20, 2023, 7:14 pm 
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If points, is the rubbing cam worn down/damaged?
If electronic, is the trigger wheel un-damaged? I've seen that happen.

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PostPosted: July 20, 2023, 8:19 pm 
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Has it always run that way? If not, what was done right before that?

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PostPosted: July 20, 2023, 9:22 pm 
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Have you pulled #2 plug, hook HT wire back up, ground plug, crank to start, and prove the plug is not firing? Remove 1,3,&4 HT wires from plugs to prevent start up.

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PostPosted: July 21, 2023, 12:57 am 
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This is a stretch, but:
If the coil is very weak, and cylinder #2 happens to have the highest compression of the four, maybe that plug can't fire due to the compressed air in the cylinder acting as an insulator. If this is true, placing the connected plug on ground and cranking the engine may show a spark, but it may not be sparking under cylinder compression.

As a test, replace the coil, and also see if it's getting 12V when running. Some cars need a ballast resistor and some don't.

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PostPosted: July 21, 2023, 8:24 am 
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All good suggestions. I'll admit that I came into the problem after a lot of the analysis was done by others. Most of what I said was hearsay. Let me assume that all reports were correct.

Sean, I am assuming that when they swapped the #1 and #2 plugs, there was nothing visibly outstanding they probably would have just put all new plugs in from the get-go.

RTz, They took a screwdriver in the #2 plug wire and drew a weak spark to the block before changing the dist cap. I saw that. After changing the cap, it was a strong spark. Some ignition improvement was seen but still not firing the #2 plug (or somehow no fuel is getting into the cylinder.)

Kurt, I can't say. I wrote that this car is relatively new to him. When he bought it, it had not run at all from years of storage. Many other issues had to be resolved before it was roadworthy, which it is now. I do not recall ever hearing it run smoothly.

Perry, see answer to RTz's above. #2 Plug must be good since when it was moved to cyl#3, it fired there just fine.

Kurt, I was told that all cyls had 120# compression. I did not personally do the test. I do not understand how a minor variaton in compression would prevent firing. This is not a misfire. It is not firing at all.



It is a puzzle for sure. That is why I presented the problem to you guys. Your collective knowledge has yet to fail me. Whatever the problem is, I am leaning toward the problem being mechanical in nature, preventing ignition to occur.

I was thinking a bad lobe on the cam or poor valve adjustment might prevent fuel from entering. But the compression would have shown that, no?

My next thought is a bad #2 lobe on the distributor cam could cause the issue. But a strong spark would seem to dispel that, no?

I'll keep on keeping on with this, one night a week, good weather allowing. Our MG weekly garage work is on Wed night and the weather has to be good for the owner to drive it 15 mins to the garage. The one that was performing the testing should know how to do them though. He and I have differed in opinions and technical approach over the past few decades, but he certainly knows these cars. Perhaps I should personally repeat all of the previous testing to be certain of the results.

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PostPosted: July 21, 2023, 9:46 am 
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rx7locost wrote:
I was thinking a bad lobe on the cam or poor valve adjustment might prevent fuel from entering. But the compression would have shown that, no?
A worn cam lobe can still show good compression, (even if the valve opens just a hair, it would allow enough air into the cylinder to be compressed at cranking speed). Process of elimination,the valve cover needs to be pulled and lobes, rockers, push rods inspected.

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PostPosted: July 21, 2023, 9:52 am 
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Kurt wrote:
...I do not understand how a minor variaton in compression would prevent firing. This is not a misfire. It is not firing at all...

If you guys didn't test compression yourselves, start there in order to establish truth, as that's most likely the actual problem. That said, assuming the compression values really are more or less equal leads to my thinking, below:

Air is a natural electrical insulator. The more of it there is, the better an insulator it becomes. A spark plug that fires fine sitting on a table will require more voltage to fire in a cylinder under compression. I am speculating that "maybe" cylinder #2 has slightly higher compression than the other cylinders, and that combined with a weak coil causes the spark to be unable to bridge the gap under the high pressure of compression.

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PostPosted: July 21, 2023, 10:40 am 
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horchoha wrote:
rx7locost wrote:
I was thinking a bad lobe on the cam or poor valve adjustment might prevent fuel from entering. But the compression would have shown that, no?
A worn cam lobe can still show good compression, (even if the valve opens just a hair, it would allow enough air into the cylinder to be compressed at cranking speed). Process of elimination,the valve cover needs to be pulled and lobes, rockers, push rods inspected.


I also tend to think the cam may be suspect. Lack of compressed fuel/air mixture will give no spark. If the motor sat for a long time, stuck valves or worn cam lobe can cause all kinds of problems. A leaking head gasket that is not allowing water in but allows loss of compression can also be a suspect.

My 1st car (55 Chevy V8) at 16 had a constant miss. Thanks to an old, retired mechanic neighbor testing the compression on # 2 & 4 cylinders showed low compression, but not extremely low. We popped the head off and discovered a blown head gasket between 2 & 4 cly. It had caused a groove to wear into not only the head but in the block. Ultimately, had to replace the motor.

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PostPosted: July 21, 2023, 5:13 pm 
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I expect the car has carbon core wires which are awful when new and get worse with age. They can be intermittent as well. Replace with spiral wound inductive wires. Accel "super stock" kits that must be assembled are outstanding value. They come with male and female terminals and a crimp die for the vice. The end that is crimped is not the spark plug end. I use them whenever possible.

Also, get an inline clear spark checker. It's quick and easy to check at either end of the plug wire with no ground clips needed and the plug itself can be bad and it still works to test the system.

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PostPosted: July 21, 2023, 7:30 pm 
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Miatav8,MstrASE,A&P,F wrote:
Also, get an inline clear spark checker.

Attachment:
finger.jpeg

This is my inline spark checker, been using it since 1977, a bit worn out but still useable, only problem I have with it's is that it hard to focus on it because my eyes are twitching and my nerves are jumpy (and sometimes I bite my tongue and my eyes roll back in my head when using it).


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"If you can't build it safe, don't build it."

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Perry's S10 Super 7 The 3rd
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PostPosted: July 21, 2023, 7:34 pm 
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You can also check whether the spark is happening on each individual cylinder with a timing light.
This is what I use.
Attachment:
timing light.jpeg


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Perry

'If man built it, man can fix it'
"No one ever told me I couldn't do it."
"If you can't build it safe, don't build it."

Perry's Locost Super Che7enette Build
Perry's TBird Based 5.0L Super 7 L.S.O
Perry's S10 Super 7 The 3rd
Perry's 4th Build The Topolino 500 (Little Mouse) Altered
Perry's 5th Build the Super Slant 6 Super 7
Perry's Final Build the 1929 Mercedes Gazelle


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PostPosted: July 21, 2023, 7:49 pm 
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The last time this issue came up on another forum a leaking intake gasket was the culprit.
Leak = Lean, try a little propane around the intake and see if the revs change.
The plug may look good from before the leak.

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PostPosted: July 21, 2023, 9:29 pm 
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MV8, I hate to be a naysayer, I fail to see how swapping the good cylinder #1 and bad cyl #2' wires, and the problem stays with #2 cylinder can be indicative of a bad plug wire.

Perry, I have been bit so many times I personally hate to pull the wire off a running engine, or even hold a wire while an engine is running. BTDT BTTS (been there, done that- bought the tee shirt.) It sounds like you have several tee shirts. I would bring my clamp on timing light next time

MV8, I have one of those clear spark testers. After well over 50 years of wrenching I finally bought one of those spark indicators last year to work on my son's Tracker outboard engine. If I were to be checking this car at my garage, I would have used it. Unfortunately. I was somewhere else's garage. Somewhere I have seen those free-air testers where you can increase the gap to check just how strong the spark is. Are they any good? The physics says yes. I have always used 50,000 volts per inch as a ball park number for arcing thru air. It has served me well in circuit design. Of course there are lots of variables that can affect that value.

Richard, you might have missed that this engine has 2 intake ports, 1 each for 2 cylinders, not to mention 1 SU carb feeding each of those ports. A leaky gasket would have presented the problem on both cylinders that share that port.

If I get to work on this car again, I will personally start from scratch and be sure of the results of any previous tests.

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“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766


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