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Compression check 1956 BN2

Martinld123

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I had the engine rebuilt about a year ago but have been having some problems with rough running. The engine has 100M larger size pistons. So I did a compression check. No too long after the engine was broken-in the compression was 1 - 4, 183, 185, 184, 183. A year later with 5,000 miles or so on the engine the compression was 1-4, 165, 170, 180, and 170. Then I put some oil in each cylinder to test it again then it was, 1-4, 172, 176, 186, and 176. What does this tell you?? From what I have read it seems like the numbers are too far apart? Thanks Marty
 
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Oil in the cylinders helps rings seal a bit better, so you have some leakage around the rings, which is normal. I'm not sure what you mean by '100M larger size pistons;' the M pistons were differentiated by having a shallower dish on top (if your cylinders were bored the pistons would be larger in diameter). Anyway, compression readings can vary some based on technique, battery condition, phase of the moon, etc. Yours are not far off--10% difference is acceptable--and not likely the cause of your rough running; I'd start with the ignition.

My BJ8 has accross-the-board lower readings, with a larger variance than yours, but runs like the proverbial 'Swiss watch.'
 
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Martinld123

Martinld123

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Oil in the cylinders helps rings seal a bit better, so you have some leakage around the rings, which is normal. I'm not sure what you mean by '100M larger size pistons;' the M pistons were differentiated by having a shallower dish on top (if your cylinders were bored the pistons would be larger in diameter). Anyway, compression readings can vary some based on technique, battery condition, phase of the moon, etc. Yours are not far off--10% difference is acceptable--and not likely the cause of your rough running; I'd start with the ignition.

My BJ8 has accross-the-board lower readings, with a larger variance than yours, but runs like the proverbial 'Swiss watch.'

Thanks, your info makes me feel better about engine. Okay, I checked wires with ohm meter plus checked out in dark with all good. Distributor is original so maybe I will send it out to have it rebuilt. Thanks for help, Marty
 
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Thanks, your info makes me feel better about engine. Okay, I checked wires with ohm meter plus checked out in dark with all good. Distributor is original so maybe I will send it out to have it rebuilt. Thanks for help, Marty

I'd try new points and condensor first (if you can find decent quality). If you have a dwell meter check that, if dwell 'wobbles around' a bit you might need a rebuild, otherwise you shouldn't. Check timing, spark at the plugs, etc. You said car 'runs rough,' but is it rough idle, stumbling, or ?
 
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Martinld123

Martinld123

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I'd try new points and condensor first (if you can find decent quality). If you have a dwell meter check that, if dwell 'wobbles around' a bit you might need a rebuild, otherwise you shouldn't. Check timing, spark at the plugs, etc. You said car 'runs rough,' but is it rough idle, stumbling, or ?

Bob, I have been using a Pertronix ignitor in original distributor with good luck until now. It does wobble some but I thought that would be less of a problem with Pertronix. I had someone look at it last year and he seemed to think it was okay. Timing has always been a problem for me with no timing marks on engine so I set it until engine pings then back off. Not the best I know but worked well in past. When driving at times it feels like it is running out of petrol. Need to think back. We just got back from Florida so being away for 6 weeks has not helped my memory, lol. I will have to get my thoughts together and report back, Marty
 

CraigC

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Martin, there are timing marks on your engine. Look closely and you will see an arrow stamped into the timing cover directly above crank pulley. On the original crank pulley you should find a 1/16" hole drilled near the edge on the rear half of pulley. It lines up with the arrow when you are at TDC cylinder #1. Use a "dial-back" type timing light to check/set timing.
 

busybrit

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When you say it runs rough is this on Idle? or when you actually drive the car. What happens when you pull the choke out a little once the engine has warmed up? With the air filters off, looking at the pistons on the carbs, do they lift at the same rate? Does one vibrate more than the other?

I'm testing for a vacuum leak !

Johnb
 
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Martinld123

Martinld123

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Thanks for responding. Out of town and just got back. I bought brand new SU carbs from Moss that are sitting on my work bench now. But I did check the lift of of pistons on old carbs and they did match. Vibration in either was not on my watch list at that time but used system that had let me see piston movement and all looked good, solid, and equal. I sent out my distributor to Advanced Distributor, Shakopee, MN, for a rebuild so hope I am covering all of my bases. Going to have a fresh start. Thanks Marty
 
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