Author Topic: B31 Poor Performance  (Read 1742 times)

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DAVE BRADY

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Re: B31 Poor Performance
« Reply #30 on: 24 June, 2021, 20:28:12 »
Hi,

A bit of a long shot but what does the bore look like on the plug side of the barrel?  Could it be that it has become glazed and is not allowing the rings to do their job.  Unusual for this to be localised but may be worth a look.  If you have an unused ring check it in the bore.  Slide it down evenly and shine a bright torch from underneath and look for light leakage.  It is possible for a bore to not be totally round or for there to be other irregularities.  Either way oil could be getting past the rings. 
Did you check the compression or did it feel OK?

Dave.
« Last Edit: 24 June, 2021, 20:29:55 by DAVE BRADY »

martinh10

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Re: B31 Poor Performance
« Reply #31 on: 25 June, 2021, 10:03:26 »
Gadorey/Bess: Thanks for your comments. All the things you mention have been checked out/complied with at some stage.
Dave: You say itís a long shot but youíve got me thinking here. The rings on the NOS Hepolite +.030 piston were each slightly pitted in one location and I decided not to use them. At the time I couldnít find +.030 rings anywhere so on the advice of a couple of trusted suppliers I purchased +.040 rings and gapped them to suit my bore. I gave them the feeler gauge and flashlight check and, if memory serves, I was fairly satisfied that there was good contact with the bore all the way round. Itís just possible I was being a bit optimistic in thinking they would quickly bed into the bore and was reassured by the fact that compression was very good. However, it now seems to be a very possible cause of the oil in the combustion chamber, if not the lack of performance.
My trusty Suffolk supplier now has +.030 rings in stock and Iíve ordered a set Ė it may make no difference but itís worth a try. Fingers crossed.

Calum

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Re: B31 Poor Performance
« Reply #32 on: 25 June, 2021, 10:28:40 »
That sounds very possible. Whilst they may have touched enough to not let a feeler or light pass, enough pressure on the wall to seal is different. I make piston rings from scratch quite a lot at work (from 1" up to around 10" bore) and I tend to 'cheat' and make them bigger on the OD which means they don't quite touch all the way around until well bedded in.  But it does mean as soon as they are ground to thickness they are ready to fit - no hot work necessary. On the applications I use them in it isn't really a problem and they do settle down.
I believe the proper way is to make the ring to the right bore size and then heat them and 'set' to give the correct spring. So your 40 thou over rings won't quite be sealing properly. It should be quite obvious on the ring I should think.

Bess

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Re: B31 Poor Performance
« Reply #33 on: 02 July, 2021, 11:33:00 »
Hi,
     Did you manage to solve the issue?

Best wishes...

martinh10

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Re: B31 Poor Performance
« Reply #34 on: 02 July, 2021, 13:20:19 »
Hi Bess,
On removing the head and barrel it was evident that about a 1.5Ē section of the +.040 oil-control ring was not making contact with the cylinder wall. So Iím pretty sure this is the cause of the oil in combustion chamber problem. The compression rings were making good, even contact all round.
Iíve now received the +.030 rings but after carefully checking the oil ring in the bore against a light source found that although it is much better than the +.040 one it is not perfect. I have been in contact with both the supplier and the manufacturer and both have been very helpful. I have returned the +.040 ring to the manufacturer for examination and he is dispatching a replacement to me. When received and tested Iíll post an update. I still have my doubts about this being the cause of the performance issue Ė weíll find out in due course.

martinh10

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Re: B31 Poor Performance
« Reply #35 on: 14 July, 2021, 13:49:10 »
Not making any progress I'm afraid. No change in performance with the new rings. I've tightened the clutch springs a bit just in case there was some slip (not always easy to detect) - no change. If it was ratcheting on the crank cush drive I think I would hear it so have no reason to suspect that it is. The mag is sparking over a 5mm gap, I've rechecked the ignition timing and valve timing. Good fuel feed.
Amal have agreed to test the new 276 carb for me. It's a long shot but I'm running out of ideas.

Bernessie

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Re: B31 Poor Performance
« Reply #36 on: 23 May, 2022, 19:42:23 »
Chatting to the chaps at Amal they said check the fuel level, I am running a 276 Pre Monobloc standard jets etc and my B31 runs out of puff at about 40-45mph. It was completely rebuilt by the guys at SRM with reconditioned mag, so I am following this thread with interest.