Low compression after rebuild, won't start

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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simonh
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Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Re: Low compression after rebuild, won't start

Post by simonh »

I just checked again to make sure that I didn't dream that it started. I have set the pilot air and throttle stop and it now starts first kick start and idles well. I also have a Model 31 and I have been a club member for a few years, so this bike will get used as soon as I can get it on the road. Thanks all for interest and support.
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clive
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Re: Low compression after rebuild, won't start

Post by clive »

If you are a club member Simonh you are not showing up as one, so are not correctly logged in. This means you will not have access to all of the website. I suggest you email Eamonn on webmaster@jampot.com to get it sorted out.
clive
if it ain't broke don't fix
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Well the saga continues. The bike again won't start. I have done nothing and touched nothing other than fit the chain guard. It is exactly the same symptoms as before Paul visited. Nothing seems to be loose or moved, nothing connected to the engine, carb or electrics has been touched. I was planning to re-try the Boyer ignition but thought better to make sure that it still started first. I thought it could be stale petrol, so I just got a fresh 5L of shell super, no change from the standard stuff I had in my shed for the mower.
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clanger9
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Re: Low compression after rebuild, won't start

Post by clanger9 »

Has it lost compression again? It did appear to do that briefly when I was there, then it righted itself.
Maybe something up with the exhaust valve lifter...
1989 Moto Morini Dart 350
1993 Ducati 750SS
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Well, I got it going. It was the ignition timing point again. Somehow, the advance unit appears to have moved slightly and the spark was coming at the wrong time. No idea how as I had not been near it. I am wondering if this was the original problem; it does however seem tight to the taper and I can't move it by hand. I am going to have a go at refitting the Boyer ignition at the weekend and see if that works. The compression seems to be good but the clutch slips occasionally; probably all the kicking. I originally did suspect the valve lifter when I originally had no compression, but it seems OK. I will update the Saga again with results. Thanks
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clive
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Re: Low compression after rebuild, won't start

Post by clive »

Simon I have not read all 7 pages of this thread but you are still logged in as a guest rather than under your membership. This means you need to sort the problem out in under 90 days or your account will be frozen. An email to webmaster@jampot.com will get your current logon linked to your membership.
So thinking without knowledge of lightweights what could cause intermittent loss of compression? What was done since it last ran? Your probably used the valve lifter to stop it so is it jamming? On a heavyweight this is relatively easy to check. Perhaps the cable end Ferrell becomes misplaced and then rights itself. You could even remove it to ensure it is not the cause and stop it by a hand over the carb or stalling it.
Other possibilities seem to me to be a slightly bent push rod jamming against the side or one where the top or bottom drift apart jam for a bit and then reseat.
A slightly bent valve which jams open sometimes. While the compression is down I would immediately check the free turning of the pushrods to exclude these possibilities.
If it's none of those then perhaps one of the rings is nipping up in the piston ring slot, I have had this after an overheating seizure (on more than one occasion, but in my defence the bike was pulling a double adult sidecar with 3 children and full camping gear!)
Good luck and apologies if these ideas have already been suggested.
clive
if it ain't broke don't fix
SPRIDDLER
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Re: Low compression after rebuild, won't start

Post by SPRIDDLER »

simonh wrote: Tue Aug 10, 2021 9:21 am Well, I got it going. It was the ignition timing point again. Somehow, the advance unit appears to have moved slightly and the spark was coming at the wrong time. ......... it does however seem tight to the taper and I can't move it by hand.
So just to be clear, what exactly did you do to correct the timing and get it running as something must be slipping for the timing to change?
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Which taken at the flood............'
simonh
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Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Re: Low compression after rebuild, won't start

Post by simonh »

Hello all. One explanation is that the advance unit was not fully tightened on its taper. That was what I checked when it would not start: In the position found, the spark would have been a bit early. I can't explain why it moved as I still needed to use a puller to get it off. My wife is cooking tonight, so I managed a couple of hours on it. I have now reinstalled the Boyer ignition and it is working. With the Boyer ignition it seems smoother and idles better; the ammeter needle is also much more stable. Thanks all.
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clanger9
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Re: Low compression after rebuild, won't start

Post by clanger9 »

I can't imagine what could cause the ignition timing to slip (??) The centre bolt was done up pretty tight!
Hey ho, at least you've got it running again with the Boyer ignition. It'll run much nicer with that fitted.

Funny things, these Lightweights :roll:
1989 Moto Morini Dart 350
1993 Ducati 750SS
Groily
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Re: Low compression after rebuild, won't start

Post by Groily »

clanger9 wrote: Tue Aug 10, 2021 10:06 pm It'll run much nicer with that fitted.
As long as the battery is kept fully-charged, anyway! A bit too sensitive to voltage (and also power-hungry) for my taste, the BB system, despite its many undeniable advantages.
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