Low compression after rebuild, won't start

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Andy G
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Re: Low compression after rebuild, won't start

Post by Andy G »

simonh wrote: Sun Jul 18, 2021 4:49 pm I will probably disconnect the Boyer and try the old points setup next weekend.
Pain to do, but at least it should isolate one possible cause.
Stick with it!
Bombling contentedly around rural Aberdeenshire
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Dear all, I can't answer the question RE the crank being from a 250. The piston comes to within about 4mm from the top of the barrel from memory. I have fitted a different barrel and re ground the valve seats which is what will have improved the comparison. I am starting to mistrust the Boyer ignition, so when I get back home on Wednesday, I will refit the points, however I have never seen them working. Thanks, Simon
SPRIDDLER
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Re: Low compression after rebuild, won't start

Post by SPRIDDLER »

simonh wrote: Mon Jul 19, 2021 6:44 pm I am starting to mistrust the Boyer ignition, so when I get back home on Wednesday, I will refit the points, .......
That's what I've been thinking. With Easy Start and even a feeble spark at more or less the right time you should get some sort of firing. At least with points you can see more of when and what is or isn't happening.

Otherwise............ ;)
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clanger9
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Re: Low compression after rebuild, won't start

Post by clanger9 »

simonh wrote: Mon Jul 19, 2021 6:44 pm Dear all, I can't answer the question RE the crank being from a 250. The piston comes to within about 4mm from the top of the barrel from memory.
Standard 350 pistons have a flat top and I believe it must reach the top of the barrel to get enough compression.

A 250 crank has a stroke of 69.85mm
A 350 crank has a stroke of 85.5mm

Should be easy enough to confirm which one you have.

350 pistons are hard to find, so if the stroke is correct it may have been fitted with the wrong piston.
1989 Moto Morini Dart 350
1993 Ducati 750SS
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Dear All, The stroke is correct because when I stripped the engine I was expecting odd shaped flywheels as per the description of 350 lightweight engines. My flywheels are round, so I measured the stroke. Also another club member told me that the tech descriptions were wrong and all the flywheels were actually round. The piston is 72.6mm and has a flat top with cut outs for the 2 valves at the correct angles, so I believe the piston is correct or at least it appears to have been made for this type of bike. I haven't been in the garage again yet because it is too hot + work for my employer. I will have another attempt this weekend using the original points and condenser. I did not measurer exactly how close the piston gets, but I think is less than 4mm
Andy G
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Re: Low compression after rebuild, won't start

Post by Andy G »

All the piston discussion aside, Simon has good compression, a spark and assumed fuel, so if the timing can be set then should get some sort of reaction?
Good luck with the original points Simon, looking forward to good news!
Bombling contentedly around rural Aberdeenshire
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clanger9
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Re: Low compression after rebuild, won't start

Post by clanger9 »

Agreed, 125 psi should be plenty. Why was it only 50 psi before?
1989 Moto Morini Dart 350
1993 Ducati 750SS
bitza
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Re: Low compression after rebuild, won't start

Post by bitza »

If the carb is set up correctly and the plug is getting wet when kicking the engine over , compression is also good,and valve timing , ignition timing set correctly , my thoughts would be a fault in the ignition system , as someone posted if you trickle a drop of fuel into the plug hole a kick her over the engine should try to fire up. Just my thoughts on this.
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Dear All, I did not manage as much time working on the model 8 as last weekend, however I did assemble a good WIPAC ignition and after stripping out all the Boyer, I installed it. The WIPAC ignition was set up as the book and like before, I got good sparks, no problem. The engine was exactly the same, no start, not even a cough, the carb is brand new, set up to the book by AMAL, last weekend, I stripped it and blew out all orifices and holes, just to make sure. Checked compression again and over a hundred after 3 kicks. I have tried squirting easy start in the carb and putting a drop of petrol down the plug hole, nothing. I am lost for ideas now and can only think that it is something so obvious or really specific to this model. I have an iron head model 16 which I recently acquired, it has been off the rode for approx 11 years and despite 'household' wiring and oil like tar, I got that cleaned out timed and replaced the cables. It took about 3 hours, fired first kick (no really). If any lightweight whisperer wants a look at it, I'd be extremely grateful. I think I have exhausted everything I could possibly try.
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clanger9
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Re: Low compression after rebuild, won't start

Post by clanger9 »

Where are you in Cheshire, Simon? I many not be far away...
1989 Moto Morini Dart 350
1993 Ducati 750SS
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