Low compression after rebuild, won't start

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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ajscomboman
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Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Low compression after rebuild, won't start

Post by ajscomboman »

Pictures speak a thousand words, post a picture of the piston in the barrel and anything else cylinder/head wise .
Red Gauntlet
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Re: Low compression after rebuild, won't start

Post by Red Gauntlet »

Just check to see if both valves are open marginally on the overlap at TDC on the exhaust stroke, it sounds as though maybe the valve timing is out.
Mick D
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Location: Aberdeenshire

Re: Low compression after rebuild, won't start

Post by Mick D »

Hi

Just to help clarify the situation:

Has the engine ever run in this configuration? Were you there to witness it? Is the engine 'standard', ie not a mixture of components from other models?

Regards Mick
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Thanks again for all comments. I bought the bike as a basket case, matching numbers etc, but the engine was said to have been rebuilt. To be honest, it looked like the truth, because the head was spotless and the valves new. The piston and rings were also new. The barrel built into the engine had a broken fin damaged in storage, so I used the spare barrel which came with the bike. This was on original bore but worn. This morning I removed the barrel and checked the ring gaps. I had ruled out the professional re-bore work but the ring gaps were actually too big. I gave the company the barrel, piston, rings and measurements, but it was not done correctly. Ring gap 0.033 should be 0.008. So, I got the original barrel and tried the ring gaps. 0.008, so spot on. I next made a new fin and attached it to the barrel. Welded with high silicon stainless wire, appears to be strong and cosmetic
simonh
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Re: Low compression after rebuild, won't start

Post by simonh »

Continued.. from above. I put all back together with repaired barrel. There is much more compression, but it still won't start. I tried a new NGK plug; big blue spark. I am certain that the timing is correct as I set it with a Vernier depth gauge 6.35mm BTDC before I put the head on. Both valves are closed. My ignition is Boyer, so I think it sparks every revolution, there being 2 pick up coils. I retimed the cam yesterday aligned the dots and 11 o'clock. Carb is brand new set up to factory settings as I didn't get one with the bike.
Mick D
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Re: Low compression after rebuild, won't start

Post by Mick D »

Hi

That sounds like you are making good progress - hope your compression returns now.

Regards Mick
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dave16mct
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Re: Low compression after rebuild, won't start

Post by dave16mct »

Make sure you haven't swapped the wires from the stator plate to the transistor box. (It would still spark but the timing would be way out).
Dave.
56G80S
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Re: Low compression after rebuild, won't start

Post by 56G80S »

I think I read all the posts, possibly missed a couple? dave16mct's comment sounds good but..........

Are the plugs getting wet with petrol?

Try putting a teaspoon of petrol down the plug hole and if see you get any action.

Johnny B
simonh
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Location: Cheshire

Re: Low compression after rebuild, won't start

Post by simonh »

I will have a go tomorrow, had enough last 2 days, at least 16 hours spent; and I went to work as well. Does anyone know the best initial pilot air screw setting for a new monobloc? It is not mentioned in the Amal pamphlet that came with the carb, nor in any AMC manual that I can see. Thanks again for all assistance.
Mick D
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Re: Low compression after rebuild, won't start

Post by Mick D »

Hi

The starting point is fully in and then one and a half turns out.

Regards Mick
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