Loss of compression

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Re: Loss of compression

Postby Pharisee » Fri Oct 06, 2017 2:21 pm

1608 wrote:And John, in the photo of your G80 rocker arm bolt do I see a 376 carb fitted. Should it not be a 389. But if the bike performs well then thats fine. Correct me if I'm wrong but arn't the inlet tracts different sizes on those carbs.
Probably talking out of my hat, but well meant observations.


You may well be right. That's what was fitted when I bought the bike 18 months ago. It runs well enough but once I've finished the G3 rebuild I'm currently working on, the M18 will be next in line for a makeover. I have a later model cylinder head that I want to fit as the exhaust port on the one fitted currently is very badly worn and leaks air into the system. I'll look into the carburettor issue then.

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Re: Loss of compression

Postby Bruno » Sun Oct 08, 2017 8:54 pm

Thanks for your suggestions JimFitz - I finally removed the valve and checked it's trueness. It spins perfectly straight in a drill chuck and there's no sign of chafing on the stem, so it looks again like that's all OK. I did re-checked that the valve clearance was OK before continuing the head strip down.

1608, your comment about build up of gunge on the stem seems to fit. The valve was not really moving freely in the guide, even sticking on some occasions. So I'm going to try cleaning up the valve stem with a very mild abraisive (chrome cleaner) and see if I can get it all moving nicely, then stick it all back together and try firing the engine up again.

While I'm at it, I noticed quite a build up of carbon on the piston head, so I'll clean that off while I'm at it.

Again, thanks for the input and I'll let you know the outcome .... hopefully a success.

Cheers, Bruno

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Re: Loss of compression

Postby Janet » Mon Oct 09, 2017 7:42 am

Far be it for me to disagree with anything that's been said before but if there's a chance of winning £1, I'll make another suggestion. You haven't mentioned it but if you've ascertained that the valve seat hasn't rotated, check to make sure the angle of the seat matches the angle of the meeting surface on the valve when it's shut. Then a little bit of crud makes a lot of difference.
I know, I'm now going to be told that it must do, but mine didn't, hence my engine having been in bits during the last 2/3 weeks.
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Re: Loss of compression

Postby 1608 » Mon Oct 09, 2017 10:23 am

Get your coat Janet :rofl: Probably meant your valve head had warped.
But Bruno, if the trouble with your engine turns out to be collect deposits of burnt oil etc. then explore the reasons why and correct otherwise the problem will return. Check the rings and bore for wear. The heavily coked piston crown indicates it may not have been apart for some time. You may get away with a hone and new rings which can last for some considerable mileage these days with modern oils and only casual mileage etc. The singles have been known to suffer seizing Exh valves due to over heating and carbon/oil deposits.

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Re: Loss of compression

Postby Janet » Mon Oct 09, 2017 1:43 pm

1608 wrote:Get your coat Janet :rofl: Probably meant your valve head had warped.
It isn't nice to laugh at someone because they don't know as much as you do. :cry:

First, what to you mean by warped? Obviously you can't just mean bent. Have you a photo?
Second, I can only go by what I was told when I was having the head refurbished. Since I'm only learning, how am I supposed to know who is telling me correct information? He said, "that's supposed to be 45°. Does it look 45° to you?" I didn't think the seat was supposed to have steps/ridges in it either but that was only an uneducated guess.
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Re: Loss of compression

Postby Rob Harknett » Mon Oct 09, 2017 2:23 pm

Valves may stick and cause loss of compression. If the valve stems are seen to be dirty causing this. So might the be the valve guide. But only cleaning the stem is mentioned. When all seems clean, I have also used a bit of solvo-autosol on the valve stem, then inserted and twisted in the guide to really clean up both parts. perhaps not really the done thing. Make sure you wash clean both parts. You will probably grind in the valves, which will also need a good clean. Note the seats on both valve and head. Both should show where they have been ground clean, with the same colour and texture all round. This will show if they are seating OK. Valves can pocket. Which means they seat very deep in the valve seat. If either or both are very worn, pocketing will result in less area around to make the seal. EG the valve is only seating very close to the lip edge of the valve head. Pocketing can be seen after grinding. Part of the seat near the valve head, & deep into the seat ( viewed looking into the head dome ) will appear to be where the grinding has produced the dull greyish Finnish.

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Re: Loss of compression

Postby Harry44 » Mon Oct 09, 2017 3:38 pm

Janet wrote:
1608 wrote:Get your coat Janet :rofl: Probably meant your valve head had warped.
It isn't nice to laugh at someone because they don't know as much as you do. :cry:

First, what to you mean by warped? Obviously you can't just mean bent. Have you a photo?
Second, I can only go by what I was told when I was having the head refurbished. Since I'm only learning, how am I supposed to know who is telling me correct information? He said, "that's supposed to be 45°. Does it look 45° to you?" I didn't think the seat was supposed to have steps/ridges in it either but that was only an uneducated guess.


Take no notice of 'em Janet. The valve seat in the head is indeed cut to 45 degrees. However the seating surface of the valve head is a slightly different angle so that as the valve is ground in it produces a thin grey line on both surfaces to show correct contact. As everything ages and the valves are ground in a few times the seat angle in the head degrades to produce a thicker line of contact which perversely provides poor sealing. And it is no longer 45 degrees.
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Re: Loss of compression

Postby Janet » Mon Oct 09, 2017 5:18 pm

Harry44 wrote:
Janet wrote:
1608 wrote:Get your coat Janet :rofl: Probably meant your valve head had warped.
It isn't nice to laugh at someone because they don't know as much as you do. :cry:

First, what to you mean by warped? Obviously you can't just mean bent. Have you a photo?
Second, I can only go by what I was told when I was having the head refurbished. Since I'm only learning, how am I supposed to know who is telling me correct information? He said, "that's supposed to be 45°. Does it look 45° to you?" I didn't think the seat was supposed to have steps/ridges in it either but that was only an uneducated guess.


Take no notice of 'em Janet. The valve seat in the head is indeed cut to 45 degrees. However the seating surface of the valve head is a slightly different angle so that as the valve is ground in it produces a thin grey line on both surfaces to show correct contact. As everything ages and the valves are ground in a few times the seat angle in the head degrades to produce a thicker line of contact which perversely provides poor sealing. And it is no longer 45 degrees.
Thank you. I feel better now.
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Re: Loss of compression

Postby 56G80S » Mon Oct 09, 2017 8:19 pm

IKBA but after getting the "new" standard barrel & piston from T&L and before fitting the head I lightly ground the valves in as there was very slight marking following failure of the gudgeon pin circlip.

I wanted to make sure that I'd done enough as I thought I'd erred on the "too light" side so put the head back together, turned it upside down and filled the hemisphere with petrol; nothing emerged past the valve seats and down the guides so I assumed all was well. 655 odd miles later all well and pulling well.

Good luck with sorting everything out.

Johnny B

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Re: Loss of compression

Postby Bruno » Tue Oct 31, 2017 8:11 am

Hello,

An unexpected delay in rectifying the problems with my G3. I managed to shred the ends of my left hand thumb and index finger on a circular bench saw - very lucky to still have them and on the mend, but I've been 'digitally challenged' for the couple of weeks.

I will shortly resume clean up on the exhaust valve and get the top end back together. It is now moving nice and smoothly in the guide since removing the deposits. I think the build up may be related to the bike being run very rich by the previous owner - when I got the motorcycle it was running really badly, mainly due to the carb set up being far out.

Anyhow, I'll give it a go and report back in due course ... hope to catch a couple of warmer days for runs before the proper Scottish winter sets in.

Cheers, Bruno

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