G2 - low compression and little power

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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G2 - low compression and little power

Postby sej » Sun Nov 11, 2007 6:55 pm

Hi, Still trying to sort my G2 which I originally bought in boxes (not always the best idea!!). I will photograph it and put in the gallery when finished. The engine ticks over nicely, carb was stripped and thoroughly cleaned. Engine runs quietly and starts fairly easily with throttle wide open.
Unfortunately after warming up, if I rev the engine it really struggles, the air lever at this point is fully forward. If I pull the air lever back a third it revs much better. I have checked the carb and everything is standard size and settings. I checked the compression which only shows 25psi (not a clue what it should be, but surely higher than that) I put a little oil down the plug hole and tried compression again it was exactly the same - 25psi. Has anyone any idea what the problem is, I really don't know but I am sort of guessing maybe a valve problem! Many Thanks.

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G2 - low compression and little power

Postby Don Madden » Sun Nov 11, 2007 9:53 pm

Search for the recent thread on compression testing, then get back after following the suggestions.

Cheers, Don.

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G2 - low compression and little power

Postby paul knapp » Sun Nov 11, 2007 10:17 pm

G'day Steve, If the motor requires wide open throttle to start, this would generaly indicate that there is excess fuel in the carburettor inlet. eg. its flooded!
If the motor struggles to rev, eg. flat spot, check that the ignition is advanced.
Is there black smoke coming from the exhaust when the engine is struggling?
You are not getting the air lever (choke) on & off positions mixed up are you? Fully forward air lever means that the choke is on, if your control layout is standard.
25 psi would be felt on the kick starter as very little resistance, if that!

Cheers Paul
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G2 - low compression and little power

Postby sej » Sun Nov 11, 2007 10:47 pm

Hi Don, I need to check the compression a bit more, thanks.

Hi Paul, Thanks very much for the reply. The wide open throttle start was a tip off from an old friend which is something he used to do in years gone past, seems to help. There is a little black smoke when when its struggling, I guess that means its too rich, the plug does look a bit dirty.
The air lever set up I think is standard, when it is pulled back it lifts up the air valve - I think that means I have been trying to set the bike up on full choke!! I thought it was the other way round as the choke cable is under full tension all the time the choke is off. I suppose it is obvious it is weakening the mixture as its letting more air in!! The pennies dropped!!
The kick starter resistance is quite good, but less when starting with the throttle wide open
Thanks again.
Cheers Steve

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G2 - low compression and little power

Postby bag-o-nails » Mon Nov 12, 2007 2:42 am

Hi Steve
as paul said the air lever should be fully forward when starting a cold engine
and then once the engine is warm
retract to the full open poistion for normal running
if you think the compression is low check the valve clearence do you have a workshop manual? it is easy to set the valves on the wrong stroke
cheers for now b-o-n

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G2 - low compression and little power

Postby jonhal » Mon Nov 12, 2007 7:45 am

Hi,
check the carb to barrel seal is not distorting, I have known them fine when cold but when warmed up will leak causing rich mixture and power loss.
John

Edited by - jonhal on 12 Nov 2007 07:47:43 AM

Edited by - jonhal on 12 Nov 2007 07:49:21 AM

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G2 - low compression and little power

Postby wilko » Mon Nov 12, 2007 8:23 am

Huh?????

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G2 - low compression and little power

Postby Biscuit » Mon Nov 12, 2007 9:10 am

Oo! John, air leak carb. to head = weak mixture.




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G2 - low compression and little power

Postby sej » Mon Nov 12, 2007 9:14 am

Hi b-o-n
Hope your G2/14 is going well. As I'm sure you may have seen I thought the air valve worked the opposite way, as it seemed likely that the tension would be off the cable when it was switched off. I have never had a bike with an air valve lever before. I think I should be able to set up the running now.
I do have a workshop manual. I set the piston in the correct position and set the valves. I could not see anywhere in the manual to tell how you which of the two positions to set the valves on! I set it on both valves shut as it was about to fire. Is there an easy way to tell if I'm on the right stroke? Many Thanks

Hi Jonhal
I previously checked the flat edge that joins the carb to the barrel with the edge of a steel rule. It bowed up towards the mounting bolt ends. I gentley filed it flat until no light could be seen. Many thanks for the suggestion.

Cheers Steve

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G2 - low compression and little power

Postby Biscuit » Mon Nov 12, 2007 10:40 am

The half time pinion is keyed to the shaft.
The half time pinion has a mark, as has the cam gear.
Turn the engine until the mark on the half pinion is at 11 O'clock,
Insert the cam with the marks coinciding.

Which stroke you want is immaterial, the keyway and mark on the half time pinion determine this.

Yours is a strange engine if the carburettor fits on the barrel, sure it's not a two-stroke?

Edited by - Biscuit on 12 Nov 2007 10:42:09 AM




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