High revving engine

Information relating to the Matchless G12 or AJS Model 31 650cc twin
ixonchitts
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High revving engine

Post by ixonchitts »

Hi,

Looking for help please. My 1960 G12 combo's engine revs fit to bust on starting. I thought the throttle had stuck open but feeling through the air intake it appears to operate fine. Reducing the air by putting a finger across the air intake slows the engine to a more sensible rpm!

I've had the carb off several times now, and can't see anything obviously wrong. It had been working ok - or at least it was until a recent ride where the engine would either die on deceleration/gear change down or rev very fast if I blipped it to keep going. This was the first longish ride since I got it mot'd and back on the road and the problem wasn't apparent until the engine was hot and I'd ridden about 5 miles or so.

The carb is an Amal Mk1 930 concentric of unknown age but it was overhauled recently and most parts, including the throttle cable replaced.

I'm fairly new to this and still feeling my way, so any advice gratefully received!

Steve
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clive
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Re: High revving engine

Post by clive »

maybe a worn slide allowing too much air through. An air leak between carb and inlet manifold or between manifold and head. broken gasket. ? Had you removed the carb before the problem starred, or worked on the engine. When refitting the manifold it needs to be fitted before the cylinder heads are tightened down.
clive
if it ain't broke don't fix
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dave16mct
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Re: High revving engine

Post by dave16mct »

As well as what Clive said, is the new throttle cable slightly too short? This might stop the slide from shutting down. You should be able to hear it as it hits the bottom of it's travel.
Dave.
ixonchitts
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Re: High revving engine

Post by ixonchitts »

I dont think its the throttle cable as it is slightly longer than previous which was a bit tight. I havent workex on the engine so have discounted a leak between engine and manifold.

I had taken the carb off following my eventful ride as I thought it might be an air leak somewhere. Also I had a paper gasket between the manifold and carb which Amal say is wrong. I put a heat reducing spacer between the carb and gasket. I also replaced the o ring seal but it seem to sit too low in the groove and may not make a good seal.

The throttle slide is new but im now wondering if the chamber is worn and letting air pass the slide.
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clive
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Re: High revving engine

Post by clive »

ixonchitts wrote: The throttle slide is new but im now wondering if the chamber is worn and letting air pass the slide.
If the throttle slide was worn enough to need replacement the body will have worn a bit oval too. On the monoblocs you can get away with one replacement slide, not sure about the concentrics.

You need to check this through logically. Was the over revving occurring before you had the concentric overhauled and fitted the new cable? If it was then the carb is either not the problem or you have not sorted the fault in it.

If it has only started since the carb was overhauled then I would strongly suspect an air leak. Replace that rubber seal with a fatter one. I personally would use some wellseal and a gasket to ensure there was no air leak either side of the spacer you have installed. Check that the carb has not been over tightened bowing the face.
clive
if it ain't broke don't fix
ixonchitts
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Re: High revving engine

Post by ixonchitts »

Thanks for the advice Clive - hope to get a look at it tonight or over the weekend. I'll post the findings.
Steve
TrevorCSR
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Re: High revving engine

Post by TrevorCSR »

AMAL concentric carbs are not the best! IF the flange nuts/bolts have been over tightened, the flange will distort. Distorted flange means distorted throttle body. The slide WILL stick. If you have fitted a new slide that will only make things worse. (That's why the Triumph T140s 'rubber mounted' their carbs. With their design, you CANNOT over tighten the nuts)
You can attempt to straighten concentrics, but you'll spend hours, and in the end, the body will still be worn. Only remedy is to buy a new carb. They are not a lot of money, (less than £150.00 on eBay now). You could even buy a new AMAL Monobloc, as was fitted standard to your bike.
For even better reliability and performance, and IF you're not worried about originality, fit a Japanese carb, such as a Mikuni. (The concentric is NOT original) There are companies that will jet a Mikuni for you, with manifold and cables to fit.
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Stuoyb
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Re: High revving engine

Post by Stuoyb »

Complete carbs are available from club parts.
Concentrics £113, Monoblocs £156
Both prices inclusive of v.a.t.
These also qualify for 10% discount if you are a discount member.
Johnobirches
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Re: High revving engine

Post by Johnobirches »

When I fitted a new throttle cable the slide seemed to move freely but on starting the engine ran VERY fast. On disconnecting the cable at the twistgrip the engine slowed to a lovely tickover.
It turned out that the overall cable length was ok but the outer was about half an inch too long.
Shortening the outer cured the problem and allowed the slide to drop completely.

Good luck.
John
ixonchitts
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Re: High revving engine

Post by ixonchitts »

Thanks all for the advice.

I stripped the carb down (again!). The flange is slightly distorted, the throttle slide was sticking closed and the float bowl also had slight distortion. Amal have a 12 week lead for a replacement. I give all parts a good clean tried a bit of wellseal and a new gasket and it seems to have done the trick. The angle the throttle cable came out the top of the carb was also causing some intermittent high revving and have sorted that too. Took it for a longish ride this evening with no problems.

I will still replace the carb when my order with Amal comes through, but hope this will keep me on the road in the interim. I did consider the monobloc but have got to know the concentric intimately now! If I was worried about originality I'd have a long job ahead of me!!
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