Engine revs rising when air lever opened

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GaryK
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Engine revs rising when air lever opened

Postby GaryK » Wed Aug 15, 2018 1:25 pm

Hi All
I have a 1957 VH fitted with a concentric mk1 carb and no air filter. Starting and idling a few months back was fine. (The bike is undergoing work and not yet ready for the road).

Recently when I start the engine, with the air lever closed, all seems fine until I open the air lever, then the revs rise very quickly so that I have to close the air lever again. I don't let it run for long before I shut it off. Spark plug is sooty but not really wet.

Do you think the carb is the problem? I have taken it off and cleaned and checked float height but no change.

Not worked on old bikes before so any help or advice appreciated

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Re: Engine revs rising when air lever opened

Postby Eero.Korhonen » Wed Aug 15, 2018 2:39 pm

What size is your carb and jets and what kind ingition do you have?
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Re: Engine revs rising when air lever opened

Postby admin » Wed Aug 15, 2018 3:01 pm

It might be obvious ...

When the air lever is "closed", it "drops" a little slide into the mouth of the carburettor, to choke it.
When you "open" the air lever, it raises the little slide.
This little slide isn't getting caught and raising the main slide at the same time ?

It is also possible that the main slide is stuck slightly higher, and when the the air slide is down it masks it ......

Easy to check from the open end of the carb.
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GaryK
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Re: Engine revs rising when air lever opened

Postby GaryK » Wed Aug 15, 2018 3:20 pm

Thanks for your replies.

The carb is a 930, not sure of the jet sizes, but can check later. Magdyno ignition.

I cannot see any movement of the main slide when the choke is opened or closed. There is no air filter so easy to check.

As I mentioned it was starting and idling OK a few months back.

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Re: Engine revs rising when air lever opened

Postby Eero.Korhonen » Wed Aug 15, 2018 7:32 pm

I have 928 in mine and main jet is 180.
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Re: Engine revs rising when air lever opened

Postby admin » Thu Aug 16, 2018 3:53 pm

GaryK wrote:Thanks for your replies.

The carb is a 930, not sure of the jet sizes, but can check later. Magdyno ignition.

I cannot see any movement of the main slide when the choke is opened or closed. There is no air filter so easy to check.

As I mentioned it was starting and idling OK a few months back.


Drop the main slide onto the bottom of the carburettor , by backing off the idle screw. The throttle cable should still be loose enough to let that happen.
That should stop the bike. if it doesn't then there is an air leak somewhere.

(forgot to mention that a sticky throttle cable will hold a carb slide up and even a little gap is enough to drive the revs up significantly)
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Re: Engine revs rising when air lever opened

Postby GaryK » Thu Aug 16, 2018 6:39 pm

OK John will try that, hopefully this weekend.

Eero, the main jet is 220.

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Re: Engine revs rising when air lever opened

Postby GaryK » Fri Aug 24, 2018 1:59 pm

I dropped the main slide as far as it could go. The engine still runs with the choke down and still revs high when the choke is lifted.

Does this point to an air leak somewhere?

I have not tried playing with the needle position yet. Would this make any difference?

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Re: Engine revs rising when air lever opened

Postby admin » Fri Aug 24, 2018 4:35 pm

if the slide is truly at the bottom, then the bike should stall.
I assume you've backed off the idle screw and given the cable enough slack to be 100% certain ?

If there is an airleak, past a loose slide or through the manifold, then the bike will run.

The needle shouldn't make any odds here, unless it was missing entirely I suppose ?
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Re: Engine revs rising when air lever opened

Postby volker.halbach » Fri Aug 24, 2018 5:04 pm

I had a similar problem on a Triumph with a 930 Amal Concentric. Of course, John is right: If the slide is at the bottom, the engine must stall. But assuming that the slide is not at the bottom because there isn't enought slack in the cable, check whether the spark plugs are sooty. In my case the main jet had become unscrewed a little bit. The mixture was too rich and with the air lever closed even richer. When the choke was lifted, revs went up.


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