Engine revs rising when air lever opened

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nevhunter
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Re: Engine revs rising when air lever opened

Postby nevhunter » Fri Aug 24, 2018 11:32 pm

Back off the idle speed screw about 4 turns and make sure there is about 1 mm slack movement of the outer cable before the slide moves. Raise the choke and observe whether the slide moves with it. Raising the choke slide should not affect the position of the main slide at all. If it does there's something wrong with the shape of one of them or how it's installed Nev

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Re: Engine revs rising when air lever opened

Postby GaryK » Mon Dec 10, 2018 1:47 pm

Update on problem - Not had much time to look at the bike since my last post in August!!. I recently installed an Amal stay up float just to make sure there was no problem with fuel level. Bike starts OK but same problem with revs rising when air lever opened. However, because it is much colder in the garage than in August I noticed exhaust smoke coming out of the engine between the cylinder barrel and head. Presumably it will also be drawing in air which would cause the high revs?

I don't believe there is any kind of gasket between barrel and hesd, so do you think it better to take off the head and check, or just try tightening the head bolts and see?

Thanks for any replies

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Re: Engine revs rising when air lever opened

Postby nevhunter » Fri Dec 14, 2018 1:58 am

The alloy head motors of that period use a gasket I believe. I've never heard of a leak at that spot so big as to make the idle speed too fast but I guess there can be a first time for everything. Spray WD on it when it's running to see if that effects it at the suspected leak area. Nev

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Re: Engine revs rising when air lever opened

Postby john.whiting » Fri Dec 14, 2018 4:47 am

If you have a big enough leak in the head barrel join to allow significant air in,you will also hear a cracking noise when the cylinder fires.,and feel the blast coming out...........Your smoke in the fins may be exhaust joint leak blowing back.......Is your carby new?....other wise there may be very worn spot on slide and body just off idle that can cause problems,even preventing the slide from bottoming....You may find the slide can rock back and forth in the wear ,causing differnt effects each time.

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Re: Engine revs rising when air lever opened

Postby GaryK » Fri Dec 14, 2018 1:27 pm

Thanks for the replies.
I will try the WD40 trick and see what happens.
The carb in not new, but I will check everything over again to make sure it all works as it should.

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Re: Engine revs rising when air lever opened

Postby James Hickman » Wed Jun 05, 2019 3:02 pm

How did you get on? My newly rebuilt 1950 NH 350 appears to be running very weak - even at idle with the mixture screw fully tight. Runs better if I partly occlude the air intake. I also have an exhaust leak at the cylinder head and am wondering if it is entraining air enough to make a difference. Mine doesn't have a gasket so I guess I may have to have the barrel and head lapped :roll: Would it be sacrilege to try a smear of high temperature jointing compound...?

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Re: Engine revs rising when air lever opened

Postby Simon.Gardiner » Wed Jun 05, 2019 11:30 pm

James Hickman wrote:....I also have an exhaust leak at the cylinder head and am wondering if it is entraining air enough to make a difference. Mine doesn't have a gasket so I guess I may have to have the barrel and head lapped...

A leaky head joint or a leaky exhaust pipe joint??
Head joint blowing (if it doesn't have a gasket) usually leads to things getting too oily rather than problems with the mixture - but in either case I'm pretty certain I could find some people (not all, but some) who would recommend that you used some sealing compound in the first place!

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'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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Re: Engine revs rising when air lever opened

Postby James Hickman » Thu Jun 06, 2019 7:18 am

Definitely the cylinder head joint that is blowing

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Eero.Korhonen
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Re: Engine revs rising when air lever opened

Postby Eero.Korhonen » Thu Jun 06, 2019 7:34 am

I think you should hear some weird sound, is there is a leak between cylinder and head.
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Re: Engine revs rising when air lever opened

Postby john.whiting » Thu Jun 06, 2019 9:38 am

if the head to barrel gasketless joint is leaking,it will spit out particles of hot oil,and generally make itself known on your trousers........I suggest first a smear of blue or red silastic.....expensive ,but definitely stronger than hardware $4 a tube stuff......i used to have a lot of trouble locating head gasket leaks in the V12 Deutz motors.......a sheet of white paper between the cylinders will generally have a black line showing the leak location.


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