carbs

[daveg]

Mike,

The engine will not rev without an air supply so if you are blocking the air intake to the carb it is getting in somewhere else. I think you have checked the manifold to engine and manifold to carb joints and not found a problem however I would try assembling these joints with silicon sealer as well as the gasket and see if there is an improvement. 

The carb throttle and choke spindles can also leak but would have to be in a really bad state to cause the problem you are having but it may be worth putting some thick grease around them to see what happens

[Hobbo]

The carb is a newly reconditioned, and have tried a NOS 30 VM 7 ,have tried sealing joint already to no avail. I think I have about tried all the suggestion Mike

[Ian Williams]

Have you checked the manifold thoroughly for cracks holes or distortion? Everything you describe still points to weak mixture, if you are 100% certain that the carb is clear of blockages, has the correct jets, fuel level and fuel supply then your problem has to be an air leak, which can only really come from the manifold, either through distortion, bad gasket or a hole or crack of some sort. Take the manifold back off, with a straight edge ensure it is flat where it meets the block, check the face of the block is also flat and do the same on the carb flange and corresponding manifold face. correct any issues, then remake both joints with new gaskets and a decent flange sealant such as Loctite 518, Hylomar or similar. I would also inspect the manifold very closely for any cracks, holes or other flaws or damage before refitting.
You mentioned valves, what makes you suspect this? I am assuming that you have checked compression's are good and even across all cylinders. Is there a history of a previous owner rebuilding the engine? I would not be immediately suspecting this but it is just possible the cam timing is way off, that said I am far more inclined to be suspecting weak mixture. You said that there was a lot of fuel in the bottom of the manifold after revving with the air intake blocked, this would be expected as the engine vacuum would be pulling almost neat fuel through the carb, but air must be getting in somewhere or it would simply flood and die when you block the intake.

[Tiger]

Hobo
Can I ask did you return to the original ignition set up ??
When a young lad I worked with a mechanic who worked on RAF PLANES during the war.
He always started to solve this type of problem remembering

90 % OF CARBURETTOR PROBLEMS ARE ELECTRICAL

Sorry if this is old hat !
Keep us informed
Regards
Tim

[Hobbo]

Hi I haven't as yet taken off the electronic ignition and returned it back to points. Will have a further look at manifold for cracks etc, thought if there was anything like that it would run rough on tick over. I reassembled everything after the carb with white exhaust paste, also checked everything was level with steel rule, will take it off again and let you know what I find. Thanks Mike

[David Stepney]

I appreciate this may be a 'long shot', but have you got the right jets in the carburettor? For a Big Seven, they should be:- Main Jet 90, Compensation Jet 50, Slow Running Jet 60.

I would also restore it to electro-mechanical (points and condenser) ignition. I don't trust all this modern "electrickery". Electronics can fail in some really esoteric ways and often a buggered circuit board looks exactly like a working one!

[AustinWood]

I still think it is a jet problem. Hand over the intake is equivalent to pulling out the choke and is often a trick used for getting a reluctant engine to start.
Normally fuel is fed into the engine via the main jet. However when the throttle is opened from tickover there is a delay in vacuum getting to the main jet and the engine will stall because the mixture goes weak.
The compensating jet holds a reserve of fuel in its gallery and is brought into play when the throttle opens, hence enriching the mixture. Once the revs have risen the main jet takes over.
Stalling on opening the throttle is classic compensating jet problem.
If it's ticking over OK try flapping the throttle open and shut and pull the choke at the same time. If it gets off tick-over it might well run OK.

[Nick Salmon]

It is a very long shot, but just in case... I posted this in a thread in Jan 2019.

I was recently asked by a chap to have a look at his Big 7 - which he said was down on power. I took the air cleaner off and found the choke cable was connected too tightly and this resulted in the choke butterfly being permanently half closed. Released the cable and normal power was resumed.

[Hobbo]

Hi thanks for all the suggestions. I have this afternoon taken off the manifold cleaned everything up checked it for cracks or holes checked for everything being level even blanked off the inlet part of the manifold and filled with water, no leakage into exhaust parts. Refitted with exhaust paste both sides of gasket which was o.k., started up seemed initially to be better but would still not rev unless choke was partially on. It is the same whichever carburettor I use, one is recondition the other new old stock but a 30 VM7 not a 30 VM4, but other people run with a VM7. Losing all confidence now that I will get it right, will let every one know what I decide to do Thanks Mike.

[Colin Reed]

HI ALL,
Would it not be possible for some one close to Hobbo to go round with a carb & dis  which they know are working OK and start from scratch and go through the setup again.
A lot of times it is very easy to miss  the obvious and the new person does not have any frustrations with the engine.
it is very hard to keep on going over the same setup.
would hate to see some one give up on Sevens
So please some one needs to step up

IT will get fixed
Colin
NZ