Erratic running at full revs?

[peter john]

I am restoring a 1936 A7 two-seater sport. It has a standard two bearing engine. It came fitted with an SU carburetor (1 inch bore at the manifold type 617p) and an electric fuel pump. The problem I have is that when the engine reaches full revs it fades after a few seconds and then picks up again only for the same thing to happen again. I assume it is to do with fuel starvation, but the pump functions well, so I am very puzzled.  Is it the wrong carb for the car, or could it be something else?
I have tried different needle sizes to no effect as well as altering float level etc.
 
Any ideas please…..?!
 

[Jack in the Box]

You could check that your fuel filler cap is vented adequately.
Alan

[Jamie]

You could check that your fuel filler cap is vented adequately.
Alan

Any in line fuel filters fitted and blocked?

Jamie.

[bob46320]

Old fuel ? Has the engine been re-built and run in yet? Ie, is it "tight". I seem to remember someone having power problems due to a bunged up exhaust pipe after the silencer, took the rear pipe off and all was OK.

[AllAlloyCup]

If there’s muck in any of the filters clean out the tank?

Or check all joints are tight and it’s not sucking air in at some point in the fuel line?

Bill G

[peter john]

Hi Bill G

Thanks for that, but no blocked filters, or air getting in. Any other suggestions very welcome.

Thanks

Peter

[dickie65]

I have a suggestion,

You might be suffering from Carb Icing, as the air goes through the venturi at full chat the temp drops and ice forms.
by the time you slow down a bit the temps come back up and the car runs fine again.
I used an outdoor temp probe taped onto the carb to find this out on my car.

[Stumpi]

I have the same set up but  1 1/8.
Is the piston moving freely.
Should be bronze with NO spring.
Check the oil in damper To much causes misfire at revs. 
                                   To little causes hesitation at low revs. and I use Sewing machine oil 

 Both  problems  i have experienced.
Float Bowl loose which changed mixture making it rev one minute and die next.
Andy

Also check rubber hose to carb is not collapsing ,Old hoses seem to fail with modern fuel.

[Bob Culver]

Not related to the problem but if that is an oil dashpot, then needle recommendations for later carbs apply rather than those for the heavy undamped original piston.

[GK5268]

Have you actually checked the electric fuel pump, at low demand it may be ok, but going flat out may be another story, is it points or solid state, if points, are they correctly adjusted and the diaphragm is set-up properly.

Is the level in the float chamber set too low?

Only so much to check so you should find the answer!