Could be a leak further down the inlet tract
spray wd 40 or the like around the manifold gaskets and listen for any change-and do the carb base too the block may not be cracked but it could still leak
check the levers on the accelerator jet diapragm arent all crudded up and sticky.

Thanks Sean willdo.

Bugster,
to add to Sean's advice, you do not need to go any larger than a 107.5 primary jet with normal induction, as the Dyane/Ami blower setup only needs a 110 to cure the flat spot on an 18-26 carburettor.

It's not really trail and error, as this modification has been done hundreds of times before, invariably with a successful outcome.
For the record, a 107.5 will pass about 10% more fuel than a 102.5. What matters as far as the engine is concerned is a jet's cross sectional area, which is related to the square of its diameter...

What float height are you using, and is the accelerator pump producing a good flow of fuel out of the little injector nozzle when its operating lever is worked by hand?

ken.

So with all being good in carb department it sounds lime it's worth upgrading the jet

Thanks ken. Your description makes it much easier to understand.

I'll take another look tomorrow/Monday and post what I find.

Possibly a little off the wall but I came across a problem with the accelerator pump lever which I hadn't seen before when helping Dan with a similar problem with his van at the National. The original problem seemed to be that the O-ring sealing the accelerator pump jet had failed meaning that when the pump was operated petrol was escaping from around the body of the jet rather than the hole. The fault had been diagnosed by a garage and they suggested he should buy and fit another carb(!), which he'd done.

The 'new' carb had the same tendency to stall upon hard acceleration from idle, and after much head-scratching it turned out that although the accelerator pump diaphragm was in good condition, and its linkages clean, the lever that operates it had been worn almost all the way through by the cam on the end of the primary choke spindle. Most of the carbs I've looked at since have had a small groove here but this appeared to be worn so far that it was affecting how much of a 'shot' was given by the pump.

I did feel a little sorry for Dan about the time and money he'd thrown at the problem that day, as all it took in the end to 'cure' the problem on his original carb was a small bit of cotton from my torn jumper wrapped around the stem of the accelerator pump jet....

EDIT: Thanks for the NGK link, Ken. It does make rather more sense now

Thanks Ken, Luke and Sean (in alphabetical order )

I've checked as follows:-

The float centre to underside of top face of carb is 18mm without gasket fitted.

The accelerator pump sprays a decent squirt of fuel (new diaphragm just fitted) though the jet is looser in the carb than I would have expected i.e. it can sometimes fall out if carb is inverted.

I sprayed the manifold joints to the heads and the flange area of the carb with brake cleaner and WD40 and did notice the engine slowing up for a second or two initially but this turned out to be a split in the rubber hose from carb to air cleaner. A smear of silicon over this and there was no change in engine note when retested.

The accelerator pump lever is clean and free and there is a slight groove where it contacts the cam.

One more thing, I pushed the boat out and put a couple of gallons of super unleaded in it yesterday with a cap full of Castrol Valvemaster lead replacement with octane booster. The engine seems much sweeter and the flat spot problem reduced since then.

Any further tips very welcome thanks

Hi Bugster
Have you checked the choke pull off device thing on the top of the carb for leaks?

Hi gaz, no I haven't. I did remove it when rebuilding the carb but then found there was no replacement bits for it in the carb overhaul kit so just replaced it. Think I read somewhere that it can fall off and a special plate can be bought to fill the gap I hate it when things fall off

you get used to the odd bit falling off i think you can make a blanker out of gasket paper but with out the air hole and put under it