Accelerator pumps on updraft carburetors

[carbking]

I have been getting more and more questions from newcomers to our hobby on accelerator pumps on updraft carbs not working.

WITH VERY FEW EXCEPTIONS, accelerator pumps on updraft carburetors are activated by engine vacuum. If the engine is not running, the pump does NOT work!

Think about it. If a novice should get in an old car, and try to start it, and pump the footfeed 92 gazillion times on a vehicle with a downdraft carburetor, the fuel pumped by the pump goes (gravity) into the intake manifold, flows into the cylinders, and washes the oil off the piston rings.

Now, what would happen with an updraft carb and a mechanical accelerator pump? The fuel runs out the air intake and falls on the ground under the vehicle (gravity). A spark, and one more old car is a statistic.

The carburetor designers knew this. Most updraft accelerator pumps have a vacuum piston which is drawn into a vacuum cylinder by engine vacuum ONCE THE ENGINE STARTS; and charges a large spring. When the footfeed is depressed on the running engine, vacuum drops significantly, and the spring drives the accelerator pump downward, forcing fuel from the pump well. There are some syringe vacuum pumps as well.

One exception to the above is the Carter type BB-1 updraft which does have a mechanical accelerator pump; and thus CAN be dangerous to the novice.

When starting a vehicle with an updraft carburetor; USE THE CHOKE!!!!!

Jon.

[sambarn]

Thanks carbking. Well great explanation.

[aussiecowboy]

My 29 Pontiac has a Marvel carb which has a mechanical accelerator pump.

[Ivan Saxton]

Mechanical accelerator pump works well on the two throat Zenith 105 D or DC used on Stutz, Isotta 8A, Lancia Dilambda, some Jordan, and perhaps others. And I have never heard of one catch fire. I probably made a dozen or so complete replicas with gunmetal castings instead of muck metal. I mean Complete, jets and venturis to specification according to the car, and even those butterfly discs with the rectangular bleed passage up the centre. There was a lot of knife and fork work in all the parts and the detail. Only original part re-used was the float. We will be able to do investment castings when there is need for more here, so the casting detail will be better. I'll have to make press dies for more floats, too.