Author Topic: Flat spot on acceleration - pump diaphram?  (Read 1874 times)

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Lewey

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Flat spot on acceleration - pump diaphram?
« on: December 16, 2005, 12:48:54 AM »
I have always had a bad flat spot on my Bis just after the second choke opens in the carb.  As I am very near to driving the thing again (just need a a dizzy- please help someone) I would like to hear your opinions on what it is.  My assumption is the accelerator pump/diaphram on the carb.  When the engine was running a couple of month back I had taken apart and cleaned the carb with compressed air, checked fuel pump pressure, cleaned and gapped points and fited new plugs, timed the engine using a strobe without vacuum advance, then connected it to check ignition advance against data I have to make sure advance was accurate.  I also mucked about with the idle mixture of the carb, deliberatly setting it wrong either way to try to force a change of habit with the flat spot.  None of these things worked, however there was a noticable boost in performance overall of course as the engine was in poor tune when I bought the car.  The car performs very well overall, easily going 70MPH on the flat (and round some roundabouts).

If it is the diaphram, where can i get one from?

Chris Lakin

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Flat spot on acceleration - pump diaphram?
« Reply #1 on: December 16, 2005, 01:24:04 AM »
hi m8, yes i have same problem with the carb on my BIS!! you have to bring throttle backup to keep going!!! i drove it all summer like that!!

your right, i believe it is the pump diaphram! ive heard loads of people with the same prob.

Carb kits are cheap, www.ricambio.co.uk

they did two carbs for the BIS a weber and a polish one, make sure you state which one yours is, as when i got the weber carb kit one of the gaskets wasnt the right shape. I think the diaphram is the same though! i bought a weber version of the carb off ebay to stick on soon, i need ot get it cleaned up!

regards

Chris
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gnash

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Flat spot on acceleration - pump diaphram?
« Reply #2 on: December 16, 2005, 09:46:36 AM »
A flat spot on acceleration?  That can't be normal...  ya mean your BIS accelerates?  Mine has no discernable acceleration :lol:
(gotta love the wee buggers)

Lewey

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Flat spot on acceleration - pump diaphram?
« Reply #3 on: January 10, 2006, 12:36:57 AM »
I only just noticed this slanderous comment!  Acceleration is my way of saying:

"As inertia builds, the car is forced along the road at ever greater speeds until it is going a little faster than it was a mile or so ago."

BTW: I have managed to iron out the problem with a slightly richer idle mixture.  It may be back, but for now it is better.

 

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