Hello,
I have a Nissan A15 fitted with a Weber 32/36 carby.
It has always had a flat spot upon acceletaing from rest, but lately has got worse and also appeared whilst on the move.

If the movement of the accelerator pedal was divided into quarters, I would say it happens on the second quarter.

We have checked the accelerator pump it is fine, rebushed the throttle shaft, cleaned the carby, and also thought it may be the timing, but no matter what we do it is stiil there.

Seeing it is getting worse, I hope we can get it fixed.

Any ideas would be appreciated?

Check all your vaccuum hoses, fittings, caps, and vacuum diaphrams. Sounds like a leak. Since it was vetter vefore, you should be able to improve it to that point.

My 32/36 always had a flat spot just off idle. never could get rid of it completely.

This is probably going to go without saying, but personally, I'd check/replace everything in the ignition system (poinrs coil and plugs primarily, leads secondarily) before even embarking on any carb related fix. It might well still be the carb, but the way you describe it worsening, is more consistent with the slow deterioration of ignition which happens inevitibly over time.

Was reading about maifold desighn and apparently the 'bathtub' desighn of the weber addaption plate is bad for throtle response and low RPM's. dosent realy help your problem but might explain DD's?

i have a standard A12 with standard carbi and had flat spot just of idle and replaced leads, points, coil etc and still there, even replaced with a fully rebuilt weber 32/36 just last week and to my suprise still has flat spot just on take off, not sure what problem is, might be vacum leak or something but same problem new cabi did not fix problem might not be carbi. same as mine just a thought.

sounds like carby problem to me
throtle plate open to far to idle
pulling fuel from the progresion slots or holes
not past idle screw
lack of fuel between progresion slot and
when main starts to flow fuel causing flat spot
do the motors have a big cam? low vacume signal?
sometimes u need to drill a hole in both throtle plates
so u can drop the idle open amount to drop the idle speed
and pull fuel from idle pasage only
a must do for sum big camed v8s
tommo

ROConner, you may be on to something. After living with the flat spot for a while, I fitted the stock manifold extension tube that extends down almost to the manifold floor and it helped noticeably.

Thanks everyone for your help.
My A15 has a standard Cam.
I played around with it on the week end taking everyones info on board, and found the following:
I check the ignition leads, they are faily new, new ish Coil, could not find vacuum leaks. So stuck with the carby.
(Forgive me if I get the names of the parts wrong)
The accelerator pump spray jets.
Well the one on my carb had very short arms and was squirting the petrol onto the wall of the throat of the carby not down the centre.
I took it to a carby repairer and showed the guy and he said he had never seen one with arms that short. He had one with longer arms but a smaller dia hole. I mentioned to him that both of the spray nozzles ( Short & Long) only have one hole, in one arm and he said they are all like that, and he mentioned if I was careful I could re drill the hole larger.Well I did drill it out carefuly to 1.5mm dia, and refitted it. At this stage the carby guy said when ever you get a flat spot it means lack of fuel.

But no differance, so I took it back out again, and noticed that the side with the hole was on the secondary carb throat not the main throat, So I (tried) to carefuly drill out the other side, But the drill came out about 3/4 of the way down the length. But I refitted it again and WoW, it has fixed most of the problem, It seems that the primary throat was not getting the squirt of petrol from the Accel Pump.

Now I still have a smaller problem with accelleration on the move, a hesitation at a certain point of throttle.

I will try and buy another long arm acc pump spary nozzle, but ask the carby guy to drill out both sides for me this time. It may fix it (?)

Thanks again everyone.

Quote:

The smaller the 'plenum' (commen chamber) the better as far as tourqe and throtle responce is concern as the effects of vacume is fellt quicker.

ROConner, you may be on to something. After living with the flat spot for a while, I fitted the stock manifold extension tube that extends down almost to the manifold floor and it helped noticeably.

Again forgive my ignorance but does someone have a photo of this? As I am not sure of what they speak.