accelerator pump install/spark arrestor out

I installed my new diaphragm in my 39mm FCR mx. The new one was slightly different than the old. The old one had a larger button portion; about 1/8". The new one was just about flat.

I installed it with the part number facing out. The smaller button on the inside and spring on the outside. My throttle response is really suffering. Just to note, I did remove my spark arrestor. I plan to put this back in to see if it is the spark arrestor removal changing the throttle response. The bike no longer wants to wheelie!!!! I also put my smaller sprocket on the front, the bike should be flipping me off!

Did I do something wrong when I put the diaphragm on? I re-lockwired the linkage as well. The only three changes are:

  • new accelerator pump diaphragm
  • spark arrestor removal
  • one tooth smaller gear in front

I do not want to go for a test ride right now, severe wobble from somewhere. Fix that, then install spark arrestor, and see what happens.

So you are saying the rivet is shorter? Confirm the linkage is correct (you did not turn the screw, right?).

Spring goes between the AP pump cover and the diaphragm. If that is not how it is, there is your issue.

Put the spring on the cover side, did not adjust the linkage, re installed the spark arrestor. (test rode, and it is the same). May try the other side of the diaphragm. I am not sure if this is a new style diaphragm or did I get the incorrect one and maybe installed it the other way around.

Really hoping to find out from someone who knows more than I.

hmmm, Maybe adjust the screw on the AP linkage? Try the diaphragm the other way round?


Confirm the chamber is getting fuel

Confirm the AP nozzle is not clogged

OK, thanks William, I will NOT adjust the screw.

You do not sound concerned the new diaphragm does not have the same size button. Can I guess this is the new style diaphragm? Or it does not matter. I read up on the slant style FCR, to file off the button for more volume. (I do not have a slant style FCR)

The rivet length affects the total amount of the squirt as well as the duration. A longer rivet means less fuel.

Who sold you the diaphram? It should of been the same.

AFAIK, all FCR-MX use the same one, only the slat style had various ones to use as an adjustment.

I got it from Winners Circle in Markham Ontario.

It makes sense I should actually have more power now, with the higher volume of fuel (no button). I cannot say the diaphragm I removed was the correct one; I purchased the carb used.

I am going to flip the diaphragm to see if there is a difference.

We are having torrential rains for the next few days, it may have to wait!

Ah, a used carb, are you sure it is actually a FCR-MX and not a slant version?

It sounds to me like the gave you a diaphragm for a slant.

Did you check to see if you even get a squirt at all?

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