Weber 40 IDA 3C accelerator pump Cover "Flatness"

sam512bb · 08-12-2018, 09:49 AM

Good day All,

I have been looking into my acceleration pump circuit, as I have been getting some fuel seeping through the accel nozzles at a constant cruise RPM of around 3K RPM. This was causing my plugs to foul after about 10 minutes of driving, as this was causing a very rich fuel mixture. As I was examining the accel circuit, I noticed my accel covers are not flat, but have a slight "bow" in them between the mounting holes. Please see the attached pictures which shows the "bow"/gap. I measured the "bow"'s peak to be about 0.6mm (.024") which does not seem right to me. Is this normal? I would have expect the cover surface to be flat...however, I thought I would ask the other members on here.

Thanks in advance!

Cheers,

Sam

sam512bb · 08-12-2018, 11:02 AM

Good day All,

You can ignore my last post, as I found my answer. I was reviewing John Passini's book ("Weber Carbs - Theory, Tuning, and Maintenance") and in Page 68 (top right paragraph) it states that the covers should be flat and in fact he suggests rubbing the cover with emery cloth on a known flat surface.

As I mentioned in my previous post, this cover should be flat the bow I am seeing is not correct. Given that the cover is part of the diaphragm edge sealing mechanism I suspect that some fuel was leaking and if not then the assembly was reliant on the diaphragm thickness to create the seal...

Cheers,

Sam

BURN-BROS · 08-12-2018, 11:30 AM

No, they bend over time. You will need to flatten them back out and re-machine the gasket area. Or buy new replacements.

sam512bb · 08-12-2018, 01:49 PM

Quote:

Originally Posted by BURN-BROS

No, they bend over time. You will need to flatten them back out and re-machine the gasket area. Or buy new replacements.

Good day Aaron,

Thank you for the info!

It looks like I have to machine them down by about 0.024"... is this too much or should replacements be the way to go? I ask, as I can see the lever pivot distance to the eccentric cam lobe will be reduced by this amount and so the lever will effectively push the diaphragm inward a bit more than what it was normally. Will this create an issue?

Thanks in advance!

Cheers,

Sam

superninety · 08-13-2018, 06:23 PM

Check your PM.

R.

BURN-BROS · 08-14-2018, 07:33 AM

If the cover has a small bend, then you can sand it down....But if it is like yours, you should bend it back carefully, then sand the cover flat.

Quote:

Originally Posted by sam512bb

Good day Aaron,

Thank you for the info!

It looks like I have to machine them down by about 0.024"... is this too much or should replacements be the way to go? I ask, as I can see the lever pivot distance to the eccentric cam lobe will be reduced by this amount and so the lever will effectively push the diaphragm inward a bit more than what it was normally. Will this create an issue?

Thanks in advance!

Cheers,

Sam

sam512bb · 08-15-2018, 05:26 PM

Quote:

Originally Posted by BURN-BROS

If the cover has a small bend, then you can sand it down....But if it is like yours, you should bend it back carefully, then sand the cover flat.

Good day Aaron,

I ordered and already received some replacement covers ... just in case. While I was waiting, I was able to reduce the arc and I sanded them flat. I compared them to the replacements and found that they were pretty spot on. The replacement covers were ever so slightly smaller in places. Not enough to cause any issue, but are just enough to be slightly different.

Thanks again for your post and the info!

Cheers,

Sam

sam512bb · 08-15-2018, 05:27 PM

Quote:

Originally Posted by superninety

Check your PM.

R.

Good day Robert,

Thanks for your PM!

Cheers,

Sam

KTL · 08-17-2018, 06:24 AM

I would suggest that you also flatten the "block" that mounts to the carb body itself. I think you'll find those pieces are not necessarily all that flat either.

I had this same problem on my former set of Weber 46 USA-made carbs. Both accelerator pumps were leaking quite badly

sam512bb · 08-19-2018, 01:30 PM

Quote:

Originally Posted by KTL

I would suggest that you also flatten the "block" that mounts to the carb body itself. I think you'll find those pieces are not necessarily all that flat either.

I had this same problem on my former set of Weber 46 USA-made carbs. Both accelerator pumps were leaking quite badly

Good day Kevin,

Indeed, I checked these and they were pretty flat. I lightly sanded them on a known flat surface and only found a few high spots...and as I said they were not bad.

I am somewhat embarrassed that I did not realize that the covers were so "unflat"... until I really dug in to look at the 2500-3000 RPM rich issue I was experiencing. I am still perplexed as to how the covers could become so "bent"... The studs are relatively small and this would limit how tight the covers could be tightened... and the distance between the studs is small. Odd...

Thanks again for your post!

Cheers,

Sam

KTL · 08-20-2018, 05:26 AM

Good to hear you checked the blocks for flatness too. You should be leak free for a long time now!

sam512bb · 08-20-2018, 12:22 PM

Quote:

Originally Posted by KTL

Good to hear you checked the blocks for flatness too. You should be leak free for a long time now!

Good day Kevin,

The funny thing is/was that there appeared to be no leaking... but... I was getting a very weird rich condition as the main circuit was coming on. Perhaps the covers and obviously lack of sealing was causing some perturbations of the float levels and/or some small dribbling of fuel through the accelerator nozzles? That being said, as I flattened the covers, etc my weird rich condition went away!

All is now good!

Cheers,

Sam