Since you have the new carb out do you mind check the jet sizes inside and tell us the numbers? Ofcourse it will be interesting to know the differences between the two, i have the same problem and after lot of adjustments i decided to leave it as is Maybe the new is correct and the old has some different jetting.

Re: Backfire at speed

Labman said:

Tried timing it. Advanced...same. Retarded...same. So I decided to swap the new Weber out with the old. Bingo! No more backfiring! So it is something related with the jetting on the new carb. It's just dumping too much fuel into the cylinders. At this point I'm inclined to just leave it now since it is purring like a kitten.

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When you say "NEW" carb was that new in terms of it being one of the modern repro carbs or an alternative carb that was new to the car. I have heard some poor reports about the quality of the carbs and one major supplier made the comment that they have been a lot of trouble and that you are better off with a good reconditioned original carb.

Toshi 975 said:

When you say "NEW" carb was that new in terms of it being one of the modern repro carbs or an alternative carb that was new to the car. I have heard some poor reports about the quality of the carbs and one major supplier made the comment that they have been a lot of trouble and that you are better off with a good reconditioned original carb.

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I guess it’s a repro. Made in Spain. Looks identical but something is up with the jetting I guess.

Labman said:

I guess it’s a repro. Made in Spain. Looks identical but something is up with the jetting I guess.

View attachment 185181

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Made in Spain did come into the conversations I had but I have never got to get my hands on one of these to form an opinion. I did get to see a new Weber 28IMB carb for a 650 engine and I could see where manufacturing economies had been made. If it is a problem with jets that would be as cheap as chips to sort out but if I had to put money on it I would suspect that something is not quite right with the carb main body.

Toshi 975 said:

Made in Spain did come into the conversations I had but I have never got to get my hands on one of these to form an opinion. I did get to see a new Weber 28IMB carb for a 650 engine and I could see where manufacturing economies had been made. If it is a problem with jets that would be as cheap as chips to sort out but if I had to put money on it I would suspect that something is not quite right with the carb main body.

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So I leave old carb on or swap out the internals from the old into the new?

If internals are different try switch them a put the new one to see what happens.
Thomas

Labman said:

So I leave old carb on or swap out the internals from the old into the new?

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Well it seems to me that you have already answered your own question if the engine is running well with the original carb then why change anything. You have a back up carb that works but does not work so well and could be a useful source of spare parts if the original carb starts to show signs of wear.

Toshi 975 said:

Well it seems to me that you have already answered your own question if the engine is running well with the original carb then why change anything. You have a back up carb that works but does not work so well and could be a useful source of spare parts if the original carb starts to show signs of wear.

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Yes you’re right. The reason I bought a new carb is that the original one was leaky and warped at bottom. I guess I will have to deal with it and use the new one as a spare parts carb as you said

Re: Backfire from exhaust at speed

All...I haven't posted in a while but this issue is ongoing. I renamed the title "Backfire from exhaust at speed" because I originally thought it was backfiring through the carb but it actually is coming from the exhaust. Also when downshifting and coming to a stop, it seems to want to stall. I have to give it throttle. This happens even if I turn the idle screw in so it idles higher and adjust fuel mixture. What gives?? Is it timing (I have a 123), fuel mixture or something going on internally? It's driving me crazy that I can't sort this out.
BTW...I installed a new carb as I thought that was the issue.