Low rev problems

I built my motor many years ago, but I've never been able to cure a basic problem. It runs great above 2500 RPM, but below it, it's a nighmare. It stumbles, spits back, and generally runs like garbage. The engine started out as a '72 case, and I added a 88 mm big bore kit, dual port heads, dual Webers and a mid-range cam. Someone suggested that the carbs should be re-jetted and I suspect that the cam doesn't help, either. When you say air-cooled up here, people either think Harleys or lawn mowers, so I'm on my own. Any suggestions? Many, many thanks.

..."stumbles, spits back" ....

sounds like a lean problem but there can also be many variables . Timing and carb jets are just a few potential sources.

I agree....sounds super lean in the idle circuit, and maybe has a vacuum leak. It runs OK above 2500 because you're getting a little beyond the idle circuit into the transition ports. A mild cam should be more than OK....I've got a pretty hot street cam and it (now) idles really nice, once I richened out the idle jets.

Spits and pukes from both sides or just one?

I've heard that Webers are really easy to re-jet, but others on here should be much better than me to help you with that.

First thing would be to find the idle jets and see what's in there (they're stamped on the side with the size), then get two or three sets, each progressively larger, from CB Performance (jets're cheap) and put in progressively larger ones until it smooths out (AFTER you change the carb base and manifold-to-head gaskets to eliminate that as a culprit).

After that, you can maybe play with the main jets, once you get idle straightened out.

There may be something in the knowlege section to help, too - go look.

gn

If you used to have a smaller engine that ran well with these carbs and now have a much bigger engine with the same carbs and jets it is most likely a lean idle circuit. The easiest thing to do would be to post your engine and carb spec on the cal-look forum. Someone there should be able to guess at jetting so you only need to buy one or 2 sets.

Bet you have an 009 distributor too --- there are some guides on the web to getting them to act half decent -- but there are better alternatives.

Ive always discovered a missed adjustment is your carb linkage. Pulling more gas feed on one side, quicker than the other,will cause it to backfire,spit, and run like crap. I bought a speedster several years ago that the owner could never get to run right. A quick linkage adj.,and it ran super. You might want to do a fresh valve adj. also.