Topic: carb learning experience

[Keith]

Not a CB, but an XL 250 (SOHC) with a Keihin 645B carb. I purchased it a while back but it had a terrible bog/hesitation when trying to accelerate, and was very difficult to start cold. (all engine specs correct, valves adjusted, new Honda points, etc.) I noticed the bike ran better with the choke pulled about half-way..I could rev it smoothly up to redline, but with the choke off it fell on its face. I pulled the carb thinking I would raise the needle but upon disassembly I could see the clip was already  installed in the bottom groove. So the PO had the same thought I did. The main jet was a 120 but the pilot/slow speed jet was unmarked. I ran a jet drill  (backwards, as a gauge) through a 245  pilot jet that was in a carb kit, then tried the same with the jet in the carb but it wouldn't fit. Even my smallest drill wouldn't fit. Aha! So I put the clip in the middle groove in the needle and changed the pilot jet. Perfect. No hesitation, no bog. Fixed. Surprising the pilot jet had such an effect.  Imagine the effect for FOUR carbs!

[Don R]

 The pilot also adds fuel all through the RPM range.

[willbird]

I think it would be extremely cool to be able to put a bike or just the engine maybe even better on a dyno and do a series of runs and make minor changes and document the fuel air mixtures that come from each minor change.

On automotive oriented engine dyno they will do timing sweeps for example and can illustrate exactly what each change does to HP and torque output.

When I was a kid my dad worked at Tillotson Carburator in Toledo, OH. I am pretty sure that he did his Tool and Die apprenticeship there. This would have been late 1960's. He related stories about the small engine dyno test cells that were running long term tests on all kinds of engines that Tillotson made carburators for.

At the very least today we have the tech to use a stepper motor to adjust the idle screws, that would make say 16 back to back runs with the screws advanced or retracted 1/8 turn each time a fairly simple process. Just one example.

[Keith]

Imagine the skill your dad developed over time. So many skills like that are being lost. Thanks!

[willbird]

I remember him talking about making prototype carburators, the production carburators would have a tapered slot in the bore where the butterfly sat near idle and as throttle was open, on prototypes they would drill a series of holes to specified sizes that did the same thing, they would enlarge the holes as directed in response to dyno test data from the engineering department.

I had a chainsaw that had a Tillotson carb, the butterfly was about 3/8" or so in dia :-).