A Little Help with Carb Tuning?

Subject bike is an ’83 Passport C70, a crusty barn find being fixed up and put back on the road for my granddaughter. Almost there.

The carb was too roached to rehabilitate, so I installed a new one from Beatrice. The idle was easy to adjust, and the motor idles well. When riding, the issue comes at about ¼ throttle, which I assume is about the point where the idle circuit gives way to the needle. There is a sudden, uncomfortable lurch forward, like the bike wants to jump out from under me. After that, everything revs and runs fine, up to full speed. The same thing happens when throttling down. At about ¼ throttle, it the bike wants to jump back. These transitions should be a lot smoother. So far, I have not fooled with adjusting the needle or jetting.

Is this a familiar issue? Is the needle setting too lean? Too rich? Improperly jetted?

Sounds like a classic needle height issue. New carburetors are rarely plug & play. Simplest suggestion, raise the jet needle C-clip two notches, retest. If it improves, or solves, the problem, you're on the right track...probably within one adjustment (because there are only five grooves). If thing get worse, work in the other direction (lowering the C-clip) and testing from there.

racerx said:

Sounds like a classic needle height issue. New carburetors are rarely plug & play. Simplest suggestion, raise the jet needle C-clip two notches, retest. If it improves, or solves, the problem, you're on the right track...probably within one adjustment (because there are only five grooves). If thing get worse, work in the other direction (lowering the C-clip) and testing from there.

Click to expand...

That makes good sense. I experimented with needle level using all five slots. Best results were slots 3 and 4 counting from the top. Starting from a good idle, with the needle at the top two slots it bogged with added throttle. At the opposite end, slot 5 from the top, the lurching back and forth was worse than at 3 and 4.

With the needle positioned at the best height, with the bike stationary and in neutral, the throttle transitions beautifully from idle all the way up through full throttle, then back down again. But here’s the thing. On the road, under a load, that lurch forward still appears at maybe 1/8 throttle and a comparable lurch backward appears in the same place when throttling down.

Sorry I’m not familiar at all with the peculiarities of these small carbs. What else could cause this? I’ve checked for any air leaks on the engine side.

One thing worth trying is to just disassemble the carb and make sure all the jets are clear. a little carb cleaner sprayed through each orfice and then some compressed air. go both directions. its possible you have a clogged jet or passage that is impeding the transition from idle to intermediate circuit.

This file at the Yahoo C70 group is helpful: https://groups.yahoo.com/neo/groups/Honda-C70/files/Honda Motorcycle Carburetion Technical Manual/

Hal

Sorry I missed your last post, until just now. Sounds to me like you have the right size main jet. Being able to go from pig-rich to fatally lean with just jet needle height changes, best results in the middle range, is close to ideal. That should give you maximum potential to dial-in your combo. Two things to consider: all adjustments should be done only with the engine fully up-to-temp and under load, i.e. actual riding conditions. Running in neutral is going to tell you very much. An engine, running freeshaft, needs very little fuel and can perform well with an A:F ratio that's too lean for riding.

In terms of discerning rich from lean:

• Rich...easy starting, especially when cold; strong throttle response; tendency to go into misfire at WOT, before the engine can reach full power; black exhaust smoke, fouled plug
• Lean...hard starting, especially cold; stumbling/bogging; weak throttle response, or having to "feather" the throttle; "flame-out" under WOT.

You won't necessarily get all of these symptoms, in either situation. There's a third aspect of tuning, mid-range throttle & rpm. That's where jet needle height comes into play, big time. Based on your posted results, I'd adjust the pilot (idle circuit) mixture as rich as is practical (no plug fouling) then find the needle height that delivers the best performance from part-throttle through WOT. Really sounds to me like you have a part-throttle mixture issue that needs to be adjusted.

Good advice, thanks. I was starting to think it might have something to do with the clutch grabbing instead, as I have zero familiarity with this kind of clutch. But I will certainly keep working on the mid-range carb tuning at operating temperature.