Topic: CB350F rich at idle to ~2K

[mikestebbins]

Clear tube method accomplished. Bike was on the center stand in the garage (should be fairly level in roll axis, but I didn't actually put a level on it) and I leveled it in the pitch axis (wood under front tire, confirmed with the level). Results are:
1: 3.8 mm
2: 4.9 mm
3: 3.8 mm
4: 2.6 mm

All measured with a digital micrometer, from the interface between the float bowl and carb upper section.

I'm surprised that there's that much variance. I set all the float heights with the same caliper, they are all 24mm (+/- 0.25mm, if I had to guess). Does that much variance seem normal? Thoughts on if these results would make the very rich idle condition make sense?

[scottly]

I'm surprised that there's that much variance. I set all the float heights with the same caliper, they are all 24mm (+/- 0.25mm, if I had to guess). Does that much variance seem normal? Thoughts on if these results would make the very rich idle condition make sense?

The variation is caused by the strength of the little springs in the float needles, which is why it's best to check with the clear tube.
#4 is the only one on the rich side, and it's not a smoking gun.

[mikestebbins]

If anyone has any other ideas for easy-ish tests, I'm all ears. I'm out of town for 2 and 1/2 weeks, so while I may be slow to respond, I'm still VERY interested in your thoughts on diagnostics. Thanks for your help!

[jaytee-nz]

In my experience, those float height variations shouldn't cause a rich issue on all four cylinders (but I would still keep it in mind).
One possibility that has not been discussed is worn needles and/or needle jets. If these are worn they will give you a rich mixture at low revs. One indicator of this problem is that the bike will start easily with no choke when cold (because it's already rich). If this is the cause, DON'T use aftermarket replacement parts.

[denward17]

Clear tube method accomplished. Bike was on the center stand in the garage (should be fairly level in roll axis, but I didn't actually put a level on it) and I leveled it in the pitch axis (wood under front tire, confirmed with the level). Results are:
1: 3.8 mm
2: 4.9 mm
3: 3.8 mm
4: 2.6 mm

All measured with a digital micrometer, from the interface between the float bowl and carb upper section.

I'm surprised that there's that much variance. I set all the float heights with the same caliper, they are all 24mm (+/- 0.25mm, if I had to guess). Does that much variance seem normal? Thoughts on if these results would make the very rich idle condition make sense?

I'm a little confused on these measurements.

You say "All measured with a digital micrometer, from the interface between the float bowl and carb upper section."

Are the values listed measured from the float bowl gasket line to toward the top of carb?


Another question:
You have new coils, wires and caps.  Do the resistance values add up to match what Honda originally installed?

[mikestebbins]

Hello all, I'm back from vacation and had a chance to be in the garage yesterday.

Jaytee-NZ,
Thanks for the idea on the needles and/or needle jets. I'm certainly not saying it's impossible, but the bike was running with them before I rebuilt the carburetors, so I'd be surprised if an ultrasonic bath would remove so much material that all of a sudden it didn't run. With that said, I'll keep in mind replacing them with NOS parts.

Denward17, sorry for making it confusing. I grabbed this screenshot off of YouTube, because I didn't take any pictures when I did it. But, it shows where I was measuring from, the interface between the top and bottom halves of the carburetor.

I measured resistances of the coils, the spark plug wires, and the spark plug caps.
Coil 4-1: 4.6 ohm / 17.02 kohm
Coil 2-3: 4.7 ohm / 17.39 kohm
Wire / cap 1: 0.3 ohm, 5.15 kohm
Wire / cap 2: 0.3 ohm, 4.91 kohm
Wire / cap 3: 0.4 ohm, 4.90 kohm
Wire / cap 4: 0.4 ohm, 4.87 kohm

I'm looking in my Chilton's manual, but I don't find values for what I'm supposed to have.

Thanks for any thoughts that anyone might have!

[Deltarider]

I measured resistances of the coils, the spark plug wires, and the spark plug caps.
Coil 4-1: 4.6 ohm / 17.02 kohm
Coil 2-3: 4.7 ohm / 17.39 kohm
Wire / cap 1: 0.3 ohm, 5.15 kohm
Wire / cap 2: 0.3 ohm, 4.91 kohm
Wire / cap 3: 0.4 ohm, 4.90 kohm
Wire / cap 4: 0.4 ohm, 4.87 kohm

I'm looking in my Chilton's manual, but I don't find values for what I'm supposed to have.

Thanks for any thoughts that anyone might have!

The values you measured are ideal!

[HondaMan]

This is a long shot, but one I have seen twice: are the angle-cuts of the carb slides installed toward the back of the carbs?
I once 'rebuilt' a set of carbs from a SuperHawk that would foul plugs below 3000 RPM and a set of carbs from a 500/4 that would blacken plugs in 1 minute of runtime: both of them had the slides installed backward(!) in the carbs (and all 4 in the 500's set!). They fit just fine when this happens, so thought I'd ask...

[mikestebbins]

Deltarider, thank you for confirming!

Hondaman, I've been very careful to keep parts for each carburetor together, so as to not accidentally mix up parts. That, combined with the slot in each slide and the little nub on the inside of the carb body only allowing the slide to go in in one orientation, makes me think I haven't accidentally flipped the slides. But, I will put it on my list when I pull the air box and other parts next to double check. Thanks for the thought!

[HondaMan]

Deltarider, thank you for confirming!

Hondaman, I've been very careful to keep parts for each carburetor together, so as to not accidentally mix up parts. That, combined with the slot in each slide and the little nub on the inside of the carb body only allowing the slide to go in in one orientation, makes me think I haven't accidentally flipped the slides. But, I will put it on my list when I pull the air box and other parts next to double check. Thanks for the thought!

Yeah, those little nubs in the slide tower are supposed to keep you on the straight-and-narrow: they were gone (with a grinder, it appeared) in the 500 set.
I still wonder 'why'?
I suppose if you work on these bikes long enough, you'll see mysterious things...

Everything in the 350F carbs resemble the "N" scale model railroading stuff of old...which was big in the 1980s...

A  little different thought: if you ultrasounded the carbs and they were dirty enough at the time: can you check the long-ish idle-air pathway from the back of the carb(s) to the idle jet's hole (air screw removed, as it blocks the way)? This takes a fine wire that can bend around a bit, like a single strand from a stranded 16 AWG wire, or the like. I'm specifically wondering if some of the dislodged crud may have found its way into those passages, depending on how the carbs were oriented in the soaking tub. This long passage is tiny in size, and it takes very little to plug it up. It supplies the air that the air screw meters into the tiny emulsifier of the top of the idle jet.