100B+ carb mystery

[JPark]

When I got the bike, the carb had a #90 main and the needle was in the second notch. Rather odd, but I assumed the jetting was for high altitude [lots of that around here] so I put the stock #95 main in and moved the needle to the fourth notch. It ran terribly like that so I assumed that the worn out piston and rings were to blame; just too much blowby to generate enough suck. I put the needle back on the second notch and it ran, such as you would expect with rather negligible compression.

After a complete teardown - bearings , seals, piston, rebore etc. - I put the carb back to spec at the fourth notch and same thing; way too rich. The middle notch is still too rich and the second notch runs okay. There is some sort of weirdness at the 3/4 point as it transitions to the main jet, but it's functional. My guess is that with the needle so low it doesn't quite work out - a bit lean - at that point.

The needle and the needle jet show no wear, just a slight polish of the original matte as you would expect. They're both the right numbers N6 and 4F6. No signs of creative molesting. All the carb parts are the right #s and the air jet is clean and clear. Idle is strong and constant, enrichener works properly, ignition timing is good and new B8S reading light tan. New carb o ring and manifold gasket. New filter element. Saw 60 mph on the speedo in 5th yesterday.

I know, just accept it and ride on. At first I thought maybe I was getting the needle slot numbering backwards and it should be fourth up from the bottom but the manual says no on that. I just can't fathom how the needle setting could end up two notches out unless the needle or jet was way worn. It's as if the PO reamed the needle jet to correct for wear but there's no sign of that ever happening, unless he was a jeweller.

Any thoughts? What am I missing?

[matt glascock]

Induction sounds great. Any restrictions in the exhaust system?

[Al Harpster]

I hate to ask, and I may be in left field, but did you do a pressure test?

After that, when you say too rich but plug "looks" right I'm a little lost.

If you've gotten 60 out of her I'd say you are in Fine Tuning Zone.

Here, some Half Size pilot jet changes might bring you closer.

There is the float level check. You might as well double check it, though it's also a longish shot.

And in your analysis of the throttle position range it seems like you have a handle on this.

IF YOU CAN FIND a new needle jet and jet needle, that would put that out of suspicion. But I think that's a long shot too.

If you are getting 60 you are getting what I'd say is Full Power.

[JPark]

The exhaust system is clean and stock. I had the same thought, but if it was partially plugged you'd think it wouldn't pull hard at high revs. Wide open and wound out isn't a problem.

I'm going to visit a friend who has some Hodaka carbs and see what I can find for needles and needle jets. Maybe just build up another carb and see how that works to isolate whether it's something other than the carb. He's going to need a freshened carb for his 100B+ project eventually anyway.

[Dale]

What are you running for fuel and premix (ratio)?

My guess is that the nozzle is worn enough that you are getting a little extra fuel past the needle. I am not sure what series the B+ uses but if they are available I would sure replace it with new.

[olddogs]

Please use caution when doing sustained wide open throttle runs while chasing jetting problems. An engine seizure and rear wheel lockup at speed on a 50 year old Ace 100 will take years off your life. Been there, done that.

[Bullfrog]

There IS the old story that jetting specs were finalized in the dead of winter at sea level on a high barometric pressure day when it was about minus 40 degrees. The story isn't true - but jetting WAS selected to be safe for engine and rider at sea level. Sounds to me like you've got it running pretty well . . . why be so determined to use the same needle clip notch as published when the facts on the ground indicate something different is "right"? That being said, I didn't read any mention of double checking pilot jet size and pilot air screw setting. Pilot system must be dialed in BEFORE doing main jet and needle tuning. Finally, the needle jet bore could be worn (as mentioned). Note if previous owner "reamed" the needle jet bore (HIGHLY unlikely) - that would be classified as "precision, intentional owner induced 'wear' " or "re-sizing/enlarging" the needle jet bore.
Ed

[JPark]

I'm putting in a 'new' needle and jet and will see if that changes anything. They don't look or measure different, but worth a try. The clip was on the fourth notch so it was presumably working like that on the last bike they were in.

I'm running 92 octane and 32:1 Maxima semi synthetic. The fact that the bike came to me with the needle two notches off stock makes me suspect something other than my choice of fuel and lube. Two notches is a whole lot of wear, which isn't visible.

I'll recheck the pilot just to be sure. Also recheck the enrichener valve seal. Is it just me or is the right hand carb mount nut a PIA to deal with?

[Bullfrog]

Nah, not a PIA . . . a focus enhancer!
Ed

[JPark]

I installed the 'new' needle and jet. Same thing; way rich at the fourth notch. I moved it to the second notch and it runs the best yet. No weirdness at 3/4 and no stumbles. I've been trying to get this thing running like I remember my old '69 Ace did and it's finally there.

Buuut....why the second notch. As I was scrabbling around yesterday in a my friend's bunch of old VM20s I found one that was different. It had K10P stamped on the flange and all the parts are just a bit different. When I got the bike from this guy it was missing a slide and we found one in his carb box. This one is missing its slide and I think I know where it went.

So. It is a 2.0 Mikuni VM20 slide, but there's no guarantee that the relationship of the cutaway to the clip seat is the same. As the needle jet on the Suzuki carb is different and non hooded and it's an 80cc rotary valve from '64 it's possible that the dimensions vary. I suppose I could measure it up but this is my explanation and I'm sticking to it. I wouldn't want facts to get in the way of a perfectly good theory.

By the way, the old Suzuki 80s were pretty cool bikes. When I worked in a shop back 'then' we'd get given old disused tiddlers that nobody wanted. There was a field behind the house and I'd get them running and my brother and I would set up a flat track and race in the snow. It was great practice for saving lurid slides at speeds that were pretty benign. My favourite weapon was a ratty 'Zuki 80 that just refused to die. I think his fave was a Honda 55 Sport.

It also shows that it doesn't take much wear on a needle or jet to upset the linearity of the midrange.