Solved - 1F w/SD40 carb stumbling at steady light throttle

[oregon fj]

I have the original but rebuilt SD40 on my '64 FST F mill. Seems to run cleanly at idle, pulls nice and clean under acceleration. But steady cruise at light throttle openings seems very rich, with lots of burble and frequent hiccups. Smells rich, too.

I'm guessing something is not right in the low speed circuit, but most things I'm familiar with would cause a lean mixture, not rich. But I'm not terribly familiar with these carbs, so feel free to school me if appropriate.

Any thoughts?

 

[whitey45]

Don’t forget to check your timing, advance, points, etc..

 

[oregon fj]

Did all that. New points, condenser, cap, rotor, wires and plugs. Timing spot on using strobe. Pretty sure it's fuel related.

 

[Pin_Head]

You can’t smell rich, because the product of running rich is carbon monoxide, which is odorless. If you smell hydrocarbons or gasoline, it is misfiring. It could be so rich that it causes misfiring, but that is unlikely.

 

[oregon fj]

Well, yes, in a perfect world a rich mixture would just increase CO. But we all know our rigs aren't perfect, especially 5-+ year old ones like mine. 40+ years of dinking with motor vehicles leads me to associate what I'm smelling here with a rich mixture.

Not misfiring most of the time. Definitely not at idle where is is very smooth. Nor when accelerating at wider throttle openings. Pulls nicely, no issues. But steady cruise at light throttle it burbles and coughs a bit. Pretty sure something isn't right. If it were an SU-type carb, or a simple motorcycle slide valve carb I could diagnose much quicker. But the SD40 is a bit more complex-looking than those.

 

[thebigredrocker]

What’s your vacuum number when this issue happens?

 

[oregon fj]

It only happens when driving, and I don't have a vacuum gauge I can read from the cab. But it happens all the time while cruising at light throttle. In support of it being rich, pulling the choke a little makes it worse. But I'm not aware of any adjustments past the idle bleed screw, which shouldn't have much affect off idle, or so I believe. Am I wrong?

 

[oregon fj]

Bump?

 

[Casey E]

what distributor are you running ?

 

[Pin_Head]

I am not that familiar with the SD40, but in general, the off idle performance depends on fuel from the “transition” slot that is just above the throttle plate. The transition slot gets fuel from the idle circuit, so the more fuel delivered out of the idle port, the less fuel in the transition slot above it. This is the typical situation with a manifold vacuum leak or possibly an obstructed idle circuit feeding them. This results in a lean situation and not a rich one and I cannot envision how this could become rich.

But what do I know? I think that you can’t smell a rich condition but that was only based on working 3 years in a CA smog shop where we actually measured how rich the exhaust we were smelling was.

 

[oregon fj]

what distributor are you running ?

Stock dizzy for a '64 F.

Will do some additional experimentation this weekend. I'd like to get a handle on this soon, as I am preparing to pull the tub for sandblast starting next week, and won't be able to drive it.

 

[65swb45]

First I’m seeing of this post Arne. Because the SD40 uses metering rods to supplement the main jet, my thought is that the metering rod may be kicking in too soon and may need an adjustment.

I don’t remember if your carb has the more common reverse-step rod, or the earlier straight metering rod on the pivot arm. Do you know?

 

[oregon fj]

First I’m seeing of this post Arne. Because the SD40 uses metering rods to supplement the main jet, my thought is that the metering rod may be kicking in too soon and may need an adjustment.

I don’t remember if your carb has the more common reverse-step rod, or the earlier straight metering rod on the pivot arm. Do you know?

Not sure, Mark. The carb date code is Jan '63, so that may be the early rod. Is this something I can see on the outside?

Betting this pic from when you rebuilt it is of the wrong side.

 

[oregon fj]

@65swb45 - Mark, it appears that I do have the early straight metering rod. I can't seem to find the height spec to double check, all my searches come up with "use the template in the kit" or others asking the same question with no answer. I also don't know which rod I have - there does not appear to be a spare rod in the carb, nor do I know if I could see the difference if there was.

I've been thinking all along that it is running rich, both just from the way it runs, and also from the fact that choking it while driving does not improve it. But I suppose it could be running lean, perhaps something clogging the low-speed jet? Still, if that were the case I should think a bit of choke would help.

 

[Splangy]

Check the external linkage for the metering rod at the base of the carb. In fact, post a picture of it here. The steel plate is prone to failure by the shaft hole wearing out. It’s possible it’s not lined up properly or it’s stuck in a weird position.

1963 should be the earlier straight rod.

Edit: just read the other post where it says Mark rebuilt it. I’m sure he would have noticed if the plate was buttered up.

 

[65swb45]

Arne

There is a small piece of linkage under the fuel bowl that connects the throttle butterfly to the accelerator pump (and thereby the metering rod.

The linkage has a kink in it halfway between top and bottom. Slightly straightening out the kink will draw a thicker part of the metering rod into the orifice, thereby leaning it out.

Make very small adjustments. Very small. For reference, when I make float adjustments, I do it a 32nd at a time .

 

[oregon fj]

No offense to carb-savvy people like Mark, but I hate carbs!

OK, I removed the kinked link, measured it with my calipers, and straightened (lengthened) it by 1/32". No improvement. Did it again, total change 1/16" longer. No improvement, in fact seemed to be a touch worse.

So ok, maybe I'm an idiot and don't know rich from lean. So I shorten it a total of 1/8", or 1/16" shorter than the original. Still no improvement.

What am I missing here? Is there anything else that could affect off-idle running? Idle is nice, and runs strong and clean accelerating with larger throttle opening. But any cruise under light throttle is nothing but stumbles and such.

 

[Pin_Head]

I don’t know the specifics of the SD40, but generally, metering rods meter the main jet which isn’t involved in off idle transition. The transition slot just gives it a blast of fuel as the throttle plate opens until vacuum starts pulling atomized fuel from the main nozzle. It only works for the few degrees of throttle opening. My experience is that carbs can be really rich without suffering much in performance (short of dumping raw gas out of the bowl vents).

 

[oregon fj]

Well, I'm not afraid to admit that I'm no longer sure that it is rich. I know that pulling the choke does not help, so it wouldn't appear to be lean. I don't want to flail about and possibly make things worse.

I can duplicate the problem somewhat now in the driveway with the hand throttle. Will work tomorrow afternoon through the ignition side one more time before tearing into the carb.

 

[Pin_Head]

Strange things can happen with a sticking centrifugal advance. Sometimes you can feel it unstick and start running better instantly.