Carburetor Tuning Questions

I'm struggling with some carb symptoms that I can't seem to explain. At idle, thee engine is running lean. It pops violently on overrun, idles a bit hot, and hovers a little high before settling back to idle after blipping the throttle. It responds well enough to a quick throttle blip. The problem I'm having is that when the throttle is initially cracked (rolling into it slowly), it goes pig rich. It is bad enough that if you go to about 5% throttle and hold it there, the engine will die. A little bit more throttle, and it's fine. This is in an application where that slow roll on range is incredibly important.

The problem I'm having is that every change I make overlaps and may fix the lean idle, but then the stumble gets worse. If I lean the idle out more to make the stumble go away (air screw adjustment) then the thing will barely idle. Timing and points are set properly. Raising and lowering the needle has no effect (as expected).

Adjustments on this carb are float height, needle clip height, idle air screw, idle speed, pilot jet, and main jet. It runs fine at higher rpm and load, so I think the main and needle are generally okay. It is the factory carb for this application, and factory settings do not work. The carb is very clean internally, though it is 40 years old.

Any idea what would cause this, or how to tune it out? I've spent about 8 hours this week screwing with it with no success.

Vacuum advance? Is it hooked to the correct 'port' on the carb? Does the vacuum advance canister hold vacuum?

You say the timing is good, but have you checked for a vacuum leak? That is what I would investigate next. At higher RPMs the vacuum leak is irrelevant because there is so much air moving through the engine, but at idle it will be most pronounced both because of the high vacuum and the overall less airflow.

What carb are we speaking of? Can you confirm the following as being in good order?

Timing?

Valve lash?

Plugs?

Points?

Compression?

Wires?

Vacuum?

What all has been done to the engine? Big cam?

Most carbs that I have messed with require that the tune be done from the idle up to the highspeed circuits. This ensures that the transitions happen as they are meant to. What you describe sounds like your idle and transition circuits trying to do their job at the same time. If your carb wont tune "As per the book" go looking for engine problems.

In a lot of cases, the throttle adjustment screw is being opened too far in order to get the engine to run right when the timing, valves or some other issue is compromising the situation. Or as the Brits like to say "Most Carb problems are ignition issues"

No vacuum advance on this carb. Vacuum leak is an interesting idea; I actually haven't checked that yet. Can't believe that skipped my mind...I'll check when I get home. Now that you say that, there is an o-ring in the intake that could leak...hmm.

NOHOME, responses to your questions are below.

What carb are we speaking of? Keihin PE24. Round slide side draft carb; very basic and very old.

Timing? Set to factory specs with a timing light. Verified mechanical advance works. No vacuum advance.

Valve lash? Set correctly to the loose side of spec.

Plugs? New, properly gapped, but very dark

Points? Gapped properly, and filed. Good contact, no arcing.

Compression? Good, and does not burn oil.

Wires? Old. Could be an issue, but also NLA so I would need to make replacements.

Vacuum? No good way of measuring.

What all has been done to the engine? Big cam? Mildly hotter cam according to the previous owner, but I don't know that I believe him. Otherwise stock.

Most carbs that I have messed with require that the tune be done from the idle up to the highspeed circuits. This ensures that the transitions happen as they are meant to. What you describe sounds like your idle and transition circuits trying to do their job at the same time. If your carb wont tune "As per the book" go looking for engine problems. In a lot of cases, the throttle adjustment screw is being opened too far in order to get the engine to run right when the timing, valves or some other issue is compromising the situation. Or as the Brits like to say "Most Carb problems are ignition issues"

Hmm. The bit about the idle screw adjustment makes a lot of sense, in that it could be getting into transition jetting too early. Idle speed is correct per factory specs, but I may be able to advance the timing some and back off the idle speed adjustment.

Get a can of carb cleaner, get the bike running and spray the rubber boot manifold and see how drastically it changes how the engine runs. I suspect you're going to find a rather profound change.

I also suspect something else amiss, like a vacuum leak.

That said, this a good tuning guide. http://www.keihincarbs.com/tips/gate.html

"What carb are we speaking of? Keihin PE24. Round slide side draft carb"

Yeah, OK...I'm out of here!

It is constant velocity like an SU or more like a real carb?

In reply to NOHOME:

Take a SU, remove the butterfly throttle and the stuff that controls position of the slide, attach throttle cable to slide. Pretty common type of motorcycle carb.

No vacuum leaks. Kenny, that's a good guide. Different carb, but the same things apply.

NOHOME, it's not a CV. The throttle cable acts directly on the slide that holds the needle, rather than that slide being controlled by vacuum. CV carbs in off road applications tend to fare poorly because the vacuum-actuated bits will bounce as you go over rough terrain. It can get quite exciting having wildly nonlinear power in those situations.

I've talked to a few oldtimers and have gotten more advice to look harder at the ignition system. Going to check the grounds and replace the plug tonight. I was also advised to try a hair more base timing advance. The only time I have gotten the stumble to go away was once when I forgot to turn the fuel on and it leaned way out as the float bowl emptied, and the other time when I set the float so low that the fuel valve never opened, again resulting in a very lean condition.

Thanks for the advice, guys. I know it isn't the typical automotive application, but a carb is a carb and Sprockets doesn't get much traffic, heh.

other than a vacuum leak in the manifold, is it possible the carb could be leaking air in somewhere? 40 year old carburetors have wear, and wear causes leaks.

In reply to volvoclearinghouse:

Yeah, I think it is possible, but unlikely. Though it is 40 years old, it was never ridden much. The slide still has the original anodizing with no visible wear at all, for example, and the gasket that seals the cap on the carb is still pliable. For kicks and giggles at one point, I left the cap of the carb completely off. This should have created a huge vacuum leak, but made only a minimal difference. That said, it did make the issue slightly worse.

OK as I have a set of keihin FCR carbs on the Datsun as well as one on the Beta I will chime in.

First if you turn the bleed screw all the way in does it kill the motor?

Next does it idle better at less than 1 1/2 turns out?

You mention a possible cam change and I suspect the problem is that particular camshaft has a good bit of overlap. If this is the case you are going to need to go down on the pilot jet.

I have two motors for the Datsun; the 1200cc motor has a cam with 104 degree lope centers 286 duration 460 lift, the 1500cc motor 108 degree lobe centers 278 duration 420 lift. The larger motor requires going up on the mains but it will still run whereas the smaller motor with the 104 degree centers behaves exactly as you describe until I change the pilot jet (in this case two steps leaner).

Tom