Chinese carb revs out of control!

[stuartrubin]

I installed the a new Chinese carburetor (ADF 34) and things were looking pretty good in the garage. Other than not being able to get the idle smooth at a good speed, it seemed to run fine, so I took it out for a test drive. For about a mile everything was running fine until suddenly the RPMs shot up over 5000 and would NOT go back down!

Turned off the car and checked under the hood and could not see anything obvious. The throttle was back in its idle position and moved smoothly. Nothing looked disconnected. Whenever I start the car, the revs shoot way up to 5500 or so.

Any ideas? Winter is coming to Cleveland and I'd like another day to ride...

Thanks!

Stuart
"Il Mostro Di Frankenstein"

[bradartigue]

Your throttle plates are staying open. Nothing else can cause 5000+ RPM - the throttles have to be open. You can't generate enough airflow otherwise. So it sounds to me like the carb is doing its job, but something in the linkage is slipping.

I know you have a non-standard throttle linkage, did the barrel shaped thing slip, leaving the throttles open but the linkage slack?

[stuartrubin]

Brad, I did install the barrel" thing (bell crank?) and as far as I can tell, it's not slipping or sticking, at least on the outside of the carb. I suppose that it's possible that when I took off the "ball" linkage part and put the barrel on that I loosened something up, or possibly over tightened it.

So, to confirm, at 5000 RPM, it can't be something like the fast idle, right?

Tomorrow (when the engine has cooled down!), I'll look for slippage and sticking. Someone in another post suggested removing the cable to get it out of the way, too.

Thanks for the tip.

Stuart

[707kevin]

I just got one of these and the lever that actuates the secondary throttle plate was bent a little so that the secondary would not close all the way every time. double check that all mechanical actions are 100% free and smooth.

A few others have reported that some adjustment screws vibrated loose right away, so double check all that as well. The stop screw for the secondary, the idle speed screw, and the choke idle speed screw could all do something like this. Something is open, somewhere.

[johnsje6]

Definitely look at the secondary - mine did the same thing, and it was the secondary that was staying open, nothing to do with the idle or linkage, was the set screw that sets the closed position on the secondary. It is located on the base of the carb on the linkage side.
Photos are here - http://www.fiatspider.com/f15/viewtopic ... 1&start=30

[stuartrubin]

Right on, John J! It WAS that set screw. Just as you described, it was just wiggling loose. Strangely, it "falls" down as it shakes, but I sort of would have expected it to "push" up (out) because of the tension on it. I think that when the pedal is held down and the secondary throttle is properly held open, the screw rattles down, then keeps it from going back up. Regardless, I unscrewed it back into position, it ran fine for about 5 minutes, and then the RPM want to about 3500. Sure enough, back in the garage, the screw was loose.

It's a good idea to put some adhesive to keep it in place, but I'm not sure exactly to where to set the screw before sealing it. Clearly the screw is fixing the opening amount of the secondary throttle during idle, but I don't why you would want it open at all! There's not a lot to go on here since the original Weber 34ADF has no such screw. I didn't test this, but I'm wondering if you took the screw out completely if it would stay in the closed position, or just open in the other direction.

Separately, I was not able to get the idle adjusted correctly, but I believe I have an air leak at the base of the carb spacer to the intake manifold. If I screw in the idle mix completely, the idle never dies. I sprayed some carb cleaner in various places and noticed that when I a certain spot, the speed/quality changes.

Other than then plastic spacer, should there be some kind of gasket(s) here between the manifold, spacer, and carb base?

Thanks everyone for their input.

-Stuart

For reference of other posters, here is a photo of with the troublesome set screw (from John J's post).

[bradartigue]

You do need gaskets between the carb and spacer and the spacer and manifold. These are paper (typically). I soak them in oil briefly before installing. Some of the FIAT OEM ones were rubber and you can use cork as well. Makes no difference what the material is, but you need something to make the seal.

[stuartrubin]

Gastkets and a new spacer have been ordered.

Looking back, I could see that the old spacer was a little deformed and took the shape somewhat of the carb. I'm thinking that the previous owner just squished the crap out of it to get a good seal, which he never quite got.

Thanks

[707kevin]

Is there a situation where you would want the secondary to never close all the way? Would it be OK to just remove the secondary stop screw?

[bradartigue]

Is there a situation where you would want the secondary to never close all the way? Would it be OK to just remove the secondary stop screw?

Without the stop screw I think you run the risk of the throttle plate slapping the aluminum, eventually grooving it.

[707kevin]

Is there a recommended measurement for that? A 0.xx mm feeler gauge in the throttle plate and set the screw with locktite?

[stuartrubin]

The baseplate and gaskets came today, but I was surprised that at the material of the gaskets. They are some sort of hard platicky/metallic/carbony material (I can't put my finger on it). It seems like they would make very poor air seals against the baseplate, manifold, and carburetor.

The baseplate is paper-backed (as Brad said they may be). Does this mean that I do NOT need the gaskets? Can I just soak the paper in a little oil and mount them? What's with these stiff gaskets?

The gaskets are http://midwest-bayless.com/Fiat-124SC-0 ... a-new.aspx

And the baseplate is http://midwest-bayless.com/Fiat-124SC-0 ... a-new.aspx

Thanks!

[bradartigue]

I'd ask the vendor. The description says "two required per side" - which can't be right (doubling up gaskets is a sure way to create a leak). I think they mean one per side, two in total. From the picture that looks like the kind of medium-hard rubberized thing that you'd use if you didn't have paper gaskets. I don't think you need both, but I've never had the opportunity to have both.

[btoran]

mine had the hard plastic thingie in the center and then one gasket each side of it (top and bottom).