The Stovebolt Site Forums The Shop Area Engine Shop Zenith 10920 in my GMC 228

I’ve been battling a rough idle but believe I have appropriately addressed everything that could be a problem. At low idle it seems very close to right for the most part but the speed is not steady, it “surges” for lack of a specific description.

My shop manual says set 228 flywheel pointer to the | mark between U and C but the posts I’ve read about any other stovebolt say engine runs best with pointer at the BB. Which is correct for a 1950 228?

When I remove the distributor cap I can move the rotor back and forth slightly. Is this normal or do I have a worn shaft pin?

to quote Carl52, "It's never the carburetor"

But I had an 80 Chevy Citation that would surge going down the highway right after I bought it new. Took it back to the dealer and they finally adjusted the float level, which fixed it up.

There would normally be a little play in the distributor drive, so it's unlikely the shaft pin is worn.

Thanks, I have seen that mentioned many times but honestly don’t think it is the carb. Of course I could have overlooked something. The Zenith 10920 carb is freshly rebuilt and I have gone back through it to be certain it is properly assembled and as clean as a whistle. What specific carb issue might be suspect?

1951 GMC Maintenance Manual says to set timing on the BB at 350 rpm (mechanical advance starts at 400) with vacuum advance disconnected and the "octane selector" set to zero.

Thanks Bill.I now have it set on the ball but it made no difference.

KL may have hit the nail on the head unless I have a bad vacuum leak I can’t find.

I removed the filter and blocked off 3/4 of the air intake with my hand. When I do this I can set the idle way down and it idles smoothly.

Can float level cause my problem?

Short answer. Yes.

Erratic idle can sometimes be attributed to a flooding issue in which fuel is over-topping the bowl.

When the engine is shut off, can you open the throttle and see if there is any fuel sitting on the intake manifold?

Can you look down the barrel at idle and see if any fuel is piddling from the jets?

Otto, thanks for the good ideas but I tried and can’t quite get a light and my eyes lined up well enough to see.

I’ll check float level next and provide an update. That’s the only thing in the carb I haven’t triple checked.

Try taking a video with your phone. It is easier than trying to line up your eyes and the light.

Check the distributor shaft for side to side play. The shaft runs against the cast iron housing without bushings, and there's supposed to be a spring and a phenolic thrust button behind the grease cup in the housing to keep the shaft pushed against the side of the hole it runs in. That spring can wear out or the grease can get solid, allowing the shaft to wobble and change the point gap. That alters the timing between cylinders and causes the idle speed to "hunt" instead of being steady. Richening up the mixture by restricting the intake and getting it to smooth out sounds like you're dealing with a small vacuum leak. Try moving an unlit Propane torch around the carburetor base and the intake manifold. If the idle evens out when the gas finds the leaky spot, fix it.
Jerry

If obstructing the inlet is better, the mixture is lean.

Thanks Jerry, since it idles smoothly when the intake is partially blocked wouldn’t that eliminate the distributor to be the problem? It idles beautifully when I block it. I have no propane handy but I plugged the wiper vacuum port on the intake manifold and sprayed carb cleaner around it, the carb gaskets, vacuum ports, vacuum advance and manifold gaskets. Could not locate any leaks.

I removed and inverted the air intake body to check the float level as best I can with a caliper and it looked like it was too high per the 1.484 +/- .031 spec in my manual. I adjusted it down as close to 1.5” as I can get. This did not help.

Thanks Twin Jimmy, good idea. I was able to get a video. I don’t know what a “proper” operating carb would look like but the fuel flow is not a smooth running stream and I see an occasional splash onto the throttle plate. I think that could explain the surging behavior but is it the cause or the effect from something else?

This problem is kicking my butt and I wouldn’t hesitate to spend the bucks to fix it, but shotgunning can be quite expensive!

What to do?

Sun Electric Co. used to have a slogan they put on all their test equipment- - - - -"We Test, Not Guess". It's as true today as it was 60+ years ago the first time I saw it.
Jerry

I think the fuel you see splashing on the throttle plate is the cause not the effect.

Check your fuel pump pressure. Is it stock mechanical or electric?

Your carb could also be leaking internally.

When the engine is cold, look down into the carb and open the throttle. Is there fuel sitting on the intake?

You could have a leaky needle and seat.

Otto, my pump is stock mechanical and new. I don’t have a pressure gauge but the bowl holds pressure and fuel squirts vigorously when the fuel line or carb filter head is loosened. I can visualize a sticking needle to be the cause and will check it next. When I pull the carb I’ll be able to see in the manifold.

Jerry, how would you check the float level? A height gauge would be ideal but there’s no way to manipulate a caliper to contact the machined surface and the float.

Thanks to all for your inputs. We have company coming today but I’ll be back on it tomorrow!

50G, Take off the air cleaner and look down the throat of the carb while holding open the throttle with your hand to see if there is gas down there. (engine off)

I set the float level when I rebuilt my carburetor by using a drill bit as a gauge.

If there is something stuck in the needle and seat, you might try putting a few puffs of compressed air into the carburetor inlet. I have done this on two occasions to dislodge debris that became stuck in there causing the carburetor to malfunction.

A vacuum gauge/pressure gauge is a pretty inexpensive and useful tool. You can get one at the local auto parts store for about $20.

Don't rely on the fact that your fuel pump is new to assume that it's working properly. I had to replace two brand new Carquest pumps recently because one failed after a few hundred miles and the second one wouldn't pump a drop straight out of the box. I ended up having my original glass bowl type rebuilt.

The 216 requires 2-4 psi fuel pressure. Not sure what your engine requires but if the new pump is putting out too much pressure it could lead to the problems you describe.

Changed thread title since I’m convinced my issue is in the carb.

The intake manifold is not exactly dry but there is no pooling. I stuck the corner of a paper towel in and had a tiny bit of damp on the very tip.

Removed needle and seat. They look clean but I sprayed them and the passage with carb cleaner and blew them out with compressed air.

I mentioned I could see some dribbling into the secondary venturi and some occasional splashes down onto the throttle plate. I do think this is likely the root of the surging behavior. I’m not knowledgeable enough to know if there should be any fuel at all through that location at idle, but I have to keep the rpm high enough for the engine to keep running.

The drill bit trick may allow me to get a good float height measurement. I’ll try it. However it appears the float is above spec and if so the bowl fuel level should be below the secondary venturi outlet.

Float level spec in my manual.

How can a 1-barrel carburetor have a secondary venturi? If fuel is dribbling out of the venturi, either the idle speed is too high or the float level needs to be lowered a little. I make an L-shaped float level gauge out of thin cardboard like the backing board of a note pad, or maybe a piece of index card with the height of the float on the long leg of the "L". That's how it's been done for the 60-something years I've been rebuilding carburetors if there's not one included in the repair kit.
Jerry

Thanks for the tip. My screwdriver in the attached pic is on what the manual calls the secondary venturi. Below it is a larger dia “main venturi”.

OK- - - -the smaller diameter venturi is the one that begins to flow fuel just above idle speed. All it takes is a couple of hundred RPM above the idle speed the engine is supposed to run to start fuel flowing through that one, so find the vacuum leak, fix it, and get the engine slowed down enough to make the idle system actually work like it's designed.
Jerry

I could probably disassemble and reassemble the carb by now! Float is set right per the manual. I cannot find anything wrong with the carb. I have no vacuum leaks and it runs like a top when I block off the intake. I don’t know what else to do. Ignore the spec and lower the float? Rebuild with another new kit?

Then it's too lean- - - - -find out why and fix it. It's possible you've got a restricted idle passageway. How are you cleaning the carb during the rebuild process? Boiling water and Dawn dishwashing liquid to soften the deposits and massive volumes of high pressure air through every hole in every casting is the only effective way to be sure they're clear.
Jerry

I have a gallon of carb cleaner to soak the parts before using a carb cleaner spray can followed by compressed air. I haven’t removed the check valve and ball since initial rebuild. Can that affect the idle?

I have plenty of Dawn. I’ll dig out my Coleman stove and get some water boiling next.

WWhere did you get the new parts for the rebuild? Could the metering rod be incorrect?

Generic rebuild kits often have needle and seat sets that are not up to spec for the specific application but rather a part that "sort of works but not quite" for a variety of applications.

Carburetor Doctor supplied the kit. There is no metering rod in this Zenith. The needle looks good using a 100x microscope. I’ve cleaned the heck out of the idle passageways and can see light thru them but maybe there is something restricting flow. I can boil the castings but there is a ball check valve in the bowl casting that can’t be removed without damage. I have nothing to lose at this point though so I’m going to leave it in and boil the castings and hope I can flush out any water residue with carb cleaner and compressed air. I live in the boonies but will either install my backup fuel pump or pick up a gauge next time I’m in town if this don’t help.

I read the entire thread twice, and did not see this; what is the setting on your idle mixture control screw?

Jon

Jon- - - -shame on you for inserting logic and common sense into this thread!
LOL!
Jerry

Proof that my wife loves me!

Well it idles much better now but the speed varies a bit and I still have some fuel feeding into the secondary Venturi. If idle adjusted low enough for the engine to be on the verge of dying the fuel no longer feeds into the Venturi. I wish someone with a “perfect” idling Zenith could take a video to see if any fuel fuel enters the Venturi.

So, it is much improved but it seems something else needs attention but I don’t know what is.

I’m questioning the float level spec in my manual since I had to adjust the float to a higher bowl level in order to meet the spec.

Do you good fellows have any more ideas?

FYI I don’t know if this could contribute to the idle speed variation issue I’ve been chasing but my truck has been driven 10 or 15 miles at most since the engine was completely rebuilt, so it is very tight.

Answer Carbking’s question please. He is our resident carburetor expert.

I see you're on that high-carb diet.

... or isn't at home.

Sorry for somehow overlooking that carbking replied and had even considered calling him today!

I adjusted the idle mix for max vacuum at the intake.

With a fresh engine, try re-adjusting the idle mixture screw turning in until very lightly seated, then opening about 1 and 1/2 turns. Once the engine has a thousand miles, you should be able to re-adjust to just 1 turn.

Jon.

I spoke to carbking who advised this for my fresh engine:

Adjust the mixture to 1-1/4 to 1-1/2 out instead of max vacuum.
Fuel entering the Venturi at idle is not unexpected.
Hunting for stable rpm at idle is more than likely since the engine is tight - set speed 100-150 rpm above normal and drive.
After break-in the idle speed can likely be reduced to normal and the hunting for a stable speed may no longer occur.

Thanks Jon, Jerry, Otto and all of the others who have helped.

I’m off my high carb diet for now!

In 60+ years of building engines, I've discovered that there's a bit of tinkering necessary between ignition timing, idle mixture, and idle speed that will eventually find the "sweet spot" and it's not necessarily right on the factory specification. A new, tight engine won't always run smooth until it's been run enough to break in properly. That can take up to 500 miles on the road at times. Have fun!
Jerry

500 miles will take a while at 40 MPH!

Thanks for sharing your knowledge!