Carb difficulties (Zenith) on a IIA

[hackneyed1]

Okay I need some help here. Here's the back story. I have a 1962 IIA SWB with a Zenith carb. For one reason or another the truck sat untouched for 2 months, but I've recently calculated I last put fuel in the tank 6 months ago, so it was rather old and varnished. Tank & lines drained, fresh fuel added. The fuel pump wouldn't pump. Pump replaced. Truck would then start and run but die out (as in starving for gas) after about 20 seconds. After about a minute, it would start up again and run for another 20 seconds. I pulled the carb off and went through it. It wasn't horrible, some dust/dirt/debris but the worst thing that I found was the large O-ring at the throat of the carb to be rotten. After reassembly the truck will now start and idle happily for however long I want it to, but as soon as I move in any direction and take my foot off of the gas, it refuses to return to idle and instead stalls. It will restart (sometimes with difficulty) and again idle happily. Until I move it. I suspect that there is something amiss with the fuel mixture screw (the idle control screw) since turning it appears to make no difference. If I take out the scew completely the truck will continue to idle for a while before eventually shutting down. I can feel a vacuum being pulled through the passage. I've looked at a spare zenith carb that I have (that also needs rebuilding) and the idle control screw just appears to open/close and air passage into the carb throat, which is not clogged on my carb. Oddly my screw has a channel cut into its threads, and that channels appears to be filled with with something intentionally. The spare carbs idle control screw does not have this channel, but it is also a different size/thread pitch. Could I have my idle too high (via throttle stop screw) to allow the mixture screw to have an effect or......
The truck seems VERY cold natured...even after idling for 15 minutes, it'll bog on throttle input unless the choke is feathered on.

[I Leak Oil]

I briefly had a zenith on my 2A after my original weber died. I wasn't happy with the zenith at all! Mine would also not idle. When I dug into the carb, one side of the float looked like it exploded!
The float was rubbing on the side of the carb body, not allowing it to move freely.
Obviously, yours isn't to that point or you would have noticed while it was apart. Did you check the float level? Made sure it moved without obstruction?
Then there's always the well documented zenith warp issue.....
I went back to the weber 34 and couldn't be happier. Easy to tune and stays that way.....
Jason T.

[Jeff Aronson]

You've mentioned a dissolved o-ring and a high idle adjustment; both lead me to suggest that maybe the issue is inside the carb.

Carbs have many tiny orifices that are not immediately visible. I run a Weber and a Solex on my two II-A's, but what I've noticed in both is that they're more complex than just the idle and mixture screw openings.

If you've not removed the jets in the carb and used air pressure to clean them out [either an air gun/can or spray carb cleaner, you might still have a bit of that dissolved o-ring stuck around one jet. The jets are usually in the bottom of the carb bowl and can be unscrewed. Clean them up and spray air/cleaner into their respective holes. Until you take the carb off the car and turn it upside down, there are orifices you won't see at all.

I've had similar problems to yours with the Weber and the problem has always been in the jets and/or their orifices being plugged up. It takes only the tiniest of particles, small enough to go through filters, too.

Good luck,

Jeff

[hackneyed1]

I did take it off and take it apart....the O-ring that I reference was the one between the top and the float bowl/bottom. I blew through all of the passages and jets...Of course I could have missed something. Of course I suspect the idle mixture since it seems to make no difference when I turn the screw. My gearhead friends have suggested a float issue but since the carb will idle and throttle fine (even wide open) I don't think its a fuel flow issue. During the rebuild, the only issue on the rebuild that I had was the badly made paper gasket for the top and bottom half....the four screw holes just do not line up!

[I Leak Oil]

Did you check it to see if it's warped?
Jason T.

[hackneyed1]

Jason- No I didn't check to see if it was warped, and at first I was simply going to reply that it couldn't be warped but then I read all about the infamous zenith warping and the associated results and it sure sounds like that's my problem. From sitting. gotta love it. Since I'll be back in there and alot of places mention it, what should the setting for the float height be (another thing that shouldn't have changed while it was parked, but....)?

[gudjeon]

A thing to try (sorry if you'd already done so) is unscrewing the jets themselves to make sure they are clear. It sounds like the low speed jet is plugged. A manual or a sticky post on the Zenith should provide you with the necessary diagram to ID it.

These Zeniths are not that complicated as far as carbs go so you have to be a bit more persistent than they are.

[Les Parker]

Zenith

Check the Zenith rebuild link:-

http://www.roversnorth.com/forums/showthread.php?t=3121

There is some good info contained there and some good tech. tips, too.

[mechman]

Sounds like a classic Zenith problem, especially given the dissolved O-ring and garbage in the bowl. You're probably having accelerator pump issues. Your main circuit is not getting fuel, at least not coming off the idle circuit.

The accelerator pump is the aluminum piston that lives under the top and aims downwards. If you pull it out, there are two issues you will probably notice. First is that the piston has many fine scratches down its length. These need to be polished out to a shine with a fine piece of crocus cloth, then gently polish its bore, too. The scratches can cause the piston to bind in the bore, making it stumble off idle.

Second is probably your primary problem - look into the pump bore and you'll see a small ball bearing in the bottom, held in with a wire spring clip. This ball bearing tends to stick to the port that it is supposed to regulate, causing no fuel to pass when it is needed. Use a straightened paper clip and a good shot of carb cleaner to gently nudge the ball loose under the clip, but DON'T REMOVE THE CLIP!! They're a real bugger to get back in. Pull your mixture screw and clean the carb body obsessively, use the wand attachment on the can of carb cleaner to blow out the ports and passages, then reassemble it carefully.

Zeniths are decent carbs if they're clean and properly set up. I've built them back from scrap parts on MANY occasions. I prefer them to the Webers, Series trucks just seem to run so much better with Zeniths.

Mech