Newbie carb questions

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Jun 19, 2012
I've lurked here for years, but only just joined up to ask questions. I have a mostly 1984 '60. This spring, it was the carb's turn to fail-- the truck bucked and surged, wouldn't idle stably, wouldn't idle below 1200RPM, spewed black junk out the tailpipe, and made a stench like something running on way too rich a diet. Knowing this was coming, I had on hand a rebuilt carb from (they would sell me one without a core, so I was not under pressure to rebuild in a hurry).

This weekend, I finally swapped the rebuilt carb on and replaced all vacuum hoses attached to the carb, and whatever else I had enough hose left to replace. I also scraped all the crud off the PCV valve and replaced it. When everything was all put together, I started it up (it went right to 1500RPM) and I did the "lean drop" method (in another thread), and got everything to stabilize at the right RPMs (650). Because my cruiser doesn't have A/C and never did, my carb is slightly different from some of the diagrams I've seen, but I figured that the "idle speed adjusting screw" was the one farthest from the firewall (and most difficult to get a screwdriver on).

Once this was done, the RPMs were stable and the exhaust was invisible (and not smelly).

First question: How do I set the fast idle screw? I'm assuming that I need to set this with the engine cold and the choke closed. Is this correct? What RPMs am I aiming for?

Next question: I'm not entirely happy with the result-- There seems to be a slight hesitation when I press on the gas in any gear above 2nd. Nothing serious, but it just doesn't seem quite right. It's more like an almost stumble. Before, when things were right, I could just idle around the block in almost any gear with no complaints, and I could accelerate with no stumbles. Is this indicating that I didn't do the lean drop quite right?

I would love any advice-- there seem to be a few possibilities:

1. I had to disable the carb jet valve on the side of the carb, because it leaked gas right down onto the manifold. I haven't been able to find another jet valve, so it's disabled (lever removed, plunger down, and encased in some permatex stuff). It's the same carb jet valve that was on the old carb-- I swapped them over, because the "rebuilt" one leaked, too. However, I don't usually press down hard enough on the gas to even open this valve (this is a tractor, not a racer), and the old carb, with the same disabled valve, didn't cause stumbling. Why would the exact same valve behave differently on two different carbs?

2. I had to do the lean drop method with the air box off. Yes, I tried with the box on, but I just can't think backwards or in a mirror, and I feared that I was tearing up the idle mixture screw by hunting for it by feel. I suspect that if the engine is running a little too rich, it could stumble like this, and the extra air resistance of the air box might cause it to run a little rich. Would it be in order to try leaning out (turning in) the idle mixture screw, say 1/4 turn? Or is it better to start over?

3. The plugs, at least, must be a real mess after all that rich running. I didn't have a chance to pull one out and look, because I'm just assuming that I should go ahead and replace them, and I didn't have any on hand. Probably also the wires, as well, because they look ancient. However, the plugs and wires haven't changed since I did the carb swap, so while I think they aren't helping things, they can't really explain the stumble.

The other thing I noticed when I had the carb off is that the valve cover gasket has clearly seen better days. Plenty of gunk oozes out onto the "wing" of the plastic carb spacer, and since this wing has a big crack in it, the gunk works its way right down onto the exhaust manifold. It's not a huge leak yet, but when I had the RPMs up for the lean drop, there was clearly a little smoke, as the engine heated up and more gunk squirted out and burned off. How difficult is this gasket to replace, and how fearful of the air rail should I be as I work in this area? The air rail looks like it could be a nightmare to work with.

Thanks for any advice you have!
I do the lean drop with the air cleaner off. I think what you need to be careful with is to plug (or keep connected) vacuum lines that connect to PCV valve T or the distributor flush VCV (these are components on the passenger side of the engine compartment, assuming your truck is left hand drive). You don't want to have any vacuum leaks while doing the lean drop method.

The valve cover gasket is really easy to replace. it is a rubber gasket--more like an o-ring--and should come off without any scraping, etc. Once the air cleaner is removed, it's just the four bolts holding the valve cover on. You may need to move some wires or hoses out of the way. You should not have to mess with the air rail at all.

You should also do the "paper clip" test to make sure the secondary vacuum diaphragm is working properly.
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No problem keeping the A/c off! My truck must be just about the only one sold in the US without a/c!

It's definitely not right.. I drove it today, and it hesitates and stumbles about about 2000 RPM.

Looking through the threads, the symptoms sound like a classic EGR valve problem. Not sure why that would happen all of the sudden, except that for a while, the exhaust must have been pretty awful, of the sort that could clog I'm going to try folling with that.....

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