And wet.

 

So what does 5 turns out on the screws indicate? Vacuum leak? Set timing to 10 degrees. Haven't driven it yet, if it starts pinging I will go to 8.

 

I would not worry about 5 turns, so long as they were making a difference in idle quality. These carbs are emissions carbs, they have very fine threads on the idle mixture screws, so I would not worry about 5 turns. I would worry if they do not have any affect on the idle, that is a sign your idle speed may be too high.

 

Thanks for the info. My old Dwell/Tach meter is in a box somewhere with my vacuum gauge. Just finishing up a new shop building and I am not quite all moved in yet. Had everything set, I thought, until I sat in the cab and idle seemed quite high. Got that lowered down some and it seems to run better. Still a bit of a miss it seems but I haven't heard one of these run before so maybe it is what it is. I will drive it some this weekend and do some more tweaking. There are A LOT of 38 year old vacuum lines that can be causing issues. Will work on desmogging as I go. If the truck is sound overall then there will likely be some tmeyer orders being made over the summer.

 

I removed all my vacuum lines tonight, old rotten things. I have the dist hooked to ported source from front choke side of carb. What source do I use for trans? Ported or constant? Fuel screws are at 5 turns and still will change idle quality. Getting approx. 17" vacuum at idle. Engine seems to be lean at partial throttle-while sitting, not driving it as of yet. Is this due to the lean stock jetting?

 

If you know anyone that plays guitar, the excess string clipped off and discarded when restrung, are perfect for poking out carb holes. I'm back onto using Pine Sol again for carb soaks after using straight gas, chemtool, and spray can cleaners. I don't know what it is about Pinesol, but it seems to work better than anything but does take about a week to soak. I also use gun brushes on various carbs at times.

 

Jetting has nothing to do with idle, it's strictly the idle circuit. In fact you could remove the jets entirely and drive around the block. Although it's called the idle circuit, it's in play probably up to around 35 mph or so. That's why it's important to get right, because on a street engine most city driving is done in this zone. Idle and just off idle. The pump shot or accelerator pump carb circuit provides extra juice to prevent bog or hesitation.

The idle is supposed to be fairly lean, or only slightly rich. If it isn't, the idle quality will suffer and the plugs will "load up" after a few minutes. 17" Hg is a little low, if you're at sea level. Not too bad, but it's worth investigating. Correct idle mixture adjustment is probably worth another full point. Want a crisp, steady idle that doesn't start to wander.

Idle mixture screws start out with a rough bench setting, say 1.5 turns out, and get turned in clockwise (lean) slowly and evenly. This is the whole point, to lean the fuel mixture at idle. The idea here is that the throttle butterflies are nearly closed, and only a sliver is exposed at what's called the transfer slot. So start with the idle itself real slow, and turn the screws in, the idle RPM should pick up, this is how to get a crisp smooth idle. "5 turns out", something is wrong, and spark plugs will get fouled out. If you turn one screw in all the way, the engine should start to stumble and stall. Just slightly richer from that point is where both screws need to be.

 

if you have vacuum leak, you have to richen things up significantly to try and mask it.

 

Coming from more of a motorcycle background 5 turns would be considered too far out but then you have pilot jets that can be changed. I have not checked transfer slot, will do that tonight. I thought that you set screws by either a) backing out until stumbling, then back in until stumbling, then setting mid way between those two points, or b) set to highest manifold vacuum. At 5 turns I can still get immediate effect but turning 1/2 turn either way. I only have ported vacuum hooked up to dist and my vac gauge hooked up to manifold port so there shouldn't be any leaks. I will test with brake cleaner tonight. I am thinking maybe I need to go through that carb again. We used to use purple cleaner to clean carbs at the bike shop. That stuff is nasty, it will remove anodizing. BTW, yes to sea level, house is probably 250'-300'.

 

The initial bench setting is just a ballpark adjustment set out far enough so that the engine will start and idle so the fuel mixture can be trimmed, backing them out farther just makes the idle mixture even more rich to a pig rich condition. They are always turned in clockwise (lean) unless it's one of those goofy smog carbs. And they are a precision cut needle & seat. If you have an O2 sensor it shows turning even 1 mixture screw just an 1/8 or 1/4 turn will change the AFR a whole point or more, if they need to need to be adjusted that far out there is a problem somewhere else.

The key overall is every circuit is tuned independently from the other in isolation, because the errors will stack up and then the mechanic is trying to compensate for that by adjusting something else off spec downstream to make up for it. The most important adjustment if you had to choose one is probably the float height, or actually the resulting fuel height in the bowl. The carburetor is calibrated around a specific column height of fuel in the bowl and fuel well, all the holes and emulsion tubes and air idle bleeds and high speed air bleeds and the rest of it is affected by this. Too high and it will run overly rich or even flood, too low and it will run lean, and either way screws with proper jetting selection.

So first check that the fuel pump pressure output is within spec, your vacuum gauge will measure this. Then measure the resulting fuel height in the bowl after idling a few minutes on level ground, the float height is again just an initial rough bench setting, the fuel height must be checked after carburetor is installed and the float adjusted up or down as required to achieve this. Also called "wet float height" the fuel height in the bowl is spec'd within a 32nd of an inch and needs to pretty much stay there at all times under all conditions.

One bugaboo to look for in this series is excessive wear or sideplay of the throttle rod & mounting holes in carb body, this will cause a vacuum leak that can't be tuned around. The fix is to drill them out and install bushings.

Try advancing the intial timing slightly and see if the manifold vacuum will improve.

 

The whole thing is going to turn out better if you get rid of that emissions carb and put a aftermarket non-emissions carb on it. They are jetted a little richer and have more latitude in their adjustments. The carb you are messing with was calibrated to work with all that stuff you just took off.

 

He's got a point. Just don't go too big, a big *** heavy truck like a 350 will respond better with better low end torque with a correctly sized carb.