Okay, to move on from my previous thread, I had a few minutes to fool with it again last night. Back story, fresh rebuild, nothing fancy, but new rings, bearings, timing chain, heads checked and simple valve job, carb soaked and rebuilt. No vacuum ports coming off the intake or carb. Truck starts right up but will only run with choke almost closed. Last night I disconnected the vacuum advance and the carb is sucking, but plugging the hole didn't fix the problem. I ran both mixture screws all the way in with no result, so I have a leak somewhere. I don't see or hear one, so looking for any Y block known issues or common mistakes to look for. I would think that big of a leak should be easy to find!
Everything was painted up, so hate to start spraying carb cleaner all over it.

 

What carb are you running? Have you put a vacuum gauge on it?

You probably know this already but Y blocks have a couple of quirks that can throw off rebuilders. Timing chain installation mistakes as well as well as ignition timing itself because of defective crank damper.

 

the Early 2100. No gauge, need to rig up a place to hook one up.

 

Check everything from square one. A vacuum leak that bad would be obvious? Make sure distributor is installed correctly and rotor points to #1 tower at TDC and firing order is correct with the wires. Find a source of manifold vacuum and get a gauge on it.

 

Dizzy, and wires were routed on the stand. I have timing set at 6° advance right now. Truck fires right up, idles fine and can be driven, just the choke plate has to be almost closed. If you try to open it, stalls. It was rougher with the advance unhooked, smoothed out when the carb was plugged, or line reconnected. Usually you can hear a decent vacuum leak.

 

What's your idle RPM? It smooths out with vac connected because it is advancing the timing. But, ported vacuum should have no signal at idle spec of 500 and thus no vac advance at the correct idle speed. Not much anyway.

Have you verified the damper has not slipped. This is common and makes timing marks basically useless.

You could try advancing the base timing by rotating distributor clockwise. The idle speed will increase smoothly to a point, and then start to wander and stumble a bit. Back off till a smooth idle and this will be close to optimum. A vacuum gauge works slick for this.

 

Vac gauge works good for a lot of things. Too low is generally timing or engine condition. Vibrating is generally valves and slow fluctuation is generally carb like float level too low. What it doesn't really work for is finding a vacuum, leak. There are other methods got that. The best I have ever found is my old stethoscope that I removed the probe from and attached a longer hose. I can pinpoint the smallest of leaks and it's much safer than spraying carb cleaner.

 

I tried using a heater hose last night as someone else suggested, and heard nothing.
I really don't think it is a timing issue as from my experience if you are off much either way it won't run. But I will look into it.

 

Smoke machine

 

A vacuum gauge will indicate vacuum leaks well because that's how they work. Won't tell where. Head gaskets, intake manifold and carb gaskets. They are useful because so many things can be done at once in a situation like this.

Maybe you know all this stuff already. No way to tell. You've been here 15 years and made less than 40 posts, so you ain't much for visitin' with folks.

 

An under the hood pic would probably not be a bad thing, might stimulate someones brain cell.

 

If you don't have a massive vacuum leak. I would look at your Carburetor's idle circuit! When you use the choke you are using the main fuel circuit to add to the idle circuit! You state that when you adjust the idle needle valve it doesn't make a change!
Just my thoughts!
Let us know what you find as we will all learn in the long run.

Here is a link to reading a vacuum gauge!
How to Use and Interpret a Vacuum Gauge

 

first off, to determine a vacuum leak using a gauge, you would have to know what it was before and it would have to be a really big leak to fluctuate the gauge any noticeable amount. As far as not responding to idle adjustment, that IS indicative of a vac leak but also if you have the curb idle screw in too far (throttle plates more open) the carb will not respond to idle mixture adjustments much, if at all.

 

Can't see much, but here are engine shots. Hope to find time this weekend to try it again.

 

Sure sounds like a vacuum leak condition. Are you sure the carb spacer doesn't have a vacuum port on the rear, possibly off a later model?