Bought a harbor freight fuel pressure guage...hose on that is smaller than the fuel line but was still able to use with a T fitting just off the carb. Guage reads a constant 8.5lbs... rule out the fuel pump I guess.

 

Sheeit, this seems to be a poser...

Good fuel pressure.

Have you tried to verify the timing...or where TDC is on the balancer?

Something else to try...I know you said she runs good (outside the stumble), no pinging, etc,....have you tried retarding the timing some and then taking her through the paces? If she runs better - no hesitation/bog/stumble then I would suspect the balancer has slipped and has to be corrected.

 

backed timing off to what indicates about 11 degrees and retuned. Had to set idle a bit higher and richen it to get vacuum up. I am out 3.25 turns on each mixture screw. Again, idle is nice but it still runs terrible when you give it anything but gentle throttle when under load. Step it down and Im getting backfires through the carb which screams too lean a mixture but dang, how far out do I need to take those mixture screws? Already seems excessive and the idle exhaust even smells rich.

Buyers remorse is really setting in. I am out of ideas...i have a truck I think I really paid too much for that just will not run.

 

OK, yesterday I hit a low. I am thoroughly frustrated with this thing. I will sum up all the things I've done:


Tuned carb with vacuum guage (every time). Getting a consistent 18" at idle
Partial rebuild of carb that included new power valve and accel pump diaphragm (Previously the diaphragm was installed backwards)
Checked fuel pressure. Making 8.5lbs
Confirmed vacuum advance on the distributor is working properly
Reset timing from 20 degrees down to about 11.
I R&R'd several vacuum lines. I can't find any more vacuum leaks unless I'm leaking on the canister itself or inside the brake booster.


With the current tune I'm getting lean misfires when I step it down under load. I have the adjustment screws out 3.25 turns each.


My choke is entirely inoperable...I think there's even pieces missing, such as the fast idle screw, and perhaps a piece of linkage. I can almost live with that since the truck isn't driven a lot and I live in a hot climate.


Truck still idles nice.


I ordered a remanufactured carb last night after lamenting over this a while. At least then I can start with something that I know all the pieces are there.


Guess we'll see how this works.


There's just nothing that complicated on these trucks. It's really surprised me that this issue has been so difficult to track down.

 

Why are you using the idle mixture screws to try to tune an under load condition? I agree it sounds lean, but that would necessitate a main jet or power valve change, not idle mixture adjustment.

 

I'm not. I'm using them to tune at idle. My point was that it seems plenty rich at idle, yet it still appears I'm getting a lean misfire under load.

 

Provided your ignition components are in working order (which I think you've already addressed) and no major vacuum leaks, then it sounds to me that you should step of a couple of main jet sizes.

In my experience, people tend to jet way too lean in search of the elusive fuel mileage, or slap on a carb from a different application that is jetted incorrectly. For example, I purchased a used Holley 750 (cheap) from a guy that said it needed a rebuild. The only thing wrong was the main jets were 7 sizes leaner than base calibration. He probably pulled his hair out trying to figure out why this POS carb didn't work. After re-jetting and giving a quick once over, it runs great on my 460.

Your previous owner could have done one or the other. Either way I would be curious to know what size jets are in it now.

 

Not sure how I would check to see what size jets it has.


Definitely a possibility though. The carb had obviously been rebuilt recently, but by a novice who I know put the accelerator pump diaphragm in backwards.


I think I have chased everything possible on this thing. A reman'd carb is on its way. I hope to the powers that be that it does the trick. Will be nice to have an operational choke anyways.

 

Jet size is stamped on the jet, you would need to pull the jet to find out the size.

Read the spark plugs?

 

What do the plugs look like?

Jets have absolutely nothing to do with off-idle acceleration or transition. In fact, you could even remove the jets entirely and drive around the block provided the RPM is kept below a couple grand.

Make sure that the _fuel level_ in the bowl is correct. Set the _float height_ wherever it needs to be to effect this.

The "bench" or dry float setting in the instruction sheet or manual is just a baseline to get started. This is fairly critical that it is set right, every adjustment downstream is based on this. Too high fuel level and flooding occurs. Too low and it will be fuel starved at highway speeds or heavy acceleration.

Idle mixture is always set as lean as possible consistent with a smooth idle. 3 turns out is compensating for some other fault.

Backfiring through the carb can result from a lean condition but it's almost always a timing issue, and this would help explain the poor performance. In these cases it's wise to make sure that the engine timing chain hasn't "jumped" timing, and also verify that the damper ring on the balancer hasn't slipped.

Verify also the spark plug wires are in the correct firing order.

20 degrees initial at idle isn't "too much" per se but it's likely to cause ping city on the open road, because the distributor mechanical advance would have to be dialed back to compensate for so much lead. Starter kickback would also likely be a problem esp. in hot weather.

You mentioned a steady 18" of vacuum at idle, this is measured at factory idle of 550 to 600 RPM correct?

 

Measuring vacuum at just under 700 RPM. Achieved 19" when I went and retuned last night. I took the timing back up to about 15-16 degrees and it runs a little better. I didn't have pinging at 20 degrees, but have read that advancing too far at idle + mechanical advance at higher RPMs can burn valves so am playing it a little safer. Symptoms are exactly the same regardless where timing is set.


I didn't pull all the plugs, but the ones I pulled are consistent with a lean condition...they're whiter than they should be.


I richened it up a little more...am out about 3.5 turns on the adjustors and the truck is driveable again, but I can't mash the gas still without the stumble. Idle is as smooth as can be once the truck is warm.


Ok, all that is said.


I had said before I didn't have vacuum leaks. When retuning last night I got out the trusty carb cleaner and hit the throttle shaft directly, and it dropped the idle, so maybe I found my problem. Carb might just be worn out where the shaft enters the carb body. Since I already ordered a remanufactured carb, I'm HOPING this resolves the issue.


I'm off on a 4 day camping trip this afternoon in the truck...that's why I at least needed it to be drivable. Should be OK for that, as it's running as good as it has for the other trips I've taken it on. I'll get the new carb on it next week and report back.


Keeping my fingers crossed! I have $1700 worth of new 5" lift sitting in my garage to go on the truck, along with 4.10 gear sets, a rear Detroit locker, and a 3.92+ front carrier. Hoping to resolve the running issues before I bolt all that stuff on and go up to 35" rubber.


After that I'll be investing in new tires and bumpers to turn this into the expedition truck I bought it for.

 

This is misleading at best. Transition to the main circuit is not dependent on RPM, but totally dependent on throttle position. With even moderate acceleration the main circuit will be active at far less than 2,000 RPM.

 

Well, OK... The point still stands - jets don't have anything to do with an off idle stumble or bog.

 

Off idle is just that....a transition to the main circuit. So yes, main jets can certainly influence off idle problems. The transition will not be smooth if there is an overly lean condition.

 

Nope, they aren't even in circuit till probably 40 miles an hour. They set the steady cruise, low load fuel/air ratio. They don't have anything to do with idle or acceleration, that's the job of the idle circuit, transition, accelerator pump and power valve (economizer).

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