Good morning! I need some help, please...

My 1986 F150 4.9L has been converted to duraspark for almost 3 years with excellent results. The 1982 Carter carburetor had a worn throttle shaft and needed replacing. I aquired a 1970 Carter YF from a F350 and installed it this past weekend. I was amazed at the nice even idle even at the initial 450-500 rpm.

My problem is this: the 1982 carburetor had three vacuum ports. As I recall, one had basicly manifold vacuum, the 2nd port had no vacuum at idle, but almost immediately equaled manifold vacuum as soon as the throttle was opened and the 3rd had no vacuum at idle and gradually increased as the throttle was opened. The distributor vacuum advance was attached to the 3rd port and operated beautifully. Once, I experimented by attaching the distributor vacuum to the 2nd port and found that the truck would buck and could not be driven. Well, the 1970 carburetor has one vacuum port and it acts like the 2nd one on the 1982 carb.

With the vacuum line attached, the engine starts easily and idles smoothly. If the engine is raced, everything is okay. But, if the engine speed is increased to approximately 1500 rpm and held steady, the engine begins to miss steadily, almost like a rev limiter. If raced again, everything is okay until a steady speed is attempted. I plugged the vacuum line and the truck drives very well, but without vacuum advance.

Yesterday, I purchased an inline vacuum restrictor [Dorman #47311] in hopes that this would soften the quick response. It might have helped some (?), but still is not driveable.

Any ideas? Thanks in advance!

 

let me preface this with the "i'm no expert"...

it sounds like your vac advance may be pulling *too much* advance for your setup. all the carbs i've worked with (older stuff) only had the 2 ports, so there are lots and lots of vehicles hooked up like what you've got currently.

is your vac advance adjustable? if so- back off some, and take it for a test drive.

 

I have not yet adjusted the timing as I was attempting to change nothing but the carburetor, so as to see what difference going to "pre-emmissions" carb calibration would reveal.

Thanks for your suggestion, rustywheel68.

 

I agree. If the advance canister is set up for a slow increase in vacuum and you're getting a quick one with your new setup, it's most likely simply out of adjustment.

Many vacuum advance canisters can be adjusted with a hex wrench in the end.



I *think* it's a 3/32".

Turn it clockwise for the advance to come in sooner and counterclockwise for it to come in later. (You'll most likely want later).

Also, it usually takes a good, full 360° turn to make adjustments, not little incremental tweaks. I remember doing that at first and not understanding why I wasn't noticing any difference.

 

X3 Another time my vacuum gun came in handy. I checked a new stock canister the other day and it took 4.5 hg to start advancing. That thing will adjust to well over 10hg, so there is plenty of room for adjustment.

 

Yep. A vacuum pump is great for setting/testing the vacuum advance!

 

Thank you all, for your collective input! The photo was very helpful.

Well, after work yesterday, I couldn't wait to solve the problem. I gathered my hex wrenches and headed for the truck. Unfortunately, my 3/32" wrench was too short. I tried the 7/64", which was long enough, but did not connect. I did notice from inside the truck with the engine running and the vacuum hooked to the distributor that the missing will begin as low as 1300 RPM or up to 2000 RPM when the speed is held steady. It revs cleanly from any point when the throttle is abruptly pressed.

Today, I borrowed longer wrenches from a co-worker. They are the type with the ball on the end. Anyway, the 3/32" was long enough, but I couldn't locate an adjustment. I tried several times and also went up and down a size, still with no success. I suppose the didtributor vacuum can does not have an adjustment. I found a tag indicating the distributor was made in July of 1982.

I'll keep after it and report back.

 

Be careful about getting a wrench that's too long or you can accidentally puncture the diaphragm inside.

The wrench should just be a little bit smaller than the opening, but not much. You'll know it when you get it. Too small and it'll pass right through like nothing's there. Too large, and the wrench won't go in very far. Just right and it'll feel like it kinda slides snugly into place.

It *is* possible it doesn't have an adjustment, but I think they did.
Stock replacement ones you get from Autozone/Napa/Schucks, etc. definitely do.