Hello all,
I was attempting tune my carb last knight but I was having a little trouble. Thought maybe someone here might point me in the right direction.
Little back story, I got a 72 f250 with a 360 in it. Wasn't running well when I got it. It has a motor craft 2bbl carb in it that I have rebuilt, new coil, wires, plugs, and distributor converted with pertronix. After a lot of miss direction and miss diagnosis(lots of help from experts) I had it running fairly well by ear. I was able to get a vacuum gauge a couple days ago so I was going to fine tune. With some more expert help, I was advised I needed to connect the advance to manifold vacuum on the back of carb instead of ported vacuum on side. After an attempt at tuning, I drove it a few times since and it ran poorly at low rpm and backfired quite a few times. After some reading, I moved the vacuum line back to side of carb ant was going to tune but the engine won't keep a consistent idle speed. Timing is set at 8 btc. It was not doing this engine speed hunting before I had changed the vacuum, and drove it around. Could the backfires have caused it? Any help would be great.

Thanks, Luke

 

i would start by looking for a vacuum leak.. have you connected your vac. gauge to manifold vacuum.. if so was it reading steady 16-19 in. at idle or a low, erratic reading

 

Engine backfire through carburetor can damage power valve in earlier (pre '92) Holleys. Autolite and Motorcraft carburetors have no power valve protection at all as far as I know. A quick way to tell if the PV is damaged, anytime the idle mixture screws are turned in to full seat the engine should stall.

 

It was holding around 19 the other day, now seems to be about 16. Best way to check for vacuum leaks? Only one vacuum line on it, to the distributor.

 

you can check for leaks around carb base and intake runners vacuum, brake booster and other places by spraying with some ether(engine start fluid) and listen for rpm changes.. ted hit on another good thing to check..since backfiring will for sure cause a power valve problem..

 

I had thought this might have happened, but info around power valves seems to be conflicting. Engine will die if I close up the idle adjust screws. Don't know if a power valve would cause surges. Any way to test it when it's out?

 

i do it by mouth.. suck on it and see if it holds vacuum with your tongue... not sure if there is a better way.. but if you have fuel in the cover plate that usually means its bad also..

 

you said you closed the idle screws..yes that will kill the engine.. you need to seat the air/fuel mixture screws and see if it dies or just stumbles

 

by closed Idle screws, I was meaning to say seated(closed) air fuel mixture(idle mixture) screws. sorry for the mix up. This is very subjective though because as it surges the rpm raises and lowers, it always just about wants to die at the low end. I had to raise the idle a little just to keep it running at all.

 

I will check on these couple things and let you know. seems odd that it would develop a vacuum leak after a about 15 miles but you never know. Or maybe it had a leak before but it is acting goofier now because something else has happened like the power valve being blown?

 

I was able to take a can of either to it this evening. It did seem to raise the idle a little. I have a new gasket between carb and spacer but the one under the spacer is suspect. I will try to remove clean and replace this weekend. I could see the vacuum leak making it run rough or make it difficult to set. How is it that a vacuum leak will make the engine rpm vary?

 

a vacuum leak will effect the idle or throttle response different depending on where the source is.. there are too many variables to list them all...for example a leak in a manifold runner will give it a miss on only one cyl. a leak at the carb base may effect only 4 cyl. or all eight depending on where it is.. a leak at the brake booster may not even affect the engine rpm at all till the brakes are applied.. or you can have internal carb leaks that are a whole different story..

i hope it is something simple on yours that will be a easy fix.... the way my luck runs problems always seem to be the hardest to fix ones.. ha ha ha ha

 

I hope it's simple also, just seems strange that it was running smooth and idling well, now after 15 miles of driving with vacuum advance wrong, it won't even hold a steady idle speed.

 

Vacuum leak causes it to run lead & when in a lead condition don't always idle smooth.

I'd check the PCV is not gummed up, and possibly the Pcv valve rubber grommet that fits in the valve cover is old an cracked sucking air.

And for auto trans check all rubber vacuum line going to the modulator valve at trans.

The carb base pcv plate loads up the caked up carbon and needs to be cleaned and replace with new gaskets.

Timing needs to be checked & reset any time the points or dizzy has been messed with. While vacuum has bee removed from the dizzy and idle at 600rpm or less adjust dizzy timing with a timing light.
Orich

 

High mileage worn out distributors cause erratic idle and advance/retard timing. Check that the vacuum diaphragm is OK, better to replace. Hang on to it, but swap out a known good one.

A neat trick btw, I got learnt somewhere's on these here intertubes, the standard Ford distributor of 57-73, the vacuum advance arm is secured on the pivot or breaker plate post by a "C" clip about the size of a flea and pretty easy to ping across the garage, never to be seen again.

Turns out it isn't even needed, if the arm gets the right bend placed on it. It will then just slip over the post and sit flat, and never cause any trouble.