Im on a roll today with questions I recon. New to us 84 F250, 460, C6. Holley 4180. Starts fine, idles fine, seems to run fine when your on the pedal hard but crusin down the freeway I feel a constant surge,miss,hesitation kind of feeling. The P/O had a new front in tank pump with relays installed, Ive installed new plugs,cap and rotor which did nothing for it. Old plugs looked pretty much right on color wise. Throw me some ideas. I was thinking power valve but doesnt sound like this. New wires going in today if the rain stops, other that that Im stumped. I dont understand the fuel delivery system on this thing, I see two in line filters on the frame under the cab along with a rail mounted pump. If it was fuel related Id think whan you nail it it would run terrible and its fine.

 

It sounds to me like a vacuum leak.

 

Take the vacuum line off the distributor and put a screw in it. Then go for a test drive. If it doesn't do it anymore, your timing is a little too advanced.

 

Timing is right at 8 degrees. Also replaced with no change, plugs, wires,cap,rotor, fuel pump, 3 new filters, timing check out, newer carb, still need to double check the float level and fuel pressure though and I keep forgetting to check the dist shaft for excessive wear or wobble. Frame rail fuel pump weak maybe??

 

If it runs fine at heavy throttle then it isn't a fuel pump issue. The surge is probably a lean condition and Maytag's suggestion would account for that. A cracked or broken vacuum line would do that, and these lines are OLD so crack easily.

 

Try my experiment, it will only take a few minutes and not hurt anything. I have had a lot of vehicles have a part throttle surging problem because the timing was too advanced. You never know, your harmonic balancer may have slipped throwing the marks off or someone has installed the wrong dist or the wrong vacuum advance unit sometime before.

If I am wrong, just put the vacuum line back and try something else.

 

Can't hurt and could easily point you to the problem.

 

Please make it easy on yourself and listen to the suggestions made, apache84. These men know their stuff!

 

Will do on the timing suggestion. Believe it or not at 50+ years old this makes vehicle number 200+. Most have been pretty much tired old 4x4 trucks. I like trouble shooting for some reason. I would think far to advanced timing would have this thing pinging like crazy which I dont hear and I have a multi mile long grade to go up on the freeway on the way home. Not to dispute the suggestion but if it sucessfully passed smog about 100 days ago and the docs show 8 btdc dont they take these numbers off actual timing from their machines and not physical timing light check? I did check the float level and dist shaft play which were on the money and good. I dont hear any vac leaks and it idles smooth. Even not plugging the one vac line to the air cleaner when removed makes it run rough. Be great to do a smoke check test for leaks but I haven't the equipment.
Tomorrow should be interesting as we bought the truck with a travel trailer which Im picking up tomorrow. The PO used the combo little and only locally, hope I make it home OK. Thanks all and keep ya posted.

 

The problem with using a smoke machine and listening for a hiss by the use of your ear is that these methods are only good for large leaks. You could have a small leak that is causing this issue. And therefore wouldn't be heard or seen with smoke.

 

I beg to differ a bit..

A smoke machine can show very small leaks. Mine has a dye you can add that will mark any leaks and can be seen with UV light.

Hearing a leak is a different matter, but I've been able to find very small leaks using a smoke machine.

 

You can have a fairly large vacuum leak, and it will not affect the engine at running speed, only at idle.
A small vacuum leak will be very large at idle, since the throttle blades are closed not letting much air into the engine. A small leak at speed will not be noticed by the engine at all, since it's so small in proportion to the large amount of air entering through the throttle blades of the carb.

When you are cruising your timing number will be the initial(8 degrees) + centrifugal(affected by rpm) + vacuum(affected by load on the engine). It's just curious to me what would happen if you took the vacuum part of it out. It may not necessarily be the problem, but if the problem goes away, it would be something to chew on for awhile. When you are troubleshooting, you can do things to the engine and it will react to these things you do, it will be almost like it's "talking" to you. When you do something and it reacts a certain way, it gives you something to think about.

 

I agree. It does take a large vacuum leak to cause a problem at cruise, but something like the brake booster will do it. Especially if the mix is already lean.

As for the engine talking to you, that is a good analogy. Trying something, like pulling and plugging the vacuum advance, is like asking it a question. It will respond and the answer helps solve the problem - something like playing 20 Questions.

 

I highly suggest 2 items.....faulty/ leaking vacuum advance unit, easy to check AND the 3 way fuel filter next to the carb. I've mentioned this several times already, rust from the old fuel tanks will collect in it and restrict the flow resulting in the surging described, been there done that with both of these. Also vacuum leaks are suspected.

 

If im not mistaken there is no filter in the "three way" set up at the carb. Its a bypass to keep cool fuel moving to the carb when running. There is however a filter in the carb itself much like a Quadrajet which I just replaced, the old one was spotless clean with no signs of rust flakes or otherwise. From what read about the "hot fuel option" system there is a changeable orifice in the bypass that could become plugged which logically would up the pressure to the carb itself as well as potentially create boiling fuel as it stops flowing when heated up working hard.