Doing a tune up on my 68 F250 390 auto. Cant get the rpm below 650 without it wanting to stall and the rpm flucuates while idleing, the timing is at 8* , I have a Pertronics installed in the dizzy and showing the dwell at 35. With the Pertronics can I set the dwell at the 26-31 range that the specs calls for?

 

You don't set dwell with Pertronix ignition.

 

what's the manual call for, as far as idle RPM?
less than 650 seems awfully low.

are you using ported or manifold vacuum?

 

if the rpm fluctuates while idleing usually means the carb needs adjusting if timing is ok.Also could be a vacuum leak.
orich

 

Manual is saying 500 rpm at idle. Are you asking where the dist. advance is coming from? If so its coming off the intake.

 

I have checked for vacuum leak like you suggested. None found. I just rebuilt the carb replaced the fuel hose and filter. When adjusting the carb for the air fuel mixture, if you turn the needles in while the engine is running shouldnt the engine want to die, then back them out to where the engine runs smooth?

 

Idle should be 650 in Park, 550 in D with vac line at dist. pulled and plugged and headlights on. As already said, up and down idle generally means a vac leak...especially if you can close both air/fuel needles without killing the engine. If you're running the stock carb with a spacer under it, recheck your carb gaskets...carb kits are generic and sometimes won't make good seal on that spacer. Sounds like you're in the ballpark though.

 

As noted above all good places to first check. When checking for a vacuum leak this also means possibly from power/booster & hose, manifold vacuum tree any rubber caps. Also auto-trans vacuum line. I use quick start fluid spraying a little at a time in all these area to see if any leaks as engine will race some if any leaks suck it in..
orich

 

68 250 , Most of the time 1.5 turns from seated on the idle mixture screws puts you real close to smooth idle. Do you have a vacuum gauge?If so do as Orich suggested and disconnect all vacuum sources and connect the vacuum gauge then try adjusting the mixture screws starting from 1.5 turns out. Did you check the compression? You could have an engine problem. Kenny Nunez

 

first thing i would do is cap off all vacuum ports and start motor if it runs better your leak is one of the lines you disconnected , reconnect lines one at a time start engine after each when one make the engine idle funny again you found your vacuum leak .

 

Ok I started the truck and sprayed starting fluid around all vacuum hoses,base of carb, and the brake booster hose. No change of how the engine runs. Next I plugged each port. Still no change. The only time the engine ran smoother and higher rpm was when I disconnected the hose on the trans module. Could it be the trans mod itself? I appreciate all the help you people are sharing with me.

 

Dont have a vacuum gauge. I will pick one up today. I turned them out to 1.5 but had to turn them both in 1.0 for it to run the best it is. Maybe I missed something during the rebuild? I wasnt able to remove the jets in the bowl, maybe thats where the problem is.

 

take the vacuum hose off down at the trans on the modulator valve. if any trans fluid is in there then your modulator has a shot diaphragm and you can get a modulator for around 11 bucks. trans fluid in your vacuum lines can mess stuff up sometimes ;-)

 

One item that gets over looked is the dist. vacuum advance those diaphragm are for about 15 yrs. If someone has every changed the card the the 1970 model carb they have a choke pull open shot diaphragm on the top of the carb. When they fail they suck air and choke hangs up causing the motor to stall or run very rough. Just putting that out for a heads up..orich

 

Are we not missing something here Guys?

Should that not be on the carb base and NOT the manifold?