Bronco Forum - Full Size Ford Bronco Forum banner
1 - 8 of 8 Posts

·
Registered
Joined
·
135 Posts
Discussion Starter · #1 ·
Got a 4 barrel motorcraft car on my 351 and it is idling to low and dying. How do I adjust the idle up and what is the proper way to adjust the air fuel mix? I know where the air fuel mix screws are but do not know which is which. The way I remember how to do it is you back them out until the engine just start to die then you take them back in a 1/4 turn. But I do not know if it is like that for all carbs or if that is even correct. Then with the idle the way I remember is that you warm up your engine, then set the secondary up a notch and then it gets blurry. But I am not sure if what I think I am supposed to do is correct. can someone either refresh my memory or tell me the right way to do it?
 

·
Registered
Joined
·
57 Posts
the way I have done it for over 35 years is to turn them both in all the way (clock wise) than turn them both out (counter clockwise)2 full turns each than I slowly turn them in untill the idle speed just starts to drop and than I find my highest idle speed and than adjust the idle up or down with the throtle adjustment screw or cable. It always helps to start with a freshly rebuilt clean carb . If I use a carb at all my first choice is a Holly . :twak
 

·
Practicing Infidel
Joined
·
15,299 Posts
Buster's right

just make sure you count the exact (to the 1/8th) number of turns in so you can get back to where you started if anything goes wrong, goes wrong (as has been known to happen from time to time).

Also, splurge on a $10 vacuum gauge and adjust those two screws, equally, until you get the highest reading you "kin git". Then back off a 1/4 turn each. That ought to get you in the ballpark.

Never forget your initial timing is uber-related to this. Since last time your timing chain has been wearing and now your stock 10 degrees is NOT 10 degrees anymore.

Slap a light on the engine and see where it "says" it's at. Then, with the vacuum gauge still hook up, loosen the dist clamp bolt and rotate the dist to say 12 degrees and watch the vacuum reading.

It should be going up. You can't go much past 14 and expect it to start easily or remain on 87 unleaded, but be sure to at least check the timing.

If you re-adjust the timing to compensate for chain wear, go back and re-set the two idle mixture screws.....surprise, surprise ! They're right back where they started (see ! I told you it was chain wear !).

****Before you do any of that I'll tell you what a mechanic buddy told me 30 years ago "it's NEVER the [email protected]#%ing carb son!". What he meant was the screws don't back out on you, something else has changed and is affecting the engine.

I'd suspect a vacuum hose is cracked or off or more likely your ignition components(wires, cap, rotor, PLUGS) have worn to the point they're affecting the way it runs.

When you're ready to graduate to the second step go here;



Sixlitre
 

·
Registered
Joined
·
444 Posts
Ok first does any one have that letter/page in a format that you could print. When I open it up it is way too big.

Second the idle mix screws two turns out, what about the throttle adj screw. Do you set it to a spec rpm? thanks
 

·
Practicing Infidel
Joined
·
15,299 Posts
Ok first does any one have that letter/page in a format that you could print. When I open it up it is way too big.

Second the idle mix screws two turns out, what about the throttle adj screw. Do you set it to a spec rpm? thanks
PM me with yo' e-mail and I'll send you somethin'

Once you warm it up and set the idle mixture screws for highest vacuum, then you set the timing ahead and re-adjust the idle mixture screws. All the while adjusting the idle speed screw "IF" required.

Otherwise wait until the engine is tuned for vacuum and the timing advance, then "IF" you're not at the idle speed you like.....Then and only then would I adjust the idle speed screw (re-check your vacuum afterwards).

Sixlitre
 

·
Registered
Joined
·
1,315 Posts
then if its never the air fuel mix screws that need adjust meant y do peple even touch them? my 86 351w ho with over a180tho still has those caps and the air fuel mixture screws that you gota break off to adjust them
 

·
Addicted to Junk
85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
Joined
·
16,371 Posts
I have a 1985 f250 with a 351w where are the fuel/air screws located?
On a stock 85 carb, you cannot adjust the mixture, or timing. It's all done via the eec-iv feedback system. Your best bet is ditching the factory carb and distributor, and converting to duraspark 2
 
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top