Well, here we go again. 78 f150 4X4 351M engine, C6 AT...

I'm still battling carb choke and mileage issues. Here's the history so far:

carb #1 (Motorcraft 2150 original): 9.5 top recorded mpg
carb #2 (Motorcraft 2150 remanned): 7.4 top recorded mpg
carb #3: (Holley 2300 new w/#61 jets) 8.43 top recorded mpg
carb #3: (Holley 2300 new w/#57 jets) 9.19 top recorded mpg

Carbs 1 and 2 had serious choke issues which is why I've been working on this to begin with. The Holley runs great when warmed up, but when choked first thing in the morning it still starts to bog if I have to stop at a light or 2 before it's fully warmed up and I'm right back in that catch-22 again of needing the choke to run it yet it wants to die with the choke on. (Note: even at this the Holley choke situation is better than either of the previous carbs but it's still irritating and makes it difficult to drive.)

I asked Holley tech support about this and their answer was less than helpful: "the 7448 [stock number for the model 2300] is a performance carb unlike the M/C carb if you are not pleased with it and you have had it less than 90 days I would return it to the store where you bought it for a refund." Great. That's swell. Yeah, that helps.

So, I may try a last ditch tweak of the idle mixture on the Holley (I have nothing to lose now). But my guess is it either won't help and/or will just make things worse. I went with the Holley to begin with since the 2300 bolts right up to the stock 351M manifold.

Before I go messing with the Holley settings or returning it, my question now is does Edlebrock make a 2-barrel, manual choke carb that will bolt on like the Holley?

Thanks

 

What other mods have you made to your engine?

If the distributor is stock, I'd say RUN don't walk out and get the distributor off a '79 F-150 or Bronco; the '78 was a "dyno" calibration, and spark advance was all in by right about 2200 - 2400 rpm, while the '79 was tuned by driving the truck (the emission law changed) so it has much more centrifugal advance, and much better driveability.
Set it to around 8 degrees advance at idle, slightly less if you get severe spark knock, slightly more if you never get any spark knock. Best will be around 12 degrees, but you'll get part-throttle knock.
This will also believe it or not improve your cold-driveability problem.

Once you get the ignition straightened out, you can work on the carb:
That Holley 2300 is just like half a four-barrel - one primary and one secondary, so you use the same tuning techniques. (Well, that Motorcraft carb is half a 4-V too, but the wrong half...)
To improve your cold tip-in, you might want to see if you can get a little more pump-shot - maybe you can open up the squirter, or get a slightly larger one.
You might also try a different power valve, one that opens at a slightly higher manifold vacuum, but not high or it'll be open too much of the time. Pay special attention to the manifold vacuum you get while crusing down a level road at say 45 - 60 mph, and be sure the power valve opens 2 - 3 in. Hg lower than this figure.

A much better bet for you, however, would be to get a Holley "Street Dominator" manifold (no longer made) off e-bay, and a Holley 4V carb, for example one off an early-'80's 460 truck motor. Then richen it slightly (because you've got a smaller displacement motor) and enjoy!
Big HP increase AND much better fuel economy than with any 2V carb you can find.

But first: the distributor!

Have fun;
- Robert

Wait a minute: Is your manifold heat stove still intact, and the heat stove hose to the aircleaner snorkel? Is the cold air valve in the snorkel working? If not, you'll never have good cold driveability, and you'll have to use the choke all the time.

 

> What other mods have you made to your engine?

None. All stock.

> If the distributor is stock, I'd say RUN don't walk out and get the
> distributor off a '79 F-150 or Bronco; the '78 was a "dyno" calibration,
> and spark advance was all in by right about 2200 - 2400 rpm, while
> the '79 was tuned by driving the truck...

Yes, stock distributor.

> Once you get the ignition straightened out, you can work on the carb:
> That Holley 2300 is just like half a four-barrel - one primary and one
> secondary, so you use the same tuning techniques. (Well, that
> Motorcraft carb is half a 4-V too, but the wrong half...

I might look at all that but it sounds like more than I want to deal with and really performance as such is not an issue at this point. The big problem I have is just getting the choke not to kill the thing at stops in the morning. I know the idea behind the choke is to make it richer (with a little fast idle) but it sure seems like it's too rich when choked. On the advice of the place that sold the holley to me I tried tweaking the idle mix this afternoon but as I was afraid of, it was already as leaned out as far as it could go. Of course now the mix is a little screwed up since I messed with it but I figured that would happen too. I may try to get them to fix that since they were the ones pushing that brilliant idea.

After the idle mix failed to help I was so po'd with it I put the old original motorcraft carb back on (carb #1) so I could return the holley first thing tomorrow. But that old carb won't hold idle, surges like mad now. Through all the messing with it I probably screwed it up. Finally gave up and put the holley back on. The holley is unacceptable but it's the "best" of the 3 carbs I have now. What a nightmare. This shouldn't be this hard. I had to do all these same basic things with a 65 Jeep but it didn't take hundreds of dollars and 3 carbs to do it!

> A much better bet for you, however, would be to get a Holley "Street
> Dominator" manifold (no longer made) off e-bay, and a Holley 4V carb,
> for example one off an early-'80's 460 truck motor. Then richen it
> slightly (because you've got a smaller displacement motor) and enjoy!
> Big HP increase AND much better fuel economy than with any 2V
> carb you can find.

I don't want to throw any more money at this. A $260 carb should run right. I just can't believe the choke can't be made to work right. This is very simple. Why does the holley run so badly when choked but so good when it's not?? I just want it to run right with what I have and not be the ongoing money-pit project that it's been for weeks now.

> Wait a minute: Is your manifold heat stove still intact, and the heat
> stove hose to the aircleaner snorkel? Is the cold air valve in the
> snorkel working? If not, you'll never have good cold driveability, and
> you'll have to use the choke all the time.

The heat stove is all still there, just not hooked up. There's nothing to hook it up to now since the holley has a manual choke. I'm assuming I don't remove that heat stove but that I don't need to plug it up either. If that's not correct though, let me know.

Thanks for the help. If you think of anything else let me know.

Stan

 

First, I've got to make this point: your engine is a "system" not just a collection of separate parts like carburetor, exhaust, distributor, etc.
You change one part, you may well have to make changes in several other parts.

Now, regarding the idle mixture: if you already have it leaned out as far as it will go, then that's part of your cold stumble problem.
Set it to run idle correctly, like it says in the manual, and not a bit leaner; you'll just get into lean misfires, and that doesn't help fuel economy at all.

Second, and here's the really important part: get someone to show you what a "heat stove" is (tip: it's not ever connected to a choke); it's a sheetmetal part on the exhaust manifold that is very important to your cold running. It warms the air - that's where it gets the "stove" name - and conducts this warm air up to the aircleaner via a flexible hose (probably called "heat stove hose").

This warm air mixes with fuel a lot better than cold air does, and will go a long way towards eliminating the cold stumble problem you have.
Trust me on this: you're going to have to use 'way too much choke if you don't have warm air in the carb during a cold drive.
There's a vacuum-controlled flapper valve on the aircleaner snorkel where the heat-stove hose connects: this flapper valve causes the aircleaner to draw air ONLY from the heat stove when the engine is cold. In the vacuum line to this valve, there's a thermostatic vacuum switch that mounts in the base of the aircleaner; when the air in the aircleaner gets warm enough, it cuts off the vacuum to the flapper, so it will now draw cold outside air for improved performance.
Check and see that the flapper valve closes when the engine is cold and at idle.
If you're missing any of these parts, get them; your truck will never run right without them - unless you use 'way too much choke...
Get a manual, and see how they're supposed to be connected. Get someone else to review it, if necessary.

Now about the distributor: the ignition is one more very important part of that "system" called your engine, but I'm only going to make this point once more; after this, you're on your own:
Get the '79 distributor: it's the single best cheapest most important improvement you can make to driveability in your truck - and your cold stumble problem is a "driveability" problem. AND it will give you significantly better fuel economy since you'll be running with a lot better spark table than with that POS distributor curve that you have now. Firing the spark plug at the right time is a rather important part of getting the engine to run right.
Trust me: I've gone through all these same flogs, on this same engine, 25 years ago.
I still own the truck, and I'm still tweaking it.

Good luck;
- R

 

> Now, regarding the idle mixture: if you already have it leaned out as
> far as it will go, then that's part of your cold stumble problem.
> Set it to run idle correctly, like it says in the manual, and not a bit
> leaner; you'll just get into lean misfires, and that doesn't help fuel
> economy at all.

No lean misfires, just tends to be too rich during cold choke. Mix is now back to where it started though. Warm running is still OK.

> There's a vacuum-controlled flapper valve on the aircleaner snorkel
> where the heat-stove hose connects

I do have the flapper valve but it's inoperative, and it gets worse...as I was reading your post the awareness of where you were going with all this started to set in. One of those "uh-oh, I'm in trouble here" things...

> If you're missing any of these parts, get them; your truck will never
> run right without them - unless you use 'way too much choke...
> Get a manual, and see how they're supposed to be connected. Get
> someone else to review it, if necessary.

Ah, OK, I know what you are talking about now. I thought you were talking about that little pipe that goes to the auto-choke setup. I do have manuals but I'm not very familiar with the heat stove stuff since I don't have any of it. I also am pretty sure I do not have the correct air cleaner unit either. The one I have fits on there but doesn't match anything in my manuals. Sounds like this is going to be critical to have but will take a trip to the local boneyard to get.

> Now about the distributor: the ignition is one more very important part
> of that "system" called your engine

Absolutely, but I only want to change one thing at a time for now. However, since I'll have to collect up the heat stove stuff anyway I'll see if I can get one of those distributors while I'm there.

> Trust me: I've gone through all these same flogs, on this same
> engine, 25 years ago.
> I still own the truck, and I'm still tweaking it.

OK, thanks. I'll let you know what happens.

Stan

 

Stan:
You're on the right track now!

You don't need the "exact" correct aircleaner, but it needs to have a RH side (passenger side) snorkel.
The bigger one off a 460 will probably work, even though it's off a 4-barrel, and it will flow a lot more air. To see what I'm talking about, check the manifold vacuum at wide open throttle; you'd think it would be "zero", but it'll creep up to 3 - 4" Hg as the RPM comes up, but wo't be over half that with the 4-V aircleaner.
Try to find one that has a functioning thermostatic vacuum switch in the base. It's a little round silver-colored metal device device with two vacuum ports sticking down through the bottom of the aircleaner, retained with a spring clip.
Do the vacuum routing as much like stock as possible. A vacuum delay valve in the line from the switch to the flapper door will keep the door from slamming open when you accelerate and dumping a huge shot of cold air into the intake.

Be sure to put vacuum to that new distributor - vacuum advance is critical to good fuel economy, and if memory serves, there's much more vacuum advance in the '79 distributor as well.

Also: if you still have the EGR spacer, check and see if it's made out of aluminum. If it is, odds are it has corroded through and is leaking, or will very soon, so I'd try to get a cast iron one from the junkyard - as long as you're headed out there anyway!
Or just do away with it altogether.
And if you've got a direct-drive flex fan on there instead of viscous-drive fan clutch, you can pick up a little fuel economy improvement with the fan clutch.

And a lot less noise; be sure to use the fan-clutch fan, though, if you make this change.

Let me know how this turns out for you.
- R

 

> Stan:
> You're on the right track now!

Well, Forrest Gump is as Forrest Gump does sometimes. In my own defense I've only had this thing for about a year and I've never seen one that's all complete and together. It's like a huge jigsaw puzzle with missing pieces. The guy who put it together did a good job but he used the minimum engine vac stuff to make it run and none of the heat riser parts are there.

> You don't need the "exact" correct aircleaner, but it needs to have a
> RH side (passenger side) snorkel.

Mine has the valve cover vent hose nipple on the wrong side (sort of up front to the left of the snorkel). I saw what looked just like what I have on some older Mustangs on a TV car show.

> The bigger one off a 460 will probably work, even though it's off a
> 4-barrel, and it will flow a lot more air...

OK, I'll try to find one if I can.

> Try to find one that has a functioning thermostatic vacuum switch in
> the base.

I've already called around town and no one has stuff for anything this old. I'll keep looking and post something on the forum here and see if anyone has something. If you know of any places I can try please post.

> Do the vacuum routing as much like stock as possible. A vacuum
> delay valve in the line from the switch to the flapper door will keep the
> door from slamming open when you accelerate and dumping a huge
> shot of cold air into the intake.

OK.

> Be sure to put vacuum to that new distributor - vacuum advance is
> critical to good fuel economy, and if memory serves, there's much
> more vacuum advance in the '79 distributor as well.

All that is there and working OK. Just have to find a source for these older parts...

> Also: if you still have the EGR spacer, check and see if it's made out
> of aluminum.

Mine's a bit cruddy but it's the iron one and basically OK. EGR is inop and frozen closed.

> Let me know how this turns out for you.

OK, thanks.

Stan

 

Follow this link to see what the air system is supposed to look like; these are the parts you need. it's at Jeff's Bronco Graveyard:
http://broncograveyard.com/images/catalog/AIRDIA.JPG
(He doesn't have the parts new, but you could check with some of the classifieds posted on his site) This particular layout has the therm vac switch in the aircleaner cover.
You might even check this particular ad:
http://classifieds.broncograveyard.c...9/1653119.html

Or e-bay...

(BTW, you can swap those valve covers side-to-side; 351C cast alum valve covers work one those heads also)