I have an on-going issue with my 1978 f150 that I need some advice with, have looked everywhere and can't seem to find anything. Totally bone stock truck other than small cap HEI igniton upgrade with external coil and open air cleaner assembly. Stock 351m engine, manual tranny. Without going into detail truck runs great with either of two carbs, a Motorcraft 2150 or a Holley 7448 (experimenting), although cold startup and warm-up is better with the Holley. Manual choke. Anyways on to the issue I've been losing sleep over for two years..

Starts/runs great at startup, running smooth, engine begins warming up so I get out and let 'er idle to warm up good. Now when the thermostat opens I get this bog - the engine bogs down maybe several hundred rpm? I can tell exactly when it opens by watching temp gauge/feeling upper rad hose. The bog may last a full minute or two then it clears up on its own. From then on it runs GREAT. I can stop/start engine all day long and it never bogs again, even after sitting for hours. Next morning odds are it will bog again on cold startup, right as the thermostat opens. It's irritating because you feel like it's going to die, it bogs down pretty low.

The only explanation I've heard that makes any sense is gas pooling in the intake as the cold coolant is introduced, but from what I understand the 351m has a "dry" intake (truck still wears the original cast iron intake) so this wouldn't apply? What else could cause gas to pool in the intake? Any thoughts would be greatly appreciated, I am no expert for sure. Any questions ask away and I will do my best to answer - Thanks in advance.

 

The flap that is closed on warm up and then opens up. Probably been deleted. I think that's what it used to be for. Vacumm door flap on the pancake housing with a tube. But I'm not sure.

 

You are experiencing a heat riser EFE problem IMO
Early fuel evaporation and the heat riser function (choke stove) is what helps your issue.
When the thermostat cycles it lets more cold coolant into the motor
That means you need more choke at that moment
So, after 2 cycles of that, your engine is at normal operating temp and the problem is gone
+1 for fuel injection
You have to live with problems like that with a carburetor
At least you have been playing with a couple and know what's up

 

Problems like these were one of the reasons why carbureted cars and trucks had stuff like pre heating for the air that enters the air filter. The thermostatic door closes, the manifold heats up quickly and the engine pulls in preheated air. After the housing warms up the door opens and allows in non preheated air. It isn't a problem that you have to live with.

The reason the cold start and warm up is better with the 7448 is because it is calibrated richer at part throttle. 7448 and 4412 are made just like 1/2 of a double pumper carburetor which means that the power valve channels are smaller than the ones in a vacuum secondary carburetor. That's why the jets are larger, bigger jet, smaller channel, same mixture at wide open throttle. This is done because those models are really intended to be used in racing situations where the camshafts have more duration and tend to have somewhat less signal at part throttle.

The Holley 2 barrels are not rated at the same amount of depression as a 4 barrel, 40.6in of water vs 20.3 for a 4 barrel. This means that a carburetor of the same rating is about 35% smaller than the same "size" 4 barrel. A 4412 is 1/2 of a 750 and flow about 335cfm, a 7448 rated at 350cfm flows about 235 cfm if rated like a 4 barrel.

 

Truckwardo, manicmechanic007, DaveMcLain.. Thanks for your insight!

I think I understand what you're referring to - the "heat riser" is the thermostatically controlled "flap" on the original air cleaner assembly, out there near the end of the "snout", am I correct? Yes that got deleted (naturally) when I went to the open air cleaner assembly like everyone and his dog has to do (looks cool).

Makes sense, I just never realized it served such an important function. As you guys all seem to point to this as the problem, I may convert back to the original air cleaner and see what happens (I still have it), would just need to come up with the necessary plumbing. If I understand what you're saying it might be worth a try - or am I missing something?

DaveMcLain - what you say about the Holley vs the 2150 makes perfect sense, nice to talk with an expert. Carburetors are fun to tinker with and I've learned a lot, still learning! Again let me know if I'm understanding you guys.. Thanks!

 

Back a long time ago when we used to work on cars we used to change a lot of those parts. Chrysler 318's were bad about the heat crossover getting stopped up in the intake. Then they would run crappy and the choke would never open. The engine would get all loaded up etc.

You could always just get an old stock air filter and use that along with the stove in the winter only. It would be easy enough to do and wouldn't cost anything. Just make sure that the little door is working. A lot of times heat or lack there of problems with carburetors are the worst when the air temperature is about 40 degrees and it is raining.

Older Fords had an air filter where the door was thermostatically controlled using no vacuum or anything else. 289's from around '65 had that setup and it would probably be pretty easy to rig that up on the 400.

Those 2 barrel Holleys, especially the 4412 are probably the most common racing carburetors in use. They are required in lots of circle track racing classes as a way to limit horsepower and engine RPM. My most developed engine combination is one of those; gauge legal 4412, 358 cubic inches, un ported cast iron heads, flat top pistons, flat tappet cam, headers and aluminum intake. 502 horsepower and 470lbs/ft torque. Target rpm is 7200.

 

One day last winter I drove for miles straight into a driving snow, got home popped the hood and that open air filter was all iced over with melting snow. I had to take it into the house to dry it out it was soaking wet. Those things come with a price for just a little more air flow. I've always thought about going back to a closed system, and if it would help cold weather starts that would be a big plus. I'll see what I can do - that would be great if that's all it is.

I'm really liking the Holley, it gets out and runs like efi. The motorcraft on the other hand was quite a challenge, I deleted all the emissions but the pcv and had to basically re-tune the carb.

Thanks Again to everyone I appreciate your input!

 

Just an update for anyone interested - I got the original air cleaner and all related plumbing up and working, and it solved my cold start problems 100%!! I ended up sourcing a temp sensor and vacuum delay valve off ebay (NOS Motorcraft parts), the vacuum motor was still good. Found a heat duct for the exhaust manifold, picked up a preheater hose at NAPA and I was good to go. What a difference!! Warms up faster, idles smoother, better throttle response when cold. I'm kind of fascinated by this old system that crudely monitors the air temp into the carb - but it works. Nary a hiccup since I converted back to OEM. I don't dread warming him up any more!! Thanks Again to everyone who chimed in, you all nailed it spot on - it was just the push I needed!!

 

Nice. Should be good to go for another 20 years or so. Then you might need a new flapper door again.

 

Yah I did find a spare for when that happens... some of these old parts are getting hard to find.