Hey guys, I am new to working with carbs in vehicles but I have worked on my dirt bike race motors a lot so I understand the basic concept so any help is appreciated. I bought a 79 f150 shortbed that had a ford 429 out of a lincoln rebuilt and put in it. Its got a performer intake and a holley 870 cfm street avenger. It sounds like its got a cam too but I don't know for sure, stock manifolds with a crack in one, true duals.

Its running rich, backfires when you get out of the throttle, some sweet tasting black smoke. The guy I got it from had a 140 shot wet no2 setup on it and said he "jetted up for it". I opened it up today and its got 73 primaries in it. How many steps down do you guys think I should to lean it out enough?

 

That is an awful lot of carb for a street truck. Try 70 main jets in the primaries. What are the secondaries?

 

I haven't had time to check the secondaries, I'm at work. I thought is was a lot of carb too but the truck has been really reliable and runs so good on the gas, but definitely rich. And when you come off the gas all of a sudden it backfires like crazy, but not really if you come out gradually.

 

Okay so my jet kit came in and i've been playing around with some different ones. The secondaries that were in it were 83s which I thought was a little odd since the primaries were 73s. Right now I have 67s in the primaries and 72s in the secondaries. It seems to start and run stronger, the exhaust smells good, but it still backfires out the exhaust if its revved up and then let off. There is a decent crack in the exhaust manifold on one side, could that be contributing to the backfires? I am going the the local yard to look for one soon. And does anyone know what this thing is just laying beside the carb, looks like it should probably go to something.



Im not sure if this thread should be here or in the 79 truck section so sorry if its in the wrong place.

 

The secondaries are almost always bigger than the primaries. The primaries have the power valve to add fuel at WOT. In most case the secondaries do NOT.

Yes the addition of air into the exhaust will cause pops in the exhaust.

That arm is the auto trans kick down lever which is suppose to be connected to the throttle linkage.

 

If you can spring for it, I'd strongly suggest adding a standalone O2 sensor/gauge setup (tons of them out there, $100-200 range, google it). You'll have to weld a thread bung into your exhaust, preferably as close to the engine as possible (where exhaust is the hottest and the O2 sensor will warm up the fastest). This will eliminate all guesswork about your air-fuel ratio and will eliminate the possibility of damaging your engine (from a too-lean mixture, but probably not an issue with that carb!).

I agree with the other poster above - 870 CFM is a HUGE carb for that motor on the street. You can do the volumetric calculations (your engine is nothing but a big air pump), assume 80% volumetric efficiency and an engine speed of 5000rpm. It takes two crank revolutions to pump/fire all 8 cylinders, so here's the math: (5000 crank rev/min) x (429 cubic in/ 2 crank rev) x (1 cubic ft/1728 cubic in) x (0.8 efficiency) = 496 so roughly 500 cfm @ 5000 RPM!

So a 600cfm carb can EASILY handle 5000 rpm (and higher), and I seriously doubt you'll be in that range very much if ever. Now if you add a turbo or a supercharger, then your volumetric efficiency number will increase, but 80% is a good number to use for normally-aspirated engines.

My $.02 on the primary jet sizes: on my 1971 Ford 400 engine (only a few cubes less than a 429) with a Motorcraft 2-bbl, I had .054" jets, and went up to .057" when I added a dual exhaust. So .067" in the primary still sounds too large to me. But the O2 sensor setup will answer that for certain, which is why I am suggesting it. You can buy the whole setup and install it yourself for less than what two hours' worth of auto repair shop labor costs!

 

thanks for the help guys, hopefully new manifolds will fix the popping, I think I've about got my mixture dialed in. The carb was on the truck when I bought it, I realize its overkill but I guess it won't hurt anything as long as its jetted right. The o2 sensor sounds like something interesting for the future but I'm a 19 year old college student and this is just my toy so thats not really in the budget. I considered the 40 bucks I spent on the jet kit to be a major investment haha