how do i get my air fuel mixture correct? my truck is back firing and im pretty sure im running too rich. is there a set way i should have my screws? like all the way in the back out 2 turns...etc...?

 

Depending on the carb, I usually lightly screw them in all the way by hand, then back them out 1 1/2 to 2 turns to get it to the point where it will start, then fine tune the adjustment from there.
Here's a video on how to set the idle mixture on a Holley, but the principal is still the same.
YouTube - Holley Carbs - Setting Idle Mixtures

 

does it backfire at idle, acceleration, or deceleration?

 

it backfires at idle, then sometimes when i take my foot off the gas.

 

hm. mine had bad ignition wires, and one was shot, so there was no spark getting to the cylinder at all, and it would idle good, but when you revved it and let off, or were coasting down the road, it sounded like someone was shotgun happy. doesnt sound like yours is doing what mine was though.

 

I tune a lot of engines, everything from bikes to trucks to hotrods to race cars and I found the best way is to invest in a wide band air fuel ratio meter. I am running the one from innovate and I think it cost me around $280 but may be a little cheaper now. I got tired of all the years of guessing and "reading" plugs etc. Now all I do is weld in a bung for the O2 sensor, connect a couple of wires and start the engine and drive around. It's nice knowing EXACTLY what your air:fuel ratio is and IMHO it was the best $280 I ever spent.

Of course you still have to know what you are after, this just helps you get there a lot easier and more precisely. BTW, I run the LC-1 with a remote digital guage and laptop connected for data logging, but you can run it with just a gauge.

George

 

I hooked up a vacuum gauge, backed the screws way out, turned them in 1/8 at a time until I started to lose power, then backed off a little bit.

That said, I'm about (like this week) to do the cheap version of what George recommends- air/fuel gauge is $30ish anywhere, and the local muffler shop is going to weld in a bung & provide a sensor for $50 (I'm not set up for welding right now). Not as precise as what he's talking about, but waay more accurate than any other method.

 

so i got my air fuel right. but it still backfires. i have a new ignition wire on it? could it be bad? any other ideas on what it might be?

 

Just so you know, the cheap air:fuel gauges are a narrow band setup. They are not nearly as accurate as a wideband in that they can only read rich or lean and not what ratio they are reading. There are some that do show the ratio, but it is extrapolated data where the gauge tries to guess how rich or lean it is based on limited input from the sensor. I know this doesn't sound very clear, but do some google on narrow versus wideband and you'll find lots of good reading. The thing is the wideband O2 sensor is the majority of the cost, they run about $160 just for the sensor.

One thing to think about is this. On 99.9% of new cars they run a narrow band sensor. This helps them to run good in conjuction with a lot of other sensors, but if you add a new cam or major engine mods to a new car you still have to retune the computer. If the new car ran a wideband setup then you could swap stuff around all day and it would totally adjust to the new conditions, thus eliminating the need for expensive programs like "EFI Live" and other programming setups.

Just a thought. Do some research on the differences and save yourself some trouble by getting it right the first time. I fought it for years, but ever since I broke down and spent the cash I am back to loving the process of tuning an engine

George

 

id check my timing first and only after that has been ruled out would i bother with fuel mixture...

 

my engine is permenatly timed from the factory.

 

Uh what? It might of been timed when it left the factory but it isn't permanent. Rotating the distributor from right to left sets the timing.

 

my distributer is rivited to my block. unless theres something im missing

 

never seen one that you couldn't adjust???,
does the motor have a air pump (smog pump) on it?? if so take the belt off and run it and see what happens

 

There are no rivets holding the dizzy to the block. Only thing holding it is a 1/2 bolt thru a clamp that bolts into the block. Take a picture of your base mount of the dizzy.