1980 C10 backfires through carb under load

[Daniel20721]

So I have a 1980 c10 with a 305 I bought in perfect running condition a few months back. I did a edelbrock performer intake, performer 1406 carburetor, plug wires and plugs, cap and rotor on the distributor, recurved it and set it to 8* BTDC and now as soon as I give it any load that's not extreme light acceleration it backfires right back through the intake and then drops really low in rpm and has no power and has no power between a certain rpm range regardless. It will also stall out under moderate acceleration. I have checked the accelerator pump and it squirts fuel just fine so no problem there. if it can get into the higher rpm it seems to run very healthy and strong and I'm lost on what the problem could be because I've triple checked to make sure everything is at the right specs and it still does it. it is super irritating because it ran perfectly fine when I bought it and I'm starting to lose patience. it has also started to bog down at what I can accelerate it to and lose speed instead of gaining it while I can hear the rpm increase. I'm pretty sure its timing I just am not 100%. Carb is in new condition.

 

[SirRobyn0]

Well you have taken an emissions era 305 and stripped the emissions stuff off. No offence, but it's no surprise it's no longer running well, it can be tricky at best to get these 305's to run decent after that. But the most common causes for backfiring up the carb, is too lean of a mixture, too retarded timing, burned exhaust valve or sometimes a flat cam. In your case I'd suspect that you have to heavy of springs in the distributor or to light of weights causing an area of RPM where there is not enough advance when you step on it. Honestly where it comes to the 305 if it's an otherwise stock motor, you'd be ahead to put the factory stuff back on it IMO. If your determined I'd look for a problem in that recurved distributor.

 

[Daniel20721]

I will look into that tomorrow. I originally did not recurve the distributor when I do the carb, intake, cap and rotor, etc... I recurved it after I drove it for the first time after to see if it would help, not sure if that makes a difference .

 

[1lejohn]

More than likely the carb mixture is lean. I would invest in a wide band O2 analyzer and go from there. I had to change the springs in my Dizzy. With the stock springs the truck would ping under throttle at low speeds. i put in a heavy and a light spring which cleared up the pinging. I bought a Jegs spring kit for the dizzy. You also should check your total timing. My truck responded best to more initial timing , and less centrifugal. The Eddy carbs are also sensitive to fuel pressure. Do you have a choke on the carb? I put an electric one on my 1406. Make sure the dizzy vacuum can is connected to engine vacuum.

 

[75gmck25]

Also buy a spring kit for the carburetor and try swapping to higher rated springs.
Stock step-up springs on a 1406 are 5”, so the carburetor goes from cruise to power mode when engine vacuum drops to 5”. The kit will also have 6”, 7” and 8” springs. For example, when you use a 7” spring it will go from cruise (lean) to power (richer mixture) when vacuum drops to 7”. This earlier transition usually resolves a lean acceleration bog.

You might also try advancing the base timing a little more just to see how it handles it. When I had stock iron heads I found I could use up to about 12 degrees base timing measured at about 600 rpm. Also use a setback timing light to check your total timing. If you rev it up to about 3000-3500 rpm you don’t want to exceed about 32-34 degrees total timing.

 

[1lejohn]

My 350 with Vortech's likes 12* base. The 305 may not like as much.

 

[Squirrely Brother]

I had a real similar issue a few years ago that @Ricko1966 guided me through. My issue with changing to an Edelbrock 1406 was way too much fuel. I installed an adjustable fuel regulator from Summit with pressure set to 5.5 lbs and haven’t had any issues after dialing in the timing. Prior to installing the regulator I had detonation, loss of power and fuel fluctuating between lean & fat, that left me stranded multiple times. If I remember correctly my timing is not 18 degrees btdc with a 7 degrees on adjustable vacuum for a total of 51 degrees all in. Also, I dialed in the left and right banks with a vacuum gauge for a steady 21 pounds. Starts right up, idles real quiet and lights up the tires. It’s been reliable since.

 

[Trucksareforwork]

1.check firing order. Ask me how I know.

2. Careful with the advice here. 305 is a different animal. Timing is more like 4-5 BTDC base.

 

[Ricko1966]

I had a real similar issue a few years ago that @Ricko1966 guided me through. My issue with changing to an Edelbrock 1406 was way too much fuel. I installed an adjustable fuel regulator from Summit with pressure set to 5.5 lbs and haven’t had any issues after dialing in the timing. Prior to installing the regulator I had detonation, loss of power and fuel fluctuating between lean & fat, that left me stranded multiple times. If I remember correctly my timing is not 18 degrees btdc with a 7 degrees on adjustable vacuum for a total of 51 degrees all in. Also, I dialed in the left and right banks with a vacuum gauge for a steady 21 pounds. Starts right up, idles real quiet and lights up the tires. It’s been reliable since.

Good to see you man,it's been a while,hope you're doing good.

 

[Squirrely Brother]

@Ricko1966 All is good, same old same old, except for got updated with a titanium hip last year. I hope all is good with you and yours.

I can’t express enough thanks for all of your direction in the past. It was driving me crazy for a bit. The only change I’ve made to the truck was a high torque mini starter, that I made sure to shim correctly, in the past while. Starts quiet like a modern truck now.

Have a Happy Thanksgiving.

 

[Ricko1966]

Okay it ran good before the carb and manifold swap? Then ran bad so you tried recurving the distributor,is this the correct order of things. P.S. 350 is a different bore Vortec is a different combustion chamber and port design,most ofwhat worked for them probably will not work well for you.

 

[Aneurysm3]

Make sure you don't have the 6 and 8 spark plug wires swapped.... ask me how I know that, lol