Mid Range Engine Performance Issue

[cannunz546]

I have a 1973 with a 302 crate motor with a manual three speed. The motor has about 1000 miles on it since we installed it and rebuilt the entire truck (about 13 years ago). It's run great since then. I put the truck in storage every year for the winter with fuel stabilizer and take out the battery etc. The battery is about 2 years old. I do routine maintenance every year and just replaced the fuel filter. When I took it out of storage a couple months ago it ran great and since then I've run a a couple of tanks of fresh gas through both tanks. About a week ago, the engine started running bad at about 25 mph and up to about 40. It almost sounds like it's missing and near back firing. But it idles fine and the engine seems to run fine after 40mph or so. Before I start digging around, I thought I'd ask all of you your thoughts. You guys have always been great and helpful and the knowledge here is unreal. By the way, the engine has the original spark plugs, cap and wires. I'll post a pic right after I submit this post. Thanks very much for any help!

 

[cannunz546]

Pics

Here are the pictures

 

[Bolt]

Check the distributor to make sure it’s secure. Then check the timing.

 

[broncochevy]

Gunk in the carb?

 

[jckkys]

The distributor is a GM type HEI. Nothing about it is original. The centrifugal advance may have a broken spring. A blockage in the AFB or fuel filter would cause more problems the faster you go and the needs for fuel increase.

 

[cannunz546]

Thanks for the tips, bolt, broncochevy and jckkys. I will let you know what I find.

 

[gr8scott]

Fixed it..............

 

[cannunz546]

Thank you gr8scott

 

[Bolt]

What was the issue?

 

[DirtDonk]

The upside-down pics were fixed, but we're not sure about the engine issue yet I don't think.

When you said "original plugs and wires" cannunz, I assume you meant original to the rebuild 1000 miles ago?
Do you have a part number on that distributor by any chance? A "small cap" HEI perhaps? It's not the standard GM HEI because the cap is too small and the coil is separate. But it might still be one depending on how it's put together inside. Curious which one that is.

Is there a PCV valve in the passenger side valve cover? Looks like a breather vent in the driver's side, but can't see the other.

Paul

 

[gr8scott]

Thank you gr8scott

No problem.

What was the issue?

Yeah all I did was fix the upside-down pics because looking at upside-down/sideways pics makes my head hurt, lol.

Curious as to which distributor that is as well.

 

[cannunz546]

The upside-down pics were fixed, but we're not sure about the engine issue yet I don't think.

When you said "original plugs and wires" cannunz, I assume you meant original to the rebuild 1000 miles ago?
Do you have a part number on that distributor by any chance? A "small cap" HEI perhaps? It's not the standard GM HEI because the cap is too small and the coil is separate. But it might still be one depending on how it's put together inside. Curious which one that is.

Is there a PCV valve in the passenger side valve cover? Looks like a breather vent in the driver's side, but can't see the other.

Paul

Hey Paul, thank you so much. Yes, those plugs are from the new crate motor about a thousand miles since installation.. It does have a PCV valve on the other side. I will get back to you on the distributor. I have to go back and look at the parts list when I had it built. Stay tuned. Thanks again.

 

[cannunz546]

We replaced the plugs and the performance has improved but it's still not where it needs to be. The plugs were not gapped properly. It's a Pertronix distributor so I'm gonna replace the cap and rotor and see if that helps.

 

[cannunz546]

After a season of replacing the easy stuff, I bought a new Edelbrock AVS 650 cfm carburetor and installed it yesterday. That did it. Engine runs perfect

 

[jckkys]

Your 1st photo shows an AVS carb. Is that the one you replaced? If not what did you replace?

 

[cannunz546]

Your 1st photo shows an AVS carb. Is that the one you replaced? If not what did you replace?

I replaced it with an AVS2. Here's some pix. I apologize in advance if they are upside down.

 

[cannunz546]

Pix ......

 

[cannunz546]

Pix again

 

[gr8scott]

I run the same carb, except manual choke version. I ended up tuning mine with a wideband, ended up jetted way down.
Was running too rich. Keep an eye on your plugs.

Also, any reason you're running full manifold vacuum to your distributor?

 

[jckkys]

Just so we all can understand. You replaced a new looking Edelbrock Thunder Series AVS with a truly new Edelbrock AVS2, and that fixed the problem. The AVS2 is the same carb as the Thunder Series AVS except for the booster venturis that are annular discharge like Ford 2100, 2150, and 4100s have used for about 55 yrs. The annular boosters do atomize the fuel droplets better. That's the reason Ford and recent Holleys like the 4180 use them. In Edelbrocks the boosters are modular parts that can be transplanted to other carbs so if Edelbrock would sell you (or anyone) a pair of AVS2 boosters you could easily install them in your Thunder Series AVS and it would become an AVS2. Edelbrock has done this before. The AFB and AVS off road carbs are simply the run of the mill carbs with spring loaded needle and seat assemblies. All that said the different booster venturis aren't likely the cause of the mid-range loss of power. If you disassemble both AVS carbs and compare the the jets, metering rods, and step up springs, you're more likely to find what the cause was.