GMC 305 v6 Holley 500 swap woes

[AngryPirate]

For those on the gmcclub, this is a repeat post...

So, I got a factory refurb Holley 500 2bbl 4412S wallered out the opening a bit to match the opening on the bottom of the new Holley carb adapter plate, torqued everything to spec, hooked up the fuel, throttle linkage, choke and vacuum advance to the dist.

It runs like ****, when I can even get it running. When it does run, the vacuum advance runs the RPMs up super high. I adjust the dist to lower the rpm but it seems to be slightly misfiring. All other vacuum ports are plugged.

I've tried everything... setting the idle mixture screws out 1 1/2 turns from turned in all the way, messed with the ignition timing and I think I smoked my coil (and I'm sure my starter is about to go too).

It ran decently with the Stromburg WW2 carburator.

This is MADDENING.

Any other ideas?

The only other thing I can think of is the timing chain skipped at some point? It there an easy way to determine the initial timing of correct?


Side question: with the new carb, how does anyone access the #3 spark plug to change it or seat the boot easily?

Thanks for any help.

[geezer#99]

Did you set the float level?

[AngryPirate]

Originally Posted by geezer#99

Did you set the float level?

I did not. I guess I foolishly assumed from the factory setting would suffice, but I'm also ignorant in these things.

Let me do some googling how to do that.

Thanks for the suggestion.

[geezer#99]

Be prepared for some fuel leakage when you open the window on the side.
And some leakage when you adjust the level screw and lock nut.
I always set my float so that when you lean on the fender a little fuel leaks out.

Any other questions just ask.

[MikeN]

From looking at the photo, it looks like maybe you have an unused and unplugged vacuum port? Make sure any ports you aren't using are plugged. Also, if the vacuum advance is causing your idle speed to be too high, switch to a ported vacuum source. This will be the port high on the metering plate in most cases on a Holley.

Checking the float level is vital - fuel should just barely dribble out of the hole with engine idling.

Finally, you'll need to adjust the accelerator pump cam clearance, and possibly the squirter size. You can google how to do that.

All that said, it's CRITICAL to make sure you have no vacuum leaks,. so gotta make sure your seal/gasket to the intake is good.

[AngryPirate]

Originally Posted by MikeN

From looking at the photo, it looks like maybe you have an unused and unplugged vacuum port? Make sure any ports you aren't using are plugged. Also, if the vacuum advance is causing your idle speed to be too high, switch to a ported vacuum source. This will be the port high on the metering plate in most cases on a Holley.

Checking the float level is vital - fuel should just barely dribble out of the hole with engine idling.

Finally, you'll need to adjust the accelerator pump cam clearance, and possibly the squirter size. You can google how to do that.

All that said, it's CRITICAL to make sure you have no vacuum leaks,. so gotta make sure your seal/gasket to the intake is good.

"it looks like maybe you have an unused and unplugged vacuum port"
There isn't but always worth mentioning.

"Checking the float level is vital - fuel should just barely dribble out of the hole with engine idling."
This was a good call. It was waaay too low. Adjusted. Thanks! And thanks also geezer#99 for this suggestion.


"Finally, you'll need to adjust the accelerator pump cam clearance, and possibly the squirter size. You can google how to do that."
I'll check this. I believe it's the nyloc nut with the spring on the throttle linkage side of the carb next to the bowl.

"It's trial and error as to which works best in most cases."
Noted. I'll try the other vacuum port on the back of the carb as well as the one on the intake manifold. The intake manifold and back of the carb vacuum ports are huge compared to the hose size that goes into the distributor.

Thanks again for the insight and suggestions!

I'm now dealing with a spark issue that started while messing with the carb. I think the Pertronix electronic ignition (points replacement) died from the non-stop starting I was doing while trying to get the carb set up.
Coil is good (also got a new one to be sure, but the resistance seemed fine on the old one), ignition switch is sending power and neg side of coil outputs voltage while starting.

[Matt's k5]

It's needs to be ported vacuum source, also make sure the coil has 12 volts while cranking and with key on, a lot of electrical issues come from bad connections at main firewall plug corrosion, this is what supplies 12v to the ignition switch from the starter,you'll have to run a switched 12v supply to coil If your running pertronix to bypass the factory resistor wire, also I've had a Holley 4412 give me grief ,it would randomly flood out for no reason nothing would help ended up switching out for another one , I believe the factory shiny coating was plugging up something, I've been running my Holley and pertronix no since 2018 everyday no issues your welcome to text me at 80-836-6353, for anything related to the V6

[AngryPirate]

Originally Posted by Matt's k5

It's needs to be ported vacuum source, also make sure the coil has 12 volts while cranking and with key on, a lot of electrical issues come from bad connections at main firewall plug corrosion, this is what supplies 12v to the ignition switch from the starter,you'll have to run a switched 12v supply to coil If your running pertronix to bypass the factory resistor wire, also I've had a Holley 4412 give me grief ,it would randomly flood out for no reason nothing would help ended up switching out for another one , I believe the factory shiny coating was plugging up something, I've been running my Holley and pertronix no since 2018 everyday no issues your welcome to text me at 80-836-6353, for anything related to the V6

Thanks for the response Matt!

I do have 12v going to the coil.

I called Holley and they said that the port I was using was the timed vacuum port to be used with a vacuum advance. I was using the oval shaped gasket that came with the adapter plate and not the one with the 2 round holes that match the carbs bottom intake holes.
it seems that the gasket closes off 2 small holes in the bottom of the metering block to create a sealed a "vein" between the 2 holes which should create no vacuum unless acceleration is applied.

It helped for sure, but it's still not running correctly.

I'm out of town at the moment but my next thing to check is to see if my timing chain is correct and hasn't skipped a tooth or something. I guess the "put something in cylinder 1 like a pencil and rotate the crank until the #1 piston comes all the way up and before it goes back down" and that should be TDC on the crank pulley marking?

Thanks again Matt and all for the information. It's been helpful.

[MikeN]

Per my note above - ported vacuum only gives vacuum when you open the throttle, as opposed to full manifold vacuum at all times. Some vehicles came from the factory with ported vacuum to the distributor, others came with full manifold vacuum. It's trial and error as to which works best in most cases.

[geezer#99]

Those uncalibrated pencils are usually off 10 degrees or more.
The only way is with a piston stop.
But first check your timing.
Make sure to plug off all the vacuum ports when doing timing.

Factory timing is 5 degrees before top dead centre.

https://www.summitracing.com/parts/cca-4795

[AngryPirate]

To follow up on what the issue actually was...

The carb, timing and vacuum were all fine. The culprit was very badly set valve lashes.

The truck ran "okay" prior to the carb swap with the old Stromberg. It never ran great, but it ran enough.

Adding this newer, higher CFM carb apparently revealed how bad the valves were set.

Once I got the valve lash set properly for all cylinders, it started up and purred like a kitten.

I hope this helps others to investigate the valves sooner than I did.

Thanks all for your replies and help.

[Matt's k5]

Glad to help anytime, happy your up and running long live the V6!