Carb Help needed....

[RottenBanana]

Hey Guys,

So my Bronco has a Holley 4412 2bbl carb on it. When I'm on the gas, if I let off it completely the truck will backfire. Other than this, the truck runs great, so I just feather the throttle down. Without knowing how old/fresh the carb is, I thought I'd have it rebuilt. At the same time, I plan on cleaning up the mess of fittings used for the fuel line.

However, while reviewing stuff online, it looks like my vacuum lines are wrong?

Heres the Holley Diagrams. There is a "Timed Spark Vacuum Port" and a "Full Manifold Vacuum Port"

https://imgur.com/a/ZVWGeEK

My truck has a line from the disti to the "full manifold" port:

https://imgur.com/a/deDZgHI

https://imgur.com/a/Vv1146p

Am I crazy or is this setup wrong? How has the truck run great except occasionally on deceleration?

 

[ared77]

I believe the way yours is connected now the advance is always there, not what you want. You want it to advance with engine RPM so it needs to be moved to the side (timed) port.

Since it's so easy to do, just swap it over to the side and cap off the other one. Test drive it and see what you think. You may have to reset timing afterward to get it all dialed in.

I'm no expert, others will chime in soon, I just wanted to get you a quick answer in case you're waiting! Good luck!

 

[ared77]

“Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

 

[RottenBanana]

So, I swapped it, and just took a quick spin around the block. Overall the truck feels about the same. Maybe a little less sluggish? But the backfire on deceleration is either gone, or will be slightly hard to reproduce.

 

[ared77]

If you're familiar with using timing light it might be time to check timing.

If you're not comfortable with that you can always mark the distributor as is and play around with it to see if you can improve things. Just put a sharpie mark on dist right where it enters block and mark block in same spot. Then loosen the 1/2" hold down bolt and slightly move dist, tighten bolt and test drive. Moving dist clockwise is going to advance the timing, ccw is retarding it. When I say move the dist I'm talking about very slightly, like a fraction of an inch.

 

[RottenBanana]

Funny enough, I have an old school timing light sitting in the garage. I think it's time to learn

 

[gnpenning]

Is the backfire in the carb or exhaust?

Any exhaust leaks

 

[DirtDonk]

Did you also check for vacuum leaks?
And if they use a power valve like most four barrel Holleys you might consider replacing that as well.
Make sure it’s correct for your vacuum level, but when I had spitting and popping problems with Holleys, it was often a blown out power valve.

 

[RottenBanana]

Did you also check for vacuum leaks?
And if they use a power valve like most four barrel Holleys you might consider replacing that as well.
Make sure it’s correct for your vacuum level, but when I had spitting and popping problems with Holleys, it was often a blown out power valve.

So the blown power valve train of thought is what lead me to investigate having it rebuilt. When you had issues, was it on deceleration? Most symptoms I read online seem to indicate it would happen frequently...

Other than the spitting and popping (under certain circumstances) it runs great

 

[Johnnyb]

I believe the way yours is connected now the advance is always there, not what you want. You want it to advance with engine RPM so it needs to be moved to the side (timed) port.

Since it's so easy to do, just swap it over to the side and cap off the other one. Test drive it and see what you think. You may have to reset timing afterward to get it all dialed in.

I'm no expert, others will chime in soon, I just wanted to get you a quick answer in case you're waiting! Good luck!

After you move the ports, you will need to re-time the motor because it will have been set to "normal" with full vacuum advance.
-JB

 

[m_m70]

After you move the ports, you will need to re-time the motor because it will have been set to "normal" with full vacuum advance.

wouldn't timing be the same?? Idle would have to be adjusted but the initial timing shouldn't change.

 

[DirtDonk]

No, it can change drastically.
When you set base timing, then use "timed" spark advance vacuum, there is zero change at idle (or is supposed to be zero when working correctly) but when using full vacuum the advance goes up immediately. So you usually set your base timing lower for full vacuum than you do for timed, or ported vacuum.
If you don't set the timing differently, you at the very least need to re-set the idle speed because full vacuum raises the idle speed.
If you're lucky, you can get the idle speed to a normal range. But some engines don't like full vacuum and must have their initial timing set very much retarded.

But at the very least, you need to check your timing.
And leave the timing light connected as you connect and disconnect the vacuum line to different sources.

Paul

 

[m_m70]

No, it can change drastically.

Oh OK. I always thought that base timing was set with the advance plugged off. Then once connected to full vacuum the idle would have to be adjusted down.

Probably why none of my cars ever ran right with full vacuum and I always use ported.

 

[DirtDonk]

Some carburetors can be adjusted far enough down. But none of mine ever could.
I always had to retard the timing at least a little.
Might depend on how much advance your vacuum canister actually applies. That used to vary by application as well.