[tr709]

All, I've got a good head scratcher for you.

First here's the issue - on my 67 firebird, mostly when outside temps are above 80 and the car has been driven for a while say 30 min, it develops a sporadic bog or carb overload at WOT(can't 100% tell), happens as it is winding out. I do smell gas when it happens.

Now here's the crazy part. This has been happening since 2005 and since then here is everything that has been changed in going from a 350 to it's current 428. Complete engine, intake, carb, dist, plugs & wires, fuel pump, fuel lines, cooling system, master cyl, power brake booster, battery and exhaust.

Here's the only things still the same - Gas tank (new in 2005), trans and converter, electrical, alternator, PS and starter.

My guess is something with the tank (venting?) or some kind of short with the wires going to the distrib.

Your thoughts?

[Formula8]

Check to make sure you have the correct gas cap on and you may as well check your fuel lines while you are
there

[bryansMtngto]

do you have a fuel pressure guage??? seeing any sporatic pressure or pressure drop?
Had something like that happen in an old Bronco years back tank liner seperated and at hard accel below 1.2 tank gas would suck liner against pick up and bog out of gas. Took me a year to figure out til we pulled tank. check all you coil and ignition connections week connection can fail under load also. look for any buldged or stretched wires.

[tr709]

Thanks for the replies. I have been trying different caps, as the 67 & 68's vented through the cap and many of the repo caps only vent one way, in but not out. I have suspected the tank, but didn't have an idea what could be.

Checked the wires before, but will look again.

[BVR421]

"sporadic bog or carb overload at WOT(can't 100% tell), happens as it is winding out. I do smell gas when it happens.'

Here are a couple ideas based on similar experiences on my car and on a friends car.
I dont have an answer but some things to consider.

It could be electrical but Id be surprised. Always a possibility of a wire shorting out as the advance mechanism moves thru its path.

Its running out of gas. It would literally feel, sound and smell the same if the tank were being run dry.
Possible tank vent is creating a vacuum the fuel pump cant overcome or the gas cap/vent tube is totally sealing the tank. The pump cant suck against a vacuum. You car may have a tiny fuel line that was barely capable of feeding a 350 at freeway speed. Now with a 428 and a lead foot its not capable of feeding the carb with a little 5/16 fuel line.
Or, possible the tank pickup filtering screen is clogged or pickup pipe is in wrong spot.
Many people have had a fuel starvation problem when the tank is less than half full. The gas all sloshes to the back and the pickup sucks air. That is most likely in my opinion.
Coincidentally, many Pontiac owners have a problem with gas sloshing out the vent at the tank cap when the tank is over 1/2 full so to fix one problem another is created. Craps! Likewise we drive our old cars so little they seldom have more than a half tank in them.

[tr709]

Thanks.

Sending unit is new - Robb Mc 1/2 inch
Lines are new - 1/2 inch to pump, 3/8's to carb and 3/8's vapor return
Pump is new - Robb Mc 550hp

[torqjunki]

Is this a q-jet? I have tried a q-jet on my car with a 550hp Robb Mc pump. I have more motor and have stock fuel lines. I realize this is not a good comparison to your set-up, but I have the same problem when trying to run the q-jet, only my problems start as soon as the secondaries open. The carb has been modified by Cliffs book with parts from him. I have run three different Holley style carbs with no issues. I would try a borrowed known good Holley carb and report back. My setup starves so quickly that I am not convinced that a 1/2" fuel line and sender will be enough. THere is a diminishing return with larger fuel line to the rear as the weight of the fuel is heavier and requires more pump to overcome acceleration forces to pump it forward. Also not crazy about the heat that the mech pump puts in the fuel close to the carb. I will be going to a Mallory comp 140 pump with return reg using stock fuel lines and will re-try the q-jet. I will go to a 1/2" fuel line if nessesary. I will also have the pump wired through a oil pressure switch so it will kill the pump if the motor dies in an accident.

[gold bird]

I have the same prob as u do with the Q jet only in to the secindaries, my 850 holley has no probs i have 1/2 fule lines to the reg with a holley blue . put in a home made picup 1/2 inch all so have not tryed the q jet with the pic up yet let us know what fix it .

[tr709]

Yes. I'm running a q-jet, funny thing is it also happened with an edlebrock carb and smaller fuel lines too.

Combo
72 400 block w/428 rotating assembly from PPR
D-port 96 heads rebuilt and ported by butler (flow approx 240cfm)
comp cams hyd roller XE276HR - 276 I - 282 E 224 I - 230 E .502 I - .510 E
q-jet - rebuilt by Cliff
RobbMc 550hp pump and 1/2 inch sending unit and lines
OE intake ported by SD performance
pertronix ignition 12 deg initial 36 total - all in by 2500
RARE long branch manifolds into duals with dynamax super turbos
Turbo 350 trans - unknown make on converter. SB 2200 stall

[Sirrotica]

Vapor lock, Q jets are susceptible to it and combined with Pontiacs running hotter than say a small block chevy. Insulating the line and having a return line to keep the fuel circulating to keep it from picking up underhood heat will go a long way towards helping stop or reduce it. Also making a sheet metal deflector for the front of the carb to keep radiator heat away from the carb helps.

When I raced oval track the class I raced in required a stock carb, Q jet and if the car sat on the track running during a yellow, when the race was restarted the car would undoubtedly run out of fuel when the green came down. I believe without insulation of the fuel line and a return line you may be experiencing the same phenomenon. This is compounded with having fuel with ethanol as it lowers the boiling point. If you have ever noticed Pontiac started insulating the fuel line from the pump to the carb on the later Trans Ams starting in 1977, so even the factory recognized there was a problem with vapor lock. The Q jets have a much smaller float bowl than a holley does and I think the added volume masks the problem that is much more evident with the smaller reservoir of the Q jet.

The gas smell is the fuel vaporizing in the carb and lines and blowing out the vent tube in a vaporized state. Remember the needle and seat can only work when there is liquid in the float bowl to float the float and shut the needle and seat off. After enough cooler fuel runs through the system it stops the vaporizing and re-fills the float bowl and the car then takes off and runs as it should.

Just some of the things I have observed in my years as a mechanic and plus using a Q jet as my choice of carbs on my oval track cars, hope this helps....................

[tr709]

Thanks,

I am running a return line and the lines are insulated where they run close to the motor on the way to/from the pump. line from pump to carb is not insulated, so I'll give that a try.