boggin at full throttle

[crm318]

lastnight i was out gouging on by truck and i noticed that when i floor it, the engine would bog down very badly. this wasnt just a stumble. the truck would not overcome the bog at full throttle. it would bog as long as i had my foot down. what might cause this? i did just put a new vacuum advance on the distributor and set the timing to 12 degrees.

[lowriden58]

sounds like your advance isnt opening or maybe you have a fuel issue

[Billla]

Timing, then carb.

Is the vacuum advance adjustable, and if so is it adjusted?
Have you set your mechanical advance?

If the timing is right, then we start looking at secondary opening - what kind of carb is it and what's it's condition (new, recently overhauled, never touched, etc.)?

[crm318]

the carb is a rebuilt qjet. im not sure if the vacuum advance is adjustable. how could i tell? how would i set my mechanical advance?

[WorkinLonghorn]

I'd check the fuel filter as well. You can tell if the vac adv, is adjustable by looking at the area right next to where the hose attaches. If that area is smooth it's NOT adjustable. If it is five (or 6?)sided,it is adj.

[crm318]

it is smooth and i assume non-adjustable. i have a clear fuel filter and there doesnt seem to be any debris in it, but ill check. the truck runs fine at all speeds accept full throttle.

[d10s69c10]

a Quadrajet has a filter right in the carburator at the fuel line. i had the same thing happen to me i just removed it and the spring and my truck ran like a raped ape.

"i couldent blow thru it to save my life"
almost feels like vapor lock?

[Yukon Jack]

Was having a similar problem with my 70 K5 when I finally got it running with a rebuilt motor. I changed out the fuel filter in the quadrajet and it was a touch better but still having a bog at full throttle. It was also vapor locking pretty bad.

Replaced the mechanical fuel pump and the bog and vapor locking problems were gone!

[haysonj]

there should not be any vacuum advance at full throttle so I doubt the problem is in the vacuum unit. You could cap it off and it will only affect cruise and idle. I would tend to think it is fuel related. may have to large of secondaries or make sure that the secondaries are fully opening when you mash it from inside the truck. Have a friend look while the truck is not running.

[Billla]

Originally Posted by crm318

the carb is a rebuilt qjet. im not sure if the vacuum advance is adjustable. how could i tell? how would i set my mechanical advance?

WorkinLonghorn nailed the adjustable vacuum advance question - the mechanical advance question is answered in my timing FAQ linked in my sig.

The vacuum advance for the Q-jet can be adjusted - it's called an Air Valve. Once the timing is right we can look at that.

Definitely agree making sure the in-carb filter is removed if you have an external filter!

[crm318]

the filter in the carb is removed. probably about 2 weeks ago i got down on it and it roasted the tires off. i dont know how it could come out of adjustment in that amount of time. as far as the vapor lock issue, the truck acts like this reguardless of what temp the motor is or if i just started it or have been running it all day. i assume vapor lock is caused by heat. ill give the dist. and the carb a look over.

[telly]

I just finished fixing a similar problem with my Monojet and inline 6 250ci.

The things I did -

1) Replaced the fouled spark plugs
2) Rebuilt the carb (although it might not have been necessary - should have done this last)
3) Attached the vacuum advance to the proper port (PO had the vacuum advance port from the carb running to the charcoal cannister)
4) Reset the timing and idle speeds

Finally this morning the truck is accelerating like it should.

Hope this is helpful - don't know if there is carry-over from my truck ('70 C10) to yours.

Good luck. I'll be watching your progress.

[crm318]

i just tore back in my distributor and i noticed that the new vac advance was very stiff. i took it off and sprayed some pb blaster lubricant in it ans now it moves more freely. we will see how that works.

[welch_41]

I'm curious if spraying lub in the vac adv. cured your problem. I'm having a similar issue with my 66 C10/250. The carb has been rebuilt and timing set to 4 deg BTDC. When I first bought the truck it had a Holley carb on it but I couldn't find a rebuild kit for it so I picked up a reman Rochester Monojet. Thats when my problem started. From reading previous post, does an inline fuel filter make that much of a difference over the filter in the carb? Thanks for any help.

[OrangeCrush1970]

Originally Posted by d10s69c10

a Quadrajet has a filter right in the carburator at the fuel line. i had the same thing happen to me i just removed it and the spring and my truck ran like a raped ape.

"i couldent blow thru it to save my life"
almost feels like vapor lock?

raped ape is good, I might have to try that one