Help Diagnose This Problem

[BigDave]

I have a completely original factory 402BB. The fuel pump, in-line fuel filter, and all rubber lines were replaced over a year ago.

Recently, after the truck has been running awhile and usually when the AC is on, I have been experiencing an engine power loss at WOT (wide open throttle). It happens more frequently when the AC is on but it has happened without it too.

I can be rolling down the road, slam the throttle, and near the top of the shift, the engine scavenges hard like it's running out of gas. If I let off and coast a bit until it recovers and re-apply reduced throttle I'm back where I was.

Again, from dead stops, at WOT near the top of shift it scavenges like crazy.

I know it has to be a fuel delivery problem. It sounds like the secondaries are just starting to open good and then, WHAM the bottom drops out of it. I was wondering if it could also be loss of vacuum. Recently I've noticed when I drive at highway speeds for awhile I have less brake pedal than when I started on completely cold brakes.

Any help would be appreciated.

[71cadc10]

does sound like a fuel delivery problem. Are you running a Qjet?

loss of vacuum won't have an effect on your WOT performance. Idle and part throttle would suffer though. It's more likely fuel related.


If so the inline fuel filter at the carb inlet could be blocked. within a year this can happen. I take the inline filter at the inlet out completely and run a better filter upstream farther.

Qjets don't have much fuel capacity with the single center fuel bowl anyway. When the secondaries are open (WOT) for a while going up thru the gears a lot of stock Qjets will run out of fuel top of 2nd gear. A restriction in the inlet filter aggravates the problem....

[draino76]

Sometimes the inside of rubber fuel ine can go bad and when the flow rate is high enough "like at wot" the inside of the line can peel back and shut down the fuel flow untill the flow rate drops, if it gets bad enough the truck will only be able to idle. I have seen this a few times and if you have never seen it before it can take a long time to figure it out.

[CHEVY & GMC FOREVER]

WHY YOU RUN'N THAT OLD 402 THAT HARD ANYWAY????

[ZOOMY]

Does it still have the "stone" filter that's behind the BIG HEX fitting on the carb. If that filter is there, make sure that you can blow air easily thru it. If it's a paper replacement, make sure it's intact & not disintegrated & blocking flow. Is your gas cap check valve OK ? If all of this is OK, then I'd temporarily hook up an electric pump to your gas line by the mechanical pump line & see how much gas it will pump. The electric should pull at least a gallon in 4 minutes. If it's less than that, check for a bent or crimped line between the tank & pump. Could also be restricted in the tank.

[John Fabris]

Does the gas tank have a vent? Next time your truck starts running bad, pull over and open the gas cap, if you hear sucking sound, you need a vented cap. Just a guess...

[BigDave]

Thanks, these are all good areas for me to start looking.

The carb is a factory Q-jet. It still has the factory original distributor with points and it still has the factory original gas cap. Since it's a big block it also has a return line.

I will check the filter at the carb and the in-line filter this weekend.

Thanks again.

[68C15]

did this happen all of a sudden or progressively get this way?
I see you have a points system so the module going bad is not the issue. a weak coil could do this. check the points spring. use a tach-advance timing light & with it in neutral bring it up to the speed where the problem occurs while noting the timing. it should be steady & smooth.
a vacuum guage would be helpful also.
could also be a problem with lifters bleeding down not opening valves fully but I am steering more towards ignition related.

[53 Chevy]

It Could Be The Sock Filter In Your Fuel Tank. Take Gas Cap Off And Remove Line From Tank To Fuel Pump, At Fuel Pump. Place Air Nozzle Into Hose And Blow Into Tank. This Will Remove The Filter And Let It Settle To Bottom Of Tank. Put Hose And Cap Back On And Drive. You Can Leave The Filter In The Tank And It Will Not Hurt Anything. Hope This Helps

[cdowns]

there's also a sock filter inside the gas tank on the pickup tube that get clogged with crap

[busterwivell]

Sounds like you want to keep it stock. When I still had my Q-Jet, I found the same type of problem and it kept getting worse. When I checked the filter in the carb, it had been overtightened (wrong filter?) and twisted like a rag. I eventualy switched to an Edelbrock carb and haven't had a problem. I also switched to HEI, but that's another story.

[Brainchild]

I was having the same problem with my truck,and a new filter solved it.It's a cheap place to start anyway

[modifiedGMC]

I had a 396 once that ran fine as long as you just drove "normally" but if you got down into it hard it would run for a coulpe of seconds then start cuting out like you were pressing the gas pedal and letting off then repeating it over and over. It turned out to be a small block fuel pump on it and it not being able to pump enough fuel due to a shorter arm. The big block pump has a longer arm on it. A small block pump will go on a big block but won't pump enough due to the shorter arm, BUT a big block pump on a small block will cause possible engine damage due to the arm being too long. I replaced the fuel pump and the problem was gone

[JIMs70GMC]

I think he said points. What your status?

[Robert1970C20rstbukt]

I just had the same problem. Driving in town going 30-35 it ran good, on the highway going 70 the engine would surge and cut out. Pulled the dist. cap and took the points out, they were burnt. Changed 'em and it's running good now.

[cduster]

Do you have points or HEI? I have had points float out in the upper rpm range. Just a thought.

Dan