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Still fighting the gremlin...

K

Kp

1/2 ton status
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Apr 5, 2006
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Location
Tucson
Well, I cracked open my carb this weekend, in my never-ending quest to figure out why my sub can't be driven more than about 25 miles.
Quick backstory, I don't drive this truck much, mostly because if I drive more than 25 miles or so, it will start to act like its gonna die, have no power, and generally act like its running out of gas. Once that happens, I can pull to the side of the road, idle for a minute or so, and then it will be able to accelerate. Sometimes I can go for a couple miles after that, and sometimes I can only go a few hundred yards.
Here is what I have done so far in my attempts to fix.
Replaced the fuel pump. First with new mechanical, then ditched that and went with electric. No change. I changed the fuel delivery from a filter with a return line, to a deadhead system. I am getting about 6 PSI to the carb. I replaced the ignition module in my old distributor(HEI), then bought a new complete Summit HEI disty. I have new plugs and wires.
The funny thing is that up until the second that the truck starts faltering, it runs like a champ. Engine is a remanned 454, with about 5K on it.
It is really hard to troubleshoot this, because it ONLY does this on long drives. I drive it to the dump every weekend, which is a 20 mile round trip, and have no problems. BUT, if I try to drive into town, about 25 miles, it will act up everytime. It acts like it is running out of gas, even when there is gas in it, or maybe like all of a sudden a huge vac leak opens up, and it seems to be worse when the ambient temp is hot. The temp gauge works, and the truck does not get close to overheating when this happens.
Any help would be appreciated. This is our second vehicle, but it would be nice to actually be able to count on it.
 
Oh yeah, reagdring the carb, it is a Q-jet. The number is 17058212, which I think means it is from a 1978 chevy truck with a smallblock, and AT.
The primary jets are 70's, and the rods are 34B's
the secondary rods are DH's. and the hanger is an "M".
As far as I can tell, that combo didn't come in that carb, so it has likely been "tuned" by a PO
The carb must've been recently rebuilt, because the gaskets, and float looked BRAND new. I did notice that the power piston was sticking in the bore. I think I fixed that.
 
Is the vent on the tank/gas cap working? Next time it happens loosen the gas cap and see what happens.
 
toomany said:
Is the vent on the tank/gas cap working? Next time it happens loosen the gas cap and see what happens.
Click to expand...

x2, check the tank vent lines and charcoal canister. Maybe just replace gas cap with a new vented one.
 
I can't remember if the cap is vented or not. There is no charcoal canister though. I am not sure if it came with one from the factory. The truck was originally a 350, but a PO had swapped in the bigblock, and when I got it, I put in the rebuild.
I'll spring for a vented cap and see if that works. I hope so, because it sucks not being able to use this truck for anything but short trips.
 
What year is it? Does it have one of those filter bags on the pickup line inside the tank? If there is dirt in your tank it can clog up the filter bag and then run like crap, and then if it settles away from the sock during low or no fuel consumption it will run fine for a while again until the dirt get's pulled back to the filter bag sock thing. (yes those are techinical terms :) )
 
Its an 83.
I haven't dropped the tank yet to check for debris in the tank. Mainly because it looks like it will be a PITA to wrestle that big thing back into place.
I think the cap may not be vented tho. It says something like "beware of fuel spray when removing" or something like that on it. I guess if it was vented, that would not be an issue. I have to admit, the gas cap is not something that I had considered before now. If that doesn't work, I guess I'll have to drop the tank, unless someone can think of a clever way to look for debris, without dropping it?
 
If your still getting fuel when it does it then it would be neither, unless it is dirt then it settles away when it dies.

Can you read the fuel pressure when you are driving it ?

That would help to diagnose.

Does the carb still have it little filter ? in it

Could it be vapor locking any fuel lines close to a heat source ?
 
I don't have an inline fuel pressure gauge to read when the problem occurs. When it first happened, I was convinced that it was vapor-lock, but after changing fuel-pumps from mechanical to electric, and changing fuel delivery from deadhead, to return, and back again, I think I have covered those bases. Not saying that it still couldn't be that, but with all that being changed, I think its unlikely.
I took the little filter out of the carb. The electric pump itself has a filter built in, and I have an inline filter as well.
 
Well, it looks like the gas cap IS vented. One way valve that will allow air in, but not out.
 
Sounds a bit like percolation. Basically the fuel is boiling inside the float bowl. I had that condition on my truck as well. I solved it by putting a heat blocking spacer between the carb and the manifold.
 
I have tried the carb spacer. I have a 1 inch Mr. gasket phenolic spacer under the carb.
I am telling you guys, I have tried almost everything. This is driving me crazy. I guess dropping the fuel tank is in order.
 
just a random thought, but have you checked that the choke isnt closing or something?
 
Kp said:
This is driving me crazy. I guess dropping the fuel tank is in order.
Click to expand...

Well I take no responsibility for this if you try it because it could be dangerous if you aren't very careful to not spill fuel. But if you run a temprorary line from a seperate container with a few gallons of gas to the fuel pump inlet just to run a small test sometime you can figure out if the tank is the problem before you remove it. If the problem goes away, then drop the tank and fix it. If the problem remains, it's not the tank.
 
Chevy305 said:
just a random thought, but have you checked that the choke isnt closing or something?
Click to expand...

this is what i was thinking, i had a similar problem several years back with a car, although i was climbing up Mt. Lemon (yes i lived in Tucson awhile back) and the air temp change drastically, that is what caused my choke to close on me and essentially choked out the motor, no power and had to pull over, it was still running, just like crap though.

pulled off the air cleaner, held the choke open by hand, all went well, so i readjusted the choke spring tension and all was gtood from then on.

run it till it does this again, pull the air cleaner lid off and see where the choke butterflies are, wide open or nearly closed.
 
When I first put on the carb spacer, the rod that connects the choke coil on the manifold, and the choke linkage, was too short, so I left it off. During that time I had it zip tied so the choke stayed open, and I still had the problem.
 
Well, I thought I should update this thread. I bought a remanned Q-jet and put it on this week.
Unfortunately, it made no difference. I am completely stumped. My wife has pretty much had it, and is ready to sell the truck.
I may switch back to the mechanical fuel pump, because I checked the electric, and it is only giving me 3.5-4 PSI. I've read that Q-jets don't need a lot of pressure, but I don't know what else to change.
I hooked up the vac gauge, and did some tuning when I put the carb on. I was able to get about 17 in hg, and a steady needle, with 8 deg. initial timing. I did notice that when I disconnected the vac advance to set the timing, that I couldn't get a steady needle on the vac gauge. It did clean up once the vac advance was reconnected.
It still runs good, right up to the point where is hit the wall. throttle response is good, and it will break the tires loose with just a quick blip of the throttle.
I don't know what I am left with? Flat cam? But wouldn't it run bad all the time? Not just on hard acceleration, or after a period of driving?
I need this thing to work. I got drawn for Whitetail, and I need to go scouting!
 
If your only getting 3-5 psi that would be your problem and the symptoms tell it too. Factory is around 5-8 if your high is the factory low it would run but when you get it going down the freeway its going to run out cuz the pump can't keep up. When you stop and idle it catches back up and everything works right again. Didn't know that you had that low of pressure or would have suggested it sooner.

Factory pressure may even be higher than that I know when I was running a guage in my Nova when you would get on it it would drop alot as the fuel got used like four or five PSI drop. so if your at 3 then you have none when you get on it.
 
Reread the whole thead replace the pump get a high flow something you can go electric or mech sounds like your motor may be built a bit if its spinning the tires so it probably needs a little more anyway. Make sure the system is a dead head one too, ends at the carb no return line won't build pressure if it has a return
 
Yeah, it seemed pretty low to me too. I check the pumps internal filter, and it was very clean, and I checked the inline filter under the hood, and I was able to easily blow thru it. (very scientific test method huh?), so they don't seem to be causing the low pressure.
I also did a test to check for volume, and it also seemed marginal. I read somewhere that your pump should be able to pump a pint in less than 30 seconds. I tried it, and it pumped it in 28 seconds. I don't know how true that is, but I had heard it somewhere, and I figured it was worth checking.
 
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