driveablity, carbs, gas tank... blah

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

spanky55amg

I'll give u $5, a hardy handshake, & 5 fish sticks
Joined
Jun 6, 2017
Posts
819
Reaction score
212
Location
Dallas, TX
First Name
Spanky
Truck Year
1984
Truck Model
C10 Short Fleet
Engine Size
V8
Ok, so I posted before that my truck has a Holley 650 double pumper which we agreed was too big for a stock 305. It runs rich and dies at stops. I went to fill it up for first time yesterday night and I could only get about 1/3 of a tank. Reading the forums for about an hour last night, I concluded that it is probably b/c of the tank vent valve, which if Im correct is behind the drivers head lights (???). Would this also create driveablity issues as described? I dont believe that the carb is set correctly for idle speeds, as in it is too low. I dont have a tach, but looks like I might get a cheap one this weekend. Also, a vacuum gauge/pump.
 

donnieray

Talks to dogs and trucks.
Joined
Oct 23, 2014
Posts
588
Reaction score
425
Location
Roanoke Rapids NC
First Name
Donnie
Truck Year
84,85,86.
Truck Model
86 C10, 84 K10 MM7 NP833 option truck, 85 K10.
Engine Size
350,355;350.
I know from an old friends experience who put a non vented cap on an old Chevelle. A couple days later the engine died and there was so much vacuum on the tank that it collapsed looked like a beer can that had been stomped. It only held five or six gallons after that. Same could happen if a vent valve was plugged.
 

spanky55amg

I'll give u $5, a hardy handshake, & 5 fish sticks
Joined
Jun 6, 2017
Posts
819
Reaction score
212
Location
Dallas, TX
First Name
Spanky
Truck Year
1984
Truck Model
C10 Short Fleet
Engine Size
V8
I know from an old friends experience who put a non vented cap on an old Chevelle. A couple days later the engine died and there was so much vacuum on the tank that it collapsed looked like a beer can that had been stomped. It only held five or six gallons after that. Same could happen if a vent valve was plugged.

I got under the truck and the tank looks fine. I know I cant see much of the top besides the lines, but it looks fine up there too.
 

1987 GMC Jimmy

Automobile Hoarder
Joined
Jan 23, 2016
Posts
5,848
Reaction score
2,387
Location
Mississippi
First Name
Jesse
Truck Year
1987
Truck Model
V1500 Jimmy
Engine Size
350
I feel like we've talked about whether or not you had an evap canister so forgive me. If you have one, and it's connected, then that's your vent system right there. The system generally consists of the appropriate gas cap, canister, canister filter, and purge valve. If something's plugged like a line or the charcoal canister, that can be an issue. Also, a clogged filter (bottom of canister), a bad gas cap, or a vacuum leak somewhere can be bad. Most evap problems result in gas smells or vacuum leak related idle/driving qualities.
 

spanky55amg

I'll give u $5, a hardy handshake, & 5 fish sticks
Joined
Jun 6, 2017
Posts
819
Reaction score
212
Location
Dallas, TX
First Name
Spanky
Truck Year
1984
Truck Model
C10 Short Fleet
Engine Size
V8
Umm. Well I know things like the purge valve have been removed. This engine has aftermarket valve covers. Im not sure if the gas cap is original, its a faded yellowish color. I should prob get a new one. I also know this truck doesnt have cats. Sooooo good question as to what else is off of the truck.

But, Im trying to figure out why it would only allow me to put in a 1/3 of a tank of gas before it spit it back up the fill tube. And it seems that I have seen the most likely cause being the gas tank vent valve....... which Im not sure if that is on the vehicle or not. When I purchased the truck, the tank had more gas in it. It seems like what happens is the hotter it is, the vent valve blocks up more. And it is about 15-20 degrees hotter than when I purchased the truck.

Im curious as to if the vent valve is blocked/plugged up, would it create an over rich running?

Gah, I need to drop an LSx engine in this thing... something with fuel injection. :(
 

1987 GMC Jimmy

Automobile Hoarder
Joined
Jan 23, 2016
Posts
5,848
Reaction score
2,387
Location
Mississippi
First Name
Jesse
Truck Year
1987
Truck Model
V1500 Jimmy
Engine Size
350
I don't think it would affect driveability that much on an older vehicle. A vacuum leak is not a benign occurrence, and any known ones should be dealt with, but your issue when fueling definitely has something to do with how the tank is venting. I would check for a clogged line between the canister and the sender. The lines should be labeled on the canister if it's still there. The trick I've seen is to blow in it and see if it's clear. I don't know how good that is if there's crud, and it blows back into the sender, but I don't think it'd hurt. You could also check the canister for clogs by doing the same thing. Just put a hose on the inlet and see if air comes out the other side. I think in the carbureted trucks, the valve was mounted on top of the canister so it should all be there and connect to the carb per the emissions diagram. If you didn't want to fool with it, you could also disconnect the line that goes to the sender, install an inline filter, and altogether delete the evaporative emissions system.
 

Honky Kong jr

Super Sarcastic Man
Joined
Jun 14, 2016
Posts
14,968
Reaction score
9,789
Location
Denver,PA
First Name
J-me
Truck Year
87
Truck Model
V10
Engine Size
Lil BB 407
I used a differential breather on my tank. Right, wrong or indifferent it works. I had a few left over ones just make sure it's above the tank.
 

74 Shortbed

Full Access Member
Joined
Aug 12, 2013
Posts
6,306
Reaction score
1,410
Location
*
First Name
*
Truck Year
*
Truck Model
*
Engine Size
*
I used a differential breather on my tank. Right, wrong or indifferent it works. I had a few left over ones just make sure it's above the tank.
Anything will work as long as it's kept away from the elements as much as possible, can always use a mini breather on the end too.
 

Honky Kong jr

Super Sarcastic Man
Joined
Jun 14, 2016
Posts
14,968
Reaction score
9,789
Location
Denver,PA
First Name
J-me
Truck Year
87
Truck Model
V10
Engine Size
Lil BB 407
Anything will work as long as it's kept away from the elements as much as possible, can always use a mini breather on the end too.
Yup.just gotta be able to breathe it ain't rocket surgery.
 

Forum statistics

Threads
41,850
Posts
903,483
Members
33,364
Latest member
Tporter79
Top