Wrong carb for 305?

[gerretw]

I have a 68 C20 with a chevy 305 in it. The gas mileage is terrible. More like gas footage. I think part of the problem is the carb. I found the number on the carb, it is a 17085212 quadrajet which I find out is for a 454. I am in a quandary what to do - swap it out? Rejet it? Or is it in need of tuning?

However, I think that isn't the only problem. I smell gas when I drive around. It has a mechanical fuel pump. It also has a dual tank that's not hooked up, and I would like to get it working, however, that's another story.

Back to the fumes - any ideas where they could be coming from? Is there a return line to the tank? If so, where is it? I only see one tube coming out of the tank behind the seat.

Thanks for any help.

G

[Ironangel]

I'd rebuild the carb to small V-8 specs and put it on a new 1/4" gasket. The Quadrajet will get excellent mileage if they're set up correctly. A Quadrajet set up for a 454 would most likely be rich and foul plugs. Your timing needs to be determined and verified when you get the carb rebuilt. There are short rubber hoses that connect the fuel and vent lines under the cab. Make sure one of them isn't cracked and leaking fuel. It's always a good idea to buy the Shop Service Manual specific to your year of truck, https://www.classicparts.com/1968-Sh...ctinfo/04-026/ It will have the carb tune specs and much more necessary information.

['68OrangeSunshine]

A '68 has only the one fuel line out of the top of the tank -- part of the fuel gage sender. No return from the carb like '71 and later trucks.
If your auxiliary tanks are dry, you should get no fumes from them. Usually they were ''blind'' [no level sensors] and plumbed direct to the fuel pump thru a selector valve.
If you get fumes in the cab [with an in-cab tank] you either have a leak from the gasket at the Sender, or the fuel filler grommet is cracked and wet gas gets in when you fill up.

There could be some old gas leaking from cracked rubber lines out of the Aux tanks. Needs checking...

Is a 305 the original engine in your C/20? Manual or automatic trans? I'm guessing with a 305 in a 3/4 ton, the rear end gear must be way low. 4.11 at least. Getting good mileage off the farm wasn't it's intended role.
If that's the original QJet for that engine, then keep it and get the experts to tune it up. If not, there are other carb manufacturers.

Ideally, you want a bigger engine with a smaller carb, and maybe numerically lower rear end gears.

[Steeveedee]

Yes, it is the wrong carb. It's not calibrated for a 305 if it's for a 454. That said, unless it's leaking in some obvious way, I'd wait on finding a correct one. Let's see a pic of the carb. It's possible that there may be an open port on the carburetor, since later engines had emissions equipment that connected to the carb, and your truck doesn't have that. Your fuel smell can be as simple as a bad gas cap gasket, btw. Is it a locking cap?

[gerretw]

Well, the carb was a rebuilt one I bought, instead of having the one on the motor rebuilt - probably not a good idea on my part. I think I thought the motor was a 350 when I swapped the carb, not knowing which block I have. Later on I found the casting numbers and looked it up - it's a 305 from an 83 buick.

When I got the truck running reasonably well, and was driving it around, I discovered how fast it eats gas. It's an automatic.

I'll check out the hoses under the truck - I had replaced one that was shot. Maybe there's another old one?

I put a O ring over the cap gasket - that stopped gas from splashing on the body outside the fill tube ( the original cardboard was shot.)

G

[HO455]

Originally Posted by gerretw

I have a 68 C20 with a chevy 305 in it. The gas mileage is terrible. More like gas footage. I think part of the problem is the carb. I found the number on the carb, it is a 17085212 quadrajet which I find out is for a 454. I am in a quandary what to do - swap it out? Rejet it? Or is it in need of tuning?

However, I think that isn't the only problem. I smell gas when I drive around. It has a mechanical fuel pump. It also has a dual tank that's not hooked up, and I would like to get it working, however, that's another story.

Back to the fumes - any ideas where they could be coming from? Is there a return line to the tank? If so, where is it? I only see one tube coming out of the tank behind the seat.

Thanks for any help.

G

You would be ahead of the game if you located a Qjet from a 305. Not only would the jets and rods be more appropriate for your engine the idle circuits would be correct for that cubic inch engine. There are no adjustable parts to get 454 idle passages down to what is needed for a 305. To make it work correctly would involve drilling, tapping and installing plugs with new smaller orifices in the existing idle passages. Not a job for a novice. Your 454 carburetor has value (Likely worth more than the 305 carburetor you need) so no need to risk it.

The link below goes to my Burban repair thread where I posted (#896) the latest on my carburetor tuning activities. The posts speak directly to what kind of modifications are needed. Do not think the steps I did apply directly to your carburetor as I did not start with a 454 carburetor. I only post it as an example. There are other Qjet posts scattered throughout the middle and last parts.

https://www.67-72chevytrucks.com/vbo...698377&page=36

A carburetor that is running extremely rich will cause the exhaust to smell like gas. If you wipe your finger inside your exhaust pipe does it come out black and sooty? If you hold your hand next to the exhaust pipe when it is idling for 30 seconds (Without touching the pipe and before doing what I asked in the previous sentence ) then smell your hand. Does smell like gas? These are indicators of an over fueled engine.

Everything else that was posted previously is good advice about tracking down the source of the fuel odor.

Good luck and keep us posted.
Oh and Welcome aboard!

[Steeveedee]

OK, looking at the pics, I see that the 3/8" fitting for the bowl vent is capped, which is good. That would have connected to the vapor canister, which of course, you do not have. Since that is capped, it's not a source of fuel stench.

For a possible increase in mileage, find a block-off plate for the EGR valve, which is on the passenger's side of the manifold. You don't need it in your application. First, just disconnect and plug the vacuum line and see if that helps. This engine is designed to run with the EGR, but if it isn't pinging when the EGR is disconnected, leave it like that.

You potentially may be getting oil fumes from the open vent cap. You can get an adapter to go to the base of the air cleaner as a source for filtered air for the PCV. I recommend this because it's just one more stink to eliminate.

[geezer#99]

Looks like he has his pcv hooked up to his air cleaner already.
Doesn’t work like that.
Needs to be hooked to the big port right in the bottom front centre of the carb.

[Steeveedee]

^ Doh, I missed that! You are correct.

[Wrenchbender Ret]

Take a hard look at the fuel pump with the engine idling. A leaking pump can send a spray of gas out the vent hole. It can be hard to see but can waste a lot of gas & cause a smell.

George

[gerretw]

The tail pipe is full of black soot. I'll call the guy who I got the carb from and see about a swap.

Thanks G