Ok I give up... Carb issues again

Okay so I drove my truck to Reso and back, not a single issue. I get back and drive around town a few days after changing the oil. So I parked it last Thursday night. I went to move it since we had a big storm coming on Friday morning. Truck starts, let it warm up, and it dies. I try to start it for a bit, then decide to push it int he garage since its pouring.

Since then I have narrowed down what the problem is. Fuel delivery. There is no fuel in the bowl. Now this is what has me stumped. The float is fine. No holes, it moves freely. The needle was sticking in the seat when the float was all the way up, but came out if you shook it. So I replaced it with a new one anyway, both needle and seat. Put it back together, crank and crank and crank, and still no fuel in the bowl. I look through the sight glass on the front of the carb. Its obviously dry. So I pull off the fuel inlet fitting, and throughly clean it, the through bolt the fuel passes through, and I marked where the holes where, on the bolt head, and made sure to line them up properly when it was assembled again. I also sprayed carb clean into the actual inlet on the carb, and not much sprayed back at me so I figure it was going into the bowl. I figured right because it fired for a second and then nothing. I can crank it to wits end and the fuel will not go into the bowl. It fires on carb clean but will not stay running for long. I have also disconnected the feed line off the fuel pump and put it into a cup and crank the motor over on bump. It sprayed out a lot of fuel. Somehow the fuel is not getting from there into the bowl.

I am absolutely out of ideas. I have been trying to get this thing to run since Friday and I can't figure it out. I have done everything short of replace the carb. This carb is professionally rebuilt. I still have the stock carb I rebuilt that I can try, but I would like to avoid it since its a day job just changing them, and then what if it doesn't work and I have to swap it back over?

I'm curious if its possible for a mechanical fuel pump to go bad? Mine has the return line right on the fuel pump. Only thing I can think of is if something internal is shot and its sensing resistance on the carb side and sending everything back to the tank?

could be the pump. I blew up 2 mechanical style pumps. one and a SBC and one on a BBC so its very possible.

Ya the mechanical pumps go out all the time..put a block off plate where the mechanical pump is and get a nice aermotive electric fuel pump. Sounds like a pump issue. My cad stalled on the freeway and the carb is a rebuilt factory and my glass fuel bowl was empty so i tried the cheapest easiest fix and sure enough it was the stupid factory mechanical pump.

Well I wouldn't mind putting an electric pump on it, but I don't have the time to get one, and install one ATM. I need to be able to drive my truck for school this week when it starts again. I didn't know it was possible for them to go bad, which is why I asked. It will be much faster to change that out than the carb so I will see if I can buy a new one today and replace it.

But you said it was pumping fuel right? Even if it isn't pumping enough it should do enough to idle. If it is cheap I would try the fuel pump, but it sounds like that is not it from what you said.

Originally posted by huskerdually



But you said it was pumping fuel right? Even if it isn't pumping enough it should do enough to idle. If it is cheap I would try the fuel pump, but it sounds like that is not it from what you said.

It is pumping fuel, but if I put my finger over the line and have someone crank it I don't feel any pressure, just the wet of fuel. I know the fuel pump is supposed to kick into return mode if it senses more than 4 PSI. I am worried that something may be wrong there. I can't think of any other reason why the fuel would not make it into the bowl. I filled the bowl with fuel and the motor fired and ran until the fuel was out of reach. So there isn't a problem with fuel delivery inside the carb. And I cleaned the fuel inlet port, and fittings, and was able to fill the fuel bowl with carb clean through that source. What I think is that something is bad internally in the fuel pump and it cannot create enough pressure to get past the needle, so its just pumping into the tank. I had a friend come help and he said he could hear fuel going into the tank while I was cranking the motor.

The ironic thing is, I just finished a class for EFI, and this semester I am taking a class on carburetors and fuel delivery systems.

I looked at the pics in your profile. I don't know a whole lot about your particular vehicle but it looks to me like the carb just returns any excess fuel. Hence the two lines on it. If you don't get any pressure it is possible the diaphragm in the pump is bad and has a hole in it or something. Check your oil and see if it looks like it is diluted with gas. I think that's where it would go with your particular motor. It looks easy enough to change and it is probably cheap so it would be a pretty good guess to make, and you wouldn't be out a lot of time and money if it wasn't it.

I have a new diaphragm. I can pull the one out of my other rebuilt carb as well. I just changed my oil, and there was no gasoline in it.

Anyways, I don't know which picture you looked at, but there is a feed line on my carb, that literally goes straight to the bowl. There is the fitting on the front of the carb, that goes directly to the needle and seat. That other fitting on the front of the carb is a vent line, that goes to the charcoal canister. The actual return line is on the fuel pump itself. Once it builds up too much pressure it starts pumping fuel back into the tank. At this point, the fuel is not making it to the carb, its just cycling through the fuel lines back into the tank.

ok. The diaphragm is in the fuel pump, not the carb. Leave it to the Japanese to over complicate a mechanical fuel pump.

I do know for a fact that mechanical fuel pumps can go bad. It happened to me on my 88
Toyota & my 82 Datsun & I was having the same
problems you were. After I changed them both
the fuel problems went away.