Carburator issue

[justinae]

My 1980 720 started stalling on me the other day. I traced the problem (i think) to the fact that when the throttle is open, it doesn't close the flap on the carburetor very much. In fact it barely moves. When I keep the throttle open with my finger and then close the flap a little more then it idles nicely.I leave the throttle open and let go of the flap, then it wants to stall again.

Brief history. About 4 weeks ago I noticed a small hissing sound coming from one of my vacuum hoses. I ignored it and then that small hole became a big slit. Then I poked the slit and it collapsed the hose and the truck continued to run fine. Well I should have fixed it then, but I continued driving it. Not sure if the issues are related or not. I did replace the hose and thought that would do it but no dice.

How do I make an adjustment to the carburetor flap? Any way to do it without taking the carburetor off?

thanks
Justin

[justinae]

I just did some online searching and perhaps it's my choke flap stuck in the open position? Is that fixable?

[justinae]

Couple of video clips:

http://www.youtube.com/watch?v=u1yYc9hG5wA
http://www.youtube.com/watch?v=aYCi-qIwyIg

[breadbox]

IDK, I wish I had more experience with carbs.

Mine right now, just doesn't close right with I let off the gas, I have to tap it so it returns to the right spot, like it get stick. but your problem seem to be only carb related.

[fastboatman29212]

Justin,
First off, I do not think your problem is related to the choke flap. I'll explain why after I describe a few things.

The flap you are speaking of is called the choke flap or simply the "choke".

Anyway, what's supposed to happen is that when the engine is cold, the choke spring will position the choke flap so that it blocks or partially blocks the opening. That causes less air to enter the carb, the result being a richer fuel mixture flowing through the carb during the warm up process. The choke spring is heat sensitive, so after a few minutes it will slowly open the choke flap as the motor warms up. The choke flap will be nearly vertical if the engine is at operating temperature. The video shows the flap like it should be if the motor was already warmed up. If your carb has a manual choke, then there will be a knob or something in the cab of the truck that you use to "choke" the carb when you start it.

By playing with the flap in the video, you were causing the fuel mixture to run a bit richer, and that caused it to idle a bit better. That, however, is not the cure to the problem.

I suggest that you decide, like many other 620 and 720 owners, if you want to upgrade to a Weber carb or keep the one you have. BTW, I have not heard of anyone who was sorry that they got a Weber.

If you were getting decent mpg with your current carb, then the problem might be as simple as finding a vacuum leak or just running some carb cleaner through the tank to unblock a passageway in the carb. In that case, keeping your current carb may be just fine.

I suggest that you:
1. Inspect all the vacuum lines and replace any that have leaks.
2. Put a bottle of carb cleaner in half a tank of gas and don't add any more gas until it's about empty.
3. Verify that the fuel pump is operating properly.
4. Replace the fuel filter (near the fuel pump)

All that should cost you less than $20 (unless you find that the fuel pump is bad).

Disregard the words on the 2nd photo.

This is what the carb flap should look like with engine warm/hot - operating temp.



Here is what the choke flap looks like with a cold motor.



Inside the "Choke coil cover" is the temperature sensitive spring which causes the choke to open or close depending on the temperature of the motor.

[justinae]

Fastboatman that is super helpful. Thank you. So I caved and called AAA. Amazingly in a 4 hour period of time they picked up my truck, the mechanic had it fixed, and now it's back. All for $80. You were exactly right though, it needed carb cleaner. I guess $80 is expensive, but I would have lost hours fiddling with it.

Maybe it's just because it hadn't been cleaned before but now it sounds more "throaty". Kinda like a supped up car. Finally, when I turn off the engine it does a little bit of a last minute rev and kinda gurgles a little before finally stopping. I'm hoping that issue was related to the carb needing to be cleaned. We'll see....


thanks again!

[fastboatman29212]

The chug-a-lug cough -burp sound after you turn off the key is called "dieseling" and I am not sure if your carb has an "anti-dieseling" solenoid or not. It's supposed to stop that from happening. If it continues, no real harm done, it's just annoying. LOL!

Edit: I went back and looked at the 2nd video and I think the red wire that is on the back passenger side of the carb plugs into the anti-dieseling solenoid.

[justinae]

come to think of it I think there was a wire back there that may have been disconnected. not sure. I'll check tomorrow and report back. Thanks fastboatman! For what it's worth, it didn't do that when I first got the truck. Why would it suddenly start to do that?

[fastboatman29212]

Mechanical things can break anytime they want, especially on a 31 year old truck. However, it could be something else.

[justinae]

good point.

[justinae]

Well I spoke too soon. When I drove it home from the mechanic it worked just fine. It sat for about a week and then I drove it the other day and within ten miles it started doing the same thing. It idles really nicely and I can even rev it up and hold it steady at 3000 rpm. But when I put it in gear I get a few feet and then it just cuts out all power.

Any ideas. I can replace the fuel filter but did that less than 10K miles ago. Fastboatman how do I check the fuel pump?

[fastboatman29212]

Your fuel pump should be mounted to the frame of the truck just in front of the passenger rear tire. First thing to do is have a friend turn the ignition key but not quite far enough to crank the motor. If you stand next to the pump, you can hear it hum as it pumps fuel. I suspect that it pumps fine, otherwise the truck would not have a steady idle.
The next thing to check is the fuel line at the carb. Disconnect the incoming fuel line and attach a piece of spare fuel tubing to the hard line so the fuel can be pumped into a gas can or similar. Repeat the key turn like before. You should get a steady flow of fuel. A gauge to measure fuel pressure is probably not necessary.
I suspect that your fuel pump is fine. It might be dirty and need replacing, but it is not likely the reason for the truck stalling. IMHO you have a dirty carb. I would add some carb cleaner to a half tank of gas and call AAA and tell them the problem isn't fixed and you think they should refund or fix it right.

[justinae]

Thanks. Now that I'm not able to actually drive, do you think I should still run some carb cleaner? Will it work if I just idle? Never used the stuff before so not sure how long it has to be running.

I also wonder if air is getting shut off or leaking somewhere, but all the hoses look fine.

[fastboatman29212]

Liquid carb cleaners that you pour into your gas tank have ingredients that help clean out blocked passages inside the carb. You might not see a difference right away because it may take 2 or 3 treatments for a really dirty carb. That means you will run through 1 or 2 tanks of gas before the carb cleaners have worked. It's important to run the tank almost dry before adding more gas. That way the carb cleaner can work at full strength until it's gone. The carb cleaner additive might not solve your problem within minutes, but if you idle the truck for a half hour and vary the throttle, it just might work. By the way, is your air filter clean?

[justinae]

Yeah air filter is clean, maybe not spotless, but certainly not dirty enough to cause this. AAA just picked it up and sending it back to the mechanic. Thanks again. I'll keep you posted.