Starving for fuel.

L

LRiddle

Well-known member
My 51 Cub runs like a top, when it's running right. The engine is strong and it purrs like a kitten all through the rpm range. The problem is that after a bit of work it will start sputtering and try to die out. The only resource at that point is to run it on full choke to keep the tractor running. At full choke it will idle at full rpm when she's acting like this, but when you try to drive on full choke it bogs way down. But eventually after it sits long enough, or you slowly take away choke and add throttle it will go back to running like a thoroughbred again. So my first thought is, if it will only run at full choke when she gets in these moods, it must be a fuel issue. I took the original IH carb off yesterday and it was nice and clean. The float moves freely, the jet is clean and everything seems to operate like it should. I put it back together, tried to go mow and the same thing happened. Ran beautiful for a bit until she remembered that she wanted to mess with me and then tried to die out on me. What am I missing here? I'm thinking of just sending the carb out to Denny's just to make sure the carb isn't the issue.

Edit: I also took the fuel line off to make sure it isn't the sediment bowl, but it seems there is enough fuel running through it to feed a hemi, so I don't think that is it. The carb is getting the fuel.
 
You should check the rest of the fuel delivery system, such as the vent holes in the cap, the sediment bowl screen and lines, ahi in-line filter which may have been added, etc. There also have been owners with a piece of trash in the tank which would bock the fuel outlet randomly, driving them crazy until they found it and corrected it.
 
Next time it starts acting up, remove the gas cap and see if the problem goes away. The vent may be blocked creating a vacuum impeding fuel flow.
 
I actually thought of that. My filler neck was leaking, so I bought an extra cap seal and put the two together and it sealed perfect, but my vented cap is no longer working and it did pull a vacuum, I haven't spent much time looking in that direction, but I have since pulled the extra seal and I don't run with the tank full avoid cap leakage.
 
LRiddle":1iwbz5he said:
I actually thought of that. My filler neck was leaking, so I bought an extra cap seal and put the two together and it sealed perfect, but my vented cap is no longer working and it did pull a vacuum, I haven't spent much time looking in that direction, but I have since pulled the extra seal and I don't run with the tank full avoid cap leakage.
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Why not just get a new cap? They're free.

http://www.ihgascap.com/index.aspx
 
I did, using the link at the top of the page. Ordered one for my Cub and one for my dad's M. That was a few weeks ago and I haven't heard anything about it since.
 
There is a small strainer in the fuel line fitting that connects to the carburetor. Make sure that is clear as well.

Bit of a dumb question: does the needle valve seat screw in? Our model H started having fuel supply issues and it was because the valve seat backed out to the point where it closed itself.
 
Puffie40":2ks6yvyy said:
There is a small strainer in the fuel line fitting that connects to the carburetor. Make sure that is clear as well.

Bit of a dumb question: does the needle valve seat screw in? Our model H started having fuel supply issues and it was because the valve seat backed out to the point where it closed itself.
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I plan on taking apart the entire fuel system to check on a few things. I have not looked at the fuel screens. I also didn't spend much ruined in the carb. I checked the float and needle for movement and made sure the jet was clear. I guess I should do some more exploratory work.
 
Puffie40":2phd5c0r said:
There is a small strainer in the fuel line fitting that connects to the carburetor. Make sure that is clear as well.
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IHcarb inlet screen.JPG Here's the IH carb screen that is referred to, you'll probably have to remove the carb to see it. Don't be surprised to find it missing, they tend to get removed by someone who wants to "fix" the problem.
 
There's a forum member, a little over an hour from you, who knows plenty about carburetors and likes to have mini-fests. :D Maybe he'll contact you soon.
 
Don't send the carb off to Denny's. You just need to do a quick google search to find out why.

I see you checked the carburetor, but did you check fuel flow from the tank? Disconnect the fuel line from the carb and let it run into a catch can for a gallon or two while you watch. The flow should be strong and steady. If at any point it changes you need to look into what is restricting your fuel flow.

Another possibility is valves. If your valves are adjusted too tight they will not seal properly when the engine heats up and cause the engine to sound like it's starving for fuel. Worst case is a burned valve, but I have never heard of a Cub with a burned valve.
 
Matt Kirsch":dqf50fc6 said:
Don't send the carb off to Denny's. You just need to do a quick google search to find out why.
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Thanks for the advice, I found plenty of reasons fairly quickly.

I see you checked the carburetor, but did you check fuel flow from the tank? Disconnect the fuel line from the carb and let it run into a catch can for a gallon or two while you watch. The flow should be strong and steady. If at any point it changes you need to look into what is restricting your fuel flow.
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I did do that, but not for a couple gallons, only for maybe a half gallon. I'll do that tonight.

Another possibility is valves. If your valves are adjusted too tight they will not seal properly when the engine heats up and cause the engine to sound like it's starving for fuel. Worst case is a burned valve, but I have never heard of a Cub with a burned valve.
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I sure hope it isn't that. Hopefully I can get this righted before it comes to valve work.
 
Matt Kirsch":2xyzdaqj said:
I see you checked the carburetor, but did you check fuel flow from the tank? Disconnect the fuel line from the carb and let it run into a catch can for a gallon or two while you watch. The flow should be strong and steady. If at any point it changes you need to look into what is restricting your fuel flow.
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You bring up a good point. Shine a flashlight into the tank and see if any sticks/pebbles/foreign objects are sitting at the bottom. all it would take is something sitting on the fuel outlet to block the fuel flow.
 
Update: Yesterday walked next to the tractor while my dad mowed and I noticed that the sediment bowl wasn't keeping up with the tractor's needs. It's one of those frustrating things you can't see because it only happens under load at full RPM. I'm not sure what the problem was exactly, since I kinda went all out in cleaning yesterday. I pulled the line from the carb and drained the tank empty. It seemed to have good flow, and I didn't notice a considerable change, but then again I've never seen how it looks when's it right. Then I pulled the sediment bowl and flushed it, the tank, and the fuel line with cleaner. I disassembled the sediment bowl and cleaned the screen. Then with the tank dry, I used my smaller shop vac to suck up anything that could be in there. It looked pretty clean, but I wanted to make sure. Then blew all parts off with air and reassembled and fueled up with new gas and put on my new vented cap. So somewhere in there was the problem, be it the gas cap, the sediment bowl, the line, dirty fuel, or trash in the tank. Either way, I mowed for about a half hour today until supper with no problem. That's far more than it could do this weekend. I'm going to mow all three acres Thursday, so fingers crossed.
 
While cleaning a tank today, I found one of the plastic seals, like you find on Seafoam cans, in the tank. Tank came from a non runner, so I didn't have any issues with it, but, when washing the tank, it floated over the outlet hole, and partially blocked it. Being clear plastic, it could be hard to see, in the gasoline, and occasionally float over and block the fuel flow.
Ed
 
I'm glad you got your fuel system all cleaned out. I am going through the same thing right now. There's a bit of rust in my fuel tank that I need to clean out, then I will red kote the tank. It can be very frustrating!

However, PLEASE do not vacuum the fuel tank! If there are any remaining flammable vapors in there, they may get drawn into the shop vac motor. A standard electric motor will produce sparks any time it is running. Potentially very dangerous! :shock:

Edit: I guess Jim Becker beat me to it...
 
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