Engine runs when bottle fed but draws no fuel through two different carbs.

[Paxman]

Hi all!

I'm helping a friend that was given an old Simplicity snow thrower with a Tecumseh engine that we have no info about.

I did the usual primary checks: spark, fuel directly into cylinder and got it to cough off. I then dunked the carb in the ultra sonic cleaner and we tried to start the thing. When spraying fuel directly into the carb it runs with good power but as soon as I stop feeding it it dies. I found an old carb that is the same with a little smaller throttle throat diameter and tested...same behavior, it starts and runs as long as I squirt fuel directly into the carb and dies as soon as I stop. I tried to cup the carb to force fuel to be drawn from the float bowl (which gets filled) but nothing.

I'm totally at a loss here. We know it has spark, we know that there is compression and obviously it has air. The only thing missing is the fuel...but why doesn't it draw it from the bowl with either of the two carbs? I have tried to see if there is any intake leaks but I can't see anything that indicates any cracks or bad gaskets. Also, when I cupped the carb there was pretty good suction.

We have ordered a new carb (Chinese from Amazon for about $10) but to me that's a shot in the dark since we tested two different carbs and there was no difference.

Any one with any idea what can be the issue?

Kind regards
Fred

 

[hlw49]

Model spec and serial no of engine so we can look at what you have. Throwing a carb in an ultrasonic cleaner doesn't necessarly clean all the passages in the carb. Sometimes you have to run something through the orfices to clean them out. Honda has a good set of orfice cleaner but pricey.

 

[Paxman]

What I know is that it is a Tecumseh hm80. Don't think I have seen any information plate.

I did use compressed air through all the passages and even some carb cleaner through the pick up pipe (don't know the name of it...is it emulsion tube maybe?) and it looked to be good flow thorough it.

Also, since the behavior is identical with two different carbs it seems unlikely that they would have the same restriction of fuel...but hey, you never know. The new carb will be delivered in a few days so then we will get the answer if it's carb related or something else. The latter is why I started this thread. What other reason can this engine behavior be if it's not carb related? I've been racking my brain but I can't really think of anything else.

 

[Scrubcadet10]

Fuel pump or gravity feed?
take the line off at the carburetor and see if you have fuel present there.

 

[Paxman]

Gravity feed. There is good flow in the fuel line before the carb and the bowl does fill up. It has the needle screw in the middle of the nut that fastens the bowl to the carb so I have been checking that there is in fact fuel in the bowl.

 

[Rivets]

Two things to check. First the needle is the high speed fuel mixture screw. Intial adjustment from lightly seared is 1 1/2 turns out. Second on the side of the carb you will see a small plastic cap. Under this cap you will find the idle mixture jet. Remove this jet and make sure that both orifaces, horizontal and vertical, are open. They like to plug causing your problem. Here is a service manual which may help you out. https://www.mymowerparts.com/pdf/Te...P-4-CYCLE-L-HEAD-FLAT-HEAD-ENGINES-692509.pdf

 

[Paxman]

First the needle is the high speed fuel mixture screw.

REALLY?!! That would explain EVERYTHING!

Now that you told me this it's 100% logical. Why would a drain screw have a needle? *face palm*

This is the first carb I have messed with with that kind of bowl screw so I just assumed that the knurled screw was for draining the bowl. It's a snow blower and I know that they run a higher risk of getting water in the tank and float bowl. The needle part did confuse me but I didn't think of that the main jet always being in the bottom of the bowl.

Wow...I feel a little stupid now. I'll have to check with my friend to see if he has ordered the new carb...

Thank you so much Rivets! I'll report back when I get the machine running...which probably will be next time I can get my hands on it.

 

[Paxman]

Ps And thank you for the service manual! Ds

 

[Paxman]

Rivets had the correct diagnosis! I turned the screw 1 1/2 turns out and my friend pulled the cord...the machine started on the first try. I did some tweaking of the high and low screws and the engine ran really good!

Unfortunately either the drive for the auger or the wheels worked as they should, and I think that spare parts for this machine is almost impossible to get here in Sweden.

This machine shows heavy signs of having been ran hard and put away wet so I really don't think it worth the time and money to fix it...but I'll let my friend decide. He got it for free and in my opinion he should sell it for a few bucks and buy another one. It's way to big for his yard anyway.

Thank you all for your help! I will never mistake the main jet adjustment screw for a drain screw again.

 

[Rivets]

Before scrapping out this unit please make one more check. Auger and wheels not preforming properly are normally caused by one of three things. Bad belts, bad tensioner pulleys, or bad tensioner arm. With these old mules I would be trying to repair before buying 90% of the new pieces of crap on the market today.