HM80 only runs with choke on

N

Nick Felt

Member
#1
#1
I've been building my Super Bronc. I've got the stock carb and the governor linkage all hooked up. It will start with the choke on, after letting it warm up, as I turn down the choke the engine starts to die, it won't idle or stay alive with throttle. Setting are main 1.5 to 2 turns out doesn't help. Low speed is 1.25 out. Idle adjustment screw is set as well. I did not check the float height when i installed the new stock Tecumseh carb. I am not running the fuel pump. I was hoping to not have to use it. It seems as I might be getting enough fuel in the bowl without the pump. Here are a couple of pics of the fuel line routing.

Any help on figuring out the issue would be appreciated.
 

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Mark G

Mark G

#7
#7
Nick Felt said:
I've been building my Super Bronc. I've got the stock carb and the governor linkage all hooked up. It will start with the choke on, after letting it warm up, as I turn down the choke the engine starts to die, it won't idle or stay alive with throttle. Setting are main 1.5 to 2 turns out doesn't help. Low speed is 1.25 out. Idle adjustment screw is set as well. I did not check the float height when i installed the new stock Tecumseh carb. I am not running the fuel pump. I was hoping to not have to use it. It seems as I might be getting enough fuel in the bowl without the pump. Here are a couple of pics of the fuel line routing.

Any help on figuring out the issue would be appreciated.
Click to expand...
I'd pull it off and check the float height first. Also check your gasket for proper sealing. With the frame mounted tank you won't need the fuel pump. What was wrong with the old carb? The rest of the bike looks pretty clean.
 
N

Nick Felt

Member
#9
#9
Mark G said:
I'd pull it off and check the float height first. Also check your gasket for proper sealing. With the frame mounted tank you won't need the fuel pump. What was wrong with the old carb? The rest of the bike looks pretty clean.
Click to expand...
I'll have to do it. It had a slide carb on it from the previous owner.
 
N

Nick Felt

Member
#10
#10
I got it run ok, it does idle but too high. I'm trying to fine to that, it's either too high or dies. I don't have a way to tell the RPM so I'm doing it by ear, it's definitely over 1,000rpm.

If I turn the idle mixture screwin, is that less air or more air?

For the main screw turning in less fuel? So far it seems to run best at about 1.75 turns our on the main and about 1 to 1.25 on the idle mixture screw.

The gas cap is venting well.
 
sparkwizard

sparkwizard

#11
#11
I would take it apart, blow through every passage and put it back together.
I have run into this pretty often on the "new" OEM carbs that have been on a shelf for decades. Bugs, spider webs, flakes of metal from machining and corrosion can clog the idle passages enough to make them run like a Chinaman built them.
Blow it out, reassemble it and let us know how it works out for you.

Dave
 
N

Nick Felt

Member
#12
#12
sparkwizard said:
I would take it apart, blow through every passage and put it back together.
I have run into this pretty often on the "new" OEM carbs that have been on a shelf for decades. Bugs, spider webs, flakes of metal from machining and corrosion can clog the idle passages enough to make them run like a Chinaman built them.
Blow it out, reassemble it and let us know how it works out for you.

Dave
Click to expand...
I took it apart and cleaned it all out. Still about the same, still having the same issue getting the idle set right, it will idle high then after a few seconds it seems to settle down ok, then after about 30 seconds or so of running it slowly dies. There seems to be no real change in the engine hunting or not turning the air mixture screw all the way in to 2 turns out, while it's running.
 
sparkwizard

sparkwizard

#13
#13
Here's a hint: If your idle mixture screw doesn't seem to make any change, but it does kill the engine when you turn it all the way in, you have some kind oh blockage inside your idle passages, where you can't see them.
If you need to idle it above 1000 to keep it running, you are pulling fuel from the main, high speed circuit because...well, I won't say it again.
 
sparkwizard

sparkwizard

#15
#15
Only because your idle is set high. You are drawing fuel from the main because your idle is plugged and the throttle plate is open enough to draw through the main circuit.
 
N

Nick Felt

Member
#17
#17
Bird Brain said:
Remove your main jet and holder and make sure this hole is not plugged somehow. This being blocked would cause your problem. Unlikely blocked on a new carb but not impossible.
Click to expand...
Thanks, it's not blocked. I'll pull the carb again and do another cleaning on it and see what happens.
 
sparkwizard

sparkwizard

#18
#18
Remove the idle mixture screw. Use a tube on a can of carb cleaner and blow into the threaded hole. Wear goggles and look into the throttle end of the carb. You should see cleaner spraying from the tiny holes next to the throttle plate, and cleaner should spray down into the bowl area.
Blow into the vent in the air cleaner side, too.
Also, the first time you install a piece of fuel hose, there may be dust and debris inside it. So, even a new, clean carb can act funny and need to be blown out a couple of times before it acts right. Good luck.
 
Charles S

Charles S

#19
#19
Make sure your fuel filter isn't too restrictive, make sure it's for gravity feed. Filters for pressurized feed are more restrictive than filters for gravity feed. When I bought a case of B&S filters, the box was marked "GRAVITY FEED". A fuel filter for a lawn tractor with a pulse-pump, for example, may be too restrictive for gravity feed. For a simple test, run a one-piece fuel hose from the tank to the carburetor, eliminating the filter and the shut-off valve, either of which may be restricting flow.
 
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