Walbro LME carb help

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nyalex

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Hi everyone, I'm first time poster that needs a bit of help. I've searched through old topics but can't find much. My problem:

I have a HM80 Tecumseh engine that won't idle. Doesn't seem to get any fuel at low rpm, and if it does, the idle mixture adjustment doesn't do anything. At high power settings it will surge a bit, but I believe that is due to the low end issues.

I've taken it apart, soaked it in carb cleaner, blown it out and put in a new carb kit. I checked the float height and put in new gaskets on both sides of the intake manifold (carb and engine side). This model does not have any Welch plugs.

I'm attaching a few images with the hopes that someone can shed some light on what does what in this carb.

4200822711_4fc6398b54_m.jpg


The brass inlet circled in blue is on the choke/inlet side of the carb goes all the way to the low speed needle towards the other end. Carb cleaner and air confirm this is unobstructed. The low speed needle is circled in red in this picture.
4200822481_31ca9ffb33_m.jpg


The final picture here is of the hole associated with the low speed needle valve. As mentioned before, carb cleaner blown in the brass fitting on picture one has no problem making its way to this hole and squirting out.
4201576332_303872ac5c_m.jpg


I am confused about the actual function of this low speed circuit. During its trip from the brass inlet on the front of the carb to the outlet hole past the throttle plate, does this low speed line have an offshoot of any kind that accesses fuel? Or does the low speed needle simply meter additional air going to the carb, with the fuel being picked up from the main jet in the middle of the carb?

If there is a fuel pickup somewhere along the way, how do I go about physically cleaning it. I'm not convinced compressed air or carb cleaner will do a proper job.

If there isn't a fuel pickup, where do I go from here? The main jet is clean with all holes free of any debris. This is an older carb and the brass nut on the bottom of the carb does not have any holes, those are entirely in the main jet. I'm sorry if this seems like a basic question, but I'm a bit stumped on getting this to run right. Thanks for the help.

Alex
 
I am not familiar with that Walbro. But here are some carburetor generalities. The idle system normally picks up fuel from an orifice or tube submerged in the fuel bowl. It then travels up a passage to an air bleed, which I think may be the orifice you've circled in blue. Then it goes down a passage first to the orifice that is swept by the throttle valve and then further down to the orifice controlled by the idle screw you've circled in red. If you block the two idle orifices with your fingers and spray aerosol carb cleaner through a straw into the bleed you have circled in blue, do you get carb cleaner squirting into the fuel bowl? If not, the pick up for the idle fuel may be blocked. Wear goggles because sometimes the idle system and main system share passages and you may get carb cleaner spraying from orifices where you aren't expecting.
 
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Can't help you much. But, when I took a small engine rebuild class MANY years ago, we used a "dip tank" carb cleaner that would eat the paint off as well as any other passage clogging build up. Maybe some shop around you will "dip" it for a nominal fee.

Warning...it will eat rubber parts too.
 
Does that carb have a main and a pilot jet? If so, did you pull them to clean them and also clean all passages in the carb body with a thin piece of wire? I've got tons of experience with small carbs and have found that usually the problem is dirty jets.

The pictures you posted a pretty small. Hard to see any detail. You might want to see if you can find an exploded view diagram for that carb. Do a search for Walbro carbs and see what you can find.
 
Also....the (sometimes) obvious: Your carb was mounted with a good air tight seal to the engine? And, VERY old carbs can sometimes have worn throttle plate shafts that allow excess air into it where it doesn't belong!

Good luck.

Here's a pdf link to a complete Tecumseh engine manual. Be patient for it to load:

http://www.cpdonline.com/692509.pdf
 
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Is the rubber and brass washer new (and correct) on the screw circled in red?

When I get in trouble with something like this I soak it in stuff that will eat your hand off. That always eats the offending gum or varnish away.

Need to see a picture from the bottom. Is the nozzle plastic or brass? You didn't say how new or old the engine is.
 
For about 24 bucks, you can purchase a gallon of GUNK carb cleanerfrom Advance Auto parts. Even comes with a screened parts tray inside. Soaking in this solvent has cured all but one of my carb issues on OPE
 
Hey Boraticus
Can you define the function of the very fine wire 'spring' which mounts on one of the throttle control rods? Reminds me of a long whisker-not sure what it does though
 
Originally Posted By: steve20
Hey Boraticus
Can you define the function of the very fine wire 'spring' which mounts on one of the throttle control rods? Reminds me of a long whisker-not sure what it does though


Can you post a picture of it? Is it attached to the butterfly valve pivot rod?
If I could se a picture of it, I might be able to help.
 
Hard to tell where one end of it is attached-since one end is broken off. From the geometry of the rod I would say it is attached to the rod that moves when you move the throttle.

The spring is as I said, about as thick as a whsiker (cat or dog, take your pick) and close to one end it is wound around the rod at least 20 times
 
Thanks for everyone's help. I took Flanso's advice and blocked off the two obvious exits for the low speed circuit and blasted it with carb cleaner. Due to the cleaner dripping around, I never could tell for sure if there was a fuel pickup anywhere.

In doing all of that, I somehow misplaced the low speed screw. I got a new one and it seemed to fit much tighter than the original one. I put everything back together it runs very well now. I'll never know if it was the low speed screw itself or all that passageway cleaning I did, but it idles well, doesn't surge, and makes good power. Although I wish I knew what did the trick, I consider this job a success.
 
Good going. Run some SeaFoam in the fuel once and a while, keep the fuel tank clean and make sure your fuel is also free of water and debris, you should be set.
 
The low speed doesn't have a gasket or washer, it just screws in and has a spring to hold tension on it. No O-ring. Parts diagram confirms this is how it was intended. I thought it was strange as well.
 
I believe it is the return spring for the throttle-that expalins why it runs either max rpm or it dies when mving hte throttle.
THANKS GUYS



Steve
 
Boraticus, I'm sure you meant well but your experience apparently doesn't include Walbro LME (and similar) carbs. Removing the main fuel nozzle will disrupt the alignment of a very small (about .020") cross-drilled passage that feeds the idle circuit. This passage is drilled at assembly, and will not align properly when reinserting the nozzle into the carb body. If the main nozzle is removed, a service replacement nozzle must be used instead of the original. The service nozzles have an annular groove that feeds the idle circuit. Alternately, a groove can be machined into the original nozzle, this is very simple lathe work. The service nozzles are no longer available from Walbro, but they can still be found out on the net.

I just performed this procedure on a carb from a '67 JD 112H. It would not idle properly and surged like crazy. The main nozzle had marks in the screwdriver slots. Telltale evidence that someone had removed it, probably to clean it when putting in a carb kit. They just reinstalled it and didn't get the idle passage aligned - there's no marks and you would have to be extremely lucky to get it exactly right. I machined a groove in the nozzle to match the service part, and the engine now idles perfectly.

In short, don't remove the main nozzle from this type of carb unless you have a service nozzle on hand or have the resources to modify the original.
 
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