Poulan 46 on ignition. Battery, switch check out good.

I have a conversion engine that is surging/cutting out. It idles well, but when the throttle is advanced (fast or slow, worse when cold) it starts to cut out. It never goes dead, and starts easily. It is worse in the air than on the ground. I can lean out the low end so that it almost goes away, but the engine will die if left to idle too long. If adjust it and seem to be running well at top end and take off, the surging returns after about one trip around the pattern, albiet less severe. Never dies, not once.

Gaskets and diaphram seem ok. Fuel draw seems ok. The carb is pressurized via a tube from a bypass port to the top of the carb, and the pulse hole in the body of the carb is closed off. I made a new gasket for the base of the carb and remounted it on the engine. I cleaned everything I could get to. The engine was run a good bit with this setup without issue.

I was going to check the idle jet passage(s) and removed the low speed needle. There is a grey plastic "washer" on the straight part of the needle above the tapered section--before the threads. The washer has a step in it. Is this the "seat"? Should it come out when the needle is removed? It fits very tightly at the top of the shaft of the needle. Since I can't lean it so that the engine dies when given throttle, I thought maybe this has come out of it's seat in the carb body, and is passing way too much fuel to the carb. The high end is not bad, 6750-6800 on a Pro Zinger 20-10.

All the Walbro pictures I can find have too poor resolution to get a good look at these parts. This carb is "enviromentally friendly" and is not on the Walbro site as far as I can find.

Any suggestions appreciated.

Hey guys, I know it doesn't add up. How about some opinions at least. Thanks

I just went through something similar, but mine would cut out and die and finally it just quit running if you did anything like move the throttle.

Mine turned out to be crud in the carb. I took it all the way apart and cleaned it up and reassembled. It run fine and today I flew it for about an hour with no problems, in fact todays the first day I've been able to get more than 2 flights in on it.

Mine was a ZDZ and a Bing carb, but all carbs can get dirt in them, especially if you don't filter the fuel. I double filter mine before it gets into the tank so the tank stays clean.

Anyhow, I vote for a passage with crud in it.

What's the model of the carb? Here is a link to the walbro site. I've had good luck with it as far as illustrations go etc.
[link=http://wem.walbro.com/walbro/group2.asp?FamilyName=WG]walbro[/link]

Check this post also as it's related to your question as far as carbs go...
[link=http://www.rcuniverse.com/forum/3W140_trouble%2C_flat_spots_in_low_range/m_1866098/tm.htm]rcu thread[/link]

WT-657 Carb from Walbro. I can't find it anywhere except the Poulan website where it is described as standard equipment on the lastest version of the saw the engine came off. Bob_nj those are some great resource sites. I'm going to the field today, and will report on any changes to my cleaning and remounting efforts. Thanks to Cummings66 as well for taking the time to help out.

Roger

I spent the better part of a day fiddling with this thing. I replaced the gaskets/diaphrams, and adjusted my ignition timing. Things improved just a tad. Here is the symptom that I believe is causing the hickup. The high end is leaned out. When the throttle is advanced, the carb sprays a good bit of fuel out the top. Eventually it stops cutting out and settles into a good steady run, but only after about ten seconds. I can cover the carb with my hand and can really feel the fuel spitting out. It can't be being pulled out by the prop blast, since I protected it from the prop blast with my hand. I can lean out the low end to avoid this, but it is then so low that the engine will usually stop when trying to accelerate. What is the brass button inside the ventri? Does it dump fuel too. There are three jets in the body of the carb. Is my popoff pressure too low? Any help appreciated.

The brass button is the main fuel nozzle, controlled by the high speed needle...Most Walbro carb have three little holes drilled in the venturi, opening one at a time as the throttle is opened from idle..Popoff pressure has NO effect on fuel coming out the carb....I have noticed the fuel coming out, maybe a velocity stack with the angle with the high part to the rear would help.
I have the latest Walbro parts book, there is no mention of the WT657 anywhere, no price and no mention in the applications section....
The carb might be a special version for a chainsaw, where the engine is operated at high rpm most of the time..
I didn't like the stock carb, don't remember if it was a 657, but used a few different models...There was some blowback, but not excessive......

Yes RCIGN1, it is frustrating not being able to pull up info on this carb. The carb on my G-26 has a boss cast for the brass fuel inlet, but it isn't installed. I wonder if I lost something out of the mechanics of the carb for this main fuel nozzle when I removed the carb butterfly shaft to solder on the horn for the throttle cable? If something is floating around in there, instead of being held by a spring, it might explain my inconsistent runs. What do you think?

Well, I added a velocity stack, and it didn't solve my problem, but it did make it easier to recognize. The carb is having the fuel sucked out by the airflow over the carb. It is like I have three adjustments, hi and lo needle, and airflow. It's like balancing on the head of a pin trying to adjust it, then when in the air it all goes to heck. I can't control the airflow well enough to get a consistent run. I tried the stack rotated every which way. You could put your hand in front of the carb and it would richen up noticeably.

Do you guys think a L shaped stack made from a copper elbow faced away from the prop would work? How about some fairly open cell foam over the velocity stack.

Thanks for your help and any experience you might offer.

fwman1

Do you have another stock carb to try on it? What are the carb settings on it now ? Why the outside pulse line? Just trying to pinpoint things. Carb gaskets in properly? On the meterng side, the gasket goes on the carb body first, then the diaphram then the cover. Fuel pump side, diaphram first, then the gasket, then the cover.

Flypaper 2, The diaphrams are on correctly. I just added a pvc 90 degree fitting to the velocity stack. It should protect the carb from the lower airpressure better than just the velocity stack alone. I think the carb was designed to run with a foam block over it and the Walbro folks have the inlet just about too big for my engine. If the 90 degree elbow does not completely solve it, then I'm going to build a little box with foam over the top to emulate the installation in the chain saw. The crankcase pressure fitting is on the top of the carb because the hole from the crankcase is being used to help hold the carb to the jug. This is something you have to do to the Poulan 46 due to it's carb attachment design as a chain saw. It works well, and I've seen other use it successfully. I have another carb in transit, but I think the elbow should solve my problem. I'll post results tomorrow evening after I get back from the field. Thanks.

fwman1