MTD Erratic/loping idle at all speeds

[jlitzz]

Having a little trouble with surging/loping idle on MTD lawn mower (#11B-106C401) with 6.0HP Briggs & Stratton engine (12V802-1776-B1). The mower surges at all speeds, more so at high idle and vice versa. I have already pulled tank and fuel line, cleaned them both and am using fresh fuel. Additionally, I pulled the carb and cleaned it several times with carb cleaner and compressed air. No obstructions in any of the passages as can tell by compressed air flow though them.

I also checked the carb to manifold gasket and this seems fine. My question is that can a bowl gasket or needle/seat cause the surging problem? Both the needle and seat look to be in good condition. I am running out of ideas on how to fix this issue and am in need of some advice. I did not replace bowl gasket but will do so tomorrow to see if this cures the problem.

Thank you in advance!

jlitzz

 

[Rokon]

Having a little trouble with surging/loping idle on MTD lawn mower (#11B-106C401) with 6.0HP Briggs & Stratton engine (12V802-1776-B1). The mower surges at all speeds, more so at high idle and vice versa. I have already pulled tank and fuel line, cleaned them both and am using fresh fuel. Additionally, I pulled the carb and cleaned it several times with carb cleaner and compressed air. No obstructions in any of the passages as can tell by compressed air flow though them.

I also checked the carb to manifold gasket and this seems fine. My question is that can a bowl gasket or needle/seat cause the surging problem? Both the needle and seat look to be in good condition. I am running out of ideas on how to fix this issue and am in need of some advice. I did not replace bowl gasket but will do so tomorrow to see if this cures the problem.

Thank you in advance!

jlitzz

Unfortunately compressed air won't remove all obstructions. Some times all jets and openings on the carb must be cleaned manually.

There is a very good youtube video on this. I believe it was for a honda gv160 engine in the video.

 

[DaveTN]

Sounds like it's running too lean. Have you tried to enrich the mixture adjusting screw? Doesn't take much turn, try 1/16 counter-clockwise turn and see if it helps level it out. If it needs more just do a little at a time till it hits max RPM. That may be all it is hopefully.

 

[jlitzz]

Thank you for your response! This carb does not have an idle adjustment screw. I was checking for vacuum leaks while it was running and noticed that spraying carb cleaner near and into the throttle shaft caused the engine to smooth out and run fine. Additionally, when placing my finger on the top of the throttle shaft and applying some pressure the same smoothing out occurred. I noticed that there is not the usual foam seal at the top of the throttle shaft and am wondering if this is causing my idle issue.

Today I am replacing bowl gasket and the throttle shaft seal to see if this solves the problem.

jlitzz

 

[DaveTN]

I wasn't sure whether that carb had mixture screw adjustment or not since I don't have files etc. But either way it's running lean and when you squirt some liquid around the carburetor mounting gasket or throttle shaft it seals and makes it pull more gas through the carburetor. And same for the top of the throttle when you push down on it. They'll suck air either place and make it run lean. Check the carburetor to engine gasket and see if it's sucking air as well. That loping is the governor trying to find the ideal spot and can't since the engine is not getting enough fuel to level out rpm. Usually in the case of throttle shaft bushing wear, they have to dis-assemble the carb, knock out or drill out the throttle shaft bushing, replace it and ream it to a certain tolerance. Or that's how it used to be, I haven't put one in for a long time. The throttle shaft seal should take care of it. Hopefully you won't have to take it apart. Good Luck!

 

[jlitzz]

Ok, so I swapped a carburetor from my working mower (similar engine/carb) onto the MTD and no running problems. Putting the questionable carb on other mower had same results--loping. I asked a technician at a local small engine repair place and he said that it wouldn't be the throttle shaft seal because this essentially a dust seal and made of foam. I confirmed this by using a rubber seal in place of the dust seal and had no change in loping. He said that I should clean the carb because it is running lean.

I have cleaned the carb several times with carb cleaner, compressed air, and small wire tools in all the appropriate holes, etc. How much more can I clean this thing? Additionally, I replaced the seat/needle, bowl gasket, and checked float for leaks (none).

Everyone's suggestions have been great and I appreciate the input!

jlitzz

 

[Mikel1]

I agree with Dave, if you have an air leak then the fuel/air mixture is going to be lean due to that extra air entering.

 

[Rivets]

Clean the carb again. I know you have done so many times. I suggest either get get a carb cleaner that you soak the carb in or take it to a local repair shop which has an ultrasonic cleaner. Cleaper than a new carb.

 

[jlitzz]

I'm going to give the soak a try. Purchased some from automotive store with dip basket and see how this works. Thanks for the tip!

 

[jlitzz]

Success!! MTD Erratic/loping idle at all speeds

Rivets,

Soaking the carburetor was the answer! I used Gunk's Chem Dip with pail and soaked the carburetor twice, each time for 20 minutes as the directions indicated. Afterwards I rinsed the carb in hot water, allowed to dry, and then sprayed with carb cleaner and compressed air to ensure no water inside. I reinstalled carb and it started on first pull with no idle issues at all. Let in run for 15 minutes and no change!

Thanks to everyone for their suggestions!

jlitzz