# 3.5 briggs and stratton rough idle



## skytronic

i have a briggs and stratton engine mounted on a lawnmower.
model 10D902
type 0129 B1
code 03021754
starts ok but runs very roughly surging and dropping, can see the governor moving back and forward with surges.
fuel,oil and spark plug ok.hope someone can help:wave:


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## tommyj3

Remove and clean carb, then install new carb diaphragm B&S P/N 795083.


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## skytronic

hi tommyj3 removed and cleaned carb fitted new diaphragm but its still idling roughly any ather sugestions. thanks


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## tommyj3

What I would do is put about 2 ozs of fuel additive in a full tank of gas. Start engine let it run, and see if the additive cleans any crud that may still be in the carb. I personally use Sea Foam Fuel Additive for this.


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## dawgpile

When you removed carb and cleaned it, did you check the fuel inlet pipe and clean it as well? I'm not sure if this has the screen on the bottom of the pipe or on the side, but in either case, a dirty inlet can starve the engine and cause it to 'hunt'.


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## skytronic

hi,yes cleaned the gauze at the bottom of inlet pipe.took it apart again and cleaned blew air through jet and body ,still idling roughly ,racing and slowing down.


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## pyro_maniac69

did you put the gasket and diaphram on the correct way? if you didn't put those on than it will not run coreectly


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## ajp

*rough running*



pyro_maniac69 said:


> did you put the gasket and diaphram on the correct way? if you didn't put those on than it will not run coreectly


remove and check spark plug. It could be fouled partially. After cleaning check gap. REinstall. if still rough then the mixture screw may be at fault. Use a screwdriver to first adjust lower screw 1/2 turn either in or out. If problem still exist go to top screw and turn 1/2 turn inand out. Your problem can be old gas also. If all these dont correct the problem them it could be the points. check back later...


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## pyro_maniac69

oh, I'm sorry, I didnt look at the model number, I was unaware it was an older style diaphram carb


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## 30yearTech

pyro_maniac69 said:


> oh, I'm sorry, I didnt look at the model number, I was unaware it was an older style diaphram carb


It's not that old, and there are no adjustments on this carburetor skytronic, so don't look for any. It is possible that old gas can cause a problem, your engine has no points either solid state ignition, a partially sheared flywheel key can cause a rough running engine too.

Pyro, this is a pulsa prime carburetor, your inquiry about gasket and diaphragm orientation is a valid question.


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## pyro_maniac69

ah ok, I saw the 10D model number and thought that it might have been an older model series

thanks 30year


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## pyro_maniac69

I REALLY need partsmart on my computer or something


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## 30yearTech

pyro_maniac69 said:


> I REALLY need partsmart on my computer or something


You can find parts look up and various sites on line, like partstree.com, jackssmallengine.com and of course briggsandstratton.com, may be a little slow if you have dial-up but otherwise pretty useful for looking up or at IPL's


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## oldstick

*Very similar question*

I think I have the same basic motor and carb. Mine is Model 9D902. This is a small Murray mower with the "throttle free" carb. Someone left behind when I moved in and looks to be used very little.

Prime it and it cranks first try and runs like a scalded dog for 5 minutes then slowly cuts off like when they run out of gas. Prime again and runs great for another few minutes and repeat.

Odd thing is it ran for 20 or 30 solid minutes the other day while I was weeding a rough, steep drainage ditch area. Tilting up/down/sideways pretty much the whole time. Then moved back to the level yard and off it goes after a few minutes.

This has got to be a fairly simple problem. Anyone know what is going on?


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## 30yearTech

Fuel pump diaphragm may not be pumping fuel into the pickup area, operating on an incline and fuel can make it's way to the pickup without the diaphragm pumping it there. Probably a new diaphragm in the carburetor will take care of the problem.


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## cajunh2s

i would also check the spring linkages to the air vain governor and carb....if you took the springs totally off to service this carb....you may not have enough tension on them to work correctly....it will surge and hunt if the tension is out of whack or if springs are installed wrong....

ask me how i know.....lol


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## oldstick

30yearTech said:


> Fuel pump diaphragm may not be pumping fuel into the pickup area, operating on an incline and fuel can make it's way to the pickup without the diaphragm pumping it there. Probably a new diaphragm in the carburetor will take care of the problem.


Thanks 30, I will give that a try. It dawned on me too that during the 30 minute run, I had filled the tank to the top for the first time. Then the shutoffs reappeared down around half a tank. Half a tank or less is what I had in every other time I ran it. 

That might further point to what you suggested. I figure with the full tank there would be a little more pressure at the intake tube which could assist if the pumping action is weak.


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## 30yearTech

cajunh2s said:


> i would also check the spring linkages to the air vain governor and carb....if you took the springs totally off to service this carb....you may not have enough tension on them to work correctly....it will surge and hunt if the tension is out of whack or if springs are installed wrong....
> 
> ask me how i know.....lol


So, how is it you know this....


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## cajunh2s

30 year....you don't remember my throttle free design 3.75 briggs thread?
you helped me find the spring hook up for this engine....

it runs like a scalded cat now.....lol
just thought i'd throw the springs linkage into the mix....since not too long ago we were dealing with my throttle free type engine...

and my springs were out of whack.....engine sounded so bad you would have thought the timing was out in it..

cajun


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## 30yearTech

cajunh2s said:


> 30 year....you don't remember my throttle free design 3.75 briggs thread?
> you helped me find the spring hook up for this engine....
> 
> it runs like a scalded cat now.....lol
> just thought i'd throw the springs linkage into the mix....since not too long ago we were dealing with my throttle free type engine...
> 
> and my springs were out of whack.....engine sounded so bad you would have thought the timing was out in it..
> 
> cajun


Oh... I remember, I just thought you wanted someone to ask!!!


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## cajunh2s

good one 30 year...ya got me......lol


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## karkev

Hey guys! I've been following this thread and was sure hoping ya'll could help me! I recently scored a very nearly mint Murray 3.5hp 20" Throttle Free push mower for $20.

I have actually mowed my lawn twice with the thing after changing the plug and oil. I went back with with the J19LM Champion plug. 

The mower started easily enough and mowed great BOTH times.

This morning the thing started fairly easily but died shortly after. I couldn't get thing to start so I yanked the plug and sure enough it was gas fouled.

No biggie - I bought another J19LM and threw it in. Initially the thing wouldn't start and I noticed I was getting very little "shock" as I held the plug wire end. I scraped some surface rust out of the plug wire electrode cap, had the wife pull the cord as I once held the plug wire and received a more powerful (but not jarring) shock for my trouble.

The mower fired up and mowed fine for thirty minutes until I shut it off to go get a drink. When I returned about thirty minutes later the mower fired up quickly but soon died.

I pulled the *new* J19LM and it was gas fouled. I also noticed that the plug wire spark sure seemed weak again.

I have a new Champion 5861 "EZ Start" plug (supposed to burn hotter) just waiting to be installed but I wanted know what the plug gap for this new plug is supposed to be and any thoughts you all might have about my Murray's plug lust?

THANKS!


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## treeboy

You're the first person I've ever heard of that checks for spark "by feel" instead of visually. Just curious how you know when you are getting a good hot blue spark? May be you judge by how far accross the room you go when your wife pulls the cord ! With this technique you probably save money on coffee !!


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## karkev

Treeboy, at least your board name is honest. You do seem quite juvenile.


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## dragonluvr05

B&S 3.5 classic throttle free design
9D902

Won't start without taking off the air filter and putting my hand over the carb. Doing this I can get it to run for a few minutes then it dies. I was thinking that it might be a gasket some where. I got this mower for free and the guy I got it from said it used to run great, but he hasn't used it in a few years. I'm hoping that I can get this fixed quick and cheap.


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## 30yearTech

Probably needs the diaphragm replaced and while the carburetor is off of the tank, I would take the opportunity to clean out the pickup tubes.


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## AthensCarpetman

*Runs Rough*

On my B&S throttle free lawn mower, it starts quite easily but runs very roughly and puffs out black smoke from the exhaust. It apparently has no adjustments so what can I do for it >


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## dehrhardt

It would help to have the model, type and code numbers from the engine. The video on this link will help you find them.


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## AthensCarpetman

dehrhardt said:


> It would help to have the model, type and code numbers from the engine. The video on this link will help you find them.


Thanks for replying, I'm at work right now but I'll get that info and post it in the morning. Okay, here's the #'s. Model#10A902, type# 2286B1, Code # 01090655.


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## dehrhardt

First, inspect the air cleaner. The foam air cleaner in your mower can be cleaned with water and detergent, then needs to be re-oiled. At the same time, I would recommend replacing the spark plug if you haven't already done that. Check the the spark plug wire to make sure the connector isn't corroded. 

If that doesn't help, you be needing to take the carburetor and tank off the mower to clean both and put a new diaphragm on it. Directions for that can be found on these two links:
http://outdoorpowerinfo.com/repairs/briggs_10l900_carb.asp
http://outdoorpowerinfo.com/repairs/briggs_pulsa-prime_carb.asp


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## 30yearTech

These carburetors have a tendency for the diaphragm to develop a leak at the impulse port for the fuel pump and allow fuel to leak past and cause the engine to run rich, lose power, and foul spark plugs. If the air cleaner is not too dirty, or if the engine runs bad without the air filter in place, then the diaphragm is most like the cause.


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## AthensCarpetman

Thanks guys, that gives me a place to start.


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## AthensCarpetman

Thanks for the info. I put a diaphragm in and it runs like new.


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## 30yearTech

Glad to hear you got it figured out. :thumbsup::thumbsup::thumbsup:


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