# Briggs Stratton motor not running



## DTX

Briggs & Stratton 3.5 hp Quantum on push mower. 
Many years old but was running great.

This mower just got where it would not run. Stopped running while I was using it. 
I can get it to run at a low rpm only with some missing if I have the filter off the carb and a screwdriver holding the auto chock flap open. But no way I can get it to increase rpm. 

I since tried replacing the diaphram gasket in between carb and gas tank. Also put in new spark plug. No improvement. 

I then went into the motor, found the flywheel shaft key broken and the flywheel a bit behind on position so I installed a new key and reasembled. Still no improvment. 

I then went into the motor. Did not see anything broken. Installed new seals and case gasket. I checked valve clearances. Exhaust valve okay. Intake valve have no clearance so I ground the end of the valve stem to give a few thousands clearance. 

Reinstalled and the motor still would not run properly. It was doing the same thing as in the beginning. idled running with a screw driver holding the choke open. 

Would anyone know what the problem might be?


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## octaneman

Hi DTX

After replacing the flywheel key, was the air gap adjusted ? If not re-align and adjust to spec.


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## DTX

I had mounted the magneto gap by site but to be sure I just readjusted to .010-.012, slightly closer than what I had. Keep in mind this problem came about before doing any work on it. I also had to flash dry the spark plug as it seems to get wet. Now when I cranked the motor for an instance reved up some rpm then proceeded to die until I held the choke flap open, then ran the low rpm sputter again. After studying that and looking down the wet carb throat, I feel that it is getting too much gas but cannot adjust out with the set screw. 

The carb seems simple. The old diaphram under it was replaced last year(ran great) and replacing it again after the problem appeared did not fix it. If it is too rich, what within or below the carb area could be the cause? 
I am just about ready to remove the carb again for futher inspection but do not know what to look for. Vaccum somehow is used to draw the gas up from the tank but I am not familiar. I think the gasket diaphram regulates the flow in some way...


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## octaneman

My understanding is you have a pulse type carburetor, there is no float adjustment or choke. The only adjustments that are made are the high speed mixture and low speed mixture screws, any off balance adjustment on both screws can set the carb into a no start condition. Has the throttle cable been adjusted ? Try re-setting the mixture screws to the fully closed position count the number of turns that it takes to go to the end, then divide by 2 and bring it to approximate center. Remember, when making the high speed mixture adjustment the engine should be HOT and throttle should be what the manual spec sheet says should be. On the low speed adjustment the engine should be WARM and turn the screws until you feel the engine is running good. Once thats done set the Idle speed.After you rebuilt it the first time you said it ran great, now it doesn't run at all which is quite strange, the parts should not have worn out that quickly. If you decide to tear it apart, don't use any drill bits or anything to poke into the jetting or any other orifice, that can damage the holes or the jetting diameter when you clean it, this can offset the carb completely. Make sure you put back the springs and diaphragm correctly ( it's happened to me ). 

post back your findings


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## DTX

Thanks for your response. This carb has only 1 mixture adjustment screw. Before removing it I counted it being almost 2 turns open. That is the setting it has been running. I put it back to that setting when reassembled. But now there is no change or improvement when I turn that screw either direction trying to get a leaner mix. 

There really was no rebuilding to the carb. Only the gasket sandwiched in between the carb and tank. There is a small linkage that hooks on the top choke flap and extends down into the gasket. Has a light spring on it. Somehow it operates that auto plastic choke flap at the top of the carb throat. There is anothe short but larger diameter spring that sits under the gasket into a recess in the tank top. Not sure what it does. 
I have to adjust the throttle cable each time I have taken the top flywheel cover off or removed the carb. Setting it back properly is quite easy for me. The carb is really quite simple but the gasket and its diaphram functions with the springs is not familar to me. 
I feel something had to break since the last time I ran it, it did not want to crank so easily. finally got it running then mowed about 15 min and it died and did not want to start again. Only runs as I described above. I am going to have to remove the carb and gasket and study it. Maybe I will spot something.


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## octaneman

I found a document online , i'm not sure if it applies to your engine . 



ENGINE ONLY RUNS ON CHOKE

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This problem is usually common with Briggs and Stratton Vacu-Jet and Pulsa-Jet diaphragm style carburetors. 

A.The diaphragm is stiff and must be replaced. This top information is for carburetors installed on Sprint, Classic and Quattro 3.0 to 4.0 hp engines. 

1. Remove the two bolts holding the gas tank to the engine. 

2. Tilt the carburetor and gas tank as a whole towards the engine to free the governor linkage from the throttle shaft. 

3. Dump the gas from the tank into a container suitable to contain gasoline. Set the tank on a work bench. 

4. Remove the 5 machine screws holding the carburetor to the tank. NOTE: **ON SOME MODEL SERIES 110900, 1119000, 113900 AND 114900 THERE WILL BE ANOTHER LONGER SCREW INSIDE OF THE AIR HORN UNDER THE CHOKE PLATE. BE SURE TO REMOVE IT SO YOU DO NOT DAMAGE THE CARBURETOR.** 

5. Pull up on the carburetor to remove it from the tank. Make sure the springs do not jump onto the floor. If you notice a pool of gasoline in the large, shallow well in the gas tank top, then the tank must be "trued". Gas in this area affects the pumping action of the fuel pump. If a local Briggs and Stratton authorized shop cannot true the tank, then the tank must be replaced. 

6. On automatic choke engines, you will have to remove the access cover on the air horn (located on opposite side of the breather grommet) to remove the link connected to the diaphragm to remove the diaphragm. 

7. Check for debris clogging in intake straw. Clean the screen with a tooth brush if necessary. 

8. Install the spring to the bottom side of the choke link. No more than two coils should be in the holder. Make sure you do not damage the spring when installing. 

9. Hold the carburetor upside down in your hand and place the wide, short spring in the correct well (caburetors needing the 391681 diaphragm use this short, wide spring). Then place its cup overtop the spring. Pulsa-Prime carburetors will use a gasket and diaphragm and a spring. The diaphragm will go ontop of the tank, then the gasket. The spring fits on a boss on the bottom of the carburetor. 

10. While holding the carburetor upside down, place the tank over the carburetor. Put the carburetor and tank assembly right side up on your bench and start the 5 screws. Do not tighten them yet. Connect the choke link to the choke shaft. Close the cover. 

11. Put a screw driver or something in the air horn to hold the choke plate open, past halfway. Jiggle the carburetor to set the choke spring. With the screw driver still holding the choke open, tighten down the screws in a staggered seqence. Then install the longer screw inside the air horn, if equiped. 

12. Reinstall the carburetor to the engine in reverse order as stated above. 


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IF YOU HAVE A 3.0, 3.5 OR 5.0 HP HORIZONTAL SHAFT ENGINE, THEN... 

1. Remove the muffler and the breather tube. If it is easier to remove the whole fuel tank and carburetor assembly, then remove the bolt at the bottom of the support bracket and the two bolts holding the carburetor to the block. 

2. Remove the four screws holding the diaphragm cover on. 

3. Take the diaphragm and a Model, Type and Code number to a local Briggs and Stratton dealer and have them identify the diaphragm as New Style (P/N 272538) or Old Style (P/N 270026). 

4. Hook the hole of the diaphragm to the pin located on the carburetor near the block. Then insert the spring first into its dish and then put the cap on top of the spring. Hold the diaphragm in place while installing the diaphragm cover. 

5. Tighten the screws tight and reinstall the breather tube from the breather on the block to the carburetor. Reinstall the muffler or the whole fuel tank assembly. 

B. High speed nozzle or jet is partialy clogged. 

On all Tecumseh float style carburetors, remove the bowl nut and clean out the holes in it. On adjustable high speed engines, make sure you find the tiny hole on the top of the first set of threads. It can only be cleaned with a piece of tag wire. 

On Briggs and Stratton, All Max and Quantum engines (3.5,4.0-6.5 hp) will have holes in the bowl nut. Clean these out. On all other Briggs engines, there will be a removable nozzle. Take this out. NOTE: **DO NOT DAMAGE THE THREADS WHEN REMOVING OR INSTALLING THE NOZZLE. PURCHASE BRIGGS PART NUMBER 19062 TO GET A SPECIAL SCREW DRIVER TO REMOVE THE NOZZLE.** Remove the nozzle and clean out all the holes. Reinstall. Opposed Twin Cylinder Briggs engines will not have nozzle. Remove the top half of the caburetor and clean the 3 holes in the front of the carburetor base. One of the holes will be the fuel pump hole and the other two will be metering holes. Clean these out with a piece of tag wire. Reinstall the top half of the carburetor. 

C. High speed adjustment is not correct. 
This is for float style carburetors only. 

1. Screw in the adjustment screw until finger tight. Now back it out 1 1/2 turns. This should be sufficient enough for the engine to run properly. This adjustment if for all Briggs float style carburetors and all Tecumseh float style carburetors. If the engine does not run correctly, let engine warm up for about 5 minutes. Screw the screw in until the engine runs erattic (lean). Then back it out until the engine runs erratic again (rich). Now turn the screw half way between and this should be good enough.


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## DTX

octaneman, Thank You. I got it running again. When I read over your last post, I wanted to know just what type of carb I have.
I did a search on Vacu-Jet and found a terrific web site that lists various types of carbs and identifies them with photos. 
The following web page shows my carb...
http://outdoorpowerinfo.com/repairs/briggs_vacu-jet_vert_auto_all-temp.asp

In that page, photo 7 shows how one needle valve controls both high and low fuel mix. Then look carefully at photo 6 and notice that there is a plastic insert that the needle valve goes into. Notice the o-ring at the end of the plastic insert. This forces the drawn fuel to go into the side hole of the plastic and out the end, controlled by the needle screw adjustment.

I had two problems with my mower.
First, was the broken flywheel key. I previously had hit a solid object with the blade while mowing. It must have broke the key. Soon after while using the mower the flywheel slowly slipped to a point where the mower no longer ran. 

In my shop, before discovering the broken key, I had removed the carb to clean it out hoping that would fix the mower. Using compressed air I blew out the tiny holes. I had forgotten that the plastic needle insert came out. I also blew out the o-ring without realizing it and put the plastic back in without the oring. (2nd problem created). This would allow fuel to flow on the outside of the plastic insert to the high and low metering holes thus causing an uncontrollable flooding problem. That is also why my adjusting the needle no longer worked. 

After some internet website education I soon found the broken key and fixed problem 1. Still motor did not run because of missing o-ring, problem 2. 

After seeing the photo at this web site, hey I started having memory flash backs. The other day I found this strange tiny oring sitting on my shop floor and could not figure out where it came from. Also started to vaguely remember the plastic insert. 
So I removed my carb and took out the needle. There was the plastic insert. Sure enough it had no o-ring. I could not find the o-ring that I previously found on the floor but found a proper size replacement from my stored assortment. Reinstalled everything and the mower cranked on the second try and runs great. (problem 2 solved)

Amazing how one little rubber ring can cause such major frustration and aggravation. Well it was a big learning experience for me about my lawn mower engine.


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## Basementgeek

Thanks for letting us know and thank for the link -excellent site

BG


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## octaneman

There's never a dull moment in mechanics thats for sure....btw thanks for the link - awesome web site.


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