Hi,
I've a B&S powered lawnmower and last week it ran out of fuel and then will not rev up properly. I think the carb must be partly blocked. It 'hunts' as it ticks over and yet will not rev up with the handle control.
Checked air filter - ok, and runs same with no filter on.
Checked spark plug...all ok but getting blacker as it now runs very rich and exhaust is more black smoke now than before.

tried a rag on air intake to cause suction but not successful to clear carb.
Will rev higher if I push rev lever on engine but not happy on tickover. Checked cable from handle control--still taut...works fine but will not rev from handle control only lever back of air filter.
Cannot see any carb anywhere...where is it?????
Anyone got a parts drawing?
Any help appreciated.

best regards

steve 23 May 2008

Steve Orton, May 2008

I have replaced the diaphragm on my 35 classic and it started straight away but then stopped. It will fire and run for a couple of seconds but then stops. I am not sure if I replaced the two springs correctly when re-assembling. Has anybody any thoughts?

Jerry, September 2012

Replacing the diaphragm and gasket on my lawnmower transformed it from a spluttering wreck to as good as new. Bought the bits on eBay for next to nothing and got full video instructions from YouTube (search for 'Briggs & Stratton diaphragm'). Thanks for all these useful posts !

Tim, August 2012

sounds like the govener spring has come off the linkage which operates the carburetor

rich, January 2012

hello i have a mountfield briggs and straton engine and the bolt snapped that holds the blade, can you explain to me how to would i replace it with a new one?

paul herron, August 2009

the bolt on the front holding the petrol tank in place came away ,therefore the tank was shifting during mowing and therefore the plastic tube between the carbarator and head cracked,
that let to much air was getting into the head sparaticaly,replace the grey tube ,tkes 1 hour ,€10

paul kennedy, July 2009

hi steve i think the problem is the deliver tube be hind the carb you will need to take the top of the mower ie the starter cord assambley the del tube is black plastic and snaps and then lets more air in if you want to ring me on 07984930521 i can explain more

martyn, June 2009

I have a 1990 engine and the fuel tank is attached to the cover. I have turned the fuel off but am not sure how to detach to get access to the carb. I believe I have this issue but I don't see a way to get the carb out without removing this cover.

Thanks guys, this post has been really helpful already :)

liam, April 2009

where can i get handle linkage for briggs and stratton 35 classic

joe from red row, November 2008

Update to below....
With VERY careful adjustment of the throttle, I can get lower revs, but with continual hunting and backfires through the carb
Any ideas please?

Colin Tayler, September 2008

I just stumbled across this site whilst trying to get answers for repairing my Briggs 3.5 hp Quantum engine.
It is running only at very high rpm or will stop, then engine speed control is ineffective. From reading this and looking at the engine, it would seem to be a problem in the linkage between the throttle cable and the thing that is acted upon by the cooling air flow. On mine there is a long spring between these two but it is loose and when operating the mechanism by hand it seems to get stuck. Is there a diagram anywhere that would show the correct positioning and method of connection for this please?

Colin Tayler, September 2008

I have just finished overhauling my B&S 3.5hp with some black electrical tape to help seal a small kink in the diaphragm. The engine was runnign really rich e.g new spark plug completely black and coked up in 1 hour and going through fuel at about 1litre per hour.

I had previously changed plugs adn airfilter and stripped cleaned carb but not noticed vey small kink. A lesson for all of us not too overlook the little things.

SimonDJ, August 2008

Many thanks to Caine and Phil.

My Briggs and Stratton engine fired up first time after replacing the diaphragm and gasket with the set I bought.

£0.82 for the repair set - what a bargain.

Thanks guys. A combination of your advice meant it took me only 15 minutes!

A million thanks to you both.

best regards

Steve 2 June 2008

Steve, June 2008

Hi Phil,

Understood your reply perfectly.
I've ordered a diaphragm kit (and replacement air filter as old one was a bit oily) which should arrive Monday so I'll get on to this next week.
Appreciate your help very much.
Thanks!

kind regards

Steve 31 May 2008

Steve Orton, May 2008

If it has a primer then there's no choke so the diaphragm replacement is easy.

The mechanism with the springs is the speed govenor. Basically, the handlebar lever does not control speed directly, but controls the tension (by moving one of the end points) of the larger one of these springs. Opposing the spring tension (when the engine is running) is a force generated by the cooling air acting on an air vane hiding inside the engine cowling. The throttle butterfly in the carb is connected to the linkage between these two. The point at which the 2 opposing forces balance is what controls the engine speed. More spring tension - engine speed rises until the increasing air flow over the vane overcomes the increased tension and engine stabilises at the new (higher) speed. The other (smaller) springs are simply to keep the mechanism's rods and levers in contact with each other (to eliminate free play)

Having understood all this, the good news is that you don't need to disturb any of it!

If you remove the 2 bolts (one at the front and 1 just behind the carb), you can manoever the whole tank/carb assembly off the engine intake pipe and leave all the complicated stuff attached to the engine. Just unhook the link to the throttle on the carb as you withdraw it.

When you replace it, just make sure that the linkage and springs don't get twisted/jammed, as this will result in a runaway engine with some really impressive RPM's.....

phil_saunders@bigfoot.com

Phil Saunders, May 2008

Hi Phil,
Thanks a million for the detailed response. Just a couple of questions.
The model looks exactly like the B&S 98900 (found drawing on internet) and it has a membrane primer you press 3x to prime the engine. Does this mean it is NOT the choke version you mention?

Secondly, there are a series of linkages sited under and between the air filter cover and the spark plug that are like fine hair springs which move in and out when you rev the engine (is this what you mean by the govenor?) whereas the handlebar lever cannot get it to do this. Are these connected to the carb and if so do I need to handle them carefully & reconn them in any way ?

many thanks for all your kind help.

best regards

Steve 27 May 2008

Steve Orton, May 2008

The carb is the device to which the air filter is attached, and it is bolted to the top of the fuel tank.

Your problem is that the diapgragm between the carb and tank is either holed, or work-hardened. It is co-incidental that it chose to fail when you ran out of fuel.

Remove tank/carb assy as a unit (2 bolts), being careful to unhook the speed govenor assy as you remove it.

Unscrew carb from tank, If your model has a choke as opposed to a primer, you will need to remove the link cover and unhook the link from the choke butterfly.

Replace diaphragm. If the diaphragm is flat, just replace it and re-assemble the carb to the engine, MAKING SURE THE GOVENOR ASSEMBLY IS FREE TO MOVE as you go.

If diaphragm has a choke link, it needs to be preloaded - proceed as follows ;-

1) Turn carb upside down. Feed choke link through carb and connect to choke butterfly.

2) Place tank on inverted carb making sure that fuel pump spring is properly positioned, and choke spring enters recess in carb body.

3) Turn assembly over. Insert screws but DO NOT tighten. Push choke butterfly beyond the fully open position so it is as far as possible 'over center'. This pre-loads the diaphragm, and the auto-choke will not work if this is not done.

4) Tighten screws. Return choke to normal position. Choke flap should fully close.

5) Re-assemble, noting the warning above regarding the govenor.


phil_saunders@bigfoot.com

Phil Saunders, May 2008

Hi Caine
Thanks for the info.
I'll strip down and see if I can find the carb under the tank.
It's not obvious from outside where the carb is, or even the fuel feed line from the fuel tank.It seems to feed fuel from under the tank somehow. Cannot see the outlet at all.
I'll have a go at it. Anyone with a parts list would help.

cheers

steve 25 May 2008

Steve O, May 2008

find fuel tank look for fuel output line follow it to carb .b&s carbs notoriousfor dirt gumming up filters jets and the supply diaprams,supply diaphram is on carb under 3 or 4 cross head screws be carefull when removingcover as there is a spring underit easy strip down if methodicaly done spray all components with carb cleaner (buy at motor factors_) some carbs on the fuel tank but hard to recognise flush out the tank with fresh fuel]

caine, May 2008