OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs plastic carby

Must be something I am doing wrong with these carbys. I have 3 here at the moment which I have put new kits in but all 3 of them for some reason are running rich. Anybody have any thoughts?

Hi Norm,

A few things you could check below but may not be the problem as I've taken the primer bubble off before and found a one way valve
has popped out and sometimes seen a jet popped out .

These sprint carbs will over fuel if the two tangs on the diaphragm are not sealing properly (2 Fuel Pump Valves ) usually you can tell straight away by how the fuel primer works ,once fuel is in the carby each press on the primer should shoot the fuel out in a strong steady
stream if not the diaphragm valves aren't sealing properly. The valves should seal as you've fitted new diaphragms but they don't
always seal.

I don't know if that's your problem without looking at the carby but if you can feel air in the primer when pushed or the primer
isn't pushing or a lot of fuel the poor sealing valves could be the problem.

Another thing I always do is remove the 2 metal fuel screens and blow everything out with compressed air.

If the spring in the diaphragm is longer than normal it can make them over fuel.

Cheers
Max.

Hi Max,
they prime as they should and start easily but they are obviously running rich because of the stuttering and black smoke coming from the exhaust. Put a good known carby on and these 3 motors run perfectly. I have just kitted another one and will see what that does in the morning.

Ok so now I have 4 out of 4 running rich. I now suspect these Chinese kits to be junk, I am so sick of Chinese junk but everything we get these days is Chinese junk

Hi Norm,

I would take the gasket and diaphragm out of the working carby after wetting the gasket with fuel or CRC so it
doesn't rip then try the working gasket and diaphragm in the carbs that are flooding that will at least tell
you it's definitely the new spare parts are the problem .

It doesn't sound like any of the carbs that are over fueling are missing the brass jet as you have 4 out of 4
over fueling.

When I was around 14 ,I would help my next door neighbours brother out doing deliveries to many Factories
which I got paid for and it was quite common for the Factories to have manufactured seconds in the skips,
we would take new parts out of the bins when the parts were at the top and it was always interesting to see
what went wrong with the parts.

The problem is people that work at some Factories take new seconds (rejects) out of the bins and sell at
markets and eBay. This doesn't happen much in Australia because factories lock their bins an have a
surveillance camera on the bins as they don't want rejects being sold and people will say their products
are faulty.

I find the selling of seconds from China is quite common these days from markets and eBay.

Cheers
Max.

Max, I thought that removing the brass jet was to make them leaner for high altitude operation

But it does sound like dodgy kits

Max, I probably have about 10 here now all doing the same but I am not game enough to pull the one working carby I have. And this is a carby I have not put a kit in. I guess I could go to the local mower shop and get a kit from them to see if there is any difference in the ones they have

Hi Tyler and Norm,


I thought it works the opposite way Tyler , the bigger the air jet the more air pressure makes it's way down to the main jet which along with the vacuum acting on the main jet this helps to drag more fuel from the main jet , the air bleeder jet will mix air with the fuel but I thought it's main purpose is to help the fuel to be drawn up the main jet.

At high altitude I would have thought you would go to a smaller jet.



There could be a problem Norm with the kits if they have shrunk or are cut wrong ,maybe worth getting a kit at the
local mower shop just to check.

Cheers
Max.

I always keep the carbs when I junk a B&S, just bung a whole carb on.....
KISS principle, I know there's not a lot of thought in my process.......
speedy

I have had it confirmed that Xi Jinping was also involved with the manufacture of this rubbish so they decided he was not cut out for manufacturing so they turned him into a president. I have been told to throw them in the bin and not waste my time with the junk.

Hi speedy, I would if I could, but I can't so I won't. I have ended up with boxes of carbys and nothing that works so I have to start kitting them. I have kitted them before but obviously this lot I got are just Chinese bottom of the barrel rubbish

Hi Max

I just checked the briggs L head service manual and they say remove the jet on high altitude

It must be larger hole allows more air bleed and less fuel in to compensate for low oxygen at high alt

Maybe if I remove the jet it may lean it out. What have I got to loose

might as well give it a crack, but if they are running that rich I don't know if it will compensate enough

I'd just put a new diaphragm and gasket over the top of an old diaphragm and gasket on a table to check the difference ,that
would tell you straight away if it's cut wrong Norm.

That's interesting Tyler ,thanks for that info, the jet must lean out the fuel mix. I was going by the guy on Youtube
explaining the operation at 38 minutes and 56 seconds in on the video.



Cheers
Max.

Hi Tyler,
drilled the jet out and a big improvement not what I would consider perfect but close enough. Running it without the cowl so once that is on I will see how it goes. I will try another one tomorrow

Just wondering have either of you removed the main nozzle and replaced the o-ring? They do go bad.

Hi AVB,
is there any way to tell if the o-ring has gone bad everyone I have removed has looked fine to me?

Advice please about how to remove that "main nozzle" mentioned above ?

Have Victa Lawnkeeper in excellent condition with 450 series 148 cc B & S motor, same plastic carby.

Problem is it is running far too rich and no matter what I do with the throttle it is revving way, way too fast even with the butterfly almost closed. Has always been like this but now I'm using it more often for mowing between shrubs because it is so light and manoeverable and don't need much power. Damm annoying to have it screaming its heart out and it is horribly thirsty.

Looks like the nozzle may be picked out but I don't want to ruin an otherwise good carby.

Would like to check the O ring or reduce the jet bore a little. Are nozzles with smaller sized bores available?

Hi MM,
Governor will be the reason it is revving too high, back that down and it will run fine

I wish it was that simple Norm.

Governor and all linkages disconnected, operating the throttle lever by hand. Wide open the motor is screaming fast enough to throw the conrod, and initially when throttle completely closed it was still running far too fast.

I took out the brass butterfly and soldered over the small airhole hoping the motor speed could be controlled by having the throttle nearly closed but it bogged down running rich. Then drilled a smaller than original hole in the butterfly which helped a bit but idle was erratic/unreliable.

I reckon the main jet is too large, hence wanting to remove it and reduce the size of the bore.

Hi MM,
you don't have another tank and carby to put on it to see what is going on, never seen any carby I could not control by shutting the governor butterfly

You just use a flat blade screwdriver and press them out from the inside the carburetor throat.

Also a few of these nozzles had a brass jet installed that would fall out over time and get lost once the carburetor was removed.

Thanks very much AVB. Removed it by pressing from inside with a 7.5 mm brass rod. O ring is perfect.

The nozzle is 1.5 mm I/D Could run a 500 cc single motorcycle on a main jet that size :-)

Noted it has some numbers, 1 and 3 Maybe other sized nozzles are available? I tried to wade through the seven pages of carby spare parts on the B & S website but gave up, it's a dog's breakfast of a website.

I do have spare carbs Norm.

Will see if my B & S dealer can help with other nozzle sizes, otherwise will start modifying what I have on hand. Wish I was still back in the old days when I was doing watch and clock repairs - would have had a suitable supply of thin brass shafts and tubes and collars to whack into the end of the nozzle to play with different fuel flow rates.

Stating the obvious it is only a 148Briggs, they are a dime a dozen, just put another carby on it in 5 mins you will find what the problem is,as I said if you shut the governor butterfly you can control the revs and then it is a problem with the governor springs. Put another carby on it and if it does the same it is the governor

Yes Briggs tries to be just as secretive as all other OEMs on the specs. I like those Nikki jets they give you the PN but not the size. I gotten a few crossed but that only what I have used myself. And our EPA here has also cause problems as they don't think the techs have the brains to correctly jet an engine.

I do admit there some out there that couldn't get out of a wet paper bag with both ends open.

Norm, on some of my mowers I disconnect the governor system and have the cable operating the throttle/butterfly valve directly. Call me a Control Freak :-)

For some mowing applications I like to be able to set the motor running at a particular speed, e.g. idling very slowly when emptying the catcher, or at a fast idle when doing a very light cut with razor sharp blades.

Pics below will show you the set up on the little Lawnkeeper.

BTW I note that B&S have carb options for High Altitude mowing. I presume that means smaller main jets - less oxygen means less fuel flow or a rich mixture would result. From contact with Muricans who are using chainsaws in the tall mountains they need to fiddle around with their carbies.

And you can't understand why it doesn't run right? I walk away from this topic

This morning mixed up some 2 pack tank sealant which is impervious to petrol. Pushed it into the end of the nozzle and after a few hours it had cured enough to run a 0.3 mm drill through. Because it was still a bit soft the hole ended up about 0.5mm. Carefully picked out any swarf and re-assembled the carb (with the original brass butterfly plate), a few pushes on the primer and gave the cord a couple of pulls and the motor fired up nicely and was able to be throttled down to a lovely smooth idle.

QED - the main jet was far too large for this motor.

The governor mechanism on these mowers pulls the throttle further open when the load slows the motor down and the cooling airflow drops.

It does nothing to the fuel flow through the carb or the air/fuel mix.

Thanks again AVB, your advice on how to remove the main nozzle was extremely helpful.