OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines COX Crusader B&S 3.75

Hello all

I have cleaned this engine up. Serviced the ignition, carby, decoked all carbon, fresh oil & valve gaps are OK.

The engine starts easily and purrs like a kitten on idle.

As soon as I increase revs it starts missing and pumping what I believe is petrol smoke out the exhaust.

I am after some idea's as to what I could try next. Thanks in advance from Mark.

The most common cause of that particular engine/carb overfuelling is the diaphragm is shot in the carb, a good clean and a new diaphragm and gasket should have it running perfect for under $10, just be sure to keep that starter cluch clean and the inside of the square part oiled, they are a pretty good motor if looked after

Thanks much Joe,
I cant remember if I replaced the diaphrapm or not now, I have a new one, I will put it in & let you know how it went.

Just remember the order they go on, rubber diaphragm goes on the tank, then the gasket then thecarby, they can wreak havoc if in the wrong order

While you are doing that, don't forget to service the air cleaner: clogged air cleaner is a perfect match for your symptoms.

I have replaced the diaphragm & spark plug and ran with the air cleaner off.
Still has the same symptoms.
I though the muffler may have been choked, as the base was covered in black soot when I got it. (I put an exhaust deflector on it)
I also tried running it with the muffler off with no change.

I am thinking I might try a know good carb and tank set up.
Would lapping valves help?

I'll butt out, this is Joe's case and I wasn't helping anyway.

I really do appreciate all comments (any comments).

I ended up fitting a completely different tank and carb from another mower I have here, took me awhile as it was filthy, so had to clean it all up.

Anyway the result was it ran wonderfully with the other unit fitted.

So I have to have a closer look at the old one.

Thanks Joe & Grumpy.

Well done, Mark. Swapping complete carburetor-and-tank assemblies is my favourite way of verifying that the fuel system is at fault with suction carburetor Briggs engines. Please post the specific fault when you find it.

Hi Mark. There is also a chance the fuel tank face is warped if fitting a new diaphragm did not help. You can check it accross the face with a staight edge. Does not take much to upset them.

Thanks rusty, will do.
I have cleaned and inspected it all again looking for why it is delivering extra fuel, havnt noticed anything different yet. Still has the same symptoms after refitting it again.
I havnt much time to look at it now for a while, I love playing with mowers but work & stuff always gets in the way.
It will be a good engine once sorted.

Hello all, I have draw filed the carby face, I tapped the filings out of the file and blew the filings away often as I went.

You can see the highs and lows in the image below.

I kept going until it was flat.

Not 100% sure how it works, to pump fuel ect. But I will start it this week and report back.

Thanks for posting that Mark. Successful draw filing requires managing to keep the file level, which is the hard part, plus achieving a fine finish. The best way I've found to achieve that finish is to use a full-sized second-cut file, and keep the teeth clean. The old guys used to rub chalk across the file frequently, which works for me with a bastard file if you are pressing too hard and causing chips to stick in it, but for draw filing aluminium I use a different system. I just keep the file card handy, and card the file every 15 seconds. You must still use very light pressure of course, and remove the swarf every 15 seconds when you clean the file.

Of course you also change from filing along the tank to across it each 15 seconds, since draw filing can only generate flatness in one direction at a time.

Remember to check the underside of your carburetor, now that you have the tank-top flat. Either or both parts can be bowed.

Your tank top looks as if it was bowed quite a long way, incidentally. Your PulsaPrime carburetor is not really sensitive to going rich due to a bowed tank the way the original PulsaJet was, but it would be a lot to ask, expecting it to work properly with a tank as curved as that. The limit for the original PulsaJet was 0.002" hollow in the center, measured with a straight-edge right across the whole tank. The same applied for the flatness of the bottom of the carburetor. I had a 92908 that went super-rich, with a bow of exactly 0.002" as closely as I could measure it, and it worked perfectly again after draw filing it flat.

Thanks Grumpy,
I got the tank flat.
The underside of the carby had a high point that I scrapped down.
Reassembled it all with no change, still misses and blows smoke when I increase the throttle to run. Idles nicely.

I also tried another diaphragm.

I will source another one.

I haven't enough experience with the PulsaPrime to go straight to a solution, Mark. Like the PulsaJet it just pumps fuel into the small well in the top of the tank so that it overflows continually back into the tank, and the thick tower with the main jet in the bottom (or the side near the bottom) sucks fuel up from the well due to venturi suction. Hence there isn't much that can make it go very rich other than actual internal damage to the carburetor. Since all of the fuel that goes into the venturi is supposed to have to go through the main jet, it may be a blocked return from the well to the tank (the return is just a big hole in the side of the well), a damaged main jet, leakage through the primer pump directly into the venturi, or some other damage that allows fuel to bypass the main jet. That is why Joe suggested a leak across the diaphragm, from the fuel pump to the area that is subject to venturi suction. On the PulsaJet that kind of leakage could happen very easily, but the PulsaPrime is much less subject to it. If you can disconnect the primer somehow, you can see whether it is leaking through to its discharge side. That primer is a bit tin-pot and AFAIK is the most common source of problems with that carburetor, though not usually by making it go rich.

OK, great,
What you have written makes sense to me.
I will have a closer look at the main jet and compare it best I can with a known good one.

Thanks all, this one defeated me, I ended up sourcing another tank and carb.
Mower now works great.