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Joined: Dec 2020
Posts: 40
Likes: 2
Andy E Offline OP
Novice
Hi all, I've got a Briggs 3.5hp, model 91202, made July 1992 with a Pulsa Jet Carb. I'm chasing a running rich issue and wondering if anyone can think of something I might be missing. I did a full carby clean including idle circuit, new gaskets, diaphragm and spring. It is running rich on both idle and higher speeds. It only has the one mix screw for the idle circuit, no high speed mix screw. Idle mix screw is set at 1 1/2 turns out from bottomed out. It runs fine, but puffs some black smoke on idle and higher revs. Less on higher revs. I'm sure I got all the airways clean and it's not the air filter, that's new and removing it makes no difference. Only thing I can think is maybe the new diaphragm has a pinhole in it and is letting fuel in to the carb throat. I did originally see a little fuel around the outside of the diaphragm cover after replacing the old one, so took it off and reseated it and it appears to have stopped I think but hard to see in the failing light I was in.. I've got a new diaphragm in the mail to try but failing that, any advice/ideas on how else the mix could be rich at all speeds in relation to the carby? Anything I could look for that I haven't thought of? I believe there could be other causes related to timing or possibly a valve but am trying to rule the carb out before I go further down that path. Thanks for any advice.
Cheers

Last edited by Andy E; 08/03/21 04:19 PM.
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Joined: Jul 2018
Posts: 2,086
Likes: 80
SENIOR TECHNICIAN
No, its a common issue with the pulsajet - stemming from a age warped mating surface (run a straight edge over the carby mounting surface and you will see issue)

They released a repair kit (with a pin and a tiny teflon washer) to seal the gap - not especially easy to come by unless you want 10 sets.

My home made solution has worked 2x now - cut a thin piece of line trimmer fuel pump diaphragm (one of the clear mylar or blue teflon ones not thick rubber) and install (holding in place with some grease

Works perfectly then

Will post a link when I find it on here

Edit; 91202 is horizontal shaft isn't it - different carby. I was thinking it was a vertical shaft. Only thing I can think of is there were 2 slightly different gasket and diaphragm designs - maybe the wrong ones?

Last edited by Tyler; 08/03/21 02:39 PM.
Joined: Dec 2020
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Andy E Offline OP
Novice
Ahhh, you know I was thinking about this, although I was wondering if it was the diaphragm cover that had maybe warped, I put an edge across it and it looked to be ok. I hadn't thought of the body, thanks for the tip.
Just saw your edit, yes horizontal shaft, will still have a look at it because I think there is some sort of mating issue / hole at the diaphragm allowing fuel to get behind it, if the body is warped it might be getting sucked in a bit by the vacuum and letting fuel past..maybe. There's only the one diaphragm for this model, but they are all after market so could be a tolerance issue maybe.
Thanks all the same, appreciate the input.

Last edited by Andy E; 08/03/21 02:55 PM.
Joined: Jan 2016
Posts: 7,466
Likes: 143
SENIOR TECHNICIAN
Here is another possible idea, cut the top out of a vertical shaft motor fuel tank and JB Weld/pop rivet that onto the top of the horizontal shaft tank. Tell me if this is a dumb idea but I have buckets of the vertical shaft tanks that I just can't bring myself to throw in the scrap

Joined: Dec 2020
Posts: 40
Likes: 2
Andy E Offline OP
Novice
Hi Norm, pretty sure the tank to carb seal is good, otherwise it wouldn't be sucking up fuel? This one has the seperate diaphragm on the side of the carb with it's own little metal screw on cover. This is not my particular one, just a stock pic to show the model I have

Attached Images
_pump_diaphragm.jpeg (23.86 KB, 49 downloads)
Last edited by Andy E; 08/03/21 03:34 PM.
Joined: Oct 2018
Posts: 542
Likes: 13
wce Offline
Qualified Senior
Hi Andy,Tyler and Norm

Was doing the same before carby service ? what did the old diaphragm look like ? new gasket, diaphragm, spring and spring cap installed correctly ? choke butterfly fully open when throttle in run position ?

cheers
wce

Joined: Dec 2020
Posts: 40
Likes: 2
Andy E Offline OP
Novice
Hi wce, it is an older Rover cylinder mower that had sat for years, so it wasn't doing anything, including starting before I serviced it. Everything is installed correctly, choke is fully open and stays that way, seperate lever. Runs good, just puffs some black smoke and soots up the plug. Will put an edge over the diaphragm body mating surface when I get the new diaphragm and failing that might swap it out with a new replacement carb just to see if it solves the problem.

Last edited by Andy E; 08/03/21 04:27 PM.
Joined: Dec 2020
Posts: 40
Likes: 2
Andy E Offline OP
Novice
I have admitted defeat and ordered a replacement carby. I think it's the air bleed that feeds around the emulsion tube in the middle of the carby, but I can't get to it without pulling the elbow welch plug and it will cost me almost as much as the new carby to replace it.
I started it up today to have another go at the idle mix, and unknown to me it was almost out of fuel, and as it leaned out it ran like a new engine, no black smoke, no vibration, so pretty sure the issue is carb related and not timing or a valve. We'll see I guess.

Last edited by Andy E; 09/03/21 04:36 PM.

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