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Hi All,

I'm working on restoring an older 3.5hp Briggs and Stratton engine with the pulsa jet carburetor which hasn't ran in 10+ years. I'm able to get the engine running, but it will only idle and not rev unless you manually flap the choke plate. I'm thinking it's not getting enough fuel pressure to run the engine at higher speeds.

I replaced the diaphragm between the carb and gas tank (non OEM model), cleaned the carburetor and gas tank very well, cleaned the crankcase ventilation system, and replaced the ignition coil. However no such luck getting the engine to run at higher speeds for mowing. It's used on a Snapper Hi-Vac lawn mower.

What other areas should I check?


Thank you!

If the carb was sitting up that long - not sure you can clean it well enough also the soft goods are probably shot.
If it idles the engine evidently is in good shape so it's most likely fuel issues - don't think that model has a fuel pump only the pulse off the crankcase working the diaphram via a small hose that's probably dry rotten

Checked the rubber elbow going from the crankcase ventilation port to the choke of the carburetor and it appears to be in good condition. No cracks were found doing a quick soapy water test with the engine idling.

Did some more research and found the problem may be with the crankcase breather system which operates the auto choke system and diaphragm fuel pump. If the valve rattles it's good; if not, replace it. I cleaned the one on the engine then the engine ran better, but not great.

Hope the new OEM one fixes it. Will post back the results.


Thank you.

What are the model, type and code numbers for the engine? Did you preload the diaphragm when installing? Did you install diaphragm and gasket in to correct order?

Model: 92502 (could be 92503?)
Type: 3136 01
Code: 8412110

I’ve been using 92502 3136 01 to find parts on Briggs and Stratton’s website.

How do you preload the diaphragm when installing?

By installing the gasket and diaphragm in the correct order, are you referring to the spring which sits in a recessed area of the carb above the diaphragm to control the fuel flow into the float chamber?

This manual will help you understand what I’m talking about. https://www.wheelhorseforum.com/fil...&t=1&csrfKey=d6af3634f2935fa124ed1ca13797786e

Fuel tank MUST be spotless.

Probably have to remove all soft parts and boil the carb body. Cheap hot plate and a pot. Little pH neutral car wash soap. Blow out with carb spray and compressed air. Might take a couple cleanings to make her run right.

Slomo, do you realize that this is a 39 year old engine? Boiling a plastic carb will ruin it.

Rivets said:

Slomo, do you realize that this is a 39 year old engine? Boiling a plastic carb will ruin it.

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Check that. Didn't realize it was plastic. Thanks for the correction. My bad.

How about 120F with a candy thermometer? Little heat to loosen up some varnish. Then blow out with carb spray.

NO!

I just worked on my personal 1980 B&S 3.5 and used an aftermarket diaphram. It ran worse that it did before I worked on it. I got a genuine B&S diaphram and it now runs great. Yours may use a different diaphram as my carb is metal not plastic. Got the spring and cup on the carb side of the diaphram?

Thank you everyone for the suggestions!

The replacement OEM B&S breather arrived yesterday; will install then try starting the engine again. If that doesn’t work then it’s time for an OEM diaphragm as well as another round of carb cleaning.

The carb is metal as well - only plastic items are the fuel pickup tubes.

Got the spring and cup on the carb side of the diaphram?

Click to expand...

Yes, all springs are in the correct spots. The spring below the choke is there, however I didn’t fully attach it to the rod controlled by the auto-choke below the diaphragm. It’s resting on top. Should it be connected?

Then the spring below the throttle plate is in the recess of the carb above the diaphragm.

If I am understanding correctly then, yes. That spring needs to be attached to the diaphram as shown in the diagram below in for lack of a better term the U shaped area on two sides of the rod.

Yes, pages 20-22 of section 3 in the manual I posted explains installation and preloading of the diaphragm. Might want to read it before going to far.

We may be onto something with the choke and how it pumps fuel into the carburetor. I adjusted the spring on the auto-choke plate to keep the choke plate fully open then the engine was able to maintain a running speed just above idle. It didn’t rev to full speed, but it was enough to adjust the mixture screw.

I‘ll study the manual Rivets provided further then try again. This time, perform the preload procedure to the diaphragm.

set 3/8 bolt in carb to hold choke in correct position then tighten screws to set. Check gas pickup tube by blowing and sucking on it. (some have check valve in) or replace the tube.

volvofan94 said:

We may be onto something with the choke and how it pumps fuel into the carburetor. I adjusted the spring on the auto-choke plate to keep the choke plate fully open then the engine was able to maintain a running speed just above idle. It didn’t rev to full speed, but it was enough to adjust the mixture screw.

I‘ll study the manual Rivets provided further then try again. This time, perform the preload procedure to the diaphragm.

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Something I forgot to ask. Are you running the engine with the air filter off? If so, put the air filter hold down screw back in. Many of those older Briggs would run rich if you tried running them with that screw removed as it would let unwanted fuel come up through that open screw hole.

Check the butterfly for throttle to be sure its not binding or loose. I have seen the screws come out of these. Just a thought.

Have you checked for warpage of the top surface of the fuel tank? The bottom of the carb where it seals to the tank? As I understand it can be necessary to add a second gasket to get them to seal up if there is warpage. Without the seal, it won't pump the fuel properly.
tom

grumpyunk said:

Have you checked for warpage of the top surface of the fuel tank? The bottom of the carb where it seals to the tank? As I understand it can be necessary to add a second gasket to get them to seal up if there is warpage. Without the seal, it won't pump the fuel properly.
tom

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warpage is a problem but a second gasket wont work on these!

It would be a very long reply for me to post everything I know about these carburetors but will try to post something helpful.
I'm not a professional, but I have worked on these engines with these carburetors for many years now as a hobby.

First off, the model number you posted for your engine indicates that this engine originally had a vacu-jet carburetor, not a pulsa-jet.
Both are interchangeable on these engines so there's a good chance you might have a pulsa-jet on your engine if someone has swapped it.
Side note info:
A model 92502 indicates a vacu-jet carb.
A model 92902 indicates a pulsa-jet carb.
5 = vacu-jet (Also indicates that the engine has a vertical crankshaft)
9 = pulsa-jet (Also indicates that the engine has a vertical crankshaft)

So this being said, if you are unsure what carburetor you have you will need to verify for sure what carb. you have.
The easy way of telling the difference if you don't know what your looking at is:
(With the carb. removed from the fuel tank) A vacu-jet will only have ONE fuel pickup tube.
A pulsa-jet will have TWO pickup tubes and a second short spring and a dished washer thing between the diaphragm and carburetor.
You will want to verify what carb. you have before ordering parts, specifically the pump diaphragm, as the vacu-jet and pulsa-jet diaphragms are totaly different.

Number one problem on these is the pump diaphragm going bad, but these carbs also have a problem of warping and not sealing to the tank properly (also the tank surface will warp too)
So sometimes you may have to replace the carb. or the tank or both.
I will post another reply with a list of part numbers that may be helpful to you or others reading.

Something I would like to add is that years ago My brother and I had a 3.5hp briggs that would not throttle up from idle without dying.
It ended up being that the ignition coil had come loose and was scraping the flywheel and wobbling around, probably messing with timing and what not. Just something to check.

Do you wan to to restore it, or just get it running? If the latter, you can also swap that pulsa-jet with a pulsa-prime carb & tank. I did this when my pulsa-jet tank rotted. Its basically a direct swap except for 2 possible issues on old equipment. You may need the longer screw for the air cleaner and the breather tube may need to be shortened.

Amazon sells the chinesium version of pulsa-prime with the steel tank and plastic carb for around $25. I have one and it works decent, but there is a strong chance you may get a turd. Luck of the draw with that stuff. If you go the knockoff route you'll also need a metric bolt where the tank attaches to the bracket. 6mm I believe.

This is a list of part numbers for the carburetors used on most 3 to 3.5 (and some 4) HP briggs and stratton vertical crankshaft engines from the mid 1960's to the early mid 1990's
So first off I will list part numbers for the pulsa-jet carbs
Carburetor pump diaphragm:
391681
Carburetors:
394063
390065
390126
391642 These 4 all require the longer breather tube/pipe
395957 This one is a later variation and requires the shorter breather tube/pipe.
Breather tubes/pipes:
230802 Long
691245 Short
231272 Short
231774 Short
Be carful as ebay sellers sometimes don't know the difference between the two sizes and will list them with the wrong part numbers.
Fuel Tank:
391694

The following is for Vacu-Jet carburetors:
Carburetor pump diaphragm:
299637
I seem to have lost or messed up my info for the part numbers for the vacu-jet carburetor.
I will edit and post when I get that sorted out
Fuel Tank:
396778




.

If the tank top is warped, prep it and use Bondo, abrasive paper and a sheet of plate glass. Use one gasket sprayed with high tack sealant. Little skim coat is all you need. And don't over torque the carb when you reinstall. Use your weaker off hand with open fingers to lessen torque. Don't need lug nut torque at all.

slomo said:

If the tank top is warped, prep it and use Bondo, abrasive paper and a sheet of plate glass. Use one gasket sprayed with high tack sealant. Little skim coat is all you need. And don't over torque the carb when you reinstall. Use your weaker off hand with open fingers to lessen torque. Don't need lug nut torque at all.
View attachment 66115

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Sounds like a good idea except for the high tack sealant, might work on a vacu-jet but not on a pulsa-jet. Seems like it would glue the little diaphragm flaps closed and cause a whole world of trouble.

As was just posted, never use any type of sealer around the diaphragms used on pulsa jet carburetors. You’ll be replacing it very fast, as most sealants will also eat the thin rubber. Experienced techs have learned this the hard way.

Quick update…no change.

I tried preloading the diaphragm per the shop manual link Rivets’ provided as well as replaced the choke shaft spring. The engine still won’t run properly. I have a genuine B&S diaphragm on order. If it still won’t rev after replacing the diaphragm, I‘m considering throwing in the towel then move onto other projects. Plus I‘d like the garage space back too! lol.

I should add I removed, re-cleaned, and inspected the two fuel pickup tubes. Those are good and clean as are the passageways they support in the carburetor.

Something I forgot to ask. Are you running the engine with the air filter off? If so, put the air filter hold down screw back in. Many of those older Briggs would run rich if you tried running them with that screw removed as it would let unwanted fuel come up through that open screw hole.

Click to expand...

The air filter has been on and off while running the engine, but I’ve always had the air filter hold down screw installed.

Sorry I’m catching up on replies the from last week. I appreciate everyone’s suggestions!!

Thank you SmallEngineHead160 for your detailed description between the vacu-jet and pulsa-jet carbs! The carb I’m working with is 100% a pulsa-jet. It has the two pickup tubes. I’m not sure if the carb (and possibly tank) was replaced sometime in the engine‘s past, but it’s looking more likely now. Also, when I started looking for parts, that could explain why my engine didn’t exactly match the diagram on B&S’ website. I verified I ordered the correct B&S diaphragm. Whew!

Ocean909, my plan is to get the mower running then sell it on.

Update #2: got the new diaphragm installed, the engine started, revved up, then died. Can’t seem to win with this engine! lol.

Well, suppose next step is to check the tank for flatness. I may be getting fuel into the tank‘s vacuum chamber. The manual Rivet’s provided has the steps I’ll perform on page 19.

Im assuming you followed the proper mixture screw adjustment procedure?
If not you're gonna want to do that before anything else.

I’ve set the mixture screw 2 turns out from the fully closed (tightened) position. I’ve tried a variety of settings for the mixture screw, but none have worked to date. The tip of the mixture screw is in good condition. Looks like the manual says 1 1/2 turns for initial. Will try that then post back.

My carb doesn’t have the idle speed screw mentioned in step 4 of the Final Adjustment procedure. May have gone missing over the years. Will replace it.

Yes one & a half turns out should be your initial setting to get it running to make additional adjustments.
Sometimes if it won't stay running with the initial setting you may tweek it a 1/4 turn in or out but you should never have to turn the screw all the way in to get it to run.
If the engine only runs with the screw all the way in than that means you still have a problem with the carburetor.

If you want the engine to idle at a set speed than your gonna want to have that idle screw but as long as your starting/running the engine in the fast position it won't matter for now.

@volvofan94 I know you plan on selling this, so spending +$25 isn't ideal for your profit margin, but swapping out for a pulsa-prime may be less frustrating. There is no auto-choke, no adjustment, and its basically a direct swap. Just thought I'd add to your options.

@Ocean909, I like it, that’s the way I’ll go with the project! At this point I’m at my wits end with getting the Pulsa jet to work correctly. lol. Tried starting it yesterday, still no luck. Even checked the carb/tank mounting surface for flatness before starting the engine; it looked good.

I’m ~$40 into the project now, not a lot of money, but having the garage space back is plus at this point.

What are the part numbers for the fuel tank and pulsa-prime carburetor?

I believe the part number for the carb is 795477 the tank 494406. Searching for the tank # at the usual online suspects will get you the tank and carb combo. Couple of things to note. Its aftermarket so it may work may also be junk. Depending on your mower age you may need to cut the end of the breather tube or order the shorter one. You also may need the longer screw for the air cleaner. The mount on the tank may need a 6mm bolt.
Hope this helps

The modern pulsa-prime carb/tank is a good cheap alternative I have done it myself and it seemed to work fine the only issues are the ones that Ocean909 stated plus I would like to add that the pulsa-prime carbs do not have an idle adjustment screw so on a mower with a throttle control if you put the control in the idle position the engine could ither idle to low or the engine could stall. This can be worked around by just slowly adjusting down the throttle until you reach a proper idle speed.
There is a another fix but that involves changing out the control bracket that mounts to the engine and fuel tank with a correct pulsa-prime control bracket which with the correct governor spring is designed to only allow the engine to idle at a set speed.
Some of the aftermarkets where selling pulsa-prime carb/tank kits that included that control bracket but I don't know you if can still find those. You can find used control brackets on eBay but this may all be out of budget for you.

All this said, if the idle issue isn't a problem then the pulsa-prime is definitely the way to go.

Got the pulsa-prime carburetor and fuel tank kit ordered. $28 and it should arrive next week.

Hopefully the speed control bracket I have on the engine will work with the pulsa-prime carb setup. Looking at the photos of the pulsa-prime carb and tank online it should match up no problem. The mower has a handle mounted speed control (looks aftermarket) which connects to a cam that adjusts the throttle plate and controls the governor speed on the pulsa-jet carb. Hopefully those parts can be reused! Will keep my fingers crossed as I begin to install the new tank and carb.

Got great news; the new pulsa-prime setup worked well!! 3-4 primes and one pull later, the was engine running! Even fixed the handle mounted kill switch so it turns off nicely too.

I appreciate everyone’s help and suggestions with this project. Couldn’t have done it without your help.

Am I able to reuse the pulsa jet carb air cleaner on the pulsa prime carb? I’m going to purchase a longer screw to hold it to the tank through the throat of the carb tomorrow.

That's great news, yes you should be able reuse your original air cleaner

Glad to hear it worked out. SmallEngineHead160 is correct, the old air cleaner should bolt right on with the longer screw. Also Some of these replacement carbs have a small black screw blocking the threads where the air cleaner screw threads in.