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Briggs and Stratton 3.5hp problem


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08-05-13, 06:34 AM   #1  
Briggs and Stratton 3.5hp problem

Hi all,

I am hoping from looking at other threads that one of you gurus can help me. I have a simple push along mower with a 3.5 hp B&S engine - the one with the plastic pulsa carb. Someone posted a picture of the exact carb in another thread, so here it is below.

The engine starts OK, but will only run on about tick-over revs, even with the throttle lever fully open (directly adjusted on the carb, so it's not a linkage problem), and just sounds out of shape - lumpy and smoking exhaust. I thought it was a fuel / carb problem, and have now stripped that carb down so many times I could do it blindfolded!

The carb is now really clean, the gas is fresh, the diaphragm is nice and supple (not been changed, but it really is fine), and all the holes look good.

A couple of clues maybe:

1. Just before the engine runs out of gas in the tank, the revs pick up really well, and then obviously it dies. Maybe that means the mixture is too rich? However there is no adjustment on this carb. The main jet looks good - I've had the jet out and cleaned it, and re-sited it nicely, properly clicked in behind the fuel screen.

2. If I run the engine with the air filter off, if I blow hard into the top of the carb the revs pick up briefly - another indication of too rich a mixture?

3. The only carb parts I didn't understand were 2 x 3mm (ish) ball bearings in a couple of the carb holes - one near the fuel pickup tube, and one near the main jet. I have no idea what these are for?

This has become a personal crusade, so it would be great to get some help!

Cheers, Nick.

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08-05-13, 09:00 AM   #2  
The rpms picking up right before it dies can be interpreted on either side of the problem

One is the mixture evens out and the rich mixture gives way to a more efficient normal air/fuel mix. That would allow it increase, but that would also allow it to keep running, and not die.

The other side is more the normal case where the engine runs out of fuel, either due to a vacuum leak or fuel blockage or the the diaphragm pump isn't flexing like it should. The leaner mixture right before a complete run out would promote the high rpms, then the run out of fuel would lead to the dieing. This scenario is much more common.

If the fuel lines and filter are good, and there isn't a vacuum leak (check by spraying WD40 around gaskets and mating surfaces, etc.) and the path to the carb from the tank is good, you probably should change the diaphragm, although it looks good. It may not be flexing like it should.

 
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08-05-13, 09:16 AM   #3  
You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.
Here we go:

Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Check the intake tube to see if the O ring seal and plastic retainer are still on the intake tube, if so remove them and re-install them into the carb. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble), now with a small screwdriver pry out the main jet(carefully) and clean it, check the O ring on the main jet for damage, if it is damaged it must be replaced, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem(not necessary if the fuel tank was clean). Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo

 
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08-05-13, 09:17 AM   #4  
Change the diaphragm and be done with it! It may look good but in certain area it probably isn't.

 
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08-05-13, 09:57 AM   #5  
Thanks for the replies!

Marbobj I'm not sure I follow you. What I'm saying is that there are two situations where the revs do actually pick up nicely:

1. When the gas tank runs out - or just in the moments before.

2. When I blow hard into the carb with the filter off.

In both of these cases the mixture is getting leaner and the engine seems to like it, which makes me think that for the rest of the time the engine is actually getting TOO MUCH fuel, i.e. the mixture is too rich. This is what makes me wonder about the ball bearings, as there seem to be no other moving parts on the carb.

Another clue is that after running the engine (to the extent that it does), when I take the carb off the tank, the fuel bowl on top of the tank (into which the main jet and screen hang) has a good amount of gas in it. If this was a fuel pumping problem (i.e. bad diaphragm), I would expect that bowl to be looking a bit dry don't you think?

Nick.

 
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08-05-13, 12:32 PM   #6  
What you see in the tank running dry and the rpms picking up would be normal and not an indication of an overly rich mixture.

Blowing into the carb throat doesn't really correct a fuel air mixture problem. It forces more fuel air mixture into the cylinder - like a supercharger. The extra mix boosts the rpms. The air pressure differences inside the carb that allow it to work would be disturbed to the point of not knowing what the carb was actually doing.

I believe it will come down to the diaphragm, but you can check it by feeding the carb through a gravity feed and bypass the diaphragm.

The other thing you can do is run it as much as you can, then pull the plug. It should be black and sooty if the mixture is too rich.

 
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08-05-13, 03:05 PM   #7  
Aha - I have pulled the plug a couple of times and it IS very black and sooty. So I guess 2 questions:

1. Given that, does it take you down a different path? If the mixture is too rich, what can I do about it?

2. Picking up on your point about checking the carb using a gravity feed, can you elaborate on that? I'm guessing empty the tank and then inject fuel into the carb throat to run the engine? Sounds difficult to achieve the right fuel rate?

Nick.

 
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08-05-13, 03:46 PM   #8  
The sooty plug, when the diaphragm is not working properly as the engine surges it is a no gas, too much gas situation, it is not getting an even flow, save yourself some time and replace the diaphragm, it is a $5 fix. Have a good one. Geo

 
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08-05-13, 04:32 PM   #9  
No you'd have to feed that one through the pickups that pull fuel out of the tank. But I don't believe I would go to that trouble. That's little different animal than most carbs, but that's a pretty broad subject of comparison.

It would be easier to just replace the diaphragm and see where you're at. I suspect that will take care of it.

 
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08-05-13, 04:57 PM   #10  
Do what Geo says,,,,,,,,,,,,,the back side of the diapragm is more than likely leaking causing it to run rich and uneven. It's not rocket science here.

 
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08-05-13, 10:31 PM   #11  
The diaphragm is your problem as Geo mentioned.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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08-06-13, 02:06 AM   #12  
Well, it seems unanimous that a new diaphragm is the way forward. I will order one up and report back ...!

Thanks so much for your thoughts so far - really appreciate it.

Nick.

 
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08-12-13, 03:44 AM   #13  
Disappointment. The parts came today and I fitted the new diaphragm and gasket (diaphragm nearest the tank). I also cleaned up the spark plug. However, whilst the engine does now run better at idle (with the governor lever pushed to the left), but when moving the lever to the right, the engine is running really badly - slow, lumpy, sooty exhaust. With the air filter off, I can see fuel kind of sputtering about in the jet area of the throat of the carb, although I can't see the jet itself because of the angles.

I'd really appreciate any further suggestions.

Nick.

P.S. Would it help to post a video of the engine running?

 
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08-12-13, 09:25 AM   #14  
Sounds like the diaphragm is not quite in place properly or maybe even on the wrong side of the gasket.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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08-12-13, 09:41 AM   #15  
Follow the instructions in post #3 very closely. Have a good one. Geo

 
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08-12-13, 01:46 PM   #16  
Hi guys,

I did follow all those instructions very carefully. I have been through the procedure a lot over the last few days, so I'm fairly familiar with it all. The diaphragm is definitely on the correct side of the gasket, i.e. it goes carb, then gasket, then diaphragm, then tank. Because of the shape of both, it's not possible to get them the wrong way around or anything. I was also very careful not to get any creases or misalignment, getting all the screws in before tightening any of them.

Any other ideas?

 
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08-12-13, 07:32 PM   #17  
A loose main jet in the carb is possible.


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God bless!

 
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08-13-13, 12:20 AM   #18  
I don't think it's that. I have taken out the jet, run a paperclip through it to make sure it's clear, checked the 'o' ring and snapped it back into place in the carb. I do think you're on the right lines though. Any tricks for experimenting with the jet on a basic carb like this?

Also, I still don't know what those little ball bearings are for. There are two of them, both in channels which come out on the side of the carb. One leads to the top of the fuel pickup tube, and one to the main jet. What are they for? Could they be part of the problem / solution?

 
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08-13-13, 08:29 AM   #19  
When it's running rough, try loosening the gas tank cap just to the point of taking it off and see if it evens out the running of it. That would tell you if the pulses are pressurizing the tank and forcing extra fuel up the main jet.

 
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08-13-13, 08:58 AM   #20  
I have had "hanging shrad" on the main jet, it had not been stamped out completely, so at times it would bend/flop up and block the jet, only saw this one time. Maybe there is a crack in the intake tube, with it running spray/dribble some carb/brake parts cleaner behind the carb to see if the engine tempo changes, if it does you have an air leak either in the intake tube or the "O" ring gasket in the carb is failing. Have a good one. Geo

 
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08-17-13, 09:56 AM   #21  
Thanks for your latest ideas.

Marbobj, I tried loosening (and removing) the gas tank filler cap while running, but it made no difference. Nice thought though.

Geogrubb, I have had the main jet out a couple of times, and it's really clean. The engine is about 15 years old, so I don't think I would suddenly get some swarf in there.

I had my daughter video me starting the machine and running the engine a bit so you can see and hear what it's like - I'm hoping that this might throw up a couple more ideas before I give in!!

IMG 0964 - YouTube

 
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08-17-13, 10:35 AM   #22  
Try crutching it with the primer bulb while it's running poorly and see what that does. It doesn't look like it's flooding as much as fuel starving. For that the fuel cap needs to be tight.

 
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08-18-13, 11:22 PM   #23  
marbobj thanks - I tried that, but pumping the primer bulb makes the engine run worse rather than better.

Just one thought - the picture right at the beginning of this thread shows 2 springs on the governor lever, whereas I only have one. Makes me wonder - is the engine even supposed to run with the throttle open this wide, or should I be making physical adjustments to the throttle linkage and spring(s) so that full throttle leaves the governor lever somewhere in the middle?

The engine doesn't run great when the lever in the middle, but it's a whole lot better than when it is fully open.

I am about to give up on this machine, so last few suggestions very welcome!!

 
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08-19-13, 04:54 AM   #24  
The governor is driven by rpms which, at present are very erratic, so it may wind up anywhere. The position of the governor arm wouldn't make the engine run like it is.

The problem is in the carburetor. When you hit the primer and got a reaction, it's an indication the carburetor is delivering any overly rich mixture which is consistent with diaphragm/gasket issues with that pulsajet carburetor. Priming it compounded the problem.

There must be something with that diaphragm. Did it match the original perfectly, hole by hole?

 
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03-22-14, 09:58 AM   #25  
hi i am new to all this
but i have just read your review drummernick
i am having the same problem do you know where i can get new diaphram and a new link as i snapped my one trying to take it off
cheers

 
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03-22-14, 11:14 AM   #26  
I got mine on EBAY - I am in the UK, so a specific link probably not useful, but let me know if you want me to dig it out?

 
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