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Hello folks,
so one of those engines with the plastic carbs without a primer bulb, with the white "cartridge" in it, a 625EXI engine has issues with the RPM/throttle surging up and down.
Before this happend, the engine refused to run properly at all, so I took the carb apart, cleaned it, the ports too, replaced spark plug, placed a new filter and it run great. But after a hour or so after mowing, it started surging heavily, no matter if there's a filter on or not. Only thing I noticed is that the carb bowl is very oily/wet overall.
Not sure what to try since the carb has been cleaned already. No way to tune it or anything, so I'm out of ideas.

Video and image: (don't forget to turn sound on)


Thanks anyone in advance for any help given!

Pull the breather tube off the back or the airbox and see if it runs better and how strongly the gasses come out of the breather ty=ube
The breather could be faulty or the head gasket could be blown.

bertsmobile1 said:

Pull the breather tube off the back or the airbox and see if it runs better and how strongly the gasses come out of the breather ty=ube
The breather could be faulty or the head gasket could be blown.

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Hi and thanks for your reply.
So, pulled off the breather tube and it ran actually good for a minute or so. Next start, and it ran bad (still, better a bit) yet again. It definetly did help, but not for too long, I imagine until it warmed up fully.

Nothing can be noticed to be coming out of the tube, even though the old filter was very oily, just like the whole air filter casing.

Suddenly, it started running okay-ish again (it still was surging, but a lot less, more like the surging everyone complains about on these engines, not even close as shown in my video), and it ran great until fully warmed when it started surging/bogging down again. So I went out and cleaned the outter carburetor casing and I noticed some grass stuck on the end of the choke lever. Afterwards, it ran good, even under load. It starts surging randomly (doesn't matter anymore if the breather tube is on or not), or when I go uphill with it. But I think it starts going bad again when it gets fully, like fully warmed up, when the choke is fully off.

But I'll try to check my fuel line and the in-tank fuel filter, as the randomess of the issue has given me suspicion on the filter. Next I'll probably re-try cleaning the carb.
Any more tips? It's giving me a headache.

I have needed to tighten the choke return spring on a few of these engines because it would flutter every now & then.
This carb would have to go down in history as being the absolute worse design ever made
The choke lever can foul on the side so not fully open and this gets worse with age as the plastic gets soft from heat and slumps a bit

bertsmobile1 said:

I have needed to tighten the choke return spring on a few of these engines because it would flutter every now & then.
This carb would have to go down in history as being the absolute worse design ever made
The choke lever can foul on the side so not fully open and this gets worse with age as the plastic gets soft from heat and slumps a bit

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Hi once again.
So I tried a different tank, w/o the fuel filter and the same issue. It starts running nice up until fully warm or put slightly uphill.
Currently I'm cleaning the carb, once again. I'll see if that helps anyhow.
Can you also please explain how you managed to get the choke return spring to be tighter, and which spring it is exactly?
Also, can the governor be bad, or out of tune? Thanks in advance for any tips!

Snip off one end then bend down the next 2 turns to make the mounting ring

XGamer223 said:

Hi once again.
So I tried a different tank, w/o the fuel filter and the same issue. It starts running nice up until fully warm or put slightly uphill.
Currently I'm cleaning the carb, once again. I'll see if that helps anyhow.
Can you also please explain how you managed to get the choke return spring to be tighter, and which spring it is exactly?
Also, can the governor be bad, or out of tune? Thanks in advance for any tips!

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Since it runs well until it is fully warm on an auto choke model, that would mean that the carb is definitely running lean, and the period while the choke is still partially closed is compensating for this problem. Make sure the carb is clean, clean, clean. Then, I would check for vacuum leaks around the carb with brake or carb clean (safety nazis hate this), and if you still have the problem I would get carburetor drill bits and drill out the main jet roughly 1 or 2 thousandths. On these carbs the jet is brass and is cast into plastic, the jet and emulsion tube assembly can actually be removed from the carb body with a prying tool.

(safety nazis hate this)

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OK in the states where there is next to no social security so stupidity is largely at costto the stupid.
however I work hard for my money so the idea of the government using it to support a blithering idiot for the next 40 years because a flashover blinded them is one of the few things that makes me cranky .
I am all for letting idiots chop their fingers / feet off in fact I would like to see it happen more often then perhaps the brain dead might come to the understanding that mowers are dangerous
But I just don't want it to cost me money

bertsmobile1 said:

OK in the states where there is next to no social security so stupidity is largely at costto the stupid.
however I work hard for my money so the idea of the government using it to support a blithering idiot for the next 40 years because a flashover blinded them is one of the few things that makes me cranky .
I am all for letting idiots chop their fingers / feet off in fact I would like to see it happen more often then perhaps the brain dead might come to the understanding that mowers are dangerous
But I just don't want it to cost me money

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Found the safety nazi, lol. But have you actually done the spray test for vacuum leaks before? I have never had anything catch fire and it generally works very well. The only alternative is propane, which is still dangerous. Being a mechanic is a dangerous job man. Besides the occupational hazards, you work with dozens of chemicals that cause cancer.

bertsmobile1 said:

OK in the states where there is next to no social security so stupidity is largely at costto the stupid.
however I work hard for my money so the idea of the government using it to support a blithering idiot for the next 40 years because a flashover blinded them is one of the few things that makes me cranky .
I am all for letting idiots chop their fingers / feet off in fact I would like to see it happen more often then perhaps the brain dead might come to the understanding that mowers are dangerous
But I just don't want it to cost me money

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What about smoking? Obesity? Alcohol? Type II Diabetes? Motorcycles? Skateboards? Soda pop? Roundup? Carpet cleaner?

I can sympathize, but if you make society collectively responsible for health, you have to accept that some people are going to do stupid things that might cost you money. Otherwise every vestige of freedom is directly erased.

artemjemmy said:

Found the safety nazi, lol. But have you actually done the spray test for vacuum leaks before? I have never had anything catch fire and it generally works very well. The only alternative is propane, which is still dangerous. Being a mechanic is a dangerous job man. Besides the occupational hazards, you work with dozens of chemicals that cause cancer.

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On cars I spread motor oil over the manifold/head join to see if it brings the vacuum up. I've never seen a mower that had any place to attach a vacuum gauge, but I bet you could hear the running difference if there's a leak.

There are unavoidable hazards
There are unnecessary hazards
A wise mechanic understands the difference and acts accordingly.
During the drought I got called to a customers house to quote on purchasing all of his petrol powered equipment
Why ?
Well he was in hospital IC with massive burns and his wife wanted all of the petrol powered gear gone before he came home.
This is yet to happen he is still in a recovery ward.
How did this happen ?

Well the engine was hydrolocked but in place of turing he engine over by hand after the plugs were removed he cranked it.
Fuel shot out of the cylinders splashed all over the mower & himself then ignited, setting fire to his cloths & the mower.
As luck would happen the mower went out but his cloths & hair did not .

Now what were the chances of this happening ?
About the same as winning the lottery
However the consequences were not pretty and it was just dumb luck that the burning petrol did not end up burning the entire house down
He will be blind in one eye at least .
The house is up for sale at the moment as without his income they could not meet the loan repayments and she has moved in with her mother while their kids are with another relative.

bertsmobile1 said:

There are unavoidable hazards
There are unnecessary hazards
A wise mechanic understands the difference and acts accordingly.
During the drought I got called to a customers house to quote on purchasing all of his petrol powered equipment
Why ?
Well he was in hospital IC with massive burns and his wife wanted all of the petrol powered gear gone before he came home.
This is yet to happen he is still in a recovery ward.
How did this happen ?

Well the engine was hydrolocked but in place of turing he engine over by hand after the plugs were removed he cranked it.
Fuel shot out of the cylinders splashed all over the mower & himself then ignited, setting fire to his cloths & the mower.
As luck would happen the mower went out but his cloths & hair did not .

Now what were the chances of this happening ?
About the same as winning the lottery
However the consequences were not pretty and it was just dumb luck that the burning petrol did not end up burning the entire house down
He will be blind in one eye at least .
The house is up for sale at the moment as without his income they could not meet the loan repayments and she has moved in with her mother while their kids are with another relative.

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So, Bert, what would you say is the OSHA approved alternative method for finding vacuum leaks, hmm?

I use WD 40 out of a trigger sprayer.... I don't care to be in the middle of an aresol fireball.

Scrubcadet10 said:

I use WD 40 out of a trigger sprayer.... I don't care to be in the middle of an aresol fireball.

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WD-40 is quite flammable as well though, especially with you spraying it in a mist form.

AFter opening up the main jet a few thousanths with a micro drill. Do you still have any running issues? Also did you check the valve lash? plus..... Make sure the oil is not overfilled.

artemjemmy said:

WD-40 is quite flammable as well though, especially with you spraying it in a mist form.

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not as easily flammable like the aresol. it will burn but not as intense

Scrubcadet10 said:

not as easily flammable like the aresol. it will burn but not as intense

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Would diesel work?

Richard Milhous said:

Would diesel work?

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¯\_(ツ)_/¯

Richard Milhous said:

Would diesel work?

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Yes but it would be super messy. Part of the benefit of using carb clean or brake clean is that it will evaporate and leave no mess. Both diesel or WD40 will leave a mess wherever you spray it that won't really go away

artemjemmy said:

So, Bert, what would you say is the OSHA approved alternative method for finding vacuum leaks, hmm?

Click to expand...

Like scrubby,
I use either wd40 or inox from a trigger pack depending upon which is closer at the time
No idea is it is OH&S compliant but a lot safer and cheaper than anything from a pressure can

XGamer223 said:

Hello folks,
so one of those engines with the plastic carbs without a primer bulb, with the white "cartridge" in it, a 625EXI engine has issues with the RPM/throttle surging up and down.
Before this happend, the engine refused to run properly at all, so I took the carb apart, cleaned it, the ports too, replaced spark plug, placed a new filter and it run great. But after a hour or so after mowing, it started surging heavily, no matter if there's a filter on or not. Only thing I noticed is that the carb bowl is very oily/wet overall.
Not sure what to try since the carb has been cleaned already. No way to tune it or anything, so I'm out of ideas.

Video and image: (don't forget to turn sound on)

https://imgur.com/a/X6fmTR5

Thanks anyone in advance for any help given!

Click to expand...

Low speed orfice restriction clean it with a piece of fishing line

DOB said:

Low speed orfice restriction clean it with a piece of fishing line

Click to expand...

Typically I would suggest exactly the same thing, it is the cause of probably 75% of surging problems, but I believe these carbs do not have a low speed circuit. There is a plastic piece for an idle fuel pickup in the carb and a cast depression for a welch plug on the side like an emulsion well, but it isn't drilled out. Seems like briggs added it as an afterthought if they wanted to add an idle circuit.

bertsmobile1 said:

This carb would have to go down in history as being the absolute worse design ever made

Click to expand...

I'll second that. There is a term for designs like this. It is called SFB engineering ($h!t for brains). I wonder what moron dreamed it up.

Hammermechanicman said:

I'll second that. There is a term for designs like this. It is called SFB engineering ($h!t for brains). I wonder what moron dreamed it up.

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Personally I don't find them too bad. Aside for the throttle shaft breaking because its thin plastic, I love how you can remove the carb jet and emulsion tube easily to really clean it out. When I do a carb clean it is almost always like back to new on these carbs.

The brake kleen i use is non flammable. Works great to find air leaks. Engine wants to die if it gets in.
And i have 2 fire extinguishers in the shop. If you ever have an experience where you really needed a fire extinguisher it will make you a believer in having 2. A small one you can keep handy and a larger one hanging by the door. Trust me on this one.

artemjemmy said:

Personally I don't find them too bad. Aside for the throttle shaft breaking because its thin plastic, I love how you can remove the carb jet and emulsion tube easily to really clean it out. When I do a carb clean it is almost always like back to new on these carbs.

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Every one of these customers bring in are either hard to start and/or surging.

Hammermechanicman said:

Every one of these customers bring in are either hard to start and/or surging.

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I have found these carbs to surge far less than any of the previous briggs aluminum vertical shaft engine carbs, just my personal experience. If they are hard to start that pretty much just comes down to the autochoke not functioning. I have noticed that sometimes it will stick open when the choke opens all the way because the choke shaft gets wedged against the left steel support piece that the air filter backing cover screws to. I end up just trimming the choke shaft piece with side cutters and it works fine.

Hammermechanicman said:

I'll second that. There is a term for designs like this. It is called SFB engineering ($h!t for brains). I wonder what moron dreamed it up.

Click to expand...

And what is even worse is when Briggs took over Victa they got the rights to the LM plastic carb that never clogs, rarely ever floods and in many cases has run for better than 40 years on customers mowers without a single problem save perishing of the primer bulb if left in the sunlight all day every day.
It had been modified to make it teir II & III EPA compliant ( We have just gone to tier II ) for 2 strokes so should have been fine for 4 strokes.
But they ignored it in favour of the horrid POS currently fitted in all 4 variations.
They are the only carbs I regularly throw away as down here the bodies continually warp .
Don't know if B & S sent all of the trash ones down here but just about every one I have worked on required the choke spring to be shortened because it would flutter,
Bodies warp and cause the butterflys to stick either fully open or fully closed

Another shop i know won't even work on them. They just replace them. At $75/hr owner says cheaper for customers to just put on a new carb and HOPE it work better than the POS they took off than try to fix the old one. Of course these are on engines you don't change oil in. Too bad i can't fit a Chonda chineesium $10 carb. They work fine.

artemjemmy said:

Typically I would suggest exactly the same thing, it is the cause of probably 75% of surging problems, but I believe these carbs do not have a low speed circuit. There is a plastic piece for an idle fuel pickup in the carb and a cast depression for a welch plug on the side like an emulsion well, but it isn't drilled out. Seems like briggs added it as an afterthought if they wanted to add an idle circuit.

Click to expand...

Think these are fixed rpm engines as in 3600 or 0. No need for a pilot jet with a primer.

I use a pump spay bottle with soapy water for finding vacuum leaks. Cheap and in every kitchen cabinet. Doesn't have to be flammable.

That soapy water trick is also great for finding bad plug wires on cars and trucks ect....

Hi everyone, sorry for later answer.
Thanks everyone for their answers!
Thankfully, I think it was (once-again) the dirty carb, atleast for now - it works (as much as the bad carb design allows it to).

I opened it up again, cleaned it throughout, completely took apart the white "cartridge", gave it a nice clean. After that, it runs okay. Only time it stutters is when you just start it up, but for like 3-4 seconds, once or twice, but then the RPM even out. It again starts to drop and rise RPM when it gets hot, but it isn't even close as it used to be. It runs fine even with the breather tube on.

I really feared it was a vacuum leak on the gaskets, as some of you suggested. Thankfully, they're okay (I really expected them to be bad since the process of opening the carb itself may damage the bowl one). Since no parts available in my country for this specific carb, I'm glad they're fine.

Also, someone filled double the oil than it should've been. I brought it down to a normal level.

I stil don't know how it runs under load, as grass stopped growing.
Here's a video preview of how it sounds now, not perfect (turn sound on)... but atleast it runs! Thanks everyone for your help, I really appreciate it.

Keep cleaning on the carb. Might take 3 or more cleanings to get her to run correct.

Far as the gaskets go, I put a film of 100% silicone grease on all sealing gaskets. Carb bowl, carb to intake and so on. This moisturizes them and aids in removal if needed. Keeps the gasket sticking slash tearing way down to zero.

slomo said:

Keep cleaning on the carb. Might take 3 or more cleanings to get her to run correct.

Far as the gaskets go, I put a film of 100% silicone grease on all sealing gaskets. Carb bowl, carb to intake and so on. This moisturizes them and aids in removal if needed. Keeps the gasket sticking slash tearing way down to zero.

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Thanks for the tip! For now it's "fine". I don't really expect much from it anymore.