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Last summer I watched some youtube videos and fixed some lawn mowers.

Generally what I'd do with a mower, is flush the old gas, get rid of the old fuel line, buy an aftermarket carburetor on Amazon (15-25 dollars). And put the new carb and fuel line on, replace the oil, clean or replace the spark plug, and I would say it would work great about 80% of the time.

For that 20% when it doesn't work.....why? I know that aftermarket carbs are not necessarily bolt n go, but many of them aren't adjustable at all. So with new fuel, fuel line, carburetor, and spark plug, why would a mower still not start? It starts up just fine with a shot of starter fluid, but around 20% of the time with an all new fuel system, it won't start!

Thoughts on why? Or what to do to fix this?

Choke not closing completely.

either what ILENGINE said, i've also ran across a few aftermarket carbs that won't run right or start well, and i give them a quick clean, spray out the main and idle jet etc., and it works like it's supposed to when i put it back on.

ILENGINE said:

Choke not closing completely.

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This is the butterfly valve not closing all the way, just need to adjust it so it does?

I’ve found that about 25% of aftermarket carbs are junk, remember there is a reason they are sold so cheap.

Scrubcadet10 said:

either what ILENGINE said, i've also ran across a few aftermarket carbs that won't run right or start well, and i give them a quick clean, spray out the main and idle jet etc., and it works like it's supposed to when i put it back on.

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Is the choke just the butterfly valve not closing all the way? Just adjust it until it does?

I still haven't why so many carburetors are replaced in the first place as I repair over 99% of them.

What I suspect is that 20% that op says wont start without a shot of fuel are primer systems.

carbnclean said:

Is the choke just the butterfly valve not closing all the way? Just adjust it until it does?

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The one nearest the air filer, assuming hat they have one and are not primer start as this is a generic question
We seem to be getting worse carbs down here as the failure rate would be closer to 50%
So perhaps all of the returns in the USA end up on Evilpay down here.

StarTech said:

I still haven't why so many carburetors are replaced in the first place as I repair over 99% of them.

What I suspect is that 20% that op says wont start without a shot of fuel are primer systems.

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well don't you know waiting 2 days or 5 days is less time than 20-30 minutes?

StarTech said:

I still haven't why so many carburetors are replaced in the first place as I repair over 99% of them.

What I suspect is that 20% that op says wont start without a shot of fuel are primer systems.

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I am not a mechanic by any means, like I said I just watched some videos, so how would you fix the primer system?

We could use a little more information.
which Briggs engine is this? do you have the model and type numbers?

I would look for a giant air leak. When the engine is running, it sucks air through the carb and the air picks up gas along the way. If there's an easier way, or even just an extra way, for air to get into the cylinder without going through the carb, it will. And then you don't get enough gas to run. You give it a shot of ether and it fires, so you know fuel is the missing ingredient. Fresh gas, new carb, it ought to run even if its ragged. So you're not getting any fuel with the air going in and a big vacuum leak is the place to start. Was the gasket the right size? A bolt didn't bottom out before things were tight, maybe?

carbnclean said:

I am not a mechanic by any means, like I said I just watched some videos, so how would you fix the primer system?

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What we got to know is which setup you have which even with the model and type numbers sometimes will not tell us as may have been several different setups used.

But with primer type systems the biggest problem I have found is a warp primer base caused by either age or by someone over-tightening the mounting screws. Both cause the seal between the primer base, gasket, and the carburetor not to be complete allowing air to leak out instead of being pushed into the carburetor. Some try using two gaskets to seal this but here I use two gaskets cut into wedge design to apply pressure on where it is needed (a build up in the warped area).

And of course there are 3 different primer which is why we keep asking WHAT THE BLOODY HELL ARE YOU PLAYING WITH >
1) squirts fuel directly into the carb
2) pumps fuel into the float bowl to overfill it
3) pumps air into the float chamber to make the float open & overfill the bowl

carbnclean said:

Last summer I watched some youtube videos and fixed some lawn mowers.

Generally what I'd do with a mower, is flush the old gas, get rid of the old fuel line, buy an aftermarket carburetor on Amazon (15-25 dollars). And put the new carb and fuel line on, replace the oil, clean or replace the spark plug, and I would say it would work great about 80% of the time.

For that 20% when it doesn't work.....why? I know that aftermarket carbs are not necessarily bolt n go, but many of them aren't adjustable at all. So with new fuel, fuel line, carburetor, and spark plug, why would a mower still not start? It starts up just fine with a shot of starter fluid, but around 20% of the time with an all new fuel system, it won't start!

Thoughts on why? Or what to do to fix this?

Click to expand...

Most of those carb manufacturers from China have little to no quality control. So the jet sizes and passageways through the carb may not be correct. This is something I learned on this forum a few years ago.
THE only time I'll use one of these is if the unit doesn't have much life left it it. As in he just wants to get him through the season cause he has plans on replacing it. Or if the OEM carb is too pricey for the customer. And sometimes when an oem carb isn't available in the time frame the customer needs his equipment.
Plus there has to be enough wrong with the bad carb to warrant a new one.
Like this tecumseh 2 cycle I finished up yesterday. I bought the rebuild kit for it, but I didn't notice the choke spring was bad, until after I'd rebuilt it and installed it back on the digger. Luckily, I'd ordered a new Amazon carb when I ordered the rebuild kit. Just in case.
Well, strike 2 with the first Amazon carb. It just wouldn't adjust. I told Amazon about it and they had a new carb the next day. Now it's running great.

Another piece of advice I learned here, was to never warranty an Amazon carburetor.

We’ve installed a few of those cheapo carbs at work and so far haven’t had any trouble with them. But we only install them after attempting to clean them or if kits/parts aren’t available. For us, the cost of labor for our customers to disassemble and clean/rebuild vs. just replacing them is a fine line.

As for adjustments - I’ve found the adjustment screws are usually hidden by plugs on the new carbs. If you look at the old carb for the location of the idle mix screw, it’s probably in the same position on the new carb, but has been sealed off by a tin plug. However, the throttle and choke cables also typically need adjustment when the carb has been removed, so I’m making sure that’s been done and that the carb barrel gaskets are sealed correctly before I start messing with the carb adjustment.

Gumby83 said:

We’ve installed a few of those cheapo carbs at work and so far haven’t had any trouble with them. But we only install them after attempting to clean them or if kits/parts aren’t available. For us, the cost of labor for our customers to disassemble and clean/rebuild vs. just replacing them is a fine line.

As for adjustments - I’ve found the adjustment screws are usually hidden by plugs on the new carbs. If you look at the old carb for the location of the idle mix screw, it’s probably in the same position on the new carb, but has been sealed off by a tin plug. However, the throttle and choke cables also typically need adjustment when the carb has been removed, so I’m making sure that’s been done and that the carb barrel gaskets are sealed correctly before I start messing with the carb adjustment.

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Alright dumb question, I don't know what a barrel gasket is..

Scrubcadet10 said:

We could use a little more information.
which Briggs engine is this? do you have the model and type numbers?

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No specific engine, I worked on probably 50-60 mowers total, just noticed that sometimes a new carb would not work and looking for common reasons why?

ell if you have worked on that many mowers you should know the problem with one will not necessarily be same as the problem with the next even when they have the same symptoms.
If you are going to do this long term the best thing to do is keep a known good carb of each type so you can quickly isolate the problem
There is a well known trueism
"HALF OF THE IGNITION PROBLEMS WILL BE FOUND IN THE CARBURETTOR"
HALF OF THE CARBURETOR PROBLEMS WILL BE FOUND IN THE IGNITION SYSTEM "

Carb barrel gaskets are most often called intake or manifold gaskets.

Rivets said:

Carb barrel gaskets are most often called intake or manifold gaskets.

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“Yes, that!” -Lord Helmet, Commander of Spaceball 1

carbnclean - if you don’t know, the barrel of the carb is the primary air passage through the carb - more commonly known as the venturi. Technically speaking, however, a venturi should have a reduction in diameter and, in my experience anyway, is not a typical feature on these small engine carbs.

Almost every 4 cylinder small engine carb I’ve worked on the last 50 years has a Venturi. Some may not be a necked down as you would think, but it is there. Most of those that look straight are slightly smaller and with the extended main nozzle helping, they are smaller in area producing the Bernoulli effect or principle To draw in fuel.

Gumby83 said:

“Yes, that!” -Lord Helmet, Commander of Spaceball 1

carbnclean - if you don’t know, the barrel of the carb is the primary air passage through the carb - more commonly known as the venturi. Technically speaking, however, a venturi should have a reduction in diameter and, in my experience anyway, is not a typical feature on these small engine carbs.

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Get your caipers out & measure the inlet opening then measure the outlet.
Guess which one is smaller ?
Usual method of working out which gasket goes where.
The one with the smaller hole goes on the engine side

Rivets said:

Almost every 4 cylinder small engine carb I’ve worked on the last 50 years has a Venturi. Some may not be a necked down as you would think, but it is there. Most of those that look straight are slightly smaller and with the extended main nozzle helping, they are smaller in area producing the Bernoulli effect or principle To draw in fuel.

Click to expand...

bertsmobile1 said:

Get your caipers out & measure the inlet opening then measure the outlet.
Guess which one is smaller ?
Usual method of working out which gasket goes where.
The one with the smaller hole goes on the engine side

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Ok, I get it - I retract my previous statement. It was based on observation only - I’m used to seeing a clear reduction in size.