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Guru
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Lawnmower Engine Can't Throttle on Choke

03/16/2009 12:23 PM

Hi, I have a lawnmower engine, that every time I choke, starts fine, but then when going off choke, or trying to throttle when it is on choke, just clonks out. Any ideas how to stop this? The petrol is new, the spark plug is new and my patience is wearing...

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#1

Re: Lawnmower Engine Can't Throttle on Choke

03/16/2009 1:47 PM

Sounds like the carb needs to be cleaned, possibly a stuck fuel bowl.

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#2

Re: Lawnmower Engine Can't Throttle on Choke

03/16/2009 2:17 PM

Choke just means you have reduced the airflow.

Running requires a ratio of fuel and air.

This one only runs, even when warm, with reduced air.

So not getting enough fuel, which is usually caused by crud in the carb or a stuck, gummed up float in the carb bowl. *Could* be reduced flow through a filter which would mean crud in tank.

I have actually cleaned a 4 barrel carb only to recrud it from a dirty tank. Leaves you cleaning the carb again, and then cleaning the tank out.

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#3

Re: Lawnmower Engine Can't Throttle on Choke

03/16/2009 9:49 PM

Clean the carb..... take an afternoon, take it apart and get the junk from the inside. Make sure you have a can of carb cleaner also. Pay attention to all the pins and remember how it goes. Careful not to ruin the gaskets. Spray it out real good and let it dry. I live in a very family oriented neighborhood. All us guys cut the grass on sat or sun, and all the kids know each other. Amazing how mechanicaly challenged the guys are though! 2 years ago I collected 6 lawnmowers over a 4 month period. Dumped curbside for trash. Most in good shape. I did just what I just told you to do. All ran fine. You leave your mower ouside, water gets in it, don't use it for a while, gas turns to turpintine, valves stick, motor fails. If you know you won't be using the mower for a while, empty the gas..especially from the carb! (take the screw out from the bottom and take the bowl off) you can buy some gas saver from your hardware supply store also, this prevents the gas from turning. GOOD LUCK!!

Kevin

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#4

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 12:54 AM

As they say, an ounce of prevention is worth a pound of the cure, Lawn mowers generally get a rough time of it.

I concur that you have a carburettor problem, and may well have crud in the tank compounding the situation. You don't mention what breed of mower or the engine either, or whether its a 2 or 4 stroke.

Some Briggs and Stutter (misspelling intentional) motors have the fuel tank as the fuel bowl. Generally there is not much to go wrong except there is much that can be made wrong.

Be careful make notes, and remember it did work the way it was before it didn't work. So don't modify anything unless you really understand it.

You will need to remove and disassemble the carburettor in a methodical way. If you remove any adjusting screws make careful notes of how many turns it took to remove. You will encounter the results of old fuel and moisture along the way. Use an old tooth brush and carby cleaner to scrub to remove the varnish (its like flakes of shellac) and corrosion. Use compressed air to reverse blow any orifices clear of crud. reassemble in reverse order of disassembly but don't over tighten anything.

Remove the fuel tank and clean it out too or else you will undo your hard work, time to replace the fuel line as well while your at it. You will potter about with this for at least 3 hrs to do a good job of it.

The current fuels available don't store much more than a month before you get grief with it, and woe betide you if you start using E10. I've just overhauled a generator fuel system which someone had used E10 because it was cheap (its not but thats another thread). The E10, had dissolved the old varnishes in the tank as well as stripped rubber from the fuel bowl seals, and the fuel line and deposited all in the carby. What a mess, Cleaned the tank, fuel filter, carby and new lines took me 4 hrs. Now it runs like new.

Two Stokes fuel systems suffer because the oil separates out of the mixture and glugs up everything. It's a bit easier to clean out, it is still a physical disassembly job but quicker.

I now close the fuel tap off and run the engines "dry" of fuel, then I decant the remaining fuel back into my fuel can before I put them away. If the fuel in the can gets close to its use by date I throw it into the cars tank (yup 2 stroke as well)and buy fresh fuel for the job at hand.

It means being a bit disciplined but it saves a lot of aggro in the long run.

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Guru
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#8
In reply to #4

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 3:25 AM

It's a four stroke engine, I will find out the make/model later.

The tank seems pretty clean. It's off a cylindrical mower. Do I need to put any new varnish in the carby?

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#21
In reply to #8

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 5:11 PM

Bondy,

Another and better link

http://www.repairfaq.org/samnew/lmfaq.htm

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#5

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 1:18 AM

I had the same symptons with a 2 stroke chain saw, turned out to be a leaking crankshaft seal.

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Anonymous Poster
#6

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 3:01 AM

Check/replace the breather.

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Anonymous Poster
#7
In reply to #6

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 3:02 AM

And the breather mounting gasket.

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#9

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 8:56 AM

Mr. MarHarg is correct. A two stroke works on vacuum to draw the fuel into the cylinder. If the main crank bearing seal has failed the volume of air needed to draw the fuel from the carburetor is displaced by the leek from the seal. Most of the time the seal actually pops out from its seat, which is why the engine quits entirely. The motor will not leak oil since it receives its lubrication from the fuel.

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#16
In reply to #9

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 4:40 PM

Dear NiCrMoNoMore,

Thankyou for supporting my answer.

If the mower is a 2 stroke and so far we have not found this out, then I would cleanout the carby first, reassemble the engine and if I cant get it to tune properly, I would then check the main shaft seal. This is best done by blanking off the carby port with a plate containing a bike tyre valve. Pump up the crankcase and listen for air leaks around th main shaft. You could use a bit of diluted detergent in water to look for bubbles.

Best of luck

Lleros

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Guru
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#17
In reply to #16

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 4:44 PM

It is 4 Stroke

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#18
In reply to #17

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 4:50 PM

The main jet Primary circuit is clogged.

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#19
In reply to #18

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 4:59 PM

How do you get to that? I cleaned a jet today, but it already seemed to be clean

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#23
In reply to #19

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 5:43 PM

The one question not asked here, is this a all of a sudden problem or has the problem become more noticeable over time. The problem is very simple. The choke circuit is independent of either primary or secondary circuits. Once you open the throttle your either getting to much fuel or not enough. Theres no other thing that could possible cause a problem. If there was one, the motor would not run on the choke circuit. If this is not a slow or overnight problem or you just picked up this mower, I would suggest that the carb itself has been messed with and that the jet you have has the wrong orifice sizing. I would suspect that if the motor and cabling have been messed with, a simple to little movement of the throttle cable would explain the low RMP's.

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#25
In reply to #23

Re: Lawnmower Engine Can't Throttle on Choke

03/18/2009 4:36 AM

You appear to be misinterpreting. It appears, reading between the lines, that what OP maker is saying, is that the engine will start (with choke enrichment) but will not run...essentially what you have said or, rather, have called into question. As to not other thing...you have overlooked the air system. With each cycle pressure must be relieved via the breather. A breather problem does not appear during start but, rather when attempting to run as compression builds. Sometimes--not always--breather defect will be accompanied by oil control problem; perhaps oil or fuel drawn through the bypass tube and into the carb throat...leading to stall or flooding as engine "attempts" to rev at de-throttle.

Would recommend to open main jet with view to getting engine to run without choking. From there further tweaking, or further symptoms (other than "engine wont' run...or start without rich assist) could help reveal fault.

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#10

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 10:40 AM

All good answers here. Just a tip; take the carb apart inside a cardboard box (seal the bottom first). Be careful blowing out the orifices, one of them has a tiny ball in it that will ususally wind up in the trash next time you sweep out the shop. Take pics before you look for a screwdriver and keep taking them along the way. Split fuel lines will give you the same symptoms.

Carl

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#11

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 11:41 AM

Open the mixture screw about 1/2 turn.

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#12

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 11:46 AM

A blocked or partially blocked fuel cap vent can cause this.

A trick (not recommended for safety or health reasons) is to find a length of tubing that will cover the cap with the cap in place and gently blow into it to apply slight positive pressure to fuel system. Be sure fuel tank is not full when you try this!

This action sometimes displaces varnish deposits and dirt in the fuel systems.

P.S. Don't let anyone see you do this, if you value your credibility.

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Anonymous Poster
#13

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 11:53 AM

Is this what you really mean?

... every time I appply choke, [the (cold) engine] starts fine; but then afterwards, when going off [i.e., while opening the] choke—or trying to throttle if de-throttling to speed up engine when it is on with choke closed—just clonks out the engine dies. Any ideas...?

...however, the fact that you're attempting to dethrottle a choked engine, perhaps that you are applying choke at all, indicates a thinness of patience even before encountering the "choke" (but not really choke) problem...such as an "engine won't start" or "engine won't run" or "engine won't power up" problem.

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#14

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 1:59 PM

I cleaned the fuel float, changed the fuel tank, cleaned the fuel line, cleaned the jet and depending on how far the jet is screwed, it either can run fine, with full range of throttle on choke, when I go off, it has a huge power surge, and then conks out, or with the jet screw turned 2 turns from that to let it run without the choke, but on minimum throttle, or it will conk out.

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#20
In reply to #14

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 5:06 PM
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#22
In reply to #14

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 5:16 PM

Sticking (or badly adjusted) float level will cut off air supply and cause flooding and stalling of engine. Also, you need to take the choke out of the equation if possible. Unless cold, engine should start without choke. If choke required (just) to start, open choke very slowly in stages to try to get normal idle...then test/adust throttle/governor. Throttling-dethottling which choke applied (yes I understand that this might me necessary just to keep engine running) is largely self defeating...in part because choke seems to be working fine.

Also, did you check the breather yet? If its hardened or leaking, throttle will not work smoothly only erratically with stalling especially at start and under load.

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#15

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 3:05 PM

Most likely the high-speed jet is restricted. It is usually located in the fuel bowl and near the bottom of the bowl. Leaving fuel in the carb for periods of inactivity long enough to allow the fuel to evaporate results in the fuel additives and parafin to precipitate inside the jet and/or mounting (siphon) tube. The carb will have to be removed, disassembled, and all compnents cleaned. Use of fuel stabilizer during inactivity (off-season) will prevent this from happening.

It could also be that the fuel supply line from the tank to the carb has a restriction or leak. Check for kinks, holes, and junk in the fuel tank or lodged in the line.

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#24

Re: Lawnmower Engine Can't Throttle on Choke

03/17/2009 5:48 PM

G'day Bondy111,

No need to put varnish anywhere, its a byproduct of stale dried fuel. It needs to be removed. E10 Unleaded has a habit of dissolving the "varnish" in the fuel tank and depositing it unhelpfully in the carby, usually where it will cause the most grief.

The jets in these motors are very small and don't take much to block them up.

You might have to soak the disassembled carby in some Shellite or similiar (once upon a time you could have used Trichlorethylnol 1.1.1, but the greenies stopped that. Damn!) for a while and then in reverse flow, use compressed air to clear the jets out again.

Hope that gets you going again.

Oh one more thing, the shaft that the throttle butterfy hangs off. If there is too much clearence, i.e. its really worn, it will upset the air fuel mixture. The only affordable fix is a replacement carby. Check to see if there is any appreciable radial movement of the shaft , unhitch the return spring to unload the assembly first.

Its amazing how something so simple can really cause such grief, except for women who are hopelessly complex and designed to cause grief.

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#26

Re: Lawnmower Engine Can't Throttle on Choke

03/18/2009 9:02 PM

Greetings.

I have had similar situation situations.

One was a plugged air filter.

One was the gasket between the carb and the block. Get the engine to run and then spray WD40 ( low flammable lube spray) at the area where the carb, gasket, block meet.

If while you are spraying you hear the engine change rpm or sound you probably have a leaking gasket.

Hope this helps.

Oly

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#27

Re: Lawnmower Engine Can't Throttle on Choke

03/19/2009 9:20 AM

Had one (2-cycle) that would start and idle, but would bog down if I tried to accelerate. Finally found the exhaust port 70% carboned up. Cleaned the carbon barrier out and it ran fine. Discovered this after I had bought a replacement.

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#28
In reply to #27

Re: Lawnmower Engine Can't Throttle on Choke

03/19/2009 12:27 PM

That's the problem, you don't want to take things in fear of breaking them further, until you have a replacement. The exhaust is half falling off, so I don't think that could be the problem. Yesterday it started making sparks but I think that may have just been from it rattling against the carb

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#29

Re: Lawnmower Engine Can't Throttle on Choke

03/26/2009 8:14 AM

The butter fly valve inside the carb is made of brass the carb made of aluminium .

The brass heats and hold heat longer than the aluminum so it will bind from time to time. Clean the throat of the carb with some super fine sand paper to keep the butter fly valve moving.

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#30

Re: Lawnmower Engine Can't Throttle on Choke

03/26/2009 6:20 PM

Replace the carb on the worn out mower...cheaper than repairing the carb.

Replace the mower...cheaper in the long run than replacing or repairing the carb...and everything else that's broken or badly worn.

Stop wasting time that can't be repaired or replaced.

If you're trying to help friend or neighbour, give them a straight answer: the above advice.

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