Backfiring Engine when under increased load

[Mower Man 9]

I was wondering if I need a carburetor replacement.  I

 

t starts up ok but then it will back fire when I engage the mower belt or shift into second to go forward.

It sputters and back fires and usually stops.  I then wait a min or less and try to restart it and eventually it will start (5 min) maybe some more back firing and repeat the restart and slowly increase the RPM and then it might work for 30 min or more.  

Sometimes it backfires under load of tall grass and then I try to restart it.

 

I  have dumped the old fuel, replaced the fuel filter, spark plug, cleaned the air filter and put fresh gas into it.

I use 100LL Aviation Fuel (no alcohol).

 

I took it to Tractor Service and he  cleaned  out the fuel line and removed a couple black pieces of dirt in the carburetor.  He charged me 2 hrs labor.   I have changed the fuel filter, spark plug and cleaned the air filter.  

 

Do I need a new carburetor?

 

The fuel filter is never more than 1/4 full.  Could it be the fuel pump?    It is a 1998 model 773348.  It has 275 hrs on the engine.

 

I would like to get it fixed so that the mower works reliably like it did last year.  

 

Thanks for your advise.

 

Rodger

Edited May 1, 2017 by Mower Man

[Thisguyisnew 114]

Did it ever work well on 100LL Aviation Fuel? Try regular Gasoline with no additives and see how she runs first 

Edited May 1, 2017 by Thisguyisnew

[953 nut 55,837]

       Chances are the  has been run on fuel containing ethanol, having replaced the fuel line and filter is a great first step. I am a real strong advocate of Seafoam products. Buy a can of the fuel additive and a spray can of their carburetor cleaner, add the liquid to your tank at the rate suggested on the can. Move it outdoors where you have good ventilation, start  the engine and nurse it up to half throttle, with the air filter removed spray a little of the carb. cleaner in the air intake; allow it to recover RPMs and repeat several times. I have done this on a couple of problem carbs with good results. 

[Jerry77 1,218]

6 minutes ago, 953 nut said:

I have done this on a couple of problem carbs with good results.

Me too.........

[6bg6ga 272]

I'm a fan of  spray carb cleaner for linkages as it does a heck of a job. Internal carb cleaning requires actually running the cleaner thru the carb. Gum out years ago made a kit which was simply an plug for the fuel line and a plastic adapter that threaded into the carb bowl. One then simply attached a plastic flex line from the can of cleaner to the carb adapter and inverted the can and started the engine up. THIS type of cleaner actually does clean the internals of a carb and the jets. With all due respect like I said the spray stuff is ok to clean linkage in my opinion.

 

So, what I'm suggesting is this....go to the store and purchase a CAN of carb cleaner that is meant to be injested by the engine and make your own little setup for a gravity feed cleaner and see if that does some good.

 

Before I would actually try anything I would do the following. Start engine and warm up. Take air cleaner off to expose the carb throat. Run the rpms up and manually apply the choke and at the same time pull the choke completely shut while opening the throttle ALL the way. The engine will try to quit and when it does return the choke to open position and keep the RPM up until the engine recovers and then let the RPM go down. This little trick will in most cases open up all the passages inside the carb. It may have to be done several times but it usually helps unblock the carb. It doesn't cost anything to try it either.

Edited May 1, 2017 by 6bg6ga

[pfrederi 17,887]

1 hour ago, 6bg6ga said:

I'm a fan of  spray carb cleaner for linkages as it does a heck of a job. Internal carb cleaning requires actually running the cleaner thru the carb. Gum out years ago made a kit which was simply an plug for the fuel line and a plastic adapter that threaded into the carb bowl. One then simply attached a plastic flex line from the can of cleaner to the carb adapter and inverted the can and started the engine up. THIS type of cleaner actually does clean the internals of a carb and the jets. With all due respect like I said the spray stuff is ok to clean linkage in my opinion.

 

So, what I'm suggesting is this....go to the store and purchase a CAN of carb cleaner that is meant to be injested by the engine and make your own little setup for a gravity feed cleaner and see if that does some good.

 

Before I would actually try anything I would do the following. Start engine and warm up. Take air cleaner off to expose the carb throat. Run the rpms up and manually apply the choke and at the same time pull the choke completely shut while opening the throttle ALL the way. The engine will try to quit and when it does return the choke to open position and keep the RPM up until the engine recovers and then let the RPM go down. This little trick will in most cases open up all the passages inside the carb. It may have to be done several times but it usually helps unblock the carb. It doesn't cost anything to try it either.

I still have one of those rigs but you can't find gumout in metal cans and the cap does not fit the new plastic bottles

 

You can approximate the process by draining most all of you gas and putting in a bottle or 2 of gumout (or you favorite witches brew) run it for a few minutes to make sure the carb is filled with cleaner stuff then shut it off and let it sit for a few minutes .  run it breifley stop again and let it sit...do that a few times and you may solve your problem.

[6bg6ga 272]

I would suspect after checking the availability of the Gumout cleaner is the formula has been changed. The new stuff is less favorable to the development of cancer and certainly less of a product. Anything injested into the carb bowl and internals (witches Brew) is certainly better than simply spraying a can of crap down the throat of a carb. My opinion here again.

 

I certainly would agree with your method of cleaning and would insert one thing... I would drain most of the gas out start the engine and insert the cleaner into the tank while the engine is running. I would let it run 15 minutes or so or until good an warm and then shut off. I would let it sit a day and drain the tank and save the contents if they were clean. I would then put a small amount of straight gas into the tank and get it started up and then add the gumout again and run it out and re-fill with straight gas. 

[Mower Man 9]

100LowLead  Aviation gas has no ethanol and no additives.  I have added an ounce per gallon of Sea Foam to the aviation gas and it still backfires under load.  I will try the Gum out.

Thank you all for your suggestions.

8 hours ago, Thisguyisnew said:

Did it ever work well on 100LL Aviation Fuel? Try regular Gasoline with no additives and see how she runs first 

It always has run well on 100LL Aviation Fuel.  I only used Premium Auto Gas rarely.  So very little ethanol containing fuels have been used.  

[6bg6ga 272]

Changing the fuel isn't going to change the fact the air/fuel ratio isn't correct. The problem lies in the engines not getting the gas it needs under load or to be more specific the power circuit of the carb. Like I mentioned it might be as simple as a high speed jet adjustment or the cleaning of carburetor passages.

[illinilefttackle 399]

Have you adjusted the high speed jet?  Run at 3/4 throttle and adjust for most rpm- then go back slightly rich- Good Luck!- Al

[6bg6ga 272]

I'd run it under load adjust it.

Edited May 1, 2017 by 6bg6ga

[oldredrider 2,549]

Given the history of it running fine last year, plus what you have done as far as maintenance/repairs, I don't think you have a fuel problem. My guess is a bad condenser. At such a low price, changing it out could be an easy fix.

[6bg6ga 272]

A bad condenser would likely pit and burn a set of points in no time. It is easily tested with a cap tester or the cap test function on your meter.  If it  tests within spec leave it alone. If you wish to go old school a visual inspection of the spark color is all that necessary. 

[953 nut 55,837]

14 hours ago, Mower Man said:

I took it to Tractor Service and he  cleaned  out the fuel line and removed a couple black pieces of dirt in the carburetor.  He charged me 2 hrs labor.   I have changed the fuel filter, spark plug and cleaned the air filter.  

 

Do I need a new carburetor?

 

The fuel filter is never more than 1/4 full.  Could it be the fuel pump?    It is a 1998 model 773348.

    Going back to the original post; it is a 1998 so I don't think we are going to find points and condenser. the mechanic removed some junk from the fuel system already, my thought is there is more in there and Seafoam has proven itself to me and others. I don't own any stock in the company, just know it has helped.

[Mower Man 9]

3 hours ago, 6bg6ga said:

Changing the fuel isn't going to change the fact the air/fuel ratio isn't correct. The problem lies in the engines not getting the gas it needs under load or to be more specific the power circuit of the carb. Like I mentioned it might be as simple as a high speed jet adjustment or the cleaning of carburetor passages.

That makes the most sense.  I tried all the above tricks with the carburetor cleaner  (I even found a Gum out product) and it still backfires and then runs for 20 min and then backfires under load.  I even have an audio file for you to hear the engine.

Tractor back Firing.m4a

[6bg6ga 272]

I don't want to come off wrong here but its air/fuel ratio problem AK lack of fuel that causes it to backfire and run lean under load.  I didn't catch the year but instead commented on the condenser comment.  Which by the way I've read in several different threads its always replace the cap. My answer will always be to check the cap and replace ONLY if its bad.

 

There is a cap however in the ignition system and if it were open it would cause no problem as its only there in a smoothing capacity on the 12volt supply going to the coil. If it were shorted you would have taken out the fuse on the circuit its in.

 

As to the carb cleaner.... the ONLY cleaner that is going to help possibly is the kind that is going to be mixed with the gas and certainly not anything sprayed inside the carb because all that does is go down the throat of the carb into the intake manifold. Sprayed cleaner does nothing to clean vital parts and passages inside the carb. CArb cleaner that is mixed strong in other words as I discribed in another post start the engine on a small amount of gas in the tank and dump in the cleaner and let it run a while it will smoke and run rough as it contains chemicals to cut thru the junk. The reason I say to start on gas is most cleaners may not have enough umpth to ignite on their own. Some here may understand some old engines from years past that would run on gas to start and when warmed up they could switch to diesel. Once the engine has started and has warmed up a bit then there is enough heat on momentum to keep it running on a lesser fuel source.

 

When not removing the carb for a boil out the next best way is to run cleaner THRU the carb not simply sprayed down its throat.  I think I remember there being a youtube video of a guy cleaning his motorcycle carbs using seafoam product.

 

Second option....... remove the carb from the engine, take it apart ak remove any gasket, float, needle and seat and emerse in laquer thinner for the night and reassemble and re-install.

Edited May 1, 2017 by 6bg6ga

[illinilefttackle 399]

Can you stop the Backfiring by slightly choking the engine when it starts acting up? If so- it's a fuel mixture or supply problem- Al

 

[Thisguyisnew 114]

Set carb adjustments back to factory and then dial it back in using your models recommended procedures

[6bg6ga 272]

Keep in mind...with keeping with EPA specs they set the engines a tad lean. Adjust UNDER load for smoothest run without being too rich or backfiring too lean.  Now's the time to drill and tap that exhaust manifold for your 02 sensor so you can set the air /fuel to spec.  Just kidding. I doubt any here have anything like that..

[rmaynard 15,610]

Lets get back to basics here. What is a model 773348 and what kind of engine does it have? 

[6bg6ga 272]

When I google the model number I come up with a Briggs engine. To be blunt it doesn't matter at all. Its running lean. Nothing else to dwell on.

[Racinbob 11,232]

@Mower Man I'll repeat what @rmaynard Please check the model # again. It should be 5 digits. Wheel Horse didn't use any Briggs motors in 1998. The more info you can give the engine gurus here will help.

Edited May 2, 2017 by Racinbob

[6bg6ga 272]

I'll be blunt....the model and or make isn't going to change what is happening. Still sounds to me like the engine is lean.

[Mower Man 9]

23 hours ago, illinilefttackle said:

Can you stop the Backfiring by slightly choking the engine when it starts acting up? If so- it's a fuel mixture or supply problem- Al

 

No.  It eventually just quits.

 

10 hours ago, rmaynard said:

Lets get back to basics here. What is a model 773348 and what kind of engine does it have? 

It has a Kohler Command CH 14

 

Ok.  I called the Wheel Horse Dealer and he said to replace all the fuel with 87 octane auto gas.

I did it and it seemed to run just fine....

for about 30 min.  I shut it down and tried to restart it 10 min later and I could not start it.

I let it sit for 3 hrs and then it started right up and I ran it for 20 min without a problem.

Then today at noon I started it right up and under load it back fired like crazy and stopped.

I waited a min and restarted it and then mowed for 40 minutes without a problem.  

The engine behaved fine even under load.  I was about to bring it to the Tractor Service but

since I was able to mow and do all lawns and weeds I figured I would wait.

Maybe the Chevron  89 octane auto gas with Techron is cleaning out the high speed jet in  the carburetor?

 

Edited May 2, 2017 by Mower Man

[6bg6ga 272]

I sincerely doubt the cleaning action the techron had anything to do with it. There are other passages inside the carb not knowing the carb I won't comment. It is very possible there was a small piece of something partially clogging the main jet.  I would be interested in seeing what the gas looked like that you took out of it. Gas is cheap. I would dump it and refill the tank. Dump some seafoam or something else into the tank start it and let it run until its warm and you see it smoking a bit, turn it off and start it tomorrow and run it again and do this several times.

 

If it still exhibits the same behavior we may be looking at ignition too.  Pull the spark plug take a picture and post please.

 

 

Edited May 2, 2017 by 6bg6ga