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Hi guys. I'm fairly new to this forum. I have read some about surging and running lean engines. I guess when you say drill a nozzle or a jet you drill down into
the jet, opening the hole a little bigger. Do you ever drill out the little holes that run cross ways in the jet? Also do you just take your next bigger micro drill bit
than the one that will fit into the hole? One more thing, do you ever git little pieces of metal down in the jets? Will spraying with carb cleaner and blowing it out
with compressed air clean them out good? I was thinking about helping a friend with a surging Duraforce. Thanks in advance for any helpful hints.
Jerry

You never ever drill a jet.
Even if you are deliberately trying to increase the size to one you have carefully calculated.
Jets get reamed to size & smooth finish .
Next the carb comes off and gets a bath in carb cleaner for a week or so.
Not the junk you buy in a spray can the stuff that comes in a paint can with a dipping bucket.
Then it gets blown out with compressed air at low pressure.
And no you don't touch the cross drillings in the emulson tube

Hi Jerry,

I too was hesitant to drill the jets on a Duraforce until I tried it twice. You drill the pilot jet with a #77 drill (.018"). For the main jet use a #67 drill (.032"). You drill axially, no need to drill the side holes.

The mower will obviously use more gas, but this mod does seem to work.

Hi Bertsmobile1,

The Duraforce engines were intentionally designed to run extremely lean ( I presume to get them to pass ever strickter emission control standards the US government placed on outdoor power equipment, which ultimately led to Lawn-Boy dropping the 2 stroke altogether). This caused the engine to surge, sometimes a slight surge, other times a very bad surge. Me personally, I don't usually mess with carbs other than cleaning them, but the Duraforce is an exception.

Regards

Jeff

Regardless of the reason.jets should always be reamed our to size not drilled.
Drilling is not accurate at those sizes and leaves a very rough surfaces.
Even if it is done submerged in cutting fluid it is just plain the wrong thing to do.
If you drilled 20 f them with the same bit you would end up with 20 different sized holes of which less than 1/2 would be truly round.

The reasoning might be quite valid but the method is a bit too random.
If it makes a difference to the performance of the engine I can not say but when working on motorcycles, jet drilling was quite unreliable.

Using a synthetic oil with a leaner ratio will most likely achieve the same result.

The "step sizes" available in fractional and numbered drills typically do not cater to engine needs, but I have done it and overkilled. Sounds like you have done well if plugs read right and you are not washing the cylinders with gas which can also wash oil off cylinders.

motoman said:

The "step sizes" available in fractional and numbered drills typically do not cater to engine needs, but I have done it and overkilled. Sounds like you have done well if plugs read right and you are not washing the cylinders with gas which can also wash oil off cylinders.

Click to expand...

In a Lawnboy 2-stroke engine, the oil is IN the gas, so more gas to the cylinder, the more oil. No way it will "wash the oil" off the cylinder.

Thanks for the update.

I have been resizing carb jets for years.
I use quality American made number drills.
I use a 0-1 calibrated micrometer to check the actual size of the number drills.
I make sure that I have extra stock jets in the event the jet gets damaged during resizing.
I use a pin vice to hold the drills.
I check the actual jet size before resizing it.
I usually hold the jet in one hand and turn the pin vice with the other hand.
I have the correct jet removal tools.
I have had to totally disassemble carbs and heat the carb body to be able to remove the stuck jet.
I have had to drill out a few jets that had been damaged by improper attempts at removal. I set up the carb body on my milling machine and carefully drill out the damaged jet.
There have been a few situations where the number drill size selection will not allow me to resize to the dimension that I want.
I research the parts manuals for the possibility of a production jet that will work.:laughing::biggrin::smile:

I always wondered if the jet hole is a clean drill out or if there is a venturi type profile to the hole.... like an angle or like countersunk (I choose for better words here...)
Yes, as I peer through it as I hold it up and look at the intense bright sun, I can see a round hole in there, but is that all that it is, a simple hole in the bottom of a threaded blob of brass?

I read somewhere that you shouldn't even try to poke it clean with a piece of fine wire for worries that you might scratch the surface....

It is dead strait and even more important, very very smooth.
Jet drills have a needle point on them and look like a tapered reamer with a parallel reamer on the end.
They look nothing like a normal twist drill.
If thee is any roughness in the side it makes a drastic change to the volume of liquid that can pass through the jet.
AFAIK, now days they are water jet cut.
When I was involved with motorcycle racing we had a set of jet drills which allowed us three sizes between std Amal jets.
Each one cost more than a piston, they were very sharp,and very easily damaged.
They were used in a tool makers lathe which was accurate to 0,0001" and secured with collets because a jaw chuck was not accurate enogh.

Thus my comments earlier about drilling which is very much a hit & miss operation.

Latter on we went to plain drilling and reaming with tiny reamers that one of the groups dad made.
This was a lot lot lot cheaper than using genuine jet drills.

On a mower running at 3000 to 4000 rpm it might not be as important as on a motorcycle doing 9,000 rpm and a person sitting on it.

A fair amount of brass jets have a countersink leading in.
With the drill bit in a pin vice, I start in a counterclockwise rotation and sometime go all the way thru in the same direction.
Holding the pin vice steady, I work the drill bit in and out smoothing up the new hole as best can be done.
I usually dip the drill bit in cutting oil before hand.
I don't recommend pushing thru a jet with a torch tip cleaning tool.
I have never got into motorcycle carbs, but I have a friend that does and he uses the same procedure that I do, in fact, I'm the one that showed him how back in the 70s. He is a retired machinist/aircraft specialty welder and now does rework of off road motorcycles used in competition. He has a watchmakers lathe and has made jets on it.
Most of my jetting work was done on two stroke outboard motors along with some high performance inboard/outboard boats, some of which turned up to 7,000 RPM and governed.
Number drills bits are manufactured to closer tolerances than jobber drill bits and can vary in size slightly between manufacturers. I use this to my advantage when resizing jets.
Sometimes all that is needed is .0005" more in hole size and this can sometimes be difficult to do.

Thanks guys for all the input. I haven't started yet. This will probably be an early Spring project. I have been working on cleaning an rebuilding a Tecumseh
carburetor for my snow blower that surges real bad. I can not believe how many lawn mowers that I heard surging real bad last summer. Probably one out of every four or five. My neighbor got a brand new portable generator and it surged up and down terrible right out of the box.
There are not many Lawn Boys in my area. I rarely see one. My friend's wife's grandpa had a Lawn boy repair shop years ago and he still has a few.
We have to keep the old boys running.
Merry Christmas.

Back to the reason for drilling jets. Surging because of a lean condition on the duraforce Lawn Boy. First are there any gaskets leaking in air causing a lean mixture? Have you replaced the top and bottom crank seals that are old and hardened /worn and allow air to be sucked into the crankcase and again leaning out the mixture? These all need to be corrected first. These will take care of 99% of surging motors. Drilling jets is putting a band-aid on the problem instead of fixing the source. I have tried all of the internet solutions. On a motor and nothing worked other than drinking more fuel. I went and put every mod back to factory stock condition and the motor had new crank seal and gaskets and block resealed and checked for leaks. The cause was the way the factory mounted the throttle plate in the throttle shaft. They only relied on a press fit to hold the plate centered in the bore of the carb. As the motor aged clearances changed and the plastic shaft's slot no longer holds the plate in position in the bore and the plate then is free to move and cause the shaft to bind. Motor running vibration then will vibrate the plate and let it free up and then restick, starting the surging cycle. I placed the shaft and plate and the surge was gone. This may be a common source of the surging than any other. It is a very poor way to build the carb. instead of a brass shaft and plate with a screwed in place plate.

Thanks for the reply LB8210. I haven't layed eyes on this machine yet. I just told a friend of mine that I would help him with his mower. I have ordered new crank oil seals and have watched a couple videos and read how to change them. I also got a Briggs and Stratton carb jet part no. 801308 to switch out the one in the side of the carburetor under the sticker. Several guys on the lawn mower forums say that helps. I have also read about the throttle plate that you were talking about being a problem. Some one said that there are 2 different throttle plates with different size little holes in them and to make sure that the duraforce engine has the one with the bigger hole and that the snow blowers take the ones with the smaller hole. I will make sure we put new carburetor mounting gaskets on too when we put it back together. We will do all of these things first before even thinking about drilling out anything.
You guys are a big help to us Lawn Boy owners that aren't mechanics.
Thanks again.