Drilling out jets

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.