drilling jets

[andrew81]

i have a 2000 20hp merc with a 25 carb. i can't purchase larger jets for this carb. but i'm willing to experiment with the ones in it. anyone know anything about drilling them. like how big.
andrew81

[Bill Huson]

jet drilling . . .

my only experiance in jet drilling was when I helped a young friend drill the jets (nitrious & fuel) for his ha-ha gas rig on his car. We used a micrometer to measure #drills in very small sizes, I had them from #80 on up, calculated the area of the hole each drill made, and calculated the percentage of flow increase. Then we drilled the jets, he ran the car, we drilled some more, he ran the car, and at some point the flow rate of the aircraft soleniod valves was reached. But that was nitrious/fuel and the more you could pump inthe engien the more horseypower it made. Punching jets on a carb that has to stay in balance with a fixed air flow is a bit more problematic.

If it's any help, thye numbering the set of jets I have for my Harley corosponds with number drill sizes - and I seem to recall the jets used in Lectron Hot Rod carbs were likewise numbered to corrospond with number drill sizes - experts? Please step in and correct my foggy memory. Thinking causes dust bunnies to fly out my ears . . .

[nbranews]

jet drilling

Most jets I've seen the number on the jet is corrispondent to drill number sizes. Check your jets first to see if some one hasn't already opened them up.
We most times drill our own jets on our johnny-rude 3 cyl. If you do it take the time to be sure you get it streight and don't leave any spiral grooves in the bore of the jet.

[sam]

You can make half sizes by filing them with a welding tip cleaning file

If you have made no other changes to the motor and it isn't running lean all you will do is use more gas .... in fact, too large will slow the motor back down

[andrew81]

thanx boyz,
i've made some modifications to my motor already, as you know i have a 25 carb, i've also drilled the exhaust and opened the reed butters a couple of millimeteres. last year i had my block split and polished the cast marks off the head, i also cleaned the cast marks off the area behind the reed block. and down the exhaust pipe. i was going to attempt to port the exhaust and the fuel ports but that was "over my head". i didnt know how big to go on the ports and it looked like a hard place to do it. the biggest problem with these motors is the fact that they don't have a "head". the block splits down the middle and you cant just "plain" to get the compression up like most motors. wish you could. is it possible to get the compression up to get a few more horses on these motors?

thanx atk

[Bill Huson]

so you're saying . . .

you have a Merc with the combustion chamber cast in with the top block instead of a seperate head piece. I have heard that some race folks who like to push the stock outboard inspection rules have pressed the block. The block is set up on a plate under a press with the spark plug holes up and a fitting is screwed into the spark plug hole, assuming the plug is verticle to the combustion chamber - won't work with angled plugs. Then you crank on the press. The pressure deforms the combustion chamber downwards thus increasing the compression ratio.

Caveat: I've never attempted that and have no clue what the success rate is RE not cracking the dang block etc etc. Hopefully a few engine *tweakers* will jump in with more accurate info.

[hydroplay]

Drilling is not accurate enough for carburetor jets. Reaming is necessary to have any real quality control. But Mercury jets are available at any Merc dealer or service place, at least to #98 or .098 inch

[andrew81]

well bill thats quite the theory. sounds like it would work, like you say i hope someone else would jump in who has actually tried it, sounds like it may be a tricky operation, hahahaha.
but i'm willing to try if i get enough threads on this
thanx

[andrew81]

just hoping someone might have a few more tips on this topic, so i can start modifying

[andrew81]

my buddy has a 40 hp merc. when he went to purchase bigger jets he found out that the jets out of my 25 carb are bigger than his. the motor only has a single carb. i just thought that was odd. wondering why.........
atk

[Mike Schmidt]

Jet size...

At one time I ran both a Yamato 250 and a Yamato 350 ( 15 C.I and 20 C.I.) twins. You would think that the 350 would run much bigger jets...Wrong. The 350 ran about 2.5 mm jets and the 250 ran 3.5 mm jets. It is all about "signal". The larger motor "pulls" a lot harder on the fuel in the jet, so even though it has a smaller jet it is getting more fuel. Hope this makes sense.

Michael D- 1

[Richard K.C. Mo.]

Tell Me more!!!

At one time I ran both a Yamato 250 and a Yamato 350 ( 15 C.I and 20 C.I.) twins. You would think that the 350 would run much bigger jets...Wrong. The 350 ran about 2.5 mm jets and the 250 ran 3.5 mm jets. It is all about "signal". The larger motor "pulls" a lot harder on the fuel in the jet, so even though it has a smaller jet it is getting more fuel. Hope this makes sense.

Michael D- 1

I really like these kind of signals, fuel/air thats what i'm talkin' 'bout.!!!
Richard.

[andrew81]

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