Have You Checked Your High Speed Jets

**keith scotton** · 12-15-2020, 01:09 PM

Had A Customer complaining about fuel consumption on stock 175 carb motor. Checked the high speed jets were worn 11 sizes bigger than marked. Install new stock size jets almost cut fuel use in half.
Enjoy Life Keith

**Dave S** · 12-15-2020, 05:05 PM

WTF.......now we can keep our 2 strokes.......

I never checked that........

**Capt.Insane-o** · 12-15-2020, 05:44 PM

I think someone is reading the jet numbers upside down...or is this like 1 vote for Trump is 11 votes for biden that happened a month or so ago.

**BUZZIN' DOZEN** · 12-15-2020, 06:02 PM

Or it's victim of how we got enuff fuel through our dinky OMC jets.

**Markus** · 12-23-2020, 02:45 PM

Originally Posted by keith scotton

the high speed jets were worn 11 sizes bigger than marked

Worn?

**Capt.Insane-o** · 12-24-2020, 03:28 AM

Originally Posted by keith scotton

Had A Customer complaining about fuel consumption on stock 175 carb motor. Checked the high speed jets were worn 11 sizes bigger than marked. Install new stock size jets almost cut fuel use in half.
Enjoy Life Keith

**keith scotton** · 12-24-2020, 08:44 AM

So have you checked your jets or are you just flagging your gums. Most jets after several years of running will be larger than stock size. The cleaners in gas oil and things like seafoam plus fuel going though them will cause the bore to open up. Put some dirty discolored brass in your favorite outboard oil for a couple of days comes out bright and shinny. Go buy a vermont gage set or numbered drill set and see. You might learn something. 11 is a lot but have seen it more than once no drill needed just time and use of the things we run in our motors. I like plain Doritos try to find them.

**BenKeith** · 12-24-2020, 09:05 AM

Every time I build a carb or doing any tuning on one, one of the first things I do is measure the orifice in the jets. I've never had on that much over size, I guess because they get checked from time to time, but I've had the two or three thousandths over. I've been doing that from back in my go-cart and car drag racing days back in the 60's. To answer your question, yes, I always check them before installing them. I have a number of OMC jets in my jet kit that measure larger than the number stamped on them. One whole set of #72d's measure to be 74's.
I generally just use numbered drill bits, or music wire, just don't assume they are the diameter they claim, use a mic and mic the bit or wire for the true size.

**tlwjkw** · 12-25-2020, 01:34 AM

jus thinkin' 'bout "how many hours" you would need ta put ona motor for it to wear a jet out.. think that would leave Merc out accordin' ta a lotta folks....

you can swear to it that they do get bigger (maybe justa smidge) with age but two, three or MORE size increases, i ain't buyin' it............. jmo

would think its more of merc's "quality control" more than anything else...............

**Markus** · 12-25-2020, 03:26 AM

Jets don’t wear out. I thought you meant something else by worn. Hence the question mark.

If the main jets are too big, they were either too big from the factory, or someone modified the engine.

Jets can get restricted as crud builds up. Crud in the main air jets will make the engine run richer. Maybe you compensated for restricted air jets by making the main jets smaller.

**Chaz** · 12-25-2020, 02:44 PM

I warned you guys about running that mid-grade gas .. with the extrude hone additive ..

This is what it looks like running thru your jets ...

**tlwjkw** · 12-25-2020, 02:56 PM

looks like 2:47 am after a 2:22 am Jack in tha box treat.............

**BenKeith** · 12-25-2020, 04:57 PM

Ok, if they don't wear, I guess the hole in the orifice must just expand from use. Mabe it's a chemical reaction with the brass and fuel used, I don't know, I just know I've had some mighty good mechanics that will tell you to always check them, and I've checked more than a couple that were larger than the number stamped on them over the past 60 years or so.
And this was before the Chinese started making everything we buy so can't blame it on that junk.

**Chaz** · 12-25-2020, 09:57 PM

See picture in post #4 to find out why the orifice and the number don't correlate.

To say : They twiste as big as they sposta be .. with out doing the area of orifice math , is misleading and only part of the true story . Before I knew better I drilled plenty of 70 thru 90 Holley jets out to .150" to run Alky in 750 and later 1050 Dominator carbs.
The rest of the story is that it depends on entry and exit angle, length of orifice and chamfer , to determine the flow rate of a "jet" ..

And yes, I own pin bore gauges and jet drills ...

jkDub ... Eeewwwwwwwwaaaaaaaah