Why not drill the jets?

[A.J. Pastwa]

O.K. I'll ask, what's wrong with drilling the jets?------Also, what all the talk about the air bleed---should I have it richened up before I run her? I don't ever expect to turn the ole girl over 6200, probably 6000 is a better quessimate. I run the thing between 4000-5000 more than I ever run 6000. Once I get where I'm going, I'm not in too big a hurry to get anywhere else! She's going together tomorrow, I want to do it right the first time! Thanks, A.J.:p

[Rusrog]

You and I both know that there is a particular someone who will dispute this but here goes....
While you can drill a particular orifice to a particular size, there is no guarantee that the flow will be the the same. There may be small burrs in the orifice, there are any number of imperfections that can cause problems and limit flow. Most of these are impossible to see in the small confines of a .084 orifice. I also think that the OMC jets are flowed so that they are all equal. IE...
an 84D jet may not actually have an .084 hole in it. But all 84D jets are supposed to flow the same. The OMC gurus may correct me on that because I am not sure but it seems like I heard it somewhere...

Russ

[B.Leonard]

I think Russ pretty much has it summed up.

I would drill low speed jets/orifices if you couldn't get the jets but replacing the jet would be a first try. Never high speed except when switching to alcohol because the jets need to be so much larger you don't have a choice. I converted a Quadrajet to alcohol one time and had no problems drilling the jets, but I stayed on the rich side just to make sure.

Sometimes I've had to fabricate jets out of 8-23 brass all-thread for low speed circuits that have pressed-in orifices from the factory. The part throttle circuit on a Qradrajet is like this, they make it so the orifice is at the end of a brass tube 1.5" long. There is no way to drill it except replacing the entire tube with a screw-in jet.

-BL

[Ray Neudecker]

You will have to drill to go above 79. There have been larger factory jets in the past. Last time i ordered, 79 was the largest available. Yamaha's don't go that large, so drilling is a fact of life here. I go up in stages with orifice drills and then use a reamer for the exact size. This has worked for me.

[racer]

Russ said what needs to be said an 84D does not have an 84 hole more like .091. Jets are available thru normal channels to 81D and to 89d thru sea way marine in seattle. The air bleed /pull over determines when the high speed starts to dump fuel it will also make the same size high speed jet dump more fuel. I would install a set of 106`S before running your engine. Also as Ray mentioned a reamer should be used preferably in a lathe if you need to go beyond factory jets.

[A.J. Pastwa]

O.K. so I need to go to my local OMC dealer and order some (6) --106 air nozzle inserts? What can I call them so my local knucklehead will know what I mean? A.J.

[Markus]

I use a reamer on Yamaha jets with great success. I bring it with me in the boat and open up the jets if I see the slightest sign of detonation on the plugs. I jet by looking at the plugs, though, not by any fixed jet size, so I do not care if I get an exact size and I do not even want all jets to be the same.

[Jay Smith]

Russ sumed it up about the swirles and "threads" a drill bit would create in the soft brass , even if you did it on a lathe it would probally still have a twist to it like a rifle bore. Marcus is got the ticket with the reamer that is the way it should be done even at the factory.
If I were gonna start enlarging the jet sizes I would invest in a pin gauge set as this is the only way to determine the exact size you are at and maintain exact tolernces. The smaller size sets are very affordable and can be bought from any of the tooling suppy folks ( Enco ,Msn, Rutland airgass , ect.....


Good luck and good boating,
Jay @ JSRE

[Rusrog]

THe only jet you will need to get that sled of your to run is made by Pratt& Whitney and it requires no drilling.....hehehehehehehe:D