Topic: main jet 20mm keihin carb

[dbscots]

I've got a question for any carburetor expert out there. I wrote awhile back about the differences not only in the internal configuration but also the difference in the size of the orifices in the stock#75 main jet compared to the #75 main jet in an aftermarket kit I bought off e bay. The aftermarket is about half the size of the original.  The response I got suggested I shouldn't trust the jets from an aftermarket kit. I started to reinstall the originals and discovered I  had lost one. So I ordered another Keihin jet from Honda and when it arrived I compared it to the original and the aftermarket and it looks just like the aftermarket. I'm confused. Is this some improved #75 main jet or what. Does anyone out there have any ideas?

[phil71]

maybe someone drilled that old jet.

[lucky]

All this stuff about aftermarket jets is not true.

But there is ONE exception. The 1977-78 PD carb slow push in jets can be undersize where they push into the carb body. they need to be .180 thousandths diameter  to have a slip fit. IF it is .178 you will have to epoxy them-no good.

[dbscots]

Thanks for your replies guys. No, it doesn't appear that they have been drilled out. It's not the outside diameter that's in question but the inside. The original jet is basically bored straight thru with a diameter of approximately 0.060 while the new Keihin and the aftermarket jet both step down in diameter to about 0.030 and then flare out making a larger diameter exit hole. Is this some trick physics to improve fuel flow by altering the internal configuration of the jet? Anyone else got any ideas?

[Tim2005]

I think the old one has been drilled.  The hole through a 75 main jet should be 0.75mm, which is about 30 thou in imperial, if yours are 60 thou they must have been drilled, they never wear much.

[phil71]

that number isn't the size of the opening. 

[lucky]

The numbers on jets are in mm.

You need to convert to thousandths to be able to visualize
 the size of  hole if you were born in the USA.

So a #75 is .295 thousandths ...lets cal. it .030 (1/32) so if it was drilled out to .060 which is almost a 1/16 I would say that is not in the ball park at all.

[phil71]

That number is how many cc's of fuel will flow through them in 60 seconds.

[dbscots]

I suppose it's possible that the original jets had been drilled out. But I'm not sure if that would totally account for the difference in the internal appearance of the old vs. the new jets. The new jet both factory and aftermarket start with a hole diameter of approximately 0.120". They taper down to their smallest diameter which is just about 0.030". This basically forms a cone open at both ends, 0.030 at the top and 0.120 at the bottom. The same is true at the other end except the cone is not as tall.
Now the original jets start out at the same 0.120 diameter. This diameter remains constant until about the middle of the jet where the diameter steps down to about 0.060. The hole then just steps back up to 0.120. No tapering on either side. If the old jet has not been drilled out do you think it's possible that the tapering is something that Keihin discovered provides better fuel flow and atomization and now incorporates in their jets since the jets of 1976.
On the other hand they were probably just drilled out somehow.
Any thoughts?

[phil71]

Dbscots, it's very possible that they superceded this part with a design that's tapered, or graduated. it's possible they both flow exactly the same CCs/minute as one another, but perhaps the taper somehow increases velocity or something. It'd be a slightly complicated rig, but if you really wanna know, you can test to see how long 75ccs of gas takes to go thru 'em.

[Bodi]

Mikuni jets are numbered according to flow, not diameter.
Keihin jets (OURS) are numbered by hole size. A Keihin 40 jet has a 0.40mm hole.

[phil71]

oh.. #$%*, that's right. Well Dbscots, now I have NO idea what the heck is going on with your jets.  Now... what does dynajet base its #s on?

[dbscots]

Well I finished rebuilding the carbs this evening. Tomorrow I'll try to get them installed and just see how the bike runs. I'll keep you posted.
Thanks for all your input.
dbscots