Topic: Cutting Jets to size!

[NewOldSchool]

I figured posting this question here would get a better answer than in the regular SOHC bikes section.

I need a #42 pilot jet for my 1974 CB750 with 657B carbs. The problem I am coming across however is that Keihin no longer makes the 28mm long pilot jets and only 32mm long jets are available. I have done as much research as I can, but there seems to be a limited amount of info on whether or not I can use the 32mm long ones. The only other option I have is to cut them down to 28mm.

Any one else have this problem or have experience here?

The photo shows my new #42 jet next to my old #40 jet. The only difference is the length of the body, so the new #42 will stick deeper into the fuel in the bowl.

[Sam Green Racing]

The number stamped on the jet usually refers to the size of the hole through it.
If the size of the hole is the same, there shouldn't be a problem cutting it down.

Sam.

[NewOldSchool]

The number stamped on the jet usually refers to the size of the hole through it.
If the size of the hole is the same, there shouldn't be a problem cutting it down.

Sam.

Hey thanks, I do understand the bore size and jet numbers and all that. I guess my question would be is it necessary to cut them down or could I run them as is, or if anyone has done just that. Thanks!

[POPS 911]

SUDCO, has many VINTAGE CARB PARTS.       OR        www.carbparts.com    phone 216-635-1099 CLEVELAND, OHIO             RIGHT SAM?

[Seabass]

I not much help with the length issue but am following.I am wondering if you are interested in selling one or all four of your old 40 sizejets .I misplaced one during disassembly.

[NewOldSchool]

I not much help with the length issue but am following.I am wondering if you are interested in selling one or all four of your old 40 sizejets .I misplaced one during disassembly.

Hi I would no problem. PM me, do you have paypal?

[cougar]

As far as cutting the jet to match the length of your original, NO ! Unless you have the ability to Match the exact slot and design that's inside of what your cutting off. I don't have a pilot jet in front of me right now but I think there is a machined area with maybe a taper there. I'm sure someone will correct me if I'm wrong, LOL ! As far as the extra depth it will set in your bowl, I'm not sure if that would have any bad effects unless it hit something. Others may be able to expand on that.   ...cougar...

[BLUE71TURBO]

 Exactly Cougar,  Most gear heads don't understand that  drilling a jet to a larger size will not make it perform as well as a factory made size jet. The taper is very important in the designed flow of the jet.     As long as the above mentioned jet does not bottom out in the float bowl, i'd leave it alone.

[Cortydean]

Ye definitely try it as is first.

[bwaller]

Maybe that's why there are tapered jet reamers.

[NewOldSchool]

Thanks for the opinions and info all, I was just confused as sites like Jets R Us and Carbparts literally say to cut the jet to size in their sizing and info charts. See below.

http://www.motorcyclecarbs.com/Keihin_Pilot_Jet_N424-24_C92.cfm

[iron_worker]

The longer jet will cause the bike to be drinking through a slightly longer "straw" so the overall friction effects will be a bit higher. However in this cause I believe the size of the hole has a much more predominant effect so I don't think it will drastically change how it runs.

I would be leary to cut the jet due to the fact that it would be hard to leave a perfectly smoothe edge around the jet entrance and cutting the slot in it nicely may be tricky as well.

I would run it and see how it is. I would think the small amount of difference in length could be tuned out with the idle mixture screws if they made any noticeable difference at all.

This is just what my gut tells me though. I don't have experience with the shorty vs long jets.

IW

[cougar]

BLUE71TURBO ; Thank You !

[POPS 911]

Where can I buy a TAPERED JET REAMER, I called K&L SUPPLY and they said they never sold anything like that = must be a tool sold in Canada. Will buy one if you can get me the name of the part or the number who handles this tool. Must come in a few sizes for the many Keihin jets 6.5mm to 32mm long ?.

POPS BK911

[Tim2005]

I wonder if the longer jet will have a richening effect on the mixture as the inlet to the jet it will be deeper in the bowl so the fuel pressure will be higher. If not, why is setting float level accurately so important?

[BLUE71TURBO]

 Yes POPS,  I myself have only seen jet reamers that are small drill bits, no taper anywhere.   

[bwaller]

Well BLUE71TURBO I suppose the wink is intended as a slight in my direction, whatever.

Pops, tapered reamers are in my opinion far more accurate than using a drill. They are very tiny & fragile and it takes patience to be accurate. I used guage pins to make sure the pilots all ended up exactly the same. I see they sell jet gauges below to check...cheaper than a set of guage pins believe me.

http://www.dansperformanceparts.com/buggy/intake/buggyintake3.htm  bottom of the page.


Sorry to get this off track.

[Tintop]

These guy are less expensive Brent (also near bottom of page) - http://www.piercemanifolds.com/category_s/327.htm

[BLUE71TURBO]

Well BLUE71TURBO I suppose the wink is intended as a slight in my direction, whatever.

Pops, tapered reamers are in my opinion far more accurate than using a drill. They are very tiny & fragile and it takes patience to be accurate. I used guage pins to make sure the pilots all ended up exactly the same. I see they sell jet gauges below to check...cheaper than a set of guage pins believe me.

http://www.dansperformanceparts.com/buggy/intake/buggyintake3.htm  bottom of the page.


Sorry to get this off track.

                            The wink face was not directed towards you. If i had anything to say to you i would state it in words. I have no problem with ya in any way. I just posted what i know about jet reamers. I don't know everything and havn't seen everything out there.   

[bwaller]

Yes Brian those two sets look the same, just trying to show they're available. It would be wise to check what size reamers are available in the kit to suit Keihin pilot jets. I seem to remember one I used was as tiny as 0.005" so breaking a tip off is damned easy but they are surprisingly tough.

[NewOldSchool]

I wonder if the longer jet will have a richening effect on the mixture as the inlet to the jet it will be deeper in the bowl so the fuel pressure will be higher. If not, why is setting float level accurately so important?

Exactly what I was thinking.

[Tintop]

I wonder if the longer jet will have a richening effect on the mixture as the inlet to the jet it will be deeper in the bowl so the fuel pressure will be higher. If not, why is setting float level accurately so important?

I stand to be corrected.....however my thought on float level is that it only effects the main jet.  It relates to how and when the holes in the emulsion tube become uncovered, and therefore the emulsification of the fuel.

[MRieck]

I wonder if the longer jet will have a richening effect on the mixture as the inlet to the jet it will be deeper in the bowl so the fuel pressure will be higher. If not, why is setting float level accurately so important?

I stand to be corrected.....however my thought on float level is that it only effects the main jet.  It relates to how and when the holes in the emulsion tube become uncovered, and therefore the emulsification of the fuel.

Tin...it effects the pilot too. God....I f'd around enough with spacers on CR pilot jets to know.

[Tintop]

I stand to be corrected.....

OK Mike corrected.   So if your using spacers on the pilot jet is that to richen the progression to the mains??  Inquiring minds would like to know.

[NewOldSchool]

I stand to be corrected.....

OK Mike corrected.   So if your using spacers on the pilot jet is that to richen the progression to the mains??  Inquiring minds would like to know.

Same here! Now that I have the bike running great with my current setup, I would love to fine tune it to the max.

[POPS 911]

My son in law is a Master Tool and Die worker and I use his pin gauges with my jet drills to back up the sizse , if it's .0002 on then for a run down 1320' then it will be OK.  Drilled many NOS pills over the pass years and no problems with these jet drills that come in 41 sizes and are not cheap to buy. These jet drill come with there own handels for easy finger drilling into brass jets.

POPS BK911