Pilot Screw Question



14 replies to this topic
  • ducdodger

Posted September 09, 2002 - 08:20 AM

#1

Sorry for being green about the WR - I just got one. Question - the pilot screw under the float bowl on the carb...does it meter air or fuel? Thanks.

  • sabin

Posted September 09, 2002 - 08:24 AM

#2

fuel

  • Taffy

Posted September 09, 2002 - 09:20 PM

#3

no, it meters the mixture of the fuel and the air.

you adjust the two via a PAJ and a PJ and then the PS meters the resultant mixture.

Taffy

  • sabin

Posted September 10, 2002 - 12:34 AM

#4

Thant is some interesting point fo view.

If you screw it in the mixture is leaner and out richer, isn't that so?

I think that was what ducdodger wanted to know.

  • Taffy

Posted September 10, 2002 - 03:29 AM

#5

sabin

i see your point. for better understanding though it's important to understand that the mixture is a very rich mixture but only a tiy drop compared to the main venturi.

so if you screw it in you're leaning off the total mixture and vice versa.

consider it like a river nile with blue water running by. the small river that joins it has red water. we can only limit the red water not change it's colour. BUT we know that in the nile it will change it from blue to a + or - purple colour.

hope that makes sense.

Taffy

  • Mark_Cantrell

Posted September 10, 2002 - 04:43 AM

#6

Taffy,

That makes sense but it's dead wrong.

mwc

  • ducdodger

Posted September 10, 2002 - 07:48 AM

#7

Good information. In the overall scheme, in is lean, out is rich.

My real problem is I'm trying to rid of an off-idle bog, but that's a whole other topic. I'm back to WR timing from YZ (no comments - the type of riding I do demands gobs of low end power) and everything works great with exception to the bog. I've found a ton of info on jetting with the YZ timing, but not for the WR with my specific setup. What is my setup?...

WR timing, FMF PowerBomb, White Bros. E-series w/8 plates, 168 main, 45 pilot, 60 starter, DTM needle clip-4. Carb is clean, air filter is clean and dry, starts easy, etc.

Any help would be appreciated - I know the jetting subject has been saturated... thanks.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Pyroted

Posted September 10, 2002 - 07:55 AM

#8

I don't see the BK mod on your list. If your AP is squirting to much fuel it can cause a hesitation or bog.

  • Mark_Cantrell

Posted September 10, 2002 - 10:23 AM

#9

I appologize for the blunt reply. Maybe this will help.

Air weighs about 1.2KG/M(3) (sea level, standard) or it takes about 13 cubic feet of air to weigh a pound. The stoichiometric ratio of air to fuel is 14 pounds of air to 1 pound of fuel (we usually run slightly rich of the stoichiometric optimum). Therefore, at standard conditions it takes about 182 cubic feet of air to burn 1 pound of fuel or 8190 cubic feet of air to burn 1 cubic feet of fuel.

By volume 8190 air to 1 fuel. This makes some sense when you look at the cross section area of a main jet vs the cross section area of the bore of the carb. Sounds about right.

The slow air jet is used to emulsify the fuel passed through the pilot jet. The goal is to get about a 1 to 1, by volume, bubbled flow of gas/air flowing into the carb. There are several reasons, I just understand a few. The same amount of gas will be delivered at a higher velocity and the capillary effects are almost eliminated. Changing the slow air jet will change the ratio of air bubbles in the fuel and therefore indirectly the amount of fuel delivered through the pilot circuit. The ratio is around 1:1.

That 1:1 emulsion of fuel to air is then mixed 1:8189 with air in the bore of the carb. It is a fantasy to think that the amount of air in the approx 1:1 emulsion affects the 1:8190 mixture going into the engine. What it does is affect the amount of the emulsion delivered to the air stream in the bore. The amount of air in the emulsion stream is completely negligible on the final mixture.

This leads to the second point, the emulsion being created by the pilot air jet through the cross drilled holes at the top of the pilot jet is delivered by the pilot circuit. I think we would all agree on that. Another question is whether the fuel screw draws raw fuel from the bowl and meters that fuel or whether it draws the emulsion from above the pilot jet and meters the emulsion.

According to the carb logic diagram in the back of the the 250F manual, the fuel screw draws fuel directly out of the bowl. The next time I have the carb off, I will take the fuel screw out, plug the hole in the bottom of the bore it feeds, spray carb cleaner up the fuel screw and see where the spray comes out. If it is out of the pilot jet, there is a good chance it is emulsion, otherwise it is fuel. So far I only have the logic diagram to go by.

The short answer to the original question is the fuel screw or slow screw in the front bottom of the carb meters fuel. If it meters emulsion, the effect is exactly the same. It meters fuel.

Good luck,
mwc

  • neWRiver

Posted September 10, 2002 - 12:34 PM

#10

Mark, hey great discussion of the relationship between the PJ, PAJ and the PS. You sure have a knack for relating theoretical material in concrete and understandable descriptions. I definitely learned something new. Interesting that this idea of a constant desirable emulsion ratio tends to support Taffy's earlier findings that PJ and PAJ settings should be "matched" to each other as opposed to considering them as independently adjustable.

I may have missed something, but it seems that you and Taffy are saying basically the same thing here with respect to the answer to this question.

Sabin's initial answer is fundamentally correct yet leaves us pondering this PJ/PAJ relationship. Your comments are more thorough, but Taffy also clarifies that we are talking about a mixture of fuel and air which is being metered by the PS. Your comments support that either this is actually the case or that the PS is metering raw fuel into the emulsion. In either case, I think you both have indicated that the end result is a metered mixture of air and fuel (Taffy indicates this as a "very rich mixture" and you indicate it as an emulsion). Taffy's "red water" analogy seems consistent with your analysis of the air/fuel ratio in the emulsion (i.e., it is always "red", in other words it is always primarily fuel and not a combustable air/fuel mixture).

Anyway, jeez, you guys are smart! Thanks for taking the time to share your knowledge and experience with us. :)

  • Taffy

Posted September 10, 2002 - 01:47 PM

#11

thanks mark and new river.

yes we are saying just about the same thing. i couldn't have put it better though new river.

like mark says the real difference is is it an emulsion or neat fuel that you're metering? i would suggest that brass valves do fuel and passageways do mixtures.

but you'll all forgive me if i say that i haven't read a book since the day i started and i go completely by memory and lessons learnt.

good luck.

Taffy

  • Mark_Cantrell

Posted September 10, 2002 - 02:29 PM

#12

OK. The point I was trying to make was that the emulsion is not the same as the mixture we feed the engine with. Essentially the emulsion is raw fuel. It is slightly modified (air bubbles added to it) to change its characteristics for transport, not ignition.

The 'mixture' made by combining the PJ and PAJ streams is over 8000 times richer than the mixture we get by adding the emulsion to the air in the bore of the carb.

It's all good.

mwc

  • MN_Kevin

Posted September 10, 2002 - 02:38 PM

#13

I don't see the BK mod on your list. If your AP is squirting to much fuel it can cause a hesitation or bog

The 98/99 accelerator pump is totally unlike the 2k and >.

This AP can be modified using either the "Taffy" mod or the "KL" mod.

  • neWRiver

Posted September 10, 2002 - 03:24 PM

#14

Originally posted by Mark Cantrell:
OK. The point I was trying to make was that the emulsion is not the same as the mixture we feed the engine with. Essentially the emulsion is raw fuel. It is slightly modified (air bubbles added to it) to change its characteristics for transport, not ignition.

The 'mixture' made by combining the PJ and PAJ streams is over 8000 times richer than the mixture we get by adding the emulsion to the air in the bore of the carb.

It's all good.

mwc

It is an excellent point, as a matter of fact. :D

Something I never did previously grasp quite as clearly as I do now thanks to your explanation.

Thanks! :)

[ September 10, 2002, 06:26 PM: Message edited by: neWRiver ]

  • neWRiver

Posted September 10, 2002 - 03:25 PM

#15

oops...

[ September 10, 2002, 06:25 PM: Message edited by: neWRiver ]




 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.