Changing jets (new guy question)

Christofer Hoff · Posted: Mon Jul 30, 2001 6:54 pm Post subject: Changing jets (new guy question)

Charlie:

In order to 'set the baseline' does one perform this one time given a known set of environmental elements or once a day, per session, per track, etc?

Specifically, and as mentioned below, how does one know when the baseline has been achieved -- set of the pants and good plug color?

It occurs to me that given what you mentioned below, you need only set your 'baseline' once (anywhere) and then adjust tuning based upon the differential of the RAD gauge, yes?

If your baseline was 'X' and the RAD gauge read '95' (for example at a local track on a test/tune day) and then you re-sampled on a race day at the same or different track and it read '98,' one would increase jet sizes to accomodate the 2% differential, right? No real need to baseline everytime?

Thanks,

Chris

quote:
Originally posted by Charlie Tackett:
Steve and Dan, I usually try to operate on the principal of KISS. Like Blaney points out...just get a RAD gauge. Takes into acount temp and barometric pressure. You'll have to find your own baseline.....plug/piston/pipe reading (hey...the three P's of tuning!!), but once you got your baseline established, you just follow the changes in your RAD and adjust the jetting accordingly.....but you do have to be sure you can read the #'s on your jets, of course.
(Tim, that's why there's a magnifying class in my jet case....som,etimes even teh bificals aren't enough)

I have found that changes in humidity are rarely result in having to have a significant jet change, usually has come out to less than 1 jet size, so don't really worry about it, especially at this point. Also remember, if you make any changes in the engine...diff pipe, differenet reed cage, etc., re-evaluate your baseline.

Hey Dan, was good to meet you at Grattan! Got another name that actually has a face connected to it! We will have to work on those 1:34s, though.

[ July 30, 2001: Message edited by: Charlie Tackett ]

Dan Meray · Posted: Mon Jul 30, 2001 10:14 pm Post subject: Changing jets (new guy question)

As you may have seen in my recent post where I was asking for weather information at Grattan, you need to get some "weather station" equipment that will tell you what the relative altitude is at any particular moment during the day. But first you have to baseline the setup and read your plug or piston to avoid the dreaded metalic noise that occurs when the engine sticks because it is too lean or hasn't been warmed up properly. Once the top of the piston or plug color is correct, you need to keep logs and track the weather. If you are tuned for 3000 feet and suddenly a front comes through and drops the relative altitude to 1000 feet, without a richer jet, you will be going back to your motor builder. Now the trick is to get someone with a similar setup to help you along the way. This forum is a good start for that.

Hope this helps. With proper jetting I am sure you can get to third gear at the end of your driveway.
Dan

Steve Clark · Posted: Mon Jul 30, 2001 10:51 pm Post subject: Changing jets (new guy question)

Thanks for the reply.

What do you use for altitude equipment?

So 'weather log' would include things like temperature, humidity and pressure and then the corresponding reading on plug or piston. I do understand what you are saying but I have some work to do putting it all together.

I understand a bit about reading plug. Pistons? No clue. I am going to enjoy the challenge of figuring this out. How do you arrive at an altitude number?

Sounds like I could be changing the jet between cool morning practice and warmer races later in the day.

Does monitoring water temp help in any way?

Thanks. I want to find out as much as I can before going for my first test and practice session.

Steve

Steve Clark · Posted: Wed Aug 01, 2001 8:12 am Post subject: Changing jets (new guy question)

More new guy questions . . .

As I do more research I see that my Keihin PWM carb has slow and main jet. Am I correct in that I would only be changing the main jet?

Keihin has a LONG list of jets. What would be a good range of jet sizes to have? I think there is a 180 in it now. I'll have to check that.

Thanks for helping the new guy.

Ronnie Swyers · Joined: 23 Jul 2001 Posts: 95

Steve,
On the main jet a good selection would be a 160-195. On the pilot 48-62. A DGJ needle seems to be a good all around needle to have in the carb. While your buying jets, make sure to purchase a couple of spare clips for the needle. You should now be in business! Happy carb tuning

Ronnie Swyers www.RSwyers.com

Charlie Tackett · Posted: Wed Aug 01, 2001 4:03 pm Post subject: Changing jets (new guy question)

Not exactly a "slow jet" and a main. You have the pilot jet, the needle and the main. ALl play a factor in tuning the carb and are mostly related to throttle position.

Setting the baseline is tuning the old fashion way. Seat of the pants feel, sound of the motor, reading the plug, piston crown and pipe. Once you get it running they way you want it, it feels and sounds good and you have good colors/residue on the plug, tc. Then that is your baseline....permanent baseline for the way you got the engine set-up. If you change anything in the engine set-up, be sure to re-evaluate your baseline! But as long as the set-up is the same, you only have to do the baseline eval that once.

Once you have that establishe, take the RAD reading at the time and note the main jet you have in. As the rad changes, you change the main the same direction...if RAD goes up to a higher number, the main goes up to a higher number; down on the RAD 3, then down on the main number. Like I said, KISS! TO be safe starting off, if you change the main one size for any 2 pt change in the RAD, you should be close. SO if you have a 175 main at a RAD of 100 and the RAD goes to 102, put in a 178. Goes down to a 98, put in a 172. Will keep you close at first.

Oh, and always keep teh RAD in the shade in a well ventilated area and always in the same spot. DOn't stick on a shelf in the trailer once, then on the seat of the tow vehicle the next time. (Cars make wonderful greenhouses...and can throw off the RAD)

Steve Clark · Posted: Fri Aug 03, 2001 3:11 am Post subject: Changing jets (new guy question)

Thanks to all for the information. Turns out the Keihin Carb has a 195 jet in it so now I know what jets to buy and I also understand how to baseline the motor. Just have to learn to drive.

Steve

John Wehrheim · Posted: Fri Aug 03, 2001 9:23 am Post subject: Changing jets (new guy question)

I am glad we are talking about this. But are we saying that 2 points on the RAD equals one main jet size? Is this for 125 only, what about 80cc jets. Does the 2 points or 4 points or what ever effect the pilot? If so, how and when should you change the pilot.
Also, I have heard that an Altitude meter is a better way to go then an Air Density Gauge. What are your thoughtsnull