125 Engine Technical/Performance Issues

[Dr. Thunder]

Thought I'd start a new thread on the 125 engine subject. The more inquiries I make the more interesting the subject becomes. Thanks to all of you for your responses to my 125 questions.

Found this website for Dell'Orto http://www.dellorto.it/. If you click on > english language > Racing > Racing Products > VHSB34 > esploso (bottom right)
you will see an exploded view of the Carb that I believe is used on the Rossi 125 engine. From the exploded view can any of you tell me which "jet" is changed to achieve proper "jetting", basis for making a change, and what size jets are used as a replacement and for what racing conditions? I assume jet sizes are referenced in millimeters (tube diameter)?

[Composite Specialties]

VHSB Carb

This is the carb that is used on the 125cc engine.

[sam]

Parts 2, 3, & 4 affect mid and top end together with #4 being the most important for wide open.

#4 is the "main" jet

#2 is the tapered needle that rides with the throttle slide

#3 is the "needle jet" that the needle works in

The basics of the slide & needle carb are that above idle at low throttle settings the fuel flow is restricted by needle in the needle jet. The top of the needle is fatter than the middle and bottom (and usually straight for a short length). As the throttle slide is raised the needle - which is attached to the slide (by being dropped part way thru a tiny hole in the slide) - raises also. Once the straight fat part of the needle is fully out of the "needle jet" the taper on the needle can let more fuel flow up around it. The exact taper and length of the needle affects exactly when and how much the transition happens. The as the throttle approaches wide open only the tiny tip of the needle is in or near the actual restriction part of the "needle jet" and a lot more fuel can flow out of the needle jet into the air flow. When the throttle is fully open the needle is no longer restricting the fuel flow ... the only restriction at that point is the main jet (#4).


I do not know the details of the Dell'Orto parts, my experience is with Mukinis. On racing Mukinis you can adjust how soon the mixture becomes richer by raising or lowering the needle (different places you can put circlip #18 on the needle to raise or lower the needle in relation to the slide). There are also different diameters and tapers of needle available.

The main adjustment is #4, the needle is a mid range fine tuning feature. #3 would probably not be changed unless you just could not get the needle right.

This link goes directly to the exploded carb view Hover your cursor directly over the number to make the part name and number appear

[Bill Huson]

Lectron carbs . . .

a Mikuni copy or vice versa. You could also change the needle for one with a different taper. I don't think that did diddly squat for top end but it sure made a diff in midrange and the ability to come on the throttle.

[pro350hydro]

All pro racers have a different way of tuning carbs. Since you are new to pro, the best thing you can do is listen to Rex Hall (I'm assuming you bought the motor from him) and learn when and why you are making the changes. Then soon you will have enough knowledge to tune yourself and your way. Tuning is a secret of Pro racers. Also what works for one motor may not always work for another.

Good Luck

Thought I'd start a new thread on the 125 engine subject. The more inquiries I make the more interesting the subject becomes. Thanks to all of you for your responses to my 125 questions.

Found this website for Dell'Orto http://www.dellorto.it/. If you click on > english language > Racing > Racing Products > VHSB34 > esploso (bottom right)
you will see an exploded view of the Carb that I believe is used on the Rossi 125 engine. From the exploded view can any of you tell me which "jet" is changed to achieve proper "jetting", basis for making a change, and what size jets are used as a replacement and for what racing conditions? I assume jet sizes are referenced in millimeters (tube diameter)?

[Dr. Thunder]

Pipe differences ...

Just noticed a slight difference in size between the VRP and Rossi supplied pipe for the 125 engine. What would be the reason(s) for this ... remember ... I know nothin'

[Mike Schmidt]

Simple...

Two diffrent designers. Go test and see which one works best for you.

Michael D-1

[Dr. Thunder]

Towerhousings ... who uses what???

Just curious about what towerhousings 125 drivers are using/prefer ... what are the pros and cons? Who is fabricating towers? Are they all interchangeable between VRPs, Rossis etc.? Thanks.

[Composite Specialties]

Tietze

I was going to use a Lee Tietze housing for my 125cc but I switched to a 250cc engine instead and still will be using a Lee Tietze housing, very nice equipment.

[Dr. Thunder]

VRP 125 Engine Owners/Drivers ...

Any of you 125 guys/gals that are running VRPs ... I need some technical help. Please email me at Dr.Thunder@verizon.net

[kws]

do you HAVE to use a water cooled engine? or is any 125 cc engine legal?
just another idiot wondering

[pro350hydro]

Any of you 125 guys/gals that are running VRPs ... I need some technical help. Please email me at Dr.Thunder@verizon.net

What do you need help with?

[pro350hydro]

do you HAVE to use a water cooled engine? or is any 125 cc engine legal?
just another idiot wondering

PRO = Anything goes, as long as you meet the class size requirement

[m58rr50hydro]

I was going to use a Lee Tietze housing for my 125cc but I switched to a 250cc engine instead and still will be using a Lee Tietze housing, very nice equipment.

NOW MARC BE CORRECT!
ITS CALL MACHINED COMPONETS LETS DON'T XXX OFF THE "BOSS" AND SOON TO BE POSTED MACHINED COMPONETS NEWEST PRO PART THE FOOT THROTTEL/PIPE PULL WEIGHTS LESS THEN 2 LBS WOW! NICE TO HAVE TWO HANDS ON THE WHEEL
MARC
M-58 RR50H
R-58 350CCH
YOU CAN LIVE IN YOUR BOAT BUT YOU CAN'T RACE YOUR HOUSE!

[Smitty]

Actually, Dr. T, what you are asking for is real basic tuning instruction. Unfortunately, it can't just be handed to you in a couple of paragraphs here or in a private message. This would be a whole chapter in a book, or a long article in a magazine, and that's just for the basics. If you really want to learn this stuff, you have to "pay your dues" like others here have done, starting with going to the library and finding books on tune-up and engine-building. Look for books by David Vizard, Smokey Yunick, Gordon Jennings. I could give you more names and be more specific, but You need to do the search yourself. There is a phenomenal amount of information available in print now, as compared to forty years ago. Check it out.