100cc classic production build

[roflhat]

Anyone have first hand experience of carb jetting at Bonneville? The online calculator say go down 10 numbers on the main, but others I've spoken to say you may even a need larger main jet to aid cooling

Is it worth using a dyno in Salt Lake City to set the carb up before records week?

[Seldom Seen Slim]

Scott Horner's shop - Heads Up Performance - is in Sandy, a south suburb of SLC.  He's a Bville racer, and we've many times had him do a final tune on our bikes before we head out to the salt.

[wobblywalrus]

The largest jet that produces good power is determined on the dyno.  Then, a Mikuni slide rule is used to calculate the change in jet size for the local conditions.  It is usually no more than one size leaner and maybe two sizes in some cases.  This method works for me where the dyno is near sea level. 

[Seldom Seen Slim]

Further on retuning for the salt altitude:

Steve Knecum built and tuned the motor for Nancy's ZX14 at his shop near Philadelphia.  His dyno showed something like 215 hp.  We stopped at Heads Up a few days later and Scott dynoed it and found that we had lost a dozen* or more HP on the cross country trip.  Scott gathered up some HP he had lying around the shop and installed 'em in the motor. and Nancy got a record at the event.

*Yes, we considered that the two dynos almost certainly were not matched, that comparing two dynos is more of a "relative" than an "absolute" thing, when we came up with the power loss numbers.  My point is to offer my experiences in re-tuning for Bville altitude.

[salt27]

I have been tuning my 2-stroke at 500' altitude and then taking it to Bonneville with intensions of slowly jetting leaner until I find the best tune.

What I have found with my 500' jetting is that it seems to be a bit rich for the first half mile then as the heat builds it leans out.

The plugs have no deposits and the EGTs are in the 1250f degree range.

I may be chicken and leaving HP on the table but so far no seizures.

Someone once told me the secret to running 2-strokes at Bonneville is to bring plenty of barrels and pistons.

Results may vary, Don

[roflhat]

Interesting, so in reality the jetting changes required are generally not as drastic as the calculator says?
Been playing with the jetting already, got it in the sweet spot, main jet is good just the needle jet to tune. If it's going to be close to the same I may just adjust on the salt

[wobblywalrus]

The underside of the front of the Triumph fairing is shown in the fuzzy photo.  It is shaped to be an air scoop and to capture and direct air flow to the front of the cylinder and head.  Maybe this idea would work on the little two stroke engine.  Some race fuels work better in two stroke race motors than others.  Your fuel supplier should know about this and be able to give recommendations. 

[wobblywalrus]

The smoke test is attached to the latest post in my build diary.  It shows how the scoop shape of the fairing traps air and directs it to the engine.

[roflhat]

That's seriously cool! Not sure I'm allowed any aero changes at all for my classes but I love seeing the effort that goes into the "serious" builds

[roflhat]

Just looking at the FIM records, this appears to be the same as the guy on the same bike as the P-P 100cc record.


Might be achievable for me, can I go for an FIM record at BMST?

[Doc B.]

You can run FIM at BMST. Read all the requirements - EKG/stress test required if over 50, FIM approved helmets with the appropriate tag, two hour turn round for record runs are things to be aware of. AMA is a lot simpler to run - simple medical form, Snell/ECE helmets, same day turnaround for record.

If SCTA is on the radar too ( i.e., you decide to live in Utah for a month and attend both BMST and an SCTA event), requirements for leathers are different than AMA/FIM - no perfs or stretch panels allowed except armpits (and maybe backs of knees?). It's harder to find off the rack leathers for SCTA than leathers for AMA/FIM, which can be standard road race type with stretch panels.

So if you want to run in all three organization's events you would need an FIM $approved$ helmet, SCTA approved leathers, and a stress test if over 50. I would maybe put that money into more go fast parts instead and just run AMA. Fingers crossed we have a BMST this year. Otherwise I'll be looking for some SCTA leathers myself.

[salt27]

SCTA leathers requirement changed in 2017 and are a bit more lenient than in the past.
I would quote but it's more than I'm up for one finger typing. 

  Don

[Doc B.]

Don't have an SCTA rule book handy, but I did google something about an "update to the 2016 SCTA rules for leathers". Presumably that's the 2017 change you mention -

7.C.2 LEATHERS

One-piece or two-piece 350 degrees (zipped together) all leather is required and the zipper must be sewn to the leather. The use of the leather manufacturer's engineered materials, such as stretchable Kevlar, perforations, or any other of the manufacturer's materials, in non-critical areas is permissible. In critical areas (knees, elbows, forearms, back, shoulder, hips, etc.), armor or two layers of leather is highly recommended. Riders of motorcycles in the fuel classes are recommended to wear Nomex underwear or something similar in nature. Suits of synthetic materials are not permissable. Fairings (speed humps) on the back of leathers will be allowed in partially streamlined classes only.

Here's the AMA rules -

2. P. (IV) LEATHERS
Suits made of synthetic material are not permissible. The use
of stretchable Kevlar and perforated materials in non-critical
areas are permissible. Leather suits shall be one-piece design
or joined by a full circumference zipper at the waist.
V16.1 25
Leathers cannot be too big or loose. Critical area (knees,
elbows, forearms, shoulders) armor or 2-layers of leather is
highly recommended. Due to invisible flames, riders? of
motorcycles burning fuels of Methanol content are
recommended to wear Nomex underclothing or something
of similar nature.

So it does appear that the regs are more or less the same these days and an SCTA approved suit does not have to be solid leather everywhere. I stand corrected.

[salt27]

Doc, you are right on the SCTA 2016 changes but there were a few minor tweaks added in 2017.

Thanks for posting.

  Don

[Doc B.]

The difference in the 2017 SCTA rules is simply that you can run a speedhump suit in non streamlined classes if it is an airbag suit.

Fairings (speed humps) on the back of leathers will be allowed in partially streamlined classes only, this requirement does not apply to leathers equipped with air bags.