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Starting backwards on occasion is a prime symptom of advanced timing combined with a slightly less than firm kick.

The piston heads up the bore (a bit more slowly than usual) and the spark comes wayeeee before it is supposed to (relatively speaking), and the piston is powered back down the bore without ever getting to the top of the stroke.  The spark/fuel powered motion is strong enough to drive the engine backwards all the way around and bring the piston up to and thru TDC, the spark comes a bit after the piston starts going down . . . but things are "close enough" for it to simply keep running in the reverse direction.

This can be a relatively frequent occurance on vintage trials engines when the timing is set strongly advanced for crisp throttle response.

Definitely re-check/adjust your ignition timing, that should cure the problem.

Ed

Keep the rubber side down!

Is the article correct in stating that the 1973 ISDT and the 2013 ISDT Reunion Ride were centered on the very same farm?  I thought the '73 event was headquartered in Dalton (or Pittsfield).

Ed

Keep the rubber side down!

Thanks for putting up that link!  What a hoot!!

In many ways a desert race is a desert race.  I got to ride a few 100 milers in Eastern Oregon and Eastern Washington back in the 1970's.  My first one was on a 100cc Hodaka Super Rat (chrome tank) . . . almost everyone had gone home by the time I got to the finish line.  I was really bummed when I got there and the place was empty . . . cuz I thought I was going purty fast!  :-)

The video also brings back memories of the Barstow Two Day Qualifier - dang those sand washes going uphill were no fun on a small bore!

Ed

Keep the rubber side down!

Didn't Dennis get conflicting advice?  One response says fill to 6" from the top with the forks extended and the other says fill to 6" from the top with the forks collapsed.  I'm pretty sure I know which is correct . . . but I'm also pretty sure you guys know better than I do.  So which is it?  Extended or collapsed?

Ed

Keep the rubber side down!

Didn't Dennis get conflicting advice?  One response says fill to 6" from the top with the forks extended and the other says fill to 6" from the top with the forks collapsed.  I'm pretty sure I know which is correct . . . but I'm also pretty sure you guys know better than I do.  So which is it?  Extended or collapsed?

Ed

Keep the rubber side down!

Here is what I was taught.  The header pipe length is the center-line length of the bend (plus any straight sections).  You can use math to derive the center-line length of the bend . . . or you can physically measure around the outside of the bend, then measure around the inside of the bend.  The average of those two physical  measurements is the center-line length of the bend.

Ed

Keep the rubber side down!

Here is what I was taught.  The header pipe length is the center-line length of the bend (plus any straight sections).  You can use math to derive the center-line length of the bend . . . or you can physically measure around the outside of the bend, then measure around the inside of the bend.  The average of those two physical  measurements is the center-line length of the bend.

Ed

Keep the rubber side down!

[throttle settings]

To be slightly more accurate, the pilot jet flows fuel at all throttle settings - so changing the pilot jet in a Mikuni has an effect on jetting at ALL throttle settings.  To be sure, the effect at WOT of a 1 step change in the pilot jet will be small . . . but it could be very important if Wide Open Throttle was already on the ragged edge of being too lean and then the pilot jet was reduced by a step.

Yep, get the pilot jet right . . . and then check/re-check jetting at all other throttle settings.

Ed

Keep the rubber side down!

I get a bit nervous reading this thread.  If the pilot jet is so lean that causes significant problems with starting (and it isn't plugged up), then it is lean enough to cause things to get extremely noisy then extremely quiet in quick succession whenever a low throttle position cruise situation is encountered with a Mikuni (and probably with a Bing or Amal).

A quick, though not completely conclusive, check for the Mikuni would involve:
1.  Identify the pilot air screw setting.  How many "turns out" is it?
2.  Check to see if stable idle speed changes when the pilot air screw is adjusted "in" 1/2 turn.  Do another check with the pilot air screw adjusted "out" 1/2 turn from the original starting position.  Note that "in" richens mixture, and "out" leans mixture.
3. If pilot air screw adjustments do nothing . . . either the pilot jet or pilot air passage (or both) are plugged.
4. If best idle is achieved between closed and 1 turn out, this is an indication that the pilot jet is too lean - get a larger pilot jet.
5. If best idle is achieved at 2 turns (or more) out, this is an indication that the pilot jet is too rich - get a one step smaller pilot jet.  (NOTE: it is not at all likely that a hard starting engine has a too rich pilot jet -- unless it is plugged.)
6.  When the proper size pilot jet is installed, best idle will be achieved with the pilot air screw somewhere between 1.0 and 2.0 turns out.

Ed
PS: I don't know Pentons very well . . . but am pretty familiar with the innards and operation of the Mikuni.
PPS:  Being lean on the pilot jet is not a good thing.  No.

Keep the rubber side down!

Reckon I'd have to disagree a bit.  I've raced Washougal and "The Farm" . . . "The Farm" is wayeeee more fun.

Ed

Keep the rubber side down!

Also check for signs of leakage at the head gasket.  Oily residue gathers dust and is a sign of leakage at both the head and base gasket.

But, as already advised . . . do a pressure test.

Ed

Keep the rubber side down!

Also check for signs of leakage at the head gasket.  Oily residue gathers dust and is a sign of leakage at both the head and base gasket.

But, as already advised . . . do a pressure test.

Ed

Keep the rubber side down!

In addition to the cut-away number, you need to be absolutely sure the short slide won't be lifted above the guide pin at full throttle.  If the slide lifts above the guide pin, it could then turn and will not be able to snap shut.  (Hmmmm, that's one way to keep from backing off after the start!)

Finally, the slide material is "matched" to the body material.  Chrome plated brass slides are matched to aluminum body carbs and anodized aluminum slides (usually green color) come in "pot metal" (zinc?) body carbs.

Ed

Keep the rubber side down!

I too was interested in the Transit . . . till I actually walked up beside one and peered inside.  Carrying one bike would require removing the passenger seat.  OR you MIGHT get two bikes in behind the seats by removing front wheels and front fenders.

Still searching for an Astro replacement.  My Astro has about 130,000 miles and is still behaving in a civilized manner . . . so I have a little time yet to find a bike hauler.

Ed

Keep the rubber side down!

There is no way the engine will run correctly with the short piston skirt opening the exhaust port from 3mm BTDC till 3mm ATDC.  After you get that problem solved, then you can address other possible issues.  This situation is a PRIMARY problem which must be properly resolved -- i.e. new piston which does not open the exhaust port to the crankcase volume at Top Dead Center.

Ed

Keep the rubber side down!