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A little more on the IOM Le Mans

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Just to share my observation with my LeMans 2 and 3 (different PHF36 carbs) and the California 2 with VHB30. The california has an old paper air filter. When it gets dirty (7000km+) and i ride full throttle in 5th gear, it starfs a bit on not enough air. Closing the throttle a tiny bit gives it just the extra 5 km/h. This phenomenon fits to a to big main jet. For a touring motorcycle it's okay. It protects the engine for overheating.


Especially with the LM3 (850), I reduced the main jet to 125 in order to increase topspeed. It is running a little over 200km/h, which I think is quite alright (almost 8000rpm, redzone). So you could check both a larger or smaller main jet to be sure.


Additional anecdote. This is for Super Single race-machines. The engine is right underneath the tank and heats-up the fuel in the tank. This causes a decreasing density of the fuel and thus a decreasing amount of energy/ liter. A racing motorcycle dealer from Amsterdam used an oil-cooler to cool the petrol. This gave him an advantage in the second stage of the race, when others suffered from heated fuel.

Ha there's always 1 out there I guess. Unfortunately the real world doesnt agree. If a MotoGP or Superbike with the massive amount of heat they shed via the radiator and engine with the fuel tank sitting basically right behind the engine getting the full effect dont neeed fuel cooling then nothing will.

Still I love the fringe people, they make life interesting:)



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Checking back in.  And again thanks for the ideas.  The bike had spend quite a lot of time on the dyno and had never had a fueling problem so under these conditions  I ask myself what things are different from the dyno run and running on the race track ?


1.  Ambient temperature is different  (high 80's verses 100's) BUT when we raced at Willow Springs it was in the 90's ??

2.  We were at 5000 feet in altitude Willow Springs is around 2000'

3.  We were getting our fuel from a MotionPro fuel bottle rather than the Moto Guzzi tank

4.  The Miller course was lots of short straights and tight corners.


Because the motorbike spent so much time on the dyno I can't think how the carburetors could be at the bottom of the problem.  Not once did we see the engine act as if it was fuel starving.  The last few pulls were all right on the money as far as the sniffer readings indicated  and the engine kept pulling as many RMP as we dared to ask of it.


One thing we found as we pulled the motorbike apart was that the petcocks that came with the fuel tank (they looked to be fairly new) were very small 1/8" and the ones for the big motors are twice as large.  When I asked the local MG guys I was told they came off a fairly small engine bike maybe no bigger than a 650cc. 


At this point all we can do is attack all the things we thing might be at the bottom of the problem and test again at the last AFM race at T-hill in North Ca,


Thanks again for the input.



At what altitude did you do the dyno runs ?

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  • 4 weeks later...

I have returned !


Bikes have all been loaded into their respective crate (Just a FYI we get our shipping crated from a PDI shop in Williams ,Ca  Triumph crates seem to be the best) we dropped them off at the Brokers end of the week and they'll soon be on their way  to Australia.  Gina and the team will meet again on January the 22 at Phillips Island.


There was quite a lot of discussion on the fueling problem we were having.  As is often the case it proved to far simpler  than one might imagine.  The fuel cap did not match up correctly with the tank flange thus closing off the vent hole between the two.  The early Le Mans tanks are quite large ( 27 L the largest allowed for the IOM) so when we were racing in rather short races (5/6 laps on 2.5 mi. track) we only fueled them with about 2.5 ga.  At Miller's we were racing in two back to back event so we fill the tank to the top.  There had been starting problems so the races were not getting off without extra laps and time sitting around,  I figured heck what could go wrong filling the tank to the tippy top.........Silly me;  Guzzi's get good mileage and we never got to a place where the venting was really not working.  It didn't matter when there was all that air space   "BUT" when there wasn't ! "cue the Bronx cheer" she ran outa gas.  I thank my lucky stars that we found; and fixed this before we tried racing 6 laps at the Island .  Long ago I was taught that stampedes were most often horses rather than zebras.


Again thank you all for your input.  We did change several things within the fueling system.  All these changes will, without doubt help the performance.  We are being more aware of fuel temps entering the carbs by using heat resistant materials ( coolmat)    on the uderside of the tank and protecting the lines and filters with heat wrap.


Next stop is Phillips Islands !! No Worries Mates see ya Down under.  Last time I was there was 1968 can't wait.  This time I bring a Shelia (Gina)  with me.



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I'll do what I can with getting pic's.  This; I will admit is not my forte but I'll see what I can do/  Thank you all for the support.  Racing IS FUN; but having folks rooting for you make it that much better.


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