TRIUMPH T20 CUB - MODIFIED

[Old Scrambler]

Thanks for the heads-up

We think we have identified the so-called 'rev-limiter' that makes top-gear on the Tiger Cub feel like a Sunday Ride. We believe our spark is not hot enough at full-load. The GM electronic ignition is a total loss system and this year we only had about 1-amp of power coming from a 12v EV02 BALLISTIC battery meant to run a 50cc scooter. We are thinking we may need a 4-amp or higher battery to assure a full coil load at high-rpms.  The motor pulls into the mid-7000 rpm range in 3rd but only climbs to about 6500-6700 rpms in 4th.

Any thoughts..................

[Freud]

Use less gear.

FREUD

[Koncretekid]

Thanks for the heads-up

  The motor pulls into the mid-7000 rpm range in 3rd but only climbs to about 6500-6700 rpms in 4th.

Any thoughts..................

Dennis,
Why only 7000 rpm?  I've heard that BSA 250s can rev easily to 10,000 rpm.  I am using a 250 crankshaft stroked to 75mm (is your stroke just 70mm?) and I can rev easily to 8500, even with a much bigger heavier piston.
Tom

[Old Scrambler]

We tried a larger rear sprocket and ran slower...........at the same max rpm in 4th. Each tooth is roughly 2 mph.  We are fairly convinced its the ignition system........so hunting for a replacement.  We got this far on used parts .........even my new leathers were an ebay close-out Time to get serious.  We are thinking of adapting a mag system from a Honda CRF-series bike.

Tom.........we agree the motor should easily rev into the 8s.................Current bore is 69.3xx mm and the stroke is standard at 64mm. 

Maybe I just weigh too much

[wobblywalrus]

What about a beefier battery or an ignition system that uses less juice?

[Old Scrambler]

No battery equals another lb. or 2 off of my belly

Yes, the coil, modulator and battery all have to be in sync...........but a mag will give us all the spark we need and will run off of the crank, instead of the timing side.

Just exploring for now..........open to ideas..............

[Koncretekid]

Dennis,
Insufficient amperage will definitely limit the rpms.  To try it, hook up a voltmeter and run the motor up thru the rpms as you watch the voltage.  I've had it happen to my CB350 roadracer.  The motor would rev to 8,000 and just stop there.  Turned out to be a faulty battery, which started at 12v and dropped to about 9v at 8,000.  New battery, problem solved.  (I seem to be a slow learner.)
Tom

[Old Scrambler]

Tom...........I just began to learn about these things at age 64.........talk about slow

More juice is planned for 2014.........in more ways than one

[Old Scrambler]

2014 was an expensive experiment at the salt. With so little opportunity to get runs in, I tried to run the Tiger Cub only to find frozen shift-linkage. So it got repaired in a few minutes but I had quickly substituted my new build so the Cub never made a run

The new bike is a CB750 sohc put together from a mix of parts and some left-over racing pistons. Dave and I built the bike to compete in the Classic category and proceeded with a plan to break it in at the salt. We were way off on carb-tuning and timing............so not much to talk about.........until I figure out how to post the dyno-charts from yesterday's session at a local shop.  More later..............someone is calling my name

[Old Scrambler]

The bike as it sat in line at 2014 BMST


Its a 1mm overbore so legal in the Modified Classic 750 class. Now we are a lot closer to knowing what we have:


Plans are now underway to take both bikes to Bonneville for the BMST...............

[wobblywalrus]

Dennis, try dropping your needles one notch.  It might get rid of some of that dip in mid range power.

[Old Scrambler]

Needles on the RS34s are in the middle slot..................if anything I would lower the clip to raise the needles..............they have a long slender tip and then an abrupt taper that effectively closes the main when at rest. Right now the mid-range is good if the throttle is rolled on rather than yanked to WFO.

[Old Scrambler]

Just to let you know we are still working and planning to attend BMST...........the Tiger Cub should be READY with minor mods and a NEW battery........now playing with timing.

The Honda CB750.........I call it the CBR for Classic Bonneville Racer.............has a new megaphone muffler; new intake stacks; and a revised air-intake system with only a screen in the box. We tried a lot of different ways to get an effective ram-air system to the box..........but too many seams and joints caused leaks...............This box retains the OEM intake on the bottom. If the new side intakes work, the bottom could be taped shut. The carb-bowl vents will go to the lower part of the box.

A few pics just to give you an idea of what this motor is all about................

[Koncretekid]

Looks good, Dennis.  Looking forward to seeing you guys again.

Ram air?  My bike actually ran with the ram air up to about 133 mph; then just wouldn't go any more.  I decided that there are probably too many variables like position of intakes, size, corners, and air box size and shape.  Today while waiting for ignition parts, I cut the suckers right out of my fairing and tomorrow I'll fill in the holes and paint some eyeballs or something where the holes used to be.

Tom

[Old Scrambler]

We need to get together and have good discussion about the ram-air. I have discussed it with 2 engineers. Our biggest problem was the OEM frame

Dave Murre is not making the trip this year. So parts may be flying I will have Jerry from Colorado Springs to help...........and 2 or more spectator friends along for the ride.