CDI Conversion Timing Verification Question

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Geoffrey Adams

New Member
Afternoon folks,

I recently converted my 1977 CT70 with a 1974 XL70 engine to a CDI ignition, in addition to a CHP 88cc kit, and was wondering if anyone had some information of what the factory timing values are at idle and full advance for a stock 4 speed CT70?

I can't seem to find anything in the manual I have that relates to degrees of advance. Obviously it's all about points gap values.

I'm trying to verify my setup with a timing light as I'm questioning the amount of power i am getting from this 88cc kit. It seems to fall on it's face in the upper RPM's
 
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lukelaw1

Active Member
where did you get your cdi kit from? There are many out there and most are junk.

More info on you setup would also help, stock head, hotter cam, carb size, jets, etc.

I don't know if there is any documentation on degree of advance that was made public. The 4 speed Hitachi/Mitsubishi magnetos/flywheels "advancer" was mechanical and advanced with rpm/centrifugal force when the bob weights flung out. 3 speeds never had them, ran fine and revved out.
 
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Geoffrey Adams

New Member
The CDI kit was purchased from xrarespares on ebay, seemingly a reputable source based on forum posts here. The unit functions as it should, maintains a steady timing value when viewed with a light. The coil is fired right at the "F" mark on the flywheel. Just curious what that value is.

The bike originally had the centrifugal advance unit and I know the 3 speed models had a fixed timing curve. I was hoping this CDI unit had a timing curve built into it but it seems it doesn't. Being a OHC 4 stroke, while a fixed timing value like a 3 speed may work it can't be ideal. I'm probably expecting a bit much from the old girl.

The engine setup is a CHP 88cc kit (head, cam, bore, piston, small carb w/ 60 main jet connected to a stock airbox). 4 speed XL70 bottom end. I have tried 16/44 and 16/38 gearing.

It seems to run fine and the plug looks happy but doesn't want to do more than about 40 mph.
 
OLD CT

OLD CT

Well-Known Member
16-33 would be my choice. I don't think the x rare spares cdi is the issue, it's the gearing. I like the x rare spares kit, it does perform better than points. Report back speed #s after you change the rear sprocket. Should do 50.
 
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Geoffrey Adams

New Member
I will report back. I'm trying to wrangle the carburetor at the moment.

I attempted to leave the CHP carb the way it came out of the box (60 main, 35 pilot, needle clip middle position) but it doesn't seem the happiest. I haven't done any full throttle plug chops yet but it falls over in the upper rpm range. It will will rev out fully but is pretty lame up top.

I wasn't able to find too much info on this CHP "stock looking" kits performance but possibly the small carb and stock airbox are holding it back. As a note changing from 16/44 (was far too short) to 16/38 (better) didn't increase the top speed at all (38-39 mph). Which leads me to believe it doesn't have to power to overcome the bigger gear.
 
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Geoffrey Adams

New Member
I made this gear ratio to speed calculator also if anyone is interested in that.

docs.google.com

Honda Gear Ratio Calc

XR Gear Calc Gear,XL70,Gear,ST90,Gear,CRF110 w/ ST wheels 1,2.692,1,2.538,1,3.272 2,1.84,2,1.64,2,1.66 3,1.3,Full Gear Spread,3,1.045,3,1.19,Full Gear Spread 4,0.958,0.36,4,0.36 Primary Ratio,3.722,Primary Ratio,3.722,Primary Ratio,3.722 Front Sprocket,16,Front Sprocket,15,Front Sprocket,16 Rear...
docs.google.com docs.google.com
 
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