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Old 02-05-2015, 01:38 PM
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mapbike mapbike is offline
Motorized Bicycle Elite Member
Join Date: Mar 2010
Location: Central Texas
Posts: 5,379
Default Re: Curious about the engine, :question:

Originally Posted by cannonball2 View Post
Great pictorial Map! So sum it up whats the best combo of parts for a good daily driver low/mid range engine and the combo for a high rpm engine?
Well, there are so many ideas on this, and I'm not at all a pro at it and don't claim to have even close to the same amount of knowledge on all of this as many on here, but I can tell what I have done and what has been the best for me so far.

I'm still learning and plan to do a few of the other things that have been suggested in the future like using a degree wheel and getting the port timing set to the best of my ability to where others have said it seems to work the best for them such as Davezilla and a couple others who I can't remember right now....??? have suggested.

My best current running engine has a cylinder like the one I show in the pix that has the larger rectangular transfers, I just cleaned then up good and "de-bird" the intake and exhaust port, I didn't raise or lower them other than the slight amount that will accur when they are cleaned up and evened up across top and bottom if needed.

On the engine I speak of I didn't shave the deck, only trued it up on some 320 grit wet/dry sandpaper taped to a glass pane to get it flat for a good gasket seal.

The piston has ramps filed and finished with Dremel drum sanders on the transfers, the transfer ramps extend 0.60"/aprox. 1.5mm down and taper back about the same amount. they are the exact width of the transfer openings.

The piston is also ramped on the exhaust port side, it's also the exact width of the exhaust port and at it's max depth down from piston top is 0.40"/ aprox. 1mm

It has the 6cc Fred Head on it, an NT Speed Carb with jet drillwith #72 wire gauge bit which works good at my elevation with the 100:1 Opti2 fuel mix and my set up.

The current exhaust is a pocket bike type thath has been modifies a little for better flow and the header pipe is 1" Id exhaust flex pipe and is about 12 or 14" long if I remember right.

the lower engine is original version 40mm stroke Dax GenIV that has a very good balance.

This bike will cruise on the flat with no head wind at 46-48 mph with a 32T rear sprocket, I currently have a 34T on it and with the current pocket bike pipe it will cruise at 43-45mph on the flat with no head wind.

It is running a stock kit CDI which I have found to do anything the "custom made units" will do and actually out performs some of them which I wont call out here in open forum since it always gets a flaming argument started..LOL!

Main thing I have learned on all my engines since the first build in 2009 is no matter if they're mechanically capable of high rpms or not, is that and easy rolling bike, a well tuned carb and an exhaust that flows as freely as possible is the key to getting the highest rpm the basically stock engines geometry and balance will allow for what I will call a "Common Man Build", I know with more extensive port timing correction work and all that stuff like a correct expansion pipe, correct header size and length blah blah blah...etc one can get even more from the engine, but I like to focus on what I call a "Common Man Build" since that is what 90+% of people who come here will have the skills, tools and finances to accomplish with their motorized bike build or builds that will either be done at the kitchen table, small garage or under a shade tree in the yard.

My personal fastest build was a bike with 24" wheels, 30T sprocket, Dax GenIV 40mm stroke lower, all the same basic mods as the other bike I listed above but with this one having a Puch 70cc Hi Hi Head, piston ramped the same but with 12 3/16" holes drilled for making it lighter and an RT carb, this bike hit 52 MPH on a slight declining stretch of road and 50.3 on the flat, it had a stock type exhaust pipe that had all the guts removed, was port matched as good as possible to jug port and then had some extra 3/8" holes drilled in end cap and the tiny pipe stinger removed.

This engine would twist up right at 9000 RPMs, it is retired for now since I got it a little hot and cooked the tension out of the rings, the jug on it was not the large transfer port jug like I listed in the pix earlier either, it was the common PK80 type I got from BGF and I just did a little work to get ports clean as possible for better flow.

Well, before this gets any longer I'll quit, but this is the simple things I have done to get much better performance on what I call my "Common Man Build" engines, neither of these engines have or had any machine work done on them, all work was done by hand or with a Dremel type rotary tool, some drum sanders, rubber polishing wheels and carbide burrs that can be had fairly inexpensively at two different sources where I have been buying my stuff for port work.

I dont always ride WOT, but when I do..I do my best to keep the rubber side down..Ride safely my friends!
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