DoodleBug Performance Jackshaft
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They are good torque multipliers and make the most of the available power the 2.8 makes. I've seen the 2.8s last but really only as you are using it with a jackshaft or just a kid 100# or under riding. An engine that just is not powerful but if used for light duty will last, I've sold a ton of the removable engine parts so someone is wearing em out pretty good. The sweet part is you can use that jackshaft on any engine with a standard 4 bolt PTO pattern. I have some sweet briggs engines in 3hp and 3.5 to 5hp it will bolt right up to. More importantly is that they work on a 6.5hp clone well when you are ready for the swap, making the most of it's power as well.:thumbsup:
JS application info
The JS plate is the same for all applications that have the 2.5 inch industry standard bolt hole pattern on the PTO side cover. This accessory bolt hole pattern is found on almost all 2-7 hp engines that have been manufactured by various companies for decades.
The plate will bolt up to a Honda,Predator,Clone,Briggs,Tec and many other engines as well.
Tip..
On some engines there is an extra oil fill plug in the rear of the PTO side cover that will hit the JS Bearing housing when you try to install it. No Worries..Simply tighten the plug good and trim the flat plastic thumb tab off the plug with a hack saw. You are not going to ever fill the engine oil there anyway as the front plug is more accessible.
Some engine brands also have the lower cylinder head tin protruding out around the bottom of the cylinder a little more than others. This can contact the bearing housing keeping the plate from moving all the way forward via the slotted holes. Simply install a couple of the JS/side cover mounting bolts and gently tap the plate assembly forward a few times. The tin is very soft and will push in slightly allowing you to have full travel while setting up the primary chain tension.
Over the past 3 years we have made a few slight changes in the plate allowing for more
adjustment in this area. The original versions of the plate still worked fine.
They are just now a little more refined.
The difference between the 97cc, 4hp , 6.5 Honda/Clone or Predator engine application is just sprocket arrangement and gearing. Also the Predator has 5/16 NF thread in the side cover so you need to use different mounting bolts. Almost all the other Honda and Clone engines have 8mm-1.25 threads in the side cover. You may also need to use machine shims to adjust the Clutch out a bit more on the quirky Predator, so the clutch will not to rub the JS mount bolts. The Predator PTO crank dimension is slightly different than all the other Honda or Clone engines.
We sell both 3/4 and 5/8 machine shims for primary chain and clutch alignment on the website. No rocket science degree needed, just use a straight edge/ruler and the shims to line up your primary and clutch sprockets. And you can always call me for help.
For the 97cc engine we have found that a good all around gear ratio is 8.8 to 1
11t clutch mounted sprocket inboard, 18t primary sprocket mounted inboard, 13t mounted outboard driving the rear chain.
(stock ratio is 6.36.( which as you know performs horribly, is extremely hard on the engine and clutch)
For the 4hp Lifan engine we sell
(This is a clone of Honda GX120, widely used in 1/4 Midget racing)
A great all around ratio is 7.59 to 1
12t clutch mounted sprocket outboard, 17t primary sprocket outboard on JS and 13t mounted inboard and driving the rear chain.
This little motor with the JS is my favorite all around rider, my go to bike for running around the pits at the racetrack, getting the mail or letting the grandkids take it for a spin. Great power balance for the bike. It does not have all that whoopaz torque right when the clutch engages that requires you to always lean forward expecting the front end to get light (as with the larger 6.5hp engines). Yet it is still a peppy, high reving, smooth and fun engine for the DB chassis when combined with the JS.
Our headers and performance air filters for the 6.5 work on this engine as well taking it into the 5hp range. (with these mods it likes around a .030 main jet)
For the 6.5 or Predator JS application.
12t clutch mounted sprocket outboard, 15t primary sprocket outboard and 13t inboard driving the rear chain. = 6.73 to 1 for high torque application.
(this is also a blast when combined with a modified high RPM engine for short coarse racing)
You can also swap the 15t and 13t sprocket positions on the JS resulting in a 5.03 to 1 ratio for high speed gearing. Caution is advised as the short wheel based DB does not handle well at high speeds. A pot hole in the road can send you into a what we used to call a "Tank Slapper". (uncontrollable bar shake/shimmy wreck and road rash) or worse...
The higher ratio in combination with a stock engine/governor however can tame down the tendency to wheelie although it is harder on the clutch.
To switch from the 97cc application to the 6.5.. just requires a shim kit, 15t 5/8 bore sprocket and different chain length. ( 5/16 NF bolts and an extra shim or two for the Predator.)
The JS plate is the same for all applications that have the 2.5 inch industry standard bolt hole pattern on the PTO side cover. This accessory bolt hole pattern is found on almost all 2-7 hp engines that have been manufactured by various companies for decades.
The plate will bolt up to a Honda,Predator,Clone,Briggs,Tec and many other engines as well.
Tip..
On some engines there is an extra oil fill plug in the rear of the PTO side cover that will hit the JS Bearing housing when you try to install it. No Worries..Simply tighten the plug good and trim the flat plastic thumb tab off the plug with a hack saw. You are not going to ever fill the engine oil there anyway as the front plug is more accessible.
Some engine brands also have the lower cylinder head tin protruding out around the bottom of the cylinder a little more than others. This can contact the bearing housing keeping the plate from moving all the way forward via the slotted holes. Simply install a couple of the JS/side cover mounting bolts and gently tap the plate assembly forward a few times. The tin is very soft and will push in slightly allowing you to have full travel while setting up the primary chain tension.
Over the past 3 years we have made a few slight changes in the plate allowing for more
adjustment in this area. The original versions of the plate still worked fine.
They are just now a little more refined.
The difference between the 97cc, 4hp , 6.5 Honda/Clone or Predator engine application is just sprocket arrangement and gearing. Also the Predator has 5/16 NF thread in the side cover so you need to use different mounting bolts. Almost all the other Honda and Clone engines have 8mm-1.25 threads in the side cover. You may also need to use machine shims to adjust the Clutch out a bit more on the quirky Predator, so the clutch will not to rub the JS mount bolts. The Predator PTO crank dimension is slightly different than all the other Honda or Clone engines.
We sell both 3/4 and 5/8 machine shims for primary chain and clutch alignment on the website. No rocket science degree needed, just use a straight edge/ruler and the shims to line up your primary and clutch sprockets. And you can always call me for help.
For the 97cc engine we have found that a good all around gear ratio is 8.8 to 1
11t clutch mounted sprocket inboard, 18t primary sprocket mounted inboard, 13t mounted outboard driving the rear chain.
(stock ratio is 6.36.( which as you know performs horribly, is extremely hard on the engine and clutch)
For the 4hp Lifan engine we sell
(This is a clone of Honda GX120, widely used in 1/4 Midget racing)
A great all around ratio is 7.59 to 1
12t clutch mounted sprocket outboard, 17t primary sprocket outboard on JS and 13t mounted inboard and driving the rear chain.
This little motor with the JS is my favorite all around rider, my go to bike for running around the pits at the racetrack, getting the mail or letting the grandkids take it for a spin. Great power balance for the bike. It does not have all that whoopaz torque right when the clutch engages that requires you to always lean forward expecting the front end to get light (as with the larger 6.5hp engines). Yet it is still a peppy, high reving, smooth and fun engine for the DB chassis when combined with the JS.
Our headers and performance air filters for the 6.5 work on this engine as well taking it into the 5hp range. (with these mods it likes around a .030 main jet)
For the 6.5 or Predator JS application.
12t clutch mounted sprocket outboard, 15t primary sprocket outboard and 13t inboard driving the rear chain. = 6.73 to 1 for high torque application.
(this is also a blast when combined with a modified high RPM engine for short coarse racing)
You can also swap the 15t and 13t sprocket positions on the JS resulting in a 5.03 to 1 ratio for high speed gearing. Caution is advised as the short wheel based DB does not handle well at high speeds. A pot hole in the road can send you into a what we used to call a "Tank Slapper". (uncontrollable bar shake/shimmy wreck and road rash) or worse...
The higher ratio in combination with a stock engine/governor however can tame down the tendency to wheelie although it is harder on the clutch.
To switch from the 97cc application to the 6.5.. just requires a shim kit, 15t 5/8 bore sprocket and different chain length. ( 5/16 NF bolts and an extra shim or two for the Predator.)
I turned my baja doodlebug into a reverse trike and installed a 6.5hp engine. I use a simple jackshaft and repositioned the chain tenisioner. It now has more than enough torque.
If you want to see the full details of the build, a walk through with pictures, go to
redbeardlabs.com
If you want to see the full details of the build, a walk through with pictures, go to
redbeardlabs.com