Twin Pocket Bike Engines Build

[5-7HEAVEN]

On an old thread, another member installed high-performance pocket bike engines on his ride. He had problems with "blowby", compression leaks between the cylinder and the engine block. To remedy this, he installed extra long 90mm cylinder bolts and screwed them deeper into the bolt holes. Along with thread locker, he even installed M5 nuts re-threaded to bolt onto the M6 threads behind the flywheel (There's no room for any sized nut behind the clutch, because of the interfering crank bolt heads).

I decided to do the solution one better and replace the head bolts on my engines with M6 studs, like these:
https://www.amazon.com/uxcell-100mm-...words=m6+100mm

While removing the 84mm head bolts, I noticed that two of them seemed slightly bent and rubbed the cylinder fins. So, before installing the studs, I took a few minutes to file down the top fins.

Problem solved, and the 100mm studs slipped in easily. No bending or rubbing the fins.

In fact, I double-nutted them and screwed the studs 10mm deeper past the bolt holes in the block (On the flywheel side, I managed to screw the studs in more than 10mm).
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I did this for practice on my spare engine, not the high-performance one in my twin-engine project.

On my unmodified pocket bike engine, I'll replace the 19mm cylinder bolts with 40mm studs.

 

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My son taught me how to post pictures.
These are random pictures of my twin-engine build:

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Today, I drilled my two 89-tooth chainring sprockets to match the freewheel.

They're made for #25(small) chain and are the same diameter as a 44t bicycle sprocket.

Instead of using Stainless steel(SS) discs as chainguards, I'm using plastic chainguards made for 44-tooth bicycle sprockets

They fit perfectly and save 1.5lbs off the bottom bracket.

I did fab two 5" SS discs as chainguards for my 24t x 3/32" bicycle sprocket (They add weight, but they're good insurance from chain jumping).

 

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I was having problems getting clearance for the exhaust on the top-side engine of my twin-engine build.

Installing one engine was relatively easy.
Installing that second engine on any frame created a whole slew of problems!

But it's worth it to me.

I don't want to notch the angle iron motor mount or the bike's frame (aka seat tube).

The seat tube and my "bolt-on" angle iron motor mounts were in the way of the head pipe.

eport this image

I could not find an easy way of installing an exhaust, except for using loud, low-performance canister mufflers.

While brainstorming last night, I turned this engine's circular engine mount 180 degrees and mounted the engine upside-down.

Doing so would allow easy access to the exhaust port.
However, it would raise other issues, maybe with the carb.
And the pipe would be in my lap.

Then, I flipped the engine mount over and hung it from the top tube.

)!

Now the top engine is right-side up, and will be bolted onto the top tube.

The beauty of this new engine location is that I'll have 5" clearance for the top engine's exhaust port!

And the engine does not have to be flipped upside down and cause many issues.

Either this HT pipe needs bending, or I'll fit in two SBP expansion pipes.
One for each engine.

Right now, I'm unable to attach the sparkplug to its wire. When I lower the top engine 1" or less, I won't have a sparkplug clearance issue.

The top engine's chain still aligns perfectly with its outer 89-tooth sprocket on the bike's pedal/jackshaft directly below.

 

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Well, I thought it'd be more pleasing to the eye if I could position the engines in the form of a "Vee", inside the bicycle mid-frame.

To look like a V-twin engine.

Both engines would be mounted to form a 60-degree angle. The new carburetor locations would be inverted, at the bottom of the "VEE".

It'd require different carburetors, since the stock ones won't work in those positions.

The first things to find were aluminum manifolds to mount aftermarket carbs.

Which I located and purchased.

Next would be finding carbs which would work upside-down.

I remember mounting my CY460 engine upside-down on my Diamondback bike.
It had a 19mm Walbro 223H carb.

Too much carb for my 16mm manifolds.

So I found Walbro HD-48 carbs, which have adjustable High/Low jets.

Perfect for my upside-down pocketbike engines.

Now it was time to trial-mount the engines into a "Vee".

Both exhaust ports had excellent positions to rig up two SBP expansion chambers.

After several hours of manipulation, I realized it'd take a massive effort to fabricate motor mounts to position the Cag engines in a V-twin position.....

and line up with both chainring sprockets.

So it's back to the horizontal "sandwich" position.

They align well with the chainrings in this position.

At least I'll be using my new Walbro adjustable carbs I'd found.

 

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I had a spare genuine Walbro carb and manifold.....

so I bought these:
http://www.ebay.com/itm/Carburetor-...325388?hash=item238bfacf0c:g:7QAAAOSwX61ZDJcI

and these:
http://www.ebay.com/itm/Mini-Pocket...m564e793e31:m:meLttxscXsWJvZhdUiYk1DQ&vxp=mtr

This vendor's price was too high,
so I bought 3 of the exact same Walbro-style carbs from another seller for $18.49 each.

 

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With the engines in their normal mounting positions, the exhaust head pipes hit the angle iron that
I'm using for horizontal motor mounts.

Maybe I'll add 1" spacers or find another one of these mounts:
http://scooterpartsrus.com/X1_X2_engine.htm

With engines mounted in upside-down positions,
the exhaust head pipes clear magnificently.

I just need to route the top engine's
head pipe lower. away from my thighs.

The SBP expansion chambers I'm using
should fit well.

 

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Another engineering design change.....

To save reciprocating weight, I had chosen #25 chain & sprockets.

When I ordered the 89-tooth pocket bike sprockets from ebay, the picture illustrated trim, lightened steel discs.

The difference in size and weight between the #25/15t gears and the T8F/11t sprockets were negligible.

What arrived in the mail were heavy, solid steel discs, not unlike the larger 72-tooth sprockets on my shelf.

So I'm back using the tried-and-true T8F 72t/11t combination.

I've used T8F drivetrains in my other mb's
and never busted a chain.

 

[MEASURE TWICE]

Nice to see the work as it is progressing! The frequency beat of twin engines, hum hummmm hum hummmm... haha!

 

[5-7HEAVEN]

Thanks for asking, MEASURE TWICE.

I've had a twin-engine mb before.

Two things I'll always remember is the additional torque that pushes you uphill and/or punches through the headwinds.

Another thing I enjoyed was the humming drone of two engines at low, middle and cruising speed.

At one time, I had friction drive on my front tire and rear chain drive.
Both engines were identical Mitsubishi TLE43cc powerplants with
ADA C1 expansion pipes.

The friction drive was geared lower than the chain drive, unintentionally.
If I used only the front engine from a standing start,
then switched to the rear engine(only) at midrange,
it sounded and felt EXACTLY like shifting gears on a
2-speed Powerglide transmission on an old Chevy.

That was very cool.

 

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The most labor-intense and spatially challenging issues
in this pastime has been positioning two engines in
my cruiser bike's center triangle.

In retrospect, hanging a single engine in the mid frame is relatively easy.
Mount the bottom bracket, align the sprockets, make room for the
exhaust pipe and carb, fab your motor mounts and you're almost there.

Mounting two engines in a small space is like I would imagine to be keeping two mules in a stall intended for one animal.

 

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Building the bottom bracket(BB) assembly took a little brainstorming, but worked out well.
Fortunately, there were many BB units in varying lengths available.
There is an abundance of sprockets to choose from.
Each engine has its own driven chain ring sprocket,
appropriately spaced away from two other chain rings.
There are two guards per sprocket. The 24t sprocket has stainless steel ones,
and the larger gears have plastic ones.

Besides being guards, they act as chain ring guides,
to prevent chain jumping from engines and bicycle chains.

Both 72t chain ring sprockets are linked to a single freewheel,
which is mounted on the outboard chain ring.

Five M5 studs, 60mm in length, with nuts,
washers and nylox nuts keep everything together.

The studs must be perfectly trimmed to accept
the round-headed nylox nuts near the 24t chain ring.
If the bicycle chain jumps off and lands between
the sprocket and bottom bracket, it likes to
grab onto the protruding studs and wreak havoc to
the derailleur and cassette.

Ask me how I know this.

 

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Calculating the gear ratio was a no-brainer.
Since pb engines like to scream,
I simply used the same ratios as I do
on my mb with the Tanaka 47R racing engine.

Let's see:
72t driven/11t drive = 6.5454545:1
5:1 tranny x 6.5454545 = 32.727272:1
Rear 34t cassette/24t chain ring = 1.416666:1
32.727272 x 1.416666 = 46.3636:1
Comparing it to a motorized bike with China Girl's(CG) gearing:
CG gearing = 4.2:1.

46.3636/4.2 = 11.03896

So my first gear is akin to having a
China Girl engine with 110-tooth rear sprocket.

That will climb trees.

Fifth gear on the 8-speed cassette is 11-tooth.

11t/24t chainring = .458333:1
.4583333 x 32.727272 = 15:1 gear ratio.

15/4.2 = 3.5714

That's like a 36t rear wheel sprocket.

Awesome versatility!

 

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Each pb engine runs independent of the other engine.
The only things they share are the gas tank,
the BB axle and freewheel, 24-tooth chain ring,
bicycle chain and the rear cassette.

If one engine fails, its centrifugal clutch will
disconnect its link to the rest of the drivetrain.

If one clutch fails and locks up,
then that engine's chain is removed.

The bike can "limp home" on
the second engine.

 

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Thankfully, pocket bike(pb) engines are compact and relatively flat.
They're also modular and versatile.

When bolted to the front of the pb engine's block, their 5:1 transmissions can be mounted in 4 different positions,
90 degrees apart.

The 4 mounting holes for the tranny can also be used as engine mounts. It can be hung outside the transmission, ala Scooterguy.

A motor mount can also be sandwiched between transmission and engine block,
then bolted down below the engine.

 

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There are also tons of aftermarket manifolds and cheap carburetors available for pb engines. Some permit these engines to operate in any position, as horizontal, vertical,
upside-down and anywhere in between. Those have low and high speed adjustments, which is fabulous.

 

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I've realized that one of the most difficult issues in this build is
positioning both engines so that its exhaust systems can be fabbed in a sensible, efficient manner that is pleasing to the eye.

The easiest, but loudest exhaust system would've been OEM
canister systems used in almost all small engines.

Pipes designed for pocket bikes might've optimized the pb engines' potential(or not).

Too much work involved, especially since routing
the long pipes in an eye-pleasing manner has to be considered.

I was very satisfied using an SBP expansion chamber with my
loud CY460 engine. It made it so quiet, I swear
I could hear the piston ring scraping its cylinder at idle speed.

After countless hours of working the puzzle, I believe SBP pipes mounted just below and to the side of the rack-mounted tank would be symmetrical and out of the way of the rider's legs.

On with the build.

 

[MEASURE TWICE]

About fitting engine, I needed to go with short spark plug. The tilt of the engine by about 15 degrees back and the bike weighing 130 lbs, I found changing oil without removing engine or tilting the whole bike, I use a hand pump. I got a better slightly newer 3.5 Briggs and it was slightly taller, but managed to fit it and the spark plug boot outside actually touches the frame, but does not short out.

 

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Well, I'm still thinking about hanging
the Cag engines in a "V" position.

I just MIGHT have the correct design this time!

On to the build.

 

[5-7HEAVEN]

About fitting engine, I needed to go with short spark plug. The tilt of the engine by about 15 degrees back and the bike weighing 130 lbs, I found changing oil without removing engine or tilting the whole bike, I use a hand pump. I got a better slightly newer 3.5 Briggs and it was slightly taller, but managed to fit it and the spark plug boot outside actually touches the frame, but does not short out.

That's a tight fit.

How heavy is the engine?