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Premium Member
941 Posts
Discussion Starter · #1 ·
So this post is not mean to be "Gospel"

But, it is meant to help some of you understand A/F, BBK, Heat....etc..

If you have read this forum enough, then you know the Grom Runs Lean........ approx. 14.7 A/F

That's ok for a bone stock motor....... But it also causes a lot of heat.

As soon as you install an intake and Exhaust, the bike seems to run leaner. So much so that some riders have actually burned up their stock piston. Yes, we know many are riding around just fine ...... so obviously there are many other conditions that come into play.

Gas being used
Spark Plug being Used
Air Temp
Riding Conditions
Throttle input.. (WOT All the time... or just cruising).....etc..

That being said, this post is meant to explain why STOCK Setup, just doesn't work with a BBK.

BBK are performance parts.
They are NOT Meant to just "Bolt on and go"

Yes..... you can do that. But eventually..... your going to burn up your piston.

What happens... ?

Normally....... the Piston over heats because it's not getting enough fuel, OR.. your getting knock (too much ign. adv) , which usually causes overheating. Or very high Compression ratio... which also causes heat.

Then the Piston Swells

Then the rings fail.

Then there is no oil on the side walls, and you get ridiculous amounts of heat which makes the piston fail.

NOW...... this of course, is assuming the Piston is CAST Alum (which most BBK are).

FORGED Pistons can handle a lot more heat and tend not to fail as often.

That doesn't mean ... Just put a forged piston in and forget it.

They will also fail eventually.

............... That leads us to the next ways to prevent this. (There is no guarantee...... it's just Helpful tips)

1) Install your Motor Properly. EXACTLY As the directions state.... or watch the Videos... a few times. Yes, it's boring. But it helps.
We know the Videos make it look easy. And it is......... When you know what you are doing, and you follow the steps.
But, if you skip steps... like Clocking the rings, or using Assembly Lube, or if you use METAL objects to install parts and scar the cylinder, or bearings... or any of 100 other things, you can do wrong................ then your BBK doesn't have a chance. Take your time, make sure you do it right.
Or have someone help you that knows what he is doing. (NOT someone that "Thinks" he knows what he is doing.. )

2) Clean your Filter screen and Spinner

You have to drain your oil anyways to install a BBK... While you have the Oil out, you might as well clean the spinner and Screen to ensure no crap is floating around in the oil.

3) RUN a proper Fuel Controller and PROPER A/F Ratio. (most important is
in the 80 TP @ 12.2 & 100% @ 11.8 A/F Ballpark )
A Fuel Controller that can adjust the A/F Ratio to a much cooler running 11.8-12.2 A/F (preferably one that has a map that you can see and adjust)

4) RUN a Air/Fuel Monitor (recommended) ...... this will tell you what A/F you are running so you can be sure it's in the safer zone.
Without this....... you are hit or miss. Yea, a Dyno can sort of tell you...... in a Static room. But real world, and Dyno are not always the same. And Sharing your buddies BBK map from 2000 miles away with a totally different setup......... NOT a Good idea.

5) RUN Cooler Spark plug... and REMOVE the "P" from your choice... ie CR8E instead of CPR8E
STOCK plugs are NGK CPR6E or CPR7E ...... it's recommended to switch to cooler running plug..... at least a CPR8E or even CPR9E. In the Finbro Kit, the compression is soo high you actually have to switch to a CR9E and add 2 washers to back it out a little. Spark Plugs are cheap.. like $4-$5
What is P stand for ?
P stands for Protruded insulator.
In a High Compression motor, you don't want a protruded Insulator. it just causes problems. Better to get the NON P Version of the plug.

RUN Premium Fuel or higher High Compression Motors need higher octane Fuel to reduce knocking.

7) Remove any Ignition advance you have.Many maps out there for STOCK Motors have 4-6^ of Ign. advance built in.... to help increase power. That's fine for a Stock motor.
But, it's not needed for a BBK, and it can cause overheating, which hurts your motor.

8) Break in your Motor Properly
Pretend it's a brand new motor....... because it basically is. As hard as it is to do this....... Take it easy for the first few hundred miles.
We know it's soo tempting to just go out and Dog the crap out of your new BBK, or take it to the Dyno the same day you installed it.
Don't do that.
Most important, do NOT just go ride down the interstate. if you want your Piston rings to seat correctly, you need to put load on the
motor, in small spurts. And you need to vary the RPM and TP. Best is to ride around town in traffic. (Yea, it sucks.. but it's best way to
break in your motor)...... Preferably.. NOT Sitting for long periods at idle at the stop light.

9) Once you have broken in your motor, Change the Oil to full synthetic, and CLEAN the Spinner, and Screen.
Give your motor a fighting chance, Synthetic oil lasts longer, doesn't break down as easy, less friction....etc. And CLEAN the Filter Screen and spinner to make sure no crap build up is in there.

10) Install a Digital Temp Gauge.
Cheap Insurance. For only a little over $80 ... you can monitor your Motor/Head Temps. You can even set the Warning to flash at a pre-determined Temp. This will at least give you a chance to shut it down, BEFORE something bad happens. That way you can say.. "Hmm.. why is my motor overheating.." and go through the list above to see if there is something you overlooked or forgot, or a change you can make, BEFORE something bad happens.

Again there is no guarantees..... Sometimes these BBK just fail. Bad Gas, overheating, Knocking....etc....etc

But using these guide lines will at least increase the odds you have a happy Experience with your BBK.

Remember, There are thousands of guys out there running BBK, and loving them.

Yea...... sometimes one fails. But, often... not always... it's something that could possibly have been prevented.

Here is a Great Link for more details on PISTON, Rings gap:

JE recommends

.004 ring gap per inch of Cyl. Bore

So example on the finbro 183cc BBK which has a 63.5mm (2.5") bore

Your Upper Ring Gap would be 10 thousands (.010)

Lower Ring Gap = 12 thousands (.012)

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Premium Member
941 Posts
Discussion Starter · #2 ·
One little extra bit.

REMOVE the "P" from the Spark plug of your choice... ie CR8E instead of CPR8E

What is P stand for ?
P stands for Protruded insulator.
In a High Compression motor, you don't want a protruded Insulator. it just causes problems. Better to get the NON P Version of the plug.

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