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Discussion Starter #1
So I recently picked up a used 2015 for $2000 with 511 miles on the odometer. I looked for a while till this gem popped up and drove about 165 miles each way to pick it up. It had a Vagabond fender eliminator kit, TST front flush mounts/electronic flasher relay, and some cheap Aprilia style mirrors. Besides those mods it was completely stock, hadn't even had the first service done. My plan for the Grom is to build it up to be a bike I can ride to my local Kart track, spend a few hours there, and ride back. The track is about a 10 minute ride from my house, so its super convenient. I can pack some snacks/drinks, a few tools, mini tire compressor, camping chair in a backpack, wear my track gear over there, remove the headlight/taillight/tag and be good to ride.

It passed inspection without issue and I got the reg/tags/title/insurance all sorted out. The first thing I did was swap out the stock Vee Rubber tires for a pair of Pirelli Diablo scooter tires. I hadn't found many reviews of these tires but I figured I'd give them a shot. The literature on them makes it sound like they are just a downsized version of the Diablo Rosso Corsa 3 tires.





Front tire profile


Rear tire profile


These things are miles better than the stock Vee Rubber which felt like I was riding on some Lego tires. The profile on the tires makes the Grom turn in really fast, and they hold a line pretty damn good. I feel like I can get on the brakes without fear of the Vee Rubber tire just washing out. It's a very confidence inspiring tire. I'll be able to give more feedback on the tires once I get some more miles on them, but initial impressions are very good.

I wanted some lower and wider bars to be able to get down and off to the inside of the bike, and get a bit more weight on the front end. I ordered up a set of the ProTaper Honda Mini bars as they are lower/wider/and should hold up to a crash way better with the center brace. I topped them off with a pair of the Domino MotoGP grips.



I went simple on the footpegs and went with a set of PSR's. I may go with a set of Driven or Woodcraft rearsets in the future, but these made a huge improvement in grip, which was my major gripe with the stock footpegs.



Next on my list for track prep was getting some real suspension on the bike. I'm kind of a suspension snob and need a really tuneable suspension and have it setup correctly. I also ride an R6 with a full RaceTech setup and it's amazing. After doing lots of research I went with the Racing Bros fork kit and shock simply because of all the adjustability. For most people the Ohlins will probably be suitable as an upgrade in the forks, but I think it would drive me crazy not being able to tune it. The Racing Bros shock only adds a high speed compression damping circuit over the Ohlins, and its a good deal cheaper too.









Fork tubes clamped in the vice using the Racing Bros foork tool.


Heating up those fork lowers to soften up the threadlocker.


Nasty crud between the lower and tube.


Cleaned the crud off.


Removing the spring cap off fork tube.



Done installing the fork kit.


Rear shock is a breeze to install.

The suspension REALLY made a big difference to the Grom. Tires were a big jump, the forks and shock were a leap. The suspension doesn't just bottom out with me bouncing on it. With stock suspension at pace if you hit a mid corner bump, I felt like I was getting bounced off of my line. Now the suspension just moves and bumps barely upset the chassis. Under hard braking I could pretty much get the fork to almost bottom out, now there is actually support under hard braking, and I'm not afraid of bottom out, as we all know once the fork is bottomed out, the tire is the next thing to slide. This Grom is just a pleasure to ride around town, or in anger on the kart track. Next time when I service the forks I will use 10w oil as I'm close to the end of the adjustments on the forks using the recommended 7w oil.

While I had the rear cowling off to install the rear shock, I went ahead and installed the new CustomLED BlasterX tail light and Targa fender eliminator kit. I know this is an aesthetic mod, but for me, it was more of a quick change setup for the track. Two nuts off the back of the tail light and the plug and it comes out. Two nuts and bolts for the tag and it comes off, and we all know how easily the headlight comes off.



Part 2 in the next post.
 

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Discussion Starter #4 (Edited)
After the suspension was done I spent some time getting a really good feel for it, tuning it and really getting it dialed in how I like it. I really wanted to see if there was a noticeable difference for when I swapped out the rubber swingarm bushings for the MNNTHBX bronze bushings. The install was straight forward, it took me a little time to figure out a simple way to get the rear wheel off the ground NOT using a rear stand. I basically just left the bike on the side stand, and leveraged the back wheel off the ground, and shoved a jackstand under the right side of the frame where the lower rear set bolt is. The installation with the bushing tool is cake, just follow the YouTube video instructions.








I left the rear shock settings as I had used them on the stock bushings to see if there would be any difference in setup. Right off the bat I noticed how stiff the suspension felt and there was barely any sag with me sitting on it. This told me how much the stock bushings were causing the swingarm to bind and not want to move at all. Previously I had about an 35mm of sag setup and I was barely getting 10mm after the bushings. After taking about 2 full turns out of the preload adjuster, I got it back to around 35mm of sag again. The rebound and compression were also way stiff. The rear suspension definitely feels livelier and more active vs. the stock swingarm bushings. After spending some time tuning the Hi/Lo speed compression I was able to get it feeling plush on the harder hits like on potholes and bumpier roads, while be very supportive while cornering hard. These bushings are a "must do" mod IMO if you want the best suspension action out of your Grom.

As you can see, I removed the stock exhaust when the swingarm came out for the MNNTHBX bushing install. Well I wasn't just going to put that heavy ass stock exhaust back on after I just took it off.




Not the greatest scale, but it says the stock system weighs 11lbs.



And the Yoshimura system weighs 5lbs.




I love the sound this thing makes without the baffle in place, the baffle just makes it sound airy and farty, not so much quieter. I have a few videos of the exhaust, but there are so many Yoshimura exhaust videos on YouTube, I don't think we need anymore. The top end gain in power was noticeable, you can feel the power build a bit more and then kind of levels off vs. just falling right on its face after 6k rpm.

I finally got around to safety wiring my oil fill cap and oil drain bolt for tech at the local track, they let it go the first few times, so I wanted to make I got it done before the next time I went. I installed the PSR oil fill cap and a generic magnetic oil drain plug. I did the first service on the bike cleaned a ton of crap out of the oil spinner and mesh screen. Checked the Valve clearances and they were on the tight end of spec, I'll check them again in about 700 miles when I plan on doing the next oil change. While I had the clutch cover bolts out, I also installed the TST Industries frame slider kit. It's a robust piece of kit and it bolts together perfectly, just follow the YouTube video tutorial and you wont go wrong. hopefully I wont have to test them lol.






The next thing I wanted to address was the terrible headlight, I did a swap to a SF regulator/rectifier unit and H4 LED bulb. Made a huge difference in light output with no hit to the electrical system. I did a write up in the "How to guide" section of the forum for those interested in how to cleanly install the SF R/R without making a bracket AND still keep the rear brake reservoir in the stock location. The high beam indicator on the dash still works too.







I'm currently working on the beginning stages of a stock airbox intake mod. I don't like any of the intake options on the market right now with the fairing rod support setup, not a fan of it at all. I took elements from a bunch of different airbox mods I had seen online. I don't think anyone has done an airbox mod quite like this. Once I get all the parts and figure out how it will all go together I'll post up about it. IMO when installing any intake system, that gets rid of the factory throttle body inlet tube, should be what I call velocity porting the throttle body. Using the stock throttle body without porting causes a ton of turbulence in the intake tract. I bought a used stock throttle body on eBay and did the port job on it. I started with a carbide single cut bit, a bastard cut file, then finished off with 180 grit sand paper. I may go smoother on the finish but not sure if the effort is worth the gain.




 

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Discussion Starter #5

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I'm currently working on the beginning stages of a stock airbox intake mod. I don't like any of the intake options on the market right now with the fairing rod support setup, not a fan of it at all. I took elements from a bunch of different airbox mods I had seen online. I don't think anyone has done an airbox mod quite like this. Once I get all the parts and figure out how it will all go together I'll post up about it. IMO when installing any intake system, that gets rid of the factory throttle body inlet tube, should be what I call velocity porting the throttle body. Using the stock throttle body without porting causes a ton of turbulence in the intake tract. I bought a used stock throttle body on eBay and did the port job on it. I started with a carbide single cut bit, a bastard cut file, then finished off with 180 grit sand paper. I may go smoother on the finish but not sure if the effort is worth the gain.




Lookn good man, You are definitley off to a good start... Awseome porting of your TB, dont forget the intake manifold and isolator. A big fan of DIY airbox mods, cant wait to see what you've come up with
 

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Discussion Starter #7
Lookn good man, You are definitley off to a good start... Awseome porting of your TB, dont forget the intake manifold and isolator. A big fan of DIY airbox mods, cant wait to see what you've come up with
The stock manifold entry and the throttle body exit look to be pretty close in diameter, if anything, the manifold is a tad larger. The manifold exit is definitely way smaller than the isolator. I haven't measured the cylinder head intake port but that seems to also be really small. I have a used head on the way that I am going to have some light port work done to, mostly just port matching the entire intake and exhaust tract. It will likely get a valvetrain upgrade and definitely a camshaft upgrade. But I'm not looking to go any further on the engine than the top end at the moment. Looking at brake upgrades at the moment, thinking about the Brembo p2 34 front caliper upgrade and just doing a line and pad for the rear.
 

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The stock manifold entry and the throttle body exit look to be pretty close in diameter, if anything, the manifold is a tad larger. The manifold exit is definitely way smaller than the isolator. I haven't measured the cylinder head intake port but that seems to also be really small. I have a used head on the way that I am going to have some light port work done to, mostly just port matching the entire intake and exhaust tract. It will likely get a valvetrain upgrade and definitely a camshaft upgrade. But I'm not looking to go any further on the engine than the top end at the moment. Looking at brake upgrades at the moment, thinking about the Brembo p2 34 front caliper upgrade and just doing a line and pad for the rear.
Carpimoto is still running their sale on P4 calipers

https://www.carpimoto.com/en-US/42408_20516589-Brembo-P4-30.htm

Stock master works very well with this caliper. Night and day leap in performance.
 

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Discussion Starter #9
Carpimoto is still running their sale on P4 calipers

https://www.carpimoto.com/en-US/42408_20516589-Brembo-P4-30.htm

Stock master works very well with this caliper. Night and day leap in performance.
I thought about the P4 calipers, but the fact you need to remove the rotors from the wheel every time you want to remove the wheel or calipers was a huge turn off for me. I'm thinking the P2-34 will be a great upgrade long with a SS line and the better pad compound in the P2. I'm not really big on using the rear brake much during normal riding. I use it mostly when braking really hard to settle the chassis. I'm thinking a line and pad upgrade will suffice.
 

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Discussion Starter #10 (Edited)
Spent a few hours at the local kart track today and brought along my GoPro. I need to record more when on track as I can see where i'm taking shit lines and where I can improve.




Shredded these tires pretty good on the track


 

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Discussion Starter #11
I'm still waiting on one last part for my airbox mod, USPS says it will be delivered tomorrow. I got that spare head in on Saturday and eyeballed up the intake tract between the throttle body, manifold, insulator, and head.

The throttle body transition to the manifold is pretty decent, the throttle body has a slightly smaller diameter than the manifold so the flow of air is relatively smooth. The exit of the manifold seems to neck down and is actually smaller than the insulator. I decided to do a little port work there to open up the exit and get smooth flow out of the manifold to the insulator.

You can see the large step from the manifold to the insulator.



Where I scribed how much larger the port needs to be opened up to match the insulator.


After porting.





The Insulator matches the manifold exit.




The insulator matches the intake port in the head OK, it's not great, but better than the intake manifold exit does. I'm gonna leave the intake port on the head alone for the time being.


I think when you add an intake, velocity port the throttle body, and the manifold exit is matched to the insulator it has a pretty good path for airflow without going further and pulling the head. Doing these small things to the intake tract probably don't add much power if any, but it cant hurt to do them. The spare head I have will likely be sent out in the near future for porting
 

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I thought about the P4 calipers, but the fact you need to remove the rotors from the wheel every time you want to remove the wheel or calipers was a huge turn off for me. I'm thinking the P2-34 will be a great upgrade long with a SS line and the better pad compound in the P2. I'm not really big on using the rear brake much during normal riding. I use it mostly when braking really hard to settle the chassis. I'm thinking a line and pad upgrade will suffice.
What forces you to remove the rotor from the wheel before removing the wheel from the bike?
 

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Discussion Starter #13
What forces you to remove the rotor from the wheel before removing the wheel from the bike?
It's been noted that the Brembo P4 30/34 40mm calipers are just so physically large that you can't fit the caliper over the rotor with the rotor attached to the wheel.
 

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That rear tire looks like the rebound on the shock is too slow, and the pressure was too low. Too much heat on the back of the sipes and the edge. Looks like a fun track.

Sent from my Pixel 2 XL using Tapatalk
 

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Thanks for the info. That would be a royal PITA to deal with.
Its truly not that bad, but annoying for sure. 3 bolts off the rotor to allow it to rotate, 2 bolts for the caliper, and remove.
 

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Discussion Starter #17 (Edited)
I got the parts needed for my airbox mod in and had some free time to mess with it. I like to think it's a cross between the old radiator hose mod, and Brock's Performance velocity stack.

Everyone knows what the inside of the factory airbox looks like with the entire inlet pipe, IAT sensor, and filter removed.


I cut these two nubs off.


Eyeballing up the placement of this velocity stack.


Cut the hole.




Stick the velocity stack in the hole. I had to clearance the edge of the stack as I wanted it closer to that one side.




Use some 5 minute two part epoxy and glue that velocity stack into the airbox.







The silicon hose is ready to go once the epoxy is fully cured. It should require a bit of trimming to have the best fitment, but it should be pretty damn close.



I have a standard run of the mill BMC drop in filter for the airbox once everything is done. The filter clears the velocity stack by about a half inch so no worries there.
 

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This looks interesting, sadly i just ordered a stock air box cause the class i want to race in doesn't allow air box mods.
 

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Discussion Starter #19
I swapped over the ported throttle body and intake manifold, and then test fitted the modified airbox.






The fitment was pretty spot on, I didn't even need to cut the hose at all. The hose fits really tight on the velocity stack, but I'm still going to throw a hose clamp over it when I get over to the parts store. The intake sounds a bit louder, but its not crazy different. I can definitely hear every time the intake valve opens while idling. I left the airbox snorkel off, but I may experiment with it. I have a suspicion that the snorkel will give better low and midrange performance because of the super long intake tract.
 

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Discussion Starter #20
So I tried my hand at a club race over the weekend and had a blast. I'm definitely gonna be doing more racing next season as sadly it was their last scheduled race of this season.


I did some more mods to the Grom right before race day and I'll do a proper post for the build thread in the next few days. I also had a lowside crash during a heat race, but the bike held up really well with the TST crash protection I had in place. I was really impressed with how well it crashed.
 
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