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Old 03-07-2018, 06:54 AM   #1
BuckNut
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XR-70 88cc Big Bore Project

Our last XR-70 was left completely stock. The last TMGP round my nephew decided to go through the race school and throw it around for fun. Well, he finished dead last in each of his races.

He stated the bike was extremely easy to ride. He put it in 2nd and pinned the throttle.

When we got home I took the bike around the neighborhood and noticed the bike was just tired and worn out.

What better way to bond than with installing a big bore kit?

The oil pump mod was more work than planned, but Chad suggested it needed done.

We are waiting for a new intake to accommodate the bigger carburetor.

Jettech suggested some porting and polishing of carbs, intake and to open up the exhaust.

We will also play around with some smaller front sprockets.















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Old 03-07-2018, 08:59 AM   #2
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Which kit did you go with?
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Old 03-07-2018, 09:09 AM   #3
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Which kit did you go with?
ABC Motorsports.

$124 bucks.

Had to buy the highflow oil pump and upsized intake separately.

This was the bike I was going to let your son ride when you get sorted. Iíve got some leathers to lend!
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Old 03-07-2018, 09:18 AM   #4
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Yeah we are gonna get out there when my schedule frees up. You gonna have to upgrade the carb or just re-jet?
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Old 03-07-2018, 09:31 AM   #5
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We need more of these threads.

Good stuff.
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Old 03-07-2018, 09:41 AM   #6
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Iím upsizing the carb to a 19mm Mikuni, hence why Iím awaiting the intake.

TBH Iím completely ignorant about the jetting. After we do the first round of exhaust and carb/intake polishing we will look into jetting.

Everything combined with the gearing change, we are hoping to get closer to the Groms and 110ís.

Cypress kids are on spring break next week. Planning on another test n tune Monday. Bring your son out, or we can pick him up. Id suggest first track day should be done without you. Kids more likely to listen and learn. (Iíve learned my lesson being overbearing)
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Old 03-07-2018, 10:25 AM   #7
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Quote:
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Iím upsizing the carb to a 19mm Mikuni, hence why Iím awaiting the intake.

TBH Iím completely ignorant about the jetting. After we do the first round of exhaust and carb/intake polishing we will look into jetting.

Everything combined with the gearing change, we are hoping to get closer to the Groms and 110ís.

Cypress kids are on spring break next week. Planning on another test n tune Monday. Bring your son out, or we can pick him up. Id suggest first track day should be done without you. Kids more likely to listen and learn. (Iíve learned my lesson being overbearing)
Doubt Momma would be down with that. My son doesnt need a whole lot of instruction, hes been riding concrete and dirt for 6 years and hes only 8. I think seat time and a little body position, line instruction and braking technique would get him going. Its all for fun anyways right?
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Old 03-07-2018, 10:38 AM   #8
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Doubt Momma would be down with that. My son doesnt need a whole lot of instruction, hes been riding concrete and dirt for 6 years and hes only 8. I think seat time and a little body position, line instruction and braking technique would get him going. Its all for fun anyways right?
Oh itís fun! Those lil XRís arenít winning anything. Teaching kids ďlinesĒ has been challenging because they often donít see the big picture. Iíve often put down blue tape and chalk marks on the track to get in the general race line.

Kids tend to learn more chasing faster bikes and their peers or other riders giving them hints and tows.
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Old 03-07-2018, 11:44 AM   #9
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I went with a 125cc in an xr50. I figured by the time I went big bore + trans/clutch I'd be pretty deep. Thus, I swapped in a whole motor assembly.
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Old 03-07-2018, 12:37 PM   #10
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I need another 110. I regret selling mine with every bone in my body. Stupidest thing I've ever done. The xr70 was my first bike as a kid and I really wouldn't mind another.
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Old 03-07-2018, 12:47 PM   #11
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I need another 110. I regret selling mine with every bone in my body. Stupidest thing I've ever done. The xr70 was my first bike as a kid and I really wouldn't mind another.
You need a 110.

That lil XR has a hard time pulling my girth. Plus they are totally uncompetitive. Although itís fun hounding the bigger bikes through the corners.

But if you want a project Iíll throw you one for $300 with half an engine.
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Old 03-07-2018, 12:50 PM   #12
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Quote:
Originally Posted by BuckNut View Post
You need a 110.

That lil XR has a hard time pulling my girth. Plus they are totally uncompetitive. Although itís fun hounding the bigger bikes through the corners.

But if you want a project Iíll throw you one for $300 with half an engine.
Pm me some details!!!!
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Old 03-07-2018, 06:34 PM   #13
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Pm me some details!!!!
$300 firm, we will use the money to complete 12Ē rims for my nephews CRF-100.

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Old 03-07-2018, 07:45 PM   #14
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Itís alive! And way stronger than before. Appears Iíve might of missed some HP. The 88cc big bore requires an upgraded CDI box for max performance. Two CDI kits on order!

#gromsgoingdown



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Old 03-07-2018, 08:24 PM   #15
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I went with a 125cc in an xr50. I figured by the time I went big bore + trans/clutch I'd be pretty deep. Thus, I swapped in a whole motor assembly.
My nephew has mentioned this. Apparently the 125 requires a clutch, right?

XR-70 perfect size to learn clutches!
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Old 03-08-2018, 12:54 AM   #16
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throw a big cam on there!
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Old 03-08-2018, 08:26 AM   #17
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throw a big cam on there!
Iím already at the extreme of my mechanical ability.

A big cam would require splitting the case, right?

I wonder what the performance gain would be. Slowly edging closer to the Groms is a very motivating factor. Ha
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Old 03-08-2018, 08:40 AM   #18
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Nah, cam was in the top end you just rebuilt but would require re-setting valves which means replacing valve seats and checking valve clearance, basic head work. Its not terribly difficult but probably wouldn't see much difference in HP
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Old 03-08-2018, 11:24 AM   #19
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Quote:
Originally Posted by BuckNut View Post
My nephew has mentioned this. Apparently the 125 requires a clutch, right?

XR-70 perfect size to learn clutches!
You can find a 125 with an auto clutch. In my case I came across a deal on a complete motor that I couldn't pass up. It'll do ~50+mph with 10" street tires.
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Old 03-08-2018, 11:30 AM   #20
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Quote:
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Itís alive! And way stronger than before. Appears Iíve might of missed some HP. The 88cc big bore requires an upgraded CDI box for max performance. Two CDI kits on order!

#gromsgoingdown




Canít believe I never thought of direct 5 hour energy injection. Genius
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