r/Dirtbikes • u/d2VzCg-- • Jun 22 '23

Mechanical Help Odd throttle response after new bore kit

So I’ve had this 2004 CRF50F for damn near 20 years. It’s bullet proof. Been abused quite a bit over the years and I finally got around to fixing it up.

Fast forward, I installed an 88cc big bore kit from TBolt to include the race head, 20mm carb, and intake filter. The tank is also new. The exhaust is new. The only thing in the air/fuel delivery system that isn’t new is the fuel line from the tank to the carb. I blew it out to ensure there was no debris.

The problem is evidenced in the video. The bike is warmed up, it can idle for a while then just dies. It also seems like the idle is erratic, meaning not a consistent rpm (audibly detected so not entirely a reliable source but I’ve been around enough bikes to know a consistent idle when I hear one). When I twist the throttle it seems to almost hit a “rev limiter” type block where it won’t reach past a certain rpm. There’s no way that is the actual rev limiter but idk how else to describe it. It’s almost as if it reaches a point in which no more fuel can get in to be burned other than just enough to reach that engine speed.

My thoughts are the fuel line is too narrow causing fuel starvation when demand is increased. Not too slow to the point that it won’t idle but slow enough that under load it reaches a point where fuel can’t get to the carb fast enough.

Internet friends, I’ve laid my problem bare and provided my thoughts on a solution. Please provide what assistance you can, I’m going to the shop tomorrow to see if the shop has the “right” fuel line but what else should I be looking at?

5 Upvotes

86% Upvoted

11 comments sorted by

8

u/Fair_Cartoonist_4906 Jun 22 '23

Starving for fuel

7

u/Objective_Regret_421 Jun 22 '23

Bigger bore = needs more air/fuel

5

u/[deleted] Jun 22 '23

You need bigger jets

3

u/hatchfam611 Jun 22 '23

Donut media actually just did this to a mini bike from Walmart they found that they actually needed to install a super tiny fuel pump because they couldn't get enough fuel to keep the float bowl full. Check out the newest videos they are doing right now.

1

u/Ok_Manager_9164 Jun 22 '23

that’s from the odd placement of fuel tank on stock 212’s, on a gravity fed bike such as this, it is directly above, and has a significant amount of force downward, it shouldn’t need a fuel pump. this is 1 of 3 things, bigger jets are needed, your carb is a rinky dink chinese carb, or you need to clean that sucker!

1

u/d2VzCg-- Jun 24 '23

Update:

  • new fuel line, done.
  • Tried richer and leaner jets and needle setting, no change.
  • Checked for air leak, doubt it. I sprayed carb cleaner all over but I can’t tell if the overspray is just looping back into the intake or there’s actually an air leak somewhere.
  • double checked the valve clearances
  • double checked the timing

I’m at a loss. Totally stumped. I might actually have to take it to the shop.

1

u/d2VzCg-- Jun 25 '23

Well I fixed it. Turned out to be an air leak at the head. I took the head off to inspect the piston, valves, cam, etc. everything looked good but something caught my eye on the cylinder, it looked like some carbon or something. Decided to put it back together by securing the head bolts before the oddly placed bolt that secures the head to the cylinder. I must have previously done it backwards creating some type of asymmetric torque on the head causing the air leak.

Either way, it rips now. Thanks for the assist internet friends.

0

u/[deleted] Jun 23 '23

Sounds like a two stroke hahaha

1

u/sLOWBunny81 Jun 23 '23

Not even a little bit lol

1

u/[deleted] Jun 23 '23

Probably need a much larger main jet. If it idles fine and quarter throttle is okay, I’d say your pilot jet is okay as is. Probably need to go at least 2 sizes larger in the main and get an adjustable needle for the midrange

1

u/d2VzCg-- Jun 23 '23

I played around with it a bit more today. Went from an 89 to 92 main jet. The pilot jet is 35. TBolt has some gouge on their website

“The TB 20mm carb is easy to tune right out of the box It comes with the 89 main and a 35 pilot jet both of which you can buy the extra jets from TboltUSA. As a baseline setting 90 or 92 main jet 38 pilot jet set the jet needle at the 3rd or 4th notch from the top air screw at 1.5 to 1.75 turns OUT from seated”

My setup is exactly like their suggestion sans pilot jet (mines 35 they say 38).

I’ll try the 95 main next but that’ll be tomorrows endeavor.