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This topic contains 33 replies, has 11 voices, and was last updated by  Stephen gleason 1 year, 4 months ago.

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    Andy Kiker

    Larry, To start I currently run a gas gas and have for the past many years. Before that that we ran a 2001 and older version of the Honda with very good results . I have never ran the case induction Honda. It was  Anderson prepared with Dellorto carb, Anderson pipe and silencer (very quiet), digital PVL non programmable with Anderson ignition curve. (stock Honda ignition curve is the worst for our application), Stock Honda piston with stock clearance. I think it is 2.5 thousandths. any time we put a new piston in we always ran 2 to 3 easy sessions for break in. Head volume I think was around 22 to 22.5. v force reeds. Top end with ours was a matter of  keeping an eye on the bottom edge of the skirt at the center it would start to crack. time of this to happen varied. I will say RPM was the key to making them last longer. if you kept it under 11000 things last much longer. over 11000 and I was replace much more frequent. I am no expert on this stuff. I always left the motor setup to the engine builders. I just replaced parts as needed.

    You guys are very lucky to have your own expert right on here with Ian. He is very knowledgeable and willing to answer almost any questions. and is spot on with his answers.

    as Anthony said above getting your chassis to carry more turn speed will do much more in cutting your lap times than anything you can do to the motor.            


    Chris Reinhardt

    This year should be a real learning experience, but my Cobalt will have a CRF450 EFI on it.  Once I get it dialed……..  It “should” adjust itself…….

    Another thing you guys should be discussing is what carb, and what fuel system (pump, tank, pickup, return, etc), and if there’s a g load area, ie left or right sweeper that’s causing a fuel starve….  Study the pistons, lean 4 corner stick, cold seize, ignition deto seize, over heating, etc…



    Dan Davis

    I’m back!
    Well, it’s been a year since I’ve worked on my case induction CR250 project….it’s just been sitting there patiently waiting on me to get back to it while I drove a 125 and a TZ250 kart at select events last year.   I am currently working on resolving my cylinder head issue for a 2004 CR250 cylinder for which I intended to fit a 2005 domed piston.

    As I began modeling a combustion chamber to cut into a 2004 cylinder head to use the domed piston, I realized that I would need to cut the chamber excessively deep into the OEM head to obtain a 20cc CCV (without spark plug hole, approx. 22cc’s with) and a 1.2-1.3mm squish clearance.  This is mostly due to the piston dome consuming approximately 7cc’s of volume keeping in mind that my chamber must begin flush with the cylinder deck rather than inset into the bore due to the existing porting in the cylinder. The piston depth in the cylinder at TDC then is equal to the squish clearance value.
    I have concerns that this will make the combustion chamber casting thickness too thin and possibly even break through since I have not cross sectioned a head to verify casting thickness.

    This leaves me with a couple of options to use this cylinder:
    1. Use a flat top piston to gain approx. 7cc’s of volume.  This is probably the easiest option but I really would like to use the 2005 single keystone ringed piston.
    2. Explore a different head.  Expressly, the 2005-2007 cylinder head for which I know nothing about except that it was designed to be used with the domed piston I would like to use.

    Based on previous posts by our favorite technical expert on CR250’s, I think I already have a clue as to the answer of my question….which is no but don’t have a definitive answer, so here it goes:
    Can a 2005-2007 cylinder head be fitted to a 2004 cylinder?   I’m guessing the answer is no because of the offset bore/chamber relative to the stud pattern on a 02-04 cylinder (assuming that the pattern and or bolt circle is even the same) but I haven’t seen one to be able to tell.
    Thanks in advance,


    Dan Davis

    Never mind……I just looked at some decent pictures of a 2005 cylinder for sale….it appears that the bolt circle and bore are concentric unlike the 2002-2004 version.

    Flat Top piston and chamber it is!


    Ian Harrison

    Never mind……I just looked at some decent pictures of a 2005 cylinder for sale….it appears that the bolt circle and bore are concentric unlike the 2002-2004 version. Flat Top piston and chamber it is!

    That’s right Dan, the ’05 went back to the bore being concentric with the bolt circle.
    Flat top piston and chamber is the way to go!

    Ian :-)


    Dan Davis

    Experimenting with 3D printing to make a carb. rubber adapter for 2005 CR250 VF3 reed valve.  It is angled slightly downward but is tough to see in the photos. The material is a bit on the brittle side but seems to be impervious to intended race fuel.  It is a urethane acetate material.  There is a better materiel available that has similar properties and is less brittle but I don’t have access to that at this time.  Hopefully, thickness of adapter will overcome any potential damaging stresses and vibration.  Time will tell.  Fun-Fun!


    Chris Reinhardt

    Looks real nice Dan, I like it!!!!  I took a couple of Solidworks classes last year, we had access to the 3d printer….  The stuff we could make…..

    I would be inclined to think that was a work related freebie?  There’s plenty of isolation for the carb, you could certainly have that model CNC’s out of some billet alum, or maybe delrin….

    Also, I would seal around the two bolts that hold the rubber adapter on, a potential air leak….



    Dan Davis

    That’s a pretty accurate inclination there Chris 😉 ……In fact, I need all the freebies I can get to even hit a couple races throughout any given season but this kind of stuff is just about as much fun for me as piddling about behind the wheel.

    Yep, I plan on using Yamabond between boot and adapter as well as the bolt heads.  The boot is a Mikuni/Kimpex 07-105-01 which are common on snowmobiles and cost as little as $10 delivered.   If I were to use Aluminum in the future, I could eliminate the nut pockets on the inside and just tap the holes.

    Next up, build fixture to deck couple of cylinder heads, cut o-ring grooves, and comb. chambers on a CNC machining center……luckily I have a buddy that sells those things and has one in his showroom to demo at the moment…..that equals free rent and maintenance for him on one of my CIK125’s for a couple of events. 😉


    Stephen gleason

    I’m at the point where I wanna work on timing curve anyone have any to suggest??

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