All Things CR250

[Jeff Pickel]

Ian
Did you buy the carb new or used.
Sounds like your restrictor is missing in the fuel pump.
Did you check the pop off pressue with a small hand pump.
pressue the gas tank with some air to see if any fuel comes out of the nozzle.

[Ian Harrison]

Hi Jeff

Thanks for the input

Did you buy the carb new or used. New

Sounds like your restrictor is missing in the fuel pump. The restrictor is just a driling in the casing, not removeable. Even if I fully block the return, the pump pressure still won't open the fuel cut-off with the motor spinning. By a simple "blowing effort test", I think the restrictor is equivalent to around a 70 size Dellorto idle jet. However, if I restrict the air intake (so the vacuum increases), the fuel cut-off opens and fuel flows. Here's a diagram of the carb function.



Did you check the pop off pressue with a small hand pump. Yes, with my leakdown tester. 14/15psi with the diaphragm removed.

pressue the gas tank with some air to see if any fuel comes out of the nozzle. Not tried that, fuel pressure with the motor running off "sprayed fuel" is about 6psi and if I pressure the fibreglass tank above this, it will blow apart!!, but if I put a plastic tube so it seals round the "vent to atmosphere" drilling in the diaphragm cover and blow very slightly (and I mean "very") the fuel cut-off opens.

So from the above, it's not far off, and I'll report when I've tried the lightest float arm spring. But as I say my worry is reliability and calibration at the mid/top end, if I should ever get it to run at idle!!

It would be interesting to know if anyone else has run a Mikuni SBN44 on a CR250 (or any other 250 2-stroke single) and what settings they had to run. . . . . or perhaps nobody else is daft enough to try

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Racer Johnson]

Ian have you had the metering side of your carb apart yet?? almost sounds like something is astray with it, at that low of pop off It should be flowing fuel like crazy. some skis Ive built are way too rich below 55 psi!! Where are you pulling your pulse from?

-Eric

[Ian Harrison]

Hi Eric

Yes I've had the carb down (many times now) and everything is fine and checked all the gallaries/jets are clear.

Perhaps it's just a duff carb, but having said that, I can't see anything wrong.

Diaphragm is intact and no fuel leaks anywhere. Pump is superb as I say runs at 6psi, so no shortage of fuel or pressure at the carb. If I fit the carb as close to the other way up as I can, it will run above about 4,000 rpm, but if I let the revs drop, the motor just starves of fuel and dies.

Pulse is taken out of the redundant gallery across the front of the crankcases, cross-drilled into the crank chamber as we do with all our CR250's. As I say fuel is up there and at pressure.

Mikuni send them out with the settings I have and make comment about the pop-off being lower than standard, due to the fact that they are usually fitted as a performance upgrade in combination with a less restrictive filter, so I can only assume that with (at present) no filter and a small engine, that an even lower pop-off is needed.

I have to make up an adaptor to fit our CIK airbox, but this is an open box anyway, so wouldn't offer any more restriction than an open choke at low rpm's


Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Jim McMahon]

Ian it might be worth putting a post on the 2cycle board of 4cycle.... These guys build some incredible open two and four stroke stuff for dirt. Including 250 singles.
http://karting.4cycle.com/forumdisplay.php?f=26

There's a lot of guys with significant experience with these SuperBNs that hide out there. Not sure if they run them with 250's but its worth a shot.
_________________
CES Grattan pre-entry is open. TaG, Shifter, KPV, KT100, Animal\LO206, enduro or superkart.
Click Here to enter Grattan online

[Chris Reinhardt]

So Ian, you're not getting any fuel to the carb? Would it be worth while to plumb a Mikuni round pump just to get fuel to the carb? I know that's redundant, and may not even get past the main pump in the carb, but if it does, maybe at least you can check the jetting and see if that carb will work with your motors.

CR
_________________
East Coast Super Kart Series
"This is how we roll!"
www.eastcoastsuperkart.webs.com

CR2 Motorsports
"Home of Cobalt Superkarts"
www.CR2MotorSports.webs.com

[Ian Harrison]

Hi Jim

Thanks for the heads up. I'll do exactly as you suggest.

Hi Chris,

Loads of fuel up to the carb and at good pressure as stated.

Adding an extra DF52 fuel pump in series or parrallel won't improve the delivery pressure. The inbuilt pump capacity is a bit better anyway. Simply put, the problem is that the vacuum in the venturi is not sufficient to open the fuel cut-off. Question is . . . . is that a carb problem or just the way it is with a CR250 motor and no filter. My belief is that what I'm seeing would be typical.

I did consider removing the inbuilt pump and running on a DF52 (for a lighter carb and neater installation), but I could of course use a DF-62 octagonal pump, which has a slightly higher delivery pressue and much higher flow rate (not that I'm flowing anything at present . . . apart from back through the restrictor ).

Let's see if the guys on the Forum Jim suggests have any ideas. I've just registered and awaiting activation before I can post.

I'll post a link to the thread for anyone interested.

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Jeff Pickel]

Ian,
Did you check you check valves in the fuel pump.
when we ran 60 gram spring and the big valve it would leak with at idle. we would run 48mm bn with big air filters. is the big diapham in the right way.

[Jim McMahon]

Sounds like all you need is a starter that will turn the motor at 6,000 RPM and then set it to idle there LOL.
_________________
CES Grattan pre-entry is open. TaG, Shifter, KPV, KT100, Animal\LO206, enduro or superkart.
Click Here to enter Grattan online

[Ian Harrison]

Hi Jeff

As I said it's brand new, but yes the flapper valves are A1 and fuel bleedback is absolutely minimal once the motor is stopped.

Diaphragm is in the right way round with the little anodised "tit" contacting the float arm. In fact due to the way the edge of the diaphragm is formed into the sealing 'O'-ring and the 'O'-Ring groove being in the carb body, you would have to be a real dork to get it wrong.

Jim, that may well be a joke, but could also be right!!

Here's a link to my "plea for help" on the Forum that Jim siggested: http://karting.4cycle.com/showthread.php?p=4195365#post4195365

Been mad busy today and forgot to order up the 65grm spring.

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Jeff Pickel]

Ian,
have you try turning the carb 90 degrees so the diapham is not at the buttom of carb. Most of the jetski have the carbs with the shafts vertical.

[Ian Harrison]

Hi Jeff

Mikuni says that the carb works best as a pure downdraft, but if mounted as a sidedraft it must have the pump at the top and diaphragm at the bottom, otherwise fuel will drain from the fuel chamber into the venturi after the motor is stopped.

It can't run with the shafts vertical, anyway as it then fouls the cases.

See my comment about running it as near to the other way up as possible (actually at about 45 degrees).

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Robert Lawson]

Ian,

I'd be more than happy to ship you one of my 38mm CDKII Keihin's to try.

I know it works on a CR 250, took it right off my RS 125 and bolted it on.

The only issue we have ever had is a bog or fuel starve at low revs to WOT. If the revs were kept up....no problem. Never happened to me but my bro and friends had this issue. Tuning was simple, nothing like you explained above. No jet changes, no spring changes and it was mounted engine on right/needles straight left. We did remove the choke shaft and butterfly & plug the holes.

Maybe it's all the HP your force feeding it!!!!!

RPM

[Ian Harrison]

Hi Robert

Thanks for your kind offer, but let's see how we get on when I get the softer arm spring.

38mm is a bit small regarding airflow to maintain our power at high rpm.

I certainly wouldn't want to compromise our current performance in terms of throttle response and/or power.

Many Thanks and I'll shout if I want to borrow your carb.

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Steve O'Hara]

Ian,

I've been following your thread here and on Bob's forum but I haven't read everything so I hope my suggestions are not redundant.
Oversized carbs are challenging to get to idle but not exactly for the reason most people talk about on these boards. The focus in much of the thread is on the "weak signal" due to the huge throttle body but the discussion fails to recognise that when the throttle butterfly is closed the engine, pump, fulcrum arm etc don't know that the bore is 44mm or 24.
The system should idle just fine with the std pop off and arm settings if the volume of air flow needed for the desired idle speed is constrained to the immediate vicinity of the fuel discharge hole. To that end I suggest you set the carb up with the butterfly set to completely seal against the bore and then machine a small notch in the butterfly immediately adjacent to the discharge hole. The modification will cause all of the air entering at idle to flow over the discharge rather than a long thin arc of opening the majority of which is a long way from the discharge hole. Start small... probably only needs a half circle of 1mm radius.... maybe less.
My second suggestion may or may not be relevant depending on how you have your fuel system set up. The carb has a fuel return outlet path that diverts unneeded fuel back to the fuel tank but I did not see any discussion in the posts I read discussing if you are using that feature. If you are, my first suggestion is to block it off. If you do that and find that you have over rich conditions up in the power band consider placing a pressure sensative valve in the return line that only allows flow back to the tank if the pressure exceeds a certain value. I would suggest a 4psi release pressure valve in the return line if you find that just blocking it off causes problems. The idea is to build and retain residual pressure on the pump side of the inlet needle to get some to flow at idle speed. The problem with the pump carbs is that the pressure pulse from the pump is out of phase with the low pressure in the throttle bore caused by air speed and if you have a return system plumbed the pressure produced by the pump can just bleed off to the fuel tank rather than build up to move the fuel when the piston is on the up stroke.
Good luck.

Steve O'Hara