Topic: CR31 setup

[gschuld]

Ok, next challenge.

AHRMA requires carbs be restricted to 29 size for 1025cc engines in Heavyweight Superbike. They allow restrictor rings to reduce a larger carb down to 29mm.

So 1/8” aluminum plate rings will be installed just behind the carb slide bottomed out where the spigots normally seat on the bodies on a fresh never run set of CR31s.  So it goes: carb body, plate ring, spigot.  29mm ID in a 31mm carb means there will be a 1mm bump 1/8” wide around the circumference of the carb body just aft of the slide.  The leading edges will be relieved with a slight rounded edge with the trailing edge sharp.  It’s a quirky thing, but is required to stay legal in class.

Engine specs pertinent for carb tuning.

CycleX ported head. Factory 29mm ID head intake spigots. No raised port intakes or wild one off porting, but a nicely done head from a 392 casting, thin stem 34/28 valves, beehive springs
125-75 cam
Carrillo rods
Standard weight but race prepped crank
Motogpwerks 4-2-1 exhaust
Nothing attached to crank on the charging side.  Just a cover blocking the end of the crank.

What I’m looking for is a best guess initial setting for the carbs.  Not much time to play around and test before go time, so the closer we are on start up(on a new engine) the better.  We have a full supply of jets/etc, for a full range of options.

Thanks in advance.

George

[Old Scrambler]

What are the IDs of the ports, spigots, manifold rubbers? If all are 29mm or smaller I would tune to the 29mm specs.........but if all IDs are 30mm or larger, I would tune to the 31mm specs and treat the restrictor rings as velocity-enhancers during high-rpm pulls.

[gschuld]

29mm head spigots, rubbers, but 31mm carb spigots.

George

[Old Scrambler]

Head Spigots.......Those alloy roundy things with the large hex-nut that screw into the head. I reamed my 31mm motor and manifold IDs to 33m to work with 34mm carbs.

My thought is you should be prepping for the upper rpms (6-9,000) for best results. Piston speed requires plenty of intake volume for adequate cylinder-filling to achieve maximum power.

Are you using a 6-speed tranny?............

[gschuld]

Head spigots are factory unbored 29mm ID for a 392 head.  The CR31 carb spigots are 31mm ID.

6 speed trans?  No.  CycleX PRO CUT f model trans.

George

[Old Scrambler]

I suggest reaming to increase flow volume...........the restrictor ring should help to increase velocity at the needle-jet while in the high rpms

[slikwilli420]

I suggest reaming to increase flow volume...........the restrictor ring should help to increase velocity at the needle-jet while in the high rpms

No time for reaming this time around OS. Bike is going together now for a shakedown race the week of the 18-20 of June. As long as we get close on carb setup (safe side of close) we should be good to go.

[Tintop]

My concern would be that the restrictor is going to disrupt fuel atomization without blending.  Know you mentioned rounding the leading edge, but I would be inclined to make it more of a blended ramp to a very small flat section at back edge to comply with the rule.

You may also want to enlarge the head spigots to help increase air velocity after the restrictor.

[slikwilli420]

My concern would be that the restrictor is going to disrupt fuel atomization without blending.  Know you mentioned rounding the leading edge, but I would be inclined to make it more of a blended ramp to a very small flat section at back edge to comply with the rule.

You may also want to enlarge the head spigots to help increase air velocity after the restrictor.

No tapering on the restrictor plate allowed. Must be 1/8" thick and flat inside.

[scottly]

I doubt that a 1mm speed bump will make that much difference; how is the transition from 31mm to the spigot 29mm? Regarding jetting, the size of the venturi is what counts, not any downstream restrictions, whether a plate or the spigot, so jet 31mm carbs as 31mm carbs, not 29's. How big is the motor in question?

[gschuld]

1025cc, 10.5:1 compression, 125-75 cam

George

[simon#42]

dont think you are going to have much luck with this . is there no dyno local that you could use . its always better turning up to the track with a bike you know is right . you will have plenty of other things to think about when you  get there . poor preparation seldom makes for good results

[slikwilli420]

dont think you are going to have much luck with this . is there no dyno local that you could use . its always better turning up to the track with a bike you know is right . you will have plenty of other things to think about when you  get there . poor preparation seldom makes for good results

Much luck with which part? We are getting the bike prepared now according to the rules so we don't have to mess with as many things at the track. I do not have any reliable dynos anywhere near me. The bike will be very well prepared, but tuning is another matter.

[scottly]

Based on Jerry's experience, I would try a 110 main to be safe, with 105's on hand in case the 110's are on the rich side of "safe".
http://forums.sohc4.net/index.php/topic,83748.200.html

[simon#42]

dont think you are going to have much luck with this . is there no dyno local that you could use . its always better turning up to the track with a bike you know is right . you will have plenty of other things to think about when you  get there . poor preparation seldom makes for good results

Much luck with which part? We are getting the bike prepared now according to the rules so we don't have to mess with as many things at the track. I do not have any reliable dynos anywhere near me. The bike will be very well prepared, but tuning is another matter.

Much luck with the jetting . Do you think turning up at a race meeting with a set up someone on the internet has given you is good preparation ?

[slikwilli420]

dont think you are going to have much luck with this . is there no dyno local that you could use . its always better turning up to the track with a bike you know is right . you will have plenty of other things to think about when you  get there . poor preparation seldom makes for good results

Much luck with which part? We are getting the bike prepared now according to the rules so we don't have to mess with as many things at the track. I do not have any reliable dynos anywhere near me. The bike will be very well prepared, but tuning is another matter.

Much luck with the jetting . Do you think turning up at a race meeting with a set up someone on the internet has given you is good preparation ?

While it's not the ideal situation as long as it's not so lean it blows a hole in the piston then it's a safe starting point. I recognize how long it took to get my current bike dialed in so I don't have expectations of being perfect the first time out. It just needs to be close. If I had the access to a dyno then that's where I'd be but I don't.

[simon#42]

That’s fair enough but I would beware of using other people’s jetting . Your needs are very specific , just start rich !

[slikwilli420]

That’s fair enough but I would beware of using other people’s jetting . Your needs are very specific , just start rich !

I wholeheartedly agree. As long as I can be on the safe side, I don't care if we foul plugs and lean it out, it's starting too lean and ventilating a piston that worries me. Weather is looking like mid 80's all weekend, so the carbs can be a bit leaner jetted anyway and still be good, but always better to be safe than sorry.

[Old Scrambler]

Check your elevation and jet up .................Practice laps will help

[slikwilli420]

Check your elevation and jet up .................Practice laps will help

Just checked elevation because I was curious, 43' above sea level, so I don't think Ill need to be jetting for thin air this time around!

[gschuld]

Good news.  We were able to secure a dyno tuning session next Thursday evening.  It’s a little out of the way, the time isn’t at all convenient, but I’m very happy to be able to test the new engine properly and make sure it’s set up in a safe range.

Thanks Simon, I needed that extra push to get on the ball and secure a dyno session👍

FWIW, the dyno is a Factory-Pro EC997 Eddy-current low inertia type, not the typical Dynojet.

George

[simon#42]

I think that will save you a lot of time and problems on race day . Good luck !

[dragracer]

Good luck with the tuning. Best to have that out of the way before hitting the track.

[gschuld]

Yes, it’s a huge relief to have a dyno session before hitting the track.

 George

[Jerry Rxman Griffin aka MuthaF'er]

It's always good to get a general consensus but only rely on the dyno for your setup.

I haven't touched my 915 with 125/75 and CR31's since the last dyno session in Sept (?). I don't remember my bigs (120?) but according to my print out at 6000 - 6900' I was fat in the needles at about 10:1. The needles were in the 3rd slot from the bottom so I dropped the needles yesterday 1 notch to the middle notch. Time for my butt dyno. 83hp at elevation for a start. It revs FAST and really starts coming on around 7K so just be ready with your shifting foot. Gonna reset my timing to 40* from 35*. I have the Dyna2000 set on curve 4 based on the Dyna info that came with it and not brave enough ie too stupid to hook it up to my laptop. Rev limiter at 10,000. Looking forward to your print out. 

Next up is the 1000 with the RS34's and the 315 cam.

Fukn love both these carbs. SOOOOO easy to tune.