Play bike big bore kits

[hoosierf]

Okay, I need some advice from the minibike crowd. I'm getting ready to sink an unreasonable amount of money in to our DRZ 125 and I need some advice on options. I already have a a BBR pipe and oversized carb. I've opened up the air box, put a crank breather on it and I have it jetted pretty well. Should I:

1. Have Powroll stroke the crank and put the 174cc kit in with their cam? This motor is race gas only and I suspect it will immediately reach the limits of the meager ignition. Does anyone make a minibike aftermarket ignition?

2. Put the BBR 150cc kit in it with the Powroll cam. This motor would not require race gas and is non-lower end invasive.

3. What about lightening the flywheel? BBR told me that some people have reversed course on this option and changed back to the original flywheel.

We want the motor to produce good horsepower, especially on top, but we want it to be reliable and not too finicky on the carb. Any advice from you TTR hop up guys? Displacement for class racing is not a concern.

thanks for your comments,

Hoosierf

 

[minifig-phil]

Well if you want power in the higher RPM range then don't go with the stroker crank.

 

[Brianc]

The stock bare and stroke is 54x54. The 150cc big bore will make it 60x54. The stroker is 60x62. Piston speed won't be a concern, so I would go for the 174cc set-up. You will get LOTS more power from bottom thru top with that combo. I thought BBR offers an ignition set-up for most models. How big is your carb? you may want to go to a 26mm carb when the engine is ready.

 

[hoosierf]

I have a 26mm carb now. Powroll tells me they are working on an ignition unit. The ignition is critical to cam selection, so I'm hoping they work something out yet this winter and I'm good to go. I'm going with the stroker motor. Brian, what are your thoughts on removing about 8oz from the flywheel?

BBR has no ignition solution for the zuki as of today.

 

[dirtfreak]

If you take 8oz. off the flywheel its gonna stall a lot easier. If you don't want to abuse the clutch then either don't take that much off it or just leave it alone.

 

[Brianc]

Originally posted by hoosierf
The ignition is critical to cam selection,

The cam should be chosen to suit your power range and performance preference. It is not dependant om the ignition system to provide a performance boost. Unless your trying to increase the RPM limit, I don't see how the stock ignition would affect the cam choice. Doesn't your stock ignition go to around 11,000 RPM?

' Brian, what are your thoughts on removing about 8oz from the flywheel?

'

I was talking to Pete at Powroll about our flywheel ( they are doing a 9.5 mm increase on the crank of my son's CRF 150 ). He said they remove most of the weight close to the hub to lessen the overall weight without a large reduction in the damping effect provided by the flywheel. On our CRF flywheel, there is a starter gear on the hub for the CRF230 'e' start, they share the same flywheel . That gear will be cut off and the hub area will be machined to get a cleaner surface as well as reducing the weight. The CRF flywheel is in the engine oil, so the removal of the starter gear will reduce oil friction loses and foaming; I am not be concerned about the potential of stall problems occuring. The only time stalling could be an issue is when starting from a complete stop and only then if it is a slow start. Once the bike is in motion and in gear, the entire motorcycle will provide the anti-stall inertia, the flywheel weight won't matter. On the big bores, the heavier flywheel is prefered in loose dirt to limited rear tire slippage while accelerating; that is hardly a problem with 150cc 4 strokes.

BTW, what cam(s) are you considering?

 

[hoosierf]

The cam selection is important in my application because, according to Powroll, their high rev cam will bump the rev limiter as soon as the bike comes on the pipe. I want to make more high RPM hp, but if there is no ignition solution I'll go with Powroll's midrange cam. I'm not worried about stalling or clutch abuse after removing weight from the flywheel. I'm worried about severe compression braking and/or a valve failure in the event of a missed shift. However, now that the flywheel is out of the motor, I'm not quite sure if one could even remove 8 oz. I'm going to check with BBR Monday to see what they're doing.

 

[ACS]

A big bore stroker will feel a bit "flat" no matter what cam you have. You will also find the limits in the head design that will most likely impact on performance before the rev limiter cuts in. That means you really need good porting. I personally like flywheel weight reductions but my bikes are built for junior MX. Trail riders / beginners prefer the softer delivery that you get from a heavy flywheel.

The gear Powroll sells is very good for developing a strong mid range bike that will be reliable and bolts together easily. Their TTR cam and big bore is very nice. Full blown race stuff is technical to fit, really expensive as a lot of associated parts need to be upgraded and can be a bit of a grenade.

I personally would upgrade the DRZ suspension first then look at big $$ in the motor. RM85 stuff fits easily. I did it the wrong way on the TTR!!

Pics of what you can do to a CR/XR and a TTR are at http://acsracing.tripod.com

 

[Brianc]

I wouldn't worry about any valve problems due to a missed shift if you put the stiffer valve springs in the engine. Valve float on an overhead cam engine is extremely rare. Pushrod engines are more likely to have that problem simply due to the larger mass of valve train components. If you decide to go to the midrange cams, check out www.webcamshafts.com they may have some more choices. Powroll can advise on your choice. I personally would go with more lift and not worry about the rev limiter issue. Someone will offer a replacement in the near future. More lift will give an increase the power through the entire engine range, it's the duration that affects the upper and lower limits of the power range. More duration decreases the lower rpm power and less duration increases lower rpm power. I can tell you the midrange cam for my son's XR80 is a little disappointing. Definitely too mild. We put their bigbore high compression piston kit in it last summer.
Since we are talking a four stroke here, I'm a little perplexed about the comment they made ' when it gets on the pipe ' . I do agree with ACS about the power being more linear, that is another advantage of increasing the stroke of the crank. That's why we are going with the crank modification on the CRF.
Its really a person preference issue of light vs. heavy flywheel. I prefer the quicker throttle response of a light flywheel. You can always add weight back to the flywheel if you don't like the results of them lightening it. As far as your engine braking issue, your idle speed will affect that, if you want more engine braking, lower your idle rpm, if you want less engine braking, then raise your idle rpm. 100 rpm change either way will make a noticable difference in the engine braking. Also a higher idle will make the engine more responsive when exiting corners and twisting the throttle.