Topic: CB750 AHRMA Sportsman 750 Racer

[Retro Rocket]

My concern with carbs is needing to stay with stock round tops, although there is no mention in the rules about internal modifications. I need to get some stock carb gurus in here to help me out on that one.

Sounds like you need a set of bored out stock carbs, I can't remember George's forum name,  but Contact Bill Benton and ask him about them, he should be able to get you headed in the right direction.....

[Bill/BentON Racing]

That oil hole in crank snout is restricted to 1mm/.040!  Frank's, Dragracer  been running his 750 uncovered for years.

[slikwilli420]

That oil hole in crank snout is restricted to 1mm/.040!  Frank's, Dragracer  been running his 750 uncovered for years.

The side hole or the one at the end where the rotor bolt goes?

[slikwilli420]

Hey Bill, any leads on a set of bored out round tops? I can PM you and we can discuss there.

[Bill/BentON Racing]

Yep, brother George Schuld has a set of mine, 4-1 cable carbs off early 69/70. If interested I can see if he's done with them, let me know , thanks,  Bill

[slikwilli420]

I would definitely be interested. Could they be converted to the later bellcrank style with one cable? One less thing to break on the track would be best.

[Bill/BentON Racing]

Don't think converting would be  feasible or possible.

[slikwilli420]

I might still be interested despite that. I may have a line on another set. Bill, do you know anyone who still does this? With so many other carb options out there I'm sure demand is really limited for this mod.

[gschuld]

http://forums.sohc4.net/index.php?topic=147149.0

Slikwillie,

   You can look at the thread link above, to give you an idea about bored factory round top carbs.

In short, there isn't much out there regarding readily available data on how best to set these modified carbs up.

Two things should be done.

1- a proper carb only flow bench comparison, with perhaps a factory stock round top carb, a CR29, and the bored carb body.
2- a minor alteration to the fuel metering system will be needed to reduce the current need for the huge main jets Bill Benton needed to get the air/fuel withing range on his highly modified 915 motor (which made 101 RWHP with the bored carbs)

I do still have plans for both my sets of bored carbs, sorry.

The factory round top carbs will perform fairly well until an engine is seriously modified for big power.  Say 85 RWHP or so.

I don't recall what you had planned engine wise.

To do a run of a few sets of carbs would require a fairly involved custom carb mounting fixture and guides to ensure accuracy and repeatability.  Even then, there is a fair amount of hand blending to smooth the transition from the milled surface to the entry and exit, the polishing.  It's totally doable, but frankly I wouldn't consider it much of a worthwhile "business venture" if you know what I mean.

There are good carb options available off the shelf.  The only reason I'd see to use bored out carbs is for class restriction reasons for racing (like your issue) or someone making a "sleeper" engine.   

George

[slikwilli420]

http://forums.sohc4.net/index.php?topic=147149.0

Slikwillie,

   You can look at the thread link above, to give you an idea about bored factory round top carbs.

In short, there isn't much out there regarding readily available data on how best to set these modified carbs up.

Two things should be done.

1- a proper carb only flow bench comparison, with perhaps a factory stock round top carb, a CR29, and the bored carb body.
2- a minor alteration to the fuel metering system will be needed to reduce the current need for the huge main jets Bill Benton needed to get the air/fuel withing range on his highly modified 915 motor (which made 101 RWHP with the bored carbs)

I do still have plans for both my sets of bored carbs, sorry.

The factory round top carbs will perform fairly well until an engine is seriously modified for big power.  Say 85 RWHP or so.

I don't recall what you had planned engine wise.

To do a run of a few sets of carbs would require a fairly involved custom carb mounting fixture and guides to ensure accuracy and repeatability.  Even then, there is a fair amount of hand blending to smooth the transition from the milled surface to the entry and exit, the polishing.  It's totally doable, but frankly I wouldn't consider it much of a worthwhile "business venture" if you know what I mean.

There are good carb options available off the shelf.  The only reason I'd see to use bored out carbs is for class restriction reasons for racing (like your issue) or someone making a "sleeper" engine.   

George

Thanks for the great info George. This really gives me good insight into the modified stock carb world. My setup will be a stage 4 head from MRieck as well as the Dynoman 12.5:1 pistons, so power output should be pretty healthy. From what I have gathered 80HP should not be asking too much from that setup. Can stock carbs accommodate that? Are their other mods to stock carbs I could do besides boring them out to help out? I have heard people mention modifying the slide cutaway but never saw any details or testing.

[gschuld]

A near stock displacement motor with high compression and a JMR stage 4 head (with an appropriate cam to match) could be a potent motor.  Mike could probably give you a good estimate of power and rpm, and ideal carb size.

I know of a few guys with strong 836 motors road racing with cr31s.  Being stuck (by the AHRMA Sportsman rules) with factory carbs, I'd think that the extra flow available from a set of properly bored and tuned factory carbs could suit you very well, especially if you are going to be running it past 10,000rpm on a regular basis .

I would expect the dead stock factory round top carbs would start to hold you back at the top end of the rpm range with your motor.  It might not be a huge difference, but I'd think it would be at least measurable on a dyno.  I simply do not have the comparative data from dyno/flow bench testing that would be helpful to support my assumptions.

I am fairly confident that Billy's stage 4, +/- 10:1 compression, 915cc, RC 327 cammed motor would not have reached 101 RWHP (with MR12 "Crack fuel", which can bump up the power +/- 5% over regular race gas) with dead stock factory round top carbs.  But I still am curious if my bored carbs were flowing enough to get the most out of that engine at that time.  Pretty close I'd guess.  So as a rough guess I'd think your WOT cfm requirements would be about 15% less than Billy's hot rod 915 motor at 10,000.  Assuming the bored carbs flowed enough air/fuel to handle that power level, I'd think the bored carbs would be plenty to handle your motor.  Someday I'll have to arrange a flow bench comparison and a dyno comparison.  I've been a bit distracted from my hobbies lately

But don't confuse me with the real wrench spinners around here, I am primarily full of well intentioned BS.

The two inspiration pics are of Tony Faole's custom monoshock framed racer, early 70s vintage.  He put a LOT of work into building that forward thinking bike, and he ran both factory round top carbs and the factory airbox rubber  velocity stacks.  Probably a good bet these carbs were bored out and blended/polished like mine.

George

[slikwilli420]

George, thank you again for all the great info. The historical context is really interesting to see. I will keep this info in the back of my mind and now I have it in my thread for future reference.

I need to keep in mind that my primary concern is a well handling frame with good suspension, stiffened structure, and great brakes.

To that end, I ordered one of the RPE cast iron rotors in the drilled flavor that will be mated to a custom billet carrier. Why? Because I can!
I need to come up with nice master cylinder that fits the ratio outlined on Vintage Brakes, which looks to be 11 or 12mm when mated to a single Grimeca opposed piston caliper, which is on the way from England as we speak. Any suggestions there that won't cost a small fortune?

Here is a shot of the mockup caliper on the old disc. I am waiting for my 4-bolt Grimeca and rotor right now.



Next up is getting geometry set in that magic trail sweet spot. I am going to have a set of mock trees cut from HDPE so I can have the frame sit up on its own to get measurements.

I plan on going about 1" over on the rear shocks to steepen the neck angle and with the mock up trees in place I can bring the trail back to an ideal range. Since I am using GL1000 forks and they are longer than stock I will have some adjustment there as well by sliding the forks up and down in the trees.

The plan for suspension is full Race Tech with non-reservoir shocks out back (per AHRMA rules) and springs/emulators up front.

The frame will be going to the welder this week for round one of the bracing process which includes the gussets I showed earlier and having the upside down "U" where the down tubes meet welded up. After that the rear hoop will be modified by removing the stock stamped one and adding a low tubular hoop in its place so I can mount my seat with a pivot as bwaller did for his race bike. Its a simple design and seems to work well for him so that's my approach as of now.

That pretty much covers the handling/braking aspect unless there are other suggestions from racers out there on what I might be missing.

[Bill/BentON Racing]

Great thread, G'luck,  looking real good, Bill

[Haybus]

Sounds like a good plan for suspension. For comparison, I've moved up to 1.5" over stock length in the rear and I'm pretty happy with the handling. That puts me just under 24 degrees rake and 3.4" trail with 45mm offset trees. Not magic, just works well enough for me. If you can get shocks about 1" over with some height adjustability you could have enough range to play with. Otherwise here's what I did, +1, +1.25 and +1.5.

Alan

[slikwilli420]

That's a pretty slick setup for lengthening the shock. I'm thinking of going with 14" race tech shocks with height adjustment which should get me there. The longer GL forks will give me some room to play as well.

Glad you mentioned 45mm offset to since that seems to be a magic number and I will start there and see if I can get my trail right.

[scottly]

What diameter are the Grimeca pistons?

[slikwilli420]

What diameter are the Grimeca pistons?

41mm. Why do you ask?

[slikwilli420]

So although a bad ass exhaust shouldn't be on the top of my list, it is. I have a very nice used Delkevic header that I want to pair with something unique. I found these on eBay, but unfortunately, aside from being Chinese, they are only a 2" inlet instead of the 2.5" I need.

http://www.ebay.com/itm/51mm-Stainless-Steel-GP-Exhaust-Pipe-Muffler-Sport-Motorcycle-Scooter-Motor-Bike-/191873608325?hash=item2cac8eea85:g:JWIAAOSwcvdXOYIw&vxp=mtr

Is there anything out there that would work better? I realize I wont be able to match the very attractive Chinese price, but something that isn't astronomically priced would be nice. I was looking on Cone Engineering's website and I think I could get pretty close with a cone and some straight section. Is this more difficult than it looks or could this be replicated pretty easily?

[540nova]

I don't see why you couldn't do the Cone and straight pipe, it's what I did



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[slikwilli420]

Cause that has been done many times already and I want something unique for this racer.

[bwaller]

Plus you need ground clearance. Just as an option you could check with Kemp about a Hindle can. If he didn't have the exact size you're looking for, maybe he'd make what you need.  http://www.ripplerockracers.com/product/hindle-mufflers-2/

I talked to Kemp & saw his wares at Mosport & it's very good quality.

[scottly]

What diameter are the Grimeca pistons?

41mm. Why do you ask?

Because you asked about a suitable master that wouldn't cost a fortune. From a hydraulic pressure standpoint, it doesn't matter if the active pistons are opposed, parallel in the same caliper, or dual single piston calipers. I would try the stock 14mm master; they work well with two 38mm pistons, but are approaching the small side with two 42.8mm pistons, IMHO.

[slikwilli420]

What diameter are the Grimeca pistons?

41mm. Why do you ask?

Because you asked about a suitable master that wouldn't cost a fortune. From a hydraulic pressure standpoint, it doesn't matter if the active pistons are opposed, parallel in the same caliper, or dual single piston calipers. I would try the stock 14mm master; they work well with two 38mm pistons, but are approaching the small side with two 42.8mm pistons, IMHO.

Wouldn't a 14mm master put me at a potentially dangerously low ratio according to vintagebrake? A 14mm master with a single caliper with 2 41mm pistons gets me to a ratio of 17.15 while, according to him, I want to be in the 23-27 range, which would be an 11mm or 12mm master.

[Tintop]

Something doesn't seem right about that calculation.  My tendency would be to go to a 16mm master to get better feel, with shorter lever travel.  The bigger the caliper piston(s), the more fluid that needs to be displaced by the master piston.

[slikwilli420]

I'm really confused now. Not that I'm not absorbing what you so are saying but it speaks directly opposite of the chart on vintage brake. Is his chart that far off from real world experiences?