Topic: drag motor, 850 overbore wait maybe stock bore.

[MasterChief750]

Thanks mc. Im thinking undercut gears and new primaries. Maybe lighten and balance crank. Im going no charging system on this. Will have to figure out what kind of cam to run. Hopefully ill tear into the motor later next week

[MCRider]

Thanks mc. Im thinking undercut gears and new primaries. Maybe lighten and balance crank. Im going no charging system on this. Will have to figure out what kind of cam to run. Hopefully ill tear into the motor later next week

Those would be good upgrades. The undercut trans would solidify the shifting, an issue with consistency. I'd go easy on lightening the crank. Good for roadracers, not so (maybe) for dragracers. You need the weight to give you a good launch without bogging the motor. Even without the charging system you may find you want the rotor weight.  You don't need the throttle response as you're holding it wide open anyway. Balance would be good, as you'll be holding the throttle open at the line waiting for the green. On a stocker, put your money into port cleanup and run a stock cam. IMO  Sweet motor.

[MasterChief750]

and stig 1 im watching your thread like a hawk

[fang]

and stig 1 im watching your thread like a hawk

+1  I stayed up late and read all 39-something pages of the build discussion.   I actually was occasionally moved.  Congratulations Sam and team on a fantastic build, excellent racing and safe trip home.

MasterChief.  I want to soften my earlier comment a bit.  You should never let something as simple as money get between you and your dreams.  If your dreams are sufficiently motivating (and expensive) you will figure out how to earn more money.

Best wishes.
Fang

[dragracer]

As much as motor work is necessary to building a dragbike, the chassis set up should be your number one priority. I say this from prior experience in playing with a bone stock 78 K model. The only change i made to the K initially was gearing which made it more agressive on the launch. It was at that time the bike began to stand up off the hit although it didn't make a much power down track. Given this bit of data, i realized those motors, in close to stock form, will get the most benefit from the front half of the track and not the backhalf. I knew i needed to set the chassis up to allow high rpm launches since  i was not using a wheelie bar. The first thing i did was to lower both the front and rear of the bike as much as possible. Normally most sanctioning bodies require 2 inches of ground clerance to the lowest point on the bike so this was my limit. I installed a V&H sidewinder on the bike ( CB1100F) so the exhaust wouldn't create clearance problems once the bike was lowered. I then slide the front forks up through the triple trees but only after i internally lowered  and stiffened them to reduce extension travel at launch. My next step was to make a set of adjustable rear struts to replace the stock rear shocks( i later purchase a set of shorter shocks- never installed) to lower the bike in the rear. Next step was to take a stock seat and have it cut down in the front as low as possible so i would be decreasing the center of gravity for the rider and to create a stop for my butt to keep from sliding rearward. I added drag handle bars. All of these changes were very inexpensive but netted the best results in lowering my ET without ever touching the motor itself. I intended to rejet the carbs and add pods along with installing an airshifter but i came across an F model chassis that i extended and raked. I never finished those changes to the K after i completed the F project. The K sits in a corner awaiting a resurrection- i can hear it calling my name as i type this post-lol. I will likely put a mono shock arm on it with a 17" tire so i can get a sticky compound. Don't waste your time on putting an 18" tire on a hand clutch bike. You will never find a sticky compound 18" tire. This is the reason my F model has an 5.5" X18" rear slick  on it today. It limits me to racing the bike in pro ET only events and not in street ET. My swingarm is not wide enough to put a wide 17" rim on it. In hindsight i would have built it a little wider.

  Why not go with the APE 1000cc kit. No replacement for displacement.

[MasterChief750]

Although the ape kit is tempting I would rather not drop 2 k on just pistons and the needed. Then with the ape kit id need head work and a hot cam and valves and springs and...... I can keep going. Pretty soon I have a 7 k motor in a 200 buck bike.

[Sam Green Racing]

and stig 1 im watching your thread like a hawk

+1  I stayed up late and read all 39-something pages of the build discussion.   I actually was occasionally moved.  Congratulations Sam and team on a fantastic build, excellent racing and safe trip home.

MasterChief.  I want to soften my earlier comment a bit.  You should never let something as simple as money get between you and your dreams.  If your dreams are sufficiently motivating (and expensive) you will figure out how to earn more money.

Best wishes.
Fang

Thanks Fang, It's been a wonderfull experience and I'm home and been back at work for 2 days, got to earn more pennies for the next foray.

Sam.

[dragracer]

Although the ape kit is tempting I would rather not drop 2 k on just pistons and the needed. Then with the ape kit id need head work and a hot cam and valves and springs and...... I can keep going. Pretty soon I have a 7 k motor in a 200 buck bike.

Understood. Thats why i say spend your money wisely and put it into the chassis set up FIRST. A big motor in a crappy chassis will yield very bad results. Get the chassis right and you'll net an instant drop in the ET even with a stock bore motor. I've done this many times in various bikes so i can say without a doubt, chassis set up is important in a dragbike. Ok, i think i've beat that horse into submission and you get my point- lol.

Use the 836/850 kit and top it off with an F model head with mild port work. A degreed stock cam should also be worth more power. Gearing is extemely important also.

[MasterChief750]

ok will start on frame/motor soon i want to get the motor completely stripped first. im thinking heres what i do for the motor. from top down please make suggestions if i should change someting. stock cam, stock springs, aluminum retainers, stock valves,hd cam chain, hd studs, forst or second over pistons, stock rods, balanced crank(maybe polish it), no alternator?,new primary chains, undercut gearbox, kick only, aluminum sprockets(open to ideas), all new gaskets and seals, what size rear?(i want to run street ET). struts on the rear, -2 tubes in the front and slammed, im thinking sidewinder exhaust.

[jweeks]

     There's no recipe for doing this exactly right. The bigger the motor, the bigger the strain on the stock rods. They break/bend over drag racing time. The best ARP rod bolts won't help with that. The DOHC rods are a little better materials, but different in lengths. If your rods don't bend/break, the added piston weight will eventually oval the small end of the rods. You can run stockish stuff for a while, but you will eventually run out of luck.
     The carbs are another area not covered by anyone. The old 29mm smoothbores work fairly well on low horsepower cb750's. As you evolve the bike (bigger displacement, bigger cams) the carbs will need replacing. 32-34 mm carbs work well with bigger displacement and cams. Finding something that will work right in that displacement will be a challenge.($$ and tuning) Over on the automatic side, I'd be recommending the Yamaha Seca 32 mm CV carbs. They bolt up, and work well with a little tuning. Not the easiest things to find, but far from expensive.
     Street ET class requires a street legal rear time. Shinko makes the currently preferred street legal strong biting tires. They come in 17" rim sizes. There's nothing good in 18" rim sizes. The object of your launch is to go forward. As the power increases, your front end has the tendency to go up when you launch. Lowering the center of gravity and firming up the front fork springs will help to keep the front end down. As the power increases, most racers extend the swingarm to reduce wheelies. If you buy/build an extended swingarm, you should consider a wider swingarm. This will allow you to run a 17" wide rim with the latest sticky rubber. CycleX has offset front sprockets that may be needed to mate with the wider rear rim.
     My personal preference is for aftermarket pistons made within the last 10 years. They are a lot lighter than the old big bore pistons still available. Upping the compression will also wake up the motor a lot. You can get 10.5 to 1 and 12 to 1 piston sets from multiple places. You can make horsepower above 12 to 1, but you will overheat your motor getting back down the return road to your pits. A tow bike becomes necessary. You want to control your oil temperature when running. This helps with your consistency. Charging systems aren't necessary for drag racing. Charge your battery between rounds with a 6 amp/hr battery charger to keep the voltage up. (highest rate you can charge a battery without venting by removing cell caps) Aftermarket ignitions are far better than factory. Pick your choice.
     Dragracer has it sorted out. Below you can see his how/why. The quarter fairing helps with the 100+mph trap speeds. Just my 2 cents!

                                                                                Hondamatic

[dragracer]

ok will start on frame/motor soon i want to get the motor completely stripped first. im thinking heres what i do for the motor. from top down please make suggestions if i should change someting. stock cam, stock springs, aluminum retainers, stock valves,hd cam chain, hd studs, forst or second over pistons, stock rods, balanced crank(maybe polish it), no alternator?,new primary chains, undercut gearbox, kick only, aluminum sprockets(open to ideas), all new gaskets and seals, what size rear?(i want to run street ET). struts on the rear, -2 tubes in the front and slammed, im thinking sidewinder exhaust.

Sounds ok for an economical build but why not go 836? Do use HD valve springs in place of new stock springs. At high rpm's you don't want the valves needlessly bouncing off the seat even with a stock cam. Get a good valve job with some mild port work if your budget will allow. CycleEx does sell some decent spec cams for not much $$$ so go that route also or look for a good used cam on Ebay. I recently picked up a nice used RC315 cam myself off Ebay as a back up( i run a RC327 in my motor). In my opinion, you will find all of your power in the head so spend your money getting it done right. Although a good aftermarket ignition controller is a plus, you will do just fine with a simple Dyna S pickup with Dyna coils and good wires.  Follow Jons recommendations on the carbs- he is good in that area along with other wealth of knowledge about the SOHC.

Try to find a used set of HD steel rods if you aren't ready to spend big dollars on new aftermarket rods. The bottom end is a very important part of the build- remember the old saying- pay me now or pay me later. Yeap, do completely away with the alternator assembly. I would use steel sprockets and a 530 chain.  Use shocks if you can to help with weight transfer- there are short ones available.  Consider the mono shock set up. I just did one on another F model street bike using a Katana 600 arm, shock, linkage and frame mounts.

[BLUE71TURBO]

 Hey MasterChief,  Just bolt on a turbocharger and HOLD ON !!       

[MasterChief750]

Could be fun but I dunno. Looking more for consistency than brute force

[dragracer]

Could be fun but I dunno. Looking more for consistency than brute force

Now you're thinking. Consistency comes with a good chassis set up. Damn, i said i wasn't going to beat that horse anymore-lol .

[MasterChief750]

not much of an update but threads are worthless without pics. heres said motor. its a 78 k motor. we shall see where we go from here

[dragracer]

Take your time, you want to do this right the first time. Don't hesitate to ask questions from others who have already gone down this path like the Sam Green crew, Jon Weeks and Mike R. Listen closely but take what you need and leave the rest behind.

[MasterChief750]

thanks for the advice. might have some time friday to go off with its head.