View Full Version : Rebuilding a CB750A motor bottom end
11-16-2008, 09:40 AM
I'm elbows deep in the rebuild of a CB750A motor and I've found some bad stuff. This motor must have been run hard before I got it, and then left set for a long time. Both clutch packs seem to be shot. The friction discs are worn way thin and the edges of the material is all cracked off. The metal clutch pack plates are worn through the chroming to the material underneath. The baskets are rusty. Basically, time for a really thorough rebuild. Anyone got any parts
I know there are a couple dragracers of 750A's here. Have any of you guys rebuilt the bottom end before Any tips, tricks, pitfalls, etc
The other (larger) problem is that all the springs on the damper of the input shaft (behind the torque converter) have shattered. I did a little reading and it seems these are a very weak link in the bottom end. What have you guys done about this I'm certain that these springs wouldn't last one pass in a drag bike, since I've found guys that broke them at 2200 miles in a street bike. I've gotten the damper apart and pulled out the ruined springs. I've got several options now. I could replace them with steel rods, but what would the lack of damping do to the rest of the motor I had also considered polyurethane rods in there to provide slight damping, and lastly I could put springs back in so long as I found some strong enough and the right size. Anyone discovered a good solution to this one
I'm redoing the bottom end very strong because I'm turboing it with an old Rajay setup.
Thanks for the help.
11-17-2008, 04:20 PM
HELP WILL BE COMING : CHAD- A - MATIC [ CHAD INSLEY ] IS ON HIS WAY HOME FROM CALIFORNIA AFTER THE BIG DIV. POINT RACE RUN OFF = HE KNOWS THEM MOTORS. I MY SELF GOT MORE THAN A FEW 5-6AND KNOW WHERE A BRAND NEW UN-RUN MOTOR NEVER IN A BIKE IS SITTING. TRUE STORY..................... GOT TO DRIVE A LITTLE TO PICK IT UP WHEN THE SPRING WEATHERCOMES BACK AND GAS IS STILL $1.66 A GALLON. GOT TO SEE THAT TURBO RUN A MATIC AND I'M GOING TO THE SALTS TO RUN 120 -130 MPH IN A MILE ON A ALL MOTOR SOHC.http://forums.dragbike.com/smileys/smiley2.gif
POPS HONDAMATIC MAN BK666
11-17-2008, 07:07 PM
Did ya check EBAY yetI know you will need to beef up the tranny(2nd & 3rd gear)Make sure the head stays bolted down on the cyl.`s.....Weld the basket (rivets) when you get the springs changed.....With a turbo you will need to make sure she stays sealed up good,maybe a stud kit upper and lower....JMO...
11-17-2008, 08:17 PM
paul, he's talking about the automatic. heavy duty cylinder head studs and cam chain are a must. they are still available. i don't know if you can replace those springs with something else or not. i just always find some good ones and change them. broke 1 tranny and i bet my 750a has over 5000 passes on it. 2 people raced it before me. all stock parts in the tranny.
11-17-2008, 08:39 PM
Aaaaah, ok....Ya whole different critter....Never mind,LOL..... Edited by: Stuartracing
11-17-2008, 10:12 PM
Honda3303, you got any good springs (or a good input shaft) I'm surprised you guys haven't had problems with those springs. I've talked to several people who've never drag raced there's and they broke those springs.
Anyone got a good bottom end (or clutch packs)
I'm definitely putting the APE cylinder studs in the engine. The actual plan is DOHC750 rods (1.5mm shorter, but stronger than SOHC rods). A Wiseco836 forged big bore setup (10.25:1 CR) which drops to 8.4:1 with the DOHC rods. I'm going to deck the jugs to bring the CR back up to 9:1. Dyna2000 ignition for the programmable timing curve. All new consumables (cam chain, bearings, etc). I'm getting the adjustable cam sprocket to run the cam timing about 1.5 degrees advanced on the stock 'matic cam. Going for a stout street motor at 6-8 psi.
11-17-2008, 10:14 PM
'couple pics of the early mockup (Amen rigid frame)
Edited by: JLeather
11-17-2008, 10:14 PM
I got an e-mail from Ken at Cycle X today. Check out their web site here (http://www.cyclexchange.net)
Pretty cool Honda SOHC stuff
11-18-2008, 12:59 AM
Check this site out;
sohc Honda (http://cbrzone.com/sohc.html)
I WOULD REALLY SUGGEST SOME FLAT TOP PISTONS AND THE RODS AT
THE BOTTOM OF THAT PAGE.
11-18-2008, 06:36 AM
I had considered that route but I don't know anyone who makes forged flat-top pistons. Also, I've spoken with some guys who race these engines in mini-sprints (cut the transmission off and put a driveshaft off the alternator side to a 4-speed box). He said he's tried (and broke) pretty much all the different rods and that the stock CB750DOHC and CB900DOHC rods held up about as well as any of them.
11-18-2008, 11:57 AM
My two cents. I'm one of those 750A dragracers. I've never had any problems with the springs on the dampener. Old age and lack of maintenance may kill them. In thousands of passes, not one problem there. Replace the primary and cam chains. Yes, I've lost both of those on different occasions goind down the track. My fault for lack of checking them out regularly after a few seasons. Everybody's opinions on rod replacement is based on the clutch versions. Stock rods don't take shock loading well. The automatic tranny limits the shock factor. With relatively low boost, I don't think that you'll have a problem with the rods you chose. Check the rod dimensions before installing. New rod bolts won't hurt your peace of mind. The weakest link I've found is the high gear clutch pack. Fresh fibers with roughened up steels should hold up. Full throttle shifts with horsepower strains that clutch pack. It's smaller than used on clutch bikes. Both clutch packs cushion the drivetrain. They were built for comfort, not speed!
My local yard would sell me a motor for about $150 today. It sounds like you need another parts motor. No, I don't have one to spare now. Yarded motors are either very low mileage or very high. I've seen dozens under 10K miles and less with 30K+. Nothing between... If you get into a yard for parts, try a standard clutch cam instead of your stock one. No piston work, but a slightly stronger horsepower unit. I shim the bypass spring for a little more oil pressure. The most important addition is a good external oil cooler. You're putting a lot of heat into an air cooled motor. You can't use synthetic oil unless you want a lot more slippage.
If your motor is as shot as it sounds, please check out the shaft bearings before reassembling the motor. They do wear out with neglect and high mileage.
The 900F crank does drop in, but is a lot of work to self start on the street. You get a 3mm stroke increase for your efforts. Balence your rod/piston combinations and keep the stock redline. The power is falling off about a grand before redline anyway. The triumph juice generating system can and has been adapted to the 750's. It cuts the weight hanging on the end of the crank. The inexpensivealternative is to chuck up the end "flywheel" in a lathe to cut down some unnecessary OD weight. Don't ding the ignition trigger nubs! Spend a little time on a chassis dyno to get your carb setup correctly. On the street, keep it a little on the rich side. Your motor will thank you for thousands of miles.
Just my 2 cents of random thoughts....
11-19-2008, 08:20 AM
Hondamatic, thanks for the insight. It seems that this engine has just been run hard for too long. Even the steels in these clutches are shot, they're worn through to the base metal. There are a complete set of clutch packs available on eBay from a low miles bike that I can get for less than it would cost me to rebuild these. If I can find replacement springs (or even a replacement damper) I can rebuild this bottom end.
Cam chain is a must. I might have to take a link out of it to compensate for decking the jugs (hopefully the tensioner can just take up the slack of the .060" loss). New tensioner is also going in cause the tensioner wheels are hard as a rock. I've got several stock K-motors to scavenge parts from. A few 77/78F's, a couple early K's. I've got plenty of cams.
How do I check the shaft bearings The needle bearings on all the shafts seem ok. No unusual slop in the races, no worn spots on the shafts where they ride. Is there a better way to check The ball-bearings at each end also seem ok, but if I can get replacements I'll do it.
Do you have instructions or pics for shimming the bypass spring Sounds like something good to do.
I'm gonna call my local bike boneyard today and see if they've got any better bottom ends to start from. Anyone know the cheapest place to get new fiber-disks from
11-27-2008, 07:34 AM
ON THE HONDAMATIC'S : The stator is already lighten from the factory = I got a hop up "old time 1978 book" on these motors and the book gives you many tips on HP for free and other things you can do to get the most out of a CB750A. The cam from a clucth bike is the fix but I have a grinder guy who took my stock 3,000 miles cam and made it a high lift cam, then I had it and cam followerscryogenic by a company in OHIO that I met at a trade show.http://forums.dragbike.com/smileys/smiley2.gifThis big smile guy [ Chadmatic HONDA 3033] twisted my mine years ago about this SIMPLE MOTOR and every day I think about how,what,where and when you can squeeze a little more out of these FARM HORSES pulling in those heavy frame rails. http://forums.dragbike.com/smileys/smiley5.gif Some day soon I will be selling a TROY SMOCK BUSA motors just like GM crate motors, damm I wish that boy would build me a HONDA STROKER MOTOR.
POPS BK666 El. Diablo
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