Author Topic: CB750A Performance Mods  (Read 23730 times)

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Offline raymond10078

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CB750A Performance Mods
« on: January 24, 2013, 10:37:31 am »
I'd like this thread to concentrate on cheap/moderately priced performance mods for the CB750A.  For a detailed Hondamatic build for drag racing (lots of $$$ spent) go here:  http://forums.sohc4.net/index.php?topic=61876.0

I've read a lot of info, and discussed mods with other members of this an other forums/websites.

While this information is "out there" for those who are good (and have the time to) search, let's put the info in one spot.

Most importantly, SHARE WHAT HAS WORKED FOR YOU.  THE MORE THE DETAIL, THE BETTER!

Here's what I can share:

Carbs:
  • Pre-77 CB750K carbs ("round tops") will fit (and work well).  Need to buy pre-77 K carb rubbers/isolators (which are still available new).  I don't have any info on the airbox rubbers (do the K carb rubbers fit?).
  • Yamaha 650 Maxim CV carbs bolt-on using pre-77 K carb rubbers/isolators, using pod air filters.

Cams:
  • A stock K cam bolts in and works well (run it a little advanced).
  • Other aftermarket cams (41, 41A for example) work well, too.
  • The DP295 cam is regarded as the best Hondamatic cam.

Links to other feedback:

Carbs:  http://forums.sohc4.net/index.php/topic,76842.msg1006542.html#msg1006542
« Last Edit: November 18, 2019, 10:56:09 am by raymond10078 »
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Offline Sam Green Racing

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Re: CB750A Performance Mods
« Reply #1 on: January 24, 2013, 03:54:26 pm »
There are only 4 things to do to wake up the auto and you have already mentioned 2 of them. (carbs and cam)
A set of 836 pistons and you would be real surprised.  You don't need to spend anywhere near what I spent, the money I spent was for a reason.

Sam. ;)
« Last Edit: January 05, 2014, 07:32:24 pm by kandrtech »
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Offline salukispeed

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Re: CB750A Performance Mods
« Reply #2 on: January 24, 2013, 04:50:03 pm »
Who makes and at what cost for a nice 836 low to moderate compression kit. Back in around 75-76 I used to run lots of cam and compression on my K4 836 and 911 motors but heat was often the delema in traffic in 95+heat. Had one in a Rickman chassis for a few years. Seems like moderate compression of 9-1 or 10-1 max would be doable plus a big oil cooler
1974 CB750 K4
1970 CT70
1966 CA77 (305 Dream)
1984 GL1200 Interstate
1977 750A
1972 CL100

Offline raymond10078

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Re: CB750A Performance Mods
« Reply #3 on: January 24, 2013, 05:27:51 pm »
There was an ebay set for $110 plus shipping.  Reports on this site are positive about that kit.  Quite a few sets installed by members - no problem reports yet!

The seller appears to be out of stock right now.  This has happened before - he (so far) comes back with more to sell:

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-BIGBORE-PISTON-KIT-W-GASKET-/160904076629
« Last Edit: January 24, 2013, 05:41:27 pm by kandrtech »
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Offline jweeks

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Re: CB750A Performance Mods
« Reply #4 on: January 24, 2013, 06:10:48 pm »
Here's Cycle X's web page for pistons.  http://www.cyclexchange.net/Engine%20Parts%20Page%20%28Non-Cafe%29.htm

     Sometimes a piston deal will pop up on Ebay or the Forum's Race and Performance parts for sale and wanted on the high performance section

     The biggest performance improvement for most Hondamatics is ring seal. Compression improvements always help.
     Seca CV carbs are a bolt on with K manifolds and pod filters. Some main jet tuning is recommended.
     As you go for bigger pistons, remember that you need more ignition advance to fire the mixture over all of the piston surface. Ignition using a points eliminator and good coils also helps.
     There is a weight limit for bigger pistons with stock rods. You don't break the rods like the clutch versions may do. The small ends of the stock rods will oval with the increased piston weight.  Effectively you will stretch the rod a little reducing the valve to piston clearances. I've done that with a race Hondamatic 1,000+ cc motor. 836 pistons near the stock OEM piston weight should last for many years. 10.5 to 1 pistons will run on pump gas without issues. You'll have difficulty running lower compression since most pistons are now 10.5 to 1. Thicker head/base gaskets will allow you to drop the compression ratio if you want to run cheap gas.

     As you improve power, your oil's heat will increase. A bolt on oil filter adapter to an external oil cooler from a parts yard or Ebay/Craigslist is insurance for warm climates and or higher rpm extended running.

     Bolt on is a simple term that may end up not so simple if you ignore physics.  Bigger pistons, more compression, bigger carbs, better ignition, and a good cam will really impress you when you get it done. The challenge is to not wind it too tight for too long without balenced pistons, rods, and good valve springs. That's where Sam spent serious money to allow him to wind it really tight. (10,500 rpm shifts)
     If you have very high mileage on your motor, the clutch packs will need replacing. Usually that shows up initially on the shift with an rpm spike. Old motors with top end improvements may still have worn out bearings and clutch packs.
     I would look into the 836/850 cc area with overbored stock sleeves. A 'K' clutch stock cam, and at least stock K carbs and manifolds with a good head that has valves that have been freshly touched up will really wake it up. If you put more air into the motor, you need a way to get more air out of the motor. There's lots of exhaust options beyond improving the sound of it. The stocker will work if rust hasn't weakened it too much. See Jim F. for the gasket sets. They're here now, but may not be in the longer run.
     Sam's stock Hondamatic ran low 18's in the quarter mile with a lot of rust in the tank. With an 836 kit, good CV carbs, good ignition, the 295 aftermarket cam, aftermarket rods, a Mike R. ported head, and an 4 into 1 exhaust pipe it ran a best of 12.30's in the quarter. That's with streetable 10.5 to 1 pistons. If you hear of Bill or Sam racing his Hondamatic, go check it out someday. You'd be very surprised at how docile the motor is. It revs much better than a stocker, but it will run 5 mph around the pits for hours if you want.
     I've raced Hondamatics since 1984. I used to drive it to and from the track for a couple of years. The gearing wasn't so street friendly, so I started hauling it in a pickup.  Sam's bike is now up to  915cc and will eventually run 11's in the quarter. My stock '84 GPZ1100 will barely break into the 11's in the quarter. They will surprise you with even mild performance improvements.

                                                  Jon Weeks
« Last Edit: January 30, 2013, 06:57:09 am by jweeks »

Offline Mr Freeze

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Re: CB750A Performance Mods
« Reply #5 on: January 27, 2013, 04:50:52 pm »
I have a question about carbs. Since the K Carbs will work, which is the better bet? Early model carbs or later model carbs? I have a 1978 CB750A so that year K or A has the accelerator pump. I know that feature was added to give the the bike a little shot of fuel to help with throttle response. Also, how does the idle pickup attach to a K set?

I guess I really want to just know which is the better K carb set to look for, 69-75 or 76-78?


Offline cwilli78

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Re: CB750A Performance Mods
« Reply #6 on: January 28, 2013, 04:58:51 pm »
If you were to prioritize the 4 mods mentioned earlier in this thread, which would be first? K carbs, 836 kit, K cam, and K head/valves...On the fence about engine mods on my hondamtic..Thanks!

Offline Sam Green Racing

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Re: CB750A Performance Mods
« Reply #7 on: January 28, 2013, 05:49:34 pm »
When you say prioritize Willi, are you asking what you should do first?
In my mind I would do them all at the same time, what's the point of pulling the rocker box to put a cam in when you are going to have to pull it all again to do the pistons or head.
If you cant afford to do all the 4 mods at the same time, just save your money until you CAN afford all the parts.

Sam. ;)
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline raymond10078

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Re: CB750A Performance Mods
« Reply #8 on: January 29, 2013, 05:36:10 am »
Prioritization depends on your time, your money, your capability, and what you want.

IMO, if you're starting with a good runner, and you want to do things on the cheap (or easy - as is no major engine disassembly), then:

  • Do the carbs first.  Why?  it is the only mod that can be made that doesn't "need" other mods to get a decent benefit, and it won't break the bank.  In addition, if you're thinking about rebuilding your carbs, I'd recommend just getting a different set - rebuild them, and swap them over (along with new carb rubbers/holders).
  • Next would be the cam.  Why?  First remember, that I'm in the "do things on the cheap" mentality - AND a cam with stock carbs isn't going to work well.  You'll need more air than the smaller auto carbs can provide.  The big plus is, if you are hard on cash, you can buy a good used K or F cam for $50 or less.  The cam can also be installed without major engine surgery.

The carbs and cam at the same time would be even better.

These two "firsts" will wake up your bike, and you will enjoy the difference.

Beyond the first two, it again depends again on what you want, the "health" of your bike, and your pocketbook.

The biggests problem we have (again, my opinion) - is that we know that we want more - but we don't know how much more.  My CB750A gets dusted by my sons Honda CM450 off the line, and I don't like it!  So, I know I want more.  I've never ridden a non-auto (and stock) CB750, so I don't know if that will be enough.  But I do know that the cam and carbs can be installed without major engine surgery (and expense).  I'll make this change, and see how I like the bike.

If I still want more power, then I will be going the 836 route, and will also have some head work done (just a mild clean-up).  In the mean time, I'll be riding a more enjoyable bike.  At most, the only money "wasted" would be the cam - as I would most likely put in a good aftermarket cam to make the most of the 836 set up.

I chose the carb+cam route based on cost AND I knew that I wanted something better sooner rather than later.  I can't spend the $$$ for the 836 pistons, the boring work, the head work, right now.  Looking down the road, this may be a fall/winter 2013 or 2014 project, but not today.  So, my choice is - can I get more now - on the cheap - the answer is YES - and that's what I'm doing.

If you've got the money and the time, then certainly do all four at once.
« Last Edit: February 04, 2013, 05:12:22 am by SemiAutoFun »
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Offline jweeks

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Re: CB750A Performance Mods
« Reply #9 on: January 30, 2013, 07:54:29 am »
    Yes, the K carbs and cam are a cheap, noticeable improvement.   

     If you change the pistons, the cast $110 Ebay specials are about a point lower compression than the other ones out there (Wiseco, Cycle X, etc) Yes there will be an improvement with either one but more from a 10.5 to 1 than from the 9.5 to 1 pistons.
     Cams are a very interesting challenge. The modified clutch motors like a far different camshaft than the Hondamatics will. It's the classic tale of horsepower vs torque. Most clutch 750 based motors like the big horsepower numbers at high rpms. Hondamatics seldom if ever run at high rpms. What you want for the street is more torque at the rpms that you ride at. A lot of the clutch street CB750's like the -75 cam. It's the wrong cam for a Hondamatic. The 41 or 41A would be better, but the Dynoman 295 just flat works on mild modified Hondamatics. Midrange torque is what that cam gives you. Sam's bike started with a -75 cam. When the 295 got swapped in place of it, the bike was over a half second quicker in the quarter mile. Less top end horsepower, but better torque where you need it. It was quicker on Sam's bike in both the 836 and 915 cc versions.
     For more midrange torque, I recommend advancing the cam using an adjustable cam sprocket from APE and others. About a 5 degree advance will use up about 80% of the slot adjustments. If you go big on your motor (over 1,000 cc) there's a different cam that I'd recommend. The pistons need proper valve pocket clearances when you go to aftermarket cams.
     If you are changing the pistons, check the head for good valve seat seal. If it smoked on start up before, it only gets fixed when the head is checked out properly. If you're spending the money on a new aftermarket cam, I'd recommend some stronger than stock valve springs. Even though you won't regularly ride your 'matic at high rpms, most people will still test it out now and then. A valve not closing quick enough will make shrapnel out of your dollars invested very quickly.
     
     Since I run 450A's as well, the weight of the 750A's is a handicap against the lighter, smaller twins. The Honda 450A's lose a roll on contest to the Suzukimatic 450's due to the Suzuki's much longer stroke, giving more torque over the Honda at similar weights. Even the 450A Hondas can be significantly improved from stock. A friend running my modifications has run mid 13's in the quarter with a streetable 450A based combination. I also own a Suzukimatic that I haven't had the time to "improve".  ;)

                                                    Jon Weeks
« Last Edit: January 30, 2013, 08:18:40 am by jweeks »

Offline Mr Freeze

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Re: CB750A Performance Mods
« Reply #10 on: January 30, 2013, 07:32:30 pm »
So are earlier or late model K Carbs a better bet?

Offline salukispeed

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Re: CB750A Performance Mods
« Reply #11 on: January 31, 2013, 09:34:56 am »
The old Round top K model carbs were completely tunable including the Pilot jets but I suspect  some of the later model ones are limited since the jets can not be removed due to emission requirements. I do not know if this is fact. Some of the goldwings are very limited this way and real hard to get cleaned properly if badly gunked up from sitting. We used to bore the throat of the round top carbs almost 1 MM and get even a little more by boreing the roof of the throat and were able to jet accordingly.
1974 CB750 K4
1970 CT70
1966 CA77 (305 Dream)
1984 GL1200 Interstate
1977 750A
1972 CL100

Offline dave the welder

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Re: CB750A Performance Mods
« Reply #12 on: January 31, 2013, 07:55:50 pm »
ok this is my disclaimer  I AM NOT AN EXPERT . now with that out of the way swapping a set of round tops really made riding my 77a so much more fun.it starts and idles so much better than the stock carbs.now if you yank it wide open it will stumble,however i know this so i don't.i ride it like the old guy i am and it gets out of its way with no problem.even if you start off in high gear its fine.don't know if i will swap cams as it works good enough for me.
don't buy it build it

Offline cwilli78

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Re: CB750A Performance Mods
« Reply #13 on: February 03, 2013, 08:00:50 pm »
i just wanted to say thanks for the info you gents put up, it really clears things up..much appreciated!

Offline cb750aowner

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Re: CB750A Performance Mods
« Reply #14 on: February 04, 2013, 07:18:54 pm »
ok this is my disclaimer  I AM NOT AN EXPERT . now with that out of the way swapping a set of round tops really made riding my 77a so much more fun.it starts and idles so much better than the stock carbs.now if you yank it wide open it will stumble,however i know this so i don't.i ride it like the old guy i am and it gets out of its way with no problem.even if you start off in high gear its fine.don't know if i will swap cams as it works good enough for me.

Give us the details:

What did you do for jetting?  Did you just use the "stock" jets that came with the carbs?
I assume that you bought round-top carb holders.  What did you do about the airbox connection?

Offline dave the welder

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Re: CB750A Performance Mods
« Reply #15 on: February 06, 2013, 06:35:45 pm »
use the rubbers that come with the round tops.also use the air box that comes with the round tops.i left the carbs pretty much stock.just cleaned them up.i am running 115 mains.i think stock were 120
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Offline Mr Freeze

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Re: CB750A Performance Mods
« Reply #16 on: February 06, 2013, 08:03:17 pm »
Will any year camshaft work?

Offline raymond10078

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Re: CB750A Performance Mods
« Reply #17 on: February 06, 2013, 08:19:33 pm »
Most will work.  I'd stay away from the 77/78 F cams.  A jweeks has said, he believes that the K cam is a better cam than the 75/76 F cam - based on where the peak power will be on a F versus a K.

Hondaman's book talks about advancing any cam for more torque/power lower in the RPM band, which should work for us.

My thought (this is what I'm going to do) is the buy a K cam, and either run it "straight up", or advance it some - maybe like 5 degrees.
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Offline Retro Rocket

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Re: CB750A Performance Mods
« Reply #18 on: February 06, 2013, 09:06:48 pm »
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Medyo Bastos

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Re: CB750A Performance Mods
« Reply #19 on: August 05, 2013, 07:55:49 pm »
i have run the f2 carbs with great results!  i was quite surprised what that matic was able to do with just a simple carb swap. 

Offline notn

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Re: CB750A Performance Mods
« Reply #20 on: January 03, 2014, 08:15:32 pm »
Thank you for this thread.  I have a 77 750a trike built for my wheelchair and trike, me and power wheelchair run right at 1500 pouinds. I want to get some bolt on help with little or no dependability issues.  This thread has been a load of help but one question.  are the 750a pistons different from J or K? seem they would be skirt wise.  I see pistons on ebay but most are for k.  I would rather just bore out one over and stay close to stock to get better comp on old motor. but can't see any pistons specified for the "A"..thanks again for the thread.
neal
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Neal Chamberlain
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Offline jweeks

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Re: CB750A Performance Mods
« Reply #21 on: January 04, 2014, 04:59:46 am »
Aftermarket pistons will fit the A, K, or F models. Using the stock block, you can go 838cc or 915cc resleeved. Go with a big block (like from Mike R.) and consider the APE +5mm stroker crank if you want to go over 1,000cc. All of them can be reliable if you control the oil temperature.

Offline raymond10078

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Re: CB750A Performance Mods
« Reply #22 on: January 04, 2014, 07:44:49 am »
The A pistons are different from the K pistons.  The lowered compression ratio of the A bike is due entirely to these differing pistons.

So - you can either go up one size using K pistons, or make the bigger jump to 836 pistons (or go with wilder stuff per jweeks).  Your cam choice will depend on how big you go.

As far as oil temps - yes these changes, combined with the heat of the converter - will raise your oil temps, so an oil cooler is a good idea.  An I recommend using a good motorcycle (non-automotive) oil (don't want to start an oil thread, here ;) ).
« Last Edit: January 04, 2014, 07:48:23 am by kandrtech »
1978 CB750A (upgrading very, very slowly)

Past bikes - Honda: SL350, CX650C, CB900C, CB1000C, CM450A; Kawasaki: several 1972 750 H2's; Suzuki: TC90J.

Bikes I want: CX650ED, a mid-sized japanese V-twin with ABS.

Offline broshae

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Re: CB750A Performance Mods
« Reply #23 on: June 04, 2014, 08:27:24 am »
jweeks, what is the cam you would recommend for an engine over 1000cc?

Offline Yoshimatic

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Re: CB750A Performance Mods
« Reply #24 on: July 11, 2014, 04:01:54 am »
Can the f2 head bolt straight onto the Hondamatic engine??
1969 GL175 twin
1970 CB750 K0
1971 CL350
1975 CB750F1
1977 CB750F2
1977 CB750A (original condition)
1977 CB750A (836 Cafe Racer)
1978 CB750F3