Topic: CB750A Performance Mods

[dave the welder]

back in jan 2013 I posted about swapping round tops on my 77.well a couple of weeks ago I broke down and swapped in a stock 74 k cam.no using the SWAG method(Scientific Wild Ass Guess)of butt dyno tuning i have the following observations. the bike starts up like it did before, idles like it did before,accelerates slightly better from a stop.the biggest improvement is from 25mph or so on up.it is stronger and will pull to 65mph where the stock cam runs out about 60.was it worth the time and energy to swap???maybe.i thought it ran fine before  now its a little better.maybe its that SWAG thing but it seems like there is more compression braking when you let off the throttle   

[jweeks]

jweeks, what is the cam you would recommend for an engine over 1000cc?

#88 regrind from Web Cam

[TurboD]

I am currently tinkering with a automatic and wanting to increase performance. I've owned a few stockers in the past and being a person that always has to try to improve something I am thinking about taking on a auto as a winter project.

I have a extensive 30 year background building racing engines for bikes, cars and about anything with a engine.

That said. I have questions about the auto for more performance and or racing.

My first questions are cam related. It appears to me that there were 4 different stock cams for the CB750. The K<6, K7-8, the A and the F2-3. After looking at the specs in the manual, wouldn't the K7-8 and F2-3 be the best choice when staying with a stock cam? I can see a improvement when using the K<6 over the weak stock A cam.

What kind of RPM are the guys drag racing turning?

[jweeks]

     Sam's 915 cc Hondamatic shifted at 10,500 rpm. My 1,080 cc motor has seen 10,900 rpm at the finish line with a 11k chip in the MSD to limit it. Why do we rev them so high? Because we have only two gears. The powerband on both motors is falling away rapidly above 10k. The bikes go quicker in the quarter mile when the only shift allows the motor to go into high gear in the meat of the powerband. I use Quarter Jr. prediction software to figure out how to get the lowest ET's possible. You overwind the powerband on the top end because you gain a little more at the start than you lose on the top end. Without proper porting and valve springs, you're wasting your time above 10 grand.
     Street Hondamatics need torque. That's where the fun is! Torque improvements come from ring seal, compression increases, better midrange cams, and more stroke. More air in and out of the motor helps everything with the right air/fuel ratio. Yes, either cam you've suggested will make an improvement. I like to use an adjustable cam sprocket to advance the cam to improve the midrange at the expense of the top end. Several members have tried the K carbs and cam. Read the feedback. It makes a difference. If you want a more noticable improvement, you have to spend more $$. 

[Medyo Bastos]

     Sam's 915 cc Hondamatic shifted at 10,500 rpm. My 1,080 cc motor has seen 10,900 rpm at the finish line with a 11k chip in the MSD to limit it. Why do we rev them so high? Because we have only two gears. The powerband on both motors is falling away rapidly above 10k. The bikes go quicker in the quarter mile when the only shift allows the motor to go into high gear in the meat of the powerband. I use Quarter Jr. prediction software to figure out how to get the lowest ET's possible. You overwind the powerband on the top end because you gain a little more at the start than you lose on the top end. Without proper porting and valve springs, you're wasting your time above 10 grand.
     Street Hondamatics need torque. That's where the fun is! Torque improvements come from ring seal, compression increases, better midrange cams, and more stroke. More air in and out of the motor helps everything with the right air/fuel ratio. Yes, either cam you've suggested will make an improvement. I like to use an adjustable cam sprocket to advance the cam to improve the midrange at the expense of the top end. Several members have tried the K carbs and cam. Read the feedback. It makes a difference. If you want a more noticable improvement, you have to spend more $$. 

great info for sure!  i am stuffing a B-25 on one currently...

[jweeks]

Can the f2 head bolt straight onto the Hondamatic engine??

     Yes, and it will pour oil onto the cylinders. You have to sleeve a few head bolt holes to match the K style head to eliminate the draining of oil onto the outside of the motor.  It isn't recommended for several reasons:
Bigger combustion chamber will lower the compression ratio
The bigger intake ports won't like the low air requirements of the stock displacement, small cam, and small carbs. It will probably run worse than with the stock head.
     The F heads like big displacements (I like them for 1,000 cc +) better than stock cams (I like the DP-295 for Automatics) and bigger CV carbs (34 mm is the biggest you can fit with stock intake manifolds). A little porting also will help. Otherwise, you need lots of revs to take advantage of the air it can flow. That's not what most automatics do, especially on the street!       

[122JG]

is there a shop, or individual builder, that one could commission to build a "turn-key" performance 750A ...?

[M@gneto]

When you put the CV carbs in, what did you do for throttle cables? I found a set of Seca carbs I could use, just wondering if I need Seca cables or if the Hondamatic cables will work. Also, the choke, stock cable or something else?
Thanks, Mag

[jweeks]

Hondamatic cables will work. Pod air filters needed for the street. I also like 4 into 2 into 1 exhausts to improve the midrange. Seca 750 carbs bolt up with clutch version intake manifolds without issues. Just a main jet change and you're in business.
Sorry that I haven't been reading this topic for a while.

[kmb69]

jweeks, what is the cam you would recommend for an engine over 1000cc?

#88 regrind from Web Cam

Can the f2 head bolt straight onto the Hondamatic engine??

.....
The F heads like big displacements (I like them for 1,000 cc +) better than stock cams (I like the DP-295 for Automatics)
.....

Jon, Are you flip flopping? Learn something new? Did I miss something? Please explain.

[HondaMan]

If I might dream for a moment about the "A" I would build (for street) if I could lay my hands on one...
If I could not afford a RetroRocket turbo (perfect!), it would go about like this:

1. Low-compression 836cc kit from Cruisinimage, unless I could lay my hands on an 811cc 10:1 CR kit somewhere, the older Yosh style with 1-piece oil rings...if the 836 bore, I would mill the head 1mm to get into the mid-9 CR range, then flycut the valve notches in the pistons a bit. Either one: use 0.0008" to 0.0010" piston clearance, and set the ring gaps to the middle of their spec ranges.
2. Cam: (remember, street...city traffic and all that...) 1970-71 K0-1 and make sure it opens at 3-5 degrees BTC as it originally did. Or, a Megacycle 125-00, dialed to similar spec.
3. Carbs: no matter to me, whichever ones have the little 'idle solenoid' to keep it alive automatigally at stoplights. Jetted as need be, in the end.
4. Airbox: Using the OEM box, I would make a long tunnel to the rear of the bike and breathe down a tube to feed the bottom of the box. I've outlined those reasons in other posts.
5. Head: I'd install 3-angle seats and if I could find them, 30-degree face valves for the intakes. The ports need a little help directly over the intake valves, and the exhausts usually have little stumbling blocks in them that should be removed. Polish the valves, skip the exhaust valve seals, but use bronze guides (like APE) and set the valve clearance close, like .0006" to .0008" (like I just did on my K2). Remove the little retaining bolts from the rocker shafts.
6. Bottom end: make sure the crank and rod bearings are all at 0.0008" to 0.0010" clearance, no more.
7. Cooling fins: deflash everything. Then do it again. After 30 minutes, this makes a surprising difference on the street.
8. Coils: I'd use the Sakura versions for the CB750 1976-1978. They make more volts than stock, and have the same spark duration (PartsNmore sells them). I'd stay with points, but the opening ramp of the spark advancer would get smoothed down a bit (it is abrupt on the "A", for better starting and low-end torque), then use my own Ignition.
9. Chain/sprockets: switch to 530 size, use the Diamond Powersport type. Gearing to be set according to what I was going to do....
10. Tires: no more than 100/90x19 front (lower weight), 110/90x18 rear (got some lacing to do...) also for weight, will still have adequate traction.

This would not be a mountain bike, but a fun stoplight cruiser and to-work-daily commuter! I have found that the tighter I have built the tolerances, the more power, life, and lower oil temps they have. I have even built top ends to 0.0004" valve clearances with no issues whatsoever (bronze guides, though), and pistons to 0.0006" on 2 occasions, and those engines are the most torquey of the bunch I made like them. Since the "A" lives on [mostly] torque, this would be my 2 cents' worth!

[122JG]

If I might dream for a moment about the "A" I would build (for street) if I could lay my hands on one...
If I could not afford a RetroRocket turbo (perfect!), it would go about like this:

thank you, this was very helpful...
I have a 750A motor that I'd like to have redone for a bobber I'm putting together.
now I need to find a builder

[jweeks]

I've been consistent with my cam recommendations. Under 1,000 cc, I like the DP-295 especially with a 4 into 2 into 1 exhaust. Over 999 cc, I like the Webcam #88 regrind. Either cam likes a well ported head, good compression and CV carbs. 32 mm CV's for up to 915 cc and 34 mm for 836 cc up to what your wallet will allow (1200+ cc).  If you put heavier pistons than stock in, steel rods will make it live. No, these motors don't break rods, they just stretch the small ends of stock rods. I don't know of any good engine builders but there are a lot of engine assemblers around. Find an assembler and tell him/her what you want in it. Leave it to someone who hasn't built any Hondamatics and you won't enjoy the bill and/or results, in my opinion.

[Jerry Rxman Griffin aka MuthaF'er]

Well then who do you use to build yours?

[jweeks]

     122JG wanted a commercial performance Hondamatic engine builder. None of my friends that I know of does that. I never have built any bike motor for anybody but myself. The last motor that I own that someone else did some work on was Rick Stetson. He no longer wants anything to do with building Hondamatics. That's what he told me in person. Mike R. has stated that he's not a motor builder for others; he does head porting and related. Do we go down the list of friends who have built many fine motors but isn't a commercial engine builder? None of my friends should take exception to that.
     My point is that there are very few people who have built performance Hondamatic motors. Most on this forum have built clutch CB750 based motors. For an automatic, the carbs need to be different; the cam needs to be different; etc. Until Sam's build, there was very little information about what a Hondamatic motor needs on this forum. I'm not sturring the pot, but others may be.

[Tintop48]

Hello to this forum, especially this topic. I own a 77a which I installed a k1 engine in a few years ago,fun but am getting up in years and would like to re-install the a-engine. My concern is the Carb issue.finding Carb boots are impossible for the smaller a-carbs.I read earlier to use k boots with k carbs,but the spigots on the a-head are smaller.can someone explain how to do this. Thanks

[Medyo Bastos]

the spigots are the same tintop.  you can use k model carbs with k model boots just fine.  what will you be doing with your k motor?  where are you located?

[Jerry Rxman Griffin aka MuthaF'er]

I'm thinking all the A's used the -392 head from the 75/76 CB750F. Therefore the spigots are the early head size spigots.

[Tintop48]

Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs. Yes I'm going to get rid of the k1 reside in kalifornistan and the engine weighs 225+- so its a bit## to ship

[kmb69]

1. Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs.
2. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs.
......

1. Carb sizes, 24mm and 28mm, are correct. But the A head has the same spigots as the K head and will easily mount the K1 carbs.
2. The K1 carbs are definitely less complicated IMO. No need to Mc Gyver, stock K1 carb boots will work.

[Medyo Bastos]

Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs. Yes I'm going to get rid of the k1 reside in kalifornistan and the engine weighs 225+- so its a bit## to ship

specific boots are what adapts the different carbs to the head.  you need matching boots for matching carbs.  i prefer the f2 carbs on matics...

[Yoshimatic]

I have K1 carbs on my 77 magic and it goes like hell. Mainly because of the 836 kit, F2 cam, race valve springs and Yoshi pipe though.

[jweeks]

First pass on Vic Force's record holding Hondamatic twin. Perfect reaction time. Any Hondamatic can be modified to run better on the street or the track.   

[kac3177]

Any tips for tuning the carbs? I am doing basically the same exact set up. Any initial setting recommendations? First time usin the round tops. They are jetted to 120.

     I used pd41a's on my 76' jetted to 115, very fincky and cold blooded, that motor only has an aftermarket cam, otherwise stock. Once warmed up it runs good, definite change, lot more top end however I wish it ran a little better when cold.

[dave the welder]

kac read back through the older post several of us posted how we set up the carbs