Topic: CB750 Racing

[Retro Rocket]

There is more performance locked up in the heads on these bikes than any other part of the engine, all good performance Honda's have ported heads...

[Roach Carver]

Doesn't the A motor have a stronger crank? Or did I just dream that

[bwaller]

Hey Alan, we have to take the good with the bad, but sounds like you enjoyed yourself anyway. What ignition were you using. PM if you prefer, maybe I have parts.

The 392 head uses the same size valves as the rest of the K heads. However the combustion chamber is smooth as opposed to the shrouding in earlier K heads. This is considered a good head to work with. Porting will certainly unlock a power potential with or without larger valves. You have a few choices but it's a good head to start the process. 

[Haybus]

Finished my winter motor build a couple weeks ago. Fired her up without issue. Fiddle a bit with jetting and needles to make it rideable. Took her out for a ride and what a different bike now. Off to the dyno today and I'm pretty happy with the results.

Rewind a few months. Lightened and balanced crank, super rods, CX7 cam and Keihin CR31's from Cycle X. JE +1mm 12.5:1 pistons from Dynoman (max allowed in our rules). Of course all the other fiddly bits are new too. If they have an HD version, it's in there. I build a rudimentary flow bench and set about porting the head. Mostly followed Hondaman's instructions in his book. I learned a lot in the process, that's for sure, and I had fun doing it. I've never built a high performance motor before (I've rebuilt stock motors though), so lots of research (this forum!!), lots of head scratching, and surprisingly few mistakes. My biggest regret was bead blasting the cases instead of just stripping the paint. I pressure washed, soaked and scrubbed those cases so many times only to find more beads after the cases dried. I just knew there were beads hiding in places I couldn't reach that were going to get into some unfortunate places upon start up. Well, didn't happen so I'm relieved.

Was hoping for a little more peak HP, but I'm pretty happy with the curve. Still slightly rich on the mains, but they didn't have the next step down so I'll leave it there for now.

[turboguzzi]

going by the spec, you should indeed be getting better power.... my 0.02$ ? your torque is peaking at 6000 something? looks too early for me.... would retard the cam a bit and see if you can move that higher up.

[PeWe]

+10hp hidden in the head?
I measured 80 vs 83 rwhp, 836 CR 10:1 something. Cams were Action Fours SS-1 (moderat cam like webcam 41a), Megacycle 125-20 that need more CR and less acc pump stroke wich I did not try before dyno.
 
I did not try my CX-7 cam on the dyno. I'm sure it would not make below 83rwhp. It make the engine to scream very loud!
My head is ported for 34mm inlet valves.  Ported 1983-84. The port design can be different today.
My goal next dyno is +85 closer to 90 rwhp

Send head to Mike Rieck for a removal of obstructive alu and 5mm conv valve kit, 34/28mm. Might be easier to find an extra head to send so you can use bike while waiting. I guess Mike is very busy...

[bwaller]

Good job, you'll feel a big difference. It should be bullet proof, plus you're ready in plenty of time, bonus. When is your first race?

Re blasting, I always bolt an empty engine together with all the covers, screws, plugs, etc. with as little taped off areas as possible to keep bead out of the engine. Of course it takes a big enough cabinet!

[gschuld]

I'm not an expert on these things but I prefer soda blasting.  It has the benefit of being washable afterward and the leftover soda dissolves safely.  Or at least that is what I have been telling myself.

Good news on the new engine Haybus.  I hope it runs great for you on the track.

George

[Haybus]

5mm valves, 34mm seats, professional porting. I'm sure those would net me some impressive numbers. Maybe next year with one of my spare heads. I dialed that cam in as close as I could get to the number I could find and what Kenny said. Another first degreeing in a cam, so I spent a few hours going over everything.

First race is April 23rd. I still have to make an oil tank and mount the Setrab cooler, but I'll be ready for sure.

If I was to do another set of cases I think I would just use Napa spray on paint remover. I've found it works pretty well and gets into nooks and crevices quite nicely. Followed up with those plastic brush wheels on a drill and you get a nice brushed look. A little Blue Magic or Mother's to keep the corrosion at bay. For parts without small oil passages I love my blast cabinet.

Alan

[bwaller]

Soda blast would be nice & the new vapour blasting looks great for cleaning. Originally (correct or not) I wanted a coarser surface to help dissipate heat so I sand blasted the cases, and original cylinder. Mike beaded the head, twice actually and that's a very nice job. I have since beaded different cylinders but the case are still original.

The nice thing now Alan is, if you decide in the future, you can go nuts on a different head while this engine runs.

[Old Scrambler]

Allen...........The piston domes like a slightly retarded cam.......AND......a few degrees of retard on the ignition. Depending on your gas choice......the flame may need more time to get over the hump to put out the most power. On 110-Octane Race Gas, I gained 5-HP by retarding the timing.

If your interested..........I have a CR750 replica oil tank ready to go with internal diffusers and bungs for standard lines. Your choice, flip-lid or standard cap with dip-stick.

[Haybus]

Yeah, that's the plan Brent. I have a few projects in line before starting another head, like building a proper flow bench this time 

Dennis, thanks for the offer, but I already have the shape, the material cut and ready to bend, and all the bungs. Besides, I don't get enough chances to TIG weld. Gotta keep my skills, ahem, sharpened, which mostly consist of me sharpening tungsten, a lot! 

That run was on Suncoco 110, but just to prevent detonation. I don't think it has any oxygen adders, though technically it's race fuel. We didn't play with timing yesterday, though it was on my mind before we started. Never had a bike on a dyno before, so there was a lot going on in my brain during testing. I was mostly looking to the dyno operator to tell me what to do (he's a racer and a builder). Now that I've been through it I'll make a check list of things I want to test next time. I realize it's up to me to get the most out of my time on the dyno. I need to decide if I'm staying with the 110 and dropping the mains another size to lean it out a bit more, or moving on to some oxygenated race fuel. I'll probably be back to the dyno at some point I'm sure. I think it would be prudent to race it as is and go from there.

Thanks for all the advice and encouragement.

Alan

[Haybus]

A few changes over the winter. Biggest change is the 4-1 exhaust which also increases the primary pipes from 1.375" to 1.5" OD. Unfortunately it now has a cough when snapping the throttle just off idle. Revs fine if I pick the rpms up a bit first. Likely leaned it out quite a bit. It's been raining so much here it's kind of hard to get out on the road for a bit of tuning. Would come back a muddy messy just getting from my driveway to pavement. I'm thinking a trip to the dyno is in order. Cheap insurance for sure anyways.

Alan

[bwaller]

Looks great Alan.

[Medyo Bastos]

Looks great Alan.

I agree! Can you post a closer look at your fork brace?


Sent from my iPhone using Tapatalk

[Haybus]

Here's an old picture. I've since machined the underside of the brace as it was picking up a lot of rubber. Now has about 1/4"clearance. Would be better with later legs, or GL1000 legs!!. The clamps are machined to fit the grooves where the fork gaitors would fit. There's washers at the pinch points to keep from binding the legs too.

Alan

[bwaller]

It would make a great remover of foreign objects when the tire is hot. 

[Medyo Bastos]

Here's an old picture. I've since machined the underside of the brace as it was picking up a lot of rubber. Now has about 1/4"clearance. Would be better with later legs, or GL1000 legs!!. The clamps are machined to fit the grooves where the fork gaitors would fit. There's washers at the pinch points to keep from binding the legs too.

Alan

thanks for the pic, and you answered my question  i was wondering about the tire growing

[gschuld]

Alan,

Looks great....good luck this season.


For another reference, here is the Tarozzi fork brace on mid 70s lowers (12.5") and 110/80/18 BT45 fronts.

A good 3/4" clearance.

George

[754]

Haybus, did you notice more torque in midrange when you ran the smaller pipes..?

[Medyo Bastos]

Haybus, did you notice more torque in midrange when you ran the smaller pipes..?

wondering same thing

[slikwilli420]

Race bike is looking good this year Alan. Good luck to you. I was a bit surprised on your numbers until I saw you didnt have any porting done. With the same pistons (12.5:1) and an appropriate cam, I have been told its not unreasonable to get into the mid- to low-80's for rear wheel power. My head was done to the 9's by JMR so I should be covered there.

Remind me, will you be at Barber this year?

[kmb69]

Haybus, did you notice more torque in midrange when you ran the smaller pipes..?

wondering same thing

Me too ? ? ?

[Haybus]

Well, since I've only bench raced it in the garage it's kind of hard to tell  . I'm planning on a trip to the dyno this Saturday morning so I'll post the comparison chart. BTW, I made the exhaust change to fit my new belly pan and allow better cornering clearance. The 1 and 4 pipes run outside the frame rails with stock head pipes, which doesn't work with the Air-tech pans.

I left it jetted a little fat on the dyno, could have picked up a couple more ponies by leaning it out, maybe. A couple more switching to oxygenated fuel, a few more with a properly ported head, and so on. I've never seen any dyno's of a 760cc in the 80+hp, but I don't doubt it's possible. The only other dyno from a similar setup, with a pro ported head, was only a few hp more than me. I think working on track flow, corner entry and exit, and smoothness, with some decent power, is way better than going for absolute hp numbers.

My head is mostly stock, homebrew ported by me with a make shift gage (large manometer) to measure change (referenced back to a standard, always) and to make sure all ports flowed about the same (that's fairly hard to do, for an inexperienced dummy). I got pretty close to TurboD's numbers he posted on homebrew porting (delta increase), so I felt pretty good. Of course new guides, CycleX super flow valves, and a homebrew seat grind.

Would have been better off sending my head to someone who knows what their doing, but that's not really my style. I think my style is to spend way more time and money to make something worse!! Ha, ha, groan.

Barber is on my bucket list, but not this year Matt. I have 2 girls in college and one is getting married. Feels like I'm back in college, ya know, when we had no funds but lots of plans!!

Alan

[754]

The reason I am asking is, I went from a Hooker 4-1 to a unequal 4-1from a turbo bike.
And it didnt seem to suffer.
 Then I noticed that I had a surprising amount of low and mdrange power, could short shift and it just kept pulling.
 The turbo pipe has 1 3/8 primaries, it was on a 836 with a 315 cam and Webers with big squirters..