Topic: CB750 AHRMA Sportsman 750 Racer

[slikwilli420]

Not much of an update, just really stoked with how my clutch cover came out. Its subtle enough to look like something that may have been a factory part or a custom aftermarket.



25 days...

[Rocketman]

Nice.  An example of form following function!  Keep making progress and checking items off your to do list.  Is Gingerman on your schedule this year?  May try to make it and get some time on my engine before Road America.

[slikwilli420]

Thanks Martin! Gingerman is up first and with a new engine I'd like to get it on the dyno or to a track day first but depending on time of final assembly it might have to happen on the road outside the track and during practice. Not ideal but may happen that way. As long as I'm running enough fuel to keep from blowing the engine I'll be happy.

[slikwilli420]

Making progress on the chassis. I was going to wait until the engine was done then slide the bare frame over it for easy installation, but time is running too short and I know that as I assemble things will need to be fitted and massaged so I started there first. The frame assembly is in a really good spot, with the swingarm pivot (new grade 5 Titanium with Ti nuts) fitted to frame and arm and swinger shimmed perfectly for smooth pivoting but no side-to-side play. Shocks are fitted (need some adjusting) along with new swingarm bobbins.



The front has been assembled, waiting to be put back on the bike, with neck bearings greased and bearing nuts snugged up appropriately. Wiring is also back on the bike except coils and ready for action. I was running into space issues on the brake handle bar where I wasn't able to fit everything on the bar AND get good position on the brake lever with 2 fingers. I realized Woodcraft does bars that are 11" instead of 12" so I swapped both sides out for 12" bars and now can get 2 finger braking further out on the lever which should help with hand fatigue and increase brake pressure. I also had some looseness of the bolts in the holes for both the caliper lugs on the fork and the holes in caliper so made up some very thin spacers for the bolts which take up all of the gap and now with the bolts even finger tight, there is no movement in the caliper at all.

The wheels are still in progress and have not been trued, but are laced and the front is pretty close but needs more truing and final tightening of spokes.

Pistons should have shipped last week to Kenny at Cycle X and I should have an update from him this week on progress. I sent a vapor blasted cylinder to him provided by member Ekpent so he has everything he needs. The cylinder will be bored with torque plates, honed, decked, and have the rings fitted before coming back. Once back with me, it will get the m6 holes on top fitted with Timeserts and APR 6pt bolts, as well as have the windage window slots cut in the sleeves between 1-2 and 3-4.

A little side project (not that I really have time, but, ya know) was to build a heat shield for my carbs. With the heat of the engine able to enter the intake tract so easily especially with open filters and the engine making more power, it seemed like something that was worth the effort. I used some 0.080" Kydex (popular for pistol holsters) which is a thermoform plastic that is very chemical resistant. I cut everything to shape to fit around the carb insulators and other items (cam chain tensioner). It turned out really well, looks race and should be function to some extent.





Still to do:
- Finish wiring install (coils, kill switch)
- True wheels, install tubes/tires, balance
- Fit rear master cylinder and plumb brake line
- Mount seat and tank (tank getting some mounting changes)
- Engine top end assembly and cam degreeing

19 days until track day, 31 until first race...

[MRieck]

Great job on the heat shield (and everything else). You have a good eye and good hands.....a great combo.

[slikwilli420]

Great job on the heat shield (and everything else). You have a good eye and good hands.....a great combo.

Thanks Mike! Coming from you, that means a great deal. Your stage 4 head is patiently waiting to be mated above Kenny's pistons. Going to make  sh*t load of power, me thinks.

[gschuld]

I like the heat shield.  I’ve seen this idea before.  I lost track of the pics of the cb750 race bike I saw that used one.  I’ll find it.  There was an article on the bike in question.  They used a polished brake bent aluminum sheet heat shield on theirs.

It would be an interesting experiment to put a temp probe right in front of the carb inlets and run the bike back to back with and without the heat shield.  This way you could test the difference in temps entering your engine.  But for now, you best concentrate on getting on the track.😉

George


Edit:  found the article:

http://shotzphoto.blogspot.com/2011/04/wayne-nolan-and-his-honda-cb750.html?m=1

[simon#42]

i would also put a temp probe on the middle two cylinders to make sure the shield does not form an air dam and prevent air flowing through the cylinders .
i have spent a lot of time trying to get hot air away from engines over the years !

[seanbarney41]

I thought about that too.  My slabbie had one originally or someone made one for it.  Glad some one more knowledgeable brought it up.

[slikwilli420]

i would also put a temp probe on the middle two cylinders to make sure the shield does not form an air dam and prevent air flowing through the cylinders .
i have spent a lot of time trying to get hot air away from engines over the years !

Good point and worth checking out. I figured with the large oil cooler up front that it wouldnt be that big of an issue, realizing this is an air cooled, not oil cooled bike, but it should help a lot, especially with increased output. The air passages between cylinders are pretty small, has anyone opened them up for better 2-3 cylinder cooling?

[gschuld]

I did some modest research on this in the past.  David Vizard is a good reference as he tests like it’s a pharmaceutical drug study.  He seems to have come to the conclusion that keeping the carbs themselves cool offers little in lowering air intake temperature.  The big thing is getting the initial inlet air temp as low as possible.  It seems that the lower intake air temps will cool the carbs by themselves.  If you have a long engine heated intake tract like a iron intake on a  v8 engine, effort should be given to reduce heat absorption.  But the intake tract is so short in a cb750, this shouldn’t be much of a concern.

If you are really keen to lower intake temps, a worthy goal, I’d suggest concentrating on getting the lowest possible air intake temps at the stacks.  Such as from a plenum box of some sort.  Like the upper section of the factory Airbox, blocked off at the bottom, and pull in air from the coolest location possible into that upper Airbox.

I’d temp probe in front of the 2/3 velocity stacks and ambient air.  The larger the average difference between the two, the more it might be worth considering a proper “cold air intake” system.  If your difference is 10-15 degrees, not such a big deal.  If you’re talking 30+ degrees....🧐.  A cold air intake system is better defined as ....“as close to ambient air temp as possible system” 😉

George

[turboguzzi]

i do go over the smaller air passages in the head with a file, often there's casting flash that reduces the flow.

[MRieck]

i do go over the smaller air passages in the head with a file, often there's casting flash that reduces the flow.

Don't know how tight you are with Rolf but the GS1150 Endurance Bike they are racing is the bomb. I will proudly say it is my headwork helped but HP and Torque is amazing. Rolf is very good.

[turboguzzi]

i do go over the smaller air passages in the head with a file, often there's casting flash that reduces the flow.

Don't know how tight you are with Rolf but the GS1150 Endurance Bike they are racing is the bomb. I will proudly say it is my headwork helped but HP and Torque is amazing. Rolf is very good.

rolf is good is understatement of the year his sohcs are simply jaw dropping stuff, met him at a swap market in milan, we spent an hour looking at his builds, shame he is not posting here

[slikwilli420]

... with the swingarm pivot (new grade 5 Titanium with Ti nuts)...

Ti!?! You love yourself, Matt! How sexy does that look on the bike? Are you sparing any expense on this bike?

That I do Cal! It looks so sexy. I am having as much fun building as I do racing this machine. I already have plans for next year on another diet for the bike and myself (bike is down 25lbs from last year, probably more, and I am down 30 so far) like a chromoly swingarm, different brake caliper, maybe an adjustable ratio AP master, more Ti and more lightening all over wherever possible. The engine is about as good as can be component wise, so now its about tuning what is there and making it all sing with round top carbs. The next frontier would be a copy of my frame in chromoly as well as more extreme engine work, especially in the head, and would have to bend Mike's ear on his thoughts there. Ti rods and Nikasil bores would sure save some weight and take a bit out of my wallet as well!!

[gschuld]

You are a man possessed Matt😜

It’s interesting that I have heard from a bunch of sources that often the British bike contingent (Triumph, BSA, Norton, etc) have grumbled against the Japanese bikes(especially the Hondas) because they are comparatively inexpensive to prepare and make competitive for vintage racing. 

Some consider it diluting the prestige or whatever.  You should keep good records of your mods and expenses.  You can at least prove your “Jap” bike wasn’t cheap🙄😁

George

[slikwilli420]

You are a man possessed Matt😜

It’s interesting that I have heard from a bunch of sources that often the British bike contingent (Triumph, BSA, Norton, etc) have grumbled against the Japanese bikes(especially the Hondas) because they are comparatively inexpensive to prepare and make competitive for vintage racing. 

Some consider it diluting the prestige or whatever.  You should keep good records of your mods and expenses.  You can at least prove your “Jap” bike wasn’t cheap🙄😁

George

Yeah I could see that. Fortunately in my class the dominant bike is the XS650 Yamaha, bored to 750, which is a bit cheaper given it has less cylinders. There are an equal amount of trick bits out there given there long life in flat track and road racing at a club level. Some guys run Triumph engines in the class and have a hard time keeping them together at the power levels the XS can achieve with reliability, which is around 65-70hp. From what I have gathered, 75 is about the ceiling for an XS engine if you want to get any longevity out of it.

I have detailed records on what I spent and where I got parts from. I even set out with an expected price on something and sometimes I beat that price and save a buck, other times I don't. I don't really stick to a budget, its more about how much can I throw at this thing before I am happy? I am far happier this year with the improvements, but I believe there is always room to make it better. The "K" in our K model bikes means Kaizen, and if continuous improvement was good enough for Mr. Honda, it will be my recipe as well.

When I have something that really makes the grade performance wise, I have thought about ways to quickly convert to Formula specs. There are really only 4 rules that separate my bike from that class: I have to have some form of fairing (easy), I can run a second disc (bit more difficult), I could have a road race frame (can't do and stay within Sportsman rules) and I could run CR carbs (staying with round tops until the rule change, if that ever happens). Being able to drop the number plate and add a small fairing with my number on it would be cake as long as the racers weren't back to back, though Im sure I could come up with something.

[slikwilli420]

MY PISTONS ARE AT KENNY'S RIGHT NOW!!!

Kenny wanted to get back to work but he did leave me with a feeling that these are the most next level stuff he has seen in a while. Also, bare piston weight is 142g!! I am waiting for pictures now but things will start to move pretty quickly once those get in my hands.

More soon.

17 days to track day, 30 until first race of 2018.

[gschuld]

Great.  Please share the pictures when you get them.

George

[slikwilli420]

Here we go boys, likely the finest piston you can put in a CB750 for 62mm:











You saw it here first guys.

[livefast_dieold]

NICE!!! Looks like jewelry to me :-)

Dome seems even higher than Dynoman's 62mm pistons. How much do they weight?

[slikwilli420]

142g bare weight. The specs are on the cycle x page. They spec out to 12.5:1 compression with a 22cc chamber.

[slikwilli420]

Lots going on in my final push to get this thing ready to race.

Wheels trued:



Base for dial indicator made:



Rear brake fabbed and mounted, still need to figure out lever stop:



Tasty new pistons compare to stockers:





Its hard to see in the photos, but the area under the piston is completely machined out in a "V" so there is minimal material under the valve pockets. These things are so light its unreal. They will be running on the XA ring pack from Wiseco as well as tool steel wrist pins.



I will be working on the cylinder tonight which needs the windage windows cut as well as the 6 time serts installed into the top. After that I can start to mock things up and check the deck height.

I also have my piston stop made which took some doing but ultimately came out quite well. I will only install rockers on the #1 cylinder and leave plugs out so its very easy to rotate and get cam timing set.

[gschuld]

For clearancing for the windows on the cylinders, I assume you are referring to notching the sleeves to match the cutout area for the windows in the case?  How much are you talking about here?

I’m curious to whether this will cause you grief when installing the thin oil rings.

George

[slikwilli420]

For clearancing for the windows on the cylinders, I assume you are referring to notching the sleeves to match the cutout area for the windows in the case?  How much are you talking about here?

I’m curious to whether this will cause you grief when installing the thin oil rings.

George

The windows are about 1.25" wide and will start at about 0.150" below where the sleeve meets the cylinder base.

I have been thinking about that, and have decided to try to install pistons from the top. Even though there isnt a bevel like the bottom, with the windows cut it will be easy to bend a ring. I will then be able to slide the pistons down the cylinder, doing 2-3 first, slide the pin through and put in the wrist pin retainer. I will have to block it up slightly to get in there but this is how Captain did his big bore DOHC race bike, so Im going to try it.