Topic: CB650 big big big bore kit

[speedracer741]

Hello all. This is my first post since the greenspun days! I have stumbled across some posts in the past regarding performance options for the 650 mill so I figured I would share this project that I have been playing with on and off for the last few years now (the result of having too many projects at once). I wanted to build the ultimate middleweight SOHC bike so I came up with this combo. A '71 CB500 frame lightened and braced. Fuel tank, swingarm, forks, wheels and brakes from a first year CB750F resulting in triple disc brakes and an even shorter wheelbase with that swingarm. I got an '81 CB650 engine for the project and started looking through the Wiseco book for possible piston choices. I figured heck.....let's throw 65mm slugs in there! I ordered up the 12:1 version of the K836 kit, knowing I would be doing lots of machining to the tops of the pistons and would need plenty of material to work with. Out came the stock sleeves and I bored the block until it was paper thin between the fins. There were no sleeves available that would do the job since I couldn't have a step in the bottom as they would be very thin already. I had to make a set just for this project and have only .085" wall thickness meaning there is no overbore for this thing, rebuilding means a re-sleeve every time. The upper case half had to be bored out....a lot! The oil passage o-rings are right on the edge. I also machined the tops of the sleeves for copper o-rings for use with the copper head gasket I had to make. The bottom end also got new everything and some serious clutch springs. So the result....741cc without doing up a stroker crank! Not bad with an engine platform that began life as a 500. Hope these pics upload fine, it's my first time. I will post the cylinder head stuff in a bit. 

[MotoRico]

I know I'm not alone in wanting to see and learn more about this endeavour.

[speedracer741]

Ok the head pics so far. When I first did this it was based on stock valves. Since that time I have developed a set of oversize valves based on available 5.5mm blanks so this thing will be going under the knife again before it gets deck surfaced and assembled. The intake ports were milled off and machined for the new steel spigots so this head will now accept the 36mm carbs!

[Pinhead]

I'm so excited I can't hardly type! Definitely post all the pictures you can. I'm VERY interested in this project!!

This could prove that the 650 motor is the true example of SOHC4 performance!!

[bill440cars]

         Between this one and the one that paulages is doing, I believe the 650 motors are making a stand!


                                                             Later on, Bill  

[Ibsen]

You have done some really impressing work here. And some of it right on the edge of what seems to be possible.

What kind of carbs have you planned to use for this engine?

[speedracer741]

I'm not fully decided on the carb setup yet but I do have a set that I will use for it's first run. They came from a cycle salvage inventory I bought out back in 1997 so I'm not 100% on what they are from. I will post a pic tomorrow. The port spacing on the head was moved out to be compatible with carbs from larger engines. I think the set of 36mm I have are from a CB1100? They are the CV type that were used on the twin cam CB's but most of those are 34mm. I think the 36's were just on the big boys. My reason for the use of CV carbs is due to the fact that this project was designed for use on the street. This allows the use of high flow rates but still retains some manners if set up correctly. Back in the late '90s when I did most of the work on this engine (yeah it's been sitting for a long time) I was heavily involved in the street racing scene so everything I built had that in mind. Of course I have grown up now........right, anyway uh I wanted some driveability. Though I think scattering this engine all over the street would have been more of an issue. Funny thing, I found a story on the net about myself at those very races. Really makes me miss the good old days:

http://www.sanjuanislander.com/columns/old-squid/6.shtml

The story was posted in '02 but the night written about took place during the summer of '98. One of these days I will get this thing finished. I'm trying to finish up a '77KZ1000 (yes I know, traitor!) in time for summer but I think after nearly 9 years of slow progress this 741 needs to fire! Perhaps it should be my goal before I turn 30? 

[Pinhead]

You've obviously got to cut valve reliefs in those pistons.  You ought to try the Omni Valves on the bike. Especially since you're going for horsepower but still want street manners. They let you go with as big of a cam as you want without sacrificing torque, mileage, and driveability.

http://www.sohc4.us/forums/index.php?topic=18002.0
http://www.sohc4.us/forums/index.php?topic=18002.msg187976#msg187976

[speedracer741]

I have heard of those before, pretty trick indeed. Most likely I will just throw the big valves I made in this one for now. I can always spend the next decade making improvements..lol. Speaking of variable valve timing, it is incredible! Not to change the subject to cars or anything but I experienced vvt for the first time this last summer with an engine swap in my car. It is a 1990 BMW 325i, stock it had a 2.5l SOHC 2 valve straight six. Great engine and quick but a local BMW performance shop I have done head work for in the past showed me what I needed. My car now has the engine from a 1999 M3. It is a 3.2l DOHC 32 valve engine with what BMW calls VANOS:

http://www.bmwworld.com/technology/vanos.htm

Holy crap! It is a completely different car to drive. Pulls like a freight train everywhere. Ok, I won't talk about cars anymore, promise.

I may be conservitave on the cam I use in the 650 and try to take advantage of the torque I will most certainly have. The rods in this build are stock and RPM is what will kill them. I did get a lot of weight out of the slugs but they are still heavier than the stockers. The CB550 I am also working on (yes yet another project) will be the revver. Just a run of the mill 592cc upgrade but the real project is the head. I have plugged the ports completely and am making new 30 degree downdraft intake runners with big valves. I have some pics of that one to but I need to figure out how to lower the file size so I can post them.

[Ibsen]

Using CV carbs to make it street drivable sounds like a good idea. Btw, 36mm Keihin CV carbs wasn't reserved for the big boys only. The early Kawa KZ400 twin had them as well.

[pae]

Great quality engineering, there Speedracer. Like it a lot. Keep us posted

regards, Phil

[speedracer741]

Actually it's funny you mention the KZ400 carbs. When I first started this project I was going to build a set using the ones from the 400. I had like 5 sets of them laying around. The linkage I would have had to make would have been a nightmare so I eventually went with the set from the honda. The 750 in my avitar has a strange setup as well. They started life as 2 sets of 34mm CV carbs from the Yamaha XS750 triple. A carb was liberated from each set resulting in 2 sets of 2. Throttle cable is a 2 into 1 splitter from a CB350. The CB750 has 2 types of intake boots with different offsets, two for 1,4 and two for 2,3. I used all four of the same type (I can't remember which at the moment). The result is perfect port allignment. A bit of a chore to set up initially but a much better runner path than some of the CV carb installations which distorts the manifolds to make them fit.

[Sam Green Racing]

How about some pics of the bike in your avitar ?

Nice project as well.

Sam.

[speedracer741]

I only have the one picture of this bike right now. It was completed in 2002 and put a rod through the case last summer. I will try to get some pics of it going back together this spring. This one was a team effort of my friend and current owner of the bike and myself. The fiberglass tailsection is from a mold I made back in '97. The engine is an 836 with a mild Megacycle 125NR cam. Fully ported with stock valves. It makes incredible power considering the mild nature of the build.

[cafe75-550]

Speedracer-

I'm working on a similar project with the 650 and was wondering if I could ask if you would consider making another set of those sleeves? I have a set of 750 sleeves that I was going to modify, but if you're set up to do it already and are willing it would certainly be the better way to go for me....If you're willing, you could PM me with a price tag 

[Sam Green Racing]

I only have the one picture of this bike right now. It was completed in 2002 and put a rod through the case last summer. I will try to get some pics of it going back together this spring. This one was a team effort of my friend and current owner of the bike and myself. The fiberglass tailsection is from a mold I made back in '97. The engine is an 836 with a mild Megacycle 125NR cam. Fully ported with stock valves. It makes incredible power considering the mild nature of the build.

You might want to think about a set of decent rods in your new motor, then you won't have a reocurrence of you're 750 exploits

Sam.

[speedracer741]

Definitely! During the first build my friend Jeremy was in a hurry and was on a budget. I dropped this topend on a non-rebuilt bottom end with an unknown history. I pretty much closed my eyes, pointed at a pile of old 750 engines and selected the lucky victim. I predicted that it would fail during the first season, but it took over 4 years of constant abuse to finally let go! The topend suffered one piston and sleeve. I have decided to try something new and have the new rods done by Crower. I have not heard of anyone using them before for a CB750 but they will make custom bike rods. I have a set of Falicons for one of my 836 engines. If Crower has too much lead time then we will just go with Carillo but I like trying new stuff

[mrbreeze]

I only have the one picture of this bike right now. It was completed in 2002 and put a rod through the case last summer. I will try to get some pics of it going back together this spring. This one was a team effort of my friend and current owner of the bike and myself. The fiberglass tailsection is from a mold I made back in '97. The engine is an 836 with a mild Megacycle 125NR cam. Fully ported with stock valves. It makes incredible power considering the mild nature of the build.

Its been ages since I shifted one up- four down!!!!!

[Jerry Rxman Griffin aka MuthaF'er]

OK, you gotta give up all the info on Crower now!

[speedracer741]

OK, you gotta give up all the info on Crower now!

I will have to check the pricing but they can make just about any rod from billet steel or titanium in both I and H beam designs. In my day job I do cylinder heads for an automotive machine shop. Crower is one of our biggest suppliers. There catalog shows that they actually stock them for all modern sport bikes and there is a section stating the will do custom applications in sets of 4. This can also be seen in the catalog at the Crower website. I can vouch for the quality of their product which is one of the reasons I want to give it a shot

[Ibsen]

Actually it's funny you mention the KZ400 carbs. When I first started this project I was going to build a set using the ones from the 400. I had like 5 sets of them laying around. The linkage I would have had to make would have been a nightmare so I eventually went with the set from the honda.

They wouldn't have been my first choice either. To many parts that has to be modified. And another problem is the total lack of spare parts availability. The later Honda carbs are a much better choice.

[speedracer741]

Actually it's funny you mention the KZ400 carbs. When I first started this project I was going to build a set using the ones from the 400. I had like 5 sets of them laying around. The linkage I would have had to make would have been a nightmare so I eventually went with the set from the honda.

They wouldn't have been my first choice either. To many parts that has to be modified. And another problem is the total lack of spare parts availability. The later Honda carbs are a much better choice.

I still have all of those carbs. Internally they have a better venturi design and would have probably performed better after the tedious set up they would need. Plus they are really heavy as they use that wierd alloy that has a high zinc content, I think its zinc?

[paulages]

hey speedracer--i'm here in portland also, and also building a 550/650 combo with 64mm pistons. i'm machining cb750 sleeves to fit in the 650 cylinders, sitting on 550 cases and trans with 650 crank and rods. me and several other members here have a shop in n. portland, almost exclusively sohc's. you should swing by sometime. gimme a call.

[speedracer741]

hey speedracer--i'm here in portland also, and also building a 550/650 combo with 64mm pistons. i'm machining cb750 sleeves to fit in the 650 cylinders, sitting on 550 cases and trans with 650 crank and rods. me and several other members here have a shop in n. portland, almost exclusively sohc's. you should swing by sometime. gimme a call.

I read the thread on your project....wow! What are the chances that two of the biggest middleweight SOHC engines would be taking shape in the same town! You will be hearing from me shortly.

[paulages]

gotta get yours back out of the boxes!