Topic: CB750 stock carbs with headwork, Wiseco 836, and Megacycle 125-65

[brandEn]

I am building up my motor for my 750 K4.


Mreick head with Cycle X valve train
Wiseco 836 piston Kit
MegaCycle 125-65 cam
Ignition yet to be determined


I am considering rebuilding and using my stock K4 roundtop carbs with this build along with a modified stock airbox. Before I waste my time and money on these old carbs can anyone tell me if they have any experience running these carbs on a motor similar to this? If so what needs to be done to these carbs to make them work correctly. I would like to know if I am wasting my time or if they can be jetted and adjusted correctly to work with this setup. Jetting and needle adjustment specs greatly appreciated!

[Retro Rocket]

Big Jay at APE used to bore the stock carbs out for performance engines, it may be better to use something better, {bigger} though..?  Send Jay a PM and see what he says...

Recent thread on this subject...

http://forums.sohc4.net/index.php?topic=118330.msg1338253;topicseen#msg1338253

[brandEn]

Thanks Mick. I am interested in using the stock carbs for my wallets sake. I would really like RS34's but can't swing it right now.

[Jerry Rxman Griffin aka MuthaF'er]

LMAO! Yeah, it's kinda tough to do 2 REALLY nice bikes huh?! Not that it's not going to be nice though. Mine won't be a junkyard dog but it sure as hell won't be the restoration! I'm putting it together first then seeing how much cash I have left to play with. Carbon Guy is on my list of new best friends.

While the stock carbs may not be CR29 or 31's they should do fine. I have/had always run my stock carbs. My thoughts say the limit to power is obviously the air flow but you'll make good power. Just get the A/F right. I ran the 812 with Yoshi full race cam (little stronger top end than the 125-65??) all these years at 300' and 7000'. I did not have Mike's head though. You going standard size valves? And I never dyno tuned it either so I don't know how optimal I had it set up. I hate carbs! There, I said it  They have 115's with K&N in stocker airbox that I drilled cool looking holes in. Slightly rich at 7000' but it wound up when cranked. I had a friend that had some serious untold #$%* in his engine with stockers on his too and he would make mine look bad. You'll see stockers used on RC engines if you watch the pictures. I say put it together, get em jetted and see how you like it. That's my initial plans for the 1000 with 33.5 intakes but I'll have to go larger than the 115's LOL. Maybe we can get a deal on 2 sets of RS34's later?   

[brandEn]

Yup it's tough on the pocketbook for sure!

Thanks for the input Jerry.

[brandEn]

If anyone has any feedback at all I would love to hear what you have to share...

[Oakleyguy]

Hey guys, I wanted to bump this thread back up as I've been wondering the same thing.

Similar plan: Hot Cam, 850kit, High flow valves, ignition, intake and exhaust.

It seems like people haven't been limited much by the stockers, but it raises the question of is it worth spending the extra money on all the upgrades if the carbs will be holding you back.

[Medyo Bastos]

i run round tops with a big bore, cam and a powerarc ignition.

[Retro Rocket]

Hey guys, I wanted to bump this thread back up as I've been wondering the same thing.

Similar plan: Hot Cam, 850kit, High flow valves, ignition, intake and exhaust.

It seems like people haven't been limited much by the stockers, but it raises the question of is it worth spending the extra money on all the upgrades if the carbs will be holding you back.

On a worked engine you will pick up round 10hp from a good set of performance carbs....

[Doctor_D]

Hey guys, I wanted to bump this thread back up as I've been wondering the same thing.

Similar plan: Hot Cam, 850kit, High flow valves, ignition, intake and exhaust.

It seems like people haven't been limited much by the stockers, but it raises the question of is it worth spending the extra money on all the upgrades if the carbs will be holding you back.

On a worked engine you will pick up round 10hp from a good set of performance carbs....

The head is also a big restriction on these engines, so add some porting to your plans or you're going to leave power on the table.

If you want something stock-looking, plenty of people run older Mikuni smoothbore 29's.  Unlike CR's, they can sometimes be found gently used for reasonable dollars.  But, boring the 28's and cutting the bottom of the slides to enrichen tip-in is a valid option if you aren't going to spin up much past 8k.

[Retro Rocket]

Hey guys, I wanted to bump this thread back up as I've been wondering the same thing.

Similar plan: Hot Cam, 850kit, High flow valves, ignition, intake and exhaust.

It seems like people haven't been limited much by the stockers, but it raises the question of is it worth spending the extra money on all the upgrades if the carbs will be holding you back.

On a worked engine you will pick up round 10hp from a good set of performance carbs....

The head is also a big restriction on these engines, so add some porting to your plans or you're going to leave power on the table.

Absolutely, even just cleaning up the ports will make a difference...

[Sprocketwerx]

I was running an '72K 836 10.25:1 minor cleanup on the ports, webcam63A, upgraded valve springs. Roundtops with 130 main, 40 pilot, Emgo pods and unmodified airbleed.
Ran like a raped ape until an oiling issue in the top end (still ran like a raped ape even with a seized wristpin and wasted 3 cam lobes and a few rockers).

[MRieck]

K&N supplied instructions for jetting with individual filters. They were very close. I ran stock carbs for years with K&N's....no problems EXCEPT for rain.....really.

[brandEn]

bump for anymore input....

[bear]

I ran round tops on our race motors for the first couple of years, before moving to the CR' s when we started to use methanol.
If I were building a good street motor on a budget, the round tops would be my choice with out a doubt.

Cheers,
Brian

[brandEn]

Got any input on jetting specs bear?

[Jerry Rxman Griffin aka MuthaF'er]

Get all the stuff you are doing sorted out first. Tune the round tops. Once it's purring like a mechanical kitten and you've saved some dough THEN worry about better carbs. Sounds like you are almost there! At this point I don't think you are beyond stock carbs but we all know bigger carbs can make bigger HP with the right goodies.

Except for Mike's head work, which you didn't elaborate on, I'm thinking your stuff is like 2 bumps up in jet size with stockers for starters. I'd try 120's and go from there. I'm also thinking the 125-65, which will give stronger mid range vs top end will require one, maybe 2 notches of richer needle but start with one.     

[jdnova]

I can't speak for running them down the road for miles on my 1055  but I can tell you that I had to run them on the dragbike cause the cr33s had issues and I ran out of time to get the parts to fix them. This engine is or was a 1055, head worked over well, big cam, and altho it was without a doubt restricted it still ran good and put down some good 1/8 mile times all things considered. As for running them on the road I ran this same set of carbs on my 836 and never had an issue.

[brandEn]

So what is the stock needle position anyway?

As for my head it has been worked over by MRieck. He mentioned it was his stage 1 but is as good as most peoples stage 2 heads.  It has Cycle X valves and springs and Kibblewhite guides.

I have 120, 125,and 130 size jets on the way. I didn't order slow jets so stock size whatever that is.

[Medyo Bastos]

start with 120 and do your plug chops!

i run k&n pods, and had to drill the slow jets in mine to 41, and drop the clip 1 notch.  I currently run 127.50, but its a little lean on the top end.  I just havent had time to move up on the mains, but otherwise runs like a champ! 

[brandEn]

Thanks for the input! Every bit of info helps..
I'm gonna be running a K&N in the stock airbox. The stock airbox will be modified with a hole in the bottom. It was recommended to me to use a hole saw to cut a single 1" hole.

[Bill/BentON Racing]

K&N supplied instructions for jetting with individual filters. They were very close. I ran stock carbs for years with K&N's....no problems EXCEPT for rain.....really.

+1, only problem was getting caught in the rain,especially heavy rain! Bill
130-140s went in a lot of bikes,especially with air box removal.Good starting point.

[bear]

Got any input on jetting specs bear?

Sorry Branden,

My application was a little more radical than  what you are contemplating.
But Billy & the crew seem to be providing some good advice on that front.

Cheers,
Brian

[PeWe]

I used stock carbs on my CB750 K6-76.
836, ported head with over sized inlet valves (34mm), cam Action fours SS-1 (same numbers as WebCam41). KN styled pod filters and  4-1. See my avatar.

Worked very well. I think that the mains were 140, needles one step up

[brandEn]

Thanks PeWe. With all you experienced HiPo guys saying you ran KN pods filters instead of the airbox has me rethinking things. Maybe I will just get the KN pod filters. Anybody have a part number handy?