Topic: Types and makes of replacment carburetors for the 750.

[Soos]

http://forums.sohc4.net/index.php?topic=30624.0;topicseen

In this post there is a fuel injection system talked about, I know Cycle-X offers a dual carb setup, as well as the  DCOE Webber setup I have seen for sale on E-bay.


Does anyone know of any other setups that are either a drop in part?


l8r

[mkramer1121]

Kehin CR's (New, around $700), Mikuni Smoothbores (old, vintage, require carb boots, from $250~$500, plus $150 for the boots)...Mikuni also makes newTMR34's as well, but they are over $800

[Terry in Australia]

For my money, the CR's are about the best thing you can use on a 750. The Mikuni smoothbores are difficult to tune, as are the Webers and Dellorto's, not to mention that the DCOE carbs weigh a ton. Cheers, Terry.

[mkramer1121]

yeah, but CR's are sooo....UGLY

[754]

Terry , obviously you have never lifted a DCOE..I am thinking..


The jetting is very quick to change, but there are more circuits to deal with, meaning you can pretty well tailor any part of the throttle opening.plus you dont have to  pull them off to change stuff, can change you venturi sizes in 5 minutes, and they have ball-bearing throttle shafts..

[Terry in Australia]

Terry , obviously you have never lifted a DCOE..I am thinking..


The jetting is very quick to change, but there are more circuits to deal with, meaning you can pretty well tailor any part of the throttle opening.plus you dont have to  pull them off to change stuff, can change you venturi sizes in 5 minutes, and they have ball-bearing throttle shafts..

Ha ha, I carried a set of Dellorto 40DHLA's, (which I understand were copies of the DCOE's) all the way to the Post Office when I sold mine on Ebay mate, they weighed in at around 10 pounds!

The CR's are a snap to change needles and jets on the bike, so I just couldn't see any point in keeping the DHLA's. Any "race" carbs that need a bracket to hold them to the frame, so they don't fall off the engine, is a little too heavy for my liking.

Sure, they look nice, but if they were any better than the CR's, the factory teams would have used them, rather than CR's or Mikuni Smoothbores, which seemed to be the racers choice back then. Cheers, Terry.  

[754]

I dont see a lot of imported stuff on Works bikes in the early 70,s....

Mikuni did make a dual throat that was a bit smaller physically that the other 2 brands.. may have been Mikuni-Solex.

Of course there sould be a  brace they go back a lot further and are not held up with the airbox.. not real hard to move stockers up and down with pods on them..

I know stock 750 carbs weigh about twice as much as modern ones though..

[Terry in Australia]

I dont see a lot of imported stuff on Works bikes in the early 70,s....

Mikuni did make a dual throat that was a bit smaller physically that the other 2 brands.. may have been Mikuni-Solex.

Of course there sould be a  brace they go back a lot further and are not held up with the airbox.. not real hard to move stockers up and down with pods on them..

I know stock 750 carbs weigh about twice as much as modern ones though..

Yeah, quite right mate, the new CR's are very light indeed. If you use the DCOE's and a magneto, you'll be adding about 20 pounds to the weight of the bike. Cheers, Terry.

[mkramer1121]

Hey DG, how do you do a carb sync on the VM29's?

[754]

Terry, the carbs may weigh a few more lbs, but yours will probably never spin the tire w/o the clutch at  15 -20 mph!!

 Not sure what CR,s get for gas mileage, but the Webers are pretty good.

 I will weigh a set of stock carbs, they never felt very light to me..

[Terry in Australia]

Terry, the carbs may weigh a few more lbs, but yours will probably never spin the tire w/o the clutch at  15 -20 mph!!

Ha ha, you better come over and take it for a ride mate, it does that already! There's a lot more work in it than just CR carbs though, and I hope you're not saying that a set of DCOE's are gonna make that kind of difference to a stocker? Remember, they're carbs, not a supercharger, but if you're saying they will, maybe you should be posting here........

http://forums.sohc4.net/index.php?topic=15282.0

[754]

Well Terry , I have not seen any of the carbs anyone mentioned on here referred to as, "The Closest thing to Fuel Injection".. but that is a pretty common reference to the Webers.. However when I start hearing that I lot I will pay close attention..

Ask any Ford GT40 owner..


I did see the PO of my carbs dust off a Z1 pretty badly on a stock bore and cam CB750...

[Terry in Australia]

Well Terry , I have not seen any of the carbs anyone mentioned on here referred to as, "The Closest thing to Fuel Injection".. but that is a pretty common reference to the Webers.. However when I start hearing that I lot I will pay close attention..

Ask any Ford GT40 owner..


I did see the PO of my carbs dust off a Z1 pretty badly on a stock bore and cam CB750...

Yeah, I know mate, back in the day there were a lot of claims made about bolt on performance products like carbs and pipes, and considering that the stock carbs weren't all that good, I can understand why "they" raved about Webers and Dellorto's back then.

I had a bone stock Suzuki T250 "smoke generator" a few years ago that would kick asss off the line, and I beat a few guys on much bigger bikes too, but I think maybe they were asleep.............. 

[754]

Well, methinks you need a scale!!

2 dellorto dual throats w/o manifold =9 lbs

Stock pre 76 carbs w/0 rubbers = 8lbs

 Complete Magneto, less plugwires, including beltdrive pulles and adaptor to crank = 6lbs

 Stock pointplate with point condensers w/o advancer  but includes stock coils between 2 and 3 lbs

So maybe 7lbs more, but I save that much on my smaller battery.

 So I am surprised you can spin the tire with throttle only, yet other CR owners are saying they cant open the throttle hard at lower rpm..see the dyno run thread..

[Terry in Australia]

Well, methinks you need a scale!!

2 dellorto dual throats w/o manifold =9 lbs

Stock pre 76 carbs w/0 rubbers = 8lbs

 Complete Magneto, less plugwires, including beltdrive pulles and adaptor to crank = 6lbs

 Stock pointplate with point condensers w/o advancer  but includes stock coils between 2 and 3 lbs

So maybe 7lbs more, but I save that much on my smaller battery.

 So I am surprised you can spin the tire with throttle only, yet other CR owners are saying they cant open the throttle hard at lower rpm..see the dyno run thread..

Ha ha, well if you go back to my original post, you'll remember that I was comparing the weight of those weber DCOE's to modern CR carbs mate, I don't have a set of Webers handy, and my CR's are on my bike, so I couldn't do a comparison, but I'm guessing they're around half that of the webers?

And being the honest feller I am, I just went out to my garage and weighed an ARD MK2 Magneto housing with no pulleys, but Morse Fairbanks magneto, and the weight according to my new digital scales was 5.2 KG, or 11.44 pounds? I'm sure the extra long copper cored plug leads would add maybe another pound?

But hey, I was prepared to believe that your bog stock Z1 killing CB750 with just a set of Webers would allow you to spin the back wheel at 15-20 MPH, so why wouldn't you be prepared to believe that my engine with the following specs wouldn't? Here we go:

836cc 12:1 Arias pistons,
Mike Rieck stage 3 head,
Megacycle 125/75 cam,
CR 29mm carbs,
K&N pod filters,
Dyna S ignition,
Dyna 3 Ohm coils,
NGK Iridium plugs,
4 into 1 race pipe,
RC rods,
APE studs,
M3 racing cam chain tensioner,
Tsubaki cam chain, etc etc.

I mean, it'd be disappointing if this engine couldn't spin the back wheel at low speeds? Still, if you say that a stocker with just those webers will do all that, then who am I to question your integrity? Have a good one mate, I eagerly await your reply! Cheers, Terry.

[Jay B]

Terry, How much hp are you putting out with that setup?

[754]

I was talking 2 different bikes

Before I had a chance to buy the carbs I saw the PO dust the Z1..he had dragbars, 17/54 gearing and  a header on a stock motor.. I was simply illustrating that they seemed to work on a stock engine.. I ran them on a few stock motors as well.

But my bike  would spin the tire  with a lighter rider at low speeds..it is an 10.5cr 836, with stock valves & crank, unported head and RC cam...running points.

The mag that I have not run yet, was what I weighed last night.

I have read several times on here  about the built motors not  starting to really work till in the upper rpms, and a lot of them are running CR's.... they are not talking gobs of power down low.

[mkramer1121]

I think the issue is with how the motors were built, not the carbs themselves.  A number of people have built motors and gotten the wildest cam that they could.  Unfortunately, this doesn't always work for a streetable bike.  I'm pretty sure that cam profile, carb size, valve size, etc., all determines what RPM power comes on at.

[Terry in Australia]

Terry, How much hp are you putting out with that setup?

G'Day Jay, no idea mate, Mike Kramer very generously gave me a crank which he sent to Big Jay at APE (and should be in the air by now thanks Mike) to lighten and balance, so the engine is about to come out again so I can install it, and then I'll try to find a good tuner to put it on his Dyno and see what comes out. I'm hoping to get around 80 BHP at the rear wheel, any more would be a real bonus! Cheers, Terry.

I think the issue is with how the motors were built, not the carbs themselves.  A number of people have built motors and gotten the wildest cam that they could.  Unfortunately, this doesn't always work for a streetable bike.  I'm pretty sure that cam profile, carb size, valve size, etc., all determines what RPM power comes on at.

 

Very true Mike, not having any tuning "gift" of my own worth a shiite, I relied heavily on Mike Rieck's advice so when he built the head, he built it with an 836cc kit, Megacycle 125/75 cam, and 29mm CR's in mind, and they work well together. The engine is very reliable, and pretty mild mannered around town, my biggest problem is that the K0 "restification" keeps dragging me away from it, I wish I was wealthy enough to retire so I could give all my bikes a little more attention, ha ha! Cheers, Terry.

[mkramer1121]

I was talking to an old racer a while ago, and he said too big of carbs can have a huge effect as well.  They'll come on later then smaller carbs.  He would use one size larger carbs at Daytona due to the amount of throttle you could grab there in the straights.

[Terry in Australia]

Yeah, it's an interesting thing Mike, Keihin infer in their booklet that the CR31's are more suitable to a bike over 750cc, but most of the posts I read from Sean and Art is that the 29's are actually running a little rich, and Sean has reduced his main jet size. The last thing I want to do is "over-carb" it.

I raised the needle two notches originally to richen the mixture, but it's a tad too rich now, so I'll drop the needle 1 notch and it should be about right for my purposes. When I build my 1060cc engine I'll probably go a set of 31's, I've got some Mikuni 34mm flat slide race carbs (which are lighter than the CR's) that I'd like to try, but I might not be able to get the spacing right. (they're for a GS Suzy engine)

I'm also tossing up about the CycleX single phase alternator? It's supposed to reduce the weight by a few pounds, albeit at the loss of the starter motor? That doesn't worry me as the kicker starts it fine, but has anyone tried one yet? Cheers, Terry. 

[754]

I tried 32mm carb throats..  turned pretty well the same ET,s but harder to ride and  noticeably less bottom end..

Must suck to have to pull your carbs to go bigger on the venturis.  10 min job for some of us      

I  never held a Honda ARD mag in my hand, so I did not know how much they weighed.. I did notice though the newer generation CDI type for Kawi is only 6.3 lbs... alway figured those wopuld be adaptable to a 750 Honda..

[Terry in Australia]

I tried 32mm carb throats..  turned pretty well the same ET,s but harder to ride and  noticeably less bottom end..

Must suck to have to pull your carbs to go bigger on the venturis.  10 min job for some of us      

I  never held a Honda ARD mag in my hand, so I did not know how much they weighed.. I did notice though the newer generation CDI type for Kawi is only 6.3 lbs... alway figured those wopuld be adaptable to a 750 Honda..

Have you ever actually changed the venturis "on the bike" mate? There's barely enough room in the gap betwixt carb and frame for the "pan cake" air filters, so best of luck getting the venturis (that span the width of the carb, of course) out. Of course, WHY you'd need to change venturis is a whole other question...............

I've got several ARD MK2 CDI magneto's for Z1's in my garage, I sold an ARD MK2 for a Honda to one of the guys here a year or two ago, they're a lot lighter than the MK1, but 30 year old CDI's don't inspire much confidence, (in fact I've got two that have been converted back to points, but they weight more again) they also need special external ARD coils, and you've got the problem that you'll have with most magnetos, you have to start it on full advance. Can you say "kick back"? Ha ha, Cheers, Terry.

[754]

I changed venturis on the chopper without a problem, running out to look at my stock frame right now..

Looks like they will come out easy.. The first part is the bombsight, pull the screw & pull it out with 2 fingers.. them pull the Venturi.  As far as I know that is it, unless the venturi comes out the front (been a long time) them you would have to pull 2 bolts. Proper name is actually chokes. the 2 pieces are near the length of the carb but the longest is about 2 inches.

On the stock frame the clearance problem for aircleaner is at the top, the rest has lots of room with sidecovers off.. I been running without..had bugeyes on it but last time I went to use them the black foam was crumbling so I ran without..

Why would you want to change venturis?? same reason as you went to 29mm carbs or are thinking of going to 31mm on a bigger motor.

[Terry in Australia]

Why would you want to change venturis?? same reason as you went to 29mm carbs or are thinking of going to 31mm on a bigger motor.

Yeah I guess, but the bigger engine is going into another bike, so I'll still need a second set of carbs, regardless. Cheers, Terry.

[754]

The difference is in the manifolds, I have a splayed one on the bike and shorter  rubber intakes, so I have more clearance.

Straight, you dont have to notch sidecovers, splayed you do.

As any speed equipment the pain has to justfy your gain. just like you know the stiffer spring are going to beat on your cam but you decide it is worth it for the gain..it is gonna cost sometimes..like adding a second 10 lb disc to the front of our bikes..

Terry how did you end up with so much speed equipment you dont like?? Did you ever try your Dellorttos?

[Terry in Australia]

Terry how did you end up with so much speed equipment you dont like?? Did you ever try your Dellorttos?

Well, I bought the Dellorto's from a car guy, then found the only NOS manifold in Oz, and Daniel was nice enough to show me a pic of his linkages. I never ran them though, the weight really put me off, and Mike Rieck advised me to buy the CR's to suit the head he built me.

I did get to ride a Dellorto equipped CB750 based 1200cc bike, (built by Rex Wolfenden, Australia's premier Honda CB750 builder and racer) and it went extremely well, but of course, considering the engine mods it'd be hard to tell how much difference the Dellorto's made, and the new owner replaced them with Mikuni flatslides shortly after buying it, but I don't remember why.

I'm a big EBay fan, and I discovered quite early that there are "performance parts" out there that if bought cheaply enough, can generate enough money for more desireable items. The Webers, Dellorto's, ARD and RC Magneto's etc can generate quite a lot of money as they're very popular with the retro chopper guys in particular, because they do look cool. I've still got 3 ARD Mk2 magneto's for Kawasaki Z1's, but the Kawasaki guys aren't as interested in them as the Honda guys are, so they're harder to move. I must machine them up to fit CB750's.

The CR's are very well priced at around 700 bucks a set brand new, but you can normally pick up the webers or dellorto's a fair bit cheaper, and now that guys like Daniel are selling new rubbers for them, they are even more desireable. If I see another set of Dellorto's (or Webers) I might try them out on the 1060 engine, as light weight won't be a #1 priority. Cheers, Terry.

[City Boy]

Well boys,this discussion has gotten quite WEIGHTY!I only have experience with vm 29 and RC webers,the webers were clearly superior in performance.The engine had a completely different persona,also sounded more like a Cosworth V8 than a Honda at WOT.Given the chance,I would install them on my 1100F without hesitation.ROCK ON boys.

[Soos]

EXCELENT information all!

Has anyone tried any of these carb setups on a 650?

As a side note - In your opinion, what diameter venturi is needed for a given CC displacment for a 750?
The 736cc stock of a 750 for example, was it choked by the carbs?
Would say 29mm be better suited for stock, and 31mm or so for 836?
Is there a ratio of cc displacement vs venturi diameter for optimal power/performance?

l8r

[mkramer1121]

Check out satanicmechanic.org, lots of good info...

[billybobobrain]

has anyone coverted 82 cv carbs onto a early 750?

[MCRider]

EXCELENT information all!

Has anyone tried any of these carb setups on a 650?

As a side note - In your opinion, what diameter venturi is needed for a given CC displacment for a 750?
The 736cc stock of a 750 for example, was it choked by the carbs?
Would say 29mm be better suited for stock, and 31mm or so for 836?
Is there a ratio of cc displacement vs venturi diameter for optimal power/performance?

l8r

Its not an issue of cc to venturi size due to different carb efficiency, and the volumetric efficiency of your set up, head, cam, pipe. Also not all venturis are round, some are oval, so like monitor screens, one man's 19" is not anothers.

Trial and error, talk to people who have been there.

[City Boy]

Hi guys.I have experienced a direct comparison between the 29 smoothbores and the DCOE40-32 on my 1000.I have none with the CR's so I don't know how they would stack up.I road raced my machine a few times at Mosport Park ,a very fast circuit that gives ample opportunity for winding the living snot out of whatever you are on.The difference was like night and day in the Italians favour!Tuning did take longer and was more costly,the weight issue didn't bother me .I used the upper air cleaner mounts bent 90' and fitted rubber isolation studs to the carbs.The machine sits idle right now,I'm having too much fun on my 1100F,but I would put webers back on in a New York minute if I get the chance and I have any Lira.The comparo to me was valid as no other changes were made to the machine.       Rock On

[.RJ]

Has anyone fitted a set of flatslides on a CB750?  Would they still need to be ~29mm?