[fuzzybutt]

i have a 74 cb750k with the stock carbs on it, and i'm in need of something better for a mildly modified bike. i cant even come close to being able to afford the 600-900 bucks for a set of mikunis or keihins. any suggestions?

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

 

[Psyco Diver 69]

I think the 76-78 carbs are suppose to be better. I don't know for sure though, I was considering using them for my CB750, but I have to modify them to fit the spacing on my DOHC 750

22/m
1980 Honda CB750
1972 Oldsmobile 442

 

[Geeto67]

The 77-78 SOHC carbs have an added accelerator pump and slightly bigger carb rubbers. A few guys on the SOHC board have tried these on earlier bikes and found some power gains and throttle response. I have no expirence with them but it would not surprise me, they are a better design, the only problem is they are a pain to rebuild. Typically they are cheaper than K carbs when they sell on ebay. YOu will need the rubbers from a 77-78 bike (you can't use your stock 74 rubbers) to adapt them.

The other option is Mukuni VMs. Stock on kawasaki h1's, I have seen guys adapt these to cb750s before (using the K0 throttle cable). YOu are basically trading a bank system for individual carbs supported by the carb rubbers but the VM carb is a really versitle tunable carb, and was available on stock bikes in 26-32mm sizes (s3 to h2). Cycle Exchange sells a dual mukuni kit for the cb750 (about $500) with new carbs setup and ready to go for stock and modified bikes.

http://www.cyclexchange.net/Carb System Comp Page.htm

Early model GS750 (77-79) and KZ900 carbs ahve been used in the past, but they require major work to get to fit the spacing on the SOHC. The GS carbs are accelerator pumped carbs like the 77-78 so you are better off with them.

Stay away from DOHC carbs, they are CV and junk.

 

[parks61]

not to be a wet blanket...but often the stock set up is the best way to go. what sometimes seem to be simple modifications often end up causing a new problem (such as off idle surge or fatness). it all comes down to defining the problem to be solved. if you just want a faster bike, a simple carb swap won't likely be the solution (honda would have done that to begin with). bikes with racey engines often are less than enjoyable on the street. been down that road.
parks

 

[Geeto67]

quote:
not to be a wet blanket...but often the stock set up is the best way to go. what sometimes seem to be simple modifications often end up causing a new problem (such as off idle surge or fatness). it all comes down to defining the problem to be solved. if you just want a faster bike, a simple carb swap won't likely be the solution (honda would have done that to begin with). bikes with racey engines often are less than enjoyable on the street. been down that road.
parks

A simple carb swap did solve the problem and Honda did do it - it is the stock 77-78 carbs with an accelerator pump.

In a cb750K (69-76) the biggest restriction is the heads, those valve and intake and exhaust ports were very restrictive. When Honda finally redid the valves (bigger) in 77-78 the old round bodied carbs were ditched in favor of a set more inline with what Kawasaki was already doing and what suzuki had also introduced, rack carbs with better airbleeds, fuel metering, and an accelerator pump.

Parks, if we were talking about a stock bike then I would agree with you whole heartedly about the stock issue (although I would probably recommend 75-76 F carbs since honda improved the airbleeds on the otherwise identical rack for those years). Anyway, I think I remember Fuzzy mentioning this bike had a lot of engine work (810cc kit) to it which the stock carbs just do an ok job of handeling and eventually become the restriction.

Fuzzy - in the future, please detail the engine specs - mildly modified can mean just pipes and filters to some and a "small" big bore kit to others.

 

[jbranson]

I'm not so sure Honda always did what was best for power....perhaps on the larger bikes...but on bike like the 175 twins...you just throw on carbs 4mm larger and the bikes scream. About a 10-15% power increase....meaning about 2hp

JohnnyB

 

[parks61]

glad it worked for you (both); i stand humbley corrected. my tirade was based on running my racer both on the street and track for two years taking the lights on and off between events. it worked great when i rode too fast on the street, and o.k. on the track but not fast enough. years later i have a stock street bike which behaves wonderfully for all its required tasks (although the carb is 2mm bigger than stock so i guess i'm still spewing shit.) my racer now has an 8mm larger than stock carb and is all i could ask for on the track (except behind phil turk on the front straight) however, it would really suck on the street now so the shit i spewed wasn't totally off base. bottom line, if the problem is solved, you did the right thing. my mouth is now shut on this subject.
parks



Edited by - parks61 on Apr 19 2007 5:38:54 PM

 

[fuzzybutt]

i picked up 3 racks (9 carbs) of h1 carbs this afternoon at a local salvage yard. gonna try and see if they'll work any better than the stockers

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

 

[Psyco Diver 69]

what size are th 76-78 carbs?

22/m
1980 Honda CB750
1972 Oldsmobile 442

 

[fuzzybutt]

i think theyre still the 28mm like the 69-76 carbs
only difference as far as i know is they have accelerator pumps. i need to get rebuild kits for the ones i have.

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

 

[Geeto67]

quote:
i think theyre still the 28mm like the 69-76 carbs
only difference as far as i know is they have accelerator pumps. i need to get rebuild kits for the ones i have.

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

no, they are bigger and you are going to need to use the 77-78 carb rubbers to adapt them to your 69-76.

They are a completely different desing from the previous generation of K and F carbs. They are more like the carbs used on early GS suzukis. The Suzukis used Mukuni VMs, The hondas Kehins, but looking at the two carbs they are like kissing cousins.

here are the 78 carbs:

(not my auction):
http://cgi.ebay.com/ebaymotors/Hond...ryZ35597QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

and a set of 73 K carbs:

(again not my auction):
http://cgi.ebay.com/ebaymotors/73-H...ryZ35597QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Why did honda switch to these carbs? Well the Kawi Z1 900 was already kicking the cb750's ass in the power department, and suzuki's GS was slated to be introduced in 1977. Both of those bikes made in the 70's and 80's for hp (72hp @8000 rpm for the GS, 82hp @ 8500 rpm for the Z1 ) while detuned versions of the K bike (74-76) made in the high 50's. Honda's devlopment of the DOHC next generation cb750 was going slower than expected (they originally wanted a 77 introduction but the DOHC wasn't introduced until '79), so they needed to give the 77-78 cb750s a big boost of power to have a competetive product. The original 69 cb750 made approx 67hp, so the potential was there, but they suffered from chain breakage initially. For 77-78 the cb750 was given a better flowing head, higher compression (9.2:1 up from 8.5:1 approx), better cams, a less restrictive 4 into 1 exhaust, a larger countershaft with 630 chain, an oil filter spacer to help with cooling and new larger carbs with an acceleratior pump and enclosed rocker slide operation (hence why they are a pain to work on). I have a set on the bench and they look physically bigger where they meet the engine than the 75 carbs next to it, but honda rates them both at having a 28mm venturi.

When installing these carbs on the earlier cb750s you need to use the 77-78 carb rubbers to accomidate them. You will also need the 77-78 airbox (which looks the same as the 69-76).

Edited by - Geeto67 on Apr 22 2007 10:14:43 AM

 

[fuzzybutt]

the same salvage yard where i got all those h-1 carbs has a set of the 77-78 honda carbs too they said i could have for 40 bucks, theyre in need of major cleaning BUT i imagine those'll be easier to put on than adapting the mikunis which i'll use when i go to 836cc's on the 750f. also do i have to get the later model throttle cables for the accelerator pump carbs?

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

 

[Geeto67]

quote:
the same salvage yard where i got all those h-1 carbs has a set of the 77-78 honda carbs too they said i could have for 40 bucks, theyre in need of major cleaning BUT i imagine those'll be easier to put on than adapting the mikunis which i'll use when i go to 836cc's on the 750f. also do i have to get the later model throttle cables for the accelerator pump carbs?

some people are like slinkies, basically useless but they still make me smile when i push them down the stairs

I'm pretty sure you can use your stock cables on the 77-78 carbs. The only cb750 that used different cables was the 1969 bikes where they used individual cables for each carb (probably the cables you want to get to do your mukuni swap).