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):
and a set of 73 K carbs:
(again not my auction):
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