Login  Register

Re: 75 CB750 wont idle with rebuilt carbs & no airbox

Posted by Trionda on Apr 23, 2014; 12:37am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/75-CB750-wont-idle-with-rebuilt-carbs-no-airbox-tp4025625p4050370.html

I stumbled upon this thread after trying to research my friend's CB750.
I have to say--this forum looks like a really decent place full of resourceful people who want to help each other out.  That can't be said for a lot of forums.  

A little history:
The bike is super clean for a rider, and although it seems to have been well cared for, it's definitely a bitza, which makes it hard for me to know base figures for dialing it in.  My friend brought his CB750 to me 9 months after buying it from a bike shop that "specialized" in customs and odd builds.  Although their work seems to be adequate, they clearly never addressed many of the components of the bike that will need attention soon.  When he last rode it, he was complaining about a lean stumble off idle, and lack of power on top.  

The skinny on the bike:
It's a 1972 CB with a 1977K engine and 1978 carbs with accelerator pump (all carbs have a stamp just above the float bowl that reads 41BAOK7.  The A has a box around it, and the 7 at the end is stamped crooked).  
The bike has 4-into-2  Mac pipes, and Bikemaster pods.  The main jets are 110s, and I have not checked the idle screw locations or the pilot jets.  The plugs are a very light tan color, which verifies his concern over a lean condition.  The engine has either been rebuilt by someone careful, or has very low miles and is really clean.  The carbs have not been messed up by a jackleg mechanic, and look nearly new.  The valves are clean as a pin.
I have no idea whether it's been fitted with a lopey cam, but I will assume the engine internals are bone stock.

I am new to these carbs, but have plenty of experience with Keihin CVKs from the 1990s and on, and wrenching on everything from bicycles to big rigs.
These carbs look very solid, and I'm interested in learning more about them.

Since I do not have access to a dyno, or a decent place to do "true" plug chops, if you can offer any baseline suggestions as to initial jetting and idle screw configuration, I'd be grateful.