1981 CB750F SuperSport- Starting Issues

[DougB-2]

I was Told When I bought this Bike that It was a little Cold Blooded. Ive been having a hell of A time getting it started even with Ether. Any Suggestions? It May have been the cold, it was 25 degrees last time I tried. Should I take the Carbs off and soak them in a vat of Carb Cleaner?

[Mark257]

i have 1981 Honda cb750c and i have a real problem starting it when the temperature is bellow 45  , i can start her but she runs very poor until she worms up , my carbs are clean and i have a fresh fuel in it but like everyone says they are cold blooded.

[DougB-2]

Ok. Well I guess Its Good To know that That is the Nature of the Beast. It's Just Very Frustrating. I've  Ran the Battery Dow ntwice trying to get it Started.

[woodsrider250-2]

Primary main jets are generally lean to start with, and when 25 deg will be even more the case.  On the DOHC's there is a vaccuum petcock in addition to the manual one.  This won't really let the bowls fill right until you get some popping going.  I haven't tried rectifying this problem myself as of yet.  SOme will remove this vacuum operated valve completely.  You can blow on the vent line coming off the carb bank to give a shot of fuel before cold starting.

[cbRider]

I agree 100% with woodsrider250.

Just today I got so frustrated with the starting situation that I took of one of the float bowls and sure enough the gas, while cranking over the engine, just trickled down. I then replaced the float bowl and bypassed the vacuum fuel valve and the thing started with one touch of the starter button. And all this on a 70 degree day. Just remember to shut off the petcock after shutting down.
And since the idle jets are fixed (not replaceable), the next step would be to replace the primary main jets with a size 75, that would help some but better would be to ream out the idle jets just a bit.
I wonder if anyone tried that.
A detailed technical drawing of a carburetor would help here.

BTW, the vacuum petcock gets removed next time I take off the tank and it is gona stay off.. It,s not needed at all.

Happy New Year,
Karl

[Polymer-2]

Which carbs do you have (type and model#) ?  I may have a drawing.

[cbRider]

Hi polymer, I've got the Keihin VB42B. It's not even listed in the Clymer Manual.
They list the VB42A and VB42C.

[Polymer-2]

Well it doesn't look like I have that one however here is a link to a Keihin manual. This is the only one I saw on the site. <a href="http://carburetor-manual.com/keihin-carburetor-parts-service-repair-manual-ebook-p-898. html">http://carburetor-manual.com/keihin-carburetor-parts-service-repair-manual-ebook-p-898.html
He may have others if you send him the model number. Tell him Jerry (polymer) sent you.

[rtabish]

this may be a bit obvious, but do you have your choke set? and do you still have your airbox on the carbs?

[holysmokes]

are you sure your idle jets are clear and clean?a good starting place for idle mixture jet settings for a 750 is 2.5 to 3 turns out from seat, if you pump your throttle 5 or 6 times when its cold, you will prime the engine

[DougB-2]

The Choke Is Set- im going to Toy around with it a little though.

The airbox did not come with the Bike... I just have 4 foam Air Filters...

[holysmokes]

pods are garbage, go over to www.cb1100f.net and see if you can an airbox setup

[rtabish]

you need the stock airbox, and its inherent negative air pressure, in order for the vacuum actuated slides in these CVs to work properly. you may have other issues, but experience says no airbox is a biggie with poorly performing stock carbs.  

[DougB-2]

Once I get the bike Started I'll go get some Seafoam and Clean it out.

[DougB-2]

Ok. The Pods Were what came with the bike...

Hopefully a new airbox will fix it- Or at least help

[holysmokes]

pods are only good in movies..like return of the pod people or the body snatchers

[DougB-2]

Will an airbox from a 1979 cb750 fit?

[holysmokes]

quite likely k -f

[gonebiking-2]

Yes the airbox from the 79 -83 CB750 F & K models are both the same and you can get a new one from www.hdlparts.com. Thats where I Have  been getting all my parts from this website and I am in Australia restoring my 1982 CB750KZ. Just follow the instructions on the menu page and you will find what you are looking for.

[woodsrider250-2]

This is quite true for how the bike will run once running, but no real reason as to why all of these are a PITA to start after sitting for a while.

Pods=fine on a different carb, or potentially OK with specially tapered needles (K&N stage 3 jet kit).  Who knows if your bike has those in it or not...Would have to pull the needles.  

See airboxes on ebay all the time for nothin.