Dohc carbs at higher altitude

[Spencermi]

Hey i have a 80 cb750f bought it in minnesota. It was running great after i bought it. stopped running for my uncle before i picked it up which i figured out was due to low fuel anyways im in colorado now got it running but runs real rich and hard to start i have the pod filters and 4 into 1 exhaust im guessing i need to change the jets anyone know what size i should go to or if i can get away with adjusting the pilot ive also heard of people going to the gsxr carbs for the pod filters

[LukeM]

There's less oxygen at altitude, so leaning out the carburetors is what's needed.  I'd start by turning the mixture screws about 1/2 turn at a time, and then doing a plug reading after a tank of gas.  My guess is they'll be a bit black and sooty now due to the richness.  You're aiming for a light brown color on the insulator.

If memory serves, turning the screws IN on those carbs will introduce a leaner condition.

If you run out of adjustment, you may have to go one jet smaller across the board.  The FSM (on the front page of our happy little forum) should have the factory jet sizes.  Check yours when you get them off the bike, then go shopping.

Luke M

[sgtslag]

Don't know about the GSXR carbs, but I do know that the Keihin Constant Velocity Carbs will NOT work with pod filters.  They require a particular vacuum, generated by the OEM air box, in order to work properly.  My recommendation would be to get an OEM air box, and filter, to replace the pods.  Then begin working on the jet sizes, if they still are not performing properly.

For more information on using pod filters on DOHC CB750's, visit the CB750 Custom web forum, and do a search on the subject, "pod filters".  Here is the link to that site:  link.  A great many people have tried to make them work, spending a lot of money in the process, with very little success.  Cheers!

[seestheday]

I'm still not convinced that it can't be done.  I've read stories of people who have done it.  Whether or not it is worth the huge time investment to get them to work well is another story.

Regardless - pods on a the DOHC CB750 - not for the inexperienced, not a cheap modification, performance gains will probably be minimal if any.

Honestly, I think I'd rather spend a little money and just buy the cyclex carbs or carbs from an older CB750 before spending all that time trying to get the CV carbs to work.

[Spencermi]

Ya I adjusted the mixture screws and the idle adjustment got it running and actually rode it home but the next day it wouldn't start again I've heard the 80 has non adjustable pilot jets it sounds like this is what I need since it runs fine after warmed up just hard to start and keep going til that point