Author Topic: 1974 K4, Stock Bike , #45 Idle/Slow Jets??  (Read 1027 times)

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Offline bompxc

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1974 K4, Stock Bike , #45 Idle/Slow Jets??
« on: September 20, 2014, 10:28:02 AM »
Narrowing down my idle issue and I've found it to be the idle jets.  One was clogged with something so tight that I couldn't get the crud out so I went and bought the carb rebuild kits from 4into1 and for now I'm just going to replace the idle jets and float valves. My PO has left this bike full of surprises and when I took out my old idle jets, they were #45 instead of #40.  Bike was running well before all the crud got in there, but a little rich if anything.  By putting 40's back in, that should solve the rich issue right?  Or, if it was running alright before , should I just put the 45s back in?  Also, when I pulled the float valves, they were these guys instead of the stock version. http://www.vintagecb750.com/parts/480/48-1529.jpg , the ones with the little filters on top of them instead. Would it be a good idea to go ahead and replace the valves with the filterless options as that is what is stock and supposed to be in there?
1974 CB750, K4.  Stock as can be, with the exception of 1975 CB750F Keihins.

Offline bompxc

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Re: 1974 K4, Stock Bike , #45 Idle/Slow Jets??
« Reply #1 on: September 20, 2014, 10:38:49 AM »
Just ID'ed my carbs, and although I have a K4, looks like these guys are off a 1975 CB750F.  Compatibility issues?
1974 CB750, K4.  Stock as can be, with the exception of 1975 CB750F Keihins.

Offline bompxc

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Re: 1974 K4, Stock Bike , #45 Idle/Slow Jets??
« Reply #2 on: September 20, 2014, 10:50:18 AM »
Also, anyone ever ditch those little filters that go above the float valve or are they essential?
1974 CB750, K4.  Stock as can be, with the exception of 1975 CB750F Keihins.

Offline bjbuchanan

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Re: 1974 K4, Stock Bike , #45 Idle/Slow Jets??
« Reply #3 on: September 20, 2014, 06:36:02 PM »
The little filters are a nice idea but you can abandon them. The carbs are essentially the same, they have the same internal passages and orifice sizes for the main carb body. (except maybe the air screw bleed thing in the throat, don't worry about it, that is little to do with your issues)

If it is a stock bike then there is no advantage to bigger jets, they don't do anything. Even with a different exhaust it shouldn't matter. If you have pods you need to mention it.

For stock bikes and airboxes, exhaust doesn't really matter, 110 main jet, 40 pilot, middle needle position, air screw 1 turn out. That will only need small fine tuning, if it even does.

Put a description of your bike in signature line. Year, make, model, mods, condition
That way you never have to remember to include it in any thread.
The dirty girl-1976 cb750k, Ebay 836, Tracy bodykit
Round top carbs w/ 38 pilots, middle needle position, airscrew 7/8ths out, 122 main jet
Stock airbox w/ drop in K&N, Hooker 4-1

Don't trust me alone with a claw hammer and some pliers

Offline bompxc

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Re: 1974 K4, Stock Bike , #45 Idle/Slow Jets??
« Reply #4 on: September 22, 2014, 10:57:16 AM »
Sounds good.  Thank you sir!  I'll be sure to update as I take care of the issue
1974 CB750, K4.  Stock as can be, with the exception of 1975 CB750F Keihins.

Offline cherbear

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Re: 1974 K4, Stock Bike , #45 Idle/Slow Jets??
« Reply #5 on: September 22, 2014, 11:33:34 PM »
I joined the forum just to tell you... I had carb problems for years. The second I replaced the seat that had the filter with a nonfilter seat my carb problem was gone & my bike is pretty dang fast.