1971 Honda CB750 carburetor jets

MichaelBread

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Hello. I just recently purchased a 1971 CB750 and I have a 4 into 1 exhaust with pod filters. I was wondering what would be the most reliable jet setup for me...
 

dirtdigger

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There isnt a best setup, every bike and combination is a little different and needs different adjustments. Engine condition, quality of tune up, exhaust design, intake restriction amount, elevation, temperature, humidity, how you ride, etc all affect the jetting that will be needed. That said start with 1 main size larger. Check compression and verify engine is healthy, do basic tuneup....valve adjust, timing chain adjust, point and timing adjust, verify timing advancer works correct, new plugs and correct gap. Verify you needle jet position is in the stock middle slot. Verify and adjust float level and do a running sync on the carbs. Then with you 1 size larger main jet go for a ride and see how it runs and note any flat spots or boggs etc. Run wot with a good load, start slowy closing the choke....if the engine picks up then you are still lean.....if it bogs down then it is too rich. Do the same thing with the choke at half throttle or any time you have a surging or a bog and figure if you are rich or lean. Start with wot and the main jet, the work out the mid range with the needle jet position.
 

HondamanUSA

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With 'pod' air filters, there is no "best" setup. The carbs need 3 pressure zones to work correctly. In order from highest pressure to lowest these are:
1. Bowl air pressure, supplied thru 2 small vent hoses on your set that reach over the top of the airbox to the back of it where air is more still when moving.
2. Bell-zone air pressure, which MUST be lower than the bowl pressure. This is at the mouth of the carb where you can see a small brass orifice: this leads into the emulsifier tubes for the fuel being pushed up the mainjets by the (higher) air pressure in the bowls.
3. Venturi, or 'vacuum' pressure, under the slide where the top of the emulsifier becomes the needle jet. Here, the jet needle in the slide opens the cross-section of this orifice in accordance with the height of the slide opening: higher slide opening equals larger needle jet opening.

If you install open 'velocity stacks' or 'pod' air filters, the bowl and bell areas are both at the same pressure. This then requires the lower pressue in the needle jet to try to suck the fuel up the emulsifier as the vacuum changes: this is not only not possible, it is non-linear. So, as soon as the engine reaches about 1800 RPM (in the 750) and the idle jet loses its laminar flow and stops working, the mainjet, which is NOT mixing air and fuel because there is only one pressure above and below it, slugs wet, un-aerated fuel into the engine. The result is lousy throttle response, black, fouled sparkplugs, and a parked bike.

Don't use pods....find a proper air filter!
 
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