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Frankensteins Monster

Aska11415

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I have a 1978 CB750F but it has a 1972 CB750K2 engine. I didn’t do it, this is how it was when I bought it. Now here is the dilemma... the carbs are still the 78 F. What generation should I base my jetting off of?
 
Doesnt matter what engine. Jet it to what it needs based on how it runs. Every engine is different there is no way to say what jets it needs, to many variables affect jetting. Altitude, temperature, humidity, engine condition, tuneup accuracy, intake setup, exhaust setup, engine mods, fuel quality, weight of bike with extra added on weight, all of it affects jetting.
 
Hey dirtdigger, does that '78F still use the same bore size in carb (28 except for the ATX one?) the others did? That might provide some direction. I do agree with the analysis given there. Likely the idle circuits are probably pretty close, if anything maybe a main jet only change, depends. That particular engine got more cam thinking, depending on if they changed the main air bleed the main jet might get bigger or not.

So many little details can affect the carb jetting, why he says to not assume either dead stock setup works. Beginning with, the engine is no longer the one that goes with the carbs, ergo neither may work to 100%.
 
78 uses the bigger keyhole carbs. The 78F yes is a slightly more powerful sohc but the carbs are jetted lean and can be used on the k motor without much if any change. All years of the sohc are really on the lean side and I almost always end up jetting to next size and messing with the needle. The fact is run it and see how it runs and see what if any changes need to be made. After this many years nobody knows what is in the engine, could have different pistons, or different head, milled head altering compression. Earlier cam. You just never know why I always say run it see how it runs and change from there.
 
You gotta point. As many people have handled those I'm sure you're hard pressed to find one that has been left alone. It's simply asking too much.

Terrible on the DOHC too, when they begin to burn the valves most guys think it's a carb thing and why you buy so many of them with the carbs off. Messing with carbs is a male testosterone rite of passage thing or some such bullsh-t. I've also found you can tell whether any aftermarket pipe is any good or just junk, if junk the jetting stays the same (or even gets richer) but if it really works well then the jetting has to be upped a bit.
 
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