Zephyr 750 Sticks at 4000rpm

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I will give you a call tomorrow and we do some brainstorming, okay? I have a Zephyr 750 carb tuning odyssey story for you, we have to talk about.

Sounds like a plan, thanks! It will have to be Thursday evening or Friday though - tomorrow is going to be a nightmare...

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Sounds like what my KZ750 With CV34s did when it was air leaking. Any throttle at all past idle and it would jump to 4k-4.5k RPM and just stay there. It would also only happen when it was warm, exactly like yours.  

Thanks for the info. That's what I thought at first, and we did find a leak, but it seems OK now. Here's hoping it's just the bored out hole in the sliders combined with stock jets that are too big. I'll do another check for leaks though, third time's a charm...

The Zephyr 750 carb main jets (88-92-92-88) are too lean for the modern E5/E10 fuel !!!
Long story short, changing the main jets to 102-106-106-102 will un-chain more torque and top end power and better idle.
Don´t ask how i know it, the long story started with a fuel tank full of rusty chips!

 
 

Yup, vacuum leaks.  Spraying stuff on the carbs only finds the gross leaks, not the smaller ones.  

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Yup, vacuum leaks.  Spraying stuff on the carbs only finds the gross leaks, not the smaller ones.  

How do I find the smaller ones?

Some weak points are the wear out of the carb housing to throttle shaft bearing and the shaft seals!

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Yup, vacuum leaks.  Spraying stuff on the carbs only finds the gross leaks, not the smaller ones.  

How do I find the smaller ones?

Inspect the carb boots carefully.  Flex them and look for cracks.  Make sure the sealing lip that goes against the head is soft and not deformed.  Some people use RTV on the lip, but in my experience this doesn't last.  Unfortunately, the boots look to be discontinued in the USA, not sure about in Germany, and also not sure about aftermarket availability.  If possible, I'd get new boots.

Oh, and opening the pilot screw to 3 turns can help too, but it won't solve air leaks.

Checking use or old carb boots for bad sealing surfaces, cracks, hard´n brittle parts or sealant patched areas is a normal maintance in my opinion.
Replacing those old boots should be on top of the to-do-list but many people try to save some money on this have to aging parts! No seal last for ever!

Do you know anybody in the auto trade that could get you access to an exhaust gas analyser (MOT station etc) whilst using brake cleaner won't always identify the leaks, if used in conjunction with a gas analyser you will see the Hc readings spike every time you spray the problem area - This is the only way I know of to find smaller leaks that don't respond to the spray normally. Other than that it's an element of guess work, visual inspection and replacement parts that "might" cure it

Thanks guys. Carb boots are new, the o-rings from the vesrah gasket kit we had weren't sealing so we did put some RTV on the base. I have some stock o-rings now so I'll sort that out later today, I'll swap out the needles and jets and adjust the pilot screws as well. Hopefully that cures it and I won't have to start searching for a leak.

Just a quick update. We put the dynojet needles and jets back in and it was a bit better. Then we put the plates in the carb boots that restrict the airflow and knock it down to 48hp (European A2 license requirement - my son can't get his unrestricted license until March). With the plates in it was a lot more obvious that the throttle shafts are leaking - revs shot up with a bit of brake cleaner. So the carbs are coming back out and I'm going to have to dig out some felt washers...

I've never broken down any CKV carbs, but the little felt seals used on the CV34's are dust seals, not for vacuum sealing.  The CV34's use brass/bronze bushings on the throttle shaft and don't typically leak unless greatly worn.  BS Mikuni's use rubber throttle shaft seals.  They don't bush the carb body like Keihin does also.  Anyway, good luck getting it sorted.  I don't see any throttle shaft seals listed in the parts fishe so you will have to go off the playbook for this one.