1982 KZ750 Spectre vacuum reading

Hi everyone, I'm new to the forum and working on bikes so please excuse me if I'm incorrect about some things.

I've been resurrecting a KZ750 spectre lately and im trying to get the carburetors tuned. I have rebuilt the carburetors and made sure everything is clean and all parts are present. I have set float height to as level as I can by eye. I managed to get the bike running (it's been sitting since 09] after rebuilding the carbs. It has a set of k&n air filters on it. I slapped the carb on the bike and it seems to start up easily now, but I'm having high idle (about 4k). If I open the choke a bit it idles down. I know there's a fast idle adjustment but I've been wondering what typical idle RPMs are with full choke. They are keihin CV34 carburetors. 

My other question is what is a good vacuum reading at idle measured off the vacuum ports on each carburetor. I am going to synchronize the carburetor and here are my readings.

In cm HG cylinders 1-4 from left to right:

1: 20
2: 15
3: 13
4: 12

Would this indicate a vacuum leak on the other carburetors except #1, or should I go ahead and synchronize them?

PS: Pilot screws are set to 2.5 turns out, apparently the previous owner got a jet kit with the filters and they are currently installed in the carburetors.

Any help on this subject would be greatly appreciated 

Unless the carb jetting has been reconfigured to run with pods, it will be drastically lean.  A Dynojet kit will save a ton of time on rejetting.  Also, make sure your pods are real K&N's, pods from APE, or something similar.  Cheap china pods are junk, and an impediment to good running.

Syncing the carbs is a part of standard maintenance.

The rubber carb boots that attach to the head are 40 years old, and subject to cracking.  I'd replace them straight away to avoid vacuum leaks, either now or in the near future.

[deleted]

Thanks for the response

The owner says that he put a jet kit in it.  If theres any way to verify they are correct for the application please let me know. He said they were recommended for those filters. I am unsure of the specific sizes. I am sure they are genuine k&n filters.

Rubber boots appear in good shape, bike has 20k miles.

I'm still wondering, what should the RPMs look like with full choke cold?

Also, what should vacuum be at the carburetors at 1100 rpm?

I need to get things tuned correctly. If necessary I have the original jets and also the factory airbox laying around. However I'd like to avoid using those if possible.

Any answers to these questions would be greatly appreciated 

-KzJM

Does the bike have a 4 into 1 header?

If it has a header and pods then it may have a stage 3 Dynojet kit.

That type of "jet kit" includes lighter slide springs, richer jet needles, richer mains, and instructions to drill the slides.

A genuine K&N filter pod will have a big logo on the end cap.

The tuning numbers your looking for are most likely in the FSM for that model, do you have a Factory Service Manual?

Miles do not determine value of the carb holders, years and heat is what will damage them. They can look good but still have leaks.

Nobody tunes an engine off of vacuum levels. Vacuum values are not posted in the FSM. We only use vacuum values to sync carbs, the value is insignificant the important thing is all carbs read the same value.

Choke is used only to start the bike, once started you feather the choke to get about 1,800-2,000 rpm to warm the engine. No top rpm value on full choke recorded or mentioned in the FSM.

Your asking for values that are insignificant and have no use to tune an engine. I’d suggest you get a FSM and start reading.

Rubber boots could look OK but if they leak between the head and the boots you have a leak.

The fsm says 22cmHg, with not more than 2cmHg difference between all four.

This info is at the bottom of page 16. In the pdf version linked below, the bottoms of the pages are cut off, but this information is just visible.

www.kzrider.com/modules/ServiceManuals/KZ750E1&H1.pdf


Some manuals say only that there should not be more than 2cmHg difference between any two cylinders, though the manual for the 400, 500, 550 four cylinder models says that no cylinder should read less than 14.

This thread piqued my curiosity, so I perused several of fsms here at KZR. Only two or three listed a target, and the targets were between 22-27. A couple listed a minimum reading that was 13-14. All of them said there should be no more than 2cmHg between any two cylinders. Thirteen to twenty seven seems like a pretty generous range. Like Mikaw I have long understood that the actual reading was largely irrelevant, and what really matters is that they are all very close to the same.

Texaskz after you post I did the same, but only with FSM in my collection mostly big fours, and didn’t see anything listed as a value, just that they should be within 2 +- cmhg. 

Meanwhile, back at the topic———

Synchronizing the carburetors comes very late in the tuneup procedure. Two of the most important items that must be tended to first are
1. Fuel level. Eyeballing the floats is not sufficient. The fuel level in each carburetor must be within a vey narrow measurement. The clear tube method as outlined in the fsm is the only way to insure that the actual fuel level is where it needs to be.
2. Valve lash. The gap between the base circle of the camshaft lobes and their respective followers(aka buckets) must be within the specified range. 

If either of these is seriously out of the specified range, the vacuum readings will be all wackadoodle (a highly technical term). If the carburetors are synchronized first, then these other matters dealt with, they may need to be synchronized again.

A miss firing/no igniting cyl. (too lean, less compressing) create a low vaccum reading.
A good ignited cyl. (expanding burnd gas) creates an extra vaccum flow in the EX/IN over lap stroke to suck more fresh air/fuel mix.