KZ750E3 Rising Revs Issue

Hi from lock-down England, finishing touches to my café racer project and struggling with some carb issues. Here’s what we have;

KZ750 US version 1981 with the smog tubing removed. (vents in head tapped and blocked with grub screws)

The top end was rebuilt and the carbs were originally stripped, cleaned and rebuilt earlier on in the project (OK it’s taken more than 3 years…) rebuild kits, new jets (135 & 95 I think – followed recommendations from the forum). Twin K&N Filters – 2 carbs share one filter – see images attached.

After the rebuild I couldn’t get a vacuum to show on a manometer on number 4 so just replaced the intake boots with NOS, carbs slide in perfectly, clamps/clips all good and tight.

After market 4-1 exhaust (Delkevic) but no other engine mods other than usual maintenance you’d to a on a strip down – gaskets, valve grind etc.

With the original boots it was unhappy at part throttle, with the new boots it’s running significantly better and pulls strongly on anything more than quarter throttle but now when it gets hot the revs are rising and won’t drop to tickover without turning the ignition off.

I’ve tested the boots for a leak with a propane torch but no affect seen. I’ve got the pilot screws set to 2.5 turns out and moved them to 3 this morning, no change – I also tried adjusting pilot on No1 but didn’t see any RPM change. Also this am it would not fire at all on full choke – needed half choke to start. I mean not a single combustion with the choke on which seemed odd from cold…

I changed the petcock to an after-market one because the K&Ns fouled on the original – it’s still vacuum controlled and the hose from No2 to the petcock is good – all other carbs have new caps on the vacuum points. There is a secondary filter in the fuel line but I can see fuel flowing as the lines are transparent.

Did all the float heights a few months ago when the carbs were off (again)

I assume there is possibly some blockage still in the carb internals? But before I take over the kitchen table for a week again I thought I should check for anything simple I might have missed…

Any help greatly appreciated!

Stay safe.

A creeping idle means lean and or a poor sync job. Considering the pods will make the mixture drastically lean I suspect the problem could be overall leanness and/or vacuum leaks.

Thank you for the reply, carbs 1-3 were synced using the manometer and I bench synced number 4 this week. I rejetted to accommodate the filters but I need to pull the plugs and see what colour they are no. If there’s no leak and it’s still lean I guess I need to get a dynojet kit?

A Dynojet kit will get the mixture darned close, or maybe spot on. Though not cheap, it is a really quick way to avoid a lot of trial and error. Re-synchronizing all four with a manometer is certainly needed.

I pulled all the plugs, 1 looks rich, 2,3 & 4 all look lean to me...

These images show 2 and 4, possible overheating too?

The black around the base of the thread suggests it was running rich - they were all dark before I did the intake boot swap. How can changing the boots suddenly make 3 out of 4 go lean?

If you have the original air box put it back on and be sure everything is leak free, then try it, if it behaves normally you know its the pods and jetting, sadly pods and CV's are not a good combination with sacrifices to be made at either top or bottom of the rev range.

Unfortunately I don't have the airbox anymore.

What's throwing me is that this rising revs issue only started AFTER I fitted new inlet boots - so potentially eradicating a possible cause of leaks rather than introducing new ones...

Another train of thought - I'm wondering if their is a fuel starvation issue;

1) Number 1 plug is looks richer and number 4 looks leanest
2) I have a secondary paper filter inline
3) There is a high point in the fuel line with an air pocket in it
4) After market petcock
5) Low fuel level

I'm wondering if these are conspiring to fuel starvation, perhaps when the bike is on the side-stand it's filling carb bowl 1 adequately but not the other carbs that are higher?

I'm going to try pulling the filter and running a shorter fuel line direct from the petcock, switching to PRI and filling the tank up!

Will report back in a day or so.

Thank you

Well we have resolved the rising revs issue;

Removed carbs and adjusted floats to get fuel height correct - they seemed to be all over the place but I think I was making mistakes with the clear tube method - trying to get rid of air bubbles I was moving the tubes up and down without covering the end with my finger - so now I am confident the fuel height is correct.

Removed the inline filter - wasn't sure if it was an issue but wanted to eradicated all potential causes.

Replaced the fuel line with one that doesn't allow a high point under the tank (shorter and no filter)

Put more gas in the tank!

At first it was popping from the exhaust on tickover - but I think perhaps one of the needle valves hadn't settled and some fuel was running through, but after a few minutes and some revs it settle down.

Test run was good, pulling strong and smooth from 4k but fluffy on part throttle/low revs.

I'm waiting for an idle adjuster to do the pilot screws to see if that helps but I expect it needs balancing on the manometer again.

So we're nearly finished - will do a photo shoot in the next week and post the timeline on here.

Thanks for the assistance.