Z650B3, difficult to start, bogs pulling away when hot, bad misfire 5,500rpm

purgeraptor wrote:

Well, I'm still not happy with the transition from needles on to main jets.
It pulls OK through the flat spot on full throttle, but if you slowly roll it on gently, it hits a wall and misfires.
When I hold it flat out, it starts to misfire after 2 minutes, fouling the plugs. It only recovers when revs fall below 5K on part throttle and plugs start to clean and it fires on all four again.

I thought it was too big main jets #112.5, so I fitted some Dynojet DJ 106 mains on the recommendation of a tuner I trust and use for my ZRX's. A little better , but not solved the problem.

I'm thinking ignition timing now as I changed the points set-up to a Boyer Bransdon electronic unit over the Winter.

How critical is the ignition timing to good running?

Ignition is certainly critical for best performance, but don't think it's the culprit here.

Am unfamiliar with Boyer Bransdon, but if similar to oem, the fixed idle advance position quickly becomes fixed advance position with increase in rpm. Unless dealing with some sort of variable timing unit, I don't suspect a timing issue is causing the flat spot problem at 5500 rpm.

At 5500 rpm, the tapered needle jet is ruling the roost, not the main jet.

Here are some steps to approach the problem.

Preliminary Considerations (air and fuel).
Assure K&N pods aren't dirty or over-oiled.
Assure starter plunger not stuck open.
Check service float bowl fuel level using clear plastic tube test.
Check service float bowl fuel level using clear plastic tube test. (Oh... Wait... Did I say that already? )
Ignore earlier measurements in the dry up-side-down method of setting float heights, as they are unreliable and likely wrong. (All viewers may now please remove that section from manual. )
If the above checks out okay, would next consider carb main system.

Carb Main System (main air jet and air passageway to bleed pipe).

Main jet (already downsized) only meters raw fuel allowed into bleed pipe, and has no bearing on air getting into the bleed pipe.
As reported, the blocked side holes in the bleed pipe have been cleaned, so they are presumably okay.

But air must get to the bleed pipe from the air passageway, which begins at the main air jet (see small entrance hole on the airbox side of the carb) and continues along a passageway until reaching the bleed pipe (more specifically, the side or lateral holes in the bleed pipe) where the air combines with the raw fuel admitted by the main jet, whereby the fuel mixture is presented at the needle jet.

So would double-check the main air jet to assure it being open, clean and clear. And also assure the air passageway from the main air jet to the bleed pipe is clean and clear (would blast away with spray carb cleaner and compressed air throughout the passageway -- repeatedly}.

More Remote Possibilities (jet needle or needle jet).

At the needle jet atop the bleed pipe, the tapered jet needle moves up and down inside the needle jet thereby governing the amount of fuel mixture available to combine with the air being drawn through the main bore as allowed by the throttle slide, and sucked through the intake valve into the combustion chamber.

Although many different jet needle designs are available (with various taper characteristics), usually the oem needle is best. Also, sometimes a change in the jet needle clip position (whereby the needle is held inside the throttle slide) may be used to alter the amount of fuel mixture allowed relative to throttle slide position. Usually the oem position is best (often the middle position). In the present situation, I would be sorely tempted to relocate the clip position one notch higher, so as to lower the needle inside the throttle slide, and thereby run leaner at the present "flat spot" and then test run again, and do another plug reading with the different "leaner" clip position for comparison with the earlier sooty plug reading.

It is conceivable that the jet needle or needle jet is worn from the contant interaction between them, which would allow too much of the "correct" mixture. IMO, reclipping the jet needle to a higher notch, thereby lowering the needle to lean the mixture reaching the combustion chamber, is an unsatisfactory remedy when used in an effort to compensate for worn parts.

In Short.
Fuel level (clear plastic tube test).
Main air jet and air passageway to bleeder pipe.
Clip position on needle jet.

Good Luck!

Thanks again for the suggestions, it really sounds like partially blocked main air jets from your description, so that'll be my next job after checking float heights with clear tube

I cleaned and re-oiled the K&N pods corectly (just enough spray oil to colour them) with the K&N cleaner and oil kit, so I can rule them out.

Starter plungers appear to be working as the choke helps it start, and it stalls when choke is turned off when cold. Also stalls when choke turned on when engine is hot.

I did check the timing with a strobe, and it was very close (about 1 or 2 degrees to the F mark at idle. It did advance way before 5K revs, so I agree with you advice that it's unlikely to be the timing. It could still be the unit breaking down at 5K, but I'll concentrate on the carb ideas first.

Before I got the bike it had been stood with stale fuel flooding the carbs (leaky fuel tap and float valves, both of which I replaced and are fine now. The flooding stale fuel could have blocked the air jets as it trickled through and evapourated maybe?

Nothing to do with the above problem, but I also dismantled and cleaned the starter motor at the weekend and fitted new brushes and springs and power leads. Starter turns fine now, but horrible noises from the starter clutch which only turns the engine intermittently. Another job to do!!

Took the carbs apart again and sprayed / air-lined the main air jets and passageway. Didn't seem to be blocked, but you never know.

Checked the needles and they're all 4th groove down from top which I've left them at.
Needles have 5CN15 marked on them which is different to what the manual says. Does that mean anything to anyone?

Did the float height check with the clear tube and all were spot on, about 4mm below the gasket surface.

Did notice that the choke lever is loose on the shaft. looks like the plungers were not fully seated with the choke lever fully down, and not fully open with the choke lever fully up. I temporarily removed the friction screw so that I get full range of movement now. Maybe the choke staying slightly on is causing the richness?

Can't test ride it until the wekend due to work.
Will report back then.

Test rode it today and not much improvement after cleaning the main air jets.

Re-checked the ignition timing with a strobe and found the problem at last!!!!

Timing was too retarded as the ignition plate of the Boyer Bransden unit had vibrated round. Can't have done the screws up when I installed it.

Starts OK on the kick start now, and the top end is fine holding full throttle without fouling the plugs.

Still a flat spot at 5K, but nothing to worry about.

Very happy with it now