Fiddling the Mikunis on the Kat 750

[KiwiAlfa156]

I did the 1,000 Km (600 mi) oil change a week or so ago on the resurrected Kat 750 which got new rings as a part of the rebuild. I was keen to seat these properly and treated it like fresh engine with varied engine speed, no lugging, moderate revs with the occasional spin up the rev range. I happily dumped the special Penrite 15W40 Mineral Running-In Oil and filter, and poured in some fresh synthetic 10W40.

Being in running-in mode I hadn't really exercised the bike. So with the initial running-in done, I began riding it yesterday with a bit more throttle, it felt disappointingly flat from about ½ - ¾ throttle but at WFO it seemed to crispen up for the brief moment I held it there.



My highly sensitive -but not always correct- ass-dyno was telling me it was lean on the needles. So last night I popped the caps off the Mikunis to raise the needles and low and behold, the PO had dropped the needles one clip (second from the top) making it leaner mid throttle onwards -a bit of a head-scratcher. He/she must have misread the memo about raising/lowering the needles and rich/lean. I didn't check when I rebuilt the carbs, so note to self to check next time. I've returned the clips to the standard middle position on the needles. This configuration takes back the jetting to stock, with 112.5 Mains and 42.5 Pilots.



Managed to a quick spin and what a difference to yesterday. Off idle roll on greatly improved and much more lively at ½ throttle. Haven't taken on the highway to compare response at 100 kph+.

The bike came into my possession without a air-filter and box, but I like to run these bikes with a large pod filter on the plenum box anyhow (as an alternative to individual pods of which I'm not a fan). Preferable with an oiled foam Uni-filter -as I believe these offer the best filtration for performance (motocross bikes use them) or good oiled cotton gauze unit which is what is on the Kat at the moment. My experience oiled cotton in both bikes and cars is that they flow more air by filtering less and they also clog quickly and need frequent cleaning - so aren't as good as foam...

My theory for replacing the stock air-filter box with an open pod is that the small inlet to the filter box is most likely the restriction at WFO. So replacing this assembly with a large single pod is that keeping the front plenum box preserves the inlet velocity stack "trumpets" feeding air from the consistent 'ambient' air conditions inside the plenum box, and whatever resonance is possibly captured by the volume of the box. To my mind, this most likely provides greater volumetrically efficient air flow across most of the rev range than individual pods do.

It always struck me as strange that folk would spend coin of tuned exhaust systems that harnessed the pulse waves in the exhaust tract usually by combining them in a four into one, or four into two into one system to pick up a few HPs, and then remove the tuned components of the intake system. Granted, the inlet system of a GS/GSX is primitive compared to todays bikes however the fluid dynamics of air entering a pipe though a correctly radiused flared end and the significant increase in air-flow it yields is a measurable scientific fact.


So. From the stock jetting I will attempt to jet the 750 correctly for the 4 into 1 and single pod, old school, doing clandestine full throttle runs and plug chops (wish me luck). My 1100 running a 4 into 1, VM32SS, plenum and single pod required going up 2 sizes on the mains and the needle up a clip (but it might be a little rich in the mid-range). Which make sense as the air box is the main restriction at WFO at anything less the throttle butterfly is.

I intend to post pics of the plug chops to chronicle the process.

[KiwiAlfa156]

Took her out for an extended test run which included some throttle ½ - WFO motorway riding to try to get a better sense of where I'm at. I have ordered 117.5 mains jets as a starting point for plug chop testing. Being up two sizes which has worked for me before. I do have a set of 120 mains if I need to go richer. I need to get the main sorted before looking at the needles. Raising the needles back to the standard position has definitely crispened up ¼ to over ½ throttle but at around ¾ throttle at 6000 rpm she feels a little breathless. I don't know if this is a four into one induced flat spot, but possibly richer mains will impact this. We shall see.

Raising the needles has also introduced a slight off idle hesitation. I did set the pilot screw using a color tune with the needles unknowingly clipped lean. The manuals say that the pilot circuit is the boss up to about 1/8 throttle, but if you've ever had worn needle jets you know that the needle jet adds fuel from the moment the slide begins to rise. Worn needle jets cause bad running from a just cracked throttle to about 1/8 that is impossible to tune out. I've screwed the pilot fuel screws in to lean the idle mix out a tad and will see what effect that has on the hesitation.

I pulled the plugs for the first time since the first start and they have just over 1000 km (600 mi). This is after a good ride and minimal idle before hitting the kill switch. So not a plug chop. But I wanted to see the starting plug colour. The phone camera hasn't really captured the colours accurately. The porcelain is a very light tan for what's worth. More importantly the strap electrode is tan with the colour change mark about 1/3 along indicating correct heat range for the plugs. The outer ring is chocolate brown which is correct to slightly lean ... More fiddling to come.



Reference numbers lightly stamped on the plug hex


No oil and good colour.

[KiwiAlfa156]

The mixture screws ¼ turn in to lean out the pilot circuits fixed the off-idle glitch caused by raising the needles. Riding around town is a pleasure. Really good low to mid-range 3,000 - 5,000 rpm. Very responsive. Definitely exhibiting lean surging approaching 8,000 - 9,000 rpm WFO on the motorway on ramp test facility. Jet swap ahead.

[KiwiAlfa156]

Update. Pulled carbs and swapped out the 112.5 mains for a set of 120 I had in my box of jets. So 3 sizes richer on the main. Did a test ride and a couple of things became apparent. Just cracked throttle carburation (or running on the pilot by-pass) had deteriorated (which was unexpected) and while lower mid-range power was really good and clean above this, winding the throttle on past half at around 6K rpm in 4th, produced a rhythmic "blub-blub-blub-blub" so it was rich under these conditions which was always a possibility. Also it wouldn't rev out anywhere close to red line. So not much use in doing a plug chop for the main, as it ran better on the smaller main jets.

I did do a plug chop for the bad 'just cracked throttle' behaviour. Riding the bike for about 400m in the jerky, glitchy, rich feeling zone up the slope before my driveway. Hitting kill switch, pulling in the clutch and coasting down the driveway and into the garage.
These are the results.



All rich with 3 and 4 being the worst. I had tweaked them leaner when I raised the needles back to the middle clip. What surprised me was that the change of main jet had made a difference. The pilot system does draw fuel through the main jet, but I would have thought that the flow rate would be so low and that metering was done at the pilot jet, that going bigger or smaller would have a negligible effect. Wrong.

All the fuel screws were between 1¾ and 1⅞ from being seated. I took them all back to 1½ and did another 5 km ride during which she perform much better down low, and at the end repeated the plug chop as the bad behaviour was still noticeable. To be honest the plugs looked a little worse, although I had ridden in the bad carburation zone for longer.



Readjusted the fuel screws to 1⅛ on carbs 1 and 2, and 1 turn out 3 and ⅞ on 4. Test ride proved these settings were much better. Didn't plug chop as I need to get the mains and needle sorted before fine tuning the pilots. This is just to remove the just cracked throttle hunting. So now waiting for the 117.5s to arrive and wondering if 115s might be required.

[KiwiAlfa156]

OK, decided to do a full throttle top gear plug chop at the local 'testing facility' with the 120 mains fitted to see what was going on and confirm that they were over-fuelling. Pulled the plugs on the side of the road. Was initially going to only pull only 1 and 4 because there's less chance of burning my delicate hands and there is the assumption that main mix will be the same on all cylinders. But once I saw there was a difference in colour, I pulled all four. Here they are. The bike wouldn't rev out rolling the throttle on to WFO and holding it for about 1 km.




Cylinder 1, 2 and 3 are looking close to each other in colour, but 4 is really sooty. I will have to check number 4 carb when I next pull the carbs to drop in the 117.5s These plugs are all a little rich by the way. The classic diagnostic photos of the sooty rich plug thru perfect tan/gray thru to glazed white porcelain lean are relics from the glory days of tetraethyl lead, and the other additives that left these colourful deposits. Modern unleaded fuel contains fuel system cleaners (usually light aromatics/hydrocarbons) and contains oxygenates that promote cleaner burning. The porcelain isn't the gross indicator of AF ratio it once was. AFR is observed but as the colour on the ring that the ground strap electrode is welded to. It should be brown and a continuous circle around the base of the plug. There is colour ring that also forms on different regions the porcelain at different rpm/loads but this is deep within the plug and is more useful for race tuning.

The bigger question for me is what slide position have the plugs captured? As the engine wasn't revving close to redline (just as well), the butterflies would have been WFO but as the airflow created by the low rpm wasn't maximal, and its likely that not enough vacuum was created in the venturis to lift the slides all the way up, so the bike would not have been running on the main jets.

My next attempt will be in a lower gear to get it close to redline. And possibly fresh or cleaned plugs. I'll post those results

[KiwiAlfa156]

Managed to do another chop. This time in 3rd gear up a slight incline with the revs up there. I don't doubt the slides were up and we were running on the main jets. Results below



Rich confirmed.

[KiwiAlfa156]

So, 115 mains turned up, the 117.5 haven't, but the 120s look too rich. So I decided to swap them out for the 115s which are 1 step up from standard. My concern was why number 4 was so sooty. Dropping the float bowls provided the answer... the main jet and washer in number 4 was lying in the bottom of the bowl. I'd been a little careless/limp wristed tightening in the main jets... the others were a little loose too. Embarrassing.

Anyway, the 115s went in tight, and a quick trip to the test facility. Results were disappointing, but have given me more information to go on.



These are the results of the WFO plug chop.... now number 1 is rich and the others look ok-ish. The richness jumped from cylinder 4 to cylinder 1? What the...?

There was a real hesitation to rev through to 9000-ish rpm and the acceleration wasn't exactly startling. There was rich blubbering occurring after about half throttle. Curious.

So I held the bike in the blubber zone for a quarter mile or so and did the chop. These are the results.



Number 1 is carbon fouling really badly. So after some head-scratching on the side of the road, I realised that this isn't probably related to the main jet as I was riding on the needles at the time of the plug chop. So it could only be the choke circuit or the needle/needle jet. The bike runs good from idle to half throttle so unlikely to be the choke (and confirms the pilot circuit is good). So the likely culprit for the extra fuel is likely to be the needle jet. But why, or more correctly under what conditions would this flow extra fuel?

It dawned on me that if the slide wasn't opening enough, the air speed under the slide would be increased and so would the vacuum in the venturi. And that would draw more fuel through past the needle... resulting in an overly rich mixture on that cylinder with reduced air flow volume and crappy, blubbering acceleration.

I got back to the workshop and pulled the carbs, took out the No.1 slide, dropped the bowl, pulled main and needle jet, checked for obstruction with compressed air and aerosol carb cleaner both the fuel and air bleed circuits. Inspected the diaphragm and there was small signs of damage on the very edge of skirt. A pinhole or two of light was visible through the damage. I had noticed this damage - a slight flaking of the diaphragm material- when I raised the needles, but didn't see any pin holes at that stage. During the WFO plug chop with the 120s, I did experience a sudden glitch and a perceived drop in performance. This may have been the diaphragm holing and dropping the slide a bit at WFO.

Not having ever dealt with a damaged diaphragm, I don't know whether this tiny hole are enough to upset the height of the slide, but I'm inclined to think it does. The diaphragm will have to be replaced, but I smeared a tiny amount of sealer on the damage and will test to see if this makes a difference.

I guess it was a reminder to not assume causality for a set of symptoms. There might be multiple things going on.

[KiwiAlfa156]

Well did some more testing to either prove or disprove my hypothesis on the diaphragm. I've ordered a new one as it has to be replaced.

I swapped the slide/diaphragm between cylinder 1 and 4 carb, if it was the issue, the richness should shift with the slide. It didn't... .

Put a fresh plug into number 1 and did a plug chop in the 'blubber zone' at 6000 rpm and ¾ throttle. Came out sooty.

Will swap the coils to rule ignition out. But it's not a misfire and I'd expect it to with reek of gas or be wet if the ignition was cutting out at that rpm.

Gonna pull the carbs out and to a full strip, clean and reassembly. It's getting extra fuel from somewhere. More trouble shooting to come...

[KiwiAlfa156]

Thought I'd swap the plug leads between 1 and 4 to see whether the problem travelled with it. Then I thought I'd start it with lead 4 disconnected to see if that made a difference. This was the result.

https://youtube.com/shorts/04nz3nAFnbg?si=ZFMsNaq5fvqNC0Yj

Guess I found the culprit.

[slayer61]

That's an excellent find right there! Very well done Sir!

Keep it up, I've been following this thread with interest.

[KiwiAlfa156]

Thanks Slayer61. I discounted the coil itself in the beginning because I assumed that if it were the coil the problem would be on two cylinders being wasted spark. So lesson learnt.

So what I think is happening is that the coil has broken down internally under heavy load and dropped number 1 cylinder, which was the sudden loss of power, when this drama began. In this state the coil works more or less at lower rpm when the saturation and discharge rate is lower and the resistance seen at the plug is lower too. As I open the throttle the rpm rises and the cylinder pressure under compression rises to a point where resistance at the plug gap causes the external arching to occur and the ignition on cylinder 1 to become extremely poor. As the fuel is being sporadically ignited it's running rich.

[KiwiAlfa156]

Diaphragm. The fibre in the material is still intact, but the sealing layers are giving way

[KEITH KRAUSE]

I don't own your model and I do appreciate the detailed work and trial and error you're sharing.
The leak at the coil and the compromised rubber parts are basic maintenance things, meaning you'll waste time if these things aren't checked first. Carburetion tuning always comes last after everything else is certain.
Just to mention, you said "the pilot circuit does draw fuel through the main jet". That's not correct. Your carbs aren't designed that way. I also question why the plugs vary so much in color, cylinder to cylinder. Did the bike pass a compression test? Did you carefully set the float levels? Any chance you mixed up the float valves and their seats while cleaning? They wear as a unit and mis-matching them will cause issues only new replacement will cure. Did you carefully vacuum synch the carbs?
One other thing, only my opinion, changing to full synthetic at 600 miles is too soon. Change at 600 yes, but conventional oil is best for full break in. Run an additional 2,000 miles minimum.

[JMHJ]

I don't know nearly as much as I'd like to about all aspects of the ICE, but I'm scratching my head a bit on this part:

As I open the throttle the rpm rises and the cylinder pressure under compression rises to a point where resistance at the plug gap causes the external arching to occur

Cylinder pressure and resistance of the air-fuel mixture at the plug gap continue to rise with RPM? Isn't the cylinder relieved every time the exhaust stroke happens?

I'm crazy enough to occasionally think I'd like to mess with a bike project. The troubleshooting and all that is satisfying when you can find and fix the issues.

[KiwiAlfa156]

I don't know nearly as much as I'd like to about all aspects of the ICE, but I'm scratching my head a bit on this part:



Cylinder pressure and resistance of the air-fuel mixture at the plug gap continue to rise with RPM? Isn't the cylinder relieved every time the exhaust stroke happens?.

Hi there, I'm referring only to the compression stroke and the cylinder pressure at the moment the spark plug fires. The rest of the time the coil isn't generating high voltage.

Compression pressure in a running engine isn't constant from idle to redline. It might read 180psi at the speed of the starter motor on a compression gauge, but when it's running, factors such as valve duration, lift and over lap, tuned length of the inlet tract and pulse tuning of the exhaust all dynamically effect the pressure in the cylinder at the time the spark plug fires in the cylinder under compression

At a certain rpm range, all these design factors combine to trap the maximum amount of fuel air charge in the cylinder and squeeze it. It is the rpm where the engines is at its most efficient volumetrically, and maximum torque is produced. On the 750 max torque, according to the factory, is at 8,500 rpm. It is also where the highest compression pressures/fuel air density occur prior to ignition. This high pressure and fuel air density in between the centre electrode and ground strap increases the resistance across the spark plug gap and the voltage required to jump the gap. This is why the faulty coil is likely to fail at higher rpm as the electrical load on the coil is at its greatest.