Heads up to all with idle headaches on old 4-inline mikunis

Hi everyone

I am new here, but have followed this site quite intensely, as I bought an old XJ750 YICS from 84 (not the SECA, but model number 41Y), with carburetor problems.

I kid you not, I dreamt this solution, that I have not been able to find on the net, nor has anyone told me.

You adjust the mix after the idle, and not the idle after the mix!!!
I have read long and hard on various sites, but this is my simple conclusion.

Explanation:
My bike has been having trouble idling. Slowly dying out, or running to high. Different sites claim, that the mix should be set approx. 2 1/2 out. But this is not so!
I ended up setting my 4 mix screws 4 1/2 out, and finally it purrs like a kitten!!! This carburetor has been fully overhauled, with new intakes, and what not.

So the lesson is: if you have idle problems, forget what you have read so far, apart from this off course!!:

Set your mix screws to approx. 3. Take you bike for a ride. When it is nice and hot, pull over and let her run in neutral. As soon as she starts to die on you, turn up the idle screw, until she runs happily at 1000 rpm. idle. But! Now, when you try to ride her home, idle will rise to approx 3.000 rpm, or maybe more. DONT TOUCH THE IDLE SCREW!!!! Instead, sneak back home, lift of the tank, and quickly turn the 4 mix screws at least 1 full circle out (counter clockwise), thus making the mix richer, and automatically dropping the idle! After a bit of fine tuning of mix and idle screw, you should be the happy owner of an old ride with sweet idle and richer mix. Lean mixtures kill old engines anyway...

Regards from Denmark, and keep up the good work everyone. This site is the greatest!

 

Actually, it goes like this: In order with nothing left out:

VALVES IN SPEC.

Carbs religously clean, slides go "clunk."

Float levels set using fuel and clear tube gauge.

Carbs bench sync'ed.

Back on bike, carbs VACUUM SYNC'ED.

THEN we mess with mixtures and idle settings.

4 1/2 turns is excessive; it sounds like you need a good carb cleaning.

Just cranking the pilot mixtures richer is not a solution to anything, just a stopgap measure for a bike with still-dirty carbs or other (sync? vacuum leak?) issues; not necessarily sound advice to solve an issue.

The bike should idle at 1050rpm.

 

in addition to what Fitz said, you may have pilot jets too small to need 4 1/2 turns out.

I've done my bike and three others none were more than 3 or just a hair more.

Also the 750's have hitachi carbs unless you have an X...

* the Hitachi HSC-32 and HSC-33 series carbs used on the XJ650 (except Turbo), XJ700 non-X (air-cooled) models, and all XJ750 models (except "X" models), and.....

* the Mikuni BSxx series of carbs used on XJ550, XJ650 Turbo, XJ700-X, XJ750-X, XJ900RK, and XJ1100 models.

 

How about bad throttle shaft seals just like having a vac. leak. If the carbs are clean thats where i would be heading next. They won't stay in sync idle hanging then slowly cutting out all signs of the seals

 

Re: Heads up to all with idle headaches on old 4-inline miku

Hi fellow XJ'ers!

Thank you very much for your replies.
To answer one at a time:

1. The carbs/jets are religously clean and slides go clunk. No air leaks anywhere. Sync is good. Bigfitz52: I would love to have the kind of toolbox you are refering to, but I don't, and neither does anyone I know. And even considering asking a shop to undertake what you described. Well, lets just say, that the shops I know of are about making a profit, not fiddling with old bikes, and even if they did, I wouldn't be able to pay the bill... :/ But I am banking that less will be able to do it as well. Thats what I understand, anyway.

2. I'm embarrassed to admit, I dont know whether its Mikuni or Hitachi carbs. I just took what someone told me for granted. I'll check it in the morning. I know that frame and engine state the same serial number. Which leads me to:

3. Pilot jets too small? I have no idea. Translating from American to Danish, is a pilot jet the same as the idle jet? How do I know which jet sizes are right for my 1984 xj750 (model number 41Y)?
I can only guess at how many, that have worked on the bike. Wiring says 31A (XJ900).

Anyway, the big thing for me, is that, for some reason, and I have no idea why, it worked for me knowing that all the above was in order, to start with idle and then adjusting the mix accordingly, and from what I have read here and there, others out there have the same problem I had, I just never heard it solved in this manner.

I hope to learn more from you guys, or dolls, I cannot tell...

Best regards
Dan

And nothing wrong with the throttle shaft seals by the way...

 

Re: Heads up to all with idle headaches on old 4-inline miku

Years & Models: 1984 XJ750RL Seca models
Carb Manufacturer: Hitachi
Carb Series: HSC33
Carb Model ID: 41Y00
Main FUEL Jet Size: #106
Pilot FUEL Jet Size: #40 or #41 (unsure which is correct)
Main AIR Jet Size: #70
Pilot AIR Jet Size: #225
AIR COMPENSATOR Jet Size: not used.
Starter FUEL Jet Size: #43 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-18
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 1mm +/- 1mm (.039" +/- .039")
Idle RPM's: 1,050 rpms

Although not part of your carbs, your valve clearances should be considered as part of your "intake system", so here are those specs, too. Note that valve clearances should be measured with the engine "cold", meaning 70-degrees Fahrenheit or less:

Valve Clearance Intake: 0.11 - 0.15mm ( = 0.004 - 0.006")
Valve Clearance Exhaust: 0.16 - 0.20mm (= 0.006 - 0.008")

 

^- there's your answers as to what your carbs should have.

 

Re: Heads up to all with idle headaches on old 4-inline miku

Now that is what I call fast!

Thank you both. But I don't see any mention of an idle jet?... Have I been misinformed?

 

Maybe a terminology mix up. Here it's called the pilot jet but that's your idle circuit.

 

Re: Heads up to all with idle headaches on old 4-inline miku

Thank you Iwingameover. Though I beg to differ. Iwinsometimestoo...

 

I'd be interested to know how you determined the state of the throttle shaft seals as "nothing wrong". The only reliable way I know is to spray some engine start onto them while the engine is idling and see if the revs pick up, did you do this? If they are leaking they will suck air and upset your mixtures. How about binding in the linkages or misalignment of the rack? It all adds up. My XJ750 has the same 33mm Hitachi's.

 

Hi iandmac

Yup. I sprayed them, the carbs, the air intakes, everything. No leaks.

Anyway. Took a long ride today, and the engine runs fantastic. Started with a minimum of choke. Idling between shifts. Idling at intersections. Steady idle around 1000 rpm. Pulls strong and clean up througth the register. Running cooler! Its practically a new engine for me. Love it!

I'll check the sparks in a couple of days, and see if the mix is too rich. But I don't think so, and at the moment I don't really care. The bike runs better than ever.

Have you posted any pics of your bike? Sounds like it might be the same bike as mine. 750 YICS in a XJ900 frame and build.

How is your idle/engine behaving? What are your mix settings?

Regards

 

Re: Heads up to all with idle headaches on old 4-inline miku

Love your avatar by the way!

But it doesn't look like an xj750...

 

Re: Heads up to all with idle headaches on old 4-inline miku

I don't know what you're referring to; all I was talking about was the use of a hunk of clear tubing to use to gauge fuel height in the float bowls; and all it takes to check your valve clearances is a set of feeler gauges.

A manometer for vacuum sync can be made from a long piece of the same clear tubing, a yardstick, and a few CCs of automatic transmission fluid; or a pair of plastic baby bottles (and the same clear tubing.)

No super-sophisticated toolbox necessary; but it does take actually doing it.

The steps I outlined are what it takes to actually remedy any issues you're having with your carbs, be they Mikuni or Hitachi. Yes you may find a "solution" requiring less effort, but the results won't be long-lasting and you will be forced to keep fiddling with it. Or you can do it right and be done with it.

 

No worries bigfitz52.

I appreciate your by the book approach. I just don't have the knowhow or the tools for that matter.

You may be right, and you may be wrong?... Bottom line, my bike is running excellent now. The only logical explanation I can think of, is what iwingameover wrote/suggested, that the pilot jets installed are too small. Could this explain the fact, that moving the entire idle range up via the idle screw, and then turning the mix up also, actually worked for me?

I know that I should be pulling the carbs off again, to check the jets for size, but I can't be bothered now that the bike finally runs like it should...

I know that what I am claiming doesn't make much sense from a mechanics point of view, but I'm not a mechanic. I tried all the solutions mentioned in these forums. And stumbled in a dreamy haze upon the solution mentioned.

As I have written elsewhere, the bike runs better than ever. End of story. What others claim can and can't be done, life has proven may be shortsighted.

Regards