Puzzling gremlins, i feel like i've checked everything!

Hello again! Lengthy question here, trying to be thorough:
I've got a slight puzzle that i'm hoping you guys might be able to figure out!

I've spent the last 9 months or so getting my 82 XJ750 Maxim all set and roadworthy. I've run into many problems, but fixed them all with enough effort.

Now my bike is running well, except for two running problems that i cant figure out for the life of me:
Problem One: She wont start when its not warm out, the cutoff seems to be around 65 degrees! (not very cold!) When this happens, she'll sputter a little, (maybe 1-5 actual combustions) then she just cranks and cranks, without a hint of igniting the mixture. Choke doesn't make a difference. I've tried starting fluid, it made her sputter maybe a little but more, but not much at all. I've tried fuel drier additive incase of wet gas (tried new gas too), no different. The spark is healthy light blue on all plugs, all the time (i guess the spark could be a little thicker, but its not bad at all). The battery is new, charged, and strong. There seems to be plenty of fuel in the carbs as per the drain screws. Also, i feel the mixture is fine, since she runs very well when she actually starts up! On a warm day, you cant tell anything i s wrong, she'll start up instantly one turn of the starter and run great! I'm boggled by this.

Problem Two: Idle adjustment. If i set the idle turn knob to let her idle at around 1100rpm, she idles very smoothly and consistently (the knob isnt turned in a huge amount, either). Problem comes when i rev her a little, she'll stay high around 4000+ rpm until i pull out the idle screw. Then as soon as the rpm's lower a little, she'll die if i dont turn the idle screw back in or hold the throttle open a little. Thus there's no idle screw setting that works for both ranges.

Problems stated, here's everything i've already done:
- Full carb cleaning, twice, those things are spotless, fuel filter installed.
- Valve Clearances: Adjusted intake #1 quite bit, now all are on the bottom end of spec.
- Sync: Bench, then homemade manometer without YICS tool. The sync helped her to run very smoothly and accelerate well with full power through the range, but didnt help the above two problems.
- Idle mixture screws 2-3/4 turns out. (i'm operating at 5300ft elevation if it really matters)
Other than that just standard maintenance: all oils and filters replaced, etc.

I know i'm close since she run so damn well when not idling, and when she actually starts, but theres gotta be just one thing off that i'm missing. Any ideas? Thanks!

 

don't give it any throttle at all while starting it or for the first 20 seconds, just choke

 

You got a few issues to deal with:

You need to insure that the Starter Jets are clean, clear and free flowing.
This included the Brass Tube that extends into the Starting Circuit Well at the front of the Fuel Bowl.

The Meterind Ports in nthe bottom of the Fuel Bowl need to be clean and clear allowing Fuel to enter and fill the Starting Circuit Well to be picked-up by the Brass Siphon Tubes.

That you don't even get a pop out of her indicates that the Pilot Mixture Screws are in need of being adjusted to provide Idle Mixture to accompany the Enrichment Valves.

I think its all Carb work since you already know you have a strong spark.

 

I had the same problem with the idle not going down after reving it up.

My main jet was too large.. after the idle goes above 3.5k the main jet is use (at lest on my bike). If the main gives too much fuel the idle will not use only the pilot jet and will not go below 3.5k. Still working on mine, I went down to 112 jet (the original was 110), the RPMs are smooth up and down. But like I said "still working on it".

GOOD LUCK

 

definetly sounds like a pilot jet/circuit problem. there are two sets of jet the mains and then a little one take it right out of there and replace them or soak them for at least 30 min is carb cleaner then blow through them with compressed air. Have you done a compression test at all on the bike? as with low comp they are a PITA to start but het the rings a little warm and expanded and it can run ok but wouldn't have the performance that it should if correct.

 

I agree with the masses here.

But I still have a few ?'s
Are the plugs wet when you are trying for a cold weather start?

Heck, maybe the bike just knows it's too cold for your own good!?
(Ok just shooting for a yuck)

 

Thanks for the responses.
Here's some answers:
Compression was tested on a warm day, and it was fine.
Every metal piece of the carbs, jets included were soaked in carb cleaner for at least 30 minutes and then blown out.
As for the plugs, they have been a little on the dark/sooty side after riding, and this hasn't changed when its cold and she wont start, they dont get wet at all.

I believe i have the stock jets installed, is there any reason i would need to change this? (i dont have any airflow modifications, stock air filter and pipes)

 

Dark and sooty Plugs would indicate either too Rich a Mixture or Oil being introduced into the burn.

I'd try setting back the Pilot Mixtures, first ... not too much if its trying to run or runs unevenly.
This is a job for Colortune.
I think you could smooth-out your problem just colortuning the Pilot Mixture.

But, trying it after looking at the Plugs is OK.
Just don't overdo the leaning-out of the Mixtures.
A little nudge IN is all you should need.

The Mixture is sensitive to adjustments about where you are living now.

 

Because of the hard start... I just have a sneaking hunch.
Try being sure that the tiny hole between the float bowl and the chamber to the brass pickup tube is open on all 4 (layman terms)
Also check the brass pickup tube to be sure it's not restricted on all 4 carbs.

Just a hunch!?

 

aight, i pulled the carbs again, and i'll going to thoroughly check all these starter circuit things...i'll report back soon!

 

Do you have compressed air?
If so you might blow everything out.

Again if the bowls had some bad varnish in them those little holes might be plugged between the bowls and the pickup tube chamber.

Also check all you can for any vacuum leaks like in the rubber manifolds, things like cracks or if you had them off and the gaskets werent the greatest,...

 

ok, First off, i've realized that the metal retainer plates that cover up the air jets are missing in all carbs!
What purpose do these serve, and could this be causing my problems?

Also, when looking at the starter jets, looking deep into that starter well i can see brass jet-like things, but no size is marked on them, should i assume they are the correct size since they dont appear to be removable? Also, the brass tube that dips into the starter well did not appear to be removable, and had no size on it, again, can i assume this is the correct size?

Otherwise, the little holes that let gas from the float bowl into the starter circuit well are plenty clear, and sprayed through again just in case. (sprayed carb cleaner in my damn eye! safety glasses from now on!)

And all the jets are sparkling clean and clear, i swear by it.

I'll leave the carbs off for now just in case. If those missing retainer plates aren't causing any problems, any other ideas?

 

Some models used those air deflector plates, some didn't, I don't think it matters. The only thing I can think of is that they "streamlined" the airflow some and prevented air turbulence down there, and at some point in time Hitachi said "not worth it" and stop using them (they appear to be more common on the earlier model carbs than later ones).

The starter jets do not have sizes marked on them.

And the brass siphon tube for the starter jets, no, it's not removable. There is a tiny "side vent" hole in that siphon tube, right before the tube enters the carbs body, make sure it's not plugged, either.

 

thanks for the info Chacal. I didnt notice a little hole on the siphon tubes, i'll go check that tomorrow to see if they're clear, i just put the bowls back on, and need to get some sleep!

As for the rubber, it all seems to be in good shape! no cracks, seems like it should seal well. As a side note, Back before i had the carbs off, i checked for vacuum leaks in the boots and vacuum ports by spraying starting fluid, nothing (they're in decent shape as well).

Another note about the enrichment circuit: when the bike is actually running, pulling open the choke/enrichment does make a huge difference to increase the RPM's, so i believe it IS enriching the mixture, although its possible that its not quite enough i guess.

 

Check and double check the float levels.

Did you soak the carbs in the old school carb dip?
If so the throttle plate sgaft seals could be trashed!?

Ok, also the idle mixture screws are supposed to have little o-rings and washers with springs in them. (if they got lost see Chacal)
Making a small hook in the end of a small paperclip can help to get at them.

Really I'm grasping here! But it really sounds like some air intrusion.

I have an XJ700N at 5500 ft elev and I dont have those problems....

There is one other thing that I learned at this site, you really should try using a Gunsons Colortune for dialing in the mixture screws!
I like it almost as good as reading spark plugs!

Hey Good Luck there!
dont give up, it'll be the sweetest thing when it purrrrrs like a kitten!

 

Did some more checking, the pilot screws all have the washers, springs, and orings in them, and the orings looked ok. the cold start plungers seemed to have all the components the haynes manual shows, but it does seem like there could be room for air to get in that way. I may spray some starter fluid around there next time i have the bike running.

As for the throttle plate shaft seals, i dont remember taking off any seals near the butterfly valve components before dipping them, if thats where you're referring to. Where could i find these seals?
i checked the float levels once, but i'll check that again. Is it a bad idea to use water to do this, and then let everything dry out?

 

the little holes on the siphon tubes seem to be clear as well. The best i could do was to spray carb cleaner up in there, i cant SEE any blockage...

 

just a thought here. @ what elevation are you at and has this bike been running before at that elevation?

maybe cracked carb boots?

 

I'm at 5300ft.
The bike wasnt running when i bought it. The PO said it ran every now and then. The carb boots look to be in good condition, no cracks. Also, spraying starting fluid on them didnt cause a bit of difference in how she was running, so i dont think i'm getting any vacuum leaks there.

 

An update for anybody curious, with some help from a friend, i found the problem.

Turns out that cylinder 4 wasn't firing at idle due to being way too lean. I never noticed this because i was letting it idle with the screw turned in a bit far. Soon as you pull that idle screw back out though, you can feel the pipes, and #4 clearly isnt igniting. If you open up the throttle just a tiny bit, number 4 heats right up and runs well, making this tricky to diagnose! I tuned my idle mixture screws by ear on each cylinder, adjusting until the bike idled best. I was lean on all four cylinders, and pulling each screw out a little bit completely solved my problem! Now she'll idle with the turn-knob out at a reasonable setting, and revs and drops perfectly!

It is a bit strange that she was running so lean, considering i'm at altitude, and you'd think that if anything she'd be rich. I've done extensive vacuum leak checking, and there are more than a few places on the carb body that are leaking very slightly, but none of them are major. My guess is that all of these small leaks are adding up enough to lean out the mixture at idle, but not significant enough to lean things too much with the throttle open.

Anyways, now, (finally!!), i can say that 'Ol Thunder is running picture perfect! Thanks everyone for your help, i could not have gotten this bike running well without all your generous advice!

 

I know you've checked it, but it sounds like it's sucking air.

 

How do the carb boots look? cracked?

 

nah, the carb boots are fine. (i even replaced the gaskets between them and engine to be sure) Definitely NO LEAKS near the carb boots.

There are a few main leaks that i can find. They are on each side of that butterfly valve shaft, on the outside of the carbs, and also one of the pilot mixture screws seems to leak a little. These leaks take quite a bit of starter fluid before the engine speed increases, so they aren't terrible. These are, after all, carburators. I tried sealing up these areas with duct tape, and then respraying the starter fluid, now there appears to not be any leaks.

Shes definitely sucking air, but i don't think theres much i could do to fix that other than buying new carbs completely. Something tells me that going through the trouble of replacing those shaft seals would just create a new problem somewhere else. So i'm not going to bother. 'ol thunder really does run great now...its odd that the mixture screws had to come out 2 turns, but you wouldn't even know theres anything wrong as she is now.
I've got my fingers crossed for 'ol thunder to have a long healthy life with normal maintenance, next project: wheel bearings!