collapse

www.dimecitycycles.com

https://www.townmoto.com/collections/vanson-leathers


www.restocycle.com

www.CITYLIMITMOTO.com

www.jadusmotorcycleparts.com

www.bisonmotorsports.com

www.speedmotoco.com

www.lostapostlejewelry.com

www.cognitomoto.com

www.sparckmoto.com

www.Moto-Madness.com

www.pistonsociety.com

www.steeltowngarage.com


Author Topic: Help! Yellow Honda ST90 "Big Bird" Idling problem  (Read 426 times)

Offline thrillseeker

  • Posts: 361
Help! Yellow Honda ST90 "Big Bird" Idling problem
« on: Aug 03, 2018, 23:40:26 »
Hey all. I'm really hoping someone here has experience with the little bikes (70s, 90s, etc). I'm having some trouble getting my big bore & cammed ST90 to run right and I really want to get it running well so I can go riding with my wife, who just got a new Ruckus. I decided to post this because I couldnt find anything online about how to tune after getting the big bore kit and cam for these bikes so hopefully this will help someone else too. I emailed dratv but he said just a larger 75 main jet and "carb tuning" should be all thats needed.

Back story:
I bought an ST90 about a year ago with a motor that needed a top-end rebuild. Got that done along with a big bore setup all from Dr. ATV. Also got their performance cam which only extends duration, not lift. I got the bike all together and its been running fine for a while. It started having some issues after riding for a while so I did some research and found that I had float set incorrectly (measuring from gasket mating surface).  I set float hight correctly, measuring from outer lip (bottom most point) to bottom of floats like the honda manual says. The bike was running better at higher RPM before but, now that float hight is ~5mm lower, I have no full-throttle power so I've ordered a larger main jet to remedy that. But the idle I just can't figure out. It seemed to idle fine before but I noticed I had the mixture screw way out (like 3 turns). Whats happening now is RPMs hang like crazy if I blip the throttle and I cant get it to a steady "beat" while idling, it ends up stalling after about 30 seconds or so and idles very "lumpy". not like big cam lumpy but like misfire hell lumpy.

what I've done:
  • new ignition system (battery, coil, plug, points)
  • de-rusted and sealed gas tank
  • new fuel lines
  • cam chain tension set.
  • tappets set to spec
  • timing set dead on
  • rebuilt the carb with all new rubber bits, except throttle boot.
  • Float height set to 21mm (correctly this time).
  • resurfaced the deck of the float bowl because it was bowed a bit (2 bolt float bowl)
  • set idle mixture screw to 1 5/8 out
  • set idle speed screw to a comfortable idle speed (ended up being about 2.5 turns out)
  • tested for vacuum leak. sprayed all possible points of air leak with starter fluid and never got change in rpm
  • if I apply the choke about 2/3 the revs do climb a noticeable amount so, between that and the revs hanging, I assume its running lean.
  • If I unscrew the mixture screw beyond 3 turns it doesn't really have too much effect but, I assume thats because its not changing any more beyond 3 turns out. If I turn it out more than the recommended 1-5/8 it seems to rev slightly higher but doesnt fix the hanging issue

What I'm thinking
  • I think this is definitely carb related because everything else is working great (timing, spark, compression, etc.)
  • It seems the engine is running lean, though the exhaust reeks of gasoline. not sure if the smell really tells me anything.
  • I'm wondering if I need a slightly larger slow jet to make up for the increase in displacement and cam duration. I read somewhere that if it seems like you need to be able to unscrew the idle mixture screw more than it allows, you might need a larger slow jet. does that sound right? I do have a #75 main jet on the way to upgrade from the #65 because when I finally got the float hight set correctly I noticed power was cutting out at full throttle. so it makes logical sense to me that the slow jet might need changed as well.
  • maybe need to double check the seal around the deck of the float bowl. I resurfaced that deck but it might not be perfect. I dont notice gas leaking from there though so I'm assuming it should be ok.

Are there any other things I should pay attention to?

Also, I searched and searched forever for an actual honda ST90 service manual for this thing and finally found someone with an old listing from 20 years ago saying they had some for sale. So, shot in the dark, I contacted him and he actually responded. Said he didn't have it anymore (I assumed this). so I asked if he had an electronic version and he did! so I bought a PDF from him. So if anyone needs a honda ST90 service manual, here you go!

https://drive.google.com/open?id=0B8Z7L-RmXREdX2txMlF3b1RUaUU
« Last Edit: Aug 05, 2018, 10:19:27 by thrillseeker »
Don't take life too seriously, you'll never get out alive.

Offline TheCoffeeGuy

  • Posts: 636
  • Books, Books, Books, knowledge is within
    • The Coffee Guy's Blog
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #1 on: Aug 04, 2018, 08:33:26 »
Idle screw only 1 1/4 at most
www.thecoffeeguy.ca Not your ordinary Blog site.

1968 Harley Aermacchi ML 125 Rapido, restored
1972 Honda Cl 450 engine rebuild, a day rider soon, now sold
1980 Suzuki GS850GL Street Brat  My other day bike
2000 Harley 1200 Sport S My day bike
1982 Honda CB650 Chopper light

Offline thrillseeker

  • Posts: 361
Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #2 on: Aug 04, 2018, 08:52:13 »
Idle screw only 1 1/4 at most

If that's the case than there's definitely something else that needs to change. So far I'm thinking idle jet. It'll die if the mixture screw is that far in. Rev's increase as I turn the screw counter clockwise from 1-5/8 and decrease if I turn it clockwise. Manual says 1-5/8 +-1/8 so that's why I was using that number. I should also note I'm in Charleston SC so pretty much at sea level with decent humidity.


Sent from my iPhone using Tapatalk
« Last Edit: Aug 04, 2018, 10:09:57 by thrillseeker »
Don't take life too seriously, you'll never get out alive.

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #3 on: Aug 04, 2018, 10:50:37 »
One thing I'm having trouble figuring out from all my searching is how the mixture screw works. I know the general idea but how exactly does it work? Does it let air into the float bowl and cause positive pressure in the bowl pushing fuel up the slow jet? Also, what direction enriches vs leans out the mixture? Knowing this would really help me tune properly.


Sent from my iPhone using Tapatalk
Don't take life too seriously, you'll never get out alive.

Offline Tim

  • Administrator
  • DTT SUPPORTER
  • *
  • Posts: 12876
    • DO THE TON
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #4 on: Aug 05, 2018, 02:50:17 »
I need that manual!  Happy to contribute to your cost on it and host it here for people to find and download.
"Quality . . . you know what it is, yet you don't know what it is. But that's self-contradictory. But some things are better than others, that is, they have more quality. But when you try to say what the quality is, apart from the things that have it, it all goes poof! There's nothing to talk about. But if you can't say what Quality is, how do you know what it is, or how do you know that it even exists? If no one knows what it is, then for all practical purposes it doesn't exist at all. But for all practical purposes it really does exist."

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #5 on: Aug 05, 2018, 10:20:50 »
I need that manual!  Happy to contribute to your cost on it and host it here for people to find and download.
Great idea. I added it to my original post above.


Sent from my iPhone using Tapatalk
Don't take life too seriously, you'll never get out alive.

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #6 on: Aug 07, 2018, 23:50:14 »
Found a good site for pilot screw settings. https://www.howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/


Sent from my iPhone using Tapatalk
Don't take life too seriously, you'll never get out alive.

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #7 on: Aug 08, 2018, 22:50:31 »
I was able to do some tinkering with the carburetor today and found out a lot of good info. I could still use some direction though if someone could chime in.

I pulled the spark plug and it was caked black. So I've been running really rich. I took the carburetor off and, since I dont have any other idle jets currently, I decided to increase the float height to lower the level of the fuel and hopefully lean the mixture out a bit. I did that a couple times and each time it got better. I ended up going from 21mm to 23mm but the spark plug is still super black and I can unscrew the idle jet multiple turns out and it never seems to get lean enough. But I can tell its definitely running a lot better.

My question now is, does it make sense to keep increasing the float height to lean it out more? My instinct tells me I probably don't want to go any farther with the float height and its time to get a smaller idle jet.

I put this drawing together after I got a better understanding of how the idle circuit works. I believe this is correct but please let me know if something isn't right. I dont want to spread misinformation.

« Last Edit: Aug 08, 2018, 22:55:24 by thrillseeker »
Don't take life too seriously, you'll never get out alive.

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #8 on: Aug 09, 2018, 22:27:16 »
found some more good info here
http://www.keihincarbs.com/tips/gate.html

I've been searching for a leaner slow jet but coming up empty handed. This seems to be the correct one for the ST90
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_keihin_vintage_slow_1.html
Don't take life too seriously, you'll never get out alive.

Offline thrillseeker

  • Posts: 361
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem
« Reply #9 on: Aug 11, 2018, 13:26:00 »
Made some more progress today. I figured out a nice little trick to know if its running lean or rich. You can pull out the high elevation knob and if the rpms increase, your running rich. That knob just lets more air into the idle circuit. Big Bird likes running with that out and the air screw fully open (super rich). So I decided to tackle the bowed surface of the float bowl again (a little more thorough this time). It was so bowed that I could rock it back and forth against a flat surface. So I pulled out the overflow tube with pliers and resurfaced it using 300 grit and 600 grit. now she's nice and flat (photo below). Once I did that the bike started running a little better. still super rich though. So now I'm soaking it (again) in carb cleaner because Im wondering if there's some gunk still left in the air tube. I cleaned it really thorough last time but, in my photo above, the passage that is represented by the third blue arrow is impossible to get to with a wire. So hopefully soaking another 24 hours + spray + compressed air will dislodge anything that might be left in there.

« Last Edit: Aug 11, 2018, 13:30:47 by thrillseeker »
Don't take life too seriously, you'll never get out alive.