Help! Yellow Honda ST90 "Big Bird" Idling problem

thrillseeker

Been Around the Block
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
 
TheCoffeeGuy said:
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.


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Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

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.


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I need that manual! Happy to contribute to your cost on it and host it here for people to find and download.
 
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

Tim said:
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.


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Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

Found a good site for pilot screw settings. https://www.howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/


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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.

74%20Honda%20ST90%20carburetor%20idle%20circuit.JPG
 
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
 
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.

74%20Honda%20ST90%20resurfaced%20float%20bowl.JPG
 
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

TheCoffeeGuy said:
soak in Pine Sol
Pine sol? Really? Is that better than a bucket of carb soak?

I got the carb back on the bike this morning. I think cleaning helped. Now it runs better with just the mixture screw out. It doesn't need the altitude knob pulled out anymore. I'm thinking there may still be some gunk to clean out.


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I clean all of my carbs and carb parts in an ultrasonic cleaner filled with Pine Sol.
 
+2 on pine sol. Shit works well. Has to be the regular one, not one of the new flavors they make. And I use a sander duct taped to a tupperware container as a redneck ultrasonic cleaner and it works too.
 
I did some searching on this method and it seems people use 50/50 pine sol and water. is that what you guys use? I do have a bottle of original pine sol so I could probably do this tomorrow.
 
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

You know, usually when you up the displacement and use the same size carb you get a higher carb vacuum, sometimes requiring stock or smaller fuel jets. Because the bigger piston is sucking more fuel from the jets.
 
Re: Help! Yellow Honda ST90 "Big Bird" Idling problem

datadavid said:
You know, usually when you up the displacement and use the same size carb you get a higher carb vacuum, sometimes requiring stock or smaller fuel jets. Because the bigger piston is sucking more fuel from the jets.
Thank you! This is the type of info I've been looking for. This was my theory but I've been having the hardest time finding info online to verify. I looked for a 32 idle jet but the smallest they make in this style is the stock size 35. I did find one similar with all measurements the same except a longer head. Maybe I'll order that, try it out, and cut it down to size if need be. For now I'll let my carb sit in cleaner for a couple days and see if it gets any better.


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Cleaned the carb really good. soaked for 48 hours, and then plugged mixture screw passage so carb cleaner and forced air had to blow through the unreachable passage. Put her back together and it's running better than ever. Now I realize how important and difficult it can be to thoroughly clean a carb that had been sitting full of gunk for 20 years. Went for a ride and got Big Bird up to 50mph. Was only able to reach 45 before. Some more fine tuning and I'll be able to hit the highway. Ha


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