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:
What I'm thinking
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
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