Thread: New ST90 owner with problem

ok, I'll ask here before I dig into it.

I just got a nice 1973 ST90, and although it starts very
easily, and idles nicely, it stutters and pops when I try to
give it any throttle at all. It almost sounds electrical to me, but it could be carb related also??
I haven't done a thing to it yet, and figured you guys could help me out here. I'm not very well educated on these bikes, so any help would be appreciated.
If you try to ride it, it pops a lot, stutters, and breaks up, with NO power at all. I used to have a Sportster that did pretty much the same thing, it ended up being an air leak in at the intake. But somehow this seems to be ignition related, or with my limited knowledge, who knows
I just got a carb kit for it, and will undertake my 1st carb rebuild. Any pointers there?
First, I want to check the points and timing on it, since the PO was messing with the points when I picked it up.
He thought it has a bad condenser, don't know on that one.
I'll put in a new plug too, and report back tomorrow.
I also read somewhere that this could be spark advance problem, so will check the advance mechanism out too.
ok, next time, it will have new points, plug, condenser, and timing set/points gapped, etc. I'll check the intake for leaks, and maybe install new gaskets there too. I'll flush out all the old fuel, and put in some fresh gas and oil while I'm at it.
Any other ideas from you guys? or pointers on the carb rebuild?
Sorry for so many questions at once here, maybe I should have another topic on the carb itself?
Thanks
Rags

Sorry for the bad pics, I'll take some better ones tomorrow

First start with a general checking of all key elements that make the bike run.

1. Timing. Check the points. replace if necessary. IN any case clean every electrical contact as you go. I.e. the end of the condensor cable.
2. Adjust the valves with the engine cold

Then check fuel and delivery

3. rebuild and/or just clean the carb. first. its easy. have some carb cleaner, an old tooth brush and Qtips (both to help clean off varnish and the Q tips fit several passages in the carb)

4. Check both fuel lines for flow and if there are restrictions at the tank (sediment).

5. Install a clear fuel filter on the main gas line.

6. A new condensor can do wonders as well. and get a new spark plug. I have had improvements sheerly by cutting 1/4" off the coil cable and reattaching the spark plug boot.

After you do these. retest.

I pulled the carb, and totally rebuilt it.
It had some serious clogged jets, especially the #65 jet.
Don't know which jet this is, but it was almost completely clogged. I thought for sure this was my problem.
After reinstalling and adjusting the carb, the bike still runs the same, popping and stuttering. I'm still waiting for points and condenser to come in.
I did find that the carb threads were junk, and wasn't very tight on the flange, so I'm picking up some longer bolts so I can put nuts on the end, I still think it may be sucking some air at the intake, I will fix that today, and set the timing, and report back.
Going to the Honda shop on monday for new intake gaskets.
Thanks for the post, and your help.
To be continued.
By the way, the carb rebuild was easy, and now the bike starts with about 1/2 a kick! I really thought I had it, but not with my luck..
Now that I have one rebuild under my belt, I have 3 other bikes that have carb troubles, so I'm going to rebuild the worst one today, and hopefully that will fix that bike.
That bike, a 76 Ct70, runs great, but is VERY hard to start, I'm hoping the reuild will bring it back to life again!
Sometimes it takes 25-30 kick to get it running!
Peace
Rags

Dont forget to clean the petcock. It has an internal screen. On that bike Im not sure if the petcock is part of the carb or gas tank

Got the bike fixed now. I took off the timing plate,
and the mechanical advance unit was all gummed up.
It took an impact tool to get the screws out, so I don't think anyone had ever taken them off before, or serviced the advance unit. The bolt on the end of the cam was being
uncooperative, so I just sprayed the advance unit down with
PB Blaster, and with some work, got it all cleaned up and working perfectly again. It looked like gummy dried up oil on it, like glue.
I fired it up, and yahoo, it runs like a charm now.
I had it up to 50MPH on the roads around the lake here, but it didn't feel like it had much more left in it.
I still have to finish the rest of the tune up, points, plugs and condenser, maybe that will make a difference too.
Yup, I cleaned the petcock, its on right on the carb on this one.

Damn, if only I could get mine up that fast