1973 ST90 need help

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scman843

New Member
I just bought this ST90 barn find and all it needed was to clean the carb and it has no spark.

Ithe battery is bad so I was wondering does it need the battery to spark or what doi need to check to get spark

has a new spark plug also

if I need to check the points how do I do it.
 
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b52bombardier1

Well-Known Member
The points are under the silver cap with two screws on the left side of the cylinder head. Yep. You definitely need a good battery. Check the run-stop switch is on run. Key on. Make sure the battery fuse is good.

If the bike runs but the points are arcing, you need to replace the condenser that is bolted to the coil.

We can talk about the rectifier and it's testing after you get the engine running.

Rick

Two CT90 and two ST90 bikes
 
Enginedoctor

Enginedoctor

Well-Known Member
scman843 said:
...if I need to check the points how do I do it.
Click to expand...

remove the cover like rick said, it's up on the head on the 90's, you basically turn the engine over until you see the points open to their max. i don't know EXACTLY what the spec is, but on the little 70's and 50's it's .016" a feeler gauge of this thickness comes in handy here. i would make sure they're between .016-.018" so when the points are at their max opening, inserting a feeler gauge, it should 'drag' or have resistance. a larger size one will drag a lot, a smaller one will be loose. make sure the points close all the way when the ignition is at it's base circle.

and yeah you need a good battery. stress on the word good. even a weak battery won't keep a 90 running, ime. when they swapped to the 110's, they figured out that a stand alone stator based ignition primary coil was a good thing. those later bikes you can start with no/dead battery. same with the 70's. very smart.

Also make sure you use a spark plug for the 90 engine and not the 110. The plugs ARE NOT THE SAME, and one installed will contact the piston and even bend the valves. the last 90 i bought met such a fate, and required an engine rebuild (like 50% of the vehicles i end up buying)

let us know how you make out scman… lot's of ST90 fans here!
 
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