1983 gs450 cranks & won't start

[StarcrossedMoto]

Rebuilt a 1983 GS450. Was my daily driver last year until I put it away in December.

Took the winter/spring to go over the bike and rebuild, refurbish, and replace anything and everything that needed it.

Bike is back together, engine cranks but doesn't start.

Got spark
Got fuel (premium too!)
Got compression

... I don't know why she won't fire up.

https://www.dropbox.com/s/ulr4wc233bkvi6k/vid_20211006_231503.mp4?dl=0

[cowboyup3371]

What is the battery voltage before and during the time you are trying to start it?

[tom203]

Double check that plug wires are on correct plugs.
Sounds like my bike when I didn’t use petcock prime long enough….make sure “choke” is fully “on” and don’t use any throttle during cranking.

[StarcrossedMoto]

What is the battery voltage before and during the time you are trying to start it?

13.1v - 12.9v Battery is still strong

[StarcrossedMoto]

Double check that plug wires are on correct plugs.
Sounds like my bike when I didn’t use petcock prime long enough….make sure “choke” is fully “on” and don’t use any throttle during cranking.

All good points, I read elsewhere on this site. Coils are wired properly, petcock set to PRI, and choke is full send.

[StarcrossedMoto]

First things I checked were

- grounds
- battery
- starter solenoid
- starter
- coils
- spark plugs
- fuel line/carburetors
- compression

Going to test compression again as all the above are solid

[StarcrossedMoto]

https://flic.kr/p/2mAXtZz Cylinder 1 - Dry compression test

https://flic.kr/p/2mANCXj Cylinder 2 - Dry compression test

https://flic.kr/p/2mAWgME Cylinder 1 - Wet compression test

https://flic.kr/p/2mASUcd Cylinder 2 - Wet compression test

Looks like I now need new piston rings...

[Rich82GS750TZ]

Looks like I now need new piston rings...

Just an opinion from a non-mechanic, but I'd say that at 115 on both cylinders, you're good. Spec probably says that 120 is the low limit, but I've also read that 100-105 would be the absolute low service limit. Maybe that's different on the twins, I don't know. New rings certainly won't hurt, I just don't know that this is causing the bike to not even cough when you try to start it.

[Grimly]

105psi should run fine, and if the ring packs are sticking slightly, they'll usually free up with use.
Only worry about new rings if they dip below 100.
Heck, I'd only bother once the damn thing simply wouldn't start and a test revealed 70psi or so.

[Gorminrider]

1: How did the rings wear out when you weren't riding it?
2: did you have the throttle wide open when testing compression?

that is to say, the bike you stopped riding last year is the same bike this year, engine wise. So Something Else has changed: aka gummed, rotted, been "refurbished" or "replaced"

[StarcrossedMoto]

105psi should run fine, and if the ring packs are sticking slightly, they'll usually free up with use.
Only worry about new rings if they dip below 100.
Heck, I'd only bother once the damn thing simply wouldn't start and a test revealed 70psi or so.

Just an opinion from a non-mechanic, but I'd say that at 115 on both cylinders, you're good. Spec probably says that 120 is the low limit, but I've also read that 100-105 would be the absolute low service limit. Maybe that's different on the twins, I don't know. New rings certainly won't hurt, I just don't know that this is causing the bike to not even cough when you try to start it.

Hmm...so that should still be enough compression, huh?

[StarcrossedMoto]

1: How did the rings wear out when you weren't riding it?
2: did you have the throttle wide open when testing compression?

that is to say, the bike you stopped riding last year is the same bike this year, engine wise. So Something Else has changed: aka gummed, rotted, been "refurbished" or "replaced"

You're suggesting that the problem is not my engine but one of the newer components? I guess I was looking at it the other way around: since everything else was tended to, the only thing I didn't replace or touch was the engine, I naturally assume it is the engine at fault here.

What else could it be?

[Gorminrider]

If the bike was ok before you "fixed" or "improved" it, and is not ok after, the obvious conclusion is that something was disturbed in the process.

[StarcrossedMoto]

If the bike was ok before you "fixed" or "improved" it, and is not ok after, the obvious conclusion is that something was disturbed in the process.

The bike already had electrical gremlins and carb issues, plus the tank was rusted, so it all needed to be serviced and replaced or restored.

So I know you are putting fixed in quotes, but everything is far better now than it was when I acquired the bike.

[Gorminrider]

Just saying. "fixed", "improved" happens to me all the time. The last thing I 'fixed" is the first place to start.