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Weak on One Cylinder

sam000lee

sam000lee

Forum Mentor
I'm dong a makeover on my GS450 which includes an engine from another bike, UNI Pod filters, and EMGO shorty mufflers. I also just did the Coil Relay Mod and am getting great spark on both sides. The engine hadn't been run in 5+ years, and I never saw it run with stock airbox, etc.

When I start the bike up, which I can do with no choke, it feels like only the left cylinder is firing. It idles really high and the throttle response sucks. You can feel it with your hand over the exhaust, plus the right header only gets lukewarm. If I pull the left plug, the bike dies.
With the left plug OFF I can start the bike with choke on just the right cylinder, and it starts great and idles at a more reasonable RPM around 1700. In this case the right header gets normally hot. If it's running like this and I put the left plug back on, the RPMS shoot back up.

I have two sets of carbs with the following settings:
Set 1:
20 pilot jets
Needle shimmed `one washers worth'
130 mains

Set 2:
17.5 pilots
Regular needle setting
125 mains

I get the SAME reaction to both sets of carburetors which leads me to think that maybe this isn't actually a carburetion issue. I did my valve clearances a month ago before I had the engine running and double checked them a couple of days ago.

The cylinder head is from ebay and I don't have a compression guage so don't know what my compression is - I was thinking that maybe I just had bad compression on one side, but don't quite get why it seems to run better on the right when the left isn't firing.

Any ideas? TIA
 
Did you go through the carbs completely and dip them?
 
I am fairly confident there are no air leaks (sprayed carb cleaner on intake boots, which I have replaced, as well as on pod boots and neither changed how it was running). The intake boots are 2 years old and in good condition and have new O-rings between the cylinder head.

Both sets of carbs have been completely disassembled and dipped and have new sets of O-rings. One of the sets of carbs I have been using for a few years and were in perfect working condition as of last fall when I took the bike off the road for the winter.
 
It's a long shot but you might want to check the mechanical advance unit to make sure it's working properly. Twist the rotor and it should move under spring tension and spring back on it's own if you twist and release.
 
Mechanical advance seems to be in good shape. After I snapped of a carb. boot screw and botched drilling it out I bought an ebay head that's the one on there now. I'm thinking maybe now is a good time to get that head sorted out and back on (the engine only had 4,500 miles on it) since it's in better shape than my ebay one.

Doing some searching I also found that other people were worried about bad piston rings. I don't think this is the case with me. I didn't notice oil on the plugs or anything but will check the next chance I get...
 
When choke is on does it still utilize the pilot jet?
When I start on only the right cylinder with choke, it idles fine but if I give it any throttle it backfires and I can see flame coming out of the right exhaust.
After reading around it seems like some 450's have up to a 40 for the pilot. Since mine is currently at 20 I'm thinking that maybe this is way to small.

Still doesn't explain the difference between the left and right cylinders, though.
 
Could somebody give me some suggestions on these plug readings:
The left cylinder is the one that is idling high. I as worried about bad piston rings but put some oil in the plug hole and it didn't seem to make a difference. I need to buy/rent a compression tester to do a proper reading still.
The right cylinder doesn't seem to be really firing when both coils are hooked up, but does seem to work fine when it's on its own with choke.

Both seem kind of black, although after running the left one looks like its getting kind of burned up.


Left plug:
26287353714_148491abd1_b.jpg


Left plug after putting some oil in the plug hole:
26287353784_f4542395c0_b.jpg


Right plug:
26287353844_2c6baea793_b.jpg
 
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