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Another Re-Jet or Not to Re-Jet Thread

  • Thread starter Thread starter Nightmare
  • Start date Start date
N

Nightmare

Guest
Sorry to post yet ANOTHER rejetting question... but this one has me stumped.

1980, GS850 (32,000km) completely stock.

I noticed a lack of power on the highways from 1/2-full throttle so I figured plug chops could shed some light on the subject.
2/3 Throttle in 3rd gear @ 80-90km/h on a fairly steep hill
Full throttle in 5th @ 100-120km/h on a moderate incline

I was able to hold the throttle for about 2 mins each time and killed the bike without closing the throttle, pulled over and checked the plugs.
http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html
Both chops ALL of the plugs look like #22 or #23 in the above link (they reminded me of brand new plugs, smooth/shiny white insulators).

There is a fairly new air cleaner on the bike, the airbox doesn't have any mods to it (although I think one of the seals could be better). The last compression test indicated low compression on the #3 cylinder, valves were OK (probably could use adjusting now). I have rebuilt the carbs (cleaned and replaced with identical parts), adjusted the airscrews, adjusted the float heights (verified with a fuel guage) and synced the carbs (set for 2&3 lower vacume than 1&4). New intake O-Rings and boots, new airbox boots, petcock seems to be working and flowing fuel to the carbs. Dyna coils on order to replace a possible intermittant coil problem.

At first I thought there might be a fuel blockage in the line, but unless the problem is a blockage at the petcock, fuel line or "T" I shouldn't be getting the same problem on all 4 cylinders. Same if somehow the problem is caused by valve clearence or compression problems, I should see 1 or 2 bad cylinders and the others OK. I am thinking about rejetting but as per the advice from this forum, stock bikes generally don't need to be rejetted...
 
Forgot to add: Checked timing (idle and auto advance checked out) and using NGK B8ES spark plugs
 
What type of filter ? Foam? Did you oil it ? ANy fuel filter ? remove it if you do CHeck the gas cap vent
 
Oiled foam filter, checked the gas tank cap (it wasn't blocked)... :(
 
The pics you linked to said Hot or Lean But Ok (?). Could be that you are running just a bit lean. Maybe adjusting the air/fuel screws a bit could help.

Brad bt
 
Seal that airbox up. If it's stock than you shouldn't have to rejet.
 
Like others said, a stock bike shouldn't need re-jetting unless you live in a high elevation.
If your problem is from 1/2 throttle and up and you have plugs reading lean, I'd suspect fuel flow is compromised at the petcock. If you're POSITIVE it's clean/clear inside, then I'd do a quick test by running it on prime to see how it performs. Be sure you run a 5/16" fuel line and it's length is correct so you have no kinks or the hose actually bends upward and resists good flow. Also, any floatbowl vent lines must breath clear and be routed correctly, no kinks, under the seat area. I've seen PO's connect the two lines and of course fuel starvation will result.
If you still have the problem, considering what you say you've done to fix it, I'd focus on what size main jet you have and jet needle size and any possible non-factory jet needle positioning. A PO could have made changes, though I don't know why anyone would install smaller mains/leaner needle positioning, if the needle is even adjustable. But PO's do some strange things. You should be able to look up the main jet/needle info easily.
As for the airbox, it needs to be 100% sealed correctly before you can say it has little or no effect on the mixture. You make it sound like it's minor and probably not the cause but it needs fixing.
 
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