Author Topic: Yet another carb thread an new build discoverings  (Read 94 times)

Offline Armandorf

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Yet another carb thread an new build discoverings
« on: January 10, 2019, 03:36:18 PM »
Hello fellow gstwiners,


I have a 1989 gs500 that was very neglected, i bought it without running and i had been colecting pieces for a long time from ebay. I ve even bought some pieces from this forum.

Finally, i decided to start the build stage and the bike is running and i have around 900 km.
The engine was rebuilt almost entirely. (Complete bootom end and new head)
Im currently in the last and most painfully fun part of my build.  Fighting with carbs and electrical ghosts.

I have new questions as a newbie rider of these noble beasts.

I hope to use all these info i collected and organize it in a easy way.MAybe we could update WIKI or FAQ.

Now im learning a lot about the carburetors, which i had to open 4-5 times for rust from the tank.
I temporarily solved that with epoxy resin coating, i hope it will last a few years at least.
Now the gas comes out super clean. Something so simple looks wonderful. I see the clear gas and i feel satisfaction. Thats for all the work that was needed for the apparently simple things.

Going back to the point.
I will put the questions without too much order:

1 chain adjusters: Is it really neccesary to torque really hard the bolt ? in the first runs it was a bit loose and i ended up deforming the back plates that rests in the swingarm. i realized it was for that reason but the metal ends felt not that sturdy.

2 Clutch being HEAAVY: maybe i have a piece missing or some detail in the clutch assembly, i think that i will end up checking it sometime with the diagram.
The real problem is not the heavyness. It s the roughness to disengage
Sometimes is really progressive and other times it "clunks". Same thing if i quickly release the clutch, i can feel the clunk like the piece inside is hitting a bit.Like too much slack. It happens sometimes only. Realized it is best not to fully engage the clutch between gears and that doesnt happen,it is very smooth. problem is with first gear, sometimes being more difficult to put. Too much leverage? bad regulation?
I had the idea that it works best when cold. I Have to check that.
10-40 oil

3 Carbs
I have a kn 2970 lunchbox and  20 pilot , X mid and 130 mains. stock exhaust with hole in the side
It is confirmed lean, grayish white sparkplug.
So i decided to make some changes before getting the right jets.

I raised the needle with and oring (about 2mm) Is it too much?
I also untapped the mixture screws and they are 3,25 turns out. they were 2,25 in LEFT side and 3 in the RIGHT.

BEFORE the needle and 3,25 mixture:
-Rough idle- doubtful i would say, very aceptable when hot
-Popping in idle , like cycle jumps-realized that without air filter was more audible (plaaaakc sound) and saw the slide raising and gas going backwards.Left cylinder. Very rare when hot. Usual at cold idle without choke.
Hot days pretty decent.
Choke really gets the revs up  and has a tiny spot for controlling around 3k. hovers a lot with throttle inputs. maybe it falls and with a slight throttle i take it back for many seconds. i control with throttle, kill switch and choke position till i left it a at 3k maybe.
Cold days took longer to get the engine warmed up, only when realy hot it was ok.
acceleration at 1/4-3/4 throttle was too hollow sounding. like it needed to rev and when cruising was good, accelerating very hollow.(lunchbox sound???)
- WOT pulls hard till i have flat spots and the engine suddenly drop power, and has no power of acelleration, next gear brings it up. i would say that is around 9k. i know the torqeu decreases but you even start to loss speed,i imagine that at least it should cruise.

NO speedo/tach for now, i will put it to test with more accuracy.


AFTER.-still checking
better idle. heard a slight plac 3 minutes after starting without choke.still need to know and test other mixture positions
Much better start up-Choke control. choke is more progressive.With only choke, no need to use throttle-kill switch.
Acceleration is more rounded, maybe its placebo efect in terms of power, like im acellerating more with less throttle, i dont feel the bike has more power but its more responsive. pulls stronger from bottom end.
Overall i fell the sound is more healthy.Like its not struggling that hard.

i know i should change only one thing per time. Will be that way from now. Felt mixture was a very slight change.

I still have to synchronise carbs.I did it by eye. same amount of opening butterfly.adjustment is almost in the end towards one side.

CARB JETS
The question is about the mid jet, how to test it and how it affects? 62,5 seems logical and the best to compare, assuming its a 60 the unknown jet. Is it neccesary?
22,5 pilot. feels unnecesaary but maybe i will try.
bigger main jet, no doubt it will be 140+.  Thinking 142.5
If the main is a bit too big you can compensate with lower needle? just a bit?

I think about 2 mains vs 6 jets(pilot ,mid an main), even 8 if i decide to try two main sizes, not only the cost but the number of tests to try out. Too much money and time.
I think in the end will be worth it to fine tune it, i will end up using it a lot.
I have to feel and understand better my changes before making so much changes ,still learning how to tune carbs by hand and ear.

Good riding.