Questions about tuning a 990

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Mike4130

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Let me start of by saying I've watched the carb tuning video by Trip & OBR. Both of which were very helpful.


I've been trying to tune my 990 for a while now and have been noticing this,


I start at 1 1/4 out on the LSN, I slowly turn it clockwise until the rpm's lower then begin to increase again. I stop the needle at the highest RPM. This will bring the needle out about 3/4 to 1 full turn from the closed position. If I leave it @ 1 1/4 the throttle isn't very snappy (it bogs at first, possibly to rich).


At this point I do a high speed run, the plug looks good (similar color to a brown paper bag). Now, at this needle setting, it's a bear to start. I pull the cord 12-15 times to start it. Possibly not enough fuel.


So now, I set the LSN to 1 1/4 out, it starts on 4th pull (great) but on high speed runs, the motor bogs out (Carb is running to rich). Is this a improper high speed needle setting? I have it @1 1/8th out. Weird thing is, when running really lean on the LSN (3/4 turns out), this problem doesn't happen.


I'm just being cautious of running to lean & burning up my motor.


Thanks for any input.
 
My LSN on my 990 is at 7/8 out. Each engine is different.

If you are in cold weather, you can use a plastic bag to choke the engine. Once it pops, then take the bag off.
 
my 990

LS 1 +/- 1/8turn ...this is the idle set up mixing air and fuel its a balance of engine speed and fuel. in the middle is what you want.... NOT at the highest RPM it will be to lean for idle...too far out will be to fat for snap. nice balance gives a 4-6 slo full pulls , then a pop, starts on 2nd or 3rd snap pulls ..this is a cold start.

HS 1 1/2 turn... and maybe add 1/8 out for cold weather

rarely touch for tune anymore unless it fades on the top end.

gotta tweak for your conditions.
 
Set the LS needle where it starts easy then after it is warmed up turn the HS needle in a1/16 turn run it if it still bogs go another 1/16 turn this should work for you, I start at 1 1/2 turns out on HS and 1 1/4 out on LS and it is always too rich after it seems like it is running good then see how it idles, remember a very small turn can really change things, be patient I know it is easy to get frustrated.
 
Thanks everyone. It's proving to be more difficult tuning in the colder temperatures we're having now(Ohio). But this will definitely help.
 
Last edited by a moderator:
After getting mine tuned, I leave it alone.

Cold weather is a bit tougher to start, the slow pulls REALLY help.

I tried the bag with no luck.

I have a DT1 filter that is held on by a hose clamp.

If it really hard to start...

I remove the filter, cover the carb with my hand and pull once or twice, put filter on and it fires right up.
 
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