Originally Posted by snuvet75
Well bringing back to factory setting helps but it won't help him to tune it right.
reving high and then low at idle is due to rich LSN like the gentleman said.
When you look at the carb and notice you barely have any gap? Don't worry cuz it's totally normal. 0.5-1mm gap is normal depending on your setup and engine w/o venturi. So take it off and look at it again. See some gap? Then dun worry. You will have to adjust it later again anyway.
1. Set the gap w/o venturi between 0.5-1mm (just eyeball it. You can't measure it accurately anyway). Then put restrictor back on.
2. Tune the engine. (check the tuning bible here)
3. Revving high and then low at idle? Start leaning out LSN till you hear constant revving at idle.
4. Close the carb gap till RPM lowers. (there are many methods to check if LSN is set right. Pinching fuel tube, counting seconds before engine starts puttering out, listening to sound, checking smoke, etc.. Ur judgement)
5. Run it again and fine tune. Aim for 200-260 F. Dun let it four cycle. If over 260, dun be concerned too much cuz it can be totally fine depending on your engine brand and setup. 200-260 is just a guide line and that range is safer than, say, 300.
There are tons of details you need to know in between. Research and keep asking questions.
It's called a FAUX IDLE...
Too rich on the bottom, too high on the idle.
Rich idle mixture pulls the idle revs down until you go WOT and then leans out making idle mixture have more air.
Richen the top needle 1-2 whole turns to ensure ample fuel pressure
Check for all air leaks, plug condition etc.
Turn on radio, check all endpoints, braking etc and use a rubber band to hold carby shut
With 2 whole turns of HSN you cant rev at all you'll flood.
Slowly lean the bottom mixture as your hear the idle slightly rise, drop the idle screw until you can give it tiny blips without flooding and listen to the idle behaviour
Also consider the status of mixture when engine stalls. If it starts immediately, and then stalls after a few second, then it's likely it flooded on the bottom mix, so a little leaner on the bottom and drop idle screw again
You should hear the engine nicely idling with a wet "plop" without stalling.
Then put it on track and dial in the HSN.
I did this with a friend with an OS on the weekend and he too had a bad plug. It was tricky with teh OS but it purred for the rest of the afternoon
PS: And do what SNUVET SAID!!
There is a chance you are too lean on top, but this is the most common cause of a hanging idle, other than wrong endpoints