Originally Posted by
jason07
Quick question about the tune on this engine.
I just broke this in and it's on the 13th tank. I am starting to lean on this a bit more and the temps have not climbed higher than 250.
When making a high speed pass in the street (full rpm for about 2 seconds) after I let off the throttle is sounds like it's "2 stroking"for about a second and then falls back down to idle. The idle stays high for about 5-10 seconds and stops. I richened the carb up 2 hours and it stopped "2 stroking" but it was very slow on the top end.
When I brought it back to check on the fuel the engine revved up real high once I picked it up. It was low on fuel but it wasn't totally out. Any thoughts on that?
As you know, tuning is an art and much easier to do in person, but I'll give you my thoughts.
It's running-on because it's lean at that point, which can be caused by a couple of things:
1. Your carb restrictor or opening might be too large for the top end tune
2. Your top end might be too lean
3. Your bottom end might be too lean (after a long pass if you get off the throttle, a lean bottom end could be starving the engine)
4. Your idle gap might be too high, which would cause the idle to not come down right away
5. more than one of the above at the same time
First off, make sure that you're running the 6mm carb restrictor in it as a starting point (for buggy), and that the carb linkage is opening it to the restrictor size or slightly below. If your carb opening is too large, it can result in a run-on "2 stroking" behaviour after a long high RPM pass which then causes you to have to tune the top end richer than ideal. Also, I assume that you have the 2090 pipe on it with either the stock 75mm or optional 80mm header.
My first instinct tells me that your top end was fine before you richened it by 2 hours and that either your idle is too high or your bottom end is too lean. Also, 2hrs is a big adjustment. I'd use that if it was way off, otherwise less.
First off, I'd tune the top end in isolation by taking the bottom end and idle out of the equation (make the bottom overly rich to start with like 1/2 to 3/4 turn, and adjust the idle so it doesn't die). Lean out the top end to where it's making good mid-range and top end power on a high speed pass. This may mean you're going back in 2 hours or more, but use 1hr increments and then just 1/2 hr when you're closer. Since the bottom end is super rich, you should get lots of blue smoke if you let it idle for a bit first and then punch it, with good smoke at the mid-range and a light trail at the top. If you leaned it out too much, it will definitely scream and run-on as soon as you let go of the gas... since the bottom end is rich now, the cause of that lean run-on condition can only be the top tune. Even with a large idle gap with such a rich bottom end, it shouldn't cause it to 2-stroke either... it may not slow down right away, but it shouldn't 2-stroke due to a large idle.
Now for the bottom end and idle, since it's super rich at this point, you'll be leaning it to a point and adjusting the idle. (I find it always easier to tune from a rich setting as opposed to the other way around) On the box, after a few medium throttle bursts to clean out the engine, hit the brakes to stop the wheels, then go to neutral and watch what it does. Since it's still super rich with a high idle, you should see a high idle initially and then have it drop drastically after a few seconds as the rich mixture takes over and loads up the engine. The leaner your bottom end is, the smaller this idle drop will be as the engine will load up less. As you lean the bottom end, you'll have to close the idle accordingly. Do that a bit at a time and try it again. If you leaned it too much, you won't have much idle drop, or it might actually pick up speed from the initial idle. Some people lean out the bottom end to the point where there is no idle drop... for me that's too lean. The behavior I look for: after a few med-high speed bursts, when you hit the brakes, then neutral, the idle drops down immediately to where the wheels aren't spinning but it's not quite purring quietly and then the idle drops down a little further about 5 seconds later.
Another indicator of the bottom end tune that I use as a general reference is the relative temp when you're warming up... when you're just blipping 1/4 throttle on the bench, the fuel mixture is mostly governed by the bottom end. In normal outdoor conditions 70-90F, after a couple of minutes of warm up my engines get to around 160-180F. If in those conditions it's is closer to 200F, I know that my bottom end is likely too lean if the idle is set right, of if it's not even 140F it might be on the rich side.
Everyone looks for different things, but this tuning method has worked well for me.
Hope it helps you.