@dodo it was a recommendation for both in regards tuning and plugs. We were experimenting with 085 and 086 float heights with top 2 positions for needle. We were trying to keep the bottom end rich but not be too rich on the top end. Without a dyno you are really just guessing a bit though.
Dude, I can’t go leaner, haha. Like I said above, that plug is white and the needle had only one more notch left. Float is at 86. There’s no way I am too rich.
The other poster is very, very rich and could probably benefit from going leaner at the top end, so I think you might be mixing up our posts
Sounds like you are running in the ballpark of where we had been.
We would typically have float at 855-865 range with the air bleed open about as much as we can, then needle clip would be in one of the top two positions most of the time. I would adjust the needle height based on the density altitude and my sons feedback. Never went richer than the middle position even at a density altitude of 500ft. Around density altitude of 2000-2500 feet would look to lean it 1, then at 3000-3500 would look to lean it 1 more. If above 4500 might look at float height of 875-880.
Back on the clutch: Do you ever adjust your springs/engagement for different tracks? Or is yours a set and forget approach? We landed on the latter and just focused on maintenance, just curious what others did.
This is how I’ve done it. 2 black 2 white on the hillard. Changed after 2 races + practices. Race and needle bearing new after half season. Drum and shoes cleaned after every day at the track.
Usually junk them after the season is over and start fresh next.
I do have my airbleed at the recommended (by BS) 1.5 turns out, and I know that’s on the conservative side for making power. Once I get everything back together and get some good runs, playing with the bleed screw is next on the docket. Just want a solid baseline before that so I can single out the changes.
Clutch is basically set-and-forget. BUT, I have recently gone to four white springs. If my math above is to be trusted, there’s zero advantage to any slip beyond 3,000 rpm. Factory spec has peak torque of 11 lb-ft at just 2,500 rpm, so I can’t think of a logical reason to allow slip beyond that since you are just dumping otherwise useful energy. I don’t know what the really tweaked motor curves look like, just what was published by Briggs. Now, I know that is very much against the popular opinion, and I am mostly just thinking out loud and open to correction, I want to see what changes I can measure. I can come across as argumentative, but I honestly come from a place more of hands-on learning. It isn’t that I don’t believe the smart people in the room, it is that I want to understand the nuances of what led them there.
That being said, you can get a ‘hammer’ effect with standing starts where it slips the rpm up high and then grabs, basically doing a clutch dump. This is very different than the steady slip you get out of slow corners. I am just focusing on slow corner performance here, especially since I don’t see much of a ‘clutch dump’ effect with the Hilliard design and it is even harder to tune intentionally. Some, but not much.
What’s I’ve learned:
Remember the specs from Briggs are baselines. Same as chassis tuning, they provide baselines you’ll likely have to deviate from.
One thing to start with. Get the idle speed screw (throttle stop) out the airflow and use cable tension to adjust idle speed if you want it to idle. Use the air screw to make moderate tweaks to the entire fuel curve, not just idle and transition.
Needle, idle screw etc pertain to throttle position more than RPM.
Clutch engagement rpm sidebar:
What is the technical justification for wanting any clutch slip beyond 3,000rpm? If Briggs’ data is accurate and relevant, peak torque occurs at 2,500rpm and is nearly the same at 3,000rpm. It steadily falls of after that. It reads to me that the only reason to think the engine is said to be bogged down at 3,000rpm is because it just sounds like it. A single cylinder 4 stroke at 3,000rpm sounds pretty slow, but, on a dyno, it makes the most torque there when set up how Briggs recommends as a baseline. Any slip at all will reduce the torque to the wheels, so why does literally everyone recommend slipping the clutch? Remember, most engagement rpm are much higher than this, and slip happens well past the engagement point. To be fully engaged at peak torque would mean it starts engaging around idle.
To be clear, I am not saying any of this as a recommendation but as a conversation starting point, but I have not seen a technically sound justification for why no one agrees with this.
A hydraulic torque converter can increase torque beyond what the engine could do without slip, but the shoe-type clutches we run cannot. The only exception being a “clutch dump” kind of engagement where it way overshoots the engagement point and the inertia of the engine adds to the torque output as the rpm get dragged down once the clutch starts grabbing. When lapping on track, I’ve not felt anything like that. Standing starts, sometimes, but not lapping. Is there a general disagreement with this statement?