You need a richer mixture on an air cooled engine to stabilize the combustion proces and ensure the burn completes in a controlled manner. Also the intake air will actually be a bit more charged as the the fuel cooling will help to keep it from expanding on the way in. 13.5 is what a water cooled LS1 with tightly controlled EFI can get away with for a max torque curve. But our old Skool twin thumpers need some help. While the head design is good. The firing pulse, heat, packaging, and drive train load were built for nostalgia not rip roaring performance.

Here is what I run my HSR42 with and with this set up it has ran better than ever! I am getting around 40mpg in town and around 44 to 45 on the highway. I have enough power, hell its a 102 cubic inches of pure power anyway.

By the way I have never ported my manifold..... I will one day but it is running good so I am not going to mess with the manifold until I need to change it.

Needle Manufacturer: Mikuni #98
Needle Clip: 2
Jet Manufacturer: Mikuni
Main Jet Size: 170
Pilot Jet Size: 25
PMS Screw Manufacturer: Stock HSR
PMS Turns Out: 2 1/4
My Elevation (ft): 300

No fuel pump, no fuel filter, 5/16 high pressure fuel line, K&N racing air filter. AIS removed, and TPS mod. Maxair choke cable

2007 Midnight Silverado 1700

I never paid much attention to throttle position. But did look at holding speed to AFR vaues, then to mpg's. The guy I used was using a dyno with Dyno exhaust gas analyzer, probe in front cylinder, RH dooleys are a 60/40 split. He would also hold it at certain speeds so I could get a decent fix of afr. Hence my reason for my 2 speed cruise mileage tests, yes dull and boring, mundane holding a throttle steady for 100 miles or more, but gave me a lot of info. 65mph = 14 afr= 50mpg approx...80mph = 13 afr = 40mpg. wot mid 12's afr. certainly going in the right direction of richer afr numbers. The only thing different is the gearing, on a dyno he only uses 4th, my mpg correlation numbers are in 5th. I've been consistent with these numbers (mpgs) for 6yrs, w/minor tweaks due to filter changes. And these afr to mpg is also based from other data charts I've seen over the yrs, a pattern I saw.

Erbman02 wrote:
I never paid much attention to throttle position. But did look at holding speed to AFR vaues, then to mpg's. The guy I used was using a dyno with Dyno exhaust gas analyzer, probe in front cylinder, RH dooleys are a 60/40 split. He would also hold it at certain speeds so I could get a decent fix of afr. Hence my reason for my 2 speed cruise mileage tests, yes dull and boring, mundane holding a throttle steady for 100 miles or more, but gave me a lot of info. 65mph = 14 afr= 50mpg approx...80mph = 13 afr = 40mpg. wot mid 12's afr. certainly going in the right direction of richer afr numbers. The only thing different is the gearing, on a dyno he only uses 4th, my mpg correlation numbers are in 5th. I've been consistent with these numbers (mpgs) for 6yrs, w/minor tweaks due to filter changes. And these afr to mpg is also based from other data charts I've seen over the yrs, a pattern I saw.

I've had my AFR gauge for a couple yrs now. What I have found is that changing into a higher gear at the same speed makes the AFR leaner. So if your guy is checking in 4th gear and at 65mph you are showing an AFR of 14,as soon as you shift to 5th gear at that speed you will go to about 15AFR. The change is that much. You can watch the drop in the AFR at cruising speeds to set the needle clip. You get to a certain speed and when it hits that spectial spot on the needle the AFR drops like a rock. Also from what I have seen is that in order to achieve 50MPG at hyw speeds in 5th gear you need to be at about 15 AFR. The throttle position between an AFR of 14 & 15 is barley noticeable at those speeds. The 10ths on the meter is constantly changing up to a 1/2 AFR pt in any ride situation. If you watch on a dyno and see a nice smooth AFR line that is only because that is the way they have the dyno set up for that run. It is showing a smoothed average. That is not the way your AFR reads in real time on a gauge as I said it shows the constant change.

You are correct Dave.

Dave you have the right thinking.

see.....Dave !

I don't know why you said you need to richen in the mid range you are already rich across the whole range. The lower the # the richer the mix. Idle should be about 13.5 mid 13.5-14 and high end cruise 13.8-14.7 and high end cruise will even handle an AFR of 15 without a problem just better economy. The only thing that should be below 13 is WOT and idle with the choke on.

Thanks Dave



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Those readings look better but could still lean the idle a bit without changing anything else. I like it a bit leaner at idle so if I let it idle for a bit I don't have to be concerned with carbon anywhere. How do ya like the gauge and the ease of tuning with it?