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beatr911 · 05-10-2013, 10:54 AM

[QUOTE=beatr911;359925]
For what it's worth, I found a discussion on incorrect cam timing on the Concours.org forum that is interesting. The symptom was that it didn't rev past about 4000 rpm. Turns out the poster had mistakenly installed the intake cam 3 teeth retarded. No mileage indications but apparently it idled fine and ran fairly well under 4K.

Possibly going 2 full teeth retarded would be a significant step toward what we are after. I think the cam sprockets are 38 teeth for 18.95* per tooth or a retard of 37.9* for two teeth. Adding the 37.9* to the stock 54*ABDC intake closing results in closing at 91.9* ABDC. Wow, not much compression left there. The Prius retards from 72 to 105 ABDC so this is right in there to get a mild effect.

I normally get low to mid 50s mpg so am expecting to see mid to high 50s to consider it a valid improvement.
QUOTE]

OK, so I retarded the intake cam one tooth as a first test, before trying two teeth. Since compression is reduced when doing this, I increased the ignition advance from my +4* baseline to +6* which was as far as my ignition advancer would go. This to help optimize the combination somewhat.

Power was down a little but since it's pretty powerful anyway the effect was not great especially above 3500rpm or so. Throttle response was a little softer under 3500rpm or so. Overall it ran fine and idled a little smoother probably due to a little less overlap. Though smoother, the idle was more erratic, sometimes it would idle fast, other times slow. Also as the weather began to get warm, say over 75*F at very small throttle openings and under about 2500 rpm the engine would eventually load up as if it was extremely rich. It took less than a minute in the low speed/low load conditions to begin to see the loading up. It is likely that reversion is greatest at low rpm causing the rich condition as the carb meters fuel to the same air flow 3 times.

Conclusions: Fuel efficiency was the same over two fuel tanks. However, if the rich condition could be controlled by a low speed fuel jet reduction a fuel efficiency improvement would likely be seen. Because most motorcycles are designed with very long duration cams from the manufacturer there is likely little room to retard the cam and maintain accurate carburetion metering. Very short intake runners and no carburetor plenum to share air/fuel mixture between cylinders sends a very strong reversion signal to the carburetor, exacerbating the negative effects of the reversion signal to the carburetor. Electronic Fuel Injection or a carburetor with a large volume between the carburetor and intake valve are possibly the only ways to accurately tune an engine with a significant amount of intake reversion such as with an Atkinson cycle type intake valve timing.

As it had pretty similar power from say 3500rpm up, I think that Akinson valve timing has a greater effect at low engine speeds. A Harley Davidson fuel injected 883 with tall 1200 gearing and retarded intake cams might be a great combination.

For my (overpowered) bike, possibly the easiest solution would be to make a plenum and connect one or two carburetors to feed the four cylinders, or cobble together a Megasquirt EFI system. No way I could retard two teeth in it's current configuration as the carbs would be completely confused at low speeds.

For my purposes, I just sold the bike after returning it to stock. Now bikeless, I'm searching for a great deal on a bike with great potential.

05-10-2013, 10:54 AM	#62 (permalink)
beatr911 Master EcoModder Join Date: Feb 2008 Location: West Coast, USA Posts: 516 B2300 - '96 Mazda B2300 SE Focus - '05 Ford Focus ST The red car - '00 Honda Insight Thanks: 6 Thanked 77 Times in 56 Posts	Atkinson timing - Tested! [QUOTE=beatr911;359925] For what it's worth, I found a discussion on incorrect cam timing on the Concours.org forum that is interesting. The symptom was that it didn't rev past about 4000 rpm. Turns out the poster had mistakenly installed the intake cam 3 teeth retarded. No mileage indications but apparently it idled fine and ran fairly well under 4K. Possibly going 2 full teeth retarded would be a significant step toward what we are after. I think the cam sprockets are 38 teeth for 18.95* per tooth or a retard of 37.9* for two teeth. Adding the 37.9* to the stock 54ABDC intake closing results in closing at 91.9 ABDC. Wow, not much compression left there. The Prius retards from 72 to 105 ABDC so this is right in there to get a mild effect. I normally get low to mid 50s mpg so am expecting to see mid to high 50s to consider it a valid improvement. QUOTE] OK, so I retarded the intake cam one tooth as a first test, before trying two teeth. Since compression is reduced when doing this, I increased the ignition advance from my +4* baseline to +6* which was as far as my ignition advancer would go. This to help optimize the combination somewhat. Power was down a little but since it's pretty powerful anyway the effect was not great especially above 3500rpm or so. Throttle response was a little softer under 3500rpm or so. Overall it ran fine and idled a little smoother probably due to a little less overlap. Though smoother, the idle was more erratic, sometimes it would idle fast, other times slow. Also as the weather began to get warm, say over 75*F at very small throttle openings and under about 2500 rpm the engine would eventually load up as if it was extremely rich. It took less than a minute in the low speed/low load conditions to begin to see the loading up. It is likely that reversion is greatest at low rpm causing the rich condition as the carb meters fuel to the same air flow 3 times. Conclusions: Fuel efficiency was the same over two fuel tanks. However, if the rich condition could be controlled by a low speed fuel jet reduction a fuel efficiency improvement would likely be seen. Because most motorcycles are designed with very long duration cams from the manufacturer there is likely little room to retard the cam and maintain accurate carburetion metering. Very short intake runners and no carburetor plenum to share air/fuel mixture between cylinders sends a very strong reversion signal to the carburetor, exacerbating the negative effects of the reversion signal to the carburetor. Electronic Fuel Injection or a carburetor with a large volume between the carburetor and intake valve are possibly the only ways to accurately tune an engine with a significant amount of intake reversion such as with an Atkinson cycle type intake valve timing. As it had pretty similar power from say 3500rpm up, I think that Akinson valve timing has a greater effect at low engine speeds. A Harley Davidson fuel injected 883 with tall 1200 gearing and retarded intake cams might be a great combination. For my (overpowered) bike, possibly the easiest solution would be to make a plenum and connect one or two carburetors to feed the four cylinders, or cobble together a Megasquirt EFI system. No way I could retard two teeth in it's current configuration as the carbs would be completely confused at low speeds. For my purposes, I just sold the bike after returning it to stock. Now bikeless, I'm searching for a great deal on a bike with great potential. __________________ Good design is simple. Getting there isn't.