I have a 1976 R90/6. I bought it with dual-plug conversion, Dyna III, and Dell Orto 34mm PHF34 carbs. It has never really run as well as I would like it to run. I have rebult the carbs and sorted, air-cleaner, valve adjustment, cable slack, and timing. All four plugs are new. It appears to be running rich (high-speed circuit). I looked into the main jets and found they are #150 and the needle is set in the top position (leanest). Is the main jet the issue? Should I drop to 135 or 130? Other thoughts? Thanks so much.
My thoughts...
* To my knowledge, the Dellortos only came on the R90S models. Apparently, someone has "upgraded" trying to get more out of the bike. Where you'd find the correct Main Jets these days is the first question. The only other bike they came on was Moto Guzzi Sports. You might find that Mikuni jets (or some other brand) are the same size/thread, but they would use a completely different numbering system.Â
* Carbs and their fuel mixture are the only tuning bit that is set while running. That means mixture readings are a result of ALL the other tuning adjustments plus the type fuel you use. First you'd want your ignition timing set by stroboscopic light on the "Fast" mark at a very high engine RPM. (It might help to first color-in the mark on the flywheel with some bright paint.)
Additionally, I seem to remember that using the dual-plug system even changed the ignition point. So you may need to add your own timing mark to account for this.Â
* Also remember that a LOT of non-tuning bits can also darken the plugs and appear to be a "rich" condition. Things such as dirty air filters, clogged intake airways, and excessive engine blow-by also darken the plugs.
* The spark plugs themselves can lie. I always tune with NGK because their copper core enables these to cover a very wide heat range (making them easy to read), and their numbering system is easy to understand. The correct main plug for your bike is BP7ES (14mm, protruded tip, heat range 7 and 3/4" long). Some newer NGK charts are showing the 8 as the correct plug, and this is incorrect because it's one heat range too "cold". (A "cold" plug will also show up with a dark color.)
To add to the confusion, this plug is no longer made... however there ARE Chinese knock-offs available on eBay. NGK is no longer making anything except "resistor type" spark plugs, which they denote with an "R" in the code, so now the correct plug is BPR7ES. But using these newer plugs means that you need to use a non-resistor type plug cap. (The OEM equipment was a 5000 Ohm internal resistor type cap.) And these (as always) MUST be coupled with non-resistor plug wires (so called "metal core wires").
This all means that if you have the combination of ANY 2 resistor sources (example: resistor wires + resistor plugs) then that will result in incomplete combustion, which results in (you guessed it...) dark plugs.Â
* And then of course is fuel worth burning. You'll want to fill your tank with the highest octane available from the best suppliers. To do this you'll need to research Top Tier fuels. https://www.toptiergas.com/
* Finally is how the reading is taken. The Main Jet only comes into play when the throttle is 7/8 to Full open. So you'll need a 10 mile stretch of interstate highway that you can ride on for 10 miles without changing the throttle position. That will equate to around 100+ MPH. At the 10 mile mark you'll flip the Kill switch and pull in the clutch lever... and coast to the side of the road. Then you can take a meaningful plug reading. (I have no doubt that many members have ridden "really fast", but I suspect fewer than 10% have actually ridden on the Main Jet.)
Bottom Line: Changing out the main jets is the LAST thing you want to do. You have a full day's work of checking other tuning items, plus several speeding tickets ahead of you.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly - Thanks for the detailed answer. Very helpful. When I roll the throttle to wide open, the motor pulls cleanly all the way to redline. Given that, I don't think I have a main jet sizing issue, right? I am going to take a look at the needle and atomizer. When I rebuilt the carbs, I did notice a little wear on the needles, but I did not really inspect the atomizer itself. For fuel, I am running 91 octane, ethanol-free. For top plugs, I have the traditional non-resistor NGK's (which were a bitch to find two years ago). The bottom plugs are NGK B7HS. The top plugs are Bosch W6DC. All four plug wires are NGK's but I don't know if they are resistor wires like stock on non-resistor. I am trying to identify them further. Oy.
I measured the resistence of my plug wires and confirmed that they are 5000 ohms. So it appears I have the right plugs and wires. Good rule-out. Also, the bike starts up really well. Full choke - hit the starter - fires up in 1-2 seconds. Tells me my Dyna III is putting out a good amount of power.
I measured the resistance of my plug wires and confirmed that they are 5000 ohms. So it appears I have the right plugs and wires. Good rule-out.
The measured resistance of the correct plug wires should be zero, nill, none, nada.
Also, the bike starts up really well. Full choke - hit the starter - fires up in 1-2 seconds. Tells me my Dyna III is putting out a good amount of power.
The question was not, "Does the Dyna have good spark ?" The question is, "When does it spark ?"
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly Yes, I am down to timing and/or worn needle valve or atomizer. Seems to be more of a mid-range fueling problem.
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