Which Spark Plugs?

I recently did a minor tune-up on recently acquired boxer - valve adjustment, air filter, exhaust lug loosening and tightening (with anti-seize), and new spark plugs.

Clymers manual says to use Bosch W7C on my bike - so I bought them; well I bought the NGK equivalent.

A few days later I was flipping through Oak's top end manual for airheads and he say to use W5C (cooler) for bikes in the southern US. Can't get much further south than this old swamp so I dropped by and picked up the NGK version of the W5C.

I haven't put more than 65 miles on these new, cooler plugs but they seem to make a BIG difference - for the better. Bile runs a bit smoother - especially when hot - idles better, pulls away from idle better.

Does it make sense that this difference in plugs would make this difference in performance? I'm pleased with the way this bike is running.

The w5c plug is colder than the w7c and favored for engines operating in a hotter environment, under extended load or rpm. Protection of the plug and piston surface are the end goal. I'm not sure you could readily feel the difference however.

Cooler plugs are probably warranted on the bigger engines because they put out more heat. And bikes with fairings, like your RT, could also trap a bit more heat than a bike with the more exposed jugs.

Likely it doesn't make much difference in the end...I suppose watch the plug and be sure that it doesn't start to clump up with deposits. If it were too hot, the plug would end up being extremely white with fine deposits on it.

I'm sure Oak's discussion also mentioned the proper choice of resistance in the plugs. The general rule is to not have too much resistance in the spark path. Generally plugs should be non-resistor with 5K ohms in the spark plug cap. Likely that is the stock setup for your '83 RT. Early points- models had 1K ohm caps. My '78 /7 has the blocky cap which is 5K ohms...this was the transition period between points and getting ready for the electronic ignition in 1981.

Thanks Kurt.

My 87R80RT has the full fairing on it. I put new 5k spark plug caps & wires from http://www.motoelekt.com/ on it a couple weeks ago.

I'll be checking the plugs in a few weeks to see how they're doing.

Manfred said:

Does it make sense that this difference in plugs would make this difference in performance? I'm pleased with the way this bike is running.

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Yep... A lot of us hot rodders used to carry two sets of plugs, a cooler range for daily driving and a hotter range for the dragstrip.

108625 said:

Yep... A lot of us hot rodders used to carry two sets of plugs, a cooler range for daily driving and a hotter range for the dragstrip.

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I would have thought it would be the other way around...?

keelerb said:

I would have thought it would be the other way around...?

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Nope. Usually, you're leaning out your mixture and running pump mid to high octane for driving. A cooler plug and a little less spark advance helps it run clean and without detonation. At the strip, high octane (or even octane additive), richer mixture, more advance, and hotter plugs. This is old fashioned, pre-engine management computer stuff; but even the high tech guys can set up their rides with different fuel and spark maps to choose from.

A better example might be trail bikes. Altitude is more significant than lattitude; and most must rejet and replug accordingly. Of course, you can go out nowadays and by a fuel injected trail bike, with two or three ignition curves to choose from with the flip of a switch.

Hi Manfred- when you changed the plugs do you recall which ones your bike came with? Mine had NGK BPR6ES from the PO. Went to get new plugs, did my research. NGK Canada site says those are the correct ones, NGK US site said BP6ES. ??? So more research, Snowbum's site explained the difference for me and I got BP6ES. I noticed a similar increase in performance when changing from the incorrect resistor plug to the correct non-resistor.

I don't think they make hot enough plugs for riding in our winter!!

Steve

When I bought the bike it had Bosch W7C plugs. The W7C translated to NGK BP6ES. The W5C translated to NGK BP7ES, which is what I am currently running and loving to death. Certainly winter where I am is less severe than for you.