Originally when installed the NGK DCPR8E plugs I ran them a the NGK recommended plug gap of .040. The bike ran like shit from around 2800 to 3500 RPM @ 2% to 10% throttle position. There would be an intermittent miss fire or stumble. I tried to adjust the fuel mixture in that range and tried looking for any vacuum leaks etc. Nothing worked.
What I did is pulled the plugs back out and tried re-gaping them to .034 and let me tell you the bike runs great! I used to gap at the HD recommendation of .035 running the HD plugs. There is big improvement on the throttle response, power and no more miss fire.
From all the research I have done is that a smaller gap is works better in high compression motors. The Higher The Compression = A smaller gap. Some believe that larger gaps burn more fuel. Maybe you can compensate for running a wider gap if you have an high power ignition system that can produce more than 60,000 volts.
You would think. I was under the impression that NGK is a lower resistance plug than the HD plugs, and this is why they call for a .040 gap instead of the HD .035 gap. My understanding is that the Anti-Knock circuit is affected by the plug resistance.
After all the trail and error I have found that NGK plugs run better in my bike with a tighter gap. I have 30,000 miles on it now and I think it is running better than ever now. However I did not have the miss-fire before when I ran the Stock HD Champion Plugs.
Maybe next time I might try setting the gap at .033.
...I was under the impression that NGK is a lower resistance plug than the HD plugs, and this is why they call for a .040 gap instead of the HD .035 gap. My understanding is that the Anti-Knock circuit is affected by the plug resistance. ...
The ion sense circuit biases the gap with about 80 VDC and reads the current flow across the gap after each spark event to check for knock. Spark gap is critical for both the spark voltage and the measured ion sense bias current. The spark plug resistance has very little effect on the required gap and the gap shouldn't be messed with if you want your engine to run right. The only thing that might require changing the gap would be if the compression ratio is changed or a turbo is added.
Originally Posted by gator582
For what bike? NEVER trust that any plug is gapped properly!
(Edit: This is an exception that probes the rule, "never say never.")
I agree. Plug gap for a normally aspirated VRSC engine should be set to 0.035" (0.89 mm) and every plug should be checked before installing.
The H-D plugs, as mentioned are nothing more than re-labeled Champions. They are not pre-gapped. Most the time when I've been at my favorite H-D shop, the parts guy will gap the plugs for the customer before he sells them. Unless that's being done in front of you, always gap them yourself with a quality (wire-type) gapping tool. The "ramped disc" type that you see at the auto parts stores near the cash register are horrifically inaccurate. I've checked a few of them with a micrometer, and the readings at a certain indicated gap have varied +/- 0.005".