Topic: Changed the plugs

[medic09]

I changed the plugs on my '78 750K the other day.  I ran D7EAs for the cold weather to improve starting and warm-up.  Worked like a charm.  Bike ran well.  Now that we have warmer weather (highs in 70s, morning temps in 40s) and it will continue to get warmer, I switched back to D8EAs.  My bike now continually backfires when warming up, though it starts easily enough.  On the highway I noticed that it was running about 6500 rpm at 80 mph; whereas with the other plugs (D7) I would be doing 80 mph right around 6000 rpm.  It continued that way even at higher speeds (just for test purposes).

What say you all?

[eurban]

Well you should recheck your intel because changes in plugs cannot affect the RPM to MPH ratios.  Unless (since you changed plugs) you have changed gears, sprocket sizes, tire rolling diameter, speedo, tach or perhaps your clutch is now slipping then the particular RPM that you are running at in particular gear will be exactly the same as before you changed plugs.

[medic09]

Well you should recheck your intel because changes in plugs cannot affect the RPM to MPH ratios.  Unless (since you changed plugs) you have changed gears, sprocket sizes, tire rolling diameter, speedo, tach or perhaps your clutch is now slipping then the particular RPM that you are running at in particular gear will be exactly the same as before you changed plugs.

Well, that's what I thought.  All I did on Friday afternoon was change the plugs.  Kinda strange, eh?

[johnny-from-bel]

You probably pinched the tach or odo cable while changing the plugs

[medic09]

You probably pinched the tach or odo cable while changing the plugs

Well, I went and looked as long as you said that.  I don't see anything other than a worn spot in the cable sleeve.  Seems to me it would be pretty hard to pinch the cable and not notice.  I don't think it likely would stay that way, either.

[brandon]

I don't know about your MPH to RPM problems. With the winter plugs did you tune the A/F ratios to accomidate them? Perhaps all you need to do is fiddle with the A/F ratios with these cooler plugs. Have you pulled the newer plugs to look at them? What do they tell you? Also spark plug gap is a factor to consider. Try gapping the plugs to the largest specification.

[medic09]

Well on the ride home tonight, the MPH/RPM ratio was more what it usually is.  80-85 mph/6000 rpm.  What I did notice with the bike warming up in the dark in front of the ambulance bay is that #2 was running hotter than the other cylinders.  I warm the bike with throttle open for the first few minutes till it's ready.  As it warms it races before I get to releasing the throttle.  I could see that the #2 header was getting a little red, while the others don't (although they are hot).   

I haven't had a chance to pull the plugs.  These are the plugs I ran in the fall last year, until it got cold.  They're gapped about the middle of the range in the book.  I haven't fiddle with the A/F; that may need some attention since I rebuilt the carbs over the winter.

[brandon]

Sounds like #2 is running lean. Check all that stuff out and let us know.