Topic: Inductive tachometers and spark plug wires

[ABikeNamedTrouble]

Hey all,

I'm trying to fine-tune a 1980 CB650 Custom and have been having very little luck with tuning the idle screws.  I have an ESI-325 inductive tachometer (http://www.automotivetoolsonline.com/Electronic-Specialties-ESI-325-Cordless-Inductive-Tachometer-ESI-325_p_10700.html).

The problem is that the reading jump around everywhere, and I'm wondering if it's because I have sub-par spark plug wires.  They're the ones that were on the bike when I got it, so I have no idea if they're OEM or something else.

Could improper shielding cause so much interference between the wires that I can't get good readings on my tach?

- Adam.

[BobbyR]

I obseved that years ago when working on my Wife to be Camaro. I noticed the Tach jusmping around on certain wires. When I pulled the plugs attached to those wires, it was clear they were misfiring.  You have stumbled onto what sounds like the same situation. Now you have to figure what part is failing.

[ABikeNamedTrouble]

The thing is, the engine generally runs fine.  The only thing(s) bothering me is that it runs rough and backfires when engine breaking, doesn't seem to have adequate power at low throttle settings, and is getting ~40 mpg.  I figure all of these things could be the result of improperly tuned idle circuits.

[BobbyR]

OK, use your tach on your car and see what how your tach reacts. An engine that:

The only thing(s) bothering me is that it runs rough and backfires when engine breaking, doesn't seem to have adequate power at low throttle settings, and is getting ~40 mpg.  I figure all of these things could be the result of improperly tuned idle circuits.

can hardly be described as running fine. I would suggest your check your plugs, maybe clip a mm off the wire and rescrew the caps into clean copper.  It could be more than one thing. It could be a crappy carb too. Old bikes need TLC early on. 

[ABikeNamedTrouble]

Indeed, I should have specified that it only backfires when engine breaking (my wording was ambiguous).  Either way, you're right, there's definitely something amiss.

I did use the tach on my car.  It gave a slightly more steady reading, but still not accurate enough for fine tuning.  At this point I think I may return it and get a better ones that clips onto my coils.

I'm also thinking it's probably time to just replace the plug wires.  I ghetto-patched one of them with electrical tape during the rebuild anyway, so it's probably wise.

[BobbyR]

Indeed, I should have specified that it only backfires when engine breaking (my wording was ambiguous).  Either way, you're right, there's definitely something amiss.

I did use the tach on my car.  It gave a slightly more steady reading, but still not accurate enough for fine tuning.  At this point I think I may return it and get a better ones that clips onto my coils.

I'm also thinking it's probably time to just replace the plug wires.  I ghetto-patched one of them with electrical tape during the rebuild anyway, so it's probably wise.

Yeah, electrical tape and high voltage don;t mix. I had a back firing also during engine braking. It turned out to be a low float bowl level.

[ABikeNamedTrouble]

Well, the tape isn't contacting the wires themselves, just holding the wire to the plug cap.  I ordered new wires and we'll see what happens.

My float heights are set correctly, but I'm not positive one is moving freely as it should.  I'm going to do some work within the next month when it needs an oil change, so I'll check on it then.