Topic: Dwell Tach Hookup to SOHC

[Zeke]

Hey Guys:

I just bought a new "Engine Analyzer" from Sears -- it has the 50 rpm resolution that I'm looking for.

I have it hooked up to the coil and it seems to be working properly -- here's the question --

Is my bike's tach so terrible at low RPMs that it will read 1500 rpm when the dwell tach says 700?  Is this common?

I'm inclined to use the dwell tach as the manual says the bike's tach doesn't do well at low RPM but I didn't expect it to be off that much.

Just looking for reassurance that I'm doing the right thing.  With that in mind I might have to start my pilot adjustment again at 1050 RPM because it looks like I did it at too low an RPM.

ZEKE

[Bob Wessner]

I seem to recall that the dwell tach is actually for engines with 4,6 or 8 cylinders and the bike has two sets of points and wasted spark, etc. I think you have to use a different scale or something. Maybe someone can provide better details or an explanation.

[barber1303]

Zeke...I also just picked up a dwell tach, but am having the opposite problem.  The wasted spark results in a reading that is twice the actual speed.  The "low" scale resolution is 50 rpm (perfect!) on my meter, but with the speed doubled I have to switch to "high" scale to get a reading.  The resolution on the "high" is ~ 500rpm, which makes it VERY difficult to to pick up on a 50 rpm (or 100 rpm doubled) speed change.

D'OH!!!

Chris

[Zeke]

Bob and Barber:

Thanks for the replies -- this all makes a lot of sense, and should probably be put into the FAQs.

I had forgotton about the extra spark.  I wonder what dwell tachs are made for this?

With the dwell tach that I bought, my readings are essentially half of the cycle's tach.  So, 700 on the dwell tach was around 1400 on the bikes tach.  This is on the 4 cylinder setting.  I tried the 8 cyl setting but this just made the tach read even lower.

Anyway, this is still a workable arrangement.  Knowing that it's half the number means I need to set it to 525 for idle speed.  My new dwell tach has 20 rpm resolution and if you half that each mark is 10.  Makes it even easier to read 50 rpm.

Interesting that Barber's meter shows higher -- must have different innards.

In fact, the old SUNTUNE meter that I was using this morning read correctly on the high scale, but like my new one read half on the slow scale....

Can any of you old salts confirm that you've experienced this with your SOHC?

Thnaks

Zeke

[barber1303]

Zeke...which type of dwell tach do you have?  Mine is an inductive type that hooks over the #1 spark plug wire.  It registers the current traveling through the wire to determine engine speed.  Thus, two sparks (one every revolution) results in double speed readings.

Chris

[Zeke]

With this one, you hook the clip right to the power wire for the coil.  Same as most dwell tachs I've seen (which isn't very many).

It practice, the inductive type are really nice -- that's how my timing light works, and it's easy to hook up.

Not sure why you and I get the opposite effect here -- obviously they count the sparks per minute to determine the rpm.  I can only say that when the bike tach says 1500 the dwell tach says between 700 and 800 on the low scale.

Anyway, having read the carb adjust FAQ it sounds like you *could* adjust pilot screw by ear -- 50 rpm change is audible.  I just tend to be over-anal about these things, and following the repair manual to a "T" should ensure a quality rebuild, right?

Hmm, too bad it won't tell me what to do when the pilot adjust procedure doesn't work.....

Cheers,

Zeke