Tach reading tool

Okay, I have a tach issue (read under "What's the dumpest thing you have done to your bike") and I am trying to verify the tach reading. (I think/am sure its reading low.) I borrowed an automotive "multimeter" from a friend and it has an inductive tach reader that clamps around a spark plug and another that reads the terminals on the coil. So I hook up the inductive and it is bobbing around 1900-2100 at idle. My friend and I fiddle around with it awhile and it seems to be reading 2x actual RPMs. It has a 4stroke and 2stroke setting and is set to 4stroke. We then hook it up to his bike and it matches up pretty well with his bikes tach, but seems to be bouncing around a good bit. We then try the direct connect to coil method and it seems to measure 2x rpms also.

I know my bike has two coils each feeding two cylinders. (He has two coils each feeding two plugs on a single cylinder (as in two spark plugs in one cylinder of a v-twin kawasaki). Am I correct in concluding that my coils send a spark to both cylinders each time either one needs a spark (which would give it a spark near the end of compression and again near the end of exhaust strokes). This would account for the 2x RPM as it would be registering 2 sparks where a "normal" engine would register 1.

If this is the case, how can I get a true tach reading (electrical) to verify the mechanical tach. What settings might I use and should the tach number be pretty steady with one of these or should it jump all over the place?

The meter has two methods, induction with a choice of 4 or 2 stroke, and direct with a choice of 4 or two stroke and a choice in number of cylinders (1, 2, 3, 4, 5, 6, 8...yes 1-8 with 7 missing). We used 4 stroke 4 cyl when we got the 2x reading. I'm thinking 4 stroke 2 cyl might give me the 1x reading, then all I will need to work on is the bouncing thing.

Running the meter on its internal batteries.

Thanks for the help,

Ken

 

On the inductive setting it shouldn't matter what you set the cylinder number to, that's only used if you are reading the signal to the coil on a distributer type ignition.

You are correct that each cylinder fires at the top of the compression stroke and again at the top of the exhaust stroke. It's called wasted spark and is common in motorcycles.

A 2 stroke engine also fires at the top on every rev, so setting the meter for 2 stroke and using the inductive pickup should give you the correct RPM.

 

Thanks MiCarl.

Any idea about the bounciness of the reading (the motor is not really bouncing like that). Seems to change the reading once a second and it bounces around a lot (too much to be usefull for anything other than guess work as to actuall RPMs). If this is the same way that an electronic tach works, do they bounce around alot also?

Could the bounciness be indicative of an issue with the bikes spark system? (Weak spark, eratic spark, etc.). It runs and starts fine and seems fine, but could this be something that I might not notice yet...

 

I'm not clear which is bouncing, multimeter or motorcycle tachometer.

The motorcycle tach does not tend to bounce around, quite a bit of inertia to the needle.

If the digital multimeter samples fast it could appear to bounce. Also, how accurate is the meter? If you translate your numbers, your reading comes out to 950-1050. That's 5% error on a 1000rpm engine. I doubt the meter is more accurate than that.

 

it's more the fact that the inductive pickup was really designed to work better on auto ignition systems with a bigger charge coming down the wire. Take a wooden clothespin, replace its metal spring with a rubber band, and use it to "pin" the inductive pickup in place on the wire so it can't physically dance around and see what you get. Or put a big rubber band on the end of your inductive clamp such that it forces the wire tightly against the inside of the clamp. I did that with the timing light I use on the Norton, and it helped.

Electronic tachs are usually rock-steady compared to a mechanical unit.

 

MiCarl,

The meter is the one bouncing. I'll try the other settings and the trick from bigfitz and see if that works. The bikes mechanical tach is rock steady, but reading low. The original question I was trying to answer was how low and how consistantly low. I can go with its 500 low across the board or something like that if I can get the electronic to work.

According to the user manual for the tool it says using induction method it will do 30 - 9000 RPM in 1 rpm increments with a +- 2 RPM accuracy.

Direct connect to primary side of coil is 60 - 12,000 RPM with the same increment and accuracy.

That sounds like it should be pretty steady. As does bigfitz's last comment.

 

Yama Mechanical tachs are actually reasonably accurate Same Cannot be said about Yama Speedos though.
Lube the Tach, lube the cable and call it a day.
With that engine yer gonna run out of Pwer curve long before hitting redline. Tach is basically entertainment value.

 

Yea, Danilo, I had a good functioning tach (seemed to read correct, etc). However, I opened it up to clean it up and refresh it and since then it has not been acting like a good yamaha tach. (My speedo however is now dead on with the GPS and a local radar station). New cable and lubed before insertion. I FUBARd something in there and have had it open twice now. Might be time for a new one, but was avoiding that...

 

are your carbs equal??
may cause bounce