Jump to content
Sign in to follow this  

Wasted Spark in XR 400/ XR 80?

Recommended Posts

Back in the dirt after a 30 year absence-God, how dirt bikes have changed (for the better)!

Here's my question:

While doing the 100 RPM idle drop to dial in my XR 400 and my son's XR 80, I attached a tachometer with an induction pickup around the spark plug lead.

To my surprise, the tach seemed to read really high--around 3,000 RPM at idle (but, of course, my ears knew that the bike was around 1,500 RPM).

I assumed at first that the tach was bad, or the induction pickup didn't just like the spark plug lead, but I had another idea a few days later.

I surmised that the RPMs I saw on my tachometer were consistent with a wasted spark ignition system (like my old Harleys) where the plug fires every time the piston is close to TDC. A wasted spark ignition system would account for the roughly 2xs readings on my tach (since the plug fires every time the piston reaches TDC--regardless of whether the piston is on a compression or exhaust stroke. The exhaust stroke spark doesn't ignite anything--so, hence, it is "wasted").

So, what say the experts? Wasted spark, or busted tach?

Last--cool site! I've only had the bikes about a month, but have already learned a ton here.

Share this post


Link to post
Share on other sites

Uh, I dunno the answer to your question.

But welcome here. You sound a lot like me. :cry:

(XR4 after 25 - 30 yrs off bikes. XR80 for the kids - excellent. It just gets better!)

Share this post


Link to post
Share on other sites

If it were reading 1/2 the RPM, I would say it was set for a 2 stroke engine, but since it's apparently doubling, I don't have a clue. Since we know from "valve setting 101", that it shows TDC on the flywheel on both strokes, I would guess your assumption of firing on both strokes is right.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×