Before You Put on Your Helmet
No matter how much biker gear you have, you will never need it if your bike doesn’t do one
Now, there’s a lot of problems that can cause your old big twin to not roll over and catch,
but a LOT of motors have been condemned to a rebuild of a bunch of needless work
because of a “hard to start” complaint. We’re not even going to get into that here, because
that’s way too big to handle.
Instead, let’s talk about one of those old standbys for mechanics – the compression test. A
lot of folks think this is some kind of voodoo, but it really isn’t. The problem is, all too often,
the new generation of parts changers don’t know how to use the data the test gives them –
especially on older motors that aren’t computer-controlled.
Now, this article isn’t going to teach you how to do it. There are all kinds of online
resources and videos about that, but the real world is this – if you have a shop do the test,
you need to ask a few questions and realize a few things…
First thing? Ask them to clarify whether the compression test was “wet” or “dry.” This is
where a sharp tech can differentiate between a ring problem and a valve problem. If they
look puzzled when you ask, that’s a hint… take your bike and your business elsewhere.
Next, understand the numbers they gave you – you get one number per cylinder, and most
healthy Harleys should show numbers well north of 160 psi of compression in each
cylinder. That number, though, doesn’t tell the whole story. How big is the difference
between the two numbers?
Let’s say you have 138# and 142#. Honestly, that engine has some wear, but there’s really
no reason – in my humble opinion – to tear into that motor. I know of at least two
Ironheads that are still running with less than 120 psi. They’re old, but they still run fine
for what they are.
The real thing to look for is the spread of the numbers – more than 10% between the
cylinders is a good reason to start to think about rebuilding, but – and this is a BIG “but” –
engines that have been sitting awhile often need to simply be run and warmed up and have
a few gallons of fuel burned through them and “magically,” compression comes up and
For those of you with “new-ish” motors and relatively low miles, honestly, compression
testing is usually another hint the shop is grasping at straws with your money. I DO like to
do the tests on used bikes, because it helps me to understand the relative health of the
engine, but those numbers don’t exist in a vacuum.
A big challenge seen over and over again with some engines is if they have automatic
compression releases in them. These are often machined into high performance builds and
– if you don’t know they’re there (or the tech doesn’t) they can really screw up the
numbers. They’ve been around in a lot of forms for a lot of years, going all the way back to
when you still had to kick the engine over, but they can give you grey hair if you don’t know
No, really, in one case, the difference in readings between the ACR released and unreleased
was 80 psi!
Nobody likes to not have their bike running, but understanding what your mechanic is
talking about and being able to feel confident in what they are telling you is always the
smart thing to do.
Have you got any time-tested tools for diagnosing “no-starts”? If so, share them in our
biker community on Facebook – who knows who you might be helping!