Autonerdz forums

Any thoughts on this in cylinder compression waveform. 500 Single cyl ATV

It's a 2002 Arctic cat 500 Auto quad. The problem is hard starting, and when you get it running it will run for about 5 seconds and then shut off. Similar to if you shut the ignition off. However, spark still remains. It's a carb, and a fuelling problem was ruled out. You can restart it, but it will only run for 1 second or so and then shut off again. It's a bear to restart after that.

In the graph, the higher compression is wide open throttle. The lower humps are with the throttle closed. Cranking speed has been achieved via a remote starter on the end of the crank, as the electric start is trashed on the unit.

There is a decompressor, however with the high cranking speed, it's not a factor.

We checked for a plugged exhaust, however both the muffler and pipe are free flowing.

We also did a leakdown test, which had 20% leakage. Checking through the entire range of the stroke provided the same result. The air filter in the unit was trashed and there was dirt ingested in the intake. It has been cleaned out now.

Valve clearance is in spec. See attached ignition trace. This is idling and then it quits.

Trevor, thanks for looking at this. I am pretty sure that ignition isn't a problem. I think its mechanical based on the compression waveform. I think I need to pull it apart.

James, is this a 2 stroke, thought it was? That said when it is up and running the tower actually does not lean at the top 1/3 of the capture, again have never looked at 2 stroke wave form if again it is 2 stroke. If this is a reed valve, (have no idea) From the top 1/3 down it bulges out, again if coming down off compression what does this mean, vacuum changes. That also stated the base line of PSI drastically change from low RPM to high, again if a two stroke maybe that is the way it looks.

The compression release, how does it work??

Hey Larry, it is a single cycle 4 stroke carbureted engine. The decompressor is automatic and holds the exhaust valve open slightlty during the intake stroke to reduce cranking compression. You can not see it in the waveform as I am turning the motor over with a remote starter and the rpm is too high.

That hitch in the expansion stroke makes me think that the crank slowed down, not that a valve opened.

Considering that anomaly is at 135 psi and is illustrated as horizontal-ish I would take that to mean there was 135 psi for a longer period of time.

What if this engine is binding up?

What I would give to have a 60 tooth crank sensor and tone ring on every engine....

I wonder if we could have used the amperage through the auxiliary starter?

StitchB wrote on Jan 22^nd, 2024 at 3:45pm:


Thank you. I think you're onto something there.

I think all we can do now is sit back and wait for James to give us an update. 

So, for those interested it was a badly worn exhaust rocker and cam lobe. See attached picture. Piston and cylinder are in reasonable shape. Head needs intake valves and the seats need to be cut.

TrevorSchlientz wrote on Jan 23^rd, 2024 at 7:18am:


Trevor, I think you would enjoy it, and I guarantee you they are looking at this stuff with nothing but a scanner, and even that probably not very well. JMO.

Or at least offer to look at one of their problem childs, get a feel. James here is at the top of  his game but didn't come from the auto side so the learning curve is steeper for him. I notice not a lot of feed back so he is shooting in the dark.

I was helping a guy on  2012 BMW GS few months back, 02 sensor, MAP all of it as pretty much a Bosch system, and everything a guessing game and you need a BMW Scanner. But with a scope you are now coming from a different angle. JMO.

Intermittent wrote on Jan 23^rd, 2024 at 10:43am:



+1

fisher wrote on Jan 23^rd, 2024 at 1:37pm:


The upside, for in this case James, he tells the customer going in, we know we have valve train issue, do you want to pull the trigger and move forward? The wear on that cam is massive, a better question is what caused it?

And the capture will just be one more piece of history to use later on. James is a serious player, last I talked had a dyno, trying to raise the ATV world up to another level, hope he gives us an after capture.

Larry

TrevorSchlientz wrote on Jan 24^th, 2024 at 3:50am:


Just to add to that, most people just flat out run the piss out of them along with the neglect.

Thanks for the follow up!!

James H wrote on Jan 24^th, 2024 at 6:51am:


Now that is funny. If a mechanic why are you working on it.

Tight lifters. Back in the day my dad used to say about loud lifter. If you can hear them they aren't burning. A little different today but clearance is  your friend.

TrevorSchlientz wrote on Jan 24^th, 2024 at 3:50am:


Being that tight,would have thought you would have had a no start before we got this far.

Reply #4 says the valve clearance is within specs.

They could have been tight, and as the drive-ability of the machine got worse someone adjusted the valves (damage already done) to try and make it better.

I don't think that kinda wear happened overnight!