Sometimes you can take a punt....

Sometimes, a bet or a punt can pay off.

We did a pre-purchase inspection on a boat for a customer a couple of weeks ago that was very well priced. He was super keen to buy it as it met his needs for size and price. It was in average condition, not bad, but a little untidy.

When we ran the motor, a mid 90's V4 two stroke, we noticed that it had a brand new VRO (oil and fuel pump) fitted. It also idled really high, at 1200-1300rpm. That's not super unusual, but not ideal as its very hard on the gearbox when shifting.

We performed a compression test and the results weren't good. 100psi on three cylinders and only 55psi on 1 cylinder. Normally, this sort of compression result is the sign of a very sick motor and my first reaction was to inform the buyer and seller, who were both there, the bad news. The seller genuinely didn't know and the buyer was devastated, he really wanted the boat. Very low, uneven compression like this is usually a sign that the engine has cylinder bore damage, from a low oil or overheat event.

I decided to run the motor again for an extended period and let it warm up and checked each cylinder temp with my heat gun. All the cylinder temps were quite low. The spark plugs were hammered with carbon also. My brain then started to tick over, putting the new VRO pump, fouled plugs, easy cranking and low cylinder temps together and giving me another possibility to consider.....the motor wasn't damaged, it just had carbon seized piston rings.....

This can happened when the VRO pump has been failing for an extended period and has been over oiling the motor. The effective oil/fuel ratio just gets lower and lower and sometimes can be as low as 10:1. The motor will still run, but it will foul spark plugs, smoke like its on fire and use a ton of oil. The owner of the boat had these exact symptoms, hence why he replaced the VRO pump.

When the motor runs for an extended period with over oiling, the carbon from the incomplete combustion can build up in the ring grooves on the pistons, preventing them from the correct tension on the cylinder walls. This results in lower cranking compression. It also contributes to a rough idle, which someone had overcome by winding up the idle RPM. The lack of cylinder heat was a pointer to stuck rings, as a damaged cylinder bore and piston will create more friction and heat which will show on the heat gun. The sprk plugs are also a pointer to the carbon build up, as they were clogged with black gunk.

I had a chat to the seller and the buyer and gave them all the possible reasons for the compression issues and left them to work out a deal. Speaking the the buyer later, I told him that if he too a punt on the motor being carboned up and not damaged, I would come see him in a couple of weeks after he had used the boat and do another compression test to see if anything had changed.

He did buy the boat and did 3.5hrs of running in the two weeks before I visited him yesterday. He had run the boat up to WOT and had done some extended running at 3500-4000rpm as instructed. I compression tested the motor again and compression had improved, gaining around 5psi on 3 cylinders and around 10psi on the worst cylinder. This is a sign that some hard running was clearing up the carbon build up. If the motor had cylinder damage, it would have stayed the same or gotten worse after further use. The buyer was pleased with the results and now feels confident the motor wont let him down. Further use should see the carbon build up get slowly depleted even more.

We are glad that buyer is happy with his purchase and that the motor should give him years more service before it needs to be replaced. We were happy to provide the follow up check to see if the hunch we had was correct.