Getting Serious, Getting Expensive

One of the frustrations from the race July 9th was our inexperience at keeping the engine at peak performance in the 100 degree temperatures. For the upcoming weekend Weather.com was forecasting cooler temps but very similar conditions for both Saturday and Sunday. In essence, Saturday's weather looked to be a preview of Sunday's and that being the case, we could see what it would take to dial in the kart on Saturday and just repeat it on Sunday. We decided to do a real test session on Saturday, concentrating on air/fuel mixture setting and adjustment and then, if time permitted, test different tire pressures to fine tune handling.

According to Briggs & Stratton, its 4 cycle racing engine makes the most power when the head temperature (taken at the base of the spark plug) is between 390 and 410 degrees Fahrenheit. The main mixture jet is selected and installed in the carburetor before the race. Many things impact the selection of the correct jet including barometric pressure, humidity and temperature but, generally, ambient temperature is the primary factor in the selection process. Fine tuning of the air/fuel mixture, however, is done out on the track and one of the skills a Briggs kart driver has to develop is the ability to make adjustments to the mixture needle while at speed.

For most drivers this involves watching the temperature gauge and reaching back to the carb to lessen the fuel if the engine is running below 390 (too much cold methanol in the cylinder) or increase fuel if the engine is running above 410. And, yes, this needs to be done while at speed, preferably on the straights. There are remote devices, a knob connected to the mixture screw by way of a cable which would likely make the maneuver easier but, funds are getting short and we're getting to the point where we really should be finding ways to reduce the weight of the kart (we're still 14 lbs over the minimum) rather than add to it - even if it is only a pound - so we likely won't be going that route. Besides, just reaching up to the dash to make a slight adjustment is too easy. Real karters reach back over the 400 degree engine to adjust the carb.

The Ghost Of Last Sunday

It would be just Tracy and I this day, Dana was home recovering from having her wisdon teeth removed. But before we left for Circleville we discovered we had developed an oil leak. The drain plug had loosened and immediately I had my concerns. We had changed the oil between the second practice session and the first races on the 9th. That was the last time the drain plug had been out and it was very possible that Tracy or I had been distracted by some other task before we had tightened the plug completely. But whatever the circumstances, I should have verified it's security and I can't say that I did. And this is unpardonable.

Our between-session checklists showed that we had dip-sticked the engine religiously between each race and before we began practice lapping with Dana and Tracy. After that though, we have no record of another check.

When we drained the oil Saturday there was still about 5 ounces of the full 13 ounces left so it was some relief to know we hadn't run it dry. But, how much of the 5 ounces was being picked up and splashed around the crankcase was a worry.

Mysteries Revealed

The test to get more familiar with the mixture settings was very successful. As our times show, we found almost a second by the second session. In fact, the quick lap in the second session equaled to the hundredth of a second my previous personal best time. I couldn't have found that other .01 so that it would be a new personal best, huh? On, no, it was exact to the hundredth.

We learned a lot about what settings to start with on an 85 degree day and I learned an enormous amount about how quickly the engine heats up and how much of an adjustment makes a difference. The engine continues to heat up throughout the session and an adjustment may be needed sometimes every lap and by the fourth session I had done a pretty good job of keeping it dialed in.

Mysteries Remain

In sessions 3 and 4 we played with the tire pressures a bit but found no noticable difference. We only tested the effects of raising all the tires' pressures to match the stickiness of the track. Unlike almost all other forms of racing, more stick is not necessarily desired in low powered kart classes. Often too much stick results in a costly loss of momentum and some sliding will be found to be quicker. When the tires are sticking too well, more pressure will reduce the "stick" by reducing the area of the the contact patch, the part of the tire that actually touches the road.

Given more time it would have been nice to try different pressures between front and rear tires to see if a better balance is available but the four sessions I had run were taking their toll on my arms and, more specifically, the tendonitous in my right arm. I'd had enough.

The Other Shoe Drops

Tracy suited up and set out looking to improve her personal best knowing that the kart was set up pretty well after our test sessions. She was quick right away and by the second lap she was within .09 of her best ever lap. And considering her tendency to chip a few tenths off on each lap, it looked like a new personal best was a sure thing. However, her times began to fall away and suddenly she was rolling to a stop at turn Seven.

Something told me I wouldn't be able to go out there and simply restart it and drive it back so I got the track's golf cart, the resident "tow truck" and set out after her. When I got there I put the starter on the engine just to see if my worst fears were fact and they were. There was absolutely no compression. We threw it on the golf cart and toted it back to the trailer.

Poor Tracy - she was worried that it was something she had done to cause it to break. In racing, especially in racing where the machine is shared between several drivers - endurance racing for example - the driver at the wheel when a mechanical failure happens always seems to feel a little responsible. Regardless of how removed the failure is from the driver's actions. Tracy said she had just gotten back on the power coming out of the turn when it "popped" and the engine shut off. But there was nothing she could have done to cause it to happen. This was most likely the result of our oversight last Sunday. My initial diagnosis was a broken connecting rod and subsequent inspection proved this to be accurate.

And as bad as this is on our budget, it could have been much worse. Very often when one of these Briggs engines snaps a conrod the pieces get punched right through the engine block itself, requiring an expensive repair or, in a worst case, scrapping the block all together. Or, those pieces collide with the camshaft with enough force to damage the carrier boss at one of the ends. It looks like neither of these awful things happened likely because the engine was only turning about 3800-4000 rpm when it let go. Had the failure occured at 6300 it would have likely done much more damage. In addition to connecting rod, we have, it appears, lost the crankshaft and possibly the piston. I think the light scoring on the cylinder walls can be honed out.

The other bit of good fortune in this unfortunate episode is that it failed on a test day and not race day. With the connecting rod destined to fail at some point it was good that it failed on Saturday giving us time to put the spare engine in that evening and still be race-ready Sunday morning. I was looking forward to prepping for Sunday's race by relaxing Saturday night after our test session, not even unpacking. But as it was, Tracy and I got the spare engine in and everything packed back up in a few hours.
 

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