Cylinder Temps

[jep_800]

I was asked by a member to do a short write up of a trick I've learned to monitor my engine temperature while trail riding. A couple years ago, I experienced a dropped cylinder on my JD 800 while doing a vintage run in northern MN. For reference, the 800 has the Kioritz CCW 440/22 Reed Valve engine. Mine has the updated CDI ignition instead of points. The cylinder in question was the PTO side (#2 cylinder), which is the one farthest from the fan tower. The temps running that day were in the low 30's... so it was warm, but still should not have been cause for concern. There was a stiff 30 mph wind out of the south that day, and while we ditch banged to the next town 12 miles away, the engine was receiving a ton of air, but hardly any on the way back other than what the fan was pushing. My left foot was getting very hot on the way back, which should have been my first clue genius. It finally dropped the cylinder about 3 miles from where we started that day.

After I got it home to inspect it, the plug looked very lean and hot (I did a check earlier that day on my mag side, which I should have checked both. Mag looked rich so I thought I was good). After things were pulled apart it domed the top of the piston and the edge cratered by the exhaust port... Done. No other significant damage to anything, so the top end required a rebuild, which happy to say runs good now.

But, the lean condition has to be caused by something. So what was it? I checked out my needles, and I was 1 1/8 out on my low speed, and 1 1/4 out on the high speed. Shouldn't have been running lean on carburetor settings. So then I started to look for air leaks. The crank seals were installed in 2014, and were still in great shape with no leaks. Then I checked my intake manifold. When I restored the machine, I had insulated the heat shield and the small heat shield that goes over the opening where the carb is. Under one side of my gasket, a small edge of that silver insulation was wedged in there when I bolted it together. Culprit found. This caused additional air to get in through the intake and leaned the mixture. I re-sealed my intake, and while I was at it, I re-sealed the exhaust side just to make sure I didn't have any manifold leaks. So far, so good.

[jep_800]

In an effort to be more aware of my engine temps, I took advice from a buddy of mine who is a dirt bike gear head. He said he always put a temp sensor on his bike so he could see how hot he was running while riding. So I purchased a Trail Tech Temp sensor designed to monitor the internal cylinder temps of the engine. They are fairly simple in design, as it is just a probe that fits over the threads of the spark plug. When installed it takes the place of the crush washer that is found on the plug, so you have to remove that washer to use the sensor. This might not be 100% accurate, but it gives a rider a good idea of where you're at. The only other way, and probably more accurate, is to install an EGT sensor which goes on your exhaust side. These are more elaborate to install, but probably liked by race guys who are in-tune as possible to their engines. However, I've found that the TTO sensors are fairly accurate as it matches my temperature gauge at my house fairly well. And I have two of them which are installed in both my 800 and JDX8. They are within 1 degree of each other.

[jep_800]

You can see I have it mounted on the left side of my dash, directly under the tach. I chose this spot, as it doesn't interfere with anything else, and I have the probe on Cylinder #2. This one should always run the hottest since it is farthest away from the fan. If you're fine on that cylinder, chances are your Mag side is also fine. So why is there two readings? Cool thing about these sensors, it will record the highest temp that cylinder has ever reached, which flashes back and forth from current temp to highest temp about every 10 seconds. You can see inside my garage, it was -2 when I took these photos. The highest temp was 466. This was recorded at St. Germain last year running across Star Lake while it was about 40 degrees outside. Not a good day to ride, but we did it anyway.

So how hot is too hot? I've tried to research this a lot, and read a lot of forums about sled temps on various sites. From what I can gather, the consensus is if your cylinder temp rises above 550 degrees (Farenheit), you are at melt down temp. Maybe this varies depending on cylinder type, brand, etc, but most say that you need to stay under that to prevent a melt down.

Here is a few shots of what the probe looks like removed and installed. I've found it works best to thread it on the plug a couple of turns, then put the plug into your cylinder head and begin threading slowly. You have to make sure it starts well so it can thread into the head, but also guide your probe up the plug so it doesn't bind or something. If it does, start over. Also, look at the last photo. Just from a couple of seconds of handling with a bare hand, the temp rose to 1 above zero. I really like these sensors, and it gives me good piece of mind so I don't run into a disaster on the trail.

[farleyjd400]

Nice write up, and great idea.

[Steven Rempel]

Thanks for taking the time to share this. I love this Idea and to see it in action on our type of sleds is helpful in my decision to buy. I can't wait to get mine in the mail, hopefully in time to install them for the woodridge vintage ride.

[Matt]

Thanks, I've been thinking that I need something like this for drag racing. I like that it will store the max so I don't have to remember to look at the end of the track.

[Curlymurt]

I love this. I've always wondered about RPM and engine temp on mine, so this sounds like an easy and affordable way to measure the temp. I found a similar unit for the RPM but had no clue for temp. Kudos for sharing

[Matt]

I love this. I've always wondered about RPM and engine temp on mine, so this sounds like an easy and affordable way to measure the temp. I found a similar unit for the RPM but had no clue for temp. Kudos for sharing

There is one on Amazon that does both RPM and temp too.