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Onan: JB 7.5 Exhaust Gas Temp

b1miller

Subscriber
Last Subscription Date
07/23/2019
I installed a type K thermocouple on my JB and ran a few tests yesterday. Running on propane with no load I measured 1247F and with a 2.5 kW load I measured about 1222F or so. So that seem to be a reasonable exhaust temp. I am a little concerned about the muffler temp so may be looking at adding a guard or insulation.
 

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lokay5

Registered
Last Subscription Date
01/11/2015
I installed a type K thermocouple on my JB and ran a few tests yesterday. Running on propane with no load I measured 1247F and with a 2.5 kW load I measured about 1222F or so. So that seem to be a reasonable exhaust temp. I am a little concerned about the muffler temp so may be looking at adding a guard or insulation.
My NH on propane, at 1/2 load is 725-750F depending on ambient temp.
 

Charley K

Subscriber
Last Subscription Date
10/09/2019
I installed a type K thermocouple on my JB and ran a few tests yesterday. Running on propane with no load I measured 1247F and with a 2.5 kW load I measured about 1222F or so. So that seem to be a reasonable exhaust temp. I am a little concerned about the muffler temp so may be looking at adding a guard or insulation.
Very nice high dollar meter!
 

Ray Lynch

Subscriber
Last Subscription Date
08/12/2019
I installed a type K thermocouple on my JB and ran a few tests yesterday. Running on propane with no load I measured 1247F and with a 2.5 kW load I measured about 1222F or so. So that seem to be a reasonable exhaust temp. I am a little concerned about the muffler temp so may be looking at adding a guard or insulation.
Is your set trifuel so you could repeat the same test on gasoline? Curious about what the results would be vs propane.
Ray
 

YellowLister

Subscriber
Last Subscription Date
o9/30/2019
That seems very high.. what was the cylinder head temps?

My cck and jb head temps were under 550.. exhaust elbos under 750 running on gasoline with a 3500 watt load after an hour.

I suspect it would run hotter on gaseous fuel but not that hot?
 

b1miller

Subscriber
Last Subscription Date
07/23/2019
I reread my initial post and I mis-typed. I am actually not sure if the temps I was reading are in fact too high. I did notice that the EGT went down slightly when I added the 2500W load. But still is 1247 degree F too high? I did read an earlier post on EGT for a JC that if the propane mixture was leaned out the temp went down.
I installed a type K thermocouple into the exhaust elbow so it should be a valid reading. I have not made any other readings but
I can attach a thermocouple onto the cylinder head for another data point.

And yes I do love my Fluke 87V meter.
 

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VelocityDave

Subscriber
Last Subscription Date
06/23/2019
I agree with YellowLister. Aluminum melts at that temperature which your head and pistons are made of. I know Fluke meters are top quality but is the thermocouple correct for the meter? What little experience I have with thermocouple placement in exhaust it is within a inch or so of the head. Farther away would lower the temp I think. Fuel mixture controls exhaust temp but also improper (retarded) ign timing raises ex temperature.

David

9F2C4757-2395-4A0A-8C8E-FFE2DE585DF3.jpeg
 

lokay5

Registered
Last Subscription Date
01/11/2015
Time to do some more research and see if running too rich on the propane. I will check the ignition timing also. I can do a conversion back to gasoline for a comparison. It's not a tri fuel carb so all the parts are on hand to switch back to gasoline.
If it's a gasoline carburetor, how has it been converted to propane?
 

Jim McIntyre

Subscriber
Last Subscription Date
07/10/2019
Time to do some more research and see if running too rich on the propane...
Yeah - Maybe some experiments are in order. Measure EGT as you vary the mixture screw. Moreover, how do you know the mixture was set properly to begin with?

So yeah - repeat the measurement. Then open the mixture screw maybe 1/2 turn or so and see what happens. The close it and see what happens...
 

Leon N.

Registered
Yeah - Maybe some experiments are in order. Measure EGT as you vary the mixture screw. Moreover, how do you know the mixture was set properly to begin with?

So yeah - repeat the measurement. Then open the mixture screw maybe 1/2 turn or so and see what happens. The close it and see what happens...

Good Morning Mr. MIller

May I ask why you are so concerned with the JB muffler temperature? Is it because of where it is located? Generally, a hot muffler is preferred to minimize corrosion due to condensation and or weather. Your JB performance should not be dictated by the muffler or exhaust temperature. It is what it is. I trust you know this, but are just fiddling around with the carburetor mixture control to understand the relationships between mixture settings and exhaust temperatures. Very interesting, however.
 

Zephyr7

Registered
I know Fluke meters are top quality but is the thermocouple correct for the meter? What little experience I have with thermocouple placement in exhaust it is within a inch or so of the head. Farther away would lower the temp I think. Fuel mixture controls exhaust temp but also improper (retarded) ign timing raises ex temperature.
Thermocouples of the type commonly used with these meters are VERY sensitive to any corrosion on the contacts, and you also have to be careful that you use all the correct connectors and wire since any metal-to-metal interfaces the “wrong way” can introduce BIG errors. The thermocouple itself produces only a very small voltage.

I agree too, those exhaust temperatures seem pretty high. The engine datasheets I’m used to seeing don’t usually show exhaust temperature much over 1000F or so. Note that the exhaust itself will be hotter than the cylinder head (and other engine parts), since both coolant AND OIL act to carry away excess heat and help to limit the temperature of the engine parts themselves.

Bill
 

RETCPO

Subscriber
Last Subscription Date
02/29/2020
While racing karts (KT-100 Yamaha) we would tune for maximum EGT. Instrumentation used AIM MYCRON 3
Maximum EGT will very depending upon air quality and load (length of time at WOT) another factor that will effect the recorded temperature will be the distance from the cylinder to the probe as well as how well the probe is centered in the exhaust flow.
Temperature range will very from 1100-1250 degrees F
This information is from two cycle gas and oil mixed.
 

VelocityDave

Subscriber
Last Subscription Date
06/23/2019
In the late 70s (1974-1978 I raced karts and worked for Competition Karting in NC. First used Mac 91s then changed to BM 8.2 rotary valve engines. Ran B limited class. Exhaust temps ran between 1050 and about 1100 with careful carb adjustment. This is at wot. Leaner carb higher exhaust temp = more power. Anything much over 1100 you were taking a huge chance of sticking the engine. Sticking a engine was melting the piston and it seizing to the cylinder wall. Nothing worse than having a high dollar engine go from 15 or 16,000 rpm to dead stop in the snap of a finger. I ran head and exhaust gauges and ideal for me was 325-340 head and 1050-1075 exhaust. This running methanol nitro mix. 2 cycle

David
 

b1miller

Subscriber
Last Subscription Date
07/23/2019
Thanks to all for the replies and suggestions. So I do plan to verify the thermocouple is a type K. I have another known good quality type K that I will swap and repeat the testing. If I still see the same high readings then I will look at propane mixture. This JB had a propane conversion added to it before I purchased it. See picture attached. I also have all the parts to switch over to gasoline. The high temperature on the muffler exterior is only a safety concern. So will try to get more testing done during the next week and report back.
 

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RETCPO

Subscriber
Last Subscription Date
02/29/2020
I ran head and exhaust gauges and ideal for me was 325-340 head and 1050-1075 exhaust. This running methanol nitro mix. 2 cycle
Dave I agree with those lower temperatures with the fuel you identified in my case CAM-2 / oil (racing gasoline 110 octane)

Keith
 
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