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Tool Talk Discussion Forum

carb adjust via exhaust temp

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lloyd wilson

05-08-2007 17:38:28




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I Just bought a type k thermocouple thermometer unit @ harbor fright (closeout price too tempting!). I'm wondering about using it for setting carb mixture on my JD 440 crawler - at maintenance time, not during operation. Anybody have opinions/eperience/parameters about such a process?

thanks in advance

ldw




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Jerry/MT

05-09-2007 20:43:31




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 Re: carb adjust via exhaust temp in reply to lloyd wilson, 05-08-2007 17:38:28  
Unless you have a dyno to run the machine on, how do you know what the optimum mixture ratio as a f(RPM, HP) is? I guess if you do run it on a dyno and then correlate the results to the measured EGT/Tambient you'd have something that was usable.



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Tim Casbolt

05-09-2007 15:49:56




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 Re: carb adjust via exhaust temp in reply to lloyd wilson, 05-08-2007 17:38:28  
My drag car with small block Chevy seems to run best when the egt is running 1300-1400 degrees F. That's on gas. On alcohol it's more like 1200. No idea what it might be for a diesel engine.



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glennster

05-09-2007 12:09:44




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 Re: carb adjust via exhaust temp in reply to lloyd wilson, 05-08-2007 17:38:28  
one of the guys here at the shop runs a blown alcohol drag car, it has a data logger computer engine monitoring system. it has temperature probes in each header pipe in the exhaust. after warming the engine up you look to get even exhaust temps from each cylinder. we looked for 800 degrees typical. rear two were way hot, added two aditional injector lines at the rear of the blower. it would be abut the same with a diesel, check all your temps and then add fuel to your high temp cylnders to bring the temps down, the lean cylinders will always run hot.

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ldj

05-09-2007 13:51:59




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 Re: carb adjust via exhaust temp in reply to glennster, 05-09-2007 12:09:44  
Yes. it is best to have a probe in each cylinder. I have operated many with only one probe. With aircraft engines there is data that tell which cylinder is hottest and to use that if only one probe. Now if something starts going wrong with a cylinder that the probe isn't in you will never know. So a probe for each cylinder is a very good engine monitor. An engine with one probe is just the very basics for setting fuel mixture, in other words better than lean to miss then richen to smooth. I doubt anyone would want to spend the money for probes for each cylinder in a crawler.I don't think you could trust it with the engine constantly changing power.
That is just my opinion and is worth about what he paid for it---$0.00
L.D.

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IH2444

05-09-2007 07:10:41




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 Re: carb adjust via exhaust temp in reply to lloyd wilson, 05-08-2007 17:38:28  
How does one know what exhaust temp to adjust the carb for ?



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Gene Davis (Ga.)

05-08-2007 19:05:52




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 Re: carb adjust via exhaust temp in reply to lloyd wilson, 05-08-2007 17:38:28  
This has long been an accepted practice for tuning by competition go-kart racers. The exhaust temp tells them if they are lean or rich on their mixture. Lean runs hot and rich runs cooler.



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ldj

05-09-2007 11:37:18




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 Re: carb adjust via exhaust temp in reply to Gene Davis (Ga.), 05-08-2007 19:05:52  
Since airplane carbs have a means to adjust mixture in flight (during engine operation) an nothing else I know of other than a model A or T does, that may make a difference. Here is what I would do. Run the engine at the normal rpm you will be using when working it and lean out the main or high speed jet adjustment till you reach max EGT. Then you richen the adjustment till you are about 50 to 75* below max EGT. All the above is assuming you have the probe on the hottest burning cylinder and you have the probe very close, say a couple inches, from where the exhaust gas inters the manifold. The above still may not be right because now you have the engine at the right RPM but under no load.I believe I would forget it and operate it the way all the rest of them in the world are used. If you really are going to try to use the probe I would do as above but I would have doubt about it being correct.
L.D.

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