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Discussion Starter · #1 ·
How can I check engine temp cheaply? Can I do it with an oven thermometer if I hold it to the cylinder heads or something?

And how hot should these engines be after driving for 20 or 30 minutes? And does the engine size matter or should all air cooled engines run pretty close to the same temp?

Stock Honda 350 temp?
Stock Honda 360 temp?
Stock Honda 400f temp?
Stock Honda 450 temp?
Stock Honda 750 temp?

I suspect my 750 and 360 are running too hot but my plugs seem to look okay.
 

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mitchellsk said:
How can I check engine temp cheaply? Can I do it with an oven thermometer if I hold it to the cylinder heads or something?

And how hot should these engines be after driving for 20 or 30 minutes? And does the engine size matter or should all air cooled engines run pretty close to the same temp?

I suspect my 750 and 360 are running too hot but my plugs seem to look okay.
What makes you think it's running hot and during what phase of operation are you seeing the symptoms (Idle, full power, cruising, etc)? Assuming you have stock oil flow the only things I can think of that will cause an air-cooled motor to run hot are, excessively lean fuel mixture or excessive timing. You said the plugs looked good so that eliminates a lean mixture. On Honda twins, it's been my limited experience that with excessive timing the bike will not run well enough to get overly hot.

All this get me back to my question of, what symptoms cause you to think it's running hot?
 

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I've got one of those infared thermometers about $25 aimed at the head close to sparkplug or exhaust valves ports (seems to be hottest part of the engines) a air cooled engine will normally run somewhere around 225 to 255 ± 10 on twins (after running on the highway) not sure on fours but wouldn't expect much difference
 

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Checking the temp of an air cooled cylinder at idle doesn't do much for determining an over heating condition.

On air cooled 4 and 6 cylinder aviation engines we use a thermocouple that is attached to a washer and goes under each spark plug so we can monitor each cylinder head. 350 Deg temps are normal in cruise and my old Stinson used to run hot at about 400 deg or so. As important as cylinder head heat is, equally important is cylinder head cooling since we have to consider "shock cooling" when descending from a high power situation (hot) to a descent in (low power, cool).

But I digress. :)

On an air cooled motorcycle engine the determining factor on whether it overheating or not should probably be oil temperature. 200 degree oil temperature is in the ball park and maybe it will run as high as 230 or 240 without serious oil breakdown. Much higher than that and you've got an overheating problem. Maybe one of those thermometers that double as an oil dipstick would do the trick. The other option is to find a bolt on the head that's pluggin an oil port and fabricate something that could be stuck in there.
 

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Discussion Starter · #5 ·
I finally (and easily) checked the oil temp on my 750 the other day after riding 30 miles. I realized I could just stick an oven thermometer into my 750 oil tank.

Since the oil is constantly circulating it would be pretty close and the temp rose to 180 degrees but my plugs are one heat range cooler than stock.

It would be just as easy to stick the oven thermometer into the crankcase of my twins after riding and very accurate.

I like the thermocouple idea... that sounds cool for head temps.
 

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It should probably be noted that excessive idling times will not do enyones engine any good. I once witnessed a CB750A sieze up from idling for far too long. Air cooled engines shouldn't be left running without some sort of airflow, for very long. As a general rule...

GB :mrgreen:
 

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Discussion Starter · #7 ·
Yup... too hot and not enough oil moving/splashing around.

Checking at idle wouldn't be a good indicator anyway. It needs to be at operating temperature which mine was after riding for 30 minutes.
 

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So, what WERE the symptoms that made you suspect they were running too hot? Just out of curiosity...
 

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Discussion Starter · #9 ·
Only that they "seem" hotter than they should be and surely something "must" be wrong.

After driving the bike started running roughly. I assumed it was heat related but it turned out to be that the float level on one carb wasn't allowing quite enough fuel keep up with demand after running for a while.

My original post was made before I rebuilt the carbs and now everything is fine but I still wanted to know at what temperature it is actually running.

I haven't gotten back around to reading the plugs yet since I've been driving it a few weeks with no problems.
 

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It's not a thermostatically controlled motor, so the temperature depends on how much fuel it's burning and how much heat is being removed by the air around it. Riding two up downwind climbing a hill on a hot day gives you a much hotter motor than coasting downhill in the winter. Motor heat is proportional to the amount of work being done minus heat loss through air cooling. Oil doesn't really cool these motors, since there's no oil cooler - but it does carry heat from hotter parts of the motor to cooler parts. The motor doesn't do much work at an idle, so I wouldn't worry about cooking the pistons when you're stuck in traffic. These motors are sensitive to too lean a mixture under load, like high speed riding on a lean mixture, which can blow the piston crowns into the crankcase. Run a loaded bike about twenty miles at high speed, then disengage clutch, kill the motor, and come to a stop. Check standard heat range plugs - they'll tell you what combustion chamber temperatures you're running. There are lots of plug comparison charts on line. There was a story years ago that Alfa Romeo stopped including oil temperature gauges in the instrument panel (once part of their standard instrumentation) because the normal oil temperatures used to scare the hell out of the owners. If memory serves, the gauges went up to 290º.
 

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There was a story years ago that Alfa Romeo stopped including oil temperature gauges in the instrument panel (once part of their standard instrumentation) because the normal oil temperatures used to scare the hell out of the owners. If memory serves, the gauges went up to 290º.
I have a 66 Corvair Corsa and the factory cylinder head temp gauge goes up to 600. Normal on a hot summer day is around 450-475.
 

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As far as oil temp and air-cooled machines, I've learned it is most important to change the oil often. I drove a '67 VW Bus for years, lotsa Cascade Mtn roads and high desert action. And across the States a couple times. 1600cc, stage 1 cam, dual Kadrons, dog house oil cooler and headers. For instrumentation I had oil pressure, oil temp and #3 cylinder head temp, mounted in the panel just above the mirror. Sweet location if I say so myself. Clean, outta the way and ala DC3. And I did have to tap the oil pressure gauge to settle. :p Anyway, if new oil was put in, the cruising oil temp would settle just over 200F. As the oil got older, it's temp would rise to 220F or so. On some mountain climbs I would see 240F! That starts to take a tole. Much time at 240F and the engine would just start to cook, being more difficult to get cooled down, meaning you just gotta go slower. And it would reach that sooner than with new oil. When the oil started to show some signs of darkening, time for a change.
I realized having a cylinder head temp was sorta redundant, 'cause the rising oil temp would easily outpace the cylinder head temp, as far as getting into the red zone.

RIP to FCK201 Blue & White Kombi

Former Bus Pilot
 

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http://www.bus-boys.com/bbvdo.htm

they have a spark plug adapter for a temp gauge. but like the other post i used an oil temp gauge on my main bus and would have to agree with him. i cant remember how many VW i had in my life. 69 camper 66 kombi 74 super beatle stick o matic 75 SB 67 kombi 72 mudder bug that i made and numous other bugs. And those were all mine if we talked of all friends and ones i worked on this forum aint big enough!
 
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