Topic: Running hot!

[zoedyer13]

Hello,

My Honda CB750 1977 Supersport, running a 1975 engine has started running ultra hot! The garage have had it for 4 weeks; checked oil pump, spark plugs, ensured no air leaks and all the usual checks. Their pretty good with vintage bikes but feeling pretty stumped! The temperature they're getting from the engine block is 118 and 120c... how hot are your other CB750s running? Is there anything else you'd recommend checking?

The engine has been out, emptied of oil, refilled, filter and everything checked - I did a filter change and oil change before taking it in, in case that was the issue, but it still seems far too warm...even for a hearty bike like the 750. The garage are currently contemplating an oil cooling solution perhaps?

Any advice greatly apepreciated, want to get him home and cosy ready for the UK winter weather!

Thanks in hopeful advance.

Zoe

[Stev-o]

Need a little more info.  When was the engine temp taken and by what means? What was the ambiant air temp?

Overheating these bikes has never really been an issue when rode properly.  If the motor was run for 10-15 min in the garage w/o a fan blowing on it, could possibly reach the temps you describe. Air cooled motor and all...

[robvangulik]

Right! These are aircooled engines, they need to be ridden to cool down, i've seen (OIL!) temps of up to 150c while circuitriding in warm weather, the engine itself was probably warmer than that!
Take it home and ride it!

[TwoTired]

Has the engine been painted?

Was it stationary when taking readings?

I recommend a synthetic oil if it is routinely that hot, for it's heat withstand capabilities.

An oil cooler won't help when the bike is stationary.  Even then, it addresses only 10% of the problem.  90% of this engine cooling is from cooling fins and the outer surface of the engine unit.  They need air movement to take the heat away.

Running too lean a mixture?  A restrictive exhaust?  Both make the engine hotter.

Cheers,

[zoedyer13]

Thanks for all the advice so far. The temperature is taken once returned from a test ride by the mechanic, getting it up to motorway speeds (70mph) and externally gauged. 

Is there a safe operating range for the oil temperature that I can ring him and recommend when he next rides it for me (Monday). It currently has Manuel recommended semi-synthetic in it but I'll definitely suggest the fully synthetic.

Does any of that extra info help?

Thanks so much 

[ekpent]

 So what did the spark plugs look like ?  Running lean will make the engine hotter as fuel also helps to keep things cool in an air cooled engine. Are you running pods or open exhaust on stock carb jets ?

[Jerry Rxman Griffin aka MuthaF'er]

Who brought up the temperature it is running at? You or the mechanic? And why did it come up? Were there issues? I'm assuming it did not seize up?

We need someone to check their running temperature for you as a comparison. Unfortunately I can not.

[TwoTired]

Whenever my test equipment gives me a reading I don't expect, it sends me into a calibration check of the instrument.  I don't want to fix something that isn't really broken.  Been down THAT path before.

But, after a hard run, and then sudden stop, it is somewhat normal for the temps to spike.  Was there a fan on the motor during the temp measurement?  And what instrument was used for the measurement?

I still need to know if the engine has been painted.  Thick paint can insulate the heat exchange between metal and the receiving air.

I was also going to ask about the spark plug deposits.

Cheers,

[754]

Easy check if that instrument is off, pour about a tablespoon of water on the motor, above and to front of clutch..by the oil sender. Have it on the sidestand.
 If the water does not boil the instrument is way off.

[robvangulik]

Even if it boils, who is to say it is TOO hot?
I have ran several sohc's in quite hot conditions, on racecourses, in trafficjams etc, and none of them stopped doing what it was supposed to do, I think I was more troubled by the heat than the bikes!
Like I wrote earlier, get the bike home, and ride it, the warm weather has gone for this year, I'm just across the channel and having the same kind of weather as the UK, and in the next couple of months the biggest problem will be to get the bikes warm enough.....

[Dunk]

I've seen 230*-240* temps measured at hottest point on the head, after running it pretty hard with a few minutes slower riding to cool down. This is the highest I've seen, in summer weather maybe 90* ambient, give or take a few degrees. I forget oil temp but it wasn't alarming or enough to want to install an oil cooler.

If that's the highest you see, I wouldn't worry too much. Just make sure it's not running too lean.

[Deltarider]

I think I was more troubled by the heat than the bikes!

I certainly was in countries like Spain and Greece. Tip to reduce engine heat: shift down and increase the RPM.

[robvangulik]

I think I was more troubled by the heat than the bikes!

I certainly was in countries like Spain and Greece. Tip to reduce engine heat: shift down and increase the RPM.

I don't agree, the heat of an aircooled engine originates in the number of burns against the volume of cooling air. Only liquidcooled engines profit from higher revs by increasing the flow through the radiator..

[kmb69]

+1

[Deltarider]

I should have been more precise. In hot countries on entering cities and being forced to lower speeds and so less riding wind, I could feel the oil getting hotter which translated in somewhat harder shifting. Shifting down and increasing the rpm brought some relief. You can try it.

[zoedyer13]

Whoa sorry been away a day and only just seen replies.  Hopefully I can answer all questions:
1) it was me that brought up the heat issue, been running it for a year now and hasn't got this hot before.  It seemed abnormally warm and smoked INITIALLY, that has now stopped though!
2) spark plugs clean, no residue or blackness at all, equally not too pale to imply running lean.
3) water test on heads showed it boiling off and disappearing instantly when I dropped it off which added to my initial heat query
4) engine hasn't been coated at all as far as I'm aware

It's only because it hasn't got this hot prior to now that I was worried.
I'll tell the mechanic your points and hopefully it'll be back with me by the weekend. Thanks again all.

Zoe

[Dunk]

The onyl thing that comes to mind that might make it run hot is retarded timing. Check your timing and that the advancer is not seized up. Aside from that, maybe measure the oil temp as I suspect that is more likely to cause damage (bearings, cam, rockers) than the head temp being a little on the high side.

[HondaMan]

The CB750F2/3 engines, especially those in black, run hottest of all these 750 bikes. That's what leads to their early valve guide wear and oil leaks, usually from cooked rubber head pucks seals.

You can cool them off a little bit by advancing their spark timing (yes, I said that...) about 2-3 degrees, but keep an ear out for "ping" or "knock". If that happens, increase the mainjet by a #3 or #5 size from where it currently is set. You may notice a little finned adapter between the oil filter housing and the engine: this was Honda's nod to the fact that this one has hotter oil temperatures than the other bikes. The oil temps on a hot day with this bike can exceed Honda's margin of 220 degrees (F), and this plate was supposed to cool it off about 5 degrees (but seldom does). It can be wise to install a Lockhart or similar cooler in place of that plate, and mount the cooler up by the horn.

Or, just use 20w50 oil, which is a cheaper alternative, and will drop oil temps by as much as 10 degrees, all else being equal on that engine.

[robvangulik]

This  bike has a 1975 engine....

[Stev-o]

1) it was me that brought up the heat issue, been running it for a year now and hasn't got this hot before.  It seemed abnormally warm and smoked INITIALLY, that has now stopped though!

Once again, how hot did it get and how did you measure the temp?

The motor and exahust normally will feel hot to the touch, smoking could be due to an oil leak.

[HondaMan]

This  bike has a 1975 engine....

Does it also have 1977 PD carbs attached? They are the ones causing the heating.