Radiator and temperature issues.

I have an 1992 year ABS/TCS and I made a total overhaul for that bike few years ago. I bought an engine from ST1100P 2000 and I replaced also the radiator by another used one. I have no clue about the year of the radiator. I also replaced the thermostat with new part. There's a strange combination in my bike, I know, but I didn't have money then for replacing the alternator with new unit, so there's a 29-ampere unit in newer engine if that matters. Anyway, the alternator works well.

But my problem is temperature. My engine runs on lower temperature than ST's usually do. My gf bought an ST1100AP and its engine runs on higher, normal temperature. Newer radiators have different part number than older ones and if I remember correctly that radiator was difficult to install. Like it was taller or something... So I'm asking that are there really differences in radiators between different models? Of course rich mixture and many other things could make the engine run on lower temperatures.

"lower, higher, normal temperature" is a bit vague, but here's what you should expect:

At steady highway speeds the temp gauge is fairly close to the left white line. A little above it, but not much. As long as it isn't at or below the white line, no problem. If its too low your thermostat could be stuck open.

In stop/go riding the temp will rise fairly quickly to vertical. As it goes a bit past vertical the fan should come on and keep it from going any higher, or bring it back to vertical. If the needle goes a lot past vertical, then that needs to be investigated.

reply back if your bike is not operating within these ranges under the given conditions.

And I reply back, it is NOT operating within these ranges under the given conditions, it runs too cold.

I'm planning carb overhaul and I'll check the thermostat also. Fuel consumption is pretty high as well, I guess that carbs could be the key.

Try running it on the center stand ( without air flow through the radiator ). The temp should rise and when the temp gauge gets to about the 12 O'Clock position, the fan should start ( as dwalby said ).

Running it on the center stand should definitely cause the temp to rise.

It is possible you need a new thermostat. A quick test is to start the engine from cold, and keep your hand on the radiator. It should stay cold for a few minutes, then quickly get too hot to touch. The thermostat should cause coolant to bypass the radiator until the coolant is hot, around 80C. If the radiator slowly warms from the time the motor starts, the thermostat is stuck open and should be replaced.

STonewall said:

Newer radiators have different part number than older ones and if I remember correctly that radiator was difficult to install.

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A different P/N could also be simply based in change of vendor/MFG...
By logic pre '95 models (with oil cooled 26A unit) should have a slightly larger rad, then the post '96 plants with the air-cooled 40A alternator..?? (I'd need to dig through the fiche first for confirmation...)

Of course rich mixture and many other things could make the engine run on lower temperatures.

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Not to a degree that it would visualize on the temp gauge...
My bet is on an open/stuck coolant/stat...

The fact that you run an oil-cooled alternator in a post '96 engine without heat-exchanger at the base of the oil filter, could actually increase oil-, thus engine-temp marginally... dunno how this might work out while stuck in urban traffic... the 26A unit is a bit sensitive to heat after all...

I changed my engine and I installed same carbs back. Thermostat was ok, everything in the cooling system was ok and I started my "new" engine. Something strange happened, temperature rose quickly and coolant almost boiled when I rode about 40 miles per hour. I could feel the heat from the engine. So I went back to garage, removed the carbs again and I found out that pilot screws were only 3/4 turn out. I closed them and opened them 2 turns. Then I synchronized the carbs. Temperatures are ok now, like in my gf's better maintained bike, idling became smooth, response to throttle is better than ever and fuel consumption dropped from about 37-39 mpg to 47 mpg.

Hard to believe that fuel mixture would have such a large effect on the cooling system temperature. We know that rich/lean mixtures can cause lower/higher combustion chamber temperatures but it would have to be massively wrong to affect running temperature as measured in the cooling system. Also you would think the bike would have poor drive-ability issues as well. I'm with Martin in that it's likely a t-stat issue. Just because it's a new part doesn't always mean it's a good part or in this case has the correct temperature rating. On my '93 the normal running temp at the gauge needle perfectly dissecting the "E" in the word "TEMP" on a diagonal. I know newer ST's, at least in the US changed to the ISO symbols and dropped the words around '95 however.

Could also be a poor connection on the temp sending unit, mounted on the thermostat housing and, whilst working in the area you have disturbed the connector causing it to function again. Normally, the ONLY reason an engine will run cold is a stuck open thermostat.