Hello to all, today i put a new set of plugs wires(yellow) on the 1200c, bike started ok while i had the bike warming up i decided to check the temperature of the front and back jugs with a laser temperature gun, what i found was the front jug was hotter then the back one by 45 degrees (265 front, 220 back) is this normal or is something wrong?

 

Haven't a clue as to why they arent the same - a Laser Temp gun (fireman?) - sounds xpensive

Take it out for a ride, then take the temp again - I'm guessing the temps will een out

 

its a raytek (raynger st) i use it for my rc gas cars to check the temp on the heads.

 

I don't know for sure, but I heard that the back one runs cooler because it gets less airflow to it than the front one. Don't know how much cooler the guy that told me that was talking about though.

 

Umm actually I guess thats not right either he said the front one runs cooler cuz it gets more airflow.

 

You really need to take that temp after a warm up ride. Also, remember that the heads are essentially reversed, i.e. the exhaust valve is forward on the front cylinder and is rearward on the rear cylinder. Therefore to get comparable readings you should take them in the same general location but on opposite sides of the motor. The exhaust valve region of the head is going to be the hottest area of each head. If you took your original reading over the exhaust valve on one head and then over the intake valve on the other head I would expect a large difference like you got.

On aircooled H-D engines, the rear cylinder runs slightly warmer than the front because of reduced airflow. ............ BC

 

I took the reading from the left side of the bike (choke side, horn) by the engine case where the jugs connect to the bottom part of the engine.

 

It's normal that the rear actually runs cooler than the front (misconceptions about the front being cooler notwithstanding - and didn't you take the temps while the bike was standing still? No directional airflow cooling going on unless you're pointed into a stiff breeze!). The reason is a combination of the intake manifold design and the timing (assymetrical firing) of the v-twin engine - the rear ends up running just a tad richer than the front and therefore runs a tad cooler. It basically has to do with how and when the jugs are breathing and firing, with the bottom line being that the front gets a leaner charge than the rear and so the front runs hotter than the rear.

It might to help remember how a 45-degree v-twin engine fires (and gives that great potato-potato sound, by the way):
The pistons don't fire at even intervals (and indirectly results in the front running hotter than the rear as mentioned above).
A piston fires.
The next piston fires at 315 degrees.
There is a 405-degree gap.
A piston fires.
The next piston fires at 315 degrees.
There is a 405-degree gap.
And the cycle continues.
Potato-potato!

EDIT: Final thought - if something IS wrong, you'd see it in very different plug reads of the rear vs. the front. If that is ever the case, check out whether you have any intake or exhaust leaks first.

 

I don't follow your explanation of WHY the rear cylinder is supposedly running cooler than the front. Would you go a little deeper into the physical reasons as you see them? ........... BC

 

well it should read hotter closer to the head check it there.both jugs@the same time.was it runing or shut off?