O.S. .91FS overheating

[uncle festa]

hi i have an os91fs could any one give me any clues as to why it seems to run very hot im using 10% nitro double high lube from southern model craft

would i be better of using 18% nitro

[stevejet66]

3 questions,

Is the engine fully enclosed in a cowl? Answer,,, no air flow.

What size prop is on the motor. Over revving, Answer,,, Wrong size and pitch prop.

Is the engine properly run in.? Run it in!

More nitro will just give you a few more rpm.

[uncle festa]

hi steve the cylinder head is exposed the engine is second hand and its turning a 14/6 apc prop many thanks

[stevejet66]

14/6 is ok, But if only the head is exposed you need some big air flow to the rest of the engine, What ever cowl you have surrounding it make air holes! At the front if possible and a couple at the rear of the cowl.

[uncle festa]

the engine is fully exposed i thought maybe valve cleareces might be wrong but im afraid im not up on fs engines onl ever used two strokes in past

[cymaz]

Is the air able to get around the crankcase- then get out?? A tight or mis-aligned bearing? Running it too lean?

Just a thought

[Rick Tee]

If it's been setup the same as a 2S then it could be running too lean in the air. If the tappets have no clearance it might cause it to over heat but there would also be a lack of power. Tank position or plumbing could also cause it to lean out in the air.

Edited By Rick Tee on 18/08/2013 07:20:42

Edited By Rick Tee on 18/08/2013 07:20:54

[Martin Harris - Moderator]

What symptoms are causing you to conclude that it's running too hot? 4 stroke exhausts do get much hotter than 2 stroke ones, for example. If it's baking residue on to the casings this may be due to the high castor content in your fuel - I use their 15% synthetic 2% castor fuel - yours is 9%/9% and will probably end up (if it hasn't already) getting very coked up internally - a possible cause of pre-ignition and running problems if that's what you're experiencing.

The normal indication that an engine is overheating due to lack of cooling or a lean mixture is a loss of power in the air which can often be temporarily rectified by throttling back.

[uncle festa]

thanks guys i will try some of these ideas

[Steve Hargreaves - Moderator]

I'm with Martin here....a 90 4 stroke on a 14x6 in the open air shouldn't overheat per se........they do run hotter than 2 strokes though so this might be what is causing you to think its overheating.....

Its possible you are running it too lean as well...4 strokes don't have the clearly defined "peak" revs like a 2 stroke so finding the optimum point can be tricky........you should wind in the needle until it reaches max revs.....you will then find that continuing to close the needle doesn't make the engine run any faster.....at this point you are at max power & you should open the needle slightly (4-5 clicks) the trick is to get it running as fast as you can, as rich as you can....

If you can find a guy at the club who ISN'T always fiddling with his engines ask him to show you how to set up yours & then LEAVE IT ALONE.....once set an engine shouldn't need touching....I have engines with rust on their NVAs.....

[uncle festa]

thanks martin some usefull advice i will give tha a try

[crispin church]

and in this weather run it a little richer

use a taco to set it up

cowel off you cant it in air any way let the air get to it