I run mostly 4 strokes with Magnum being my main sport engine.. Recently I have been breaking in a new .70 and having one heck of a time… Probably have close to a gallon through it now… I break them in on the plane.. I have had the engine running great and then all of a sudden it won’t idle etc… I readjust the low end and it is ok for awhile then everything goes to he** again.. The good news is after thinking/saying some very unpleasant things about the engine today I may have discovered what is going on… It’s this darn hot weather… Last go around with this I set it up at home one morning and got it running good.. Took it to the field around 8am… Temp was in the mid 70’s. Engine ran like a clock… Got a couple more good flight off it that morning then around noon everything went to he** again… Temp now around 103…. I just hapened to feel my fuel can which is setting on a flight stand out in the sun.. It is very warm … Not hot but extremely warm… Now the light clicks on…. In the morning the temp and fuel are cool at the engine runs great … As the day progresses the temp/fuel warm up and engine performance starts to deteriorate… By 1 o’clock it won’t idle anymore… Then I reset the low end to try and get an idle so I can continue to fly… Next morning I come out when it is cooler and my adjustments are off again…
Has anyone else run into this problem ?… Were talking temps around 103-105 in the shade… 138 on my little temp thingy pointed at the asphalt.. You guys in Az have this problem ?… Should be some heat experts there….
I also suspect all the “goodies” are vaporizing out of my fuel when I open the can to refuel… A fresh can always seems to run noticeably better… Best solution I can come up with at this point is to put a white towel over the can etc while it is setting there which I am going to try next trip to the field along with a fresh can of fuel… Short of icing it down it seems hard to keep the temp below the ambient temp so I am wondering if fuel is ok even at 105 – 110 degrees ???… Fortunatly we have a cool down comeing in a day or too.. Mid 90’s….

I keep my fuel in the trunk of my car and I fly all times of the day and don't experience such problems with my OS engines. My guess is the MAG engine is flawed. Check your low speed needle after 2 or three flights I'm willing to bet it's vibrating very gradually, in or out. My mag .28 did this and it drove me to drink [:@]. Well Ok I already was a drinker [:-] , but it just was a PITA until I put teflon tape around the low speed needle and lowered the nitro down to 5%. GL I'm sure I will be reading more about your engine.

I've been visiting in SE Alabama. BLWBLW and I have been fighting this in the afternoon sun. The temperature would be in the mid 90's.

Enjoy,

Jim

RC-FIEND
Thanks but it is not the needle valve... I am by now very familure with the position of the slot on the low end valve... It is not changing on it's own... This engine runs so sweet when it is cooler.. I am realy convinced now that it is a fuel/temp thing.. One other guy was out there with me this afternoon... He had a Saito 100 in a H9 Mustang.. He was haveing a heck of a time getting his started and this morning it was starting with one flip.. Not just the Magnums I am thinking...

Jim
Thanks for the confirmation... Any solutions ... Besides moving north

your not the only one Iron Cross, out here in the desert its an every day thing.
its not only the fuel that is responsible its also the air going down the carb.
its just like tuning a (old) carb on a hot rod. the hotter the air temp the leaner the mix,cold air=more air=more fuel needs.
hot air=less air=less fuel needs for a correct mix
anytime you go from 70deg to 100deg your mix is gonna be way off so just get an idea where your settings are and you have a good starting point each day, whatever time it is.just adjust your idle first.

gjeffers
Thanks for the info... Do you guys keep your fuel cans coverd with something... Keeping the fuel in the trunk etc is not an option.. The cars/trucks sit in the sun and every thing in them gets cooked... Best I can do is some shade but I am still dealing with the ambient temperature..

Jim,all you can really do is keep it in the shade, thats all i do,my saito 100's low end dont change much,just a couple of clicks but my high end is about a quarter turn out farther when its cold than when its hot for an equal run(these changes are for approx 30deg diff).

Please do not make fun of me, but my field box is a cardboard Sig four gallon fuel carton. I carry a gallon of Sig fuel in there, and leave it in the carton with the lid closed to keep the sun off it. I fly in 100 degree weather here in Texas and have noticed no problem.

After thinking about this a little more I am begining to suspect the heat is cooking the Nitro out of my fuel... It seems to affect only my 4 strokes... I have a bunch of Irvine .53 two strokes that don't seem to be affected at all... The Irvines run fine on 0 Nitro.. The 4 strokes appreciate the 15 percent I usually give them... Guess I will start again with a fresh can of fuel and wrap a white towel around it or something...

Its not the fuel's temp but your needle valve setting. Our engine carbs are not self adjusting and require tweaking for changes in air density, temperature, humidty etc...etc.. As little as a 5 degree temperature change will require a carb adjustment. Unfortunately, many in the RC family are of the "set it and forget it" mindset.

I flew in lower Alabama for many years and my fuel sat on asphalt in the hot sun for many hours and never had a problem flying.

I typicaly only have to set mine a couple times a year... Winter and summer... I know it has something to do with the fuel... When they start running bad if I open a new can of fuel they snap right out of it... No adjustment necessary.. I suspect the Nitro is being vaporized by the high fuel temp.... Still have a couple more things to try though to be sure... Also as I said before this is only happening with my 4 strokes, not the 2...

i kinda think were all right, dont you guys

I think your right... I think there are a couple different things going on here which is confusing things,,

Enough speculation I emailed the fuel manufacturer to see if they can shed some light on fuel temperature, air temperature and engine settings... If they reply I will post it here..

Since the intake manifold on normally aspirated 4 strokes is directly connected to the cylinder head, heat transfers readily to the intake pipe at low airflows when the throttle is at idle. This significantly enriches the idle mixture and this enriching becomes much more pronounced at higher ambient temperatures. Try setting the idle mixture as lean as possible consistent with good throttle transition.

Try putting your finger on the intake pipe with the engine at idle, then open the throttle about 1/3 and notice the immediate decrease in temperature. The warming of mixture temperature when throttling back is partially responsible for the slowing of idle rpm after about 5-10 seconds.

Since the carb on 2 strokes is mounted on the comparatively cool running crankcase, the 2 strokes are not similarly affected.

Interesting, makes sense... Problem is I don't want to have to continualy adjust the low end throught the day as the engine is coweled in an a PITA to do.... I think what you say makes sense as does the hot air etc... This is why I have to adjust the low end for winter and summer flying... I also think with the exceptionaly hot summer we are haveing, the fuel heating up in the can, that it is expanding (thinning it) and "burning" off the nitro... I think that is why a fesh can (cooler) of fuel makes things work again.. Can't afford a fresh can of fuel every day either... I am putting a cover on the fuel can this weekend ... Hopefuly this will help... Maybe keep the can in an ice chest... Along with the refreshments that is

you know Jim, i used to drive a propane truck and have 10 yrs exp hauling gasoline and i dont think that you really loose any nitro as long as the cap is tight and sealed, just the puff that you hear when you open it and i think that would be insignificant,just been thinking[X(]

If you leave the cap off off you fuel and leave it in the hot sun, it won't be nitro cooking off that will be the problem, and no it won't be moisture from the air either. Nitro is a dense fuel that has a flash point of about 96 degrees, I don't remember what methanol is but belive its below 0. This means you can pour nitro in a cup and put a match to it the match will go out, if it is below 96 degrees. In the hot sun the alcohol will be evaporating rapidly, rapid enough that air will not be able to enter the jug. If you did this for a time, I don't know how long, your fuel will have too much oil and nitro in the mix and not enough alcohol.

wow, thats a surprise to me, i would have thought that nitro would have had the low flash, being that it is the power and idle maker, had no idea, do you have the spec sheet or anything with the specs on it?

thanks

The flash point of nitro is 109F and boils at 214F while methanol is 61F and 147F. I can't remember ever having a fuel problem even on days well over the 100F mark in the shade but I do remember putting a thermocouple down below the spray bar on one engine at full throttle and there was a temp drop of about 20 degrees. This would have been from a combination of the fuel (mainly the methanol) vapourising and the lower pressure.

Yes there is the puff.. The can is also sucking in air to replace the fuel going in the tank also as I pump the fuel... I am not sure it is the nitro cooking off... But am certain something is ... What I do know is that halfway (two to three days) through the can the engines start running like they do with low/no nitro fuel which I have tried but that's another story.. A fresh/cool can of fuel cures most of the problem... The old can never becomes usable in the 4 strokes even when cool... Perhaps it is the alcohol "burning" off. I don't know.. But something is gone... I have never knowingly seen an engine run with no alcohol in the fuel... Hopefully I will get a response from the manufacturer with some info on fuel and high temps...

sport pilot was right (not that i didnt trust you but) here are the specs. i think the most important piece of info is the last line

http://www.nitromethane.com/

So your of the opinion that the methonal and nitro are not evaporating ?... Also you have no problems with 4 strokes at 100 plus int the shade ?... Wonder if the fuel brand/mix has anything to do with it... I now have a cover for the fuel can to keep it out of the sun.. I will try it next Tuesday.. Figures though now we have a cool down started.. Mid 90's forcast for next week...

we will not grip about cooler temps Iron Cross, even if you have to wait till next year

Amen to that... Wonder what winter is going to bring though..