Small Engines cold air and Performace.

Yup... depending on the jetting it can be decent at 90 degrees outside, then you'll notice when it drops to around 60 the bike has this nice new found power,but around 30 it slows back down because the air is nice and dense but at some point the jetting will actually go lean enough to level off then it gets a little colder and it becomes lean enough to lose a little power.

I really notice that on my 1400cc intruder.... it's already mega strong, tons of torque, and I jet my carbs in the summer when it's in the three digits over here for 3 to 4 months. I jet it for making the most power in those conditions so it does get noticeably stronger when the temp drops down to the low 70's and stays this strong down to the mid 50's or so, but when it drops below 50 I'll notice that extra punch is gone, and by the time it drops into the 30's or less it's not quite as strong as it is in the high 90's. Being that the temp never stays that cold over here for more than a few days and it's back up to the 60's and 70's, there's no reason for me to re jet for the seasons.

I agree with Dave. There is a big difference in air density and jetting, but another reason is that the engine takes twice as long to get up to full operating temperature. Clearances change, rings tighten up, grease softens a bit as the engine warms up. Since it takes longer and the rides are shorter (BRrrrrr!) the engine never really gets to full output.

That's another truth to this whole thing that I didn't even mention... once the temp drops to a certain level the engine may not bae able to even reach operating temperature, which in turn will also contribute to the loss of power, especially with an unshrouded air cooled engine that relies on air moving over the fins riding on the road at cruising speeds, it'll actually warm up more at idle sitting than at cruise speed because this extra cold air is moving over the fins and cooling the engine quicker than it can heat it's self up.

In the 40s I rode my bike about 5 miles and the carb body was barely warmer than the air when I got back. Gas doesn't atomize as well when this is going on either.
My bike likes it warmer outside. Winter stinks unless you ski and ice fish.

u2smile said:

sitting than at cruise speed because this extra cold air is moving over the fins and cooling the engine quicker than it can heat it's self up
AHHH
So would there be a benifit of shirlding the engine from so much cooling? As the have an fan with the cowling as cooling. I ride mine about 20 minutes at a time more at 3/4 appx 23mph. I would have thought she'd be warm, Hotwater bottle?

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I never tried this, and don't recommend it, but if you want to experiment-
You might stick a few small pieces of duct tape on the cylinder fins to cut some air flow. Go easy here, you don't want to block it all. It's been 25 years, but in the old days flying r/c planes we put a little tape over the cylinder on glow engines to fly in winter. They won't run for crap when they're too cold.
You can always reach down and feel the cylinder, if it gets too hot peel some off.

If its a shrouded engine you could try blocking off part of the fan inlet to regulate the temperature but it would need to be blocked off inns way that the fan cant sucknin whatever your blocking it off with, and you'll need a way to monitor the temperature so you can block off the right amount, it can still overheat if totally blocked off or too muchnof the inlet is blocked. If it has a screen toncover the inlet duct tape works good for this. Try blocking off about 1/4 of the inlet and see if anything improves, but also remember it could be running lean, which could lead to detonation by the time you get it to operating temperature so it may not be worth messing with, just pay attention to what your engine is acting like if you try any of this.

I'm sure you've seen some cars and especially big trucks or 18 wheeler's driving around with part of their grille's blocked off when its really cold outside.... pretty much the same idea.

Hebe... both of us had the same idea... just the cylinder or the fan inlet... either way should work, just don't let it overheat if ya try it...

I gotta remember that everyone isn't running the same engines!
I didn't know your engine was shrouded with a fan.
The basic principles still apply, though.

All you need to do is change spark plugs between summer and winter.
For example here in the desert I run a BPR7HIX in the summer, a BPR6HIX in the winter.

The 6 and 7 are the heat index number.
The higher the number the more heat the plug transfers to the head.
If your engine just doesn't want to warm up well, lower the number.

Look at the size difference in the white insulator length where the plug threads are.



The more white you have in contact where the threads are, the more heat is transferred to the cooling fins, and the pink is just providing an air gap to keep heat in.

Yup... just look up cylinder head temp gauge on ebay... they run off a small sensor that goes under the spark plug like a washer or better yet, like a ground wire... These are fairly cheap but most of them need a 12 volt source to work.

I'm not totally sold on the spark plug heat range theory for cold weather since the air is more dense, thus leaning the fuel mixture, the last thing ya wanna do in a lean condition is put in a hotter plug.

But really, even with all the temperature swings in the wintertime in some places, do you see everyone changing their plugs and rejetting for the cold then changing back for the warm weather in their cars, trucks, buses, motorcycles etc? Nope... everyone just drives them. Some people block off part of their radiators in the winter, but that's about as far as they go with that.

You *can* do all that for the weather extremes, but it's typically not necessary unless you're just looking for maximum performance all the time, or on race day.

u2smile said:

HI KC, thanks for the info. Can you give me an example of what we run in say a honda 31cc 4 stroker. Hot and cold?
Anyone know of a small add on temp gauge?

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No clue for the 31cc but the HS 49cc 4-stroke likes the NGK 7544 (CR7HIX) Iridium here in Phoenix.

The best rule of thumb is to run as cold a plug (high number) as you can without it fouling because it's not hot enough to burn off the oil.

As for jetting I very seldom have to dink with that, Phoenix's elevation is 1,100' and seldom sees temps below 32F and the stock jets work dandy for me year-round.

And you can't compare these little air cooled 2-stroke engines to a water cooled computer controlled 4-stroke, they have their own temperature control for optimal engine temp.

Is it possible that one of you may already know- thereby saving me the time to actually find out myself- what a good jet size may be for 5000', between 30 and 70 degrees?
I've been running stock Chinese 4-stroke kits acceptably. I'd like a little more.
Thanks!

Last person I helped with a China Doll in Colorado got his dialed in with a size 64 main so that might be what you need too if using the same engine. All engines are different tho and you can have 2 identical engines side by side with one needing a jet size up or down before it'll run right.
Also, the jets coming from China aren't exactly accurate so either buy the genuine Dellorto 5mm jets or get some solder and a jet drill kit, this works good because you just solder the jet shut and drill it with the right size bit to dial it in, if it needs to go richer just drill up a size, but to go leaner, solder the hole shut again and go down a size.

Personally I prefer to have several different sized so I can go up or down at least 3 sizes if needed so I can tune for any configuration. I use only genuine Mikuni, Keihin, Weber, or Dellorto jets tho to avoid inaccuracies, others prefer to solder and drill their own and there's nothing wrong with that method either. Your engine don't care what brand is stamped on the jet or what number is stamped on there either, but it does care what size that hole is and it'll tell you when you got it right by the way it performs and by plug reading.

One more thing... These engines tend to make the most power slightly on the lean side so read the plug once you got it running the way you like it to be sure you're not too lean or it'll be nice and powerful but it could overheat or cause detonation... both very bad for the engine.

Davezilla said:

Yup... just look up cylinder head temp gauge on ebay... they run off a small sensor that goes under the spark plug like a washer or better yet, like a ground wire... These are fairly cheap but most of them need a 12 volt source to work.

I'm not totally sold on the spark plug heat range theory for cold weather since the air is more dense, thus leaning the fuel mixture, the last thing ya wanna do in a lean condition is put in a hotter plug.

But really, even with all the temperature swings in the wintertime in some places, do you see everyone changing their plugs and rejetting for the cold then changing back for the warm weather in their cars, trucks, buses, motorcycles etc? Nope... everyone just drives them. Some people block off part of their radiators in the winter, but that's about as far as they go with that.

You *can* do all that for the weather extremes, but it's typically not necessary unless you're just looking for maximum performance all the time, or on race day.

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Jus' a FYI - the 'TTO' temp gauges (CHT/plug) & similar (ebay/clones) often have a self-contained battery (hearing-aid/watch type): http://www.trailtech.net/digital-gauges/tto/temperature

I agree w/ya Davezilla. Insofar as winter rejetting & plug range swapouts... well... the American south west gets chilly n'all & folks can ofc do whatever they wish in the quest for HP - but I gotta mention it does get a bit cool up here too from time to time. The winter & summer extremes in Maine are no more then between -20°F & 100°F usually & granted, that t'aint near as much as some places but to my knowledge no one around these parts mucks about with plug ranges & rejetting in their 'smokers two or four due to the temperature change (including dirtbikes, quads, chainsaws & other year-round uses).

Yea, air density does have an effect & it can even be pronounced enough to "feel" a difference on a cool day with our engines - but consider for a moment small aircraft which routinely experience extreme variations in air density and temperature as they change altitude. At a set RPM in combination with both the EGT and CHT readings (exhaust & head temps) a manual mix control is utilized to maintain the proper fuel/air ratio as they ascend or descend... but no one worries the heat range of their plugs as they do so. This includes the not "computer-controlled" ultralight & sport/experimental classes which often fly two strokes as well as the fours, as I did with a Rotax 440 (two stroke) in a Quicksilver MX (in both winter & summer BTW, same range plugs).

So... you can "sweat" such details if ya wanna but honestly you'll be just fine picking a range/ratio that seems good for the engine & sticking with it no matter the season *shrug*





...& no, I don't bother with engine shrouds or covers either - them lil smokers ain't sufferin' near as much as me in the chilly lol

When I worked in the small engine shop here locally. Well we are a long ways from sea level here. 6000 to mebbe 8000 if your in the mountains. I am at the foot hills.

I was instructed to put a hotter plug than factory in pretty much everything we touched. Especially chain saws and weed whackers...

I was told it was for our altitude.

BarelyAwake- That second pic looks very familiar to me. We've got up to 4" sheets of ice here now; thaw, freeze, thaw, freeze...

Thanks for your advice on jetting all!

BarelyAWake said:

I agree w/ya Davezilla. Insofar as winter rejetting & plug range swapouts... well... the American south west gets chilly n'all & folks can ofc do whatever they wish in the quest for HP - but I gotta mention it does get a bit cool up here too from time to time. The winter & summer extremes in Maine are no more then between -20°F & 100°F usually & granted, that t'aint near as much as some places but to my knowledge no one around these parts mucks about with plug ranges & rejetting in their 'smokers two or four due to the temperature change (including dirtbikes, quads, chainsaws & other year-round uses).

Yea, air density does have an effect & it can even be pronounced enough to "feel" a difference on a cool day with our engines - but consider for a moment small aircraft which routinely experience extreme variations in air density and temperature as they change altitude. At a set RPM in combination with both the EGT and CHT readings (exhaust & head temps) a manual mix control is utilized to maintain the proper fuel/air ratio as they ascend or descend... but no one worries the heat range of their plugs as they do so. This includes the not "computer-controlled" ultralight & sport/experimental classes which often fly two strokes as well as the fours, as I did with a Rotax 440 (two stroke) in a Quicksilver MX (in both winter & summer BTW, same range plugs).

So... you can "sweat" such details if ya wanna but honestly you'll be just fine picking a range/ratio that seems good for the engine & sticking with it no matter the season *shrug*

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My point exactly... it don't matter if it's a big 18 wheeler or a sub compact, motorcycle or chainsaw, liquid cooled, or air cooled, 2 stroke or 4 stroke, there's no reason to sweat over what plugs you got installed or what jets you got installed, once the engine is tuned it'll run in all 4 seasons.