Tuning with temp?

GoldenMotor.com

Motakitty

Member
Feb 14, 2015
447
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San Jose, ca
Is tuning using the head temp a good way to tune a China 66cc? I'm getting 245°f at the head by the spark plug. I know I can go hotter but I'd like to keep the temps as low as possible. I haven't pulled the plug yet. I'm running a sbp pipe and the exhaust temp is about the same as the head. It runs ok, sometimes at wot it stutters. I don't mind going to a smaller jet but I want to know is going by temps an ok way to tune? I've tuned smaller 2 stroke motors using temps but what about our motors.
 

Davezilla

New Member
Mar 15, 2014
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Yes, temperature is a really good way to tune as long as you got accurate equipment and take the readings from the same place every time... something like a spark plug mounted temp probe is an excellent way to get consistant results and can be monitored by a really small digital gauge that can be handlebar mounted.
As for safe operating temps... Some say to stay under 300 F which can be done pretty easily with a Fred head or similar while others will allow up to 360F. As far as temp limits go, the aluminum really loses a lot of it's strength at around 400 F so that's definitely the "never exceed" temp to watch out for, but if you can get your engine to tune at or below 300 F it's going to be best in that range. You can get a little more power by letting it run hotter but that's where you need to monitor it while riding and back off when you see the temp aproach 360
 

Motakitty

Member
Feb 14, 2015
447
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San Jose, ca
K thanks for the info. I have a temp gun. I'd say I'm ok where I'm at temp wise then. I may go one jet size smaller and see as I have one smaller jet already drilled so I may try that one.
 

crassius

Well-Known Member
Sep 30, 2012
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I sometimes think that hobbyists wouldn't be happy if they couldn't find a way to apply one more gadget to their work. Been working on these about 7 or 8 years now, and started on real 'cycles about 55 years ago. Not once in my life have I checked the temp of any part of a motor. I used to dyno my race bikes in the 70s, but no temp checks.

If it runs good, you did it right & you're done.
 

Motakitty

Member
Feb 14, 2015
447
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San Jose, ca
It runs good, it has a ton of low end torque but kinda stutters on wot. How do you know when it's at the peak of the top end without a tach. I'm only getting like 23mph out of it but like I said it has a ton of torque. I'm thinking of going to a smaller rear gear to get more top speed.
 

Davezilla

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Mar 15, 2014
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You can definitely feel it when you get past the peak of the top end because the bike will pull and accelerate really well up to the peak and then the power will quickly start to fall off on the other side of the peak where it'll still accelerate, but at a much slower rate.
If your engine is putting out a lot of torque like this, going with a smaller sprocket will not only give you better speed but it'll also match up better with your engine's output so it don't feel or sound like it's over reved when cruising... An engine that's geared too low will always feel like it needs to be in a higher gear and it will also have more vibration and sound a lot less like it should while cruising.... When properly geared, the bike will still accelerate good and it'll be smoother and quieter during cruise as well as having a better top speed, but shouldn't feel boggy when taking off. If you got the torque, doing a gear swap will give really nice improvements all around.
 

crassius

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Sep 30, 2012
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23mph is way slow for normal weight folks & standard sprockets on 26" wheels

if rings are not yet seated, then wait till run in period is closer to finished - I suspect a larger (and simpler) problem tho
 

Motakitty

Member
Feb 14, 2015
447
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San Jose, ca
I weigh about 190 with 26" wheels and 36t rear sprocket. I only have about 160 miles on her right now. I think I just need to adjust the main jet. I'm new to mb so any input I'll take it.
 

Slogger

Member
Sep 8, 2014
544
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nohio
I'm 260 and have 29" wheels and a 40 tooth- this is actually the same overall ratio that you are running.
My bike is tuned for torque, all broken in with a stock muffler and a long manifold.
It will hit 29 anytime, and if there's a tail wind or a slight downhill grade, 32.
30 mph works out to about 5700 RPM, about what you'd expect for a long manifold stocker.
Mine once hit 36mph (6800 rpm) going down a small hill but it couldn't hold it on the flat.
I'm a big ape with a lot of wind resistance.
It has a nice relaxed sound at cruise. Pulls pretty hard from about 6 mph.
Having no 'need for speed' on a flimsy bicycle its gearing suits me.
I ride it around at about 24 most of the time.

Have fun!
 

Motakitty

Member
Feb 14, 2015
447
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San Jose, ca
I have the sbp expansion chamber, port matched intake and exhaust, nt speed carb, sbp plug wire. I plan on gutting the carb shut off valve since my pipe goes in front of it and I read it falls apart. Like I said it runs good just stumbles a bit at wot and has tons of torque at low end. I'm thinking it just needs more tinge to break in so I'm not too worried about it. Would like to cruise at about 25 and top out at a little over 30 so that's why I'm thinking a gear swap is in order.
 

crassius

Well-Known Member
Sep 30, 2012
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both of you sound a bit heavy for the ratio you have - it is possible that your motors never get quite up to their real power band (one of those confusing situations where a larger sprocket might get you both better hill climbing and more top speed)
 

Slogger

Member
Sep 8, 2014
544
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nohio
both of you sound a bit heavy for the ratio you have - it is possible that your motors never get quite up to their real power band (one of those confusing situations where a larger sprocket might get you both better hill climbing and more top speed)
The bike weighs 58 pounds, me and it make 318 pounds for the cg to drag around.
You are right, I could go faster with a bigger gear and a smaller manifold.
The way it is, though, even unloaded down a big hill it won't rev very high.
Instead it pulls really well through the middle range, especially for how it's set up.

I already got 30 and a little more downhill, so... ?
For my size and its size, it does great.
I go over the steep overpass at about 19.
That's where a lower gear would be nice.
 

Slogger

Member
Sep 8, 2014
544
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nohio
I think my bike weighs about 30-40lbs do that would make a total of about 220ish
Once the pipe is tuned for it, you should be good for those speeds. If you want more power all around and don't mind higher revs, change the sprocket.

You might need some extra links for your chain.

Mine ran much stronger after it broke in and I got the mixture leaned out.
 

Motakitty

Member
Feb 14, 2015
447
3
18
San Jose, ca
Thanks for all the responses and the info. I think I just need to put more miles on the motor and wait to see how it breaks in. I'd like more top speed but I'd hate to waste money on something that may not help our hurt the motor. I'll keep tuning the carb and get it running as best as I can before I change gears. The motor feels like it wants to be unleashed at wot but it's like it's being held back by something. So I need to figure it out.
 

Slogger

Member
Sep 8, 2014
544
4
18
nohio
Thanks for all the responses and the info. I think I just need to put more miles on the motor and wait to see how it breaks in. I'd like more top speed but I'd hate to waste money on something that may not help our hurt the motor. I'll keep tuning the carb and get it running as best as I can before I change gears. The motor feels like it wants to be unleashed at wot but it's like it's being held back by something. So I need to figure it out.
I had to go to a .60 mm dellorto jet to get mine to rev cleanly.
You may be surprised how small that main jet needs to be.
Go slow, lean it out in small steps watching for overheating.
Good luck!
 
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Motakitty

Member
Feb 14, 2015
447
3
18
San Jose, ca
K thanks. I had gone to a 73 drill bit size and it ran very hot and didn't run any better. I'm thinking there's another issue I'm having since when I did start changing jets I never noticed it running any different. I'll start by opening up the shut off valve on my speed carb. Then I'll reseal the carb to intake seal(right now I have the sbp o ring but I'll use silicone instead). Then I'll see how it runs.I'm liking the temp it's running at right now cause even after a long ride I can touch the head and not be burned.
 

Motakitty

Member
Feb 14, 2015
447
3
18
San Jose, ca
I drilled out the petcock on the carb so I'll see how it works. I'm using a separate fuel shut off valve from the auto parts store( it's black with the red switch). Hopefully it was just a fuel supply problem.I didn't like how that stock petcock on the carb worked like if it wasn't in the exact vertical position it might not supply the same amount of fuel, just seemed like a poor design. The rubber gasket inside it was in good shape though and looked like ok quality but I'll keep an eye on it.
 

Slogger

Member
Sep 8, 2014
544
4
18
nohio
Changing the main should affect the running at WOT and high rpm quite a lot.
Check your needle and seat, fuel filter and gas cap, too.
You'll be smilin when it starts running happy.
 

Motakitty

Member
Feb 14, 2015
447
3
18
San Jose, ca
I know the gas cap is vented good cause if I drain the fuel line the gas flows just the same with or without the cap on. Fuel filter is clean. I've cleaned the needle and seat. I'm gong to check the fuel float level as well. I figured changing jets should make a big difference, that's why when it didn't make a difference I figured I should look elsewhere.