Jetting and Temps

[Tyler6357]

Hey everyone. I have a question about jetting. Okay, I'm running a Dax F80, 66cc center mount, china girl on my old school steel frame Peugeot 26 inch 21 speed mountain bike. I have a stock slant head, a sick bike parts expansion pipe, a 15 mm RT (Dellorto clone) carb and I'm running Opti-2 at 100x1. I've jetted it with the .65 mm jet and it was 4 stroking quite a bit but still had pretty good power so I left it in. After riding it for about 700 miles with the .65 jet, I decided to see if I could get more out of it by jetting with the .60. The minute I put the .60 in I noticed a huge increase in performance. The 4 stroking is almost completely gone and the motor is running with more power. It's running the best I have ever seen it! My top end speed went up from 28mph to 33mph on level ground. All seems good but today I took it out on a 90 degree summer day here in CA. I rode it for about 15 miles and I rode it pretty hard and fast up some pretty steep hills with a 40 tooth rear sprocket. When I drove it home I got out my laser thermometer and pointed it at the head while it was still idling and it was 350 degrees!! I have checked it out before when I was using the .65 and it always registered about 280 at the most after a similar ride. I'm clearly running hotter with the .60 jet. It makes sense since I have so much more power, speed and higher RPMs now. My question is: Is a head temp of 350 degrees too much heat? Is my engine going to burn out fast at that temp? Should a go back to the .65 jet and the 4 stroking? Am I pushing my luck with the .60? Also, if I put a Fred Head on it will I have to re-jet and will it run cooler or will the extra compression make it run even hotter? I'm not really interested in going any faster than 33mph on this thing. I have never done any porting on this motor.

 

[Slogger]

I have a DAX 66 too.
The dellorto 60 jet is the one I ended up running, the only real difference I can see from your description is the tuned pipe.
If it's making good power it's going to run a little hotter. My advice would be to run a colder plug, ride below full throttle sometimes, and if you thrash it a few miles ride easy for a little cool down period. Coast down hills, go uphill at 3/4 throttle, shut it off at long red lights, all those would keep it from getting too hot.
4 stroking too much with a rich setting isn't good for it. The pressures are high and irregular, it's a 2 stroke engine after all.
It's only a peewee, it has to work hard. It'll get hot on long, fast rides.

 

[Tyler6357]

I have a DAX 66 too.
The dellorto 60 jet is the one I ended up running, the only real difference I can see from your description is the tuned pipe.
If it's making good power it's going to run a little hotter. My advice would be to run a colder plug, ride below full throttle sometimes, and if you thrash it a few miles ride easy for a little cool down period. Coast down hills, go uphill at 3/4 throttle, shut it off at long red lights, all those would keep it from getting too hot.
4 stroking too much with a rich setting isn't good for it. The pressures are high and irregular, it's a 2 stroke engine after all.
It's only a peewee, it has to work hard. It'll get hot on long, fast rides.

Thanks for responding! Oh, the spark plug, good thinking! I forgot to say but I've been using an NGK B7HS, should I switch to the B6HS and test? I should say that the head and the spark plug were both measuring the same 350 degrees during my temp test. Have you ever measured the temperature of you're head? What spark plug are you using?

 

[Slogger]

Thanks for responding! Oh, the spark plug, good thinking! I forgot to say but I've been using an NGK B7HS, should I switch to the B6HS and test? I should say that the head and the spark plug were both measuring the same 350 degrees during my temp test. Have you ever measured the temperature of you're head? What spark plug are you using?

It's only in the 80s around here, I've had good luck with the 7 plug. I run a 6 in the late fall and a 5 in the winter for short rides.
I also ride easier and I don't go very far, heck, my longest ride ever was only 13 miles. You may want to try an 8. Higher number=colder plug.
I have been around motorcycles so long I can actually smell an overheated engine. Since it hasn't been a problem, I've never really checked it. After being shut off for 5 minutes I can touch it without gettin burned, so IMO it's ok.
My bike runs fairly cool, but like I said I don't really thrash it, I just motor along checking the scenery and whatnot. Lots of cuteys out here in their little shorts, it's summertime!

 

[Slogger]

If you look at the electrode on the plug, you'll see an area near the end that is burned clean, to like a gray color of bare metal. If that area is only about an 1/8" out near the end of it, your heat range is about right. If it is burned clean clear to the bend, or beyond it, it is too hot. If it is fouled out to the end, it is too cold.
Just another way to rate it.
Hope this helps.

 

[Tyler6357]

Alright, thanks for the info. I will check out my plug and maybe just get an 8 and see how that is. I will also try a slower ride, I don't usually ride it that hard but I wanted to test it out, the extra power and torque are amazing and I just had to rock it! Maybe I could go 90x1 on the Opti-2 for a gallon and see if it runs cooler too. I've been reading that the FredHeads have larger fins to dissipate the heat and make it run cooler. I've been thinking of getting one for awhile.

 

[Slogger]

I would love a Fred Head but it would spoil my "It's all stock and even de-tuned, officer, this thing wouldn't pull a sick ho off a toilet seat!" story. We aren't allowed to go fast in Nohio.
I think the colder plug and riding at partial throttle now and then should work for ya.

 

[Motakitty]

Paint the head to match the motor. Someone that doesn't know about these motors will just see a stock motor I'd it's all the same color.

 

[Slogger]

Paint the head to match the motor. Someone that doesn't know about these motors will just see a stock motor I'd it's all the same color.

Maybe, but that Fred head is pretty big and cool looking. They might begin to doubt the 49cc story then..

 

[LongboardMalinois]

I have an unpainted, shiny Fred head on mine, and just yesterday an officer swallowed the 49cc story hook line and sinker! The officer pulled me over in a parking lot, which is private property, so i am wondering whether he had the right to pull me over!

 

[Slogger]

I have an unpainted, shiny Fred head on mine, and just yesterday an officer swallowed the 49cc story hook line and sinker! The officer pulled me over in a parking lot, which is private property, so i am wondering whether he had the right to pull me over!

I'd think their antenna would go up, that's a big head.
Parking lots are private owned, public access areas. He can bust you wide open in a parking lot here in Nohio.
They have to be in hot pursuit to follow you into your own garage, don't ask me how I know.

 

[Tyler6357]

Update: Hey, I got an NGK B8HS spark plug and tried it. I ran it for about 1 1/2 miles, up a moderately steep hill at WOT. When I got home the spark plug base measured at 310 degees. Wow, that's almost 40 degrees cooler!! Okay, it was a cooler day and I didn't run it as long but I ran it at mostly wide open to simulate the first ride but it still seems significantly cooler. Thanks for the tip Slogger!! I will keep checking my temps and see how it goes. I'm still thinking of getting the Fred Head but this thing is running so good right now, I don't want to mess it up.

 

[Slogger]

Cool!
The official blarney is that each number cooler decreases the combustion temp about 50 degrees C.

 

[Tyler6357]

Wow, I wonder if I should try a 9.. how cold do they make them? I didn't notice any difference in performance going to the 8, but 310 F is much better than 350 F.

 

[exokinetic]

The problem you will run into if you keep going hotter on the spark plug is plug fouling at idle, and slow, low load running.

The colder plug is not getting as hot at max load, WOT. But it is even colder at idle, and when your not running the engine as hard. If the plug is not getting hot enough at idle it will not clean itself, no matter how perfect your mixture is. You will then foul plugs constantly if you don't run the engine hard enough to get the heat in the plug.

Most of us builders seems to find that the "7" heat range is perfect for this engine, even in very high performance, high compression applications.

I would wonder what an actually cylinder head temp gauge (goes under the spark plug) is reading. Its hard to say anything concrete with a temperature reading taken from the outside of the head. You don't know how much aluminum is between where you are taking your reading, and the combustion chamber itself.



To help you out:


An aluminum cylinder head, in any combustion engine, works most efficiently at around 400 degrees F. It can safely be run up to about 425 degrees F, and even higher, depending on the silica content of the aluminum.

If that 350 degrees F were the highest temperature anywhere in that head, I would say you have no problems at all, might need to go a little leaner.

But I wouldn't trust a reading that wasn't damn close to the base of the spark plug.


Also consider:


If you are thinking about the Fred Head, you have to consider the fact that all that extra aluminum is going to remove heat from the cylinder MUCH MORE efficiently than a stock head.

If you changed nothing else in your setup, but added a Fred Head, even with its higher compression, the head will run cooler than your current stock head, I can guarantee you that.


If you have any more questions don't hesitate to ask!

 

[Tyler6357]

The problem you will run into if you keep going hotter on the spark plug is plug fouling at idle, and slow, low load running.

The colder plug is not getting as hot at max load, WOT. But it is even colder at idle, and when your not running the engine as hard. If the plug is not getting hot enough at idle it will not clean itself, no matter how perfect your mixture is. You will then foul plugs constantly if you don't run the engine hard enough to get the heat in the plug.

Most of us builders seems to find that the "7" heat range is perfect for this engine, even in very high performance, high compression applications.

I would wonder what an actually cylinder head temp gauge (goes under the spark plug) is reading. Its hard to say anything concrete with a temperature reading taken from the outside of the head. You don't know how much aluminum is between where you are taking your reading, and the combustion chamber itself.



To help you out:


An aluminum cylinder head, in any combustion engine, works most efficiently at around 400 degrees F. It can safely be run up to about 425 degrees F, and even higher, depending on the silica content of the aluminum.

If that 350 degrees F were the highest temperature anywhere in that head, I would say you have no problems at all, might need to go a little leaner.

But I wouldn't trust a reading that wasn't damn close to the base of the spark plug.


Also consider:


If you are thinking about the Fred Head, you have to consider the fact that all that extra aluminum is going to remove heat from the cylinder MUCH MORE efficiently than a stock head.

If you changed nothing else in your setup, but added a Fred Head, even with its higher compression, the head will run cooler than your current stock head, I can guarantee you that.


If you have any more questions don't hesitate to ask!

Thank you so much for sharing your knowledge on this. I have read other posts you have posted on this forum and it's clear to me that you know what your talking about! I had no idea that 425 F was in the tolerance level for these engines but that makes me feel better. I have been using the 8"" spark plug since I wrote this and not much has changed. I now have over 1200 miles on this engine. I haven't put the Fred Head on it, but I keep thinking about it. Like I said, the engine is running very well, perfect for me really, so I haven't been really very motivated to change anything or mess anything up. But after reading your post, I think I will buy a Fred Head and try it out and see how it runs, if I have problems I can just go back to stock. Do you think I will have to change the jet again with the Fred on it?

 

[exokinetic]

I had no idea that 425 F was in the tolerance level for these engines but that makes me feel better.

Well I would qualify the statement as "in tolerance for combustion engines with aluminum cylinder heads". And these engines certainly meet that standard. The important bit is that the temperature you are measuring is actually the highest temperature in the cylinder. It does you no good to look at the temperature of one of the cooling fins and think your safe when it is 350 degrees F, when there hottest spot in the head (usually right at the spark plug, but not always. For instance if there is a sharp ridge in the cylinder, it can become a hot-spot that becomes hotter that the spark plug, and it will ignite the mixture instead of the spark plug. This is called "pre-ignition" and it leads to engine destroying "detonation") is actually 460 degrees F and about to start causing pre-ignition.

Do you think I will have to change the jet again with the Fred on it?

Well if you do go with the Fred Head, to start with, you will probably have the best overall usability with the 6.0cc version.

It will work great on a stock motor, and even if you decide you want more power later, and you start going with a better pipe, or a ported cylinder perhaps, you wont have to run race gas like you might have to with the 5.0cc or the 5.5cc.

As for jetting, it really depends on where the final compression number ends up with the new head on the engine (this holds true for any head you change too, even a different stock head).


But, in general, if the new head results in INCREASED compression (witch I suspect you would see, switching from a stock head to a fred 6.0cc head) then you will *likely* have to go a bit richer.

As a general rule of thumb, any time you make a modification that is going to increase the power output of the engine, you will need to go richer on the jetting.

To be safe, always start by going a little richer than you were before the power adding modification, then start going leaner. If you start to gain performance by going leaner, keep going leaner until it starts to loose performance. Return to the setting that resulted in BEST performance, then -AND THIS IS VERY IMPORTANT- go one more step richer.

This not only keeps you on the safe side, but although you loose about 5% power output with the "slightly rich" setting, power output becomes more stable over a long ride. If you used the "best power" setting you would have the extra 5% up front, but you would loose 10-20% when the engine is run hard for an extended period. By using the "slightly rich" setting you will limit the heat soak losses to 5-10%.


These numbers are true for all air cooled engines.


-Side Question: What stock head are you running, spark plug straight up and down, or spark plug at an angle?

 

[Tyler6357]

exokinetic
Thank you. First, yes, I understand that getting temp readings off the fins are rather useless. I found that on my stock head (it's a slant head) that the hottest part of the outside of the head is at the very base of the spark plug where it screws into the head, with considerably lower ratings on the fins and less still on the cylinder fins and the crank case measuring much cooler than that. I tried to do my test on the hottest part but I understand your point about trying to get true and meaningful measurements, I guess the most accurate would be a measurement in the squish ignition zone on the inside of the head.

Also, my pipe is a SBP expansion pipe with a pretty long header (maybe 16") which gives me extra low end power but it's not as nice as other larger expansion chambers out there, like ones off of 2 stroke motorcycles. Anyhow, when I get the Fred Head I will follow your advice and jet up at first and move down until I dial it in and I will go with the 6.0cc one, that was the one I was planning on getting anyway. I have a .60 jet in right now so maybe I will try a .68 and more down from there. I've been using the only gas I can buy in my town in CA, E10, 87 octane. There is no gas station here that sells gas without ethanol in it, except at the airport who won't sell any to me. I use the 87 octane stuff because I didn't think that higher octane would make much of a difference for one of these small engines but I could be wrong about that. Here are a couple pics of my bike (more at my profile page).

 

[exokinetic]

I use the 87 octane stuff because I didn't think that higher octane would make much of a difference for one of these small engines but I could be wrong about that. Here are a couple pics of my bike (more at my profile page).

Well, when it comes to a high compression engine build, I cannot say enough good things about VP U4.4 Race Gas.

But for overall usability and utility its best to build a "street" bike engine for pump gas use (race gas is awesome, and if you are racing I wouldn't recommend anything else, but if you run out of gas on the street and your engine wont run on the pump gas the 'station has, your SOL). And this necessarily means limiting compression. Nothing wrong with that, just designing the engine to meet the desired use.


But street engine or race, I don't think it can be said enough how awesome the various 2 stroke dirt bike pipes are for our engines (KX65, KX80, RM80, CR80, KTM65... I prefer the 80cc pipes for the low end torque). And you don't have to run race gas to use 'em.

And when it comes time to fit that dirt bike pipe to the China Girl, look no farther than:

http://shop.crmachine.com/category.sc?categoryId=21

"Fire Breather Header Kit"

P.S. I raced with that pipe for a little while before I went to the KX80's, I'll bet you get pretty decent bottom end with that extended header. I think I decided the header diameter on those pipes is just too small. I'm liking 1 1/4" a lot more.


P.S.S.

BUY YOURSELF SOME CST CYCLOPS OR MAXXIS HOOKWORM TIRES ASAP!

The widest version, 2.5".

Especially at the races man, its a safety issue, for real.

Plus you will have WAY more fun with confidence that comes with running the right tires, trust me.

 

[Tyler6357]

I hear ya, I got that pipe several years ago, I didn't even know about those other pipes at the time. Had I known they were available, I would have gone with one of those. But this bike is not really a racing bike, it's more of a commuter. I first got into this because I wanted something I could put in my motorhome so I wouldn't have to tow a car. This bike currently tops out at about 35mph with a 40 tooth sprocket and that's good enough for this thing. It's a steel frame but the rims are nothing special, I really don't want to go any faster on this bike, it's a pretty old mountain bike and no shock fork (another thing I was thinking about getting). I much rather have the long header and the low end for this one, I'm up to 25 mph in less than 2 seconds.

Tires, yeah, I know, I have some cheap slick ones that I am planning to use at Willow Springs, they are 26x2.10. When the ones I have on right now are bald I plan on getting Kevlar tires, at least for the back. I was going to get THIS ONE
I was worried about the chain hitting the tire with the 2.5" ones. I'm already using a BMX chain because the 415 was hitting the tire, also I found that the thinner BMX chain seemed to go through the engine sprocket smoother and with less drag. I got a really strong BMX chain with reinforced links and a breaking load of 12,500 Newton. I haven't had any twisting issues with it at all. But even with that chain, I've only got about a 1/2" clearance with the tire. The 2.10" tires on now have no trouble with the chain scuffing the tire.