Getting all 3 Horse Power out of your GT-5 Sky? (tear down)

recon chris

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Apr 28, 2011
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pacific grove
A question for you guys. At top rpm the engine seems to stop pulling. It has the ability to accelerate faster everything is painfully smooth at this point and I can feel its not caused by friction in the system (bearings chain ect.). Also the rpms that it gets to are not its Top Out rpms it sounds like the engine can go faster. I do still have the stock exhaust on it maybe the c.d.i. is advancing the spark to fast I hear it has a linear advancement arc instead of a standard motorcycles that retards the spark at higher rpms to keep up with the engine.

What do you guys think?
 

maintenancenazi

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Oct 22, 2011
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Get an expansion chamber! After I installed my SBP exhaust, I could quickly tell a HUGE difference. Almost like it nearly doubled the HP, and helped it rev out more. Not to mention a big increase in torque. i am a pretty big guy, I weigh in at about 230 lbs, but I am able to run a 32 tooth rear sprocket with 26'' tires. And it still pulls my big A#&$ up hills very nicely! ( With some peddling assist ) I feel my engine is still breaking in, as I have only run about 3 gallons through it so far. And every time I ride it, it seems to get a little stronger! if it keeps this up, I may be entering Bairdco territory in terms of speed!

I also notice after my rebuild, it would do what you discribe and not want to rev out, or hit redline. It my need to just break in more, hope this helps.

Peace, James
 
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biknut

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Sep 28, 2010
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I think the problem could be the exhaust to some degree, but the ignition timing probably is a bigger problem. From what I've seen, the stock cdi box has no provision to adjust the timing. It's just constant at whatever advance it has to start with. Manic Mechanic said it's about 6 degrees BTDC, and needs to be more like 12 degrees BTDC for best performance.

He claims the stock magneto rotors all vary slightly in timing, because of manufacturing tolerance, but none of then offer enough advance. This is the major reason one identical motor makes more power than another, so he says.

I've ordered one of his adjustable rotors to experment with, and it should come today. I'll let you know what I find after I've had a chance to play with it.
 

recon chris

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Apr 28, 2011
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pacific grove
Thanks guys!!

The expansion chamber I know will be a big help. How fast do you go with that 32 tooth sprocket. That seems soo low. On the flats does the engine get on top of the gear and rev it out? If it does then...DAM YOUR GOING FAST. I have a 39 but am thinking about dropping a tooth or two after the power modifications.

Biknut..tell me how that magnet works. I saw it a few weeks ago and was interested in buying one. Have you heard of Jaguars' modified motorcycle C.D.I. that he sells? Its like having a real motorcycle C.D.I. but it works on 6 volts.
 

maintenancenazi

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Oct 22, 2011
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^^ Oh yea, forgot to mention that I have fiddled with the timing/rotor position as well. I think I revived an old thread about that recently. But nobody wanted to discuss it further, so I thought it was just old news to everyone lol...
 

maintenancenazi

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Oct 22, 2011
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Thanks guys!!

The expansion chamber I know will be a big help. How fast do you go with that 32 tooth sprocket. That seems soo low. On the flats does the engine get on top of the gear and rev it out? If it does then...DAM YOUR GOING FAST. I have a 39 but am thinking about dropping a tooth or two after the power modifications.

Biknut..tell me how that magnet works. I saw it a few weeks ago and was interested in buying one. Have you heard of Jaguars' modified motorcycle C.D.I. that he sells? Its like having a real motorcycle C.D.I. but it works on 6 volts.
I haven't been clocked, or even have a speedometer. But I feel I am getting very close to, if not about 40. As it seems I am keeping up with traffic very well on the secondary routes. My bike is light, it's a Trek 3700 all aluminum frame, and weighs in at just over 40 lbs. Also, that's where the tune pipe really helps getting off the line and up to speed with the extra torque.

I have also noticed, that everytime I shorten the head pipe, it wants to rev out a little more. Although I have my pipe in the UP position, I have gotten to the point where I can't shorten it any more, because the head pipe wraps around the front of the downtube.
 

maintenancenazi

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Oct 22, 2011
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Do explain/share?...
I just modified the woodruff key ( filed it a little ) to get more slop in the magnet. And rotated ( advanced ) the rotor, then locked it down. It was all discussed in a thread some time ago. Couldn't hurt to give a try!

Edit, see the thread in High performance modifications, " Adjustable ignition timing " ( Sorry, not sure how to make clickable )
 
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recon chris

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Apr 28, 2011
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pacific grove
Maintenance... do you knotice any loss of low end torque when you shorten the pipe. Im debating weather or not I should make a pipe for high end torque or more towards the low end side.
 

maintenancenazi

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Oct 22, 2011
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Maintenance... do you knotice any loss of low end torque when you shorten the pipe. Im debating weather or not I should make a pipe for high end torque or more towards the low end side.
Good question, I am by no means any kind of expert here. I can only speak to what worked for me. But in real word testing (seat of the pants) I really couldn't tell much, if any loss in low end torque, each time I shortened it. But just seemed to help it give more RPMs on the top end.

I think that's where the chamber really comes into its own. In that it gives you a lot more low end/mid range, which really helps in acceleration with my tall rear sprocket. And the extra revs on top help with speed. Again, your results and mileage may vary, but I am very happy with the results with the pipe I purchased!
Peace, James
 

biknut

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Sep 28, 2010
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My adjustable rotor did come this afternoon. I just got back from my first test ride. it came with no instructions, but luckily I already read what Jim said about how to use it. I don't have a degree wheel so I used the eyeball method. He recommended advancing the timing about half the width of the key. That's what I tried to do, but it's not easy, because you can't really see the keyway when the bolt is in place. I tried to file a mark on the end of the shaft, and made a mark on the rotor with a sharpie. All I can really say is it's more advanced than stock.

It didn't make any earth shattering difference in power, but as advertised it did seem to have an effect on the 4 stroking problem I've been having, because after the motor warmed up it was almost non existant. Before it would start 4 stroking badly after about 30 mph. I just got it up to 36-37 a few times with only the slightest hint around 34 mph or so. I didn't notice any piston ping either, so tomorrow I'll probably advance it a little more just to see what happens. Knowing me I'm sure I'll play with it till I know all it's secrets.
 

maintenancenazi

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Oct 22, 2011
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Thanks for the update, that was about the same as my result. A noticeable difference in power, but not a huge one. But every little performance mod adds up in hopping an engine up. Sometimes called the " systems " approach.
 

The_Aleman

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Jul 31, 2008
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Yes, every little mod with these can add up to a big difference, doesn't matter if it's a HT or HS or HF.

You can definitely tell the difference even between a stocker that's tuned right and a stocker that aint. Hills seem that much bigger lol
 

biknut

Well-Known Member
Sep 28, 2010
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My adjustable rotor did come this afternoon. I just got back from my first test ride. it came with no instructions, but luckily I already read what Jim said about how to use it. I don't have a degree wheel so I used the eyeball method. He recommended advancing the timing about half the width of the key. That's what I tried to do, but it's not easy, because you can't really see the keyway when the bolt is in place. I tried to file a mark on the end of the shaft, and made a mark on the rotor with a sharpie. All I can really say is it's more advanced than stock.

It didn't make any earth shattering difference in power, but as advertised it did seem to have an effect on the 4 stroking problem I've been having, because after the motor warmed up it was almost non existant. Before it would start 4 stroking badly after about 30 mph. I just got it up to 36-37 a few times with only the slightest hint around 34 mph or so. I didn't notice any piston ping either, so tomorrow I'll probably advance it a little more just to see what happens. Knowing me I'm sure I'll play with it till I know all it's secrets.
I feel kind of stupid. After reviewing what I did last night, I realize I moved the rotor the wrong way. I actually retarded the timing instead advancing it doh!

I guess it ran with less 4 stroking because it's getting more broken in or something. It did seem to lack much of a torquey feel.
 

recon chris

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Apr 28, 2011
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pacific grove
I thought "retarding the spark" or making the engine spark later was what this engine needed at high rpms? Unless you engine was already very retarded to begin with i thought that should have helped?
 

biknut

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Sep 28, 2010
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I thought "retarding the spark" or making the engine spark later was what this engine needed at high rpms? Unless you engine was already very retarded to begin with i thought that should have helped?
According to what Manic Mechanic has said, the ignition timing is already very retarded to begin with.

I'm confused though. I thought he said we need to advance the timing, but in the his picture it looks like the timing is retarded.

 

recon chris

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Apr 28, 2011
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pacific grove
That is true in that picture the timing is a few degrees retarded, If that is the original woodruff key I’m looking at. Have you seen jaguars page he says a completely different story. Another thing I think should but understood is that setting the magnet back or forward will not affect the CDI linear output. Jaguars sells a CDI that gives a parabolic curve. As the engine rpms go up it retards the timing little by little.

His link
http://www.dragonfly75.com/motorbike/CDI.html
 

Skarrd

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Oct 13, 2010
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That is true in that picture the timing is a few degrees retarded, If that is the original woodruff key I’m looking at. Have you seen jaguars page he says a completely different story. Another thing I think should but understood is that setting the magnet back or forward will not affect the CDI linear output. Jaguars sells a CDI that gives a parabolic curve. As the engine rpms go up it retards the timing little by little.

His link
http://www.dragonfly75.com/motorbike/CDI.html
i've got his cdi, it makes a huge difference in the power output of the bike
 

recon chris

New Member
Apr 28, 2011
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pacific grove
OK so im tired of waiting for my friend to cnc me a 4mm thick aluminum spacer for the jug. So i went down and bought 4mm thick aluminum plate. With a dremmel i think i should have it routed out tomorrow.
 

multipaul

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Mar 31, 2012
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Germany
While Recon Chris is dremeling, a few infos about my stock cdi. There seems to be a lot of different informations about an optimal ignition timing and stock cdis. My engine is from 2010. The magnet is very strong. It must be a neodym (and I can verify that). The ignition timing is, as I expect it for a twostroke.

500rpm 10°, that's good for easy starts
700 12°
900 15°
1200 18°
1600 25°
1700 25°
1800 25°
1900 25°
2000 25°
2100 26°

23° to 25° are normal values for such engines.
At the moment I have no possibility to measure at higher rpms.
Who has ever measured at higher rpms?

I know, that topic belongs to another thread.
Measurement is my third hobby.

@Recon Chris, this is a great thread and you made a very interesting work. Sorry that I didn't find it earlier.
I'm not sure, but I'm afraid your 4mm aluminum plate will dissapoint you.
Your transfer ports will be open about 140° and the exhaust about 166°.
IMHO that doesn't match.
You will loose lots of power at the bottom end. Could be you have to pedal behind every red traffic light. But let us see.


I wish you best results - multipaul