I've been thinking of ways to increase the performance of my motorbike without having to buy after market parts. I use it for booting around up in cottage country, And I would realy like to give it a little more kick. I can't spend more than $200 canadian on it, and all of that is going to parts for rebuilding the bike, like gaskets, washers, bolts and bushings.


I can't afford anything aftermarket at this point and I realy don't want to install aftermarket parts on such a classic bike. (Its a 77' honda)

The bike has a fairly flat torque curve, I'd like to make it more of a peak near the optimal range which is around 7000 to 10000 RPM.
It redlines at 10,500, or so I've been told. I don't have a tack to tell.
The bike has a close ratio low speed gear box, so after first, the power down low does nothing. and since it already rockets through first and almost rockets through second, I think I can sacrafice a little low end acceleration for some better performance at higher gears.
By rocketing through first I mean I can literaly shift to second and take off from a standing start almost as fast as a bigger bike.
It's only got a small 75cc engine and yet it launches off the line faster than my friends 125cc.

Things I have already considered, and may do are:

-Advance the spark timing, this will increase high rpm performance, but decrease low rpm performance. basicaly what I want.

-Run a less restricted exaust to reduce back pressure, again, more high rpm power, less low.

I think there is more I can do, but I cant think of anything else.
Anyone here done bike tuning? Is there anything I can do to give it more kick without spending the money on aftermarket parts?
I want to balance all of these changes so that I don't risk damage to the engine, I even have the shop manual and my boss to help me out

There's not really a great deal you can do for free except to make sure it's clean and running a proper fuel mixture. Use a decent 2 stoke oil too.

If you haven't already, read up on expansion chambers before you think about messing with the exhaust. The last thing you want to do is to simply put a freeflowing system on.

Quote from sinbad :

There's not really a great deal you can do for free except to make sure it's clean and running a proper fuel mixture. Use a decent 2 stoke oil too.

If you haven't already, read up on expansion chambers before you think about messing with the exhaust. The last thing you want to do is to simply put a freeflowing system on.

It's actualy 4 stroke. So it doesn't have or need an expansion chamber.

Sorry, should have said that in the original post
I thaught of it after, but you'd already posted.

Advancing the ignition = more chance of knock, especially at high rpm high load. So don't do it (the power gains would be minimal).

Can you tweak the cam timing? Or change the carbs/carb settings? If you change cam timing you'll probably need to get it setup on a rolling road again though, otherwise it won't be fuelling right. Same goes for carbs.

Have you tried losing weight too - will make a HUGE difference to a 75cc bike (essentially a moped).

http://www.motorcycle.com/how- ... -you-should-care-3423.htm

I haven't read it all but it seems pretty good.

Although you're not looking to spend big amounts, Stan Stephens is the 2 stroke tuning god.

Also I've attached a cool animated pic of how the 2 stroke works.

Edit: Just read your reply just after clicking 'submit'. You might aswell ignore my post, unless you're interested.

I've been assured by my mechanic/boss that the timing change will not cause problems as long as I don't over do it.

I guess I could loose the plastic side panels, there pretty banged up anyway.

The fenders are already gone, I was planing on buying a set though, because I kind of need them in the mud, otherwhise the engine gets covered in mud and overheats. The fenders actualy prevent splash off of the wheels and deflected mud from the tank from landing on the motor.

Other than that, there isn't any weight reduction I can do without spending the cash for a plastic tank, everything else is structual.
It's an XR75, it was actualy made for junior/introductory motocross, so there's no lights or battery or anything detatchable.

I guess I could always ride without a seat

As carbs go, I have the original 30mm carb, and a freakin huge 60 or 70mm snowmobile carb, unfortunately, that woulden't work to well.

And for cam timing, It's manufactured to stock, the only way to change it would be move it a tooth left or right on the cam gear, but that would set the timing so far off that it would blow the motor if it actualy started at all.

I was just kind of hopeing that I could get a little more power without having to buy aftermarket.

Edit: the dirt road by my friends house is basicaly my rolling road, I set the carb up there after the motor was rebuilt the first time.(not by me)

Quote from DragonCommando :

I've been assured by my mechanic/boss that the timing change will not cause problems as long as I don't over do it.

I guess I could loose the plastic side panels, there pretty banged up anyway.

The fenders are already gone, I was planing on buying a set though, because I kind of need them in the mud, otherwhise the engine gets covered in mud and overheats. The fenders actualy prevent splash off of the wheels and deflected mud from the tank from landing on the motor.

Other than that, there isn't any weight reduction I can do without spending the cash for a plastic tank, everything else is structual.
It's an XR75, it was actualy made for junior/introductory motocross, so there's no lights or battery or anything detatchable.

I guess I could always ride without a seat

As carbs go, I have the original 30mm carb, and a freakin huge 60 or 70mm snowmobile carb, unfortunately, that woulden't work to well.

And for cam timing, It's manufactured to stock, the only way to change it would be move it a tooth left or right on the cam gear, but that would set the timing so far off that it would blow the motor if it actualy started at all.

I was just kind of hopeing that I could get a little more power without having to buy aftermarket.

I think Tristan was thinking more along the lines of you losing the weight
I guess all you can do is wear tight fitting clothing and tuck your head, knees and elbows in.

It sounds like you know what your options are already - limited. And tbh, you won't notice the difference. Just get a rear sprocket that works with the sort of riding and speeds you do and leave it at that unless you want to do it as a hobby.

I think your right sinbad, but since I have to partialy rebuild the engine anyway, I might just try a spark timing tweak.

I don't realy know, I was hopeing there was something I haven't thaught of yet.

I still think you'd be better off with cam timing tweaks over spark tweaks. I doubt you'll 'blow your motor' with one tooth adjustment!!!

And any small spart advance (i.e. a safe one) will produce so little difference to the power/torque that you might as well not bother.

Can you get a new camshaft(s)? Or regrind the ones you've got?

Quote from tristancliffe :

I still think you'd be better off with cam timing tweaks over spark tweaks. I doubt you'll 'blow your motor' with one tooth adjustment!!!

And any small spart advance (i.e. a safe one) will produce so little difference to the power/torque that you might as well not bother.

Can you get a new camshaft(s)? Or regrind the ones you've got?

The gears on the cam and crank are small, and have wide teeth, I think they are 10T and 20T respectively. So a single tooth means a big change.

I considered a new cam, but again, I only have a budget of $200 and a racing or high performance cam is more than $100, I can't justify that kind of expence.

I thaught of re grinding them, but even though I have the tools, the only thing I can do is increase the open and close slope or decrease the lift.

As I've been reading and responding, I've been realizing how little anything free is going to do for power, I figured combining the spark timing adjustments and the modifications to the muffler I was going to do would be enough to change the torque curve, I don't want more power over all, I want to change where the power is.

Can the cam pulleys come off, or are they part of the cam itself? If so, could you remove them, turn them through about 175°, and cut a new slot for the woodruff key there. That way you have +-5° cam timing available (or 10° crank timing), assuming you can also turn them around [the vertical axis].

Quote from DragonCommando :

As I've been reading and responding, I've been realizing how little anything free is going to do for power, I figured combining the spark timing adjustments and the modifications to the muffler I was going to do would be enough to change the torque curve, I don't want more power over all, I want to change where the power is.

Those bikes are usually pretty well optimized from the factory.

What you "could" do with a little money is nitrous. Pick up a small bottle and line from Ebay and plumb it into the air filter like a dry nitrous system. Tape the bottle to the handlebars, plumbed directly to the carb. Back off the timing and richen the mixture. Just be very careful to not melt a piston or burn up a valve.
http://cgi.ebay.com/ebaymotors ... 92QQrdZ1QQsspagenameZWDVW this is the real kit. All you really need is the tank, a few feet of braided line, a few zip ties and a roll of duct tape.

By the way. You could also attempt a leaf blower turbo. On small engines, it really does work. http://videos.streetfire.net/v ... 16d-9cfd-a1ad23ca9564.htm

Please don't come after me if you blow up the motor or warp into a parallel universe.

Quote from PAracer :

What you "could" do with a little money is nitrous.

That cost's more then $200, though. And, it doesn't alter the power band at all. Also the fact that it adds unnecessary weight.

Quote from PAracer :

Please don't come after me if you blow up the motor or warp into a parallel universe.

I don't know why you bother saying that, because the part where you suggest the leaf blower removes all possible credibility (or lack thereof) from you're post. Suggesting nitrous doesn't exactly make you sound knowledgeable either.

I've discussed it with my boss and my dad, and they both agree it would be a good idea just to leave it alone as far as modifying the setup of the engine, however, we have discussed using gasses, like NOS or something to boost power when needed.

I also have a blower that runs off of a 110v electric motor, I was thinking of changing the motor out for something that runs 12v, 9v or 6v, the only problem is that the bike generates AC power instead of DC, and I didn't get a chance to figure out the voltage max from the generator before the bike was stored for the winter.

The good thing would be that as the engine accelerated, the voltage would rise causing an increase in pressure like a turbo, how effective it is would be something to find out through actual use. I don't think it would do much, but it would be cool to hear the blower wind up when the throttle opened. The carburator is a 30mm and the blower output is aproximately 80mm. Don't know about CFM though.

Gases I considered using for injection where:

-Oxygen, expencive and has a chance of overheating.

-Hydrogen, not sure, but I was told it would act more like diesel, combusting with more of a BOOM, might blow the motor though.

-Quickstart, not realy a gas, but my boss used it to boost his scooter, the scooter didn't last long, but that was because he used it to run the engine when he was out of gas. It flew like it had nos though.

I still don't know, I'm kind of re-considering doing anything, since it's a classic, but the blower sounds like it might work, and sound cool. Even if it doesn't do anything major.

Tell me what you think about the gas types, and if you have any more to add to the list of posible and easy to get. (my dad has a method of getting hydrogen easy, but it would be a whole post on it's own to explain it, and I'd have to have him explain it again first.

"Please don't come after me if you blow up the motor or warp into a parallel universe." -PAracer
LOL, do demo users have sigs? because that is totaly something I'd put in my sig. With your permission of course.


@tristancliffe

That is something I considered myself, but the cam sprocket has a 2 bolt setup, and is 28T. The crank sprocket is a keyed shaft and is 14T, so the only way to make adjustments is to cut another key at another location, but that might throw the balance off. Having two key slots and only one key could throw the sprocket out of wack and cause vibration. And the cam sprocket has the same resault no matter which way it goes on because the teeth actualy line up with the wholes making for 14 teeth a half turn exactly on the wholes, no matter what way it goes on.

I wish I could adjust valve timing, but I can't without taking a big risk.

Quote from DragonCommando :

I've been thinking of ways to increase the performance of my motorbike without having to buy after market parts.

1 - Diet
2 - Ride naked

Quote from DragonCommando :

As I've been reading and responding, I've been realizing how little anything free is going to do for power, I figured combining the spark timing adjustments and the modifications to the muffler I was going to do would be enough to change the torque curve, I don't want more power over all, I want to change where the power is.

Modify the intake runner. I've been running tests recently in my mechanical engineering's I.C. Engines class on the effect of modifying intake runner length to change the torque curve. We went kind of extreme, starting with a 50in long runner, and moved down to a 12in. The longer runner keeps the max torque at lower RPM, and the short runner puts the max torque at higher RPM.

Quote from DragonCommando :

That is something I considered myself, but the cam sprocket has a 2 bolt setup, and is 28T. The crank sprocket is a keyed shaft and is 14T, so the only way to make adjustments is to cut another key at another location, but that might throw the balance off. Having two key slots and only one key could throw the sprocket out of wack and cause vibration. And the cam sprocket has the same resault no matter which way it goes on because the teeth actualy line up with the wholes making for 14 teeth a half turn exactly on the wholes, no matter what way it goes on.

I wish I could adjust valve timing, but I can't without taking a big risk.

I very much doubt a small slot will cause in imbalance that could wreck an engine - it's not as though cams turn very quickly, and nor are they balanced anyway. But if you were worried, you could always weld up the original slot thus returning the balance to roughly what it was before.

Why cant you port the cylinder head, raise the compression ratio, port the exhaust system, and intake system, so air can travel though pipes smoother/faster all way to engine.
You also can do lots of thing on carb for the starter. examples:- Knife edged, thinner spindle, ported throttle ports, etc. the gain wont be much, but it all add up from modifcation there and that.
its free to do with dremel and bit of your time. but you have to know what you doing.

Actually, if you port an engine, the ports get bigger and so the flow goes SLOWER. And maintaining gas velocity remains import.

I've also read that polishing the ports reduces the volumetric efficiency by reducing the swirling effect of the charge.

Basically, there's not much point in trying to hop up a 75cc motor. If you're really lucky and it's extremely well tuned, you might get something like 12hp out of it. That's about the same power/liter as a modern motocross bike. You're more likely to get around 7 or 8hp, which is probably about what it makes now.

If you want more power, get a bigger bike.

Quote from wheel4hummer :

That cost's more then $200, though. And, it doesn't alter the power band at all. Also the fact that it adds unnecessary weight.

I don't know why you bother saying that, because the part where you suggest the leaf blower removes all possible credibility (or lack thereof) from you're post. Suggesting nitrous doesn't exactly make you sound knowledgeable either.

And thank you Captain Bringdown. Honestly, unless you try something radical, or "stupid", like I have suggeested, you will likely not even notice a difference. $200 just isn't a reasonable budget for a performance build.

By the way. I'm curious to know just why you think I'm so full of crap.

I think a boost of some sort would be the only thing that I can do to get a notable performance increase. As far as I know, the engine makes something like 7HP with the stock tuning. Right now, it probably makes about 5-ish, because it has some gasket leaks, the timeing isn't set properly, and the carb needs to be rebuilt. The timing seems to be off just a little bit, because the bike hesitates if you snap the throttle. I fixed a car with the same problem, and it was timing, so I'm just guessing there.

When it's been rebuilt, the bike should run at factory power, with all of the factory settings. But I'm realy actualy considering a blower or something, just to tinker around with it to see what happens.

I got the bike for free, so I'm willing to pay as much as it would cost for the bike used for repairs. I have had the bike looked at, and as it stands it would sell for $200, so I'm going to put $200 into repairs.
I'm actualy not alowed to sell it, and I wouldent anyway, considering the age, and the rarity.

The logic is, that if I bought a bike for $200 it would be in the same shape. So by getting it for free, and then using the $200 for repairs, I get a better bike than if I bought one used.
They have small bikes at canadian tire for around $300 new, but they are crap, and I woulden't ride one even if it was free.

I spent alot of time on this bike before it was mine, I rode it more than the owner did, so he eventualy just gave it to me. It was passed around a bit before that, and it had the piston rings and cam chain replaced a year before I got it, but they never set the carb up, and it wasn't tuned right.

Take a look at the air filtration setup on the bike. I'm not even sure what type of filter or pre card ducting was used back then. You might be able to find a K&N replacement, or velocity stack and oiled foam.

Have you been able to do a compression test? Both dry and wet. If either fails to supply near stock numbers, your engine is never going to be what it could be. You can pick up a Craftsman kit that includes a guage and adapters for various sspark plug designs.

Any exhaust leaks, or intake leaks (moreso intake) can cause resitation on throttle tip in.

Does the engine still have its original ignition system? That could be a problem right there. We've got an old Snapper lawnmower that would die if you put too much load on it. A complete new ignition fixed her right up.

Right now, the compression is fine. The piston rings where changed and the valves where cleaned last year.
I tested the ignition system my self, it sparks fine.

The engine will be configured to stock settings, I actualy just decided that with my boss on friday. He's agreed to let me use the tools in our shop to do the repairs, so I'm going to be getting everything overhauled and torqued to spec.

Getting it to stock isn't an issue, but anything after that, like performance improvements is going to be.

The air filter is a single foam cylinder, its much bigger than the actual intake system, so there's minimal restriction in the intake, it goes from about a 2.5 inch opening in the air box to a 30mm opening on the carb. I'd been running it without a filter for a couple weeks before I realized there wasn't one. I made one out of cloth as a temp fix, and it didn't change performance at all. I actualy have the filter included in the list of parts I'm getting.

I think the only thing I can do if engine adjustments won't do anything is add bolt ons. But I'd have to improvize, I can't buy anything pre-made like kits. And there isn't much room now that I think about it.

[attach]43719[/attach]
Image from google

This is what my bike looks like, minus the fenders and number plate.
I also added a fue things to the image to show how tight for space the frame is.

The exhaust is a customized version of the one on the XL version, It's been modified to work with my XR. I didn't modify it though, I got it that way. It's quieter (somewhat) but doesn't reduce power. both versions of the bike are exactly the same exept for transmission gears and exhaust. The XL75 is a road worthy version of this bike, it has lights and signals. And instead of a 5 speed close ratio, it has a 4 speed gear box, much like the one from the earlier model XR.

I added the wheel sizes so you can get an idea of the size of the bike. As you can see there isn't much space, I'm sure I could figure out a way to add a boost system somehow. It just can't be bought, I have a blower that I could use, it's just figuring out how to use it.
I could run it off of a pulley on the fly wheel, I'd have to modify the engine case on the left side of the bike though.

It would act sort of as a super charger. It's probably wishfull thinking though.

Quote from DragonCommando :

It would act sort of as a super charger. It's probably wishfull thinking though.

Yes, it might. Once you have lowered the compression ratio of the engine (decompression plate maybe, but then you'll reduce squish and increase the valve : piston relationship. Then you'll have to get the carbs rejetted, possibly with bigger chokes to see any improvement. Will the sparks be sufficient to ignite compressed fluid? Unlikely, so you'll be wanting to improve the ignition system.

All for 7hp? It's a complete waste of time and money. Get a bigger bike and sell yours as a standard-ish bike for more than you'll get once you've added some crappy, unreliable modification.