Large Sprocket | Torque Exhaust - Need Dead Stop Power

jaguar

Well-Known Member
Joined
Nov 25, 2010
Messages
3,457
Mods For Low RPM Power

11 Changes I Did To Make My 48cc Climb Hills (at 5000 ft altitude)

1. raised ports for exhaust duration of 171 and transfers of 122
2. over 120 psi engine compression by lathing metal off of head mating surface
3. large idle jet which boosted low/mid range
4. smoothed the slide bottom with JBWeld [page]
5. Iridium plug (NGK BPR6HIX)
6. 56 tooth rear sprocket
7. custom expansion chamber w/holed baffle
8. Jaguar CDI
9. Intake extension
10. put on 16mm Mikuni but according to my carb page it should have a 14mm carb (and the large engine should have 21mm)
11. I created transfer ramps on the piston
12. put on JRL piston rings which have a small (.2mm) ring end gap, unlike the stock rings which are close to 1mm. Here's the rings for the larger engine. A small end gap is necessary to not lose compression.

This engine had already been converted to having a reed valve intake. I don't consider that a mod for increased low RPM power because I did a full comparison between the two intake types and they produce the same power. Of course this depends on correct intake duration for the piston port engine which needs to be around 110. The only real advantage of using reed valves is that all aftermarket carbs have their clearance between needle and needle jet set for use with reed valves. Piston port requires a smaller clearance or else the idle mixture isn't right and you need a leaner needle (lesser taper angle).

1. The higher the exhaust port is, the smaller the true engine size, the trapped volume. But if the port is too low then it won't rev out. So my advice is to raise the ports to get around 170 exhaust duration and 122 transfer duration. Here's my page on finding port durations.

2. At high elevations the psi goes down. So I had around 1.2mm lathed off the top of the cylinder which gave me over 120 psi. This is with a standard head I modified to have a squish band but really a squish band is not needed on a low reving engine.

3. With this Mikuni a #15 or #17.5 idle jet is what gets it needing around 1.5 turns out on the air screw. But while testing different sizes I found out that #20 with 2 turns out gave the best off idle power and it could even climb hills better.

4. I found that making the slide bottom have a smooth flow transition from cutaway to slide front gives a better idle mixture and also affects mid range. I did this using JBWeld. Click here to read more about this mod.

5. Iridium plugs spark easier when there is low spark energy which is the case with these cheap chinese engines. They don't have good spark power till high revs.

6. I increased the rear sprocket size to 56 teeth for climbing streets but I prefer 36 tooth for good speed in a flat city without hills.

7. Expansion chambers increase intake flow because of the return suctin wave from the diffuser cone. At top RPM the baffle cone creates a reverse pressure wave which increases the engines dynamic compression. But below the 2000 RPM pipe powerband the baffle wave interferes with the intake. So by putting holes in the baffle and enclosing it with another section I reduced the return baffle wave which aids low and mid range power. Read more

8. My Jaguar CDI mostly helps with high RPM because the stock CDI doesn't retard the ignition timing enough at high RPM. But due to low manufacturing standards for the stock CDI sometimes it also gives the wrong timing at low RPM so my CDI gives a near perfect timing curve that assures ignition is correct at all RPM. Read more

9. Adding on a long intake tube allows the return wave from the intake suction pulse to arrive back at the engine at the right time for low and mid range revs. This increases power for climbing hills. Although this is the dorkiest looking mod I do consider it one of the best.

10. For a 48cc a 14mm or 15mm carb is ideal. Before I knew how to size carbs I had bought this 18mm Mikuni and so now by decreasing its flow area with JBWeld I have essentially reduced it to 16mm which increases flow velocity which aids in gas/air mixture.

11. Transfer ramps smooth out the intake flow which increases it. A rounded corner doesn't create the obstruction of turbulence that a square corner does. The resulting transfer port duration was 122 which is good for low RPM power for these engines. Read more


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Acefire

Member
Joined
Feb 25, 2022
Messages
49
That is way too much jaguar. I am sure someone will benefit from that much in-depth.

My problem now is 4-stroking. At WOT it really starts to 4 stroke. Already dropped to a #65 jet and I can feel it hurt low end, but improved the 4 stroking problem at WOT.

My next step I feel like is:

1. Run the needle as rich as possible (jclip at the top)
2. Run the jet leaner (#65 or lower)

After the last mods I kept the needle clip where it was (two down from the top of 4 slots). I feel like running the needle at MAX richness with a smaller jet for WOT is going to be my sweet spot.

I love this s**t!
 

bigmike4269

New Member
Joined
Apr 18, 2022
Messages
2
Mossy laughs at anything :ROFLMAO:

You said speed wasn't important and hill climbing and easy take off was

A few guys like the crawling power of a 70t

@Don_Parsons put a 70t on his fat tire bike and loves it

@Mike St put a 70t on his Rear mount chain drive and he loves it

A 70t sounded like what ya wanted It probably won't go over 20mph But IDK it might go a little faster than 20 mph
I have a 28 tooth and I do over 30mph but I also I all high performance part's to so but right now I'm having a problem with my clutch
 

DieselTech

Well-Known Member
Joined
Sep 18, 2021
Messages
4,119
I have a 28 tooth and I do over 30mph but I also I all high performance part's to so but right now I'm having a problem with my clutch
I get 38-40mph out of my 44 tooth sprocket, why such a low count rear sprocket? To unless your making huge power from these engines, I dont see purpose in such a high geared motorbike. @DAMIEN1307 gets 51mph I believe out of his 36 tooth sprocket & a stock zeda triple 40 66cc engine.
 

Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
4,444
Mods For Low RPM Power

11 Changes I Did To Make My 48cc Climb Hills (at 5000 ft altitude)

1. raised ports for exhaust duration of 171 and transfers of 122
2. over 120 psi engine compression by lathing metal off of head mating surface
3. large idle jet which boosted low/mid range
4. smoothed the slide bottom with JBWeld [page]
5. Iridium plug (NGK BPR6HIX)
6. 56 tooth rear sprocket
7. custom expansion chamber w/holed baffle
8. Jaguar CDI
9. Intake extension
10. put on 16mm Mikuni but according to my carb page it should have a 14mm carb (and the large engine should have 21mm)
11. I created transfer ramps on the piston
12. put on JRL piston rings which have a small (.2mm) ring end gap, unlike the stock rings which are close to 1mm. Here's the rings for the larger engine. A small end gap is necessary to not lose compression.

This engine had already been converted to having a reed valve intake. I don't consider that a mod for increased low RPM power because I did a full comparison between the two intake types and they produce the same power. Of course this depends on correct intake duration for the piston port engine which needs to be around 110. The only real advantage of using reed valves is that all aftermarket carbs have their clearance between needle and needle jet set for use with reed valves. Piston port requires a smaller clearance or else the idle mixture isn't right and you need a leaner needle (lesser taper angle).

1. The higher the exhaust port is, the smaller the true engine size, the trapped volume. But if the port is too low then it won't rev out. So my advice is to raise the ports to get around 170 exhaust duration and 122 transfer duration. Here's my page on finding port durations.

2. At high elevations the psi goes down. So I had around 1.2mm lathed off the top of the cylinder which gave me over 120 psi. This is with a standard head I modified to have a squish band but really a squish band is not needed on a low reving engine.

3. With this Mikuni a #15 or #17.5 idle jet is what gets it needing around 1.5 turns out on the air screw. But while testing different sizes I found out that #20 with 2 turns out gave the best off idle power and it could even climb hills better.

4. I found that making the slide bottom have a smooth flow transition from cutaway to slide front gives a better idle mixture and also affects mid range. I did this using JBWeld. Click here to read more about this mod.

5. Iridium plugs spark easier when there is low spark energy which is the case with these cheap chinese engines. They don't have good spark power till high revs.

6. I increased the rear sprocket size to 56 teeth for climbing streets but I prefer 36 tooth for good speed in a flat city without hills.

7. Expansion chambers increase intake flow because of the return suctin wave from the diffuser cone. At top RPM the baffle cone creates a reverse pressure wave which increases the engines dynamic compression. But below the 2000 RPM pipe powerband the baffle wave interferes with the intake. So by putting holes in the baffle and enclosing it with another section I reduced the return baffle wave which aids low and mid range power. Read more

8. My Jaguar CDI mostly helps with high RPM because the stock CDI doesn't retard the ignition timing enough at high RPM. But due to low manufacturing standards for the stock CDI sometimes it also gives the wrong timing at low RPM so my CDI gives a near perfect timing curve that assures ignition is correct at all RPM. Read more

9. Adding on a long intake tube allows the return wave from the intake suction pulse to arrive back at the engine at the right time for low and mid range revs. This increases power for climbing hills. Although this is the dorkiest looking mod I do consider it one of the best.

10. For a 48cc a 14mm or 15mm carb is ideal. Before I knew how to size carbs I had bought this 18mm Mikuni and so now by decreasing its flow area with JBWeld I have essentially reduced it to 16mm which increases flow velocity which aids in gas/air mixture.

11. Transfer ramps smooth out the intake flow which increases it. A rounded corner doesn't create the obstruction of turbulence that a square corner does. The resulting transfer port duration was 122 which is good for low RPM power for these engines. Read more


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Sounds like your engine is dying! 120 p.s.i. is nothing when most stock kits have at least 160 or more.
Expansion chambers aid in intake flow? LOL, NOT the draw on the case through the transfers is from the velocity of the exiting exhaust gasses it has nothing to do with the return wave, that return wave when in time is what over fills the cylinder and boosts output in that range and below it only produces blocking pressure at the open port so as not to lose all the charge out into the pipe..
170 exh and 122 transfer is 23 degrees of blow down and less pressure on the down/power stroke which will NOT aid torque and shifts up in rpm where the port timing comes into play!
 

Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
4,444
I have a 28 tooth and I do over 30mph but I also I all high performance part's to so but right now I'm having a problem with my clutch
That 28t gear is costing you usable power, stock kits can do well over 30 and even 40 when setup right, most so called performance parts don't do anything without the internal mods that actually boost performance.
 

Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
4,444
That is way too much jaguar. I am sure someone will benefit from that much in-depth.

My problem now is 4-stroking. At WOT it really starts to 4 stroke. Already dropped to a #65 jet and I can feel it hurt low end, but improved the 4 stroking problem at WOT.

My next step I feel like is:

1. Run the needle as rich as possible (jclip at the top)
2. Run the jet leaner (#65 or lower)

After the last mods I kept the needle clip where it was (two down from the top of 4 slots). I feel like running the needle at MAX richness with a smaller jet for WOT is going to be my sweet spot.

I love this s***!
The main jet does not effect the low end and a 65 main is awful lean and will run very hot with you holding wot trying to climb hills,

carb_tuning_1200x1200.jpg
 

CrispyLeaves98

Active Member
Joined
Jul 29, 2021
Messages
447
I don't care about speed. I have to run it wot to get up these hills.

Instead of focusing on different gears and all that, why not settle for a different motor? Ive put over a thousand miles on my yd100 without taking the jug off since I put it together, maybe I was just lucky but it still runs. I weigh 183 and I ran a 36T sprocket and eventually moved to a 32T after some light port work. With both sprockets and proper clutch slipping, I could pull from dead stops and chug right along hills, my pedals were busted and sucked to use so the only thing I did with em is use it to start the bike. If you need torque, get a motor like the yd100. Also if your gonna buy it in the full kit, gasbike.net sells it 100$ off sometimes when it goes on sale. Also, if you get the Amazon yd100 from the cdhpower store, it only comes with one base gasket. Order some online, because you need to run the motor with 2 base gaskets. What motor do you have anyway?
 
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