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Discussion Starter #1
Hi,

I’m in need of your help, if you don’t mind sharing some of your expertise.

I have the Danish version of the CT70, - a Honda ST50, known as the Dax, which I’ve built myself - all except for the engine that is, which I bought restored.

I would like to not alter the original engine, but instead build a new one all from scratch.
I know next to nothing about building engines, but hoping that youtube and this forum will get me a long way. The reason for doing it is pure learning, - and hopefully to get a fun ride afterwards.

The top speed of the original 50cc engine is 55 km/h (34 mph), and I wouldn’t mind reaching 70 - 80 km/h (43 - 50 mph).

The bike is 6V currently, so I would like to make the new engine 6V as well.
If you consider this a no-go please let me know why. I don’t mind the poor headlight when rev’s aren’t that high, and I must admit that I kinda do like the nostalgia and “simplicity”, even though I would like to make a few upgrades as mentioned below.

Here are the details/questions.
Hope you can give me some feedback for the build.
Questions might be dumb; I’m sorry for that.
Appreciate all the help that I can get.

Engine type
Honda XL70 (11100035100 + 11200086010)
I’ve got the engine crank L & R already. I would like to make it a 4 speed manual (bike is currently a 3 speed semi-auto). I haven’t got the complete engine, only the engine cases.

Engine bore up kits
88cc

#1 SP TAKEGAWA (Special Parts TAKEG : 17R-Stage + D Bore Up Kit 88cc (H-cylinder) [01-05-0509]
#2 SP TAKEGAWA (Special Parts TAKEG : 17R trin + D Bore Up Kit 88cc (H-cylinder) [01-05-0505]
#3 SP TAKEGAWA (Special Parts TAKEG : 17R-trin E Bore Up Kit 88cc [01-05-0528]
#4 SP TAKEGAWA (Special Parts TAKEG : 17R trin E Bore Up Kit 88cc (H-cylinder) [01-05-0527]
What’s the difference between #1 and #2?
What are the difference between #1&#2 vs. #3&#4, - stage D vs. stage E?
Which one of these four options would be preferred (if any) and why?

106cc
SP TAKEGAWA (Special Parts TAKEG : 17R trin E Bore Up Kit 106cc (Scud Cylinder) [01-05-0529]
SP TAKEGAWA (Special Parts TAKEG : 17R trin + D Bore Up Kit 106cc (Scud Cylinder) [01-05-0508]
Stage D vs. stage E again?

Clutch
SP TAKEGAWA (Special Parts TAKEG : Forstærket centrifugalkoblingssæt [02-01-0016]
Can the original clutch be used if I buy a used SL/XL engine, or would this be a good option?

Gear
Should I source an original SL/XL engine and use the original gear?

Flywheel
Should I source an original SL/XL engine and use the original gear?

High volume oil pump
https://www.japan-webike.dk/products/23465194.html

Oil cooler
Would prefer to not use this (aesthetics).
Will 88cc and 106cc be okay without an oil cooler, or is this an absolute no-go, and should one be installed under all circumstances?

Carburetor model
22mm: https://www.japan-webike.dk/products/20021256.html
24mm: https://www.japan-webike.dk/products/45674.html
Is 22mm too big if I go 88cc and not 106cc?
Please note the exhaust I’ll be using. Would like one of these Yoshimura carbs.
Are the mentioned Yoshimura carbs the correct models for my bike?

Air filter model
https://www.japan-webike.dk/products/21421757.html
Will it work with MJN22/24? Says MJN28, but reckon it’s universal?

Exhaust
OverRacing GP Performance Titanium.
I’ve got this exhaust already.

Misc
What more do I need to pay attention to, if any, and what parts would need to be acquired?

My bike:
IMG_20180624_051353_609-1.jpg
 

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I can't answer all the questions, but can tell you a bit about building these engines. The difference between the bore-up kits is that some are for 6 Volt and others 12 Volt kits, and that the D-type kits have a centrifugal decompression valve that reduces compression during start-up (makes it easier to kick.) To get to the speed range you are looking for, I would go with the 106 kit. You will also want a larger front sprocket (16-17T,) and perhaps a smaller rear as well to get a higher top speed. You will need a manual style clutch, shifter shaft ( or you can cut off the end of an automatic shift shaft shaft,) the manual style clutch cover, cable and handlebar perch. Definitely get the high volume oil pump. The cooler is not absolutely necessary unless you are riding it hard or live in a hot climate. Getting an SL70 engine for the transmission, kick-start gear, clutch cover, and flywheel is a good idea. The important thing is that if you use the flywheel, you will need the stator and stator plate from the SL70, and a crankshaft for manual 6 volt engine, as the end of the crank is different from the auto clutch versions. The difference in the air filters listed is one is for 22-24mm carbs, and the other is for 28mm carbs. Interestingly, I just started a how-to on building these engines on the blog of my website recently. I have not finished it, but will be doing it in installments, covering the different parts of the job.
 

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Discussion Starter #3 (Edited)
I can't answer all the questions, but can tell you a bit about building these engines. The difference between the bore-up kits is that some are for 6 Volt and others 12 Volt kits, and that the D-type kits have a centrifugal decompression valve that reduces compression during start-up (makes it easier to kick.) To get to the speed range you are looking for, I would go with the 106 kit. You will also want a larger front sprocket (16-17T,) and perhaps a smaller rear as well to get a higher top speed. You will need a manual style clutch, shifter shaft ( or you can cut off the end of an automatic shift shaft shaft,) the manual style clutch cover, cable and handlebar perch. Definitely get the high volume oil pump. The cooler is not absolutely necessary unless you are riding it hard or live in a hot climate. Getting an SL70 engine for the transmission, kick-start gear, clutch cover, and flywheel is a good idea. The important thing is that if you use the flywheel, you will need the stator and stator plate from the SL70, and a crankshaft for manual 6 volt engine, as the end of the crank is different from the auto clutch versions. The difference in the air filters listed is one is for 22-24mm carbs, and the other is for 28mm carbs. Interestingly, I just started a how-to on building these engines on the blog of my website recently. I have not finished it, but will be doing it in installments, covering the different parts of the job.
Thanks a lot davmo - all of the great information is appreciated. Makes it a bit easier to navigate in all of this, and make the (for me) correct decisions.

If I'm not mistaking the SL70 and XL70 crank cases should be 100% identical, and both with manual transmission, right? Then I can choose to use the XL crank cases that I've got already (they're in great condition), or I can part with them, if I manage to find a complete SL70 (XL70?) engine that looks to be in decent shape.

Seems that I should "simply" just continue down the path where I'm headed currently, - to recap:

  • Rebuild a SL/XL70 engine
  • Reuse original transmission, kick-start gear, clutch cover, flywheel, stator, stator plate, crank shaft
  • Switch the original clutch for a heavy duty one
  • Obtain shifter shaft, manual style clutch cover, cable and handlebar perch
  • Go with 106cc D stage (if I'd like to kick it over easily)
  • Oil filter isn't needed (living in Denmark, not hot sadly)
  • 16T/17T front sprocket + perhaps smaller rear to obtain higher top speed
  • Get high volume pump
  • Get 22/24 carb
  • Get K&N air filter
One question though; what size carb do you think would be suitable fora build such as this, reckon that the 22mm would be fine - or do you actually think that it might be too big?

Will be sure to check out your website, really appreciate all of the advise.
Now it seems that I just need to go on the lookout for a SL/XL engine that can be reused, - whether or not that'll be an easy find time will show.

On a final note.. Ain't I looking at a wrong 106cc D stage kit, - seems to be mentioning 12V in the description? :/
 

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You can also look for a CT70H model engine. The only difference between that and the SL/XL70 engines is the head (which is going to be replaced by your 106 kit) and the gear shift pattern is a 4-down. The 108cc kits I run from hondatrailbikes.com will perform well with a 20mm carb to a 26mm carb, if jetted correctly and matched to the exhaust. The cases you already have could be a good foundation for your engine build. You are correct that they are identical. If you can locate the transmission and flywheel, this manual clutch kit and 108cc kit with race head will get you where you want to be. Then you will not have to purchase an entire engine when the flywheel and transmission are all you need. Honda TB parts are quality parts I have used numerous times without any significant problems. I have Takegawa stuff too, and would say they are competitive performance-wise.

 

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I seen your post over on lilHonda too. But I was laying back a bit too see if anyone else might offer some help. Your first post includes a ton of questions...tired me out just thinking about all of the typing I'd be doing to start answering...lol, sorry.

You already have engine cases...hopefully they include the typical bits... oil pump? cam chain tensioner? Drain plug? Long cylinder studs? Alignment dowels for cases, and cylinder?

If you only did a simple 88cc bore up. You MIGHT reach 50mph. It's not too hard to make a 88 go past 50.

But if you're willing to go up to a 108, that is definitely what I would recommend. You will love it compared to 72 or 88cc IMO.

Either way, I guess I would recommend buying a complete engine, or at least the lower end, to include the clutch side AND stator/flywheel. I think you would save money in the long run.
If you bought a complete engine, you could use the stock head too. That would save the cost of a new head, but you would spend almost as much having the stock head rebuilt. But a stock head isn't the BEST, as far as performance. It's the starting point of GOOD choices for 108cc IMO.

A XL or SL70 will both have a 1down 3up shift pattern.
A CT70H and some ATC70s will have 4down.
Some ATC70s are 4up.

A CT70H, SL, XL, CL70 stator and flywheel has the added bonus of timing advance...something to consider.

I have parts. I have engines too. I have a ton of stuff. You could almost choose your setup, list what you need, and I probably have it...for sale. Good USED, Honda parts. I'm a bit of a parts hoarder. And a bit of a parts seller. I'd be happy to help you and give you fair prices. You first need to make some decisions. Pick your direction, and don't change it. Then we can help you to make it happen.
 

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Discussion Starter #6
Hi kirrbby,

Posted at both lilhonda and here, since I recalled that planetminis was a bit more into tuning - and I fully understand that you got a bit tired when you saw all of my questions.. I will admit that I became a bit tired writing them myself :D
Thanks for taking your time still! Also saw that racerx just chimed in. I appreciate all of the knowledge that I can get, so I'm thankful for that.

Engine cases - no, nothing. Just the cases. Had them at a good price so just bought them a while back.
Think I would be better of buying a new engine for this project, and perhaps forgetting about those cases for now...

88 vs 108 - I would be willing to go 108, no question about it.

Racerx suggested that I'd go 12V, - I would be willing to do that as well if it's recommended, and only bulbs, battery and rectifier needs to be changed - answered his post on that.
He furthermore suggest 51-52 crank + 52 bore, so trying to find some parts for that (would like to find some Takegawa parts if possible to consider as well), and a suitable engine to use.

You also carry 12V engines that I could use for a nice rebuild in your assortment? :D

You're totally right, it seems that I do indeed need to choose the direction in which I would like to head.
Originally thought I had my mind set on a 6V engine, now I'm reconsidering.
 

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Hi kirrbby,

Posted at both lilhonda and here, since I recalled that planetminis was a bit more into tuning - and I fully understand that you got a bit tired when you saw all of my questions.. I will admit that I became a bit tired writing them myself :D
Thanks for taking your time still! Also saw that racerx just chimed in. I appreciate all of the knowledge that I can get, so I'm thankful for that.

Engine cases - no, nothing. Just the cases. Had them at a good price so just bought them a while back.
Think I would be better of buying a new engine for this project, and perhaps forgetting about those cases for now...

88 vs 108 - I would be willing to go 108, no question about it.

Racerx suggested that I'd go 12V, - I would be willing to do that as well if it's recommended, and only bulbs, battery and rectifier needs to be changed - answered his post on that.
He furthermore suggest 51-52 crank + 52 bore, so trying to find some parts for that (would like to find some Takegawa parts if possible to consider as well), and a suitable engine to use.

You also carry 12V engines that I could use for a nice rebuild in your assortment? :D

You're totally right, it seems that I do indeed need to choose the direction in which I would like to head.
Originally thought I had my mind set on a 6V engine, now I'm reconsidering.
Going to a 12V model is not a bad idea. Some of the considerations are that you would have the larger magneto cover that fits differently with the chain guard, a sub harness will be necessary for the CDI box, and it will have to be integrated into the rest of the wire harness. The advancing flywheel would not be necessary. In addition to the 12V rectifier, you would need a voltage regulator (one of the combination regulator/rectifier boxes could be used.) When you build a 6 volt engine with all the parts you are talking about, it essentially is the same as a 12 V, aside from the roller bearings on the transmission shaft ends and the electrical system. You would be using the same head, cylinder, piston, and connecting rod. The trannys on both models are interchangeable. The main advantage is the loss of points, and some cool magneto covers you can put on it. Performance wise, there is little difference as long as the points are right.
 

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Discussion Starter #8
You can also look for a CT70H model engine. The only difference between that and the SL/XL70 engines is the head (which is going to be replaced by your 106 kit) and the gear shift pattern is a 4-down. The 108cc kits I run from hondatrailbikes.com will perform well with a 20mm carb to a 26mm carb, if jetted correctly and matched to the exhaust. The cases you already have could be a good foundation for your engine build. You are correct that they are identical. If you can locate the transmission and flywheel, this manual clutch kit and 108cc kit with race head will get you where you want to be. Then you will not have to purchase an entire engine when the flywheel and transmission are all you need. Honda TB parts are quality parts I have used numerous times without any significant problems. I have Takegawa stuff too, and would say they are competitive performance-wise.

You can also look for a CT70H model engine. The only difference between that and the SL/XL70 engines is the head (which is going to be replaced by your 106 kit) and the gear shift pattern is a 4-down. The 108cc kits I run from hondatrailbikes.com will perform well with a 20mm carb to a 26mm carb, if jetted correctly and matched to the exhaust. The cases you already have could be a good foundation for your engine build. You are correct that they are identical. If you can locate the transmission and flywheel, this manual clutch kit and 108cc kit with race head will get you where you want to be. Then you will not have to purchase an entire engine when the flywheel and transmission are all you need. Honda TB parts are quality parts I have used numerous times without any significant problems. I have Takegawa stuff too, and would say they are competitive performance-wise.

Hi,

Thanks for the suggestions, good idea with the engine. Don't really mind regarding the shift pattern, so both should be able to do the job for me. Sounds good regarding the carb, then I should be fine with the MJN22, if I go for the 106cc option.

Good suggestions with the TB Parts, and good to know that there are other quality parts around. I would most probably like to go with Takegawa, but I'll keep these in mind as well. As you can see in my post to kirrbby I'm actually a bit in doubt about whether or not I should go 6V now, or if I should decide to go 12V instead, - so I really need to make up my mind, and take a decision, so I'll need in which direction I'll be heading.

Thanks a lot once again - always nice to learn new things, and I appreciate the help given so far.
Took a look at your site, you sure do build some nice looking engines! :) Will need to take a closer look, once my daughter at the age of nearly 3 months gives her daddy a bit of time where he can relax in front of the laptop... ;)
 

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Discussion Starter #9
Going to a 12V model is not a bad idea. Some of the considerations are that you would have the larger magneto cover that fits differently with the chain guard, a sub harness will be necessary for the CDI box, and it will have to be integrated into the rest of the wire harness. The advancing flywheel would not be necessary. In addition to the 12V rectifier, you would need a voltage regulator (one of the combination regulator/rectifier boxes could be used.) When you build a 6 volt engine with all the parts you are talking about, it essentially is the same as a 12 V, aside from the roller bearings on the transmission shaft ends and the electrical system. You would be using the same head, cylinder, piston, and connecting rod. The trannys on both models are interchangeable. The main advantage is the loss of points, and some cool magneto covers you can put on it. Performance wise, there is little difference as long as the points are right.
Once again; thanks for all the detailed information. Building a 12V 4 speed manual engine all of a sudden seems a lot more complicated, when you're a novice as I am. I see that there are a lot of things that I didn't take into my calculation, and I'll have to do quite a bit of reading, before being able to continue this little project of mine... I'll try to piece together all of the information and see where I might go from there. Thanks!
 
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