Sweet looking bike ! But do you really want the carb and air-filter sticking out like that ?

 

I didn't have access to a welder after I did the frame, and that's the only thing I could get to work. Doesn't hit the wheel... Barely sticks out past the huge CR80 forks.

What do you think about the gearing?

 

The gearing sounds ok to me. There are a number of online gearing calculators that can help you if you are still unsure what ratio to pick.
I do remember that the Dellortos where already a pain to adjust on 2-strokes, and since that time I tried to avoid them at all costs
With the 88cc kit, a properly jetted carb + adequate exhaust pipe, you should easily be able to reach 45-50mph.

 

Unrelated but, do you fly?

 

Unrelated but I fly UAVs...

 

Id go with a 20mm carb, what size is the dellorto?

 

I've had good luck with them on 2 strokes, building a couple 60mph 70cc setups, but the feel of lean and rich as well as the sounds are different on a 4 stroke... so I'm just tuning based on performance and head temps... speaking of which... 325 was my high today. I was told 400 is push home and 350 is good for tuned setups. Right now there is an open exhaust on it with a length of about 3ft. It came off of something 4 stroke but completely different. Also, it has a honda hot cam in it with higher lift lobes...

If everyone agrees with python that the gearing is ok, I'll go ahead and keep fidling with the carb. I've got some new atomizers, needles and main jets coming in on Wednesday.

Carb size is listed above.

Is the final gearing ratio at the engine output shaft the same regardless of transmission?

 

buy a vm20 mikuni and you'll be hitting 55mph on this thing...

 

I've got 5 bikes (all two strokes and fast) with dellortos on them and hundreds of dollars invested in jets, needles, atomizers, slides, parts etc - so, I'm not really interested in swapping carbs - they came on four strokes and they can work on this one.

I'll figure it out, I just want to make sure jetting is something I should continue working on and not something else.

Does anyone know about final gearing at the ouptput shaft?

I personally think I'm geared too high at 14x35... these are 17" rims Stock CT70's came with 14-15t front and 44-45t rears and 10" rims that I imagine were under 17" with tires... so... I think I'm high... someone confirm/deny?

 

Your geared very, very tall for a 17" rim and CT70 primary drive. Ideally you'd be about 14/50. You can play with the math yourself, here: Honda50.com Gear Ratio - Speed Calculator

 

Unrelated but I fly UAVs...

Sweet. I own a small RC manufacturing company and actually interviewed with a local UAV operator. I was offered a job doing that but didn't want to be deployed, so i had to turn i down. Just retired from the airlines at 33 in March. LOL

 

Fingers crossed for 12x44 being low enough... lowest gear set I ordered

 

I played around with that calculator... guess that's what the guy was talking about earlier... Thanks raymond!!!

Which transmission exactly does the ATC70 fall under? I tried all the 4 speed transmissions and they're pretty high, so I don't really think it matters - it's still high. Is the primary drive the only determining factor?

This won't be the first (and I'm sure the last) time I over gear a bike. 1st gear is just SOOO short

 

Gear it close to a stock CL70...

 

I've been looking for 20-30mins and can't find what the stock gearing was - do you have it?

Is the atc70 transmission the same as the cl70 transmission? How many different 4 speed transmissions were made?
I don't think the primary drive is the same tooth on a ct70 is the same as a cl70.... or is it?

 

Gear RPM & MPH Calculators

 

Honda  ATC70  ATV 3 Wheeler General Specifications Prices MSRP - Motorcycle Forum

ATC 70 ratios...

1st... 3.273

2nd... 1.938

3rd... 1.350

4th....1.043​

 

I'm looking for a CT70H or SL70 four speed gearset myself. My understanding is that the ratios are as follows - much more usable IMO:

1st gear 2.69:1
2nd gear 1.82:1
3rd gear 1.30:1
4th gear 0.96:1

The "common" Chinese four speed gearing that I've seen is as follows (better than stock 3 speed gearing, but still a fairly low 1st):

1st 3.273:1
2nd 1.938:1
3rd 1.350:1
4th 1.043:1

The "AHP" four speed gearset has the following ratios:

1st 3.182:1
2nd 1.706:1
3rd 1.238:1
4th 0.917:1

 

So, in three posts, and because I'm somewhat ADD.... no, they aren't the same.

 

Alrighty then...

Looks like I'll be throwing a high flow oil pump and sl70 trans in the next time I've got it apart! Looks like they're readily available for a certain price haha

Thanks for all the help guys... Hopefully some day soon I can start answering some hard questions on here. I'll be sure to post back with results on the down gearing and a gps'd speed. This thing rips high and hard in first, so I can't imagine what it'll be like once I get it geared out proper

 

Great job with your fabbing and mixing and matching... much wider range of stuff you've got going there. Very rad bike you built! Very lean and sophisticated looking.

I built something very similar! Maybe I can help!


IMG_7953 by zorgitronn, on Flickr


IMG_7963 by zorgitronn, on Flickr

I have almost the exact same engine set up as you do. 88cc, ATC70 4speed, 20mm carb, 17in wheels, etc. I'm definitely seeing much more than just 30mph top speed.

Here's a rough top speed video. The speedo said 60+ mph (60 was as high as it could go...)

I used 15tooth on the front and 36tooth on the rear. It's geared pretty tall, but not so tall that I can't accelerate.

I don't think you need to waste any money on an sl70 trans. The ATC 4-speed is just fine. High flow oil pump, maybe, but it's debatable if they are necessary.

 

Wow... Thank you for that reply I was hoping to get a response like that.

Very awesome bike you have there too! I love the brown and tan color scheme... Is it really a mitsubishi frame? Are you using the common 67t primary drive? Which model engine did you start out with?


Perhaps I will continue down the road of tuning... I know I should be seeing more than 30 lol... But 60!?!? That's exciting... I'll keep trucking along and get her figured out. It's very helpful to have a baseline to aspire to for tuning. I guess then my gearing might be a tad lower than yours but just barely if at all.

 

Thank you for the nice words on my bike!

Heh, it's not a Mitsubishi frame. I wish Mitsubishi made motorcycles. They'd probably be pretty interesting. It's a Honda CL70 frame and Honda S65 gas tank. I didn't do any fabbing to get the engine to fit. The tank badges for the S65 are really rare and expensive, and I found some Mitsubishi wheel center caps that fit the area and look pretty cool in my opinion.

I'm using the stock primary drive. I started with a Honda 4-speed ATC70 engine. I swapped in a kickstarter and clutch cover from a Z50.

I think 50mph should be the minimum speed you could get out of this setup. Really.

The gearing is not the problem. It might be your carburetor, but it sounds like you know what you're doing with that. Just warning though, the guy down the street builds and tunes some really fast 2 stroke race bikes, but when he gives me carb advice, it doesn't work. He insists the principals are the same between 4-stroke and 2-stroke, but it doesn't seem that way.

Is there a 4-stroke stock motorcycle with similar displacement that uses Dellorto carbs? That would be a good guide for jetting everything. You've been checking your plug color, right? It seems like you know a lot about your carb and it might not be the problem.

If it's not the carburetor, it might be something with the points. I had a lot of trouble with points and went the easy route with converting to CDI. There's a well-known guy in Malaysia that sells these kits on ebay. I think it's the best investment I made on this engine. I opted for the more expensive CDI with spark advance. I think that helps the top end a lot, and it revs cleanly.

However, the 70cc Honda Passport I used to ride was bone stock with points ignition, and could reach 45mph easily. If you just get your ignition set up correctly, it should certainly go faster than 30.

I don't have the tools to test compression. I'm assuming the compression is good. The piston is a high-compression type from HondaTrailbikes. Where did you get your cylinder and piston?

That's all I can think of that would be hurting your performance so much. It shouldn't really be a hard engine to set up.

I'm talking like I'm some kind of expert, but I'm really not. I built a working engine, yeah, but only after nearly two years of stupid mistakes and ruined parts. I had no previous engine experience before this build. Hopefully someone more knowledgeable and experienced will validate or disprove my claims!

PS: Let's see some more photos of your frankenstein!

 

How much compression are you seeing with that 88cc cylinder just out of curiosity?

That's very interesting you found a CDI for that bike... Is it just coils and a box or a whole new flywheel and stator too? How did it hold up? Looked like you had the stock Hitachi ignition on there in the forum photos.

 

I agree. Gearing is not the issue here. Thank you for posting about your engine.

I agree also these are not hard engines to build. I've built several others for customers last summer when I was working as a mechanic and had no issue... This one is being a bitch for some reason.

I'm seeing 130psi... Recommended is 170 +/- 7. Cylinder was heat cycled, and new valves installed. Valve seats are clean and smooth with no pitting and seating was checked with prussian blue dye. When pressuring the cylinder to 150psi at top dead - there is NO leakage out of exhaust or intake... If you put your ear to it you can barely hear anything. Cylinder blow by at ring end gaps seems minimal when listening to the crankcase.

The only thing I can imagine at this point is that I have been sent the short piston for the long rods for this kit instead of the long one for the short rod cranks on pre 81 stuff. I have not been able to get ahold of those bastards at Beatrice Cycle. You don't happen to have the box for the piston still do you?

My piston was an "OEM" brand piston... not oem as in original lol. The p/n is "13101-GBO-910"

Also, if someone knows how far from the top of the deck the piston is supposed to stop on the 88cc kit - that would be helpful too.

 

I agree. Gearing is not the issue here. Thank you for posting about your engine.
My piston was an "OEM" brand piston... not oem as in original lol. The p/n is "13101-GBO-910"

Also, if someone knows how far from the top of the deck the piston is supposed to stop on the 88cc kit - that would be helpful too.

Ah, a low compression piston... This could be the problem actually. This is how mine looks in terms of deck height an' all. The outer edges of the piston are almost flush with the cylinder, with only the gasket making some difference. (Hmm I wonder why one side of my piston is kinda clean like that??):


IMG_7957 by zorgitronn, on Flickr

Here's a comparison with a 50cc piston:


Piston Compare by zorgitronn, on Flickr


It really sounds like a mismatched piston and cylinder is whats' screwing everything up. It would definitely make for a slow bike, but hey! Low compression = super reliable!

Do you have the links to the parts you bought from Beatrice?

 

It's not a low compression piston, different years had different strokes and the short piston is for a bike with a longer stroke.

Thanks for that pic... tells me what I need to know. You've been super helpful zorgitron...

The 2 stroke guy you know isn't a member of peddy cash are they?

 

I don't think he is part of Peddy Cash. He's got long gray hair and beard and has built a 80cc Spanish 2 stroke sport bike, a 3 Cyl Kawasaki drag bike, and he just showed me this awesome Suzuki TS125 horizontal parallel twin, decked out for racing!

 

Sounds like you have the most bitchinist neighbor ever...

The only thing it could be is a Kawasaki 750 H2... It's also know as the widowmaker. On a side note... I'm not sure we could get any more off topic lol... a 750 triple kawadog 2 stroke on a honda 70 single forum lol...

Currently exhausting new possibilities... Stay posted for updates on raised compression...

 

Bike runs like a raped ape... Turns out it was ignition timing.

I set the gap and thought it was good. Pulled out the timing light and it was firing at TDC lol... Since this bike had a fixed stator, I figured if I set the gap, I'd be fine. Guess not.

In my hunt for PSI, I polished the mating surface on the head and the compression is now 140, hopefully it'll go up as the rings seat.

The bike is spinning the clutches at about 45mph when it hits the powerband in 3rd gear with 14x35 gearing... anyone got any clutch tips before I just go buy a new performance one?

Still dialing in the carb, midrange is a little funky, but I've got some new needles I haven't fiddled with yet, so it'll probably fall into place.

You guys have been great! I'm very impressed with the knowledge base here. Especially you zorgitron!!! Happy ending, gotta love it. GPS speeds sometime next week when I've got her all dialed in

 

In my hunt for PSI, I polished the mating surface on the head and the compression is now 140, hopefully it'll go up as the rings seat.

The bike is spinning the clutches at about 45mph when it hits the powerband in 3rd gear with 14x35 gearing... anyone got any clutch tips before I just go buy a new performance one?

How did the piston and cylinder look when you took the head off? Was the piston indeed a little too deep into the cylinder at TDC? You could get a thin copper head gasket for more compression as well. I think they're about $15 or something.

Spinning the clutches? That sounds typical. A little more power than stock will overpower the clutch. A lot of people save money and just install firmer springs into the clutch, which I've heard works fine, and is only about $30 if you get new plates too. I tried to do that and failed ( I blame my lack of tools!), so I just got a TB 3-plate autoclutch. It shifts very well once it's broken in.