Need help tuning RT Carb

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Sage

New Member
Aug 9, 2015
9
0
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Boone, NC
Hi! I have been reading the forums for a few weeks now since I put together my first motorized bicycle and have learned a ton. So first off thank you all for the wonderful information posted on these forums.
I have a 66/80cc china girl on my 21 speed Schwinn mountain bike, with SBP Expansion Chamber, HD Lightning CDI, RT Carb and better spark plug as the upgrades I have done to it. I plan to get a performance head when I have a little extra money to throw at it. I also had to replace the jug and piston due to a mishap with my throttle cable getting stuck outside the old carb and running the engine too high.
After taking it out for a test ride after putting on the upgrades but before swapping out the spark plugs, i noticed a small amount of 4 stroking, so I decided to swap the spark plug out for the newer one to see what would happen. After swapping it over the 4 stroking increased so I moved on to adjusting the jets. I have tried swapping out the standard jet(which was unlabled) from the RT for the #70 that came with it. I also have a 65/75/80/85 jets that I have not tried yet. When I put in the #70 jet there was very little power and it would not crank up when trying to get things going, so I am guessing I need to go up in size, but would like some advice before I go to far along.
As I know altitude can effect the running conditions I live at approximately 3300ft above sea level, and the temps this time of year are usually around 70ºf.
Any and all help is greatly appreciated!

-Sage
 

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
Welcome to the forum. I hope we can help you.

First I would ask how much running time on your engine. Saying "a few weeks" doesn't tell us much. These little engines will perform much better after 2 to 3 hundred miles or several tanks of fuel. Also fuel/oil ratio is a consideration. 16:1 is too much oil.

I personally never advocate adding a bunch of 'go-fast' parts until after break in. Doing so gives you no base line for comparison between stock parts and the aftermarket additions. Do one thing at a time so you'll know where you're at and can better determine what works and what doesn't, and what adjustments are needed.

To answer your question, tuning the RT carb is simple. Experiment and find what jet orifice size works for you. That will involve seat-of-the-pants feel and spark plug color.

Elevation above sea level isn't as critical to tuning as you would believe. I was one of those who used to preach that it was until this past June when I took a bike from my home in Denver Colorado to central Illinois. My elevation here is 6000' MSL. The Illinois ride was at roughly 800', less than 1 thousand feet and my bike ran as good, maybe even a little better that it does at home. I didn't change my carb jetting.

Others will have advice too so stay tuned (no pun intended)

Tom
 

Sage

New Member
Aug 9, 2015
9
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Boone, NC
First off, Thank you for the quick response.

I had about 150 miles of break in on the engine before i had to replace the topend(jug/piston) and have only about 30-40 miles on the new topend. I have noticed since the rebuild I have lost some of my low end power and do not climb hills nearly as well, but mid andtop end seem to feel pretty good.

I got the bike running a lot smoother on the #70 jet, but have the needle all the way screwed in. I am about to swap down to the #65 i have, open up the needle and see if it runs any better.

If 16:1 is too much for break-in period, what ratio would you recommend?

Would you recommend putting the stock exhaust pipe back on until i have broken it in a bit more? I replaced the CDI because the old one blew out on me(as did the magneto coil at the same time when the old jug/piston was damaged) and figured it would be worthwhile to just buy the upgrade now instead of another cheapo. Also, the main reason I am trying to boost power at all, is because being in the mountains, some of these hills are quite steep and the bike cannot quite make it up as of right now. One hill in particular has been my testing ground as I have never made it more than about 600' up the road even with pedal assistance. Top speed is my last concern, as I should never have need of going over 35 in my town, and being a little below that would be ok with me as long as I can climb the hills.

I might add that this is my first real mechanical project other than a few bolt on upgrades for my old Mustangs about a decade ago, and it has been a lot of fun learning about and working on this small engine so far. Any other advice is also welcome. Thanks a bunch!

-Sage
 
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ckangaroo70

Active Member
May 13, 2011
864
126
43
Central Illinois
Welcome Sage.

If using a conventional 2 cycle oil like Lawn Boy ashless or Penzoil air cooled I would suggest 32:1 for break in mix and would probably go to 40:1 after a few hundred miles. If using an advanced 2 cycle oil like Opti-2 I would do something like 75:1 for break in and up to 100:1 after a few hundred miles(I run opti-2).
Really the trick is to find out what your engine likes without mixing in too much oil or too little. 16:1 would be a pretty heavy and dirty mix in my opinion even for break in, so at the very least I would bump that oil ratio down to 32:1 if using a good conventional oil. Just my 2 cents and conclusions I have come to after messing with countless 2 cycle engines over the course of many years.
 

Sage

New Member
Aug 9, 2015
9
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Boone, NC
Ckangaroo70, thank you for the oil ratio recommendations, I will have to transfer the rest of the mix i have into another gas can and go get some fresh gas and oil as soon as i use up this last 1/4 gallon in my gas tank. The Opti-2 sounds like a good game plan, I have seen a couple of people talk and swear by it on here.

I swapped down to the #65 jet and boy does it run a lot better. I still have to have the needle screw almost all the way in for a steady idle where it doesn't just cut off when i hold in the clutch, so considering trying the #60 jet I have to see how it feels although I am wondering if it is a little too small. I did however get rid of the 4-stroking which was my biggest worry.

Just on a side-note, I am currently using a NGK BH6S Spark Plug as I saw it recommended somewhere, and it definitely sparks better than the stock one, but I saw I think on the SPB site that with the CDI I am using I should possibly have a higher rated one and I am not sure what I should be looking for or what works best(or if it even matters a whole lot) with my particular engine, so any recommendations on Spark Plugs would be helpful as well.

Thanks again!

-Sage

Oh hey and it looks like 2 more posts and i can put up some pics of the bike!
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Stick with the plug you have. There is absolutely no advantage to those high priced plugs unless you plan on using it to accrue a hundred thousand miles before you replace it.

Like Ckangaroo, I also run the Opti-2 oil and I use it, even during break in, at the manufacturer's recomendations of 100:1. I don't recommend it to everyone but it has served me well for seveal thousand miles in my bikes. There is a good thread here that you can read on the subject, type "100:1" into the search box and settle in for an evening of reading to see the pros and cons of this particular oil.

Also you should know that jet size can be inaccurate, nebulous at best. Some of us solder the jets closed and redrill them to the size that suits our engines. Too many times we've seen the same size jet with a different size stamped into it. Jet size means little to me. I prefer to use drill bit size which I know is accurate and consistent.

Some of us are also no fans of the aftermarket ignition systems available out there. The stock CDI and magnetos seem to work fine for us and there has never been any substantial proof that the high priced igntions offer any performance improvements.

I can't see how an over rev condition could have damaged your original ignition and depending on the length of time the engine was over reving, how it damaged your original top end. What happened and how long did the engine run in that condition?

As for the power loss after you chaged the top end you must keep in mind that when you replaced those parts you then started at zero, or square one, as far as break in was concerned. New pistons, rings and cylinder would require a break in period just as a new engine would.

As for hill climbing ability, you might want to explore going to a larger rear sprocket. You didn't say what size you have now but a larger tooth count will lower your gear ratio and give you more low end torque but your top speed will suffer. 35 MPH might be a little harder to achieve with a lower gear (bigger sprocket), but you'll be able to climb hills easier. Think of a car with a manual transmission and downshifting it to climb a hill. Same principal applies here.

Tom
 

ckangaroo70

Active Member
May 13, 2011
864
126
43
Central Illinois
The Opti-2 sounds like a good game plan, I have seen a couple of people talk and swear by it on here.
Sounds like you have a good plan! That opti-2 is good stuff and at this point in time I would hate to run anything else in my china girl.
Like 2door said, there is a lot of reading on the subject here on this site if you put the word "oil" in the search engine. You can actually find some pretty heated discussion when the topic is oil, but I think you will find that opti-2 has been around and used long enough now to prove itself as a great product.
 

Sage

New Member
Aug 9, 2015
9
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Boone, NC
Well let us see what I can clarify about the unstated specs of the bike.

I am running a 44t Sprocket at the moment, and am a little afraid of putting a larger one on as there is already very little clearance between my chain and the bike fork. I would like to save a larger sprocket as a last resort option for better hill climbing unless it is really my only choice. Right now it has no problem hitting 35mph+ on straightaways, but it is just suffering on the hills. Do you think once I have it broken in a bit more it will climb better, or is that just wishful thinking?

I would like to re-drill one of the much to large jets I have to maybe a 60-63 as I think that may end up being the sweet spot just from the testing I have done so far(assuming accuracy of the label on the jets), but i lack the drill bits for the job(i know they are available on amazon, but I do not recall what the name of the drill bit set was). I have read quite a bit about re-drilling on these forums, just have not had access to tools to try it yet.

Somehow when the bike was over revving(to the best of my knowledge it was only about a 15 minute ride before it blew, although the cable could have been stuck prior to that ride) while the throttle cable was stuck outside of the carb, something managed to chip the cylinder wall and scorch the piston a little bit, and as a replacement top end was fairly cheap I went ahead and did it, it also gives me a spare top end to possibly experiment with later down the road. There was also a large blowout of oil from the exhaust manifold when the bike stopped running. I will post pics of the damage later when i have some daylight to work with. I do not think that the chipped head is what the loud ka-pow I heard was when the bike stopped running, it sounded more like a loud electrical pop than mechanical to me which is what led me to checking out the Magneto coil and CDI. I followed the guide on here somewhere for testing the Magneto coil and CDI, which told me they were blown. I also tried to re-solder the connections as i have read about "cold solder joints" and wanted to make sure that was not the problem. I had my work double checked at one of the local cycle shops and he confirmed that I had done it correctly and they were in fact blown. I am not sure if it was the over revving or something else, I just know they in fact failed on me.

I will also be sure to search "Oil" and "100:1" on the forums to see what kind of information I can dig up, so thank you for the suggestions. Will having a better ratio bring a noticeable performance difference?

If there is anymore information you would like to know about the bike, or that any of you think I should know, please let me know. I have managed to get it running much better today and am probably going to continue to tweak it a bit tomorrow.

Thank you all again for the wonderful information, it has been most helpful and I am much farther along with learning about this project and being full hands on than i thought I would be at this point because of it!

I am really enjoying this project and having a fun bike to ride around the mountains. There is also something very satisfying about getting the looks and comments from people being like "Is that really a motor on a bicycle? AWESOME!". I have met some really cool people because of it. I cannot wait until I am comfortable enough with its running condition to take it out on the Blue Ridge Parkway. I will have to bring my Go Pro with me when I do go out so I can take some good footage of the views out there to share.

So thanks again for all the help. I think I am truly going to enjoy this hobby.

-Sage
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
I also use opti2 at 100:1 but I like just a little more oil in my mix when going faster, the engines need that extra protection when running at nearly twice the stock rpm so I will run with 100:1 opti2 and add in some castor oil on top to bring the ratio down to about 75:1 or run Amsoil Dominator at 50:1 with enough castor to bring the final mix to 40:1.
I have run my engine at 10,000+ rpm on the 100:1 mix numerous times so it's just cheap insurance for me to run the higher ratios where a little extra lube doesn't hurt.
For normal cruising the opti2 works really well at 100:1
 

Sage

New Member
Aug 9, 2015
9
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Boone, NC
Hi, Dave.

What does adding in the Castor Oil do for things, and why mix it with synthetic instead of just 1 or the other?
Also, how do you tell what RPM you are running at, is there a tachometer you can get for these little 2-strokes?

Thanks.

-Sage
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
The castor oil is added as an extra line of protection since the castor oil doesn't break down at higher temps and it has a tendency to stick to the metal so it helps protect the engine and cylinder wall better when running higher rpm's and at higher temperatures. It doesn't take much and too much can leave the engine with a gummy film but if you add your normal mix of Opti2 then add about an ounce of the casrtor oil into the gas it'll be enouh to do it's job but not leave too much residue behind. If you use only castor oil by it's self it'll work but it would waste a lot of it and leave your engine a mess but by making the mix then adding a small amount into the mix it'll do what it needs to do without being quite as messy.

For the rpm on these engines, there are some small digital tachs that can read the rpm accurately or it can be calculated off your speed, gear ratio, and tire diameter. I got my rpm by calculating it out from a 26" rear wheel, 44 tooth sprocket, and 43mph. The engines have a 4:1 reduction from the crank to the clutch shaft, then with the 44T sprocket, there's another 4.4:1 reduction giving a total ratio of 17.6:1, so to use a simple formula, you could just use 17.6:1 for the final drive ratio, then the tire diameter and speed. If you got a different rear sprocket, you would need to factor this in in order to correct the rpm. Mine turns somewhere around 9750 or so rpm to hit 43mph, it can go faster if going downhill and of course, it can free rev about 1000 rpm above it's max speed rpm. I like keeping mine lower geared so it can accelerate good and the road I ride mine on mostly has a decent amount of traffic that usually moves about 35 to 40mph so I try to keep the bike geared wwhere it can keep up with the traffic but still have a little bit of head room either to gew away from a tailgater or to pass up someone who's going slower than I want to go.
 

Sage

New Member
Aug 9, 2015
9
0
0
Boone, NC
Thank you for that info. I never would have guessed or thought to use castor oil in an engine, but it sounds like it has a nice effect. I will have to experiment a little and see how it works out.

I get the parts of the formula for calculating RPM, but what is the actual formula once you have all the numbers to put in? That seems like a good thing to know for a bunch of reasons.

I am off to the store now to try and find some Opti-2, and to go hit the DMV to get my tags for the bike. Thanks again for the help!

-Sage
 

Sage

New Member
Aug 9, 2015
9
0
0
Boone, NC
Man. I swear if it is not one thing it is another with these. While riding into town today i somehow managed to warp my rear rim. does anyone have any advice on what brand of rim I should buy as replacement that might be more durable then the stock one that the bike came with. the guy at the local bicycle shop said that it was a pretty cheap rim and that they were common to bend like that even on a regular bicycle without a motor. Guess this is also a sign i should get the manic mechanic sprocket adapter thing. Oh well, I guess this is one way we learn.
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
The worksman rims are nice and tough, if you got a beach cruiser or a single speed, you can get one that's already laced with 12 gauge spokes and a wide steel rim that's a LOT tougher than most cruiser and store bought mountain bike rims, Felt also makes a really tough rim that's 50mm wide if you like the wide tire look, but the worksman 26x2.125 rims are pretty wide and meaty looking. These rims won't warp or bend unless you get T boned by a bulldozer pretty much...
 

Tyler6357

Well-Known Member
Mar 15, 2012
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Santa Barbara, CA
Sage, I can recommend getting the sprocket adapter. However, those big ones for the rims with coaster breaks will most likely be too big to fit your mountain bike speed hub. I'm not sure if manic mechanic makes adapters for speed hubs or not. I found THIS ONE, which will fit a 1.19 inch diameter hub (3.74 inch circumference) and it comes with shims so you can use it on even smaller hubs. It uses a 9 bolt sprocket but it won't line up with stock sprockets. BoyGoFast sells $15 sprockets that will fit it.

Also, Here is a link to the drill bits to drill your own jets--> http://www.ebay.com/itm/New-20pc-Mi...EE-SHIPPING-/270962704048?hash=item3f16a2beb0
 
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Sage

New Member
Aug 9, 2015
9
0
0
Boone, NC
Well, I tried calling Worksman directly and a local Felt retailer and they said they do not carry the correct type of rim for my bike in those brands, and that finding a quality steel rim may be difficult as they only carried alloy wheels that would fit my bike. I need a rear rim that can mount a 7-speed gear assembly.

I just spoke with someone at a bicycle shop and he suggested finding the outer rim piece, some heavy duty spokes and rebuilding off of my old hub because it is getting harder to find 7spd wheels as they have been replaced by 8/9/11 speed ones. I however have no idea where to begin looking for just the rim and spokes with no hub, unless it would be possible to buy the cruiser wheel from one of the two manufacturers mentioned, take off its rim and spokes and rebuild onto my hub. I know it is a 36H wheel, what other information would I need for attempting such a swap if it is even feasible?

That sprocket adapter looks perfect for what I would like to do, and the proper sprocket for it is well priced. Also, thank you for the link to the drill bits.

You guys have been a huge help and I cannot thank you enough.

-Sage
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
Yeah, your best bet for getting the wheel you want is to get the rim by it's self and lace it onto your hub with the heavy duty spokes.. Most the worksman stuff is either coaster brake or it may have a freewheel hub, but to get what you really want, nothing beats lacing your own... or having a pro lace it for you with the parts you want.
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Sage---If you are going to get that sprocket adapter make sure that the rim you get uses a standard 36 spoke configuration otherwise the 9 sprocket bolts might not clear the spokes. The MM ones are actually better since they only use 3 sprocket bolts and can clear different spoke configurations but when I was looking at them I only saw the coaster hub size. He might make smaller diameter ones, you could call and ask.