How many rpm's when stock

[Drdoesit]

Hello again guys.
Does anyone have an idea about how many rpm's a stock 66 cc 2 stroke kit should turn? I have a dept. store beach cruiser with just the parts that came in the kit on it. It starts right up hot or cold and runs as good as I would expect it too for the money. I have run well over 7 gals of fuel through it and just got a tiny tach so I would have a baseline to go by for tweaking. It seems to run about 4700 rpms wide open. I put a new autolite 216 plug in it and it seems to be a little lean. I can open the jet up a little if I need to just was trying to get an idea of what to shoot for. I am definately not going as fast as some are claiming. Also I am at sea level or just a few feet above it thankfully.

Thanks
Jack

 

[Venice Motor Bikes]

The real trick to getting more power (& speed)from these engines is to 'free flow' the exaust & re-jet the carb.

As far as RPMs go, every engine runs a little different.

 

[Drdoesit]

Thanks VMB
I am getting ready to check the main jet now. The tach is going to be a help. I noticed that the rpms don't go up from about 3/4 throttle on even though the it doesn't bog or stutter, but it was hard to tell without the tach. I have a couple of #69 drills on hand. I remember a thread that discussed the jet size somewhere as between .7 and .8 mm. That should be real close. They are .0292. I'll let you know.

Jack

 

[dmar836]

First, a drill does not drill an exact hole. Any machinist would say to be accurate with a hole you have to drill it undersize and ream it. That's even using a lathe and center-drilling it
That said, you are unlikely to get even those results with a hand drill or a press (you can't center it easily). You will likely have .300 if you're lucky.
I know it will upset many but this is NOT the way to rejet your carb. By drilling, you won't get the proper hole AND, more importantly, you will have no reference as to where you are in relation to standard jet sizes if you further tune. Jets are not "between .7 and .8 of anything". That is thinking with the "I'll drill it out" mentality and that allows huge variances.
It might help your problem or it could make it worse, then what size jet do you ask for? You also forever lose your baseline. 5mm jets are cheap($4) so buy several sizes over stock and work big to little. Then you can have a conversation using standardized tuning numbers like the big boys do.
In true 2-stroke tuning circles, drilling a good jet is a hack move and just not done. I understand the temptation for a quick and cheap fix but I would not admit it publicly if you do. You would have lot's of company here but you will not be taken too seriously by those who really know 2-stroke tuning.
Not trying to criticize, just preaching about a peeve of mine. I like to see people do it right - not hack.
Dave
KC

 

[camlifter]

we're talking about jetting a china 2 stroke with an $8 carb. not a gp race bike. it's perfectly acceptable to solder the jet shut and drill it to the size you need, theres only 1 jet and if you look at a few of them there drilled pretty haphazardly to start with.

 

[dmar836]

My point exactly. And we wonder why they have to be modded.
Acme, $8 carb? Is that the way you are thinking here? We're sending 13 year olds out there on public roads at 35mph. Please!

 

[dmar836]

As someone with something to sell you should be a leader for doing it the right way. How about selling and promoting a jet kit? $5 you start doing that.
Is the $800 Schwinn on your site run on a soldered jet? So now we aren't talking $8 anymore. This leads to the question, at what dollar amount does Acme labs start doing it "the right way"?
Not trying to be malicious but to make a point. An engine, Moto GP or Chinese HT, will be only as good as it's weakest link - and they have enough already. To promote anything else is poor craftsmanship.
JMO,
Dave
KC

 

[camlifter]

get real man. if you feel it's unsafe for youngsters to ride around on $8 carbs ($100 for the whole kit) maybe you should contact your legislator. the point is, this is a fun cheap hobby, the stock carb is so cheap and basic that drilling your jet isn't making it unsafe.
coming off with an i'm an expert elitist attitude to a newer member isn't helping the hobby any.

 

[Drdoesit]

I am well aware that a drill will not make an exact or onsize hole. I suppose every apprentise learns that in the first or second week of training or if you have not been trained sooner or later you will just figure it out if you are paying attention or have ever bothered to mic a drill bit, but I have never seen one drill a hole ten times the size of the drill. I used a # 69 drill which is .0292 I have no idea how you would expect it drill a .300 hole. When I quoted what I saw a member contribute as to what he has seen with the jets that come in these kits he was talking in millimeters. That is the .7 to .8 which you were saying there is no such thing as. If you have a decimal equivelency chart and know how to read it you will see that .7 mm is .0276 and that .8 mm is .0315. As stated earlier the # 69 drill is .0292. If you do the math you can see where it falls in that range. It just so happens that my jet was .7mm or .0276. Yes I measured it first. So you can see I haven't lost anything. My baseline started when I got a tach. That was why I didn't try to do anything with it before I knew where I was. I was tuning and racing motorcycles in the late 60's and all through the 70's. Since I reject the premise of your comment that tries to separate me from the so called big boys I will not respond to that. The newbie next to my profile is because I just joined this board,and not for any other reason. My little 2.9 cubic inch Chiese 2 stroke wonder apparently doesn't know if the main jet was drilled, reamed or bored or if the hole in it is perfectly round and since my electron microscope is in the shop I don't know if it's perfectly round either, but what I can tell you is that the lean condition went away and that I picked up approximately 250 rpm's and that is what I was trying to accomplish. And if this a public forum I am admitting it publicy.I drilled it out. What works, works whether you take it seriously or not.




Jack

 

[dmar836]

By "Big Boys" I was referring to big engines / big teams / big manufacturers. Not much about tuning HTs compared to the advances in 2-stroke tech of the 70s and 80s.
You have to realize no one knows what your understanding is when you ask a generic question like that. Just today, a guy thought he burnt a valve in his 2-stroke. He might not have known about drill bit variances and likely not even tuning principles. Some will read "I just drilled mine and got more power" and so they are on a search for a drill.

The .300 was obviously a typo. Point taken.

My reference to .7 to.8 was that it is a range - not a size. As you stated, it's very easy to drill a .03 with a .292 bit - even .315. So if you are okay with that range, fine. What exact size did you end up with and what would be the next step up?
Like I said. It may help your problem and I'm glad it did but I'll have to stand by the advice of using high quality jets for 2-stroke tuning rather than drill bits............ $100 engine or not.
250 RPM doesn't sound substantial unless 4700 is the stock max. Is it stock otherwise?
Dave
KC

 

[Bikeguy Joe]

The average stock "red line" for these chinabuilt wonders is in the 7000 rpm arena.

 

[dmar836]

Keep drilling!

 

[Bikeguy Joe]

I have successfully drilled the jets on a few of these.

I have also re-tapered the needles with (cringe now!) sandpaper and a cordless drill.

 

[dmar836]

Tim,
I am NOT a 2-stroke expert BUT this is a tuning nightmare. You can sometimes feel 10 degree temp changes.

So is this one

Guess I need to shut up and get out to the garage.

 

[Drdoesit]

The average stock "red line" for these chinabuilt wonders is in the 7000 rpm arena.

What are most of the guys using for a tach? I bought a tiny tach and it seems to be working ok. It has a 2.5 second delay but you get used to that. I don't think theres any way this ole girl will get another 2000 rpms though. Are you getting 7000 out of a stocker or is that after modifying it? That is what this thread was all about in the first place. I wanted to find out what some of the guys were able to get out of one after it was reasonably tuned.

Thanks
Jack

 

[Bikeguy Joe]

I have never applied a tach to one of my engines, but loooong ago and far away, the companies selling them stated that the redline was 7300.

Different engines rev to different levels, due to the vast differences in things like how well the ports match up, how closely dialed in the "carb" is, ect. Other things that affect top rpm are weight, type of tires, terrain, air density, ect.

They do rev better once broken in.

 

[Bikeguy Joe]

Tim,

I'll take that one off your hands.

 

[dmar836]

125cc and at about 9K it starts to pull. Great fun but I've never had it on a track. The neighbors love it!
Sorry, didn't mean to highjack Drdoesit's post.

On the subject, and FYI, most street 2-strokes can be modded to pull easily over 10K.

Drdoesit, you might check your port timing, at least see that things line up as intended. I think Chinese companies have so many suppliers you can get mismatched parts fairly easily. I have found sheet metal screws right next to machine screws all holding on the same cover and often see mismatched fasteners.

Dave
KC

 

[Drdoesit]

I have not had the motor apart yet. I also not sure yet if it is fair to compare these little guys with the Japanese built 2 strokes either. And I agree about the fasteners etc. Once I thought that I had blown it for a second and then it went to running again. I got home and pulled the side cover to find a spare woodruff key laying in there that had bounced up and run between the crank gear and the clutch gear. Thankfully it was not the best quality either. The motor runs pretty well all told, cranks right up hot or cold and probably will get 30 mph with 44 tooth rear and my 210 lb self on it. It's only showing 5000 to 5100 on the tiny tach and from the way it sounds it can't do much more. I have had 4 stroke bikes that would turn way over 10,000 rpms too. I am just getting started here and was curious. I appreciate all of the replys and help.

Jack

 

[Bikeguy Joe]

"I got home and pulled the side cover to find a spare woodruff key laying in there that had bounced up and run between the crank gear and the clutch gear. Thankfully it was not the best quality either."

That was funny.

And true.