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Discussion in 'Motorized Bicycle General Discussion' started by Henshooter, Feb 19, 2014.
You do have to rejet.
I've done tons of reading on this and the safe bet everyone recommends is between 25:1 and 32:1. I wouldn't go below 20:1 after break in. I run 32:1. The general trend with these engines is that they aren't very picky about things. Spark plug gaps can vary largely and still work, standard or premium gas works, 18:1 to 40:1 would still work and probably be a safe range.
I don't know much about the fuel they dispense at the Reno Air Races other than it smells different and makes the planes go like stink. I live in Reno and never miss the race every year.
I live in central Texas, my engine are above stock compression and have port work along with well tuned carbs and fre flowing exhaust, I run 87 octane pump gas and 100:1 Opti2 oil mix, all engines run great, dont over heat and I dont putt around because I live several miles from nearest town and ride 30+mph for many miles at a time, the high octane fuel being needed in the china girl when the compression is fairly high has not been needed for mine and I have no signs of detonation and all mine run great on the 87 octane, we have multiple summer months of 80%+ humidity and triple digit temps and almost all our gasoline around here has 10% or less ethanol, all my engines run great and no failures since 2009-early 2010 since I switched to running Opti2 oil @ 80-100:1 mix.
One more thing I'll add here, I think the best thing for reliability with the china girl engines is proper carb tuning and high quality oil mix, but tuning being the most important, if an engine is set up to run to lean its gonna have a short life plain and simple if it gets run very hard for very long.
Glad to see someone else running opti 2 oil. I'm running it too at 64:1 for the first 1/2 gallon. I'm still breaking the engine in. I'll lean it up a little to 80-100:1 after the first jug.
Hi Nevada. I broke my new engine in running opti 2 @ 80:1 from the get-go for half a tankful (approx. 1/4 gallon) then went directly to a 100:1 mix. Over 200 miles on this mix my plug is a perfect golden-brown and the engine runs surprisingly cool. I suggest raising the c clip position on your carb needle (and possibly going with a smaller jet size) because running lesser amounts of oil per gas ratio makes the mixture richer (more gas in a given gallon).
Ditto: Jonny is right on with this advice but you need to be careful and not go too lean. That is one problem running the 100:1 ratio. To get things right you really need to lean out the fuel/air mix because of the more fuel, less oil condition you are now giving your engine. Just don't over do it. A lean burn can shorten the life of any engine.
Your altitude above sea level will also play into the proper mixture of air and fuel.
Agreed, Tom. I should have clarified by advising Nevada to go easy in increments re needle clip position and progressively smaller jet sizes. Checking the spark plug color is key to making sure everything is hunky dory.
Thanks Jonny and Tom. I do live at elevation though. Around 5000 asl. Would that make a difference in the clip position?
Yes, elevation will make a bit of a difference. How much I'm not sure of, but basically the air ghets thinner (less dense) as you go up, so in theory you can lower the needle/raise the clip to run a little leaner (less fuel to mix with the thinner air). Once you mix in air temperature, then you have more 'tuning' of the air/fuel mix to think about since colder air is denser, so more O2 per stroke going in.
Once again, to put the myth of high octane fuels to rest:
The higher octane number does not give any more energy content per unit of fuel. You will not achieve more power or fuel mileage, nor will there be any better effects of burning higher octane fuel in an engine designed for regular fuel.
It ONLY means the fuel is less likely to auto ignite, meaning that the air/fuel ratio will not detonate before the spark plug fires it. You only need higher octane when dealing with an engine that has a higher energy input in to the fuel, or a higher activation energy.
The higher octane fuels are harder to ignite, meaning if you do run them in an engine that will not detonate on a lower octane fuel, it will only lead to a more incomplete combustion. This WILL lead to more deposits in the engine, and you will get less power from the engine because of the incomplete combustion. On the inverse side, running a lower octane in a high compression or forced induction (turbocharged/supercharged) engine will give you detonation and auto ignition because the energy the engine is exerting on the air/fuel mix will be higher than the combustion energy required to combust the fuel before the spark plug fires. This can quickly destroy an engine.
So if you don't have a high compression engine and you're not experiencing knock, then don't use "premium" fuel. I use quotes around premium because I don't want people to associate higher octane numbers with being better. It's a myth.
I absolutely hate hearing people say that they fill up with "premium" every once in a while to keep the engine clean and healthy. They have no idea why they're doing so. Probably because they associate premium with terms like best, clean, healthy, etc.
If the premium fuel in your country really is filtered better, then get a good fuel filter and prove it. I seriously doubt it's anything more than another myth, but then again I don't know anything about how Australian refineries process their fuel, so I may very well be wrong. Im skeptical at best.
You are spot on here Agreen,
I also have actually been in a couple of friendly debates over the years about the octane issue, with people for instants running a 8.5:1 compression SM Block Chevy with a carb that was way to big like a Holley 750 dbl pumper and telling me they went down to Chevron and pumped a tank full of 93 octane and now they had all kinds of extra power and also could tell they were getting much better gas mileage.... LOL! I don't like to hate on people and I don't like to shoot people down but, anyone that knows the facts knows that that is all complete bunk and isn't fact at all and is only a placebo affect at best because in their mind they wont it to be the case so somehow convince themselves that it did make a difference.
I don't care what anyone runs in their engines, but it bothers me when they try to convince others of things that are flat out incorrect and may cause someone to spend more money and get their hopes up for no good reason.
I have had my china girl 66cc engines with compression so high that I could barely get them started because even with the clutch adjusted extra tight it would still slip and sometimes even skid the rear tire with my 200+lb self on it and I still run 87-89 octane pump gas with no signs of pre ignition and no other issues, I say run what you want but why spend more on fuel than you need to when the stuff is twice to high to begin with, and if you don't actually need a higher octane fuel you will possible decrease your engines power output and not increase it due to the higher octane fuel not burning as complete in the low to medium compression engine.
happy safe riding all...
Read and heed this man's post. It is more conformation to what I've been saying every time this subject comes up. This not only applies to our little bicycle engines but to your daily driver. Running high octane, or 'premium' fuel above what the auto maker recommends will be a benefit, but only to the folks making and selling it to you. You and your stock engine will not see any advantage.
I'm at 6000' MSL (mean sea level). I lean my carburetors by soldering and redrilling the jets. I've never seen any consistency in aftermarket jets sizes. They seem to be nebulous at best. I don't know what their numbers mean, and they usually don't tell you.
I use a set of small orifice drills and depending on the engine, they'er all a little different, drill my soldered jets out to .026 down to even a .024. These small openings might be too lean for most elevations so be advised to start a little bigger.
I like to experiment by soldering up a stock jet and drilling it out to the small, .024 size and testing the engine. Most will require a size, maybe two drill sizes bigger to get the best performance. I also leave the 'C' clip on the needle at the second from the top in the NT carbs. That seems to be perfect for idle to 3/4 throttle. Anything above 3/4 throttle and the needle position has nothing to do with fuel flow. It is determined solely by jet size from 3/4 to WOT.
So running premium in my china girl could potentially reduce power output due to a lower energetic ignition of the fuel , I will admit with regular I get great acceleration ,whereas running the premium tank ATM ( ducks the flying beer can) at the higher end of rpm it does tend to spatter a tad , not enough to cause great consern but there is a noticeable difference between the two.
Sux just got a package from the great member THEON THANKS A HEAP MATE , BUT INLAWS ARE HERE FOR A WEDDING FOR A WEEK ARGGGGG no fitting the ported block till next week ,to busy looking after the outlaws
I'm running premium (92 octane here in the U.S.) with opti2 @ 1.1 oz. per U.S. gallon, 128 to 1.1. wow 98 octane I can only dream!!!
No worries Henshooter, worth a go on your motor I'd say, Interested how it all goes.
I'm still waiting for my new Jug lol.
A little accidental science. I raced a GSXR 600 for 3 seasons on 87 octane. One weekend i signed up for a 2 hour endurance race i had not planned to do and had to go buy fuel in a hurry. I could only find 93 octane so i threw it in and raced. The bike felt off but endurance racing is so physically and mentally hard i couldn't really tell. When i got home we put the bike on the dyno just to make sure it was ok and it was down on power from my last dyno session and I suspected the gas. We drained it completely and put my normal 87 pump gas back in and gained back exactly 5 horsepower on the dot. As described above octane is a flash retardant to prevent pre ignition in high compression engines. You will actually get the most horsepower from the lowest possible octane before pre ignition.