Sym dd w/ hona dio engine i need help please!

[jacob789]

the other day is was riding with my friend who has a honda cbr 125 and he said i was going 65mph on his speedo whilst i was riding next to him, this was on a flat, i think this is the fastest ive had out of it.
It didnt use to do this but the air filter now slows my scooter down a lot and makes it really boggy even when it isnt wet, does anyone know why? could it be the jetting?
Also does anyone know if riding without the air filter can be really damaging to my carb or engine? because thats what ive been having to do but it hasnt really done any noticeable damage so far.

[bonesv]

Jacob, you are causing a lean condition and it will burn your engine up if you run it like that.

The jetting may be a little rich and that's why it runs better with out the air filter. Put the filter back in and jet the motor correctly or you will be sorry.
That is my understanding, good luck to you. Happy Trails!

[jacob789]

ok thankyou are you sure about this?
and any ideas why it used to run fine with the air filter and now it doesnt? could it be a temperature drop? because it is winter now in england

[jacob789]

and i meant does it do any damage in terms of dirt getting into the carb?

thanks

[Wheelman-111]

Greetings:

Disregard the fact that "Air" is about 80% Nitrogen (N2) and ~20 percent Oxygen (02), the ratio is more or less constant and it's the Oxygen that we burn with the atomized fuel. The open-throated carb along with cooler ambient temperatures may produce a double-whammy effect. Both increase the quantity of "air" going into the mixture on every stroke.

First, realize that affixing a filter to the carb creates a negative pressure outside the throat. As the vacuum (created by the piston rising over the crankcase...) increases, the number of molecules being dragged into the venturi will be decreased. It takes more time for the air to bump-and-grind its way into the inner filter chamber. By the time the vacuum is done sucking air in, a number of thopse molecules "missed the double-decker bus". They'll have to catch the next one. It' won't take long - see below.

Remember, whether it was Honda-san himself or some Previous Owner, the carb jetting was selected with that filter in mind. When you remove the filter, there is less impediment for those molecules and thus a greater number of them do make it into that charge cycle.

Second, cooler air is denser - it literally weighs more for a given volume than warmer air. That's why a hot-air balloon can float upwards. The difference between say, 22 degrees C. and 7 degrees C. isn't as dramatic, but the fact is that the same volume of air will contain a greater number of oxygen molecules. Once again, the jetting calculated for X quantity of Oxygen will encounter X multiplied by 1.05 or 1.075 - someone smarter do the Math. The end result is less fuel proportionate to the Oxygen that enters that charge load. A LEAN mixture.

Now that balance of fuel and Oxygen is pretty critical. It dictates to a large extent the amount of HEAT generated by each combustion cycle, as well as how much power each burn generates. A LEAN mixture burns more quickly and more completely. Vaporizing any liquid into a gas is an endothermic phase change. Hold the button down on a spray paint can if you want an example. When there's just a tiny bit less fuel for each intake gulp, it can mean a considerable increase in average temperature in the top-end of your engine. Yes, there's a cooling system, but it too was calculated to keep things under control with a specific mixture in mind. When you lean the mixture, the extra heat from each cycle is considerable. The cooling system can't "keep up", temperatures rise to the point where the mixture begins to self-ignite, like a Diesel engine. Pre-ignition, it's called. Since this cycle is repeating one hundred times each second at say, 6000 RPM, you can imagine how quickly such a miscalculation will result in Calamity.

If you must run sans filter, then the solution is to increase the size of the jetting - a bigger squirt of fuel to offset the greater number of O2 molecules with which the fuel combines. This restores combustion temperatures to something closer to what Honda-san intended. Another option might be to REPLACE your filter element. They do get "goo-ey" over time and tend to over-restrict the flow of air through them. This results in a RICH mixture; safer but a whole different story.

The dirt-related harm to the carb running filterless is negligible. The harm to the cylinder, piston sides and rings from the silica grit sucked in may not be. Something about oil lubricating those surfaces...
So unless you're trying for a new track record at your local drag strip or a new 49cc Land Speed Record at Bonneville, I recommend you install at least an oiled UNI, K&N or some other brand of pod filter to maximize your fun with this bore. Then install a Cylinder Head Temperature gauge, or at the very least inspect your spark plug to see if your jetting is at least close. There are spark plug reading charts here on the Forum to guide you.

[jacob789]

ok thankyou thats all very complicated but i kind of get what your saying and i think my air filter has become 'goo-ey' as you describe it, do i need a new one?. I mainly use my scooter to travel at around 30-40mph for about 4 miles at a time to school every day, could it still overheat quite easily doing a short journey like this?
and do you mean i can see if the jetting is close by looking at the colour of my spark plug?
and is running a rich mixture not very damaging?

[Wheelman-111]

Greetings:

Axt-ed Jacob789:

i think my air filter has become 'goo-ey' as you describe it, do i need a new one?

Yes, this is what I am saying to you.

I mainly use my scooter to travel at around 30-40mph for about 4 miles at a time to school every day, could it still overheat quite easily doing a short journey like this?

Is it likely? Probably not unless you wind on full throttle a lot. Could it happen? You durn Betcha! Pushing a scooter 3.5 miles is less fun than you think...

do you mean i can see if the jetting is close by looking at the colour of my spark plug?

I dunno about Colour Great Britain, but here in the U. S. And A. the Color can tell many things. Search Spark Plug Color Chart for some idears. The Bear has posted a really good one several times.

and is running a rich mixture not very damaging?

The damage isn't as immediate. The spark plug can foul from the excessive soot - not such a big deal. Just swap in a new one. The longer-term harm is that the soot soon covers the head and piston crown. The space it occupies increases your compression ratio after a while, leading to overheating also. Getting the mixture RIGHT makes for a long-running, reliable ride that performs as Honda-san ordained.

[jacob789]

my garage did my jets so they got em right i think and ive ordered a new air filter which is basicly exactly the same as my K+N but with a cover so i shldnt need to change the jets shld i? the jets worked fine on my K+N before it went bad.

thanks

[jacob789]

anyone know where i can get a new body kit for a sym dd 50 or one in good condition? preferably in europe
also would an unrestricted cdi affect top speed much?

[jacob789]

??? to last post

[bonesv]

Perhaps a set could be found in the pacific islands or a search on forums hawaii. Be careful.
Is your cdi rpm limited? You said you were going 65 mph? That doesn't sound limited to me. Just say'in. You might have meant 65 kph? Anyway, there are cdi's that aren't limited, in fact, I have one that I purchased by mistake and would love to find someone who really needed it. Let me know, it has never been opened from the package it came in from taiwan. I can post a pic if you want.

[jacob789]

ok and it was definitely 65mph not kph and it was quite accurate i think because my mate told me whilst riding next to me on his cbr 125 but it wont always get tht speed although it was on a flat. how much would u sell it? and yes cld u post a pic up please? thanks

[jacob789]

URGENT! who knows where i can get a single wire throttle cable for OKO carburettors? mines damaged

thanks

[evilone]

vtcycles.com give them a call

[jacob789]

thanks but they dont deliver to uk i think ive found one can anyone tell me if this is compatible for a sym dd with an OKO carb http://www.ebay.com/itm/Throttle-Cable- ... 4cfe278126