NT Carb.... No air adjustment

[danlandberg]

I want to know what a technician thinks, or any body who knows 2 stroke engines. I had a thought about adding an air mixture screw into the intake manifold to create a slight intake leak. With my past experience on a 40mi. ride (one way) 1/2 the way back, the bike was running GREAT for about 5mi. (37 mph) on flat ground, Then I could not get it over 18 mph. I stopped at the next town, (I was in B.F.U.) and found I had developed an intake gasket failure. So when the leak begain it was small & got bigger to cause a drop in power. I used a buisnes card to make a gasket.(It worked) I made it home! It's just a question I've been pondering I have rebuilt & tuned many 2 stroke weed eaters, but not to the point they could cut down a forest with 1 role of string!

 

[motorpsycho]

an intake leak would cause the engine to run lean, even if it was made by using an adjustable screw of some sort. A mixture screw does just what it's called...it allows you to adjust the air/FUEL mixture at the carb. an adjustable screw in the intake would only allow you to adjust the air and it will always run too lean if it is sucking extra air anywhere after the carb.
you will always get more power / shorter piston life out of an engine that is running too lean.

 

[danlandberg]

Lean is lean no mater where the extra air comes from. Carb or aft. the fuel/air mixture goes into the crank case via intake manifold (where fuel & air can be mixed together) then the combustion chamber. I think that the NT carb needs improvement! But I guess you get what you pay for. I think that an adjustable jetable style manifold would work (if you are using a stock NT carb). Also a thumb screw adjustment, so you can tune as needed (on the go) The only problem I see in this, is it will be unfiltered air! I am talking about a minute bit of air! Oh by the way, at 5500 F.A.S.L. wee need to run leaner!!!

 

[danlandberg]

And how does a boost bottle work? Intake leak?

 

[AussieSteve]

Lean is lean no mater where the extra air comes from. Carb or aft. the fuel/air mixture goes into the crank case via intake manifold (where fuel & air can be mixed together) then the combustion chamber. I think that the NT carb needs improvement! But I guess you get what you pay for. I think that an adjustable jetable style manifold would work (if you are using a stock NT carb). Also a thumb screw adjustment, so you can tune as needed (on the go) The only problem I see in this, is it will be unfiltered air! I am talking about a minute bit of air! Oh by the way, at 5500 F.A.S.L. wee need to run leaner!!!

Good to see you thinking outside the box, Dan, but motorpsycho is right.
It would only allow adjustment in one direction, (leaner and leaner) and could only work if the mixture was too rich to start with, at all throttle openings.
That's where the problem lies - where we need more specific tuning of mixture to throttle opening, this method would reduce it and lean out the mix at all throttle openings.
That's where a carb with an idle circuit with pilot jet and idle mixture screw is good. You can then set the mixture independently for idle, idle to 3/4 throttle, then 3/4 to full throttle by altering pilot jet and idle mix screw, needle position and main jet size.

Sorry, forgot to add. I think that a boost bottle works by capturing the fuel that would be blown back out of the carb, to be sucked in on the next cycle. Someone else can add more or clarify, hopefully. Otherwise, my usual advice - search 'boost bottle'.

 

[motorpsycho]

yep, a boost bottle collects fuel vapor that is spit back out of the intake. the vacuum from the intake sucks the vapor back into the engine. If you fully understand how these 2 strokes work with valve ports, some exhaust actually comes out of the carb on the very beginning of the exhaust stroke. this slight exhaust backfeed is what pushes unburned fuel vapor into a boost bottle. on the intake stroke, this fuel vapor is sucked from the boost bottle and back into the engine. It's a very simple but effective way of cramming more fuel vapor into the cylinder. sort of the same effect as supercharging but in a simplistic way. it is not an intake leak because the fitting on the intake and the fitting on the boost bottle are sealed. there is no extra air involved, it's all fuel vapor and whatever tiny amount of air is backfed from the intake manifold into the bottle.

by the way, a hole with a screw in the intake will not work!!!
why do you think intakes are sealed at each end with a gasket? the air/fuel mixture takes place in the carb venturi...not in the intake manifold. my 50 c.c. 2 stroke doesn't even have an intake manifold...the carb is bolted right to the cylinder. if you want to slightly alter the way an engine respnds, long striaght intake runners (or tubes) are better for high rpms, shorter intake runners are good for low end torque. look at a tunnel ram on a drag engine. they have long, striaght intake runners that go straight into the cylinders...better for high rpm power. Look at a stock v-8 carb. intake manifold. the runners are short and curved...better for low-mid range power.
I have built many engines (cars), played with nitrous, intake manifolds, multiple carb set ups, and all that. The intake must be sealed..ANY hole will cause loss a of manifold pressure, (a vacuum leak) and will ALWAYS cause a lean condition and make the motor run terrible.
True, more air and fuel will make more power, but it's all how that air and fuel are able to get into the cylinders. adding more air will make more power, but you will lean the motor out so much, you will lose a piston sooner than you should.

 

[danlandberg]

Thanks Steve, I Hopped you would reply to this thought! And I think I had mentioned before, I am at over 5000 F.A.S.L. And had to drop the needle clip a notch to lean the mixture so it would run. I wish the NT carb did have a circuit separate for air/fuel mixture. I now have a ''speed'' carb, I can't see a difference, they are supposed to have a larger throttle and slide than stock. I know these are scavenging engines, but I think this mod would work when you need it!

 

[motorpsycho]

If you think it'll work, then why did you ask the question in the first place?
You ask the question "I want to know what a technician or anyone who knows 2 stroke engines thinks..." you get some replies and thoughts that say it will not work.
then you say "i think this could work".
so why don't you just do it and see what happens?
it's like when you have a flat tire and someone tells you that it's flat, and you respond "it's only flat on the bottom".
sometimes i get tired of wasting my energy offering suggestions /thoughts when people do not listen to what i'm saying.

 

[danlandberg]

Thanks MP; it was a thought I had after 2 experiences of intake gasket failures, the thing ran it's best! (at this altitude) I want to create the same results when I need it!

 

[motorpsycho]

That is what i'm telling you!!! even at your altitude, it will run too lean with an air leak at the intake manifold.
It ran so good with a blown gasket because there was a ton of air being sucked in...IT WILL KILL YOUR PISTON!you can only run an engine lean for so long before the piston gets a hole burned in it or, it gets so hot that the piston will seize up.
but you do what you want to do.