Engine Trouble High rev problem

[Big Red]

Well guys, I guess I'm not the MB guru I thought I was. My engine with new top end and an offset intake (otherwise stock) runs fine at low and midrange. As soon as it starts really zinging it sounds like it gets gas starved and dies. I've checked gas flow and float level. and for air leaks. Also has a good spark couse it bit me real good pulling the wire to kill it. (no kill switch yet). Spark plug is new autolite mid range. I dont know if it matters but I have the carb at a slight upwards angle. I've done this before with no problem. Is it possible I'm running the gas out of the bowl becouse it cant fill completely becouse of the angle? I've tried everything I can think of. any ideas?
Thanks,
Big Red.

 

[GearNut]

Fuel foaming in the float bowl?
How bad does the engine vibrate at high RPM's?

 

[Big Red]

Thanks GearNut, That's something I never, ever considered. Perhaps I'll try rubber mounting the carb instead of the solid offset mount I have on it now. I've been tweeking on this thing most of the day, so thats something left for tomorrow. Thanks again,
Big Red.

 

[motorpsycho]

attach a pic of the carb...i want to see the upward angle that it's sitting at.
it sounds like it;s running out of fuel and the angle could be causing the bowl to empty faster than the float can open enough to keep the bowl full.
any time you get a high rev and then the engine dies....it normally means that it's running out of gas.
not out of gas in the gas tank, what i mean is that the float bowl can;t get refilled fast enough to keep the engine running.

 

[Lazieboy]

Air

Hi, I had an air leak and or a pinched line from carb to motor intake and it gave me a similar issue. muffler was clogged up also.

 

[motorpsycho]

an air leak will usually affect the way the engine runs at all rpms.
a float bowl running out of gas will usually only affect an engine at high rpms.
i'm sure that he can ride around at slower speeds and the engine is ok.
this is because the float bowl is able to stay full longer because the engine isn't using a lot of gas.
at wide open throttle, the engine is using a lot more gas and the float may not be able to keep up.

A pinched fuel line will also affect the engine at higher rpms too because the fuel flow is restricted to the point to where it can't flow fast enough to re-fill the float bowl before it runs out of gas.

On this one, i'm leaning more towards the float being at such an angle that it's not letting the bowl get filled back up fast enough. since his carb is mounted at a slight upward angle, the float level may be set perfectly (for a level carb).
but, with the angle of the carb, the float will also be an an angle in the bowl. since the carb is at an upward angle (assuming that he means the rear of the carb is higher than the front). the needle & seat in these carbs is at the front of the bowl, so that would put the fuel level in the bowl higher in the front than the rear. the float would be at the same angle as the carb (lower in the front and higer in the rear.) once enough gas is burned the float level will drop. fuel starts to flow back into the bowl, putting less fuel in the rear of the bowl than the front. when the float rises, it's on an angle so the needle & seat is closing before the bowl can actually be filled enough to keep it running at w.o.t.
there will be more fuel in the front of the bowl than the rear and by the time the float drops enough to open the needle & seat, the bowl is almost empty....the flow of fuel from the fuel line can't fill the bowl back up fast enough.
This is my theory anyway...

 

[Big Red]

attach a pic of the carb...i want to see the upward angle that it's sitting at.
it sounds like it;s running out of fuel and the angle could be causing the bowl to empty faster than the float can open enough to keep the bowl full.
any time you get a high rev and then the engine dies....it normally means that it's running out of gas.
not out of gas in the gas tank, what i mean is that the float bowl can;t get refilled fast enough to keep the engine running.

I've got the float level set as high as it can go without hitting the primer "thingy". The needle seats well and allows a real good fuel flow when released. I've had the carb set at a higher angle on other bikes with no problem, between 90% and 45%. Not really that much. It does get a LOT of vibration so GearNut may be on to something. I'm going to level out the carb and use gas resistent rubber hose, (like smog hose) as an intake today to see what happens. It may be a combination of a couple of things, angle,vibration, causing my problem. We'll find out todayone way or the other.
Thanks,
Big Red.

 

[Big Red]

Hi, I had an air leak and or a pinched line from carb to motor intake and it gave me a similar issue. muffler was clogged up also.

Hey Lazieboy. No air leak, no pinched line. I do have the tank setting above, but close to the carb. So perhaps there is not enough gravity involved to help keep the bowl filled. combined with the angle and the vibration I might get this figured out today.
I went to the links you included thinking it had something to do with my carb problem. Imagine my suprise when they turned out to be about getting stopped by the police, And pot. I've only been on this site a few months, but don't those belong in another forum. I have no problem with the cops and I don't smoke weed, (anymore). I think it should be legal but that is for another forum, don't ya think?
Thanks for the input anyway, ALL suggestions about how to get my engine running right are welcomed.
Big Red.
Oh yeah, I forgot to add, Brand new Muffler.

 

[motorpsycho]

Hey Lazieboy. No air leak, no pinched line. I do have the tank setting above, but close to the carb. So perhaps there is not enough gravity involved to help keep the bowl filled. combined with the angle and the vibration I might get this figured out today.
I went to the links you included thinking it had something to do with my carb problem. Imagine my suprise when they turned out to be about getting stopped by the police, And pot. I've only been on this site a few months, but don't those belong in another forum. I have no problem with the cops and I don't smoke weed, (anymore). I think it should be legal but that is for another forum, don't ya think?
Thanks for the input anyway, ALL suggestions about how to get my engine running right are welcomed.
Big Red.
Oh yeah, I forgot to add, Brand new Muffler.

I doubt that it's a gravity feed problem. On one of my bikes, the fuel line from the tank to the carb is only 2 inches long. is your tank cap vented correctly and is the vent open and clear?

I assume that you have had the carb apart to make sure that the jet is actually screwed in and not laying in the bottom of the bowl .
It sure sounds to me like it's running out of gas in the bowl.
But, it seems that you are not willing to take my word for it....so let us know what you find out.
Think about how much fuel these engines use while riding at w.o.t..
When you are at w.o.t. there's more gas getting sucked up through the main jet than what is being returned to the bowl. The float bowl should never go empty while riding.The gravity feed is more than enough to re-fill the bowl as soon as the float drops and opens the needle. more fuel will flow out of the tank, than what will be used by the engine at w.o.t. for a long time.
If your float is at the incorrect angle, no matter how much fuel is flowing into the bowl, if the seat closes before the bowl gets filled, (float at an agle) it will run out of gas.
While you are riding, the float is constantly going up & down to keep the bowl full. If the float drops too low before the seat opens the fuel going into the engine will be more than what is going back into the carb. and it will run out of gas.
I had this same problem on my 49 c.c. because the carb was at too much of an angle (rear higher than the front). it ran perfect from idle to 3/4 throttle and then at w.o.t. it would run great until the bowl would run out of gas and then it would rev up and die. The gas flowing in could not keep up with the amount of gas getting burned and the bowl never truly filled up before the needle & seat would close (because the float was at an angle).
I lowered the rear of the engine and raised the front of the engine to make the carb sit level and i have not had a problem with it since. I can ride at w.o.t. for as long as i want and it never runs out of gas or dies.
the only thing i changed was the angle of the engine and carb and it fixed the whole problem.

 

[Big Red]

Hey Motorpsycho, It does it with the gas cap off too. And I am taking every bit of advice and gonna go do most all of it right now. Gonna level out the carb and use a gas resistent hose for the intake to avoid vibration. I think yer right about most everything, from the angle of the carb to the vibration causing it to foam. ANY foaming, with the carb not being able to fill all the way, Would cause it to get more air than fuel.
So, I'm headed out to the shop right now to find out. Wish me luck.