is your bike losing power at top speed?

[Dogtown Burner]

All this info is really helpful. I am new to MB and have owned engine for less than a month (and signed up literally minutes before posting this!). I have very bad foaming problems in the fuel line. I shortened the fuel line so that it is almost stretched and my RPMs increased significantly. Have not tried riding w/o the fuel cap to check for venting problems. Still don't feel like the engine's turning full RPMs at WOT. Anything else I can try (besides replacing petcock, I'll do that as a last resort, short on funds right now)?

cap HAS to vent! the petcocks don't spew fuel out. they trickle. but the engines dont drink a ton, so it seems to work. as for stretching the line and it working better? only thing I can figure is that you released a vacuum in the line when you took it apart to trim it shorter, and the fuel is perhaps flowing correctly now. the rpms thing could easily be a rich mixture. try needle clip settings, drilling 2 nicely placed 7/16" holes in the black plastic air filter cover so that it gets more air. "the nostrils in the skull"". then the main jet. pistonbikes carries stock jets.

 

[GearNut]

I am new to MB and have owned engine for less than a month Anything else I can try (besides replacing petcock, I'll do that as a last resort, short on funds right now)?

How many miles does your engine have? They do not break in completely and run at their best until after 500 miles or thereabouts.

You could try riding it some more?

 

[Dogtown Burner]

How many miles does your engine have? They do not break in completely and run at their best until after 500 miles or thereabouts.

You could riding it some more?

what's crazy is, I have 3 engines that i bought from 2 different vendors. zminglu??, and another I can't remember. they look very similar. looking for the cheapest price got me 2 very very tame engines. and one that SCREAMS! its crazy. and I bought 2 from zminglu, and the fast one is one of them. anyhow i've swapped all the components around ( intake, exhaust, plugs, plug gaps, needle settings, cdi's; everything short of grinding transfer ports and piston skirts.) funny they all return with relatively similar results. the fast one is just simply faster. weird...

 

[Rocky_Motor]

what's crazy is, I have 3 engines that i bought from 2 different vendors. zminglu??, and another I can't remember. they look very similar. looking for the cheapest price got me 2 very very tame engines. and one that SCREAMS! its crazy. and I bought 2 from zminglu, and the fast one is one of them. anyhow i've swapped all the components around ( intake, exhaust, plugs, plug gaps, needle settings, cdi's; everything short of grinding transfer ports and piston skirts.) funny they all return with relatively similar results. the fast one is just simply faster. weird...

Maybe it actually has balanced cranks. Luck of the draw remember. I get the feeling that these aren't particularly assembly line made.. You would think that if they were that every engine of say T80 would be the same. But they aren't.


Anyway, I figure it is quite easy to see if there is a vacuum problem since you can just look down at your fuel line and see if it has full of fuel or not. But I got a SBP fuel valve I am putting on this weekend. Along with the sound dampening materials they offer. I'm excited

 

[Edd]

Gearnut, I probably only have about 100 miles or so on the engine. The reason I'm pushing it is because I have a friend whose dad rides two-stroke dirt bikes, and he said they break in rather quickly. I'm running 32:1 and it runs fine. Dogtown Burner, I might try leaning the mixture a bit, but I'm convinced it's the vibrations foaming up the fuel that's causing the problem. To test it, I went to this back road in Fairfield that hardly anyone uses and opened up the throttle while occasionally glancing at the fuel line. Sure enough, as the engine picked up speed, the fuel started shaking up like crazy in the fuel line and the RPMs stabilized somewhere below what I know the engine is capable of delivering. I only weigh 120 pounds so I know i'm not overwhelming the engine or anything. It has to be the vibrations. Eventually, if I hold WOT long enough, the engine picks up more speed. Should I try mounting the fuel tank on rubber mounts, or should I try something else?

 

[breno]

After rejetting, playing with the fuel bowl setting, checking for any leaks, blah blah blah I replaced the petcock and am now finally reaching top speed. Thanx for the thread.

 

[gera229]

Breno,
Which petcock did you get?

 

[breno]

Umm well It is abit of an expensive concoction that I ended up going with as Im an impatient bugger. I end up getting a union that got me from the 10mm machine thread to a bsp thread can't remember the size right now to a bsp threaded elbow to a bsp thread with a barbed stem to a ball valve and so on...sorry if that's not very helpful.
But I can say I went from a trickle to a 1/4 hose pouring out out fuel, no problems with flow now..at all.

 

[gera229]

Anyone try the sickbikeparts petcock? And is it any good? Is it ball valve?

 

[happycheapskate]

The SBP one works great, but it is different thread than most of the china peanut tanks. Oil it and install with the rubber washer, and a quality crescent wrench. Mine had a blue handle, and the valve is simply a rounded shape with a tunnel through it. It's ON or OFF, and does not leak! Best $10 spent on the bike.

 

[Goat Herder]

Ball valves are my favorite design so far.

 

[gera229]

What do you mean rounded shape valve? Any pictures? Is the valve bigger than the stock petcock?
I need it because my fuel line always bubbles, but my bike just keeps going (never feels like it wants to turn off so I am good for now). But I just need one to see if it can solve the slight problem I am having at the moment.

 

[happycheapskate]

The SPB has a rotating part, either a cylinder or a ball, with a drilled hole through it. The hole is smaller than a bic pen tube, but very easy to see daylight through. It does not have a gasket to leak. I am not sure if it has a spring in the assembly or just relies on closely machined parts. For $10 it works awesome, and I think you will like it. For what it's worth, if you open the valve and pull the hose, it will piss gasoline so you don't have to worry about flow. Way better than the china kit petcock.

 

[Dogtown Burner]

Gearnut, I probably only have about 100 miles or so on the engine. The reason I'm pushing it is because I have a friend whose dad rides two-stroke dirt bikes, and he said they break in rather quickly. I'm running 32:1 and it runs fine. Dogtown Burner, I might try leaning the mixture a bit, but I'm convinced it's the vibrations foaming up the fuel that's causing the problem. To test it, I went to this back road in Fairfield that hardly anyone uses and opened up the throttle while occasionally glancing at the fuel line. Sure enough, as the engine picked up speed, the fuel started shaking up like crazy in the fuel line and the RPMs stabilized somewhere below what I know the engine is capable of delivering. I only weigh 120 pounds so I know i'm not overwhelming the engine or anything. It has to be the vibrations. Eventually, if I hold WOT long enough, the engine picks up more speed. Should I try mounting the fuel tank on rubber mounts, or should I try something else?

Hey Edd, i only just saw this. a week later. sorry.
I have a problem with my fuel bubbling in the line as well, but I came to realize that it isn't a problem. At first I thought it was causing power loss at high rpms and I went so far as to try the armor all in the gasoline trick. What I finally remembered was my aircraft engine rules of operation. in an airplane you have to constantly adjust the mixture for altitude, so you learn about fuel burn and resultant engine behavior. If you have a too-lean mixture going, there are a few things that can happen, and one of them is "detonation", which is an uneven burn and heavy vibration combined with power loss due to the higher than desired cylinder chamber temperature. the fuel will actually ignite prior to the spark plug firing due to the air molecule friction. causing all sorts of bad vibrations. ( sounds like hippie talk, I know) there are a couple of ways this can happen on an MB. mine was that I'd added a slant head, increasing the compression, so the fancy NGK B6Hs plug was too hot for the kitchen.
I talked with VMB Norm and simply reinstalling the cooler burning stock crappy plug solved the problem. lower CHT's eliminated the bad 'vibes and all is good. the fuel still foams in the line (and trickles at best), but the engine screams!

 

[bluenosegoat]

So I have been through all kinds of petcocks and what not to figure out what seemed to be a fuel delivery problem. I've always been curious though to rejet my stock nt carb and finally got around to it. Problem solved and wow what a difference. Picked up an easy 5mph and no more "bogging"/four stroking. I went by what I think Venice Motor bikes had said and closed and redrilled with a # 73. I know all these engines are different but re jetting worked for me. I have a x80bk engine with a milled down cylinder for a MM head, ngkb6hs, pocket bike expansion chamber and its now finally running like it should.

 

[Dogtown Burner]

So I have been through all kinds of petcocks and what not to figure out what seemed to be a fuel delivery problem. I've always been curious though to rejet my stock nt carb and finally got around to it. Problem solved and wow what a difference. Picked up an easy 5mph and no more "bogging"/four stroking. I went by what I think Venice Motor bikes had said and closed and redrilled with a # 73. I know all these engines are different but re jetting worked for me. I have a x80bk engine with a milled down cylinder for a MM head, ngkb6hs, pocket bike expansion chamber and its now finally running like it should.

Couldnt really understand that first sentence, but glad you got it running! That's a huge jet too. Most guys are running a 66 or 68. And what's a "MM" head? Manic? If so, I sadly found out from piston bikes that it makes no change on the dyno. A BGF slant works the same.

 

[bluenosegoat]

Couldnt really understand that first sentence, but glad you got it running! That's a huge jet too. Most guys are running a 66 or 68. And what's a "MM" head? Manic? If so, I sadly found out from piston bikes that it makes no change on the dyno. A BGF slant works the same.

With numbered drill bits the smaller the # the larger the diameter. My stock NT jet was really close to a #71 drill bit and filling it in and drilling smaller with the #73 made a big difference and I guess that puts the jet size somewhere around a 64 or 65. Kind of confusing really. I found this link on the forum for sizing the jets to drill bits http://www.affordablegokarts.com/Drilling Main Jets.php

Ya I've had the "M"anic "M"echanic head sitting on a shelf for a long time and I agree that it does not really provide more power but it sure looks sharp IMO and probably just in my head, but the engine seems to run smoother. It "feels" like it has a little more compression when starting but I cant quite get the deck clearance Jim specified because if I remove that much material the piston will hit the spark plug. I am happy with it for now so I wont mess the jug or plug, until I get bored again!

 

[biknut]

Anyone try the sickbikeparts petcock? And is it any good? Is it ball valve?

The SBP one works great, but it is different thread than most of the china peanut tanks. Oil it and install with the rubber washer, and a quality crescent wrench. Mine had a blue handle, and the valve is simply a rounded shape with a tunnel through it. It's ON or OFF, and does not leak! Best $10 spent on the bike.

Happy is right about the SBP petcock. It's probably the best of the best, and very reasonable in price. Mine had pipe thread that fit my stock gas tank perfectly. I used a little teflon paste on the threads, and just screwed in.

 

[Edd]

So I played around with the mixture by taking the metal tabs that the float pushes on and bending them down a bit, and problem solved!!! It seems that the float setting was too high and making the mixture too rich (my guesstimation anyways). Previously the bogging problem was so bad, I was only hitting about 20mph. Now I can do at least 30, but it feels a lot faster. It seems the petcock kept up with the demand for fuel after all, but I may end up replacing it anyways (I don't trust it). Now the only problem I have is forgetting that my bike doesn't run on free energy (ran out of gas and had to pedal it home XD)!

 

[bluenosegoat]

Has anyone had an issue with their gas cap resulting in poor fuel delivery? I had this happen to me and it took along time to diagnose. I guess the vent got plugged somehow but when I would loosen the cap while riding my bogging went away. Makes sense that if the fuel that leaves the tank isnt replaced by air then it simply wont come out. Just an experience of mine.