Engine wont idle but starts

[HemiDog]

So i have a predator 301 with a 28mm mikuni, 50lb valve springs, billet flywheel and Tav. Im lost as to why i cant get the engine to stay idling i had set both valves to .003 The bike wants to lunge when i start it and it idles very rough and only for a few seconds before if dies any idea what my issue is

 

[BaconBitRacing]

My guess—valve springs. From my (limited) knowledge of this style of engine they don’t love a ton of springs. I’m going to call in someone who’s built one, though for a kart, not a bike. Introducing, @panchothedog. Good luck and welcome!

 

[panchothedog]

I think your problem is with your carburetor. The fact that it starts probably means that your valve springs are O K. Valve lash as well. Are you running the stock muffler, or do you have a header pipe exhaust? I have built 301 on one of my karts ( rod, flywweel, cam, milled and ported head, 50 lb springs, header pipe and intake ) . I run the stock carburetor with a #40 main jet and the pilot jet drilled out to .021". With a 40 series converter with the light weights and stronger springs it surprises me how strong it leaps out of the hole and get to the upper 40's real quick. I am not a big fan of the Mikuni's, and don't know a lot about them. When you did the mods to the engine did you do everything at once.
Did it run properly before the mods? Are you getting good fuel flow. The fuel intake port on the Makuni could be higher than the stock carburetor. The gravity flow to the stock carburetor while adequate doesn't leave much room for being altered. Do you still have the stock carburetor. If so, try jetting it as mentioned above. With what you have done it won't good with the stock jets, but the #40 will only cost you $ 3.50. Wish I could be more help on your Makuni, but I don't think it would be starting and running for a short while if it were valve or timing related. How does the spark plug look. Is it black and sooty, or does it look too clean.

 

[HemiDog]

This is my current progress, i bought the bike non running and i added upgraded parts all at once its got a header pipe, i think im going to go check the fuel line again because there is air in the line not sure if that means its bad or not, it's visible in the last picture, its got brand new plug aswell as coil.

 

[panchothedog]

Air in the line. Very bad. The engine needs gas, not air to run. Are you running a fuel pump. Can't tell from your picture where the line starts. If it's from the motorcycle tank you should be OK. Is the gas cap vented. Maybe get a new piece of fuel hose. Shouldn't be air in there.

 

[HemiDog]

Air in the line. Very bad. The engine needs gas, not air to run. Are you running a fuel pump. Can't tell from your picture where the line starts. If it's from the motorcycle tank you should be OK. Is the gas cap vented. Maybe get a new piece of fuel hose. Shouldn't be air in there.

Currently no fuel pump. I may have to drill a hole in the cap for a vent, i don't believe there is one.

 

[panchothedog]

This is going to sound like bad advice, but here goes anyway. On one of my karts after installing a new remote fuel tank, I installed an inline fuel filter, looks
exactly like the one in your picture. Can't remember ( it's been a couple of years)
if it ran poorly or couldn't get it started at all. Running great before the new tank and filter. Eliminated the filter and all is good. Karts see very little use on asphalt, all of our running is in the Southern California (usually dusty ) desert.
No problems. Worth a try. Pull it and run straight from the tank to the carburetor.

 

[Bansil]

How many turns out for idle air screw? Turn in the screw clockwise, lightly seat and turn back out 1-1/2 turns, as starting point.

Take off air filter and make sure slide is opening and closing with throttle, should be a gap at the bottom. If no gap. Turn In the screw on the side of carb, it will raise/lower the slide, this is idle speed adjustment

Any difference, with filter off?