81 2 stroke backfire

[t4lude98]

i have installed the spring on the throttle stop and made sure the carb was closed and the micro switch is still on about 1 inch from top of pedal travel. i also bbq the muffler and cleaned out the carb the air mix screw is 1 1/2 turns out and i still get a backfire after i coast and then as soon as i open the throttle again pow please help

[t4lude98]

please any ideas

[racin161]

Take a good look at your exhaust piping make real sure there are no cracks and that all the flanges are tight new gaskets may be in order. Any possible air has to be sealed or your result will be a backfire.. Other than that it may be just more tweaking of the throttle cable or the accelerator mirco switch.

[Dave Box]

Just to state the obvious a backfire can only happen when you have fuel and a source of ignition. What can happen is that when you back off the throttle the engine makes several revolutions with the ignition off which dumps unburnt fuel into the exhaust pipe. If you ever run your engine without the pipe on you will see several inches of flame shooting out of the cylinder and there is your ignition source (If you do you should wear some ear protection and avoid the neighbors, it will be very noisy). The secret is to ensure that as little unburnt fuel ends up hanging about. To this end I would re-check that the spring really is clamping that butterfly shut and then adjust the throttle microswitch so it operates right on the top end of the throttle. In this way you will have the spark plug firing (and burning fuel) for as long as possible as you cruise to a halt.

If the problem persists you may need to think about replacing that carburetor with its 30 year old seats.

Dave

[t4lude98]

i will have to get new exhaust gaskets and see if that helps.
i have looked for a new carb but i cant seen to find one for a 1981.
could the fuel pump be leaking into the engine causing it to load up because i have held the throttle closed with my hand when driving and it will still backfire so im fairly sure that the carb is closing all the way but it has to be loading up with fuel somehow

[Dave Box]

It is strange that some sites show the carburetor as being 1982 - 1987 (1998 being the year of the BV 24 carburetor on the 3PG engine). Other sites show that the BV 18 is for 1976 to 1987 models. Anyway I would hate to advise you to buy the BV 18 even though I am 99% certain it is the correct unit. If you wanted to buy one then I would take your one off to double check that it is a BV 18. If you go that way then I can recommend www.everythingcarts.com. They are a sponsor here and they have the best price I found at $99. enter the code BG WILD and you get 5% off.

You raise a good point regarding the fuel pump. If you have a pin prick in the diaphragm it will suck fuel into the engine especially on overrun when the carburetor butterfly is shut. At $33 a new pump may be a wise investment.

Dave

[t4lude98]

thanks for all the info, i am going to get a fuel pump, something has to be loading up the engine and i know the butterfly is closing. it doesent leak gas out of the bowl but could it be the needle valve?

[t4lude98]

why is nothing listed for 1981 everything i find starts with 1982-1988.
the one for 82 looks like the one i need but i cant be for sure i cant even find gaskets for an 81

[Dave Box]

If it were the needle valve then fuel would be leaking from the air breather. I can only reiterate my previous advice that you remove the carburetor and see if it is a BV 18. If it is then a BV 18 is a BV 18 so it will work and so will the gaskets, but first try a new fuel pump.

Dave

[t4lude98]

i understand a bv 18 is a bv 18 which it is by the way. but i need a fuel pump and exhaust gaskets and i can only find them for 1982 and up