backfiring out the carbs?!

[Anonymous]

in my quest to revive a 1980 gs450 i have finally come to the point where everything except the original airbox is back together and rebuilt. when attempting to start her i usually get a pop out of the carbs sometimes with smoke, every so often the combustion will happen at just the right time to hit the pistons back in the opposite direction thus stopping the engine. I am really confused here. I checked the valves and their tight but the compression is good so i figured there was no point in messing with it.

once every 20 pops out the carbs or so black smoke will shoot out the mufflers (which is what i want aside from the black) but this is rare.

please can anyone diagnose this situation for me?

[gspaul]

Check your ignition timeing, sound's like the spark's not happening at the right moment!

[Anonymous]

BAH!

THATS WHAT I WANTED TO DO!, HOWEVER THERE IS NO ADJUSTABLE IGNITION TIMING ON A 1980 GS450!!!

[snowbeard]

could you have plug wires on the wrong ones?

sorry, don't know much, especially about anything not mine.

[gspaul]

Good point Snowbeard! Check the lead's.
I dont know much about the mechanics of the 450, never owned one, can the plate with the ignition pickups be installed 180o out of whack?Could the ignition advance have a problem? Have you had a timeing light on it?
DID I SAY THIS LOUD ENOUGH FOR YOU!!!!!!!..........('')

[Anonymous]

ahh thanks guys

yeah good tries but no help yet

[flyingace]

You should be able to put on a degree wheel on the large nut on igniton side (R) and mark the zero on it with a piece of wire when you have located TDC for #1 cylinder. Remove the plugs and turn the ignition on. Spin the engine using the large nut (with degree wheel on it) and note where the plug fires. If it's the proper # of degrees from TDC for #1 (see manual), you can look elsewhere than ignition/valve timing (assuming your compression is OK). I believe the plug also fires near TDC on the exhaust stroke - don't be concerened about this.

Disclaimer - I've never actually done this, but this is what I would try.

Ace.

[blo]

Put your airbox back on! You are experiencing a lean condition. Almost all engines will have a small chance of backfire when running on to lean mixture. Why? Got to ask them motor guru's.

[Anonymous]

hook up your airbox!!!!!!! everything else will be a waste of time....once you put your airbox on it will change everything!!!

[Anonymous]

airbox back on...no help

im sorry that this project is so helpless but i thought it was worth posting the progress one last time.

its got good compression, good sparks, gas can be smelled through spark holes, and last but not least: The original air filter is back on.

the results... nothing, no firing nothing, a dead engine.

should i sell her?

[LW_Icarus]

yup. to me. cheaply

I would check the following things- fouled plugs possibly? clogged pilot circuit in the carb? maybe check the cam timing to make sure it hasnt been put together off a tooth or jumped if thats possible on those. check to make sure the choke is adjusted properly. look for obvious vacuum leaks around carb boots.

is the spark a big fat blue one? if not maybe check voltage to the coils and make sure ya got 12 volts there.

don't get upset if you can't figure it out right away, keep asking questions, and trying different tests until you find what the problem is. you will get it figured out, and that first ride after all that work will be sublime.

[Anonymous]

thanks for the support

you've rekindled my motivation for firing it up. thanks for that