79 cb750K wont turn over

[big-nate]

this topic has probably been done to death, but i cant find a thread that addresses it so here goes.
 
i got the bike a while ago and it ran for a while but very poorly. i found an air box for i and that made it run a little better but not perfect. and then it just stopped running all together. i figured the carbs needed cleaning and re building. so i got that done but it still wont start. when i try to start it the pipes will puff a little. i have the screws set to starting positions the book says but i don't know what it is i should be adjusting. i haven't hooked the airbox back up yet. could that be that reason it doesn't fire at all?

[TOOLS1]

The air box would not have a thing to do with it not starting. You need 3 things for an engine to run. Compression, Fuel, Timed spark. Which of these is missing on your engine?
TOOLS

[Johnnylevi]

fouled plugs?

[big-nate]

fuel is getting to the carbs. that's basically all im sure of. as for compression and timing i dont know how to check for either of those. the plugs are a little charred but that shouldnt keep them from sparking.

[TOOLS1]

Fuel does not just need to get to the carbs. It must get into the combustion chamber. Are the plugs wet with gas? If you take out a plug, connect it to the wire, and ground it on the engine while cranking it, do you get spark?  
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[RodneyCB750K1979]

I think you're onto something there.  Verify first the spark plug spark.  Pull a spark plug or get a spare spark plug, connect the electrical lead, and lay the spark plug threads against engine metal. Turn the key to on or run, turn the handle bar kill switch to run, and turn the engine over. You should see a spark jump between the spark plug electrode.

[big-nate]

okay so i did what you said and all 4 of my plugs are sparking. but they were bone dry when i pulled them out after trying to start the bike.

[TOOLS1]

Your carbs need cleaning.
TOOLS

[big-nate]

is that the only thing it could be? i just had them re built and cleaned. i guess ill do it again.

[icerigger]

How are the insulators? Cracks in these things can suck more air than fuel. Try spraying Quick Start or other ether based product around the insulators while cranking. It won't start, but if it fires you need to replace the insulators.

[big-nate]

the insulators are brand new. i did what you said and sprayed them with starting fluid just to see if the seals weren't tight, nothing happened when i did. so i don't think that's a problem.

[big-nate]

i checked the plugs too when i pulled them out. they are a little charred but still are sparking.

[big-nate]

so i started cleaning the carbs, i was following a guide that this guy made for thurough take down and cleaning of this type of carb. and as i was taking off the air cut off valve of the #4 carb i found that the rubber gasket was completely melted. and it was like that for all 4 of the carbs.

[Johnnylevi]

some of that gunk probably came loose and is all up in your carbs in various places. Make sure to take all the pieces out you can and clean them. Shoot carb cleaner then air through your carbs and use an E string from a guitar to clean the idle jet.

[big-nate]

i had the carbs ultrasonicaly cleaned but the bike still wont turn over. but now it backfires a little when i pull the choke all the way out. i tried to start it using starter fluid too but no luck just allot of backfiring.

[free2ride]

really dumb suggestion. how is the kill switch? is it on? or has it broken in an on [or is that off?] position?

[TOOLS1]

It sounds like you have the bike 180* out of time. Try switching the wires to the coils.
TOOLS

[Black 750K8]

First off if it were the kill it would not backfire got to have spark to do that. Timing off and other things will cause back fire.

 i have the screws set to starting positions the book says but i don't know what it is i should be adjusting.

This does not compute How do you have it set to what the book says but at the same time you say you do not know what to adjust??

"the plugs are a little charred but that shouldnt keep them from sparking"

Not true They can even spark while out of the engine and not spark enough while in the engine under compression . Get a new set of plugs for trouble shooting. You can clean the set you have now with a torch later on.

 i haven't hooked the airbox back up yet. could that be that reason it doesn't fire at all?

NO

The air box would not have a thing to do with it not starting. You need 3 things for an engine to run. Compression, Fuel, Timed spark. Which of these is missing on your engine?

AIR

It sounds like you have the bike 180* out of time. Try switching the wires to the coils.

I think this is most likely what has happened. You need to make sure the correct wires are on the correct plugs. If you don't know how to do this post back and one of us can walk you through it.

[icerigger]

Try reaching under the carbs and turning up the idle screw a couple of turns. Really important if the carbs aren't well sync'd.

[Gas]

Do not use carburetor cleaner on your carbs while the air cut valves are still in the carburetor, or any rubber for that matter. That's why your air cut valve melted. 100% certain.

 It helps to have the intake tubes connecting to the carbs to ensure all of the forward vaccum ports receive equal pressure. In fact, most bikes wont run well or at all without them.

Cleaning carburetors without air compressor is futile. Make sure your pilot circuit is completely clear. pull the air screws and clean in there. Don't just spray and hope it got it. Find out where that particular circuit goes and check for air on the other side. You'll become intimate with the carburetor and how it works, and you'll know exactly what it is before you even take it apart. Be methodical.