1978 CB750K won't start

[andyvalente]

Hello guys,
I just got a 1978 CB750K left to dust since at least 20 years. I did a light but complete tune up and I was able to had it running (with full choke on of course).
New spark plugs, new air cleaner, new gas, carbs cleaning plus new floats gaskets and needles.
Put everything back in and...it won't start!
I did not check the floats height and the plugs look very dry so I guess for any reason it doesn't have enough gas.
any your thoughts/suggestions/advice is hiiiiiiiighly appreciate.

thank you so much for your help!
Ciao

 

[Dave]

The fuel petcock can easily get clogged. Pull the fuel line off the carb and turn on the gas first.

I've heard tales of needing to prime the fuel line if you run out of gas on the bikes with the single fuel line too.

After 20 years the gas tank will be nasty inside unless it was drained prior to storage. If not you'll have to clean it out too.

 

[dan-oh]

This thread might help...1981 cb750k Carb fuel line routing. I'm not sure if you need vacuum for your fuel flow. I bypassed my vacuum switch to fill the carb bowls for my first start.

 

[andyvalente]

Thank you guys!
@ dave: The fuel petcock is fine. I'll put a new fuel line from tank to carbs.
Old gas in the tank was super nasty. Drained completely and put new gas. No rust in the tank.
@ dan-oh: thanks for suggestion.

Later today I'll put carbs back in and see what happens.

 

[dirtdigger]

You really have to check float levels and adjust if necessary. Uneven float levels will cause all sorts of fueling issues.

 

[amc49]

Like said you can't change floats with no setting of them, at least on SOHCs.

 

[madmtnmotors]

The 78 CB750K had a fuel screen inside the tank. When you remove the petcock the screen will usually stay in place. Every later model fuel tank I've encountered so far appears to have had the original filter screen in place. I would recommend replacing this screen with a new one from Honda.

 

[andyvalente]

thank you guys for all your suggestions. I checked the whole fuel line. tank - petcock - hose - carbs - needles. all works. Now i check the floats height and put everything back together. I'll let you know in a couple hours how it goes.
ciao.

 

[andyvalente]

ok, fired it up for 2 sec. multiple times but for 2 sec! new spark plugs look good (little wet now compare with first attempt), new air cleaner, new gas, carbs checked (new float gaskets, new float needles, 12.5mm float height), fuel line ok (carb n. 2 and 3 little wet but no leaking).
during the last attempt it looked like there was not enough gas (spark plugs very dry). now the fuel line is ok.
what am I missing?

 

[madmtnmotors]

12.5mm float height

What am I missing?

You are obviously missing the correct manual. Float height for 1978 CB750K should be 14.5mm plus or minus .5mm:

When properly set, the straight edge of the bottom of the float is parallel (or nearly so) with the bowl flange.

 

[amc49]

Make sure while setting the spring loaded tip of needle if it has one is NOT compressed, only barely touching. Compressed is incorrect level.

 

[andyvalente]

OK guys. put a new battery in and now start right away but with full choke on. Any suggestion on how to set the right mix air/gas up? thank you

 

[madmtnmotors]

You can start by correcting the float level to 14.5mm (instead of 12.5mm) as mentioned in my previous post...