Newbie - Honda CB750C won't start!

[lusciouslemur]

Hello, newbie here, you all have saved me from hours of tinkering in the past, so thought i'd share what's going on.

I have a 1980 CB750C that is having trouble starting. The bike turns over just fine, but after a few minutes I gave it a go and it made a rhythmic puttering noise. I linked a video of the sound below:

https://www.youtube.com/watch?v=Pudx6BK4OQ4

Some notes:

1. The tubes that are essentially the crankcase vent (they were a 2-to-1 that went into the airbox) aren't on the bike in this video.

2. I performed a pretty thorough clean of the carburetors, I don't imagine all the jets are clogged or something like that.

3. The exhaust on there IS a new 4-1 exhaust. I realize that may mess with the air/fuel mixture but i'm not sure.

4. I did take the engine off and examine the piston rings and valves, but I haven't done a compression test. I got the bike to start once prior to peering into the engine so it couldn't have been that low, but not sure now if my reassembly affected anything.

Any theories on that noise?

[motogrady]

Charge the battery up real good.
Crank it, choke on, twist the throttle.
Pull a spark plug or 3, check to see if they are wet..
If not, it's a carb or fuel problem.
Pull the fuel line off at the carb rack, open the petcock, fuel should flow and fill a cup.
If it does, and the plugs are still dry, pull the air filter, shoot some starting fluid directly into the
Air box or carbs themselves.  Crank it.
It starts like this, runs for a few seconds, it's clogged carbs.

If they are wet, wait till it's dark, or close the doors and turn off the lights.
Put a spark plug in one of the spark plug wires, hold it against the fins or engine, crank it with the ignition
switch on and look for spark to jump.
No spark, it's electrical.

Yes on spark? Then I'd go to a compression test, but to me that motor sounds like it's pumping.

You have all that, it has to be timing.  A cam chain has jumped a tooth, somehow it's way out if time.


I'm betting its a fuel issue.   Had to totally go over my carbs when I first got it, 4 times to get it going.

[lusciouslemur]

You very well may be right on that. I forgot to add that I did try a spark test and she sparks on the head. I also pulled one of the plugs out and cranked, engine is dry as a bone so I really do think it's a clogged carb issue. Upsetting because I did a pretty thorough clean of them, but they did sit around for 4-5 weeks before this video.

[lusciouslemur]

Could my accelerator pump be set up incorrectly or need adjusting?

[lusciouslemur]

So, update, I sprayed starter fluid into the carburetors and when trying to start, it'll pump more but I've literally gotten backfire, flames from the exhaust. My friend thinks it's timing but I'm not so sure.

I also did a compression test on the first two cylinders...40 haha so no compression at all.

[TOOLS1]

What you have just described is classic 180* out of time. Either your plug wires have been swapped, or the ignition wires (blue and yellow I believe) to the coils have been switched. Try swapping these wires and see what happens.
TOOLS

[lusciouslemur]

Well I will confess, I did open the engine up to inspect piston rings and valves. When I reassembled it, I just found out tonight that I did not set the timing even remotely close (the little punch marks on the sprockets were everywhere). I didn't think about the actual ignition wires but i'm going to set the timing correctly then check those. Just so i'm right, I want the 1.4 T showing in the window of the crankcase cover, and the sprocket markers level with the head, correct?

[lusciouslemur]

Final update, I set the timing, bolted everything down, attached the carbs and tank and NOW the compression test reads in the 80's to 90's - more than double the previous reading. Hit the starter and she fired up within seconds. Thanks for all of your help guys!