1979 CB750K wont start

[Karl H]

Hello everyone I'm new here and am just about at my wits end with my bike.

It wont start and i'm at a loss of what to do next and would truly appreciated any help you have to offer.

Here is some background:
I got the bike for free from a good friend and it was running when I got it. The inside of the tank was rusted but otherwise it was in good shape. I pulled the engine, took off the head, lapped the valves, and inspected everything else. Everything was in great shape so i put it back together and then rebuilt the carbs.

now its all back together with a new tank but it wont start. It has spark, fuel, and air, and I've re-checked the timing. when it cranks i will occasionally get a fer pops like its trying to fire but it wont turn over and run.

I am not a professional mechanic so its entirely possible that I've missed something and I'll be more then happy to post pictures if that will help.

thank you for reading and any help would be greatly appreciated.

[sgtslag]

There are two coils, supplying spark to all four plugs, two at a time.  Two cylinders get a spark on their exhaust stroke, which is harmless, but it draws energy away from the other cylinders, which are firing.  The coils are weak, from the factory, so be sure to test each plug for proper spark.

Are you certain there is fuel/air going into each cylinder?  The rusted tank (since replaced) could have sent bits of rust into the carbs, which you stated you have rebuilt.  I guess I would verify that atomized gasoline is reaching the cylinders.  Can you start it by spraying starter fluid into the carbs (if they fire, then you have good spark, and air)?  Did you replace the fuel filter?  The OEM is a metal screen, sitting on top of the petcock, protruding into the gas tank.  Mine had been removed by a PO, and it took me about a year to discover this.  I spliced the fuel line, and inserted a disc filter into it, to prevent any crud from reaching the carbs.

The '79 DOHC is a great bike -- I love mine, but it gets ridden very little now.  Still, it's rock solid, and dependable.  My wife and I toured on ours, two up, circling Lake Superior, two years ago.  Wouldn't hesitate to take off, cross-country, on it again.  Give the starter fluid spray a try, then report back.  Cheers!

[shinyribs]

Welcome to the forum.There a lot of guys waaay smarter than me on here,they can help you fix it for sure.Fuel,air and spark.That is all any engine needs to run.As long as its timed properly.When you mentioned you checked the timing,i assume you mean the ignition timing.Is it possible the cams are out of time with the crank?Just a thought.Also,what if you accidentally have the wrong coil going to the wrong pair of cylinders?Another thought.  Stick around and the real brains will be here before long.Good luck!

[Karl H]

alright so i tried the starter fluid and nothing happened. i checked for spark and it looks like im getting spark but it does seem under powered. do i just need new coils?

about the cam shaft: i think its correct. the book says line up the punch marks on the cam gears parallel to the head with the crank lined up at 1.4T which is what i did (picture below) i dont know if that's right because of all the other lines and the picture in the shop manual is fuzzy  

[TOOLS1]

It sounds like, you have the wires from the coils to the points crossed. Try swapping them.
TOOLS

[Karl H]

i never took the wires off the coils tho and it ran before so it couldn't be those

[TOOLS1]

You could have disconnected them down by the air cleaner.
TOOLS

[Karl H]

i checked the wires and even tried switching them. still nothing =/

[shinyribs]

When you say it seems under powered,you mean the spark looks weak?If its not bright blue and gives and good sharp "snap" you may need to charge the battery real good,or even use a start/boost setting on a charger.If you boost charge the battery and still have weak spark I'd suspect weak coils if all of the connections are real clean and tight.You wouldnt think they both die together.Just my two cents

[Karl H]

I agree that the coils shouldn't both go out at the same time.
The spark is blue but its really weak even with a charged battery. I can try cleaning all the contacts. If that doesn't work il replace the coils

I would like a second opinion on the timing too, I just want to be sure the timing is right before I pay for new coils

[TOOLS1]

These bikes have a weak ignition system from the start. The quickest, easiest, and cheapest way to upgrade it is to install iridium spark plugs. Have, you checked to see if the plugs are wet with gas?
TOOLS

[Karl H]

The plugs are dry but there old so I can get new ones tommorow

[TOOLS1]

With all this cranking they should be wet. Try spraying a little starting fluid in it. I am thinking it is not getting gas.
TOOLS

[LukeM]

One of your earlier posts says it was running recently.  That means at some point this bike had everything in place to make noise.  Something happened.  Could have been any number of things.  So....

Let's back up a few steps:

The 4 elements are Suck, Squeeze, Bang, and Blow.  

The first two can be verified with a compression check. If the compression is low, then you'll need a WHOLE lot more spark for combustion to take place.  This will also vett out the valvetrain, piston rings, and camshaft timing.  If memory serves, the PSI reading for compression should be about 130-150.  All readings should be the same.

A fully charged battery will also help.  You can use a pair of jumper cables and connect to a car battery AS LONG AS THE ENGINE OF THE CAR IS NOT RUNNING.  Too much current from a running car will blow fuses and other electrical bits.

Next, we see you're getting spark.  If it's not bright blue like a welding flash, that's OK. If everything else is in place, that should be enough.  You may have fouled plugs, and you may want a new set, but I'd hold off until you check the other stuff. I'd also double check the stuff in the left side engine cover (hall sensor pickups, wiring, etc).  Be advised that the left engine cover will have a little oil in it.

Verify the main ground for your wiring harness is clean and tight.  I believe it's a green wire under the seat.

Then I'd look at the fuel in the float bowls.  Hook up a hose to the carb overflow tube, drain out some gas and take a good look at it.  Should be clear, with no little floaty bits in it.  Do the same for all 4 carbs. If you find stuff in there, clean the carbs again, then back up to the fuel line, then the petcock, then the tank. Check the manual and make sure the mixture setting on each carb is at factory spec (1.5 turns out, if I recall).

I'd start with these things, then let us know what you find.  If you don't have a manual, you can download one from the home page of this forum, under the Manuals section.

Here to help if I can.
Luke M

[lcochran]

I've always heard to clean the carbs thoroughly, then clean them one more time to be sure that no crud is in the needles, floats, etc.  Im with TOOLS thinking fuel is your problem

[Karl H]

OK so it took me a few days to find a compression tester in town that fit the treads, but now we have some results.
The compression on the #1 #2 and #4 cylinders is reading 40 psi but the compression on the #3 cylinder is 100 psi. My guess the valve timing is incorrect, but I'm unsure as to how to fix it

[TOOLS1]

Since, you do have 100 psi in one cylinder, I doubt the cam timing is off. I would do a leak down test. You can buy adapters, for it, but I make my own out of an old spark plug. Just knock all the porcelain out/off it. Then braze/weld, or even JB Weld an air hose fitting on it. Screw it into one of the spark plug holes with the piston at bottom dead center of the compression stroke, and put air to it. Listen, for the air coming out. If it comes out the exhaust, it's an exhaust valve. If it comes out the carb, it's an intake valve. If it comes out the crankcase breather it's the rings.
TOOLS

[Karl H]

alright i have the stuff to make it work but i am unsure of how to get it to BDC of the compression stroke.

[TOOLS1]

Just turn the timing mark to 1-4, and either 2, or 3 will be where, you want it. Then turn 360*, and the other one will be where, you want it. Then use the 2-3 mark, and do the same with 1-4. You can also remove the valve cover, and watch, for both valves to close on one cylinder.
TOOLS

[Karl H]

I hooked up the air and its leaking out of the carbs and the exhaust
I assume this means the valves are leaking. what should i do?