Engine running on cylinders 2/3

[Caferacer1204]

Just recently purchased a 1979 Honda cb750k
Here’s what I’ve done,
Oil/ oil filter
Head gasket
Tires
High octane gas
Carb clean
And currently in the process of replacing the valve shims
The bike simply won’t start, timing is perfect everything is tightened to spec I have spark, fuel and compression I’m not sure what to do

 

[brettp]

Hey.
High octane fuel was not meant for these bikes with a stock set up. Might be an issue there. Second, do you have the stock set up (air box and carb jets)?
Have you verified spark?
Have you checked for fuel delivery (petcock drains, hose to carb inlet shows fuel pouring in)?

 

[Caferacer1204]

@brettp
No stock air box, k&n filters
Spark yes
Fuel yes
Carbs are operating flawlessly

 

[dirtdigger]

Look at basics, obviously you dont have either compression, spark or fuel or it would start. Have you given it a shot of starting fluid to see if it fires? How do you know the carbs operate flawlessly if it doesnt run? Do you have enough compression? Spark at the right time? Not 180 deg off? Cam timing verified? If it has compression, spark..at right time, cam timing correct, and fuel the it has to run no way around that...so something is not right. get you valve clearances set to .005 to .006" and recheck everything as if you didnt know the history of the bike. Dont take anything for granted as it can lead you down the wrong path.

 

[brettp]

Do you have pods? If yes, you need a bigger pilot jet.
Make sure your coils are connected to the correct cylinders. Left coil to cylinders 1 and 4, right coil to cylinders 2 and 3.

 

[Caferacer1204]

Update bike started after valve job and checking wires and spark
Bike starts up with an extremely high Idle and only operating on cylinders 2/3 I checked back for spark on the coil pack responsible for cylinders 1/4 and still had spark could it be a carb issue?

 

[brettp]

Could be fuel or spark still. Pretty coincidental 1 and 4 don't fire. Points to coil. Pull the spark plug on 1 and press it against the cylinder head while starting the bike. You should see the plug spark. If not, there you go. Do the same on cylinder 4.
If you have spark it points to a fuel issue.

 

[Caferacer1204]

@brettp I have spark on both coils

 

[brettp]

At the plug? You can see it?

Then you do have a fuel delivery issue to carbs 1 and 4.

 

[Caferacer1204]

@brettp Rebuild carb?

 

[brettp]

When is the last time they were opened and cleaned?
Try tapping on the carb bowls for #1 and #4, maybe the float is stuck in the up position and not letting fuel in.
Next, verify the fuel mixture screw isn't seated all the way in (that would be the closed position).
If you tap on the bowl while the engine is running and fuel still doesn't make it into those carbs, then yes, I would take them apart.
If/when you do check all fuel passageways, pilot jet isn't clogged, float valve doesn't get stuck in the up position, bench sync the carbs (then sync after they're operating normally), set mixture screws 1.75 turns out from full seat.
Before the carbs go back on the bike, make sure all accelerator pumps squirt fuel into the carb throats (you will have to fill the carb bowls to do this).
It's work but it will be worth it when running well.

 

[Caferacer1204]

I checked carbs and I’m getting gasoline to each carb still only operating on 2/3

 

[brettp]

I checked carbs and I’m getting gasoline to each carb still only operating on 2/3

How did you verify that? If it were me, I would open the drain screw on the bowl with the carbs on the bike. If the gasoline continues to pour out then fuel is indeed making in there. If the bowl empties then your float is stuck.
Let's say your float operates normally; you likely have a clogged pilot jet. Poor compression on those cylinders will not pull fuel from the carbs (could be worn rings or valves out of spec).
Did you actually see the spark on the plugs?

 

[Caferacer1204]

I double verified gasoline to each carb and everything checks out checked spark again and that was visible and strong the engine is still only operating on 2/3

 

[brettp]

I would say you have a clogged pilot in both carbs. Unless somebody else in the forum has another idea, of course.
Have you tested for a vacuum leak in the rubber boots? With the bike running, spray carb cleaner around the front and back boots of both carbs. If the bike does not fire on those cylinders, I'm afraid it sounds like you have pilot jets to clean. And if that is the case, make sure your petcock is not rotten.