Topic: Troubleshooting 1977 750F KEI HIN PD Carb Idle Circuit

[Slayer]

I need some advice on how to figure out why my engine will not start. I am pretty sure the problem is on the slow circuit because I have good spark, timed correctly, and decent compression. The float bowls fill but the gas doesn't even make it to the carb boots unless I use the accelerator pump, which doesn't spit much anyway. I have cleaned them out several times and I have even taken them to an expert veteran and paid to have them cleaned, not that I am ruling it out. When starting there is suction on the air filter side so it is drawing air but no fuel. I read on previous posts (Thanks TT) that there are four passages to check for; jet hole (clear), idle mixture, (clear), carb bore (clear), and air bleed (not sure). I am having some trouble identifying the air bleed so here is the pic:



Is that it? If so, it is not along the idle circuit so I can rule that out. 

There are a couple of other ports I am also curious about:



 I can fit a small guitar string through port 1 on the accessible outside carbs, so they are clear.

 When looking at the slow jet circuit from an opened float bowl it looks like it leads to a blanked off port, which is port 2. What is that?

[punch455]

That port in the first picture is a tap for a vacuum sync gauge.  The other two ports in the second pic are permanently closed right?  Like wleded and painted over? They don't do anything.  When you spray carb cleaner into the slow jet port (remove the slow jet first) where do you see the fluid exit?  Do you see it exit?

[TwoTired]

The air jets are at the carb throat entrance.  One goes to the idle/slow/pilot system, the other goes to the mains emulsion tube.
As mentioned, what you have labeled air bleed is the vacuum port for balancing the four carbs.

The other "ports" you've labeled are plugs put in during manufacture after the passageways were drilled into the carb body. It's pretty tough to drill a right angle passage without doing that.

[Slayer]

The first port is not sealed. I can fit a tiny guitar string through. The second one is sealed. When I spray through the slow jet it exits through the bottom hole in the slide. It does the same with the slow jet installed since there is a hole at the bottom of the jet.

[Slayer]

When I spray carb cleaner through the left air jet it shoots out of the emulsion tube. When I spray out of the right air jet it shoots out of the slow jet. Looks like everything is correct on the flow path...

[TwoTired]

When I spray carb cleaner through the left air jet it shoots out of the emulsion tube.

If the carb is assembled, it should com out BOTH the main jet and slide needle orifice.

When I spray out of the right air jet it shoots out of the slow jet. Looks like everything is correct on the flow path...

It should come out the slow jet AND the carb bore exit near the rear of the slide. (assuming you have the pilot screw backed off a from seated.)

[Slayer]

Okay I changed my angle and looked through the carb throat this time while spraying. The left air jet (emulsion side) had carb cleaner running down the jet needle, and the right air jet (idle/slow/pilot system) shot out carb cleaner through the carb bore. 

[Slayer]

So the idle circuit is free and clear, and I have no idea what to look for next  Has the scientific method failed me? I don't see how everything is tested satisfactory but when they all go together it doesn't work. Whatever the problem is, it is affecting all of the carbs. Any other theories? I feel like I have come a long way and learned a lot. It sucks to give up now.

[hapakev]

So the thing still will not start?  has it run at all?  how do you know the timing is correct?  Damn dude I hope you can get this figured out.

[TwoTired]

You need Compression, properly timed spark, and a fuel/air mixture for it to run.

What is it you are missing?

[Slayer]

I have compression, properly timed spark, but I can not get the fuel to the spark, or even past the carb boots, so that doesn't even matter at this point. The bowls fill and the idle circuit is clear. I don't know why gas does not get there. What else other than the idle circuit can it be? My only hint is that all four carbs are not moving the gas from the float bowl to the throat with the exact same problem, so it must be something that affects them all.

It has never run for me. The PO said it runs, "All it needs is a new points plate that I removed for another bike." The carbs were pretty bad so I don't believe a word. I can only do static timing for now, so to check my static timing work I looked through the points sight hole and verified the 1-4 F mark lines up with the other line and a spark from the spark plug close to the engine block at the same time.   

[TwoTired]

If you crank the engine, do you feel suction at the carb mouth with your hand covering it?

[Slayer]

Yup. Good suction. Tried the manual choking at half covered, 3/4, full, and full intermittent with no difference. I also tried it with engine starter fluid and all I got was backfires. Once out the exhaust and once out the carb inlets. Also tried kick starting for a little more umf. No help either

[TwoTired]

How did you determine the spark was properly timed?

[Slayer]

http://forums.sohc4.net/index.php?topic=49623.15

You helped me out with this one. I first did it with a continuity test and then checked with a voltmeter

[TwoTired]

Do a sanity check.
Remove the # one spark plug and crank the engine over to feel for compression at the spark plug hole.  In other words physically put the piston at TDC.

Then go look at the timing marks under the points plate.

Do the the 1-4 T marks align?

[Slayer]

I did a thumb compression check and the outside cylinders are TDC at the 1-4 T marks. I am sure it is the compression stroke because it blew my thumb off. I was under the impression that the cylinders are numbered from left to right 1, 4, 2, 3.

[TwoTired]

As you sit on the bike, the cylinders are numbered 1-2-3-4 from left to right.

TDC is at the end of the compression stroke.

Maybe that's the problem, the 1-4 spark leads go to the outer cylinders, and the 2-3 spark leads go to the inner cylinders, and you've got at least two cylinders firing at the wrong time.  You'll def get backfires with that arrangement.

Cheers,

[Slayer]

You are still up man, what a trooper! I don't know why I thought it was 1, 4, 2, 3, but the 1 and 4 are on the outside making the test correct. What's next?

[TwoTired]

You still haven't convinced me that it's getting spark at the right time.

I also tried it with engine starter fluid and all I got was backfires. Once out the exhaust and once out the carb inlets.

It is possible to assemble the spark advancer 180 out of phase, getting symptoms above.
Also, verify the Blue wire from the 1-4 points goes to the same coil that has leads to the 1-4 (outer) cylinders.

Next verify that when the piston is at TDC between the compression and exhaust stroke, both intake and exhaust tappets have clearance.

[cycleman]

Even if you have a bad idle circuit you should still be able to get the bike to run with the choke on full.  It will be hard to keep the bike running and the rpms will be 2000 + but it will run.  If it does this then you know you still have an idle circuit problem.

I know this sounds stupid ( but its happened to me ), clutch pulled in & emergency shut off on when you are trying to start.  Also are the carbs getting enough fuel.

When I first got my 78 750F running if would backfire & shoot some flame back through carb #4 but I could get it too run on the choke.  Took carbs out again & really concentrated on the idle circuit, pressed in slow jets & the clear tubes - accelerator circuit.  Turns out that a couple of these where plugged. Once I cleared all these & sync carbs bike runs steady and idles at 1000-1100.  There is still a slight hesitation when you quickly twist the throttle, but this bike has sat since 1989 so I suspect a  few road miles will fix this up.

I'm pretty sure your issues are still with the carbs.  They have to be really clean.

[Slayer]

Since I had my cylinder numbers order wrong I have two coil cables going to the wrong plugs, so yea that would prevent spark at the right time. I will correct that when I get home. I already verified that the blue ignition wire goes to the 1-4 coil. I will also check on the spark advance, although I don't see how it is possible to set it out of phase since there is only one pin and one hole. I have never checked or adjusted tappet clearance so I am gonna have to read up on that a little bit before diving in. I was gonna do it eventually should I just go through all of the tappets now, or is it better when warmed up?

cycleman: The emergency shutoff is off and the bike is always in neutral. Does the clutch still have to be in? The bike does not run with the choke on either. The pressed in jets are clean and free from plugs, what clear tubes are you talking about?

[TwoTired]

I already verified that the blue ignition wire goes to the 1-4 coil.

You also have to verify the Blue wire is connected to the 1-4 points.

I will also check on the spark advance, although I don't see how it is possible to set it out of phase since there is only one pin and one hole.

The spark advancer is an assembly where the cam can be assembled on to the mandrel 180  out.   You can only put the assembly on the bike one way. But, the assembly can be assembled incorrectly off the bike.

I have never checked or adjusted tappet clearance so I am gonna have to read up on that a little bit before diving in. I was gonna do it eventually should I just go through all of the tappets now, or is it better when warmed up?

Tappets are adjusted with the engine cold.

[bubbafun101]

I just read the posts quickly. And didnt see the one thing I would look for. Float level. "Good suction but no fuel except from the accelerator pump" is what the man said. Just my .2 cents

[cycleman]

On my 78 cb750F the clutch has to be pulled in to start. Once you get your plug wires sorted out it should start, even with some of the idle circuits not working properly.

The backfire is an indication that spark is getting there at the wrong time.