Topic: What am I missing?

[hapakev]

78 CB750F

I have pulled and cleaned the carbs 3 times, checking the passages the last 2 and proving they were cleared the last time.  I have a stock airbox and filter.  New D8EA plugs.  Trimmed plug wires.  Bench sync'd carbs.  Timing set.  Mac header running without the muffler.  91 octane gasoline with about 1/4 of it 87 octane, Wife was being nice and got me some gas.  I removed the emissions equipment, but plugged the port on the top of the airbox. 

The Damn thing still wont run without the choke

[Johnny5]

Have you rejetted because of the exhaust? If it wont run without choke, it might need a richer fuel mixture.

[TwoTired]

The choke requirement proves that the carbs aren't delivering enough fuel without it.  It is symptomatic of a stopped (or starved) pilot circuit.  Since you proved that the idle circuits are clear, it is time to check the fuel level in the carbs.  The slow jet is physically higher in the carb than the main.  So, a low fuel level could starve the slow jet, making unable to deliver fuel.  The choke would force extra fuel from the main supply, fed from the bottom of the carb bowl.

Make sure the floats are not upside down and that the float height is set at 14.5 mm.

You could also run a clear bowl drain hose up the side and mark the level of fuel in the float bowls.  This would verify that the slow jet is immersed in fuel and able to deliver to the carb throat.



an aside:  Shame on you for removing the emission equipment.  (Choke, cough, wheeze, gag, expire.)

[manjisann]

an aside:  Shame on you for removing the emission equipment.  (Choke, cough, wheeze, gag, expire.)

As a member of the brown movement, I salute you, go forth and polute!! 

Brandon

[hapakev]

I set the float levels at 14.5mm.  Any other suggestions?

I am really at a loss here guys.

[BobbyR]

I you have elimnated all of the issues of the carb internals including the slow jets, you will have to look for a possible air leak. You can use WD40, Starting fluid, propane to check for leaks. Many report that these are cold blooded beasts at best. Once it is fully warm/hot will it run without the choke?

[TwoTired]

I set the float levels at 14.5mm.  Any other suggestions?

Then verify the setting by measuring the actual fuel level in the bowls, to ensure the slow jet is immersed in fuel.

The slow jet behaves like a straw in you beverage.  If one end isn't in the liquid, it will just suck air.

Does the IMS adjustment do anything?

[mattcb350f]

I had the exact same problem with a 77 750F.

As it turned out, the idle circuits were indeed clogged. For some reason, on these carbs, it's not a clear shot through the slow jet up into the throat of the carb. I could not for the life of me get a wire, no matter how small, to run through it all the way. At the same time, I couldn't get fluid through it either.

I ended up shaving down the straw on a can of brake cleaner to fit into the slow jet and them a couple of shots the eventually cleared out (SAFETY GLASSES A MUST IF YOU DO THIS)......but even after they cleared out....the wire wouldn't go through 

Anyway, Once they cleared out......problem was solved.

 Matt.

[hapakev]

I you have elimnated all of the issues of the carb internals including the slow jets, you will have to look for a possible air leak. You can use WD40, Starting fluid, propane to check for leaks. Many report that these are cold blooded beasts at best. Once it is fully warm/hot will it run without the choke?

I think my boots from the airbox may be leaking.  This is the next test I need to perform.

I set the float levels at 14.5mm.  Any other suggestions?

Then verify the setting by measuring the actual fuel level in the bowls, to ensure the slow jet is immersed in fuel.

The slow jet behaves like a straw in you beverage.  If one end isn't in the liquid, it will just suck air.

Does the IMS adjustment do anything?

I will get some clear tubing to check this.  also what is the IMS setting?  forgive my ignorance.

I had the exact same problem with a 77 750F.

As it turned out, the idle circuits were indeed clogged. For some reason, on these carbs, it's not a clear shot through the slow jet up into the throat of the carb. I could not for the life of me get a wire, no matter how small, to run through it all the way. At the same time, I couldn't get fluid through it either.

I got liquid through all holes while testing, one at a time by plugging the others.  I did not get a wire through either.

[Inkscars]

It sounds like the issue I had on mine.
TwoTired was kind enough to walk me through.
Could you please tell me what all you removed when You did your "carb" cleaning?
I believe that your idle jet may have been overlooked.
I will be removing my carbs tonight to reset my needle Height, I could document a break down on the 77/78 pd carbs if you think it would help.

[Spanner 1]

You didn't say you removed the idle/slow jets...or did I miss that along the way ?
The pressed-in jet, I'm referring to....

[Inkscars]

You didn't say you removed the idle/slow jets...or did I miss that along the way ?
The pressed-in jet, I'm referring to....

That's the one I'M talking about.

[hapakev]

I removed the idle jets and cleaned them, making sure they were cleared (i saw the light).  I also found the accel pump o-ring missing so i replaced that, now the accel pump sprays into each carb, but the bike wont idle without the choke on.

[Spanner 1]

Thanks, Hap, I now remember all the previous threads about your bike !! I think were back to gas level in the bowls then as already posted by TT.

[TwoTired]

I will get some clear tubing to check this.  also what is the IMS setting?  forgive my ignorance.

IMS = Idle Mixture Screw.  It's that screw in front of the float bowls where you burn your hand adjusting unless you have or have made a right angle screwdriver.

I think the Honda manual calls it a pilot screw on pg 257.

[hapakev]

Oh I didnt get the abbreviation.  I have it adjusted at 1 3/4 turns out.  I will adjust them out some more tonight or tomorrow.  I read that the end of the effective range is 3 turns out, is this true? 

I had them more than 3 turns out before the last cleaning with same results, does not idle without choke.

[Spanner 1]

Looks like your missing a seat to be able to ride this thing to warm it up and let-off the choke.
Is the bike rideable?.....First was 'bike won't run without choke, now (last post ).. ' bike wont idle without choke "......bike won't perform in garage without road trial....IMO.

[hapakev]

Looks like your missing a seat to be able to ride this thing to warm it up and let-off the choke.
Is the bike rideable?.....First was 'bike won't run without choke, now (last post ).. ' bike wont idle without choke "......bike won't perform in garage without road trial....IMO.

Ahh, very true.  I have a seat now.  The bike does run and idle with the choke on, but will not idle without the choke on.  I can ride it with the choke on, with my vise grips holding it in place because the hold the cable in position does not work.  I would like it to run without the choke.