Topic: 78 carb problem.

[Supersportguy]

Restoring a 78 750A. Have cleaned the carbs and still only runs on choke. I have cleaned and eventually replaced the slow jets with new #38 jets. Still have the same problem. Also have sprayed carb cleaner thru passage ways to be sure they are clear. Pilot jets are set at 1 1/8 turns out. This is crazy! Have never had this problem before. Floats set at 12.5. Main jets cleaned. The only thing I noticed is when I go from neutral to drive the accelerator pump does not activate. It does on my 76.

[newday777]

Are the accelerator pump jets all squirting properly??

[Supersportguy]

Yes, there is only one pump, on the #2 carb.

[newday777]

Yes, there is only one pump, on the #2 carb.

It should squirt out in each carb, does it?

[Supersportguy]

Can I test that with the carbs off the bike?

[newday777]

Yes if you have a carb stand and a test gas bottle to have a feed to the bowls.

[Supersportguy]

I found that number three is totally blocked.  Will clean out and retest. Hope that's my problem.

[Alan F.]

Good catch.

[HondaMan]

I had a 78F once that acted like yours after the owner rebuilt the carbs. He had put the O-ring into the idle-adjuster screw holes on the bottom of the tiny washer in there, instead of having the O-ring on top of it. It would start and run, but below 1800 RPM it would die. Sound familiar, maybe?

The O-ring goes in first, then the washer. I usually stack them on the air screw with the washer resting against the 'step' on the screw and then guide them up into place that way. Easy does it!

[Magpie]

I had a similar problem with my 78F. It would start on the choke then die. No idle possible. I went through the carbs and installed a new accelerator pump - still no go. It sounded like it was starving for gas or way too lean. New carb manifolds/insulator solved my problem. I think it was sucking air past the rock hard old ones. Cliff.

[sandcastcb750]

My CB750 K8 had the same carb issue; only ran on choke. Jets must be removed and cleaned well. This is a result of EPA requirements and Honda designed a press fit design that stinks.

[HondaMan]

My CB750 K8 had the same carb issue; only ran on choke. Jets must be removed and cleaned well. This is a result of EPA requirements and Honda designed a press fit design that stinks.

Yeah, it was the US DOT, pushed by the US EPA, that forced Keihin (and many other carb makers) to switch to the press-in type jets (witness Holley, Rochester and Carter automotive carbs of the same era). At least Keihin was smart enough during that episode to increase the authority of the air screw at the same time, giving it almost a full turn of range where it works. In the earlier carbs you only get 1/2 turn (3/4 to 1-1/4) of active adjustment, beyond which only a jet change will make a difference.