Topic: Troubleshooting my 78F

[tofan]

Hi All,

I recently picked up a 1978 CB750F with PD42A carbs. All stock. No mods. I took apart the carbs, ultrasonic cleaned them multiple times and replaced what needed to be replaced with good parts, checked for clogs, and bench sync'd before putting it back together. Replaced the plugs (NGK DEA8). Change oil and filter. Changed air filter (K&N stock type).

Here are the symptoms - I'm having a difficult time getting it started, even with choke on (pulled up). It takes many attempts to get it running, but once I manage to get it started and warm it up it seems to idle ok. Then, if I pull on the throttle hard it dies. If I pull on the throttle slowly, it'll rev up, but I can feel is slightly struggling around idle (1100rpm) - 2500rpm. If I have the choke on about 1/4 pull then I don't have this issue. Thoughts?

[denward17]

What's your float level like?  Clear tube test?

Sounds like it is running lean, what is the fuel mixture screws set to?  My '78K runs lean and need the screws about 2 turns out.

Did you verify that the choke plates are actually opening/closing when you push/pull the knob?

[lash]

Fuel lines, petcock filter, gas tank all clear?

[BenelliSEI]

Check the accelerator pump is squirting fuel down each carb when you crack the throttle?

On full choke, the mechanism should crack the throttle open a small amount. The set up specs are in the supplement for your model.

Did you pull the idle jets out to clean them?

[newday777]

Did you pull out the pressed in low speed jets and clean the emulsion tubes in the low speed jets.
And as John said, did you properly clean and check the accelerator pump circuits in each carb.

Here is a decent writeup on cleaning the PD42 carbs you should have on your 78F.

http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204

[tofan]

Check the accelerator pump is squirting fuel down each carb when you crack the throttle?

On full choke, the mechanism should crack the throttle open a small amount. The set up specs are in the supplement for your model.

Did you pull the idle jets out to clean them?

This might be the problem. I've attached photos of the workshop manual and what my bike is doing now.

All the jets were pulled and thoroughly cleaned.

[tofan]

What's your float level like?  Clear tube test?

Sounds like it is running lean, what is the fuel mixture screws set to?  My '78K runs lean and need the screws about 2 turns out.

Did you verify that the choke plates are actually opening/closing when you push/pull the knob?

The mixture screws are at 1.5 or 1.75 turns I don't recall at the moment. The workshop manual states 1.75 so I will check it against hat.
I'll recheck the float levels.

[tofan]

Fuel lines, petcock filter, gas tank all clear?

All clear

[HondaMan]

Here's the difficult question:
Are the brass parts (jets and needles) all genuine Keihin parts, and clearly marked so? If not, this IS the source of the problem. The aftermarket kits for these carbs have, for the last 10 years or so, had incorrect brass parts and jets from other bikes/carbs that don't work in these carbs.

[madmtnmotors]

What's your float level like?  Clear tube test?

Sounds like it is running lean, what is the fuel mixture screws set to?  My '78K runs lean and need the screws about 2 turns out.

Did you verify that the choke plates are actually opening/closing when you push/pull the knob?

The mixture screws are at 1.5 or 1.75 turns I don't recall at the moment. The workshop manual states 1.75 so I will check it against hat.
I'll recheck the float levels.

Verfy choke plate function too. The choke plates are spring driven with no direct link to the cable/lever. Any misalignment/drag can affect their function.

[Floshenbarnical]

What's your float level like?  Clear tube test?

Sounds like it is running lean, what is the fuel mixture screws set to?  My '78K runs lean and need the screws about 2 turns out.

Did you verify that the choke plates are actually opening/closing when you push/pull the knob?

The mixture screws are at 1.5 or 1.75 turns I don't recall at the moment. The workshop manual states 1.75 so I will check it against hat.
I'll recheck the float levels.

Verfy choke plate function too. The choke plates are spring driven with no direct link to the cable/lever. Any misalignment/drag can affect their function.

And they LOVE to oxidize around the edges, causing them to drag against the inside of the carb throat. I took mine off and filed/ sanded them to make them fit better.

[tofan]

A quick update.

I opened up the carbs - fined tuned the floats, verified all the orifices are clear and, specially the accelerator pump nozzles. Air-mixture screws set at 1.75 turns.

I tried tuning the accelerator arm, but couldn't figure out how to tune the upper part (pump arm-to carb stay clearance) so made an educated guess. If someone has photos of what it's supposed to look like it would be very helpful.

After assembling everything together it's a little better, but still feel it stumble between 1000-3000rpm.

I didn't test the accelerator pump while I had the airbox off which will be my next step.

So, if I understand the accelerator pump's purpose, it's supposed to spray fuel into the carbs at a specific rpm or throttle stage?

[tofan]

Here's the difficult question:
Are the brass parts (jets and needles) all genuine Keihin parts, and clearly marked so? If not, this IS the source of the problem. The aftermarket kits for these carbs have, for the last 10 years or so, had incorrect brass parts and jets from other bikes/carbs that don't work in these carbs.

I believe they're all original Keihin's but will verify the next time I open it up. If they're not, what replacement product do you recommend?

[tofan]

Verfy choke plate function too. The choke plates are spring driven with no direct link to the cable/lever. Any misalignment/drag can affect their function.

They seem to be good. The next time I open it up I'll try to upload a video in case anyone here sees something I don't.

[tofan]

And they LOVE to oxidize around the edges, causing them to drag against the inside of the carb throat. I took mine off and filed/ sanded them to make them fit better.

There's no oxidation, but they do drag a tiny bit when the are almost fully closed. Is this an issue?

[Floshenbarnical]

And they LOVE to oxidize around the edges, causing them to drag against the inside of the carb throat. I took mine off and filed/ sanded them to make them fit better.

There's no oxidation, but they do drag a tiny bit when the are almost fully closed. Is this an issue?

Definitely worth checking with another member but I personally would clean up the edges with an emery board or some fine sandpaper so they move properly. I don't think it's causing your issue but the drag is likely to get worse as exposed to moisture and time