All of a sudden my 300 six with the YFA one barrel is running very poorly when I drive. Oddly it idles perfectly at 700 rpm. There is some unburned gas smoke from the poor combustion. Looking for some initial fix reactions. I've never opened up a carb, always feared having leftover parts. Thx. Steve

 

Does it have a miss-fire you can actually feel, or does it run smooth but is down on power.

 

You can feel it misfiring badly, but not ignition cause it purrs at idle. To me it says the idle circuit is set right but when the throttle opens the mix is off somehow.

 

Or, ignition components [i.e. plugs, wires, etc.] are needing attention. When a load is placed on the engine, ignition weakness will show up, but at low or no loads, everything will seem okay.

 

And also the timing. If it's off, it may be ok at idle, but as soon as the rpms rise it changes, and if it's way off it can cause problems.

 

Actually there is no load....just sitting still, increasing the rpms.

 

Have you done a compression test on the motor yet?

 

Yes, it was fine. My mind steers toward the carb. Anybody got carb problem experience?

 

If it is in the carb, then it would be the main jet is clogged or partially clogged. You can test this carb theory;

Rev it up till it starts acting up. Then blip the throttle, but keep it revved like you had it before you blipped the throttle. In other words keep it above idle. If the engine suddenly clears up and then reverts back to missing, then you do have a clog somewhere in the main fuel delivery inside the carb. By blipping the throttle you are adding fuel by the accel pump, and if the engine is running lean because of a carb problem, this added fuel will "fix" it for a short time till it burns that up.

 

Excellent, I'll try it today. Thx

 

That "blip" test didnt reveal anything...it acted normal.
I found a cracked rubber cap on the vacuum tree under the carb, but no real change there either.
I remember a rich/lean test from many years ago I will try.....gradually cover the top of the carb to see how it affects its running.

 

I put a vacuum gauge on her....only 8" of mercury.
Now she barely runs or idles.
I set the idle up to keep her running.
Heat riser isn't frozen.

 

Check the EGR and make sure it's not leaking or burnt out. You can make a block-off gasket out of a piece of thin metal to see if that helps it. I usually use the thin stuff you use for flashing. It will won't last too long but it's good for a experiment.

 

Idea check the vaccume line goimg too the distributor almost sounds like its running retarted

 

The EGR is gone, replaced with a steel plate. I have turned the distributor every which way to find the solution, but none found.