All Small Six Running Rich

[Maverick57]

Recently bought a 74 Maverick with a 200. Previous owner replaced the carburetor because they didn’t feel comfortable rebuilding the original one. It’s a Carter YFA 7346. Upon purchasing the car we noticed the idle was way too high, so once we got it home I got the idle down to 700-750RPM and realized I could screw the mixture screw all the way in and the engine would still run. In fact it ran best with it all the way “closed”. I got out my vacuum gauge and timing light, and no matter what I did the best I could get was 15-16in of vacuum at best and was still pig rich. After checking for vacuum leaks I’m starting to suspect the main jet is too big. Any ideas?

Update: I measured the main jet and it is .11”

What I’ve done so far.
New carb gaskets
New needle and seat
Set metering rod
Set float
Set timing
New plugs
New cap and rotor
New points and condenser

 

[awasson]

I’m not familiar with the Carter YFA 7346. Does it not have idle or slow speed jets that work with the idle/air mixture screw?

 

[Maverick57]

Yes it has a slow speed tube that’s being fed by the main jet.

 

[Maverick57]

I’m not familiar with the Carter YFA 7346. Does it not have idle or slow speed jets that work with the idle/air mixture screw?

Yes it has a slow speed tube that’s being fed by the main jet.

 

[awasson]

I wonder if you’re right and it’s the main jet. Could be a faulty power valve too. When you screw the air/fuel mixture screw in, you should have been able to lean it right out as long as the choke was disengaged and the butterfly closes properly.

 

[Maverick57]

I wonder if you’re right and it’s the main jet. Could be a faulty power valve too. When you screw the air/fuel mixture screw in, you should have been able to lean it right out as long as the choke was disengaged and the butterfly closes properly.

Very true, the low speed tube only comes in 1 size (17-20) which makes me even more suspect of the main instead of swapping the tube. The main jet is currently a .110” which to my understanding is the biggest jet made for a YF-YFA without drilling it out.

 

[B RON CO]

Hi, since the carb was installed on the big 300 engines, I would go with either the original jet or a little smaller. Are you following the service manual for the metering rod adjustment? Good luck

 

[DON]

Make sure the butterfly valve isn’t open too far. That will render the idle screw nonfunctional

 

[Maverick57]

Back again and still struggling. I've plugged all the vacuum hoses, including the main big hose going to PCV, and I'm still getting an engine that makes 5-8psi more vacuum off idle than it does at idle. Car smokes like Popcorn Sutton and has a dead miss, but clears up after being off idle. I put a smaller jet in the carb to get control back over the mixture screw, but the trade-off is off idle surging, so I put the .110 jet back in. I'll try to post a pic of the plugs ASAP

 

[ActionYobbo]

PCV

 

[awasson]

Back again and still struggling. I've plugged all the vacuum hoses, including the main big hose going to PCV, and I'm still getting an engine that makes 5-8psi more vacuum off idle than it does at idle. Car smokes like Popcorn Sutton and has a dead miss, but clears up after being off idle. I put a smaller jet in the carb to get control back over the mixture screw, but the trade-off is off idle surging, so I put the .110 jet back in. I'll try to post a pic of the plugs ASAP

I hope I'm misreading this and that you have not plugged the hose to you PCV and have only plugged the vacuum port side. Make sure your crankcase vent isn't plugged and maybe temporarily just put a breather on the PCV vent until you're ready to hook it up again.

Where is it smoking? Out of the valve cover (blow-by) or from the exhaust?

Missing at idle and picking up later sounds like a weak ignition or plug / plug wire issue.

 

[Frank]

Very true, the low speed tube only comes in 1 size (17-20) which makes me even more suspect of the main instead of swapping the tube. The main jet is currently a .110” which to my understanding is the biggest jet made for a YF-YFA without drilling it out.

The main jet is not at fault, it plays no part in the low fuel flow of the idle circuit except a hole to let the gas into the idle circuit pick up point. There is no power valve.
Check your fuel pressure, then the float for correct height.
Is this a china carb, or a real USA carb? If it's genuine it will be stamped in plain sight.

 

[Frank]

Unless you're at very high altitude, the 110 jet is close to being right. Don't change it until the over rich condition is isolated. Jets go up to over 120 for that carb. Also, a smaller displacement engine will need a larger jet not smaller. In this case, the 110 is close to target, it's not causing the flooding.

 

[Maverick57]

Answers to the questions as I saw them.
1. Plugged the PCV hose on the vacuum side temporarily to see if there was a change.
2. Smoke is coming out of exhaust.
3. USA carb.
4. Float height is dead on.
5. 14psi of fuel pressure

 

[Maverick57]

I hope I'm misreading this and that you have not plugged the hose to you PCV and have only plugged the vacuum port side. Make sure your crankcase vent isn't plugged and maybe temporarily just put a breather on the PCV vent until you're ready to hook it up again.

Where is it smoking? Out of the valve cover (blow-by) or from the exhaust?

Missing at idle and picking up later sounds like a weak ignition or plug / plug wire issue.

Didn't plug it off at the valve cover, I plugged the PCV hose coming off the carb adaptor, only did this for a grand total of maybe 10secs to see if the PCV was causing issues with idle so no worries.

Smoking from exhaust.

 

[awasson]

I think that’s way too much fuel pressure for a Carter (or any carb for that matter). That carb should see around 4 - 7 lbs of fuel pressure to prevent the needle and seat to be overwhelmed. It may not be the problem but that is definitely a problem. I’d put a fuel pressure regulator between the pump and the carburetor.

 

[vssman]

Holley makes a decent pressure regulator that you can get from Summit. The Mr. Gasket regulator has had reports of failing on here spilling fuel.

 

[DON]

With all the problems nowadays, I think a added guage at the same time is almost a must

 

[sixtseventwo4d]

There are lots of selections for fuel pressure gauges. Since you are carbureted a 0-15 is all that's needed, be easier to read at-a-glance and have increased accuracy.

 

[Frank]

14psi of fuel pressure

This is the problem.
The above advice is correct, a quality non-return low pressure regulator will control that. I have a fluctuating 11-13 PSI into a Holley non-return and the carb gets steady 5 psi, for many years this has been trouble-free. A Summit pressure gauge mounts directly to the regulator, works great.
Set the pressure 4-5 psi.

Add some fuel line any configuration you want and mount the regulator properly. I happen to have mine on the carb side on the fender. (I have 2 gauges, you only need one.)