Suddenly Running badly

[Minkota]

It is a 350 small block V8 with a 350 transmission, MSD Ignition, New Spark Plugs, Cap, Rotor & Wires. Carb is a Carter AFB Competition Series 9605S

I was driving the truck and it was running perfectly and then out of nowhere it stuttered and died (no, it is not out of fuel). it fired right back up and runs fine (idles at around 800 RPM) but then when I put it in gear it will drop below 500 RPM and stall. when I first start it (cold) it will go into gear and drive but does seem to act a little strange but after maybe a quarter of a mile it will begin to stall when I come to a stop if I do not put it into neutral first (it does NOT have to be up to temp before this happens).

I sprayed some starting fluid around and found a small vacuum leak at the base of the Carb so I replaced the Base Gaskets and fixed the leak. I have not found any other vacuum leaks. I blocked the Brake Booster Line and the PVC Line to eliminate those as a possible vacuum leak and neither had any effect.

My next thing was going to be to rebuild the Carb but I am not 100% convinced this is the issue as it was running great and then suddenly it wasnt. I can find no other vacuum leaks I thought maybe it wiped a Cam but I do not hear a loose rocker and it does seem to run fine when not in gear so I am fairly certain it is not a bad Cam.

Any suggestion would be greatly appreciated....

[landarts]

Mine did this about a month ago and it was my choke not releasing. Once I readjusted the choke setting all was good.

[67C10Step]

Sounds a bit like junk in the carburetor. Cold the choke may be on letting it run rich enough to run OK but warmed up and choke off it leans out and dies. Have seen it too many times to count in small engines.

Those with more experience will probably chime in soon.

[Minkota]

Originally Posted by 67C10Step

Sounds a bit like junk in the carburetor.

That was kind of my first reaction so I have already ordered a rebuild kit and plan to do it either way.

[Minkota]

Originally Posted by landarts

Mine did this about a month ago and it was my choke not releasing. Once I readjusted the choke setting all was good.

I checked the choke and it did not need any adjustment. i was hoping it was that very thing as it would have been a quick fix.

[Aberdare]

Try checking the transmission modulator valve... if they go bad (and they do), This can be why the drop in RPM and stalling... Another reason why I prefer a manual transmission.

[Minkota]

Originally Posted by Aberdare

Try checking the transmission modulator valve... if they go bad (and they do), This can be why the drop in RPM and stalling... Another reason why I prefer a manual transmission.

I blocked all vacuum lines/ports from the Cab and Intake this made no change so I am fairly certain it is not the Trans Modulator.

[geezer#99]

Look inside the carb when it’s running or use a stethoscope to listen inside for crackling noises.
You might have some trash in one needle/seat and it’s flooding a little.
The crackling sound comes from a tiny fuel droplet sneaking past the throttle plate.

You can make a stethoscope from a length of garden hose or heater hose and a funnel.

[Minkota]

Originally Posted by geezer#99

Look inside the carb when it’s running or use a stethoscope to listen inside for crackling noises.
You might have some trash in one needle/seat and it’s flooding a little.
The crackling sound comes from a tiny fuel droplet sneaking past the throttle plate.

You can make a stethoscope from a length of garden hose or heater hose and a funnel.

i will give this a try, thanks

[Aberdare]

OK... interesting. The Primary Cause of Stalling at Low RPM/Deceleration is usually a vacuum leak and/or improper Carb and timing adjustments. Low Fuel Pressure can also cause the same issue. How about some tests?

1. Return all Vacuum Ports/hoses/lines to their original configuration(s) and check the Fuel Pressure. At Idle, the Fuel pressure should be between 3-6 PSI and Maintain this Pressure under LOAD (When you put it in gear).

2. Perform an ignition Timing Check. Timing for your engine should be 4 Degrees BTDC (But check your engine specs for an accurate spec value, as it could be 2-12 Degrees).

3. Check and/or adjust Engine Idle SPEED. Auto Trans should be 475rpm +/- 25rpm (NOT IDLE MIX - You'll do that later).

4. With a VACUUM GAUGE - check your engine MANIFOLD VACUUM. Any Port/fitting BELOW the CARB is acceptable to use as a test port. Engine Vacuum at Idle should be 18-22 in Hg (Inches Mercury)... although it may also be as low as 15 in Hg if your engine has high Mileage. If within range, proceed... if NOT, then you may have need to perform a Compression Test and/or check for a Manifold Leak.

5. Adjust the Carburetor IDLE MIXTURE, in accordance with your Carb's Mfr Specs. For most Carbs, turning in the mixture screw(s) until a 25-30 RPM drop is achieved, then back out 1/4 turn. Readjust Idle Speed as required to achieve proper rpm, AFTER you've finished adjusting the Idle Mixture.

6. Road TEST.

[Barnfind46]

This is probably a fuel contamination issue but mine was running bad and would stall out then run fine for a while. Drove me crazy! Come to find out it was the MSD box going bad. Put in my HEI and it straightened up. Not to muddy the waters or cause confusion but it does happen.

[Minkota]

Originally Posted by Barnfind46

This is probably a fuel contamination issue but mine was running bad and would stall out then run fine for a while. Drove me crazy! Come to find out it was the MSD box going bad. Put in my HEI and it straightened up. Not to muddy the waters or cause confusion but it does happen.

I had considered the MSD as the possible issue but I have not been able to find anything wrong with the MSD yet. I purchased a Carb rebuild kit and will try that first since it hasent been done in a year or more and the truck does sit most of the time. it seem like the logical place to start.

[Happy_dan]

If it sat that long could it be the gas itself? Maybe drain it and get fresh gas in?

[Minkota]

Originally Posted by Happy_dan

If it sat that long could it be the gas itself? Maybe drain it and get fresh gas in?

It has not sat long enough for the fuel to go bad (only been sitting for 2-3 months) and has non-ethanol fuel in it. it has been driven around the block occasionally during that time.

[HO455]

Don't forget the old adage "90% of all carburetor problems are electrical". Personally I would make sure the electrical side of things checked out before diving into the fuel side of things.
Start out by checking coil voltage and do a quick inspection of the ground straps. If all that is good and you have a good coil to swap I would try that too. I have seen coils that work fine cold and once they get warm the internal insulation fails and the misbehaving begins. Turn the power off and let it cool five minutes and all is good again.

[Minkota]

All good points, I have checked the electrical side. right now I have not found anything pointing to an electrical issue yet. Engine runs fine when in Park whether it is Hot or Cold. This weekend i am going to rebuild the carb and see if there is any change (i think it is probably needed anyway since it was sitting) then if there is still an issue I will continue to troubleshoot it.

[68 P.O.S.]

HO455 is right, your timing/ignition system has to be set correctly right off the bat or you’ll be wasting your time. The stock “warranty” initial timing for a 4 barrel 350 is 8 degrees before TDC, however SBCs run best at 36 degrees btdc total timing. I would start with the initial at 12 degrees, then once you get it running, fine tune it to 36 degrees total. Also, your idle speed with an auto is 600 rpm in drive, which works out to about 750 in park or neutral. Another thing I didn’t see mentioned, check for a clogged fuel filter or kinked fuel lines. I also agree with rebuilding the carb. It very well could have some blockages.