Engine Stalling After Short Drive

[JedIEG]

My engine is stalling out and I'm not sure what to look at.
1974 318, stock long block, mild street cam (think stock 340), performer intake, headers, QFT slayer 600 vs that has run stupid, plug fowling in 20 seconds, rich since new.
I have not been able to get the car to run right in a few months with 10.4 afr at idle. I reset the carb back to my baseline settings for throttle position to restart turning. Something has happened that I can start the car and it will idle. I can drive it about 1/2 mile to the end of my neighborhood before the AFR climbs out of control. It will act perfectly normal for that first 1/2 mile then under light throttle and load it will start going ultra lean and die.
I thought it might be a fuel pump since I already had to replace a brand new one earlier, but fuel pressure is right around 6psi when the engine stops or I bar the engine over by hand.
I think it is something with the idle/transition circuit for 2 reasons-
1. When I restart the car, once it uses the pump shot, it dies again,
2. If I pump the gas (give it constant pump shot) it will stay running
3. If I get the engine reved up and keep it in 1st (2500+ RPM) it will stay running under load (Main boosters running)

I recreated the scenario in my driveway by putting the brake on and lightly loading the engine in gear. Same thing: AFR climbs rapidly and the engine trays to stall unless I give it pump shot or unload it.
Floats are adjusted,
Idle screws are 1.5 turns off
Idle is 850rpm
Timing is 18° at idle
Plugs are very fowled within seconds of the engine running so I can't get much from that.
Not sure what to look at now and I really don't want to just take everything back apart for the 10th time.

This has almost stranded me twice now and I am getting very frustrate.

 

[hunt2elk]

Did you check that there isn't something between the needle and seat and flooding out?

 

[JedIEG]

I have a summit adjustable fuel line with a brass filter in it. The filter is totally clear. Also fuel levels in the site windows are where they should be. I don't think its flooding because if I give it pump shot it keeps the engine alive.

 

[BLK 68 R/T]

Are you sure the plugs are gas fouled? or oil possibly? Exhaust smoke? Color? How is the fuel strainer in the tank?

 

[70chall440]

Sounds to me like its starving. Are the floats set right? If it were me, I'd try and replace the carb and see what happens.

 

[JedIEG]

Are you sure the plugs are gas fouled? or oil possibly? Exhaust smoke? Color? How is the fuel strainer in the tank?

Pretty sure its fuel fouled since the carbon is really fine powdery carbon. 140-145 cranking compression on all cylinders. Smoke is gray if any. It's not blue or white. Tank and pickup are brand new and haven't ever seen gas with ethanol in it even.

Sounds to me like its starving. Are the floats set right? If it were me, I'd try and replace the carb and see what happens.

I was wondering about float levels. When I did the driveway test, I let it stall and then jumped out to run and look at the float level. It was in the middle of the sight window.
And this QFT was the replace the carb and see what happens. I tried switching back to the 4175 I had originally replaced and I could not get that carb under 2000 rpm with all throttles fully closed.

 

[BLK 68 R/T]

Pretty sure its fuel fouled since the carbon is really fine powdery carbon. 140-145 cranking compression on all cylinders. Smoke is gray if any. It's not blue or white. Tank and pickup are brand new and haven't ever seen gas with ethanol in it even.



I was wondering about float levels. When I did the driveway test, I let it stall and then jumped out to run and look at the float level. It was in the middle of the sight window.
And this QFT was the replace the carb and see what happens. I tried switching back to the 4175 I had originally replaced and I could not get that carb under 2000 rpm with all throttles fully closed.

Not being able to get it to idle under 2000 rpm suggests a large vacuum leak.

 

[JedIEG]

Yep after I had that carb on, I switched back to the qft. I could get the qft to idle at a decent speed, but found there was a leak from the brake booster. I replaced the booster with a reman from rockauto and that fixed the leak. It doesn't explain the really fast speed on the 4175. I believe the throttle shafts need bushed and that may be why that carb has not run correctly.

 

[#41]

You've have had the carb apart 10 times and are describing some weird stuff where it seems to be alternately lean and rich. If I was you, I'd save my sanity and switch to another carb. Someday when you are bored and all of your classic cars are running perfect (ha!) you can go back to this carb and try and figure it out.

 

[BLK 68 R/T]

Yep after I had that carb on, I switched back to the qft. I could get the qft to idle at a decent speed, but found there was a leak from the brake booster. I replaced the booster with a reman from rockauto and that fixed the leak. It doesn't explain the really fast speed on the 4175. I believe the throttle shafts need bushed and that may be why that carb has not run correctly.

Did the reman booster come with a new check valve or did you have to reuse your old one?

 

[70chall440]

Sure seems like there are multiple things going on here. If it were me, I would either replace the carb with something that is a known entity (as in something that works), check for or better yet disconnect and plug all vacuum ports and then see if I could get it to idle, then load it up (brakes on in drive/gear) and if all that goes well, then start to reattach things. Got to find/create a baseline.

 

[RemCharger]

I would try new needle+seats.

 

[68Moparmaniac]

I agree with BLK 68 RT, sounds like you have a vacuum leak. Maybe intake manifold, carb just flat worn out ( throttle shafts ) 70 chall440 also makes a good point, gotta find that base line. Good luck

 

[BLK 68 R/T]

It sounds like op has tried 2 different carbs already if I’m understanding correctly? If so and you get varies results, it’s time to do as suggested above. Plug off all vacuum lines and see if it improves. I have heard of a cracked intake as well before. As the engine warms up it runs worse as the crack expands wider with engine heat.

 

[JedIEG]

Thanks for the feedback everyone.
So for clarity, I originally rebuilt a 4175 for this engine and never could get it to run for various reasons. I took everyone's advice about getting a 'know good' carb- the QFT 600vs slayer carb from Summit. It has also never run right. Always stinking ric, but ok enough to get it driving. As of lately, the QFT has been having the issues I mentioned above I tried switching back to the 4175, and that was a dead end. I had a suspicion that the booster was leaking and verified that was an issue. The new booster fixed that and I verified there are no other leaks. Even used a new base gasket. I have gone down the intake manifold route in the past and that was a dead end.
I have gone through and clamped off each vacuum line and nothing happens to the engine unlike when the booster was leaking. I don't think this is a vacuumed leak, otherwise it shouldn't idle as slow as it does.
Holley suggested raising the float level so it is at 3/4 or more up the sight glass, which seems odd since that is against what they always say to do in their manuals.

 

[BLK 68 R/T]

Well it sounds like you need a new set of spark plugs for sure. I would start there and then go onto checking the dizzy to make sure it is functioning properly and not advancing and getting stuck or retarding timing at idle to much due to something gummed up or loose maybe? If that does nothing, and you verify you have good blue spark at the plugs, then next thing would be another carb. in my opinion.

 

[BLK 68 R/T]

Thanks for the feedback everyone.
So for clarity, I originally rebuilt a 4175 for this engine and never could get it to run for various reasons. I took everyone's advice about getting a 'know good' carb- the QFT 600vs slayer carb from Summit. It has also never run right. Always stinking ric, but ok enough to get it driving. As of lately, the QFT has been having the issues I mentioned above I tried switching back to the 4175, and that was a dead end. I had a suspicion that the booster was leaking and verified that was an issue. The new booster fixed that and I verified there are no other leaks. Even used a new base gasket. I have gone down the intake manifold route in the past and that was a dead end.
I have gone through and clamped off each vacuum line and nothing happens to the engine unlike when the booster was leaking. I don't think this is a vacuumed leak, otherwise it shouldn't idle as slow as it does.
Holley suggested raising the float level so it is at 3/4 or more up the sight glass, which seems odd since that is against what they always say to do in their manuals.

I’m not a carb expert by far, but I recall float levels being set so fuel just starts to run out of the bowl plug or sight plug.

 

[Plymouth71]

Did you ever resolve this issue? I have a similar concern and have rebuilt my carb 3 times so far and I'm wondering if it's a dist. Issue. Vacuum advance maybe?

 

[Jonas Nordstrom]

It is prolly not this but here is a strange story:
I had a similar thing on a 70 Cuda and I was pulling my hair to try and find the faulty part.
Then i removed my newly installed brass filter...and the car worked fine again.