850 Carter Thermoquad flooding at low rpm

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RonsSwinger

RonsSwinger

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I am not a mechanic but mechanically minded and looking for an understanding and solution. The car (had it about a year) has a 440 with 460-500hp, Carter 850 carb, with a Mopar Purple cam 296/.557. Overview….. Car fires up and runs nice. I had always noticed after going for a drive it idles rough at stop signs, and couldn’t put my finger on it, but it came to a head yesterday on a long slow drive. I live in rural Nova Scotia Canada and took my wife for a nice long drive along the coast. I probably drove the coast line for 2 hours at 1500rpm 40-45mph. After the first hour I noticed the car getting rougher as we cruised ( the tack would pulse periodically). Temp is solid 180. At the end of the 2nd hour (about 90% of the cruise). I stopped at an intersection it started sputtering badly and it was all I could do to keep the car running. I pointed the car towards home (1+ hrs)….. yep, we are in **** the bed mode. the car kicked and sputtered for the first couple miles and as I picked up the pace it felt a bit better At 2200rpm (once I got it there). The last 1/2 of the drives the car started to feel better. When I got closure to home (and braver) I gave the car a couple of good pulls and every one was better that the one before and when I returned home the car was running normal. I suspect that the fuel supply is too heavy at lower rpm and flooding the car. Looking for thoughts and education. Thanks in advance RonD
 
Fuel could be heating up in there fuel lines near the engine
 
If you suspect flooding there are only a couple reasons...
  1. Fuel pressure too high
  2. Float too high
  3. Needle not sealing correctly
  4. Internal carb issues
 
Dana67Dart said:
If you suspect flooding there are only a couple reasons...
  1. Fuel pressure too high
  2. Float too high
  3. Needle not sealing correctly
  4. Internal carb issues
Click to expand...
Well aren’t you bearer of good news LMAO……. Thanks for the response and it makes sense……..I didn’t want to hear it, but it makes sense
 
All good suggestions- I would add this: make SURE your choke is opening fully as the engine warms up- if it's not, that would explain why it's loading up at low RPMs and clears up after opening the throttle up for a bit (the throttle linkage will manually open the choke as more throttle is applied).
Always check the simplest things first.
 
Professor Fate said:
All good suggestions- I would add this: make SURE your choke is opening fully as the engine warms up- if it's not, that would explain why it's loading up at low RPMs and clears up after opening the throttle up for a bit (the throttle linkage will manually open the choke as more throttle is applied).
Always check the simplest things first.
Click to expand...
No choke. I am leaning towards dirt in the needle valve. I think I found a contact who is very good with Thermoquads in my area. If this is confirmed, I will have it rebuilt so I am not screwing around with it.
 
RonsSwinger said:
No choke. I am leaning towards dirt in the needle valve. I think I found a contact who is very good with Thermoquads in my area. If this is confirmed, I will have it rebuilt so I am not screwing around with it.
Click to expand...
Add a new high quality fuel filter
 
Maybe a quick fix, purposely run it out of gas, so the floats are dropped, take the fuel line off the carb, blow compressed air thru the fitting, worked for me
I suggest a rubber tipped air nozzle to get a good seal

16967153076856097569553007629137.jpg
 
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Foam float gets soggy after a while and sinks.
A brass one might be similar if left dormant for longer periods?
 
Congrats on using the best 4bbl carb ever made.

I doubt that you have excessive fuel pressure causing the problem; the superior design of Carter/ Edel 4 bbl carbs means that they can tolerate 10 psi pressure.

As others have suggested, leaking/worn needle & seats can cause erratic running. To check: warm up engine, engine idling, look for fuel dripping from boosters. Should be dry.
I have heard of Nitrophyl floats becoming fuel logged, have never seen one & doubt that is your problem. Float should weigh 7.5-8 gms.
If the carb has a lot of mileage, possible cause of rough low speed running:
- rubber o rings leaking
- pri main jet plastic cap leaking.

O rings. I NEVER use the Carter o rings, too flimsy. Instead but some fuel resistant O rings 7/16" OD, 1/16" wall thickness.

Plastic caps. They are expoxied at the factory & can leak if treated roughly. It does NOT affect idle [ unless the lower gasket leaks ], but it can cause a rich cruise condition. The fix is to carefully scrape out the old white expoxy out of the joint & fill with new epoxy such as JB weld. Be careful not to excessive bulging of the epoxy otherwise, the lower gasket will not fit.

Set float level to 24mm.
Unlike brand H & clones, you will not need gaskets if you are careful.

That cam is going to have low vacuum. Make sure the metering rod hangar stays down at idle, in gear if auto.

Cam will require higher than stock idle speed. Too much transfer slot may be exposed at idle, adding to the rough running.
Factory initial timing [ 10-15* ] is not going to cut it, will need at least 25-35* at idle for best performance, best idle quality. See below.


First thing to do however is to check out/fix the carb as detailed above.

img307.jpg
 
Bewy said:
Congrats on using the best 4bbl carb ever made.

I doubt that you have excessive fuel pressure causing the problem; the superior design of Carter/ Edel 4 bbl carbs means that they can tolerate 10 psi pressure.

As others have suggested, leaking/worn needle & seats can cause erratic running. To check: warm up engine, engine idling, look for fuel dripping from boosters. Should be dry.
I have heard of Nitrophyl floats becoming fuel logged, have never seen one & doubt that is your problem. Float should weigh 7.5-8 gms.
If the carb has a lot of mileage, possible cause of rough low speed running:
- rubber o rings leaking
- pri main jet plastic cap leaking.

O rings. I NEVER use the Carter o rings, too flimsy. Instead but some fuel resistant O rings 7/16" OD, 1/16" wall thickness.

Plastic caps. They are expoxied at the factory & can leak if treated roughly. It does NOT affect idle [ unless the lower gasket leaks ], but it can cause a rich cruise condition. The fix is to carefully scrape out the old white expoxy out of the joint & fill with new epoxy such as JB weld. Be careful not to excessive bulging of the epoxy otherwise, the lower gasket will not fit.

Set float level to 24mm.
Unlike brand H & clones, you will not need gaskets if you are careful.

That cam is going to have low vacuum. Make sure the metering rod hangar stays down at idle, in gear if auto.

Cam will require higher than stock idle speed. Too much transfer slot may be exposed at idle, adding to the rough running.
Factory initial timing [ 10-15* ] is not going to cut it, will need at least 25-35* at idle for best performance, best idle quality. See below.


First thing to do however is to check out/fix the carb as detailed above.

View attachment 1716151264
Click to expand...
Great info. The timing is 37. What do you think the idle speed should be. If I have to pull it off (it’s looking that way) I will do a complete rebuild. I am not sure of the life of the carb.
 
So it was running fine and then it started blubbering ?
I would go with looking at basics as described here carefully,
before u spend $ having it rebuilt
 
Brooks James said:
So it was running fine and then it started blubbering ?
I would go with looking at basics as described here carefully,
before u spend $ having it rebuilt
Click to expand...
After about a hour it started getting noticeable, tach fluctuating periodically, at the 2 hr mark it almost died at a stop. Ran extremely rough, sputtering until I got it over 2000 rpm for a period of time and started running better, a couple of hard acceleration’s it started running fine again.
 
Tach fluctuating, possible electrical issues, check all grounds and bulkhead connector
 
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Bewy said:
That cam is going to have low vacuum. Make sure the metering rod hangar stays down at idle, in gear if auto.
Click to expand...
This, was a common issue back in the day.... Low vacuum doesn't keep the metering rods down, the plugs get loaded up & soon the engine barely runs... Run the engine a little higher RPMs & the vacuum signal improves pulling the metering rods down & the plugs recover...

Pull the aircleaner & with the engine idling watch the metering rod yoke, it should stay fully down, if it's bouncing either increase the vacuum signal or reduce the spring tension on the yoke...
 
I would start with the float levels and check the needles while your in there depending on the model most thermoquads float levels are set at 29/32 in the closed position. Some books say 31/32 some models differ but 29-31/32 is a pretty safe bet.
 
That's a ton of cam for a TQ. They normally do not do well with low vacuum signal at idle.
 
dartfreak75 said:
Probably not factory im sure there is something out there to make it work. I'd sure be trying everything lol
Click to expand...
I've tried removing the spring altogether before and it still didn't work. This was with a smaller cam than what the OP has.
 
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