400 Big Block running issues

-
cletus said:
If you are able to repeat the problem with full throttle runs I would be willing to bet one of two scenarios. I'm not really familiar with the Holley carbs, someone help me out here, I think it may be possible that at WOT you are dumping enough fuel on the plugs that they are getting fouled, making it run like **** and tough to start back up, after you get it started, it could be clearing them after some running.

I bet it's either that or it's gobbling all the fuel out of the float bowls and starving at the top of 3rd gear, then the pump has to catch up.

Once you KNOW your carb and timing are correct, if it still does this same thing, I bet the pressure is dropping to the point where the float bowls are out of fuel at the top of 3rd gear. The first things I would look at would be to make sure the floats are set as high as you can get away with on that carb, pull the sending unit out of the fuel tank to check the mesh sock to be sure it's not gunked up. If it looks good you will want to get a new gasket for it anyway, the gasket likely came out in pieces. Make sure the fuel line hasn't been crushed somewhere on the bottom of the car. The tank should be vented to be sure when the fuel level is dropping it doesn't try to stop the flow of fuel with vacuum - picture dunking a glass in a full sink of water, turning it upside down, and pulling the bottom of the glass above the level of water in the sink, the water stays in the glass until the top of the glass clears the level of water in the sink. I know the tank is different from '68 to '73, but they may have had a similar setup, mine had a 2nd line from the tank that went up to the engine bay (this part I know for sure), was somehow hooked into a charcoal canister (I think, it's been years since I dealt with it) that I think was tied into the vacuum system on the engine. I removed all that and added a breather filter to where the 2nd line tied into the tank.

Those are the possible scenarios I would try to eliminate first, hopefully this helps.

Good luck! Please keep us posted on what you find.

- Steve
Click to expand...

Thats very interesting and has merit to be correct. Interestingly others have said that fuel pressure is too high and this is flooding the car.

The booster is dripping which I understand is either the needle not seating correctly in the mixture screw, worn carb or too high fuel pressure.

I've bought a regulator now and will be fitting this (though with a mechanical pump?! is it required really?!?!)

Bloody car.
 
Foreign liquid, non-missable, in tank? Pump sucking air? Fuel slosh in front bowl? Low fuel level in rear bowl? Those are my guesses. And why the high idle? Prove TDC on the balancer and Put a timing light on it.You have got to get that sorted out first. A "highish lift" cam doesnt usually idle at 15 to 16 inches of vacuum. More like 12 or less.I wonder if your PV is giving you grief.
 
With a stock mechanical pump you shouldn't need a regulator.

Definately get the timing sorted out.

Again, I don't know jack about the Holley carbs, if it was an Edelbrock I could probably help, just about anyone else on here would be better suited to help with that.

This doesn't sound like an ignition problem to me, swapping out the box might help, but I don't think it is the correct fix for the problem.

If you have fuel going somewhere it shouldn't be at the carb, fix that problem first, get your timing sorted out, then run it and see what it does.
 
Timing is all sorted now, running 17 degrees advance on initial and its very happy. idling.

The problem appears to be the secondary float.

It get stuck at the bottom of the carb, though I (and two other people that know holley carbs) can see no reason for this.

Sometimes its sticks, lets loads of fuel in and it comes out of the boosters. Take the sight out and you can see fuel flooding out.

With it off and checking it dry everything appears to work. You add fuel and it all goes tits up. The primary is fine.

Just going to buy a new carb and lob this one as far over the fence next door as I can. Shot put style.

I am going to buy a new carb.

Can you advise if a 750 DP would suite my car or perhaps I'd be better with a vac secondary?

The car is used 99% of the time on the road:


The engine is a 400 big block, low compression so I understand. The cam is a COMP Cams Magnum Hydraulic Camshafts 21-242-4. (http://www.summitracing.com/int/parts/cca-21-242-4/overview/make/dodge)
The Converter is a 2800-3000 rpm stall street/strip JEGS item
The diff is a 4.1


I may change the diff at a later date to a more lazy item.
The engine will stay the same as will the converter.

The car is on the road 99% of its life.

Does this help me at all decide on a DP or a Vac sec?

Thanks
 
There should be a little tab on the float that limits how low it can go and stop it from sticking. There is nothing on a Holley that can't be fixed. Does not sound like the person that rebuilt it knows as much as he thinks. If it still does not fix it replace that float bow.
Make sure your Ign. module is grounded. I had a mounting screw strip out and caused similar problems as yours.
 
Ok, there's a new bit of info I didn't know - that the Holleys have dedicated primary and secondary floats - I guess it would make sense with one on the front and one on the rear, I had never thought about it.

You could get lucky, maybe that's the only problem you will have to chase down, it certainly would explain the symptoms you have described.
 
Jo Diesel said:
There should be a little tab on the float that limits how low it can go and stop it from sticking. There is nothing on a Holley that can't be fixed. Does not sound like the person that rebuilt it knows as much as he thinks. If it still does not fix it replace that float bow.
Make sure your Ign. module is grounded. I had a mounting screw strip out and caused similar problems as yours.
Click to expand...

ignition module is grounded - two strong bolts to the shell, its not moving at all.

Replacement float bowls are £50+ over here, I'm looking at getting a new carb now I think.

I've already rebuilt this one god knows what else is worn on it.
 
Float bowls do not wear out. They get corroded or damaged. If float is sticking the guy that rebuilt it did something wrong, or he did not set the low float limit which is done by bending the tab on the float. Could also be wrong floats or if not replaced you could have a float that is sinking. Brass and phenolic floats both can sink.
 
Jo Diesel said:
Float bowls do not wear out. They get corroded or damaged. If float is sticking the guy that rebuilt it did something wrong, or he did not set the low float limit which is done by bending the tab on the float. Could also be wrong floats or if not replaced you could have a float that is sinking. Brass and phenolic floats both can sink.
Click to expand...

Thank you for your advise.

I've now fixed the problem.

I bought a 750 vac advanced carb, put it on the car and it ran okay but backfired when you booted the car.

So I decided to take it back off again and pinch the floats from this carb (neoprene) and put them in the 700 DP. Also used the needle valves from the 705VS.

This cured the problem, along with advancing the timing (by ear).

My timing light shows it is now running 45 * of advance at idle, so the mark must be wrong on the car because the car idles, pulls and revs well now.

No flooding or fireballs, quite happy now :)

Can anyone tell me where the timing mark on a 400 bb should be? I found TDC on Compression, marked it against the stock 'ruler' near the damper but the timing mark seems not to relate to what the engine is doing...

Damper slipping?

J
 
Hi there,

bit of an update on this.
The problem is mostly sorted, it was running a bit too advanced.
Cheers

Jon
 
Things I would check include all plug wires and discard any that show open circuit if they are resistor core. Rotor can arc through to shaft and cap can carbon track to ground or crossfire. With the problem under load ignition seems logical. If it is fuel starvation you will see it on your plugs at the end of a run. Swapping to a known good coil may also be helpful. If it looks to be a little rich I would check hood clearance or air filter volume.
 
You must log in or register to reply here.
-

Back
Top