All kinds of stuff could be wrong here. To keep it simple, see if you can swap out another carb, (any buddies around)? 600cfm would be fine. If it runs well, it's carb problems which can be addressed. If it's still acting the same, you have other issues..........fuel flow, ignition, vac leaks, etc.

Gotta run but back later. Keep us posted. BTW have you talked to the guy who did the work???? Can't get it to idle but A/F ratio is OK?????

 

http://www.youtube.com/watch?v=41rqz6gX_6I

Here is a video of how it shuts down. Usually it sounds like this, sometimes it shuts down normally, sometimes there is more knocking.


Back when it was dynoed it had 200 miles on it and yeah the idle was showing as 12:1 when it idled well, when it was brought up into the 13:1 range it wouldn't idle at all.

I don't know if its lean NOW but it completely refuses to idle. The carb is a mess too from all its problems.

I'm looking for a used carb right now, I've got plans to change the heads to some 195's or so to match the rest of the engine for ~400BHP before a 100-125HP nitrous shot, of course the engine will be properly tuned before I even think of installing my nitrous system. Will a 600CFM carb be enough to run a 350ci engine @ 6000RPM with a HP peak of 520BHP@4500RPM? Of course venturi size is NOT the issue I'm having though, I want to get this carb running properly, its only got 2300 miles on it and the thing is black after being "tuned" by this dyno shop. I forget the jet sizes but the primary with power valve is about 7 smaller than the secondaries with no power valve and both were tuned based on the A/F map from idle to 6000 RPM. I've got a copy of the dyno runs somewhere...only gained 20HP after tuning for a BHP of ~315 but drivability has suffered since then.

 

It sounds to me like you have a cracked fuel hose back by the tank, or a sunk float.

 

It sounds to me like you have a cracked fuel hose back by the tank, or a sunk float.

By this you mean:

A cracked fuel hose would cause my engine starting problem where it dies shortly after starting?

A sunk float would cause fuel to escape from the vent?

Oh, fuel line is a 1/2 aluminum line from tank straight to pump and its connection at the tank seeps a little on a nearly full tank, the tank is a plastic racing tank with bottom feed.

 

A piece of crap in the needle and seat, or a sinking float can cause major idle and running problems. They do not always pump fuel out of the vent pipe. It may not show up at idle if the secondary float is sunk(only after running under enough load to open the secondary's.

 

I watched your video. It sounds to me like you have a lack of fuel. You stated you have a leak at the plastic fuel tank, it could be sucking air there starving the engine for fuel.

That's My opinion.

 

Today I decided to take the carb apart and clean it. As I disassembled it, I tested everything. Besides being filthy from backfires, I only found one thing wrong: On the secondary assembly, there is no baffle installed by the vent but I'm not sure if I need it.

I'm currently using 65 primary jets with a 6.5 power valve and 73 secondary jets with no power valve.

I recently moved from a 1575 ft elevation to a 75 foot elevation, if the car was running slightly rich across the board, would the elevation change lean it out too much? It hasn't pinged with detonation at all.

 

What is the timing curve on this setup? It sounds to me like you need to change the plugs and start tuning to figure out what the problem is. Are the plug wires OK? The carb should not be spewing fuel out of the vent-this is a float adjustment issue, or sticking float, dirty/bad needle and seat, or to much fuel pressure. Where are your A/F mixture screws set at? How far open is the throttle blade set at? I've also seen bad idle problems when an ignition module was going bad so check that along with your cap and rotor-also check the advance weights to make sure they aren't stuck at full advance. A vacume leak can also cause your idle problem.

 

Trying a different carb sounds like a good thing to do. If you do this and still have problems, it'll save you a lot of time messing with carb tuning.

I recently had a similar problem and after weeks screwing around with the carb, I finally took apart the regulator to find metal from the braided hoses stuck in a check ball that was flooding the carb at idle. If I kept the idle high enough, the car would drive, but if you slowed to an idle long enough, fuel would fill the bowls and come out the vent shutting the car down with an ocassional backfire.

You might be able to see if it's a regualtor problem if you had a good fuel pressure gauge (the cheap ones can't be trusted).

 

carb problems

first of all what is your vacuum reading? you stated that your running a 6.5 power valve thats a power valve for stock or less than stock camshafts. im running a similar carb that your running as well. and when i first put it on i didnt know that it had a 6.5 power valve in the front metering block i had major fuel issues with this carb until i hooked a vacuum gauge on it and now i run a 3.5 power valve with no flooding or other fuel issues. my carb is a modified 650 built with the secondary metering plate,double pumper fuel bowls,nytrophyl floats,with jet extensions,size 64 main jets,rear jets are 72. and a moroso front fuel bowl extension.

 

No idle

Opinions are like noses, everybody has one! Here is mine. First things first, how is the ignition timing? Secondly, start the engine, try and hold a constant idle and then lightly spray WD40, starting fluid, carb spray along the area where the intake manifold meets the heads. see if the engine rpm increases, smooths out or any other changes in the engine running. If so, vacuum leaks.

Another simple test: install a vacuum gauge into a manifold tap. Start engine and observe the gauge. If it is wild and jumping up and down, you have valve train problems. If the reading is very low, less then 15 in. the timing is wrong and/or fuel mixture problems. Does this motor have adjustable rockers? If so another probable place for troubles.

Carb. problems, Hummm! Probably a likely suspect. I am no Holley fan! 750 cfm Holley carbs are know for crazy major malfunctions and should only be adjusted and rebuilt by Holley guys. Weekend bench top mechanics like myself have learned long ago to replace them with something that we understand and then can tune ourselves and usually don't lose a damn thing because it don't say Holley on the box. (side note: I have been repairing and building and selling cars for at least 25 years---- RULE OF THUMB-- if it says Holley, replace it! Hee- Hee!) Unless you are drag racing this car, I would wonder why you would need to run a 750 cfm carb? Most small block engines could never handle that rate of fuel with any benefit, they usually always run in a rich condition. I think you would see better performance, better gas mileage, smoother running and less trouble from a 615 or 650 cfm Edelbrock (Weber) 4 barrel. Also much cheaper and are easy to tune. (use your vac. gauge to tune it!)

Back fire in carb and over run after shutting down--- This is a serious problem and can cause serious engine harm.. Frankly I would not run the engine again until I was pretty sure this over run was addressed. If you continue to run it this way you will at least need to replace the timing chain soon. Think about the stress that has been shocked into that from this condition.

Conclusion: All things considered here if you are sure the engine is mechanically correct and the timing is close, here is the solution (me thinks, maybe) , what I would do! Remove and replace the intake gaskets and when you re-install the gaskets cover both sides with cup grease. Use no gasket maker, rtv, silicone etc. The grease allows the gaskets and the heads and the intake to move around as you start and warm up the engine on restart. ONCE THE ENGINE GETS HOT THE GREASE WILL EVENTUALLY COOK OFF. Then torque the manifold. Run engine until hot once again and torque the manifold again. Now you know that it is right and have eliminated that possibility for a problem.

Again, if it was me: I would install a different carb and test run. If everything mechanically is right, you should now be able to idle the engine and adjust the air-fuel mixture and it should be right. If this does not solve the problem then you must suspect that the valve timing is not correct in relation to the crankshaft timing..... OH BOY! But that is another post. Hope some of this helps.
I operate under the KISS method. KEEP IT SIMPLE STUPID! Good luck, Big Don

 

Ok let get back to the basics, you said there is fuel leaking out the top of the carburetor. Until you fix this anything else is a waste of time. There are a few things that can cause that.

1 Misadjustment
2 Crap in the needle and seat
3 Bad float
4 Too much fuel presure

All of these are easy to fix

If it has back fired that much, your power valve may be junk as well and adding even more raw fuel at idle. They are cheap and I would change it just for general principals.

 

Most likely the reason it is running on is also due to the fact it wond idle under 2000 RPM. I personally think all your problems point to the carb. Fix the fuel dumping out the top issue and ibet it will run fine.

 

Thanks for all the replies. Testing with a vacuum gauge tomorrow. Future upgrades will give me some power at 7500rpm so the 750cfm carb I can live with. I need this question answered though: On my secondary metering plate, do I need a vent baffle like the primary vent has? Holley says that the 110gph pump needs no fuel regulator but their next one up the 130gph does. I'm going to recheck float levels, they were perfect on the dyno and shouldn't change but I'm checking em anyways, also, both idle needles / passageways were clear, main metering valves both perfect, floats and pivot gear on em are perfect too. Power valve is undamaged as well but yeah if my vacuum is below 13in.Hg, it could be making it flood. One thing done wrong on the dyno was the idle circuit, the guy was trying to make it run at 14.7:1 A/F ratio but gave up and set it to "what ran the smoothest", which could be part of the problem. (I didnt know at the time that idle is set by best vacuum) I'm definately thinking of suspiciously low vacuum which could be the root of a whack load of my problems, all my hoses and connections are FINE though...I hope I don't have to deal with a leaky manifold gasket. I'm also going to install a shift kit and torque converter, in gear at lights is annoying even when the car is running well.

 

Low vacuum will not make fuel poor out the top of the carb, fuel level is controled by the float and needle valve, period. I think you have multiple issues to be honest.
As far as a baffle, its not going to matter one way or the other if you are just sitting still.
I have seen some carbs with them and some without.
On the fuel pump, you need to get a gauge and check it, I dont care what Holley says, it or still could be pumping too much or not enough. Until you put a gauge on it you wont know for sure.

You have some solid symptoms, try to solve them first.

Chet

 

UPDATE: I checked my ignition system and spark plugs, all good there, initial advance sitting at 12 degrees BTDC, some dust in the distributor, solid shaft. I also decided to check my rocker arm lash, found the remains of a broken guide seal in each cylinder head which explains why I was getting some extra crankcase blowby. I'm not sure what would cause the seals to break, they are new and I'm still running stock valvetrain except for valvesprings to match the cam. Whats worse, as I was on the #2 intake adjustment, I noticed there was no valve and the rockerarm was sitting on the retainer with a nice impression of the rocker arm on the top of it, the valve was pressed into the retainer about 3/8", valve locks still intact but worn out. I think I'm lucky on this one, the retainer held the locks enough that the valve never went past its peak height of .462 and my valves are non-interference. I'm pretty sure that explains the extreme backfiring and fuel squirting out teh carb the wrong way. I'm sure if I put my carb back on the engine and saw the vacuum gauge reading, it would be going from some vacuum to zero everytime #2 was on its compression stroke, of course this no doubt was activating my power valve at the same time adding fuel to the fire so to speak.

I think I'm going to upgrade to some 10 degree retainers and while I'm at it, new seals, pushrods and rockerarms as I'll be putting a pair of AFR 195's on this summer.

 

You got LUCKY! :thumbup: