Carb disassembled, cleaned, still runs poor please help!

[Black70c10]

Okay guys, I'm beyond perplexed, any insight is appreciated. So out of no where one day, my truck had difficulty idling, used to idle fine around 500-600 rpm. Then out of the blue, it would not hold idle until around 1000 rpm... It would jump around and hesitate on low end light acceleration..driving it was fine but as soon as I coasted to a stop or let off the gas it would almost die and I'd lose power breaks.

So while I had the truck in the shop getting some body work done, I asked the mechanic to take a look at the engine issues figuring it was a vacuum leak or something (I couldn't find it though). He ends up saying he couldn't find the leak and on the way home I have the hardest time keeping it running.

So, I spent the day yesterday taking the carb apart and cleaning it.. What made me nervous is the fact that overall it was really clean- clean gaskets, accelerator pump looked good, all jets were clear... So I throw it back on the truck and sure enough, same story.. It will idle at 500-600 now but wants to die as i let off the gas and has a nice hesitation off the line!

The other weird thing is that the idle mixture screw has little to no effect on the rpm until you turn it one or two whole revolutions- this is odd, normally quarter to half turns have big effects...

Please help!

[Black70c10]

Edelbrock power series 600 cfm carb by the way.. Only a little sediment in the bowls, otherwise everything looked really good.

[turp mcspray]

Had similar issues one time, that turned out to be my distributor. The mechanical advance had seized up causing idle problems. Often times ignition problems get mistaken for carb problems. May not be yours, but it is something to check out.

[geezer#99]

X2.
Check the distributor hasn't moved.
Even slightly loose and the cw rotation of the rotor and shaft will slowly move the timing to become retarded.

[swamp rat]

On the off chance take out one of the air fuel screws and grab a can of carb cleaner with the red plastic nozzel, shove it in the screw hole till it bottoms out and while holding it with your other hand give it a blast, then repeat in the other air fuel screw port. Be sure to protect your face and eyes from possible splash back. My idle circuits got plugged just from bad alcohol laden gas jelling up, i did that and could feel the pressure release as the idle circuit cleared out.

Do you by chance have an aluminum intake? did you use the correct gaskets? if you use the wrong gaskets you can create a pretty bad vacuum leak that will also give symptoms like you described, try torquing the intake bolts to spec and see if that helps.

[kxmotox247]

Do you have a fuel filter that is plugged up?

[Black70c10]

Thanks for all the input guys.

@turp- doesn't the mechanical advance only come into play at higher rpm? I do have an MSD w/mechanical advance. When you had these issues, were you experiencing issues with your idle speed screw as well?

@ Geezer- Unfortunately, the timing has not moved, I confirmed its still in the same place.

@Kxmotox- I also thought it was a fuel filter, but that is clear as well.

@Swamp rat- When I disassembled the carb, I actually removed each idle screw and did exactly as you said, blasted each hole with cleaner.As for the aluminum intake, I do have a Weiand aluminum intake, i sprayed all over the thing with cleaner to see if I could identify it.. May try one more time, but so far both myself and my hot rod mechanic could not find it...

I guess you guys can see my confusion!

[Black70c10]

Also, just to clarify on the last sentence of my original post- I meant the idle speed screw not the idle mixture. I would turn the thing one or two whole turns and it had very little effect if any... What could this be telling me?

[CC69Rat]

Pull the oil dipstick.. with your thumb / forefinger wipe the oil off the stick and smell of it. Does it have a smell like gasoline? .. it will be noticeable over just the normal oil smell.

Thinking diaphragm in the fuel pump.

[Black70c10]

I'll check the dipstick to see what that tells me, thanks for the suggestion.

On a side note, on the top left of the edelbrock 1406 (facing the engine), where the fuel enters, there is a small mesh style screen as well as another one on the right side. What exactly are these filters? I was incorrectly assuming these were fuel filters, but it actually appears I have no fuel filter down stream of the mechanical fuel pump and upstream of the carb inlet...

If this is part of the problem, why does the truck run fine at hard acceleration and WOT but have trouble holding idle and want to die as soon as I let off the gas?

[geezer#99]

You got a big vac leak. Plug everything that's hooked to the carb and see if that changes it.
Could be blown vac pot on dizzy or check valve on brake booster or a split hose.

[davepl]

Before taking the carb off as I describe later, do the idle MIXTURE screws seem to have no effect? If they're ok, ignore my post. But if that's a symptom, carry on.

From the description, it sounds like you have to open the throttle blades so much to get it to idle that you're probably uncovering the transfer port (a small slot or hole that is below the blades at idle but at or above once the blades tip in).

Since the transfer slot is an integral part of the idle circuit, you have zero chance of getting a low stable idle if this is the case.

If it were me, here's what I'd do (after checking the distributor stuff mentioned above).

Take the carb off. Check the float level. Check it again. Now go get the actual tuneup specs for your year, make, model, and transmission, and check it again. Why it matter so much I don't know, but it can.

Next, with the carb upside down, close the throttle blades (ie: reduce the idle speed by backing out its screw) until the transfer slot is covered. There's a spec somewhere for this, but I think you want it almost totally covered if not totally. I'm not an Edelbrock or AVS man unfortunately so don't know offhand.

Now, when you go back to try to get it to run, your challenge is going to be to do it without increasing the idle speed. Because remember, doing that is going to uncover the transfer circuit, and if you have to do that, you've already lost the battle.

So, you have ignition advance, mixture, and choke to play with. If you can get it fully warmed up (like thermostat open plus 5 minutes) then you can leave the choke for later and just work on the hot run before the cold start.

If you get it to where it's only bad when cold, then you need to adjust the choke and choke pulloff. That's a whole other story, so I'll pause here.

[Black70c10]

That's good info, thanks for the help. The idle mixture screws have a small change to the engine vaccum. I was able to adjust them and increase the vacuum 2-3 inches, it's mainly the idle speed screw that's not doing much.. I'm still convinced its a vacuum leak...

I'll keep your post in mind if I have to end up taking the carb apart again.thanks for the help

[Black70c10]

I started the car up and let it run. I then placed my hands over the top of the carb almost blocking all the air. The idle immediately increased in speed and remained that way until I removed my hands..

Would this support the vaccum leak suspicion? The engine is getting air from somewhere and now the mixture has become enriched and hence rpm increase?

[litew8]

No expert, but you essentially manually choked it. I could be wrong but doesn't sound like it supports the vac leak theory.

[geezer#99]

Originally Posted by Black70c10

I started the car up and let it run. I then placed my hands over the top of the carb almost blocking all the air. The idle immediately increased in speed and remained that way until I removed my hands..

Would this support the vaccum leak suspicion? The engine is getting air from somewhere and now the mixture has become enriched and hence rpm increase?

That's what it does. With a big vac leak you run lean. Too much air.
Blocking the air into the carb gets the mixture back to normal and the idle stabilizes.
I'm betting it's the check valve on the booster.

[swamp rat]

Per your reply, the possible leak could also be on the underside of the intake you won't find the leak by spraying on the top. If you Used a gasket not designed for aluminum intakes, An example would be Permatorque intake gaskets, if you go to the Edelbrock site they give recommended intake gasket numbers according to runner size and application, even tho have a Weiand I'd follow the part numbers listed.

[litew8]

Could the choke be stuck/not working?

[70SBC10BOB]

You said you lost power brakes when the truck died. You should have at least a couple of strokes after the engine dies if everything is ok.
Try testing the booster. Start the truck and let it run a few seconds. Turn it off. Hit the brakes. You should still have power assist for a couple of pumps. It not, check the check valve. It should hold vacuum. If check valve is ok, listen to the booster as soon as you turn off the truck. If it is leaking you should hear it.
Good Luck

[swamp rat]

$#@! I missed the brakes part, you can pull the check valve out of the booster and make sure its sealing ok (one direction only) and you can replace the rubber grommet for the check valve, i think the grommet is about $6 at Napa, cant remember the valve price but it wasn't too bad.

A quick way to test is to unhook the hose at the booster, inspect it for any cracks and then plug it with something (your finger may not be to pleasant) and notice any change in how its idling, and don't forget to check where it goes into the lower part of the carb or manifold depending on how you have it plumbed, but remember it needs to be manifold vacuum, on the Edelbrock 600 cfm carb the port on the lower back of the carb is manifold vacuum.

[litew8]

You guys obviously have experience with brake booster.

[chuckmc8]

Sounds like the guys may be on to it with the booster- but just in case- Establish your fuel path from the tank forward. Pull your sending unit and check the strainer on the sender pickup and that the pickup tube is clear. Then, disconnect the fuel hose at the pump and force air thru....you get the pic. Make sure it is clear path to the carb. I see lots of strainers completely stopped up . HTH

[turp mcspray]

My distributor was a million mile HEI. The advance was stuck advanced. By removing the cap and rotor, you can see if the advance stuff is moving freely. Sounds like you are getting it figured out.

[Black70c10]

Really appreciate all the input. I took the check valve off and it appears to be just fine, only allows air out (towards carb). I will have to wait until tomorrow to start it up and test it with the line plugged (neighbors complain of noise :/ )
However, based on how it was driving, this theory seems to make sense, although nothing appears cracked and looks like the line is connected up pretty well and snug to the booster and intake manifold inlet.

I am a little confused about the comment on the intake manifold gasket. If there there is no noticeable leak around the exterior of the manifold, why would this create excess vaccum? If the seal isn't compromised around the intake, I'm confused as to how this is an issue for me.

Same with the distributor, I'm not too sure how to determine if the mechanical advance is free or not..I take the cap off and remove the rotor? What am I looking for beyond this?

Sorry some of this stuff is new to me.. Thanks for the help

[geezer#99]

Sounds like check valve is in the right way. Do you have a solid line (tubing) or rubber line to the booster. Rubber lines can squeeze shut.
If the intake gasket is blown out on the underside of the intake it'll create a big vacuum leak. Sometimes you can pop a breather off the valve cover and hear a vac leak inside the valve cover.
To check the dizzy weights, pop the cap off and twist the rotor. You should hear and feel the weights move outward and the springs will pop them back.