WIGDTW: need carb tuning advice

[shelby987]

Here are a couple pics. of what I got done this weekend, before questioning the madness of putting headers and a 4bbl carb on a tired 250, understand that this engine will be pulled in a month or two in place of a built 250 I have in the garage, and I wanted to make sure the carb and any issues of installing it were ironed out before I had to fire a brand new engine.

Here is my issue (excuse me, I am not carb skilled ). I got everything setup, followed the basic rules to the letter, purged the new fuel lines several times, installed an inline filter (which is a completely different headache issue), bought all new vaccum lines, made sure everything is sealed, etc. But when I pump the gas and turn the key the truck fires up, runs to 3,000 rpm, sits there for a second or two, then dies. I figured there must be some issue with an idle control screw so I tried to ease my foot into it just a bit to up the idle.....it again ran to 3,000 rpm and my foot was not into it that much, if I don't charge the intake (tap the gas) the truck will not start.....so where do I start to trouble shoot this issue?

If it makes a difference, the headers are not connected to exhaust, so obviously we are running low on back pressure, but I didn't figure that would be the cause of the problem.

p.s. this install was much more of a pain that I thought it would have otherwise been.....take a look at the room between the body of the carb and the pb booster.....and unless you block the end of the headers to hold them in place, they constantly fall off the block....don't even get me started with getting those bolts that serve double duty holding the intake and header flanges down
yes, those pieces of metal used to join wood braces, bought from home depot, no way of welding at my shop.....they will be trimmed and painted before being installed on the new engine.

[Palf70Step]

Float problem is my guess, but I am no carb expert either.

[JAKES 68GMC]

What kind of 4bbl is that? Kinda funny looking.

[shelby987]

its an 'economaster' no float bowls, and its rated for 450 cfm, small primaries, larger secondaries.....I guess it was holleys solution to the gas crisis.

[Karl Hoh]

I don't know anything about that carb, but it sounds like you have a vacuum leak( engine running up to 3000rpm) and a fuel suppy/ delivery problem (dying after 1-2 seconds)IMO

[shelby987]

pretty confident the fuel supply is good, this was a running/driving motor before the swap with no problems, unless I need to worry about the stock fuel pump supplying a 4bbl? (I would assume it can at least support idle).

What is the best way to track down a vaccum leak?

[motocrosschump]

squirt some carb cleaner around the base of the carb and see if it revs up. if it does then you have an air leak.

[shelby987]

I will go back and check that (will carb cleaner make it rev up, it always bogs my engine down when I use it).

If I find a leak at the base, it will probably be because of installation procedure, so can someone give me an outline as to the best way to seal a carb to the intake (I have two gaskets in this setup as I needed to use an adaptor, so I wanna make sure i get it right).
Thanks

[69_hafton]

Nice setup - I'm looking to do something similar in the near future. Sounds like your accelerator pump is working, but the carb isn't drawing (or getting) any fuel through its venturis. Most carbs I've worked on before have a small diaphragm that pumps extra fuel when you push the accelerator, which is over and above what the carb should be sucking in from the float bowls. Might have to open up the top of the carb to see if there's any fuel in the bowl. Sometimes the float valve can get stuck shut, any you'll have to unseat it manually.

Oh, but I just reread and you say no float bowls? I don't know if I can help much then. If you keep pouring fuel or spraying starting fluid in the top and it continues to run, it's not pulling fuel in like it was designed to do. (Be careful if you do pour fuel in from the top - have an extinguisher handy)

BTW, That rubber fuel supply line (along with your inline fuel filter) is kind of a fire hazard. I'd stick with the original steel line from the fuel pump to the carb. You could put an inline filter on the passenger side above the fuel pump - it'd be safer than a rubber hose and glass (?) filter directly over the exhaust manifold. Not trying to tell you how to do things, but it seems like there's been an incident or two recently with rubber fuel lines?

[shelby987]

ok, tried the carb cleaner, its kinda hard to do this (no helper) so I have to coat the carb in cleaner, then jump in the truck and try to fire it up.....well, the half hearted attempt got a 'somewhat' decent idle....so now I need to go back and pull the carb, and replace the gaskets....any advice on proper sealing of these gaskets?

[_Dick_]

look at the new gas filter is it dirty already ,other wise i am asking did you clean the tank and gas lines before installing the carb?? No not saying your stupid just looking for mistakes i have done in my past

good luck

[shelby987]

The filter still looks pretty clean (a couple of pieces of debris).

But yes, when i installed the filter I pulled the top off of the coil and turned the truck over a couple of times and purged the line all the way to the coupling that is threaded into the carb. I filled a 16oz. bottle a little more than a 3rd of the way full. I am confident that the line had no debris in it...thinking I am looking at a leak from one or both of the intake/carb gaskets.

[motocrosschump]

I put High Temp RTV Silicone on my carb gaskets.

[motocrosschump]

Also...what happens if you start the engine and give a little throttle? Does it run or does it die?

If it runs, turn up you idle screw..then start tickering with your Fuel Mix screws

[motocrosschump]

BUT, only do that after you get your leak fixed

[Psycho71]

And if all else fails, call Holley's tech line. I just put a street avenger on my truck, had a few tuning issues, called Ricky on the tech line, and he pointed me right at the solutions.
That is a wild looking carb though. I have never seen one myself.

Good luck,
Jay

[shelby987]

high temp rtv on both sides of the gasket?
It revs to 3k if I touch the gas.....I think I need to get the possible vaccum leak fixed before I can do much else......
I have already turned my idle screw all the way in, and it still dies

[shelby987]

ok, I have completely dismantled the intake, I got down to the adaptor plate I installed and saw that the 4bbl adaptor the PO installed was oozing rtv (looks like he used rtv that wasn't gasoline friendly). But now I can't find a gasket to put the 4bbl adaptor on the intake (just the square one without the ear and with the 4 holes).

Should I be looking at just making my own gasket out of some gasket material for this section? Also hoping to hear back from someone about the RTV, both sides, is it the typical install, wait 1 hour, torque down? Should I be doing this over the course of 3 hours since I have 3 gaskets stacked on each other? How long should I let everything setup before trying to run the truck?

[william 70 c/20]

i would put it all back together dry...if it's running to 3K rpm's then it's getting air from somewhere..look down the carb with the motor off to see where the butterfly's are at..they should be closed up pretty tight..try taking off the throttle linkage and starting it to make sure that's not the problem..call the tech line at holley before you smear rtv all over that new carb...

[Karl Hoh]

Originally Posted by shelby987

ok, I have completely dismantled the intake, I got down to the adaptor plate I installed and saw that the 4bbl adaptor the PO installed was oozing rtv (looks like he used rtv that wasn't gasoline friendly). But now I can't find a gasket to put the 4bbl adaptor on the intake (just the square one without the ear and with the 4 holes).

Should I be looking at just making my own gasket out of some gasket material for this section? Also hoping to hear back from someone about the RTV, both sides, is it the typical install, wait 1 hour, torque down? Should I be doing this over the course of 3 hours since I have 3 gaskets stacked on each other? How long should I let everything setup before trying to run the truck?

I had another look at this and want to know does the carb bolt to the spreadbore adapter ? If so, look at the bottom of the carb and place the gasket on it. Are there any vacuum channels peeking out past the gasket? The edges of that gasket are very narrow. That spreadbore gasket looks like it could be replaced with one for a GM Q-Jet.

[shelby987]

not sure what taking the linkage off will do, when it was all assembled there was a bit of slack in the cable linkage, and its definately resting on the idle adjustment screw.

Is dry really my best bet? I have pulled this apart and put it together twice now, I would really rather this be the last time.....will holley answer questions on a carb that's 20 years old?

[motocrosschump]

Hey do me a quick favor, see if you can turn your fuel mixture screws in, should be two of them. If you can't back them out 1 1/2 turns and start again. Kinda of sounds like your air screws are all the way in.

[shelby987]

nope, tried that, I checked to see if I could turn them in when i first had the problem, I probably only got 1-1.25 turns.....I will put it back together tonight....again, and see what I get....guess I will try dry all the way, at least that way if it doesn't work, I can pull it apart and put RTV on it.

[motocrosschump]

Always good to start dry, use RTV only if all else fails

[Bowtie67]

Before pulling it all apart also spray the carb cleaner around the intake where it connects to the head, could be an intake leak as well there, another suggestion is to make sure all surfaces are flat with a staight edge, I am also sure someone around where you live has the QJet gasket. Check intake, Carb & base plate, Aluminum Intakes over time with excessive heat will warp. Considering the Exhaust is also right there you may have some warpage and a machine shop can plane these down for you. Alot of cast Iron manifolds on 6 like these used to crack due to heat.

Couple other questions are how long has this been sitting? the cab may need an overhaul. I would never use RTV on a carb base gasket, RTV is not fuel compatible, read the directions on the RTV tube.