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Old 06-03-2005, 05:00 AM
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carb discussion...

Ok guys this one got me stumped....

I am no first timer with a holley carb but i have never run across this one before....

It's a 750 dp holley. Engine is a 12.3:1 406,bullet solid roller 262/268 @.050 106 lsa, vic jr intake, msd ignition.

The carb was on another engine and worked fine. I had built it for my 66 chevelle, took the choke horn off, milled all the gasket surfaces for a good seal etc,,,,, i sold it to a customer and It had been sitting for about a year before i ran it again. The engine will start and run fine. Perfect power. no miss. pulls strong. When i idle it down to around 1000 rpm it will stay there for about 1 minute (give or take, i really did not time it) then it will start to die, (sounds like a engine with a vacuum leak) If i run the idle up to 1250 rpms or so it will just stay there for ever... So i took the carb off and went through it. I checked all the passages, replaced all the gaskets etc... Same thing.

So i am thinking it has a fuel leak some place because it does not seam as bad when cold????? I did not get a chance to take if off again. but i am trying to plan for what to look for.

I have all the transfer slots set to about .020. I make my own secondary adjustment screws so i can tweek them from the top of the carb instead of underneath like a normal holley. There is no fuel dripping from the boosters untill pull over ( 2500 rpm or so) the floats are set correct maybe on the short side but i know that will not cause this problem.... Both bowl vents are open. I have plenty of timing on it ( 25 initial) The primary throttle plates have a 1/8" hole in them...

The only other thing i thought of is it's only a 2 corner idle carb. I am thinking about changing it to a 4 corner. I really do not think it will help because it should either idle or not..... This deal where is sits there and idle fine for a period of time and then slowly starts to die got me.... the other thing when you catch it to keep it from stopping it kind of acts lazy like it's flodded????

Let the banter begin!!!!!!

Thanks guys, Keith

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Old 06-03-2005, 05:28 AM
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Quote:
milled all the gasket surfaces for a good seal etc

Good ole porosity in the metering block along with other area`s in a Holley will do it every time... One of the main reasons Barry Grant went with billet parts...
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Old 06-03-2005, 06:23 AM
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Keith,
I have a couple of thoughts. When the carb sat for that year how clean of gas was it? Could fuel have gelled up in the air bleeds? What sort of vacuum at idle does the motor hold? Could the power valve be borderline opening or bad entirely? Does the distributor have a vacuum advance pot that's allowing the timing to retard at too low an idle/vacuum?

Bill
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Old 06-03-2005, 06:49 AM
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Carb

Quote:
Originally Posted by engineczar
Keith,
I have a couple of thoughts. When the carb sat for that year how clean of gas was it? Could fuel have gelled up in the air bleeds? What sort of vacuum at idle does the motor hold? Could the power valve be borderline opening or bad entirely? Does the distributor have a vacuum advance pot that's allowing the timing to retard at too low an idle/vacuum?

Bill
I did make sure all the air bleeds were clean. I did not check the vacuum, but it's on the top of the list when i get it back ( it's off getting inspected right now) It's a 65 power valve, I did look at the boosters to see if they were dripping and since the pv feeds the booster circut i think it would have showed up there????I did think that the pv diaphram could be bad ( rotted from sitting)leaking fuel into the main body side of the carb????? ( i had a cap for my vacuum pump that i check them with but the munchkins must have gotten it because i can't find it???? ( how does that stuff get lost)The distributor is mechanical only no vaccum advance on it. I was thinking about looking at it again tho because it sat in the back of his car over the winter months out side and got damp. The inside of the distributor was rusted. I stripped it down and glass beaded the reluctor wheel and re-blued it. The pick up also had surface rust on it that needed cleaned... Maybe there is something there to look into?????? I am not positive but i always went on the theory that they either worked or didn't???

Keith
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Old 06-03-2005, 07:19 AM
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You may find you need to open the idle air bleeds. Milling the choke horn changes the air pressure right above the bleeds. Whether the air tries to rush right over the bleed(Low pressure, rich mixture) or stacks up above it (high pressure, lean mix) I can't say without seeing it. I'm guessing you have a over rich condition fouling the plugs at low speeds. The higher idle speed might be enough to generate enough heat at the plug to keep them clean.
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Old 06-03-2005, 07:26 AM
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Check the advance weights for stickiness too. They may be gummed up to the point when you set the timing, It slowly loses advance untill the engine stalls. I should'nt think it would need 25 degree's of advance. 20 Would be my start. But most of what I run and build is locked. That by itself is a indication something is wrong.

BTW, why idle this engine at 1000? Seems pretty low to me.

PS. The 2975 work's better with that combo. OR at least a two inch spacer.
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Old 06-03-2005, 07:30 AM
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Tangent vector one niner niner.

I'm going to take a shot in the dark and say the springs in the dizzy are too soft.

====================

I bet they spin out and advance the timing to 25* but when it idles and the revs slow just a bit it looses a bit of timing. As time goes by this rate of increase progresses until the timing falls off, engine tries to die, on rev it acts slugish due to slow timing.
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Old 06-03-2005, 07:34 AM
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Sounds like the atomized fuel is separating from the air below 1250. If it is, you should be able to let er die at an idle and then quickly inspect the floor of the plenum for evidence of fuel puddling.

Iam sure that you've already checked for cracks in the base plate and main body but I wonder about the intake.

How far open are the secondary throttle plates before and after the engine dies? Do they have a slight bind in them after the motor is hot and are free while cold?
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Old 06-03-2005, 07:40 AM
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carb

The distributor only has 12 degrees of advance in it. 25 initial and 37 total. The engine will run at 10 degrees initial. no problem. The only reason it's not locked is to aid in the starting. The timing is rock solid. it not retarding or advancing past where it's set. I allready double checked it.... I know the weights are ok because i had it all apart to clean it and i ran it on my distributor machine and it's all functional.......

I was thinking about the air bleeds but i wonder why it worked fine on the previous engine ( a 12.5:1 355) and now it don't??? I can't believe that the extra 50 cubic inches would make a big difference at idle????

Keith
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Old 06-03-2005, 07:48 AM
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carb

Quote:
Originally Posted by 56Maynard
Sounds like the atomized fuel is separating from the air below 1250. If it is, you should be able to let er die at an idle and then quickly inspect the floor of the plenum for evidence of fuel puddling.

Iam sure that you've already checked for cracks in the base plate and main body but I wonder about the intake.

How far open are the secondary throttle plates before and after the engine dies? Do they have a slight bind in them after the motor is hot and are free while cold?

Good idea, I'll check for the wet floor when it comes back, sounds like a good place to start. I have the gut feeling that it's fat. The secondarys are free no binding. both pri and sec are set at the correct opening for the transfer slot..

Keith
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Old 06-03-2005, 07:51 AM
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What head and spark plug?
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Old 06-03-2005, 07:56 AM
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carb

Iron 200cc pro action,
ar135 autolites

Keith
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Old 06-03-2005, 08:42 AM
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Can you monitor fuel pressure? Maybe your pump doesn't do so well at idle?

Do the plugs show rich or lean?

I think I've run out of ideas.
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Old 06-03-2005, 09:00 AM
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Right, Keith knows what he's talking about and so do we.... Nothing left but to try a different carb on that engine or that carb on a different engine or both!

You are sure there isn't a speck of grindings in one of the air bleeds. Any chance the rockers are too tight?
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Old 06-03-2005, 11:29 AM
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I had a similar problem a few years ago. BBC 12:1 comp, 256/264 @.050 108lsa Dart intake, 850 dp,MSD ing. After talking with the each rep. Decided it was the carb, so talking with a B&G tech and checking the transfer slots, 1/8"holes, power valve, mixture screws. Found that my idle adjust screw was turned into far (just to get to hold a idle which was 1200rpm)so it started working off the main system. So the tech sent out idle air bleed restrictors to help get it to work off the idle circut and WOW! I had it idling at 850rpm! I hope this helps? Maybe BG tech could add something.
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