Engine wont idle?

[Andy Gargagliano]

Iv been driving my 72 Monte all summer and has been running great.It a mild built 350 that was rebuilt last winter.Everything is new.Carb is a rebuilt Holly 650 double pumper/mec secondaries no choke.I drove the car today and it would not idle.Its been cold here in Michigan so I let the engine warm up to full operating temp. before i drove it.Had to Feather gas pedal to keep it running but would not idle.Figured if i drove a little bit it would be ok.Ran great when driving no hesitation during acceleration.Brake torqued it and broke the tires loose immediatly.When i come to a stop it wont idle?In park it will idle for about 30 seconds and rpm will start dropping till it stalls.This process takes about 30 seconds.Checked all vacume connections and all are good.Carb is tight on the intake manifold.Ran great all summer i did not change any adjustments.Maybe somthing stuck in the carb?Thanks for any help.

[Sam (Bones)]

open your idle jets another 1/8th turn and see if it gets better, remember they change the gas mix for cold weather too, I've had to richen my 750DP up some on my 454 for my idle

[Andy Gargagliano]

Sam do you mean the air/fuel screws on each side of the front float bowel?if not where are the idle jets located on the carb.Thanks.

[Andy Gargagliano]

Just watched a video on how to tune a Holly carb.think i may have found my idleing problem.Before i put the car away the last time i drove it i replaced the vacum advance hose.The hose went from the distbuter to the port on the carb just above the air/fuel screw.I thought the hose would be better at the port at the base of the carb.Sounds like this is the wrong port for the vacume advance hose.Will move hose back to port above air/fuel screw tomorrow and see how it idles.I think this will solve my problem.

[stangeba]

Make sure the initial timing is set with the hose OFF the dist and plugged!

If you have a vacuum gauge, attach to each port, one at a time, and see which one has zero (ported vacuum) at idle and the other should have perhaps 15 inches.

With full manifold vacuum attached to the dist vacuum advance, your timing will automatically advance and make the engine idle faster. Drive the car with the hose on one port then the other port and see what you feel runs better. The vacuum can on the dist could be defective and leak so that's a good thing to check also. With a hand held vacuum pump or attach a hose and suck on it, see if it holds vacuum. The engine should be off for this test.

Bruce

[Andy Gargagliano]

I switched the vacume advance hose to the upper vacume port on the carb and made no difference? With engine fully warmed up it will not idle in drive.It idles ok in park but when i put it in drive with my foot on the break the rpms slowly drop untill the engine stalls out.This process takes about 20 to30 seconds.Can someone let me know where the idle jets are that Sam is talking about?Car ran great all summer and still runs great when driving it,just all of a sudden wont idle in drive? sure would like to get this figured out before i put it away for the winter.Thanks for any help.

[Sam (Bones)]

yes Andy, the screws on the side of the carb

 

shoot Andy...I could've came down to you and helped

[stangeba]

ONE time I worked on a car with a leaking power brake booster. When you applied the brake, in gear, the engine would run rougher and stall. If you listened closely you could hear the vacuum hiss from under the dash. Rare, but if you can't find anything else and your parking brake will hold!!! apply the parking brake and put it into gear, see if it still stalls out.

Bruce

[Andy Gargagliano]

Thanks Bruce power brake booster is brand new.Thanks Sam i will open the idle jet screws a little more and see what happens.if that dosent work i may take you up on that in the spring.Its going to be a long winter!

[Sam (Bones)]

is your parking brake in good working condition? if so, set it, and play with the idle mixture screws,just note where they are before moving so you can put them back

 

have you checked your timing to make sure it hasn't changed?

[TrunkMatt]

My take: richen the idle mixture

 

I am betting you actually were idling a bit lean in the summer, and now the colder air is making you way lean... my suggestion would be to richen the idle mixture by turning out (left) all 4 of your idle screws 1/4 turn on your holley.

 

Remember, Colder weather = lean condition... especially with it "dying after XX seconds"... and especially without having a choke. For every 10 degrees down in temperature the air becomes about 2% more dense... thus making you lean really easily.

 

By feathering it, you are using the pump shot to give it enough fuel to compensate, but if you stop doing this it leans out / stumbles out.

 

If you have a newer holley, the idle screws are on the sides... 2 of them per side (front and back). This video will show what I mean:

 

 

Funny thing about motors, they are more sensitive to being lean than they are rich. Case in point In terms of Air/Fuel ratio, 14.7:1 (air to fuel) is perfect... anything higher is lean / anything lower is rich.

 

Now, a car will actually idle without dying down to 9:1 (way way rich, will be sluggish but won't die).... but a car will barely be able to run at 16:1

 

So in general cars are more sensitive to being lean... I am betting yours is leaning out. All it wil take is a 1/4 turn out (left) on all 4 idle screws... maybe even less (they are super sensitive).

[Andy Gargagliano]

Checked distributor,its tight.Is there another way the timing could change?I will play with the idle screws and see what happens.Thanks.

[Andy Gargagliano]

Worked on car today.I am now more puzzled than ever.I checked the idel screws before i started and they were both two full turns out which i believe equels one turn.Warmed engine to operating temp and hooked up rpm guage.with engine running i turned screw out(left)and rpm started to drop so i turned the screw in and reached highest rpm when idle screw was all the way in?So i turned the other screw all the way in and engine was still running and running good?Took car out for a ride and it ran better than ever.It idled and accelerated great?I am happy it is running good but on the other hand how is engine running with idle screws all the way in?Somthing is not right.To recap carb is a used holly 650 double pumper #4777.It was rebuilt by American carburetor wich has been in business forever.Primary jets are67 and sec.jets are73 and power valve is 8.5.May be its time for a new carb? if so any suggestions.My engine is a 350 arround 400HP.Trans is street/strip turbo 350 with 2600 stall torque converter.Rear end has 3.73 gears.Thanks for all help.

[Sam (Bones)]

are there mixture screws on the backside of the carb too (four idle mixture screws)

[Andy Gargagliano]

Just 2 in the front.

[Sam (Bones)]

you shouldn't be able to completely close the mixture screws,

float level, plugged air bleeds, fuel pressure, too hi of a idle speed could all cause this issue. it's so hard to figure out without it in front of me

 

tomorrow I taking the '71 to be put in storage, don't know how the rest of my day is going or I would just head down, I have some ideas but need my hands on your running engine

[dbreese]

Good read from Patrick at Prosystems

 

CARBURETOR ADJUSTING 101

 

by Patrick James

 

**** The information given in this article is intended for PRO-SYSTEMS carburetors only. Other manufacturers tuning and jetting ranges can vary dramatically from this supplied information, so follow your original manufacturers guidelines for tuning your specific design.

 

One of the biggest mistakes made by a tuner of a high performance street engine is not tracking an engines varying needs.

 

Most high performance street cars make a lot of under hood heat and engine transferred heat. As a result, this engine ingested, super heated air, can climb as high as 60-100 degrees higher than the outside ambient air.

 

But what you'll find in the spring and fall is that as outside air temperatures drop from the 80-90 degree range down to the 60-70 degree range, underhood and engine transferred temperatures will fall much more.

 

So instead of following the outside ambient drop they fall exponentially. So a 15 degree drop outside can be as much as a 50 degree drop to the engine ingested air.

 

It also works the other way, with exponential rises.

 

So in June on a hot summers day, your engine is driving around drawing in 80 degree weather, ingesting 150 degree air and in the fall you are driving around in 60 degree weather ingesting 90 degree air. Thats a 60 degree variance!

 

Obviously you can see why the vehicle manufacturers construct those air ducts that go to your fender well or grille area to reduce this variance.

 

Unfortunately, the first thing folks typically do is remove those ducts and add a chrome air cleaner. They then tune the car for the weather that day, drop the hood and off they go for months at a time.

 

Then as it warms up outside, the plugs start to foul, the exhaust is getting rich and changes have to be made.

 

Typically from cooler air to the warmer summer air ( on non-hood scooped or fresh air ducted engines) you should reduce your main jets about 2-3 sizes on all four corners and possibly increase your idle air bleeds by about .006-.010.

 

 

 

Air Pans are a great way to duct fresh air to your engine. See our "Racing Parts" section for a great Air pan to really help offset this variance.

 

This is because there is not as much oxygen available in the warmer summertime environment. If you don't pull that fuel out, you will lose twice as much power, as now the engine is running rich and creating slight misfires and incomplete combustion. The power is lost due to the lack of oxygen molecules, but if you keep pouring coal on the fire, its gonna go out. So you have to pull some coal back.

 

If you have an air density gauge that corrects back to a Density Altitude number, most Pro Systems gasoline fueled programs will require a drop of one main jet for every 1100-1300 feet in increased Density Altitude.'

 

Don't forget to adjust your mixture screws too. Make sure every corner of the engine is happy, pull the plugs out and get them all similar in color at an idle.

 

Typically you simply, turn the idle mixture screws in until the engine hesitates or requires you to squirt fuel in it to restart and then turn them back out until those problems go away.

 

Tuning a performance street car for its best performance, even if you have outside air ducted to the carburetor is a constant job (or at least seasonal).

 

 

David

 

 

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[Andy Gargagliano]

Thanks Sam and everyone else for your help.I drove my 70SS454 Monte for the last time today and then put it up for the winter.It ran great.At least i have one that is running right!I did some reading on some other forums and it seems that this is not an uncommen problem.Possible problems could be i have my curb idle to high not allowing the idle screws to work.Blown power valve.Float may be to high allowing gas to leak at idle.Secondary throttle plates need to be adjusted.Dont know anything about that one.Gives me somthing to work on next spring.