hey guys iam a little worried about my 350 in my chevelle....... i have a 750 df carb and when i put it on i just adjusted the curb idle and it was running good but rich so i tried to adjust the fuel air mixture screws and i think i messed it up. It runs good but it dont feel like i have all my power plus it doesnt always fire up after running it for a while and she blows white smoke which i know is water and my engine was rebuilt already so it is healthy and it also sputters after hard acceleration. Please help me with this problem because iam worried that ill do damage to my engine and shops or either too exspensive and non helpful.....thanks for your time and hope for an answer soon. Eric

Sounds like it has a bad head gasket, cracked head, or a cracked block. That is generally how water gets into the combustion chamber. Screw the idle mixture screws in until they are seated, and back em out 1.5 full turns, most any motor should run well with the screws 1.5 turns out unless the carb. isnt matched for the motor or there is another problem. Sounds like at least the heads need to come off the motor though:(

thank you for answering my question real quickly but i think i wewnt over board with the smoke...the engine was rebuilt and there is no cracked anything there is no bad gaskets that i no of. the smoke is very little and i know that condensation will build up over sitting over time. but what i need is how to adjust those stupid screws bc it was fine before no problems! i did now i need to fix it. so do i screw in all the way with the engine on or off?

do it with the engine off ,if it was running it would die anyway ,and i use a vac gauge to set mine highest vac setting stays

i dont have a vac. gauge but i have a tach on the car so that will help right? i heard a good idle at park is 850 and in drive it is 750 what do u think of that?....thanks eric

also by the way when nova said trun the screws in untill they are seated?does that mean turn them in untill they wont turn no more then back then out? please help!

Yes on the idle mixture screws.
Just turn them in lightly untill they stop.
Do not force them to see if they will keep turning.
You might damage the screws if you seat them hard.

Sounds like you might need to adjust the float levels as well.
There's a sight plug on both the front, and rear float bowels. With the engine running at idle, remove the plug in the side of the front float bowel. Fuel should JUST trickle out of the hole when you rock the car SLIGHTLY.
If fuel starts pouring out, the float is too high.
If you need to adjust the level, the Large screw with the nut under it on the top of the float bowel is the needle and seat adjustment. Loosen the screw SLIGHTLY, and turn the nut to adjust the float level. Once the front float is set, repeat the procedure for the rear float.
Then adjust the air/fuel mixture as mentioned previously. If you don't have a vacuum gauge, start at 1 1/2 turns out, and adjust until you get the highest idle rpm. Once this is done, then you can set the Idle using the idle adjustment to where you like it.

Not 100% on whats done to your motor but a 750 is a lil' large for a 350. I found my 750DP (4779-6) ran best at 3/4 turn out and got best vacuum.

Hotrod,

I'd like to add on to what Zman said to do.

After you get the 1.5 turns out set(which will be very close) and then once the car is running, get the floats set as close as possible first. Then return to the idle mixture for fine adjustment.

Remember that the 2 or 4 screws on the metering blocks are in fact an idle adjustment and not an air fuel mixture. The amount that they are in or out determines how much "mixture" is fed to the carb. The ratio never changes. (If you already knew that then disregard my comment).

When you do adjust the idle mixture screws try to get the idle set as low as possible before starting( the large screw on the side of the carb). Nothing will be optimal yet and the car may even try to die on you but just go with it and barely increase the idle if you really need to.

The reason that you need the idle set low is that there is a fine line between your idle circuit and your main curcuit. There is a slot down in the veturi at the base plate and that slot tells the carb to go from "idle" fueling to "main" fueling and if you don't have the blades closed enough to much of the slot is uncovered and you are always on the main curcuit. You can turn the idle screws in or out 100 times and you won't notice a change in idle or vacumn.

Once your idle mixture screws are where you like them, whether it be RPM's or vacumn. Drop the idle set screw down to the idle you like and the repeat the steps again until it's where you like it. Then go back an d check your floats just to make sure they are still right. Sometimes they will change with a mech. fuel pump.

Your idle mixture screws may be at 1/2 turn or they may be at 2 turns but it's whatever your car likes once you are on the idle circuit like I said above. It is nice to keep all the screws the same so if you have to jump back and forth a bit try that as well. It's easier to remember to get back to when you have to adjust anything else on the car. I.E. all screws at 1 turn out rather than left screw at 1.5 turns and right screw at 1/2 turn and so on.

Hopefully this didn't confuse too much, I threw a lot out there but good luck and it's easy after the first time.

Rob

P.S. There's a big blue book out called "Holley carburetors" and it has everything I just talked about in it. Well worth the price. ($20?)

ok first of all i to thank all of u guys u have been very helpful. to mfsr iam kinda confused and kinda confident that i can explain this so i understand. first i check float to see if gas comes out does this mean there is too much feul instead of air? on adjusting that needle seat with the wrench do i turn clock wise or counterclock wise? iam guessing counter to bring it down and when do i know when it is good? iam alos guessing no gas comes out. also when u said to adjusr float then mixture screws adjust idle do i do this twice for "idle" and "main" circuits or just once? and when done did u mean to bring the idle back help or when u said drop it as a typo? thanks gain and i think i can do this with all your guys help. You guys are great iam glad i joined this club......hope to hear from u soon........eric

Eric,

Try not to associate the floats and the idle mixture screws with fuel to air ratios. That's basically controlled by a different part of the carb. The book I told you about can explain that stuff. We are all here to just walk you through the basics and get you running smoothly again.

The floats(the 1/2" screw on the side of the bowl) are to control the amount of "overall" fuel the engine gets. Holley's rule of thumb is to set the float level at the bottom of the hole(with the screw out of the bowl and the engine running of course). If the level is too high, you'll see the fuel coming out of the hole as soon as you start to loosen the screw on the side. If it's not high enough you can kind of tell where it's at by rocking the car side to side.

Now if the level is too low, the engine will run out of fuel at high rpms. Not instantly because it takes a moment for the bowl to run dry(not literally). If the level is too high, the main problem you run into is at cruise or idle speeds, the raw fuel dumps right into the top of the carb. (the book would explain that in much better detail and it would take to long to type the reasons that happens).

To adjust the float,with the car running, barely crack open the 1/2" screw on the TOP of the bowl(a rag is handy to have because some fuel WILL spray out). Turn the nut (5/8" wrench) under the screw clockwise to lower the float, and counter-clockwise to raise it. The fuel should now be right at the bottom of the hole and if it BARELY trickles out that's O.K.

So the reason I said to first adjust your floats was to make sure that they weren't dumping raw fuel into the carb and messing with your idle circuits.

Idle mixture screws= 2 or 4 screws on the carb metering block(s) that control an amount of fuel to keep the engine idling.

Idle speed screw= Screw that controls the throttle or the speed of the engine at idle.

Now for the mixture screws. You can adjust them as many times as you like until you're comfortable with where they are at. Just make sure you use all of them to adjust the idle mixture and don't stop at just one. I do them by sound and rpm after I found that the vacumm gauge can give the approximate same reading with the scews in different positions.

I don't know where your engine likes to idle, but get it down there(with the idle speed screw) where you think it should be. Remember, you're trying to keep it off of the main circuit. Then adjust your idle mixture screws. Your rpm will most likely go up. Set it back down to where you want it again, and try adjusting again. If the rpm change is minimal to none then most likely your there. Set your idle speed and go.

Your floats most likely won't change so if you're confident that you set them right the first time then you really don't need to check them again.

Try this and get back to me.

Rob

rob.....this carb is brand new so i dont think they should be adjusted but ill check them like u said and see what happens. the one thing i dont understand is the whole "rocking the car thing"? do you actually rock the whole car? iam pretty sure with the advice u,the other guys and a chevelle tuning book i think i can do this...thanks and ttyl.......eric

also my main problem was that at hard acceleration it kinda bogged down 4 a second and left a big gassy smell and it wouldnt start up sometimes after running, but at cruise it ran great had acceleration and idle seemed to be ok so who knows? ttyl eric

another quick question i have is that i put brand new tall valve covers on my 350 and they came with only 2 holes why is that? i have a pcv on 1 side and a tall filtered oil/breather on the other will this affect the cars performance and if so what can i do to fix that?thanks eric

Eric, how many holes did you want?. Mine are like yours

Eric,

I think I went too far on thr rocking the car thing. The only reason to rock the car is to see the fuel level in the floats if they are too low.


With the fuel being clear and it being dark in the bowl its hard to just "see" in there.

Now I'm going to poke a little fun at you now all in good nature :D , but yes, you do rock the whole car. You can't jusr rock the right fender with out the whole car moving a bit. smile.gif Just nudge the fender with your leg while looking in the bowl plug.

Kinda like two teenagers up at makeout point. They are only sitting in the back seat, but the whole car moves when they are in there. Right? smile.gif

Hope that helps.

Rob

jim chevy,
i would like three but i was thinking of of putting a mr. gasket pcv/oil breather cap on there instead of a pcv valve to make things even, but not sure if i should remove the pcv valve, what do you think?
Eric

Rob,
very funny and thanks for clearing that up, being that iam only 21 i think i can relate to that lol. I got a letter from holley tonight letting me know what they think and he got inot detail about rejetting and reaching down inot the carb for some kind of throttle arm? He thinks that the carb might not be getting enough fuel but it is brand new hardley driven.....so who knows, i told him i would do the screws and float first. but thanks again for all your help you have been great and i think i can tackle this and i will let you know how the outcome is. One more thing though i know you said to check float first but with screws not were they should be will i get a inaccurate float reading? ttyl....Eric

The idle mixture screws don't have any effect on the float level. The float and bowl on your carb works just like the tank and float on a toilet. As the gas is going in the engine and leaving the bowl the fuel level drops and so does the float. When the float drops it opens a valve to let more gas into the carb. The maximum fuel level within the bowl should be level with the bottom of the sight plug hole located on the passenger side of the fuel bowl. Fuel level height does effect the idle mixture some because it is in direct relation to how high the fuel is in the idle feed wells. Get your fuel level/float level set then dial in your idle mixture screws and report back on what you ended up with. 1.5 turns out is the general starting point for most carbs. Most new Holley carbs come with a tuning instruction pamphlet-did yours? Bob

yes but it didnt go into detail like you guys.........thanks for you and everybody's help iam going to do this sun. because iam working all this week and sat. too =< and when it gets dark at 5:30 and no garage to work in makes things kinda difficult......but thanks again to everybody who pitched in and donated there time to help, you guys are life savers.....ill get back to you guys when i tune it sun. night....Eric

hey guys sorry i toke so long to respond but with the weather and work its tough to get online but i did adjust las week and it came out so so the front float i think needed adjusting so i did it a little bit and the back 1 seemed ok. but the fuel started to squirt out so i adjusted it back and it seems ok now but my friend told me that i have to adjust with car not running and i think my plugs are shot so that take cares my sputtering problem but i think iam going to have my friend come over and give me a hand but my other friend said my power valve might be blown but with the power valve protection and the carb being new could that be true? get back asap thanks eric