Fuel dripping out of primary's

[maddogs1965]

Hey Cliff!!!

So i will try and keep this as short as possible with supplying the most info for you..

So i have my Fathers 69 GMC truck S.W.B. fleet side 2 wheel drive with the #'s matching original 396 from the factory in it. I have just recently had the motor re-built since it had over a 100,000 miles on it, and since this truck is so rare, i am trying to keep it looking as ORIGINAL as possible, so i am trying to keep the Q jet.

Motor specs after re-build.

The motor has been bored .30 over with 9:1 compression.  I had the machine shop do some head work to the  original  cast iron heads and and they  install a set of 2.17 intake valves and 1.88 exhaust valves.  I am running  a Edelbrock Performer Z-0 intake manifold. I also installed a Comp Cam  part # 11-242-3 which has a .515 intake lift, .520 exhaust lift with 268 intake & 280 exhaust duration.

According to my vacuum gauge i am pulling between 8 to 9.5 pounds of vacuum  @ idle.

So my issues is with the Q jet is that if i turn the idle adjustment screw in past 800 R.P.M.'s in park, i start to see fuel dripping  out of the primary's into the intake manifold.   I am assuming since my vacuum is so low i am having issues keeping the needles sucked down into the jets, and i am going past the idle circuit into the primary circuit.   I am hopping if i order your Power Piston Spring kit this will address this issue, and you might be able to tell me what color spring i need to start with. The Carb # is a 17058213

I will also be ordering a new float and your Q jet manual...
 

[maddogs1965]

So i have been reading some of the threads on your AWESOME forum, and from what i see, it is looking like the older Q Jet carbs are the ones to have.   

So  besides the 17058213 that i am working on now, i also have a 17057213AWP that i rebuilt for my Father over 20 years ago and he stuck that on the truck, and i also have which i believe is the original Q jet for the 396 that i found on his shelf in his garage which is a 7029214LF.

Out of these 3 complete carbs, which one would you recommend to run on the 396?

I have already purchased a installed a rebuild kit, and ordered a float and the Power Spring kit from your shop for the 17058213, but if i have a much superior carb in one of the other two, i will start all over again with the one you recommend.

And bye the way, if i have not said so before, THIS IS ONE AWESOME FORUM YOU HAVE HERE....      i thought i new allot about these old carbs until i found your awesome Webb site 

[Cliff Ruggles]

Making under 10" vacuum is a result of the cam choice, the tight LSA and XE lobe profiles are just not well suited to a truck engine.  It would have helped to use a cam more suited to a truck engine and lower 9 to 1 compression ratio.

All of the carbs mentioned would still work fine for what you are doing, my first choice would be the original truck carburetor as it is more generous for fuel delivery than later emission carbs mentioned.  It will still need some idle system modifications to be happy with that low vacuum at idle and low speeds.

I would install one of our kits in it, and modify the idle system accordingly.  The carb will need the orange PP spring from our kit as well, stock jets and metering rods should be fine......Cliff

[77cruiser]

How do you have your timing set up? Probably need more initial timing.

[maddogs1965]

Making under 10" vacuum is a result of the cam choice, the tight LSA and XE lobe profiles are just not well suited to a truck engine.  It would have helped to use a cam more suited to a truck engine and lower 9 to 1 compression ratio.

All of the carbs mentioned would still work fine for what you are doing, my first choice would be the original truck carburetor as it is more generous for fuel delivery than later emission carbs mentioned.  It will still need some idle system modifications to be happy with that low vacuum at idle and low speeds.

I would install one of our kits in it, and modify the idle system accordingly.  The carb will need the orange PP spring from our kit as well, stock jets and metering rods should be fine......Cliff

Hey Cliff!!!   

Thanks for getting back to me.

As far as the cam choice, i have a friend with a 57 Nomad with a 468 in it, and i really liked the performance of his cam and engine, so that is why i chose the cam i did to match his and had the intake and exhaust valves re-worked. Basically i was trying duplicate his motor as much as possible minus the bore and stroke.

Thanks for the heads up on the orange PP spring, and i am looking forward to receiving you manual so i can  mod my Idle Circuit accordingly.   I will stick with the 17058123 carb for now, but if i do like the final results, i will start over with the original carb. 

 

How do you have your timing set up? Probably need more initial timing.

Hey Jim

It has been a couple of months since i timed the motor, but if i remember correct my initial timing is at 23* and total is at 32*.   Of course it does not help one bit that i live in Truckee CA. at a elevation of 6500, not much oxygen up here   

[77cruiser]

OK scratch that.

[maddogs1965]

One more question for ya Cliff!!!

With some advice from a friend who has two 8 second Camero's , i drilled two 1/8th inch holes in the primary butter fly's behind the throttle shaft (one in each butterfly) is this going to help or hinder me after i receive your manual and do the necessary mods to the carb to make it idle with the cam i am running.

I also noticed  that the main primary shaft is loose as all get out on the carb i am try to tune in. I have a couple of other donor Q jets that the shafts are in great shape. Can i just switch the base plates out and lose the drilled holes, or do i need to remove the un-drilled primary butterfly's and purchase a bushing kit from you and install it?

[Cliff Ruggles]

A cam that works well in a 468 isn't going to work well in a 396/402 trying to do the same thing.  CID and true static compression ratio are BIG players in how well any particular cam is going to work.  Using the same cam that works well in some ones 468 in your 396 isn't going to work, it would need to be about 10-15 degrees less duration to act the same way at the same compression ratio, in other words it would act like a LOT bigger camshaft in your smaller engine.

You got bad advice on drilling the throttle plates, that carb has idle bypass air and it is a LOT better to use that system to control throttle plate angle at idle speed with big cams.

Install bushings for the primary shaft, I would NOT install a different baseplate as it's not a good idea with that carb number.....Cliff

[maddogs1965]

OK!!!! well it is a little to late to turn back on the cam now 

If i am not happy with the performance, i guess i will be purchasing another one...

I will loose the drilled throttle plates when i install the bushing kit with another set from another donor carb i have....

[77cruiser]

Just a thought. Did you degree the cam? What's the cranking compression?
That seems to be a low vac. for that cam I'd expect to see more like 12 minimum.

[Cliff Ruggles]

The cam will probably have the engine "feel" pretty strong in the mid-range, but it will not be very efficient at idle and low rpm's due to the tight LSA.

For truck applications we need good idle, off idle and broad torque curve.  Here is a much better cam choice for what you are doing, if you get around to having to change the cam out:

http://www.ebay.com/itm/151429915339?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Here is another, modern grind with emphasis on smooth idle and low rpm power:

http://www.summitracing.com/parts/sum-1300/overview/

and one more that works really well in a low compression 454 truck engine, and smaller cid engines like yours if you have around 9 to 9.5 to 1 compression:

http://www.summitracing.com/parts/sum-1301/overview/

hope this helps some......Cliff