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Discussion Starter #1
Have a 750 carb with proform main body and QF billet blocks and base plate, 4 corner.
Carb works great at full throttle, jet's set on dyno.
Previously on a 396 w 10" idle vac and runing a PCV with it.
Currently using it on a 496 with 6.5" idle vac and NO PCV.
On the 396 it worked gretty darn good drivability wise. On the 496 full throttle is great
and idle is very stable, 950 and 850 in gear, all screws 1.5-1.75 turn best vacuum.
Timing 18 initial, 36 total all in 2500.
IAB's 72 HAB's 36 IFR's 31 3.5pv 77P 88S all air bleeds and ifr's square.
Problem is 2 fold. Light throttle low cruise it is rich, light sputter, but clears out when
break around 40 mph. Other problem is flooding on decel, not even hard braking but just
letting off after getting in it. Really worse when temps are over 80' but present on cooler
days as well. It will flood out, most of the time have to hold it to the floor and crank on
it until the black clouds billow.
I installed pcv, helps on light cruise, but do not want to leave it in. (plans for N2o soon)
Triple checked N/S, and lowered fuel levels with plugs out, engine running 1/16 below
front plug and now 1/4 below rear plug. Shake car and no fuel dribble with carb hot and
engine running. Previously had light dribble from squirters, but made some tiny springs
to keep valves seated, and no differance at all. Vent tubes are clear and have abot 1"
clearance to air cleaner. Tried new metering block gaskets too. If I turn in the idle mixtures
to around 1-1.25 turn, cruise is better but idle sucks.
Curious to opinions. Should I try increasing IAB, or lower IFR? How much? Obviously getting too much fuel somewhere on decel and high vacuum, but from where? Possible that not running
the pcv is causing it to suck tons of fuel through the idle circuits?
 

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What is your t-slot exposure like? If you pull the carb and turn it over you should see little squares showing, roughly .020. If it's much more than that you will have trouble with adjusting the idle and transition circuits.

Installed PCV? Do you mean power valve?

.036 is a bunch for a MAB, especially on a 496 with a 750, I would try some .026's. Once you do that you should also be able to lower jet size, 80-84 square with no power valves, 72-76 with one in the primary.

And a power valve open or closed will have no effect in idle fuel amount, if the power valve diaphragm is blown or the PV gasket is leaking it will pull raw fuel through the vacuum passage into the engine.
 

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Discussion Starter #3
Hi Mark,
1st sorry, couldnt read my own scribble very well. The high air bleeds are 31's
PCV, as in pcv valve to valve cover. I ran one in the 396 regularly, but I am not
running one with the 496 as I plan on N20 in the future and do not want any oil
to contaminate the inlet charge. I am running a power valve in front with a plug
in rear, and did replace both the power valve and plug gasket just for giggles, but not affect.
 

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Do you have the metering block gasket that has a round hole for the power valve? If you are using the typical metering block gasket that has a tombstone shaped cutout for the metering block you can have problems as it leaves a vacuum hole exposed to where fuel for the idle circuit can get into the vacuum port and dribble direct into the engine.
If you get the gasket with a round hole and place it over the tombstone gasket you will see the hole at about 5oclock (lower right side) and see where it can intersect with the idle circuit feed crossover.
I can't load pictures, if you don't understand and think you need more info you can email me at [email protected] and i'll send you some pics of the metering block, gasket and the spot where it can leak.
 

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Discussion Starter #6
Hmmmm.... I am not sure which is in the rear but know I put a tombstone in the front last time I had it off because it is all I had. Have the old circle style in spare parts that came out and tore and matched it up and see exactly what you are talking about. This could allow a major fuel leak into the vacuum side of the power valve under deceleration correct when vacuum is really high as opposed to low speed couldnt it???
 

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Discussion Starter #7
I will get some new gaskets and try that. Still curious about change to make for lack of the PCV vacuum leak too. Would I be better off trying to increase the IAB's to 74-75 or drop the IFR to 29?
 

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I will get some new gaskets and try that. Still curious about change to make for lack of the PCV vacuum leak too. Would I be better off trying to increase the IAB's to 74-75 or drop the IFR to 29?
I'd make 1 change at a time, replace the metering block gasket first. If it is leaking at high vacuum then it is also leaking at idle and playing hell with everything. Not all of them leak, sometimes it is just plain bad luck kinda thing.
If i don't have the roundhole gasket or if the carb needs the vacuum port open for a street car then i take a piece of that aluminum foil duct tape and put a small piece of it across the idle crossover to isolate it from the gasket and the vacuum hole in the gasket.
 

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Discussion Starter #9
OK so I am ready to take a hammer to this thing :)
I ordered the Quick Fuel gaskets, nice round hole and no cross over possible problem now. Increased the IAB's from 72 to 75. Pulled it back apart, made sure power valve is not ruptured. Installed new gaskets for the power valve and plug in rear. Coulda sworn the IFR's were 31 but they were 33. Dropped them to 31's and left the IABs at 75. Blew through every orifice that I can. Cannot find any signs of debris etc.. in metering blocks or other. Have 20' initial and 36 total all in by 2200 rpm. idle screw adjustment still at 1.5 turns after all the changes, that is where it idles best. Transfer slots tiny squares.
All of this seemed to make light throttle cruise better when it is cooler outside. Then yesterday it was 90 out and she raised hell. Back to pretty good sputtering cruising anything below 50 mph. Then if you get on it and let out, it will start flooding. gotta stay in the throttle to keep it going, will cough and belch for a while, eventually settle down and clean out again. So... I tried replacing the squirters too to get away from the tube style as it still seems to siphon fuel through them, same thing. The only thing I have not chagned yet and the needle and seats. I can shut the car off and pull the screws from the bowls and fuel level is fine. Fuel pressure always 5 psi.
I dont have another carb to try for giggles. Wondering if Fuel pushing past the o-rings on the needle and seats hot and higher rpm??? That still wouldnt explain it being on the rich side just cruising down the road at 15 to 30 though.
Fuel lines run within a couple inches of header tube, but I would expect that to cause vapor lock if anything not carb flooding over. Any idea's???
 

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O-rings are a possibility, and if it's an older carb occasionally the sealing surface in the flat bowls can get corroded and leak as well. On the metering blocks how many e-bleeds are there, how many are plugged and in what positions, and what are the sizes of the open ones?
 

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Discussion Starter #11
The main body is probably 7 years old. Metering blocks are quick fuel billet blocks that are about 4 or 5 years old. The bowls themselves off the original carb and likey 15 or better. I do not see any corrosion in them around the sealing sruface, but my eyes may not be as good as they used to be.
I believe the blocks are QF 34-4. there are 4 emulsion holes and they are plugged in next to bottom hole. I do not have a pin vise to measure the openings, but visibly the 3 open ones look darn close to the 31 ifr hole opening. The blocks were installed without any mods from the way they came.
 

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Discussion Starter #12
So I just pulled the needle and seat out, [email protected]# o-ring all cracked. How the hell can it be cracked and then not run fuel out with the pump on and plugs out of the side of bowl? I cannot tell if there is any pitting in the bowl where the o-ring is supposed to seat. It looks like som dried up rubber stuck to it, cleaned up aith some 400 grit but maybe I should play it safe and buy new bowls and needle and seats.
 

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So I just pulled the needle and seat out, [email protected]# o-ring all cracked. How the hell can it be cracked and then not run fuel out with the pump on and plugs out of the side of bowl? I cannot tell if there is any pitting in the bowl where the o-ring is supposed to seat. It looks like som dried up rubber stuck to it, cleaned up aith some 400 grit but maybe I should play it safe and buy new bowls and needle and seats.
That will do it. If you PM boyracer here he can set you up with any parts you need, the QF sight glass bowls are quite reasonably priced.
 

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Discussion Starter #14
Ok, back again. Please endulge me LOL.

So with all of the things completed, new bowls and needle and seats, running fuel level at the bottom of the sight glass I am not having flooding problems so far after jumping on it. I am still however dealing with the sputtering while driving low speeds just like there is a choke partially on. Pulled multiple plugs, all dark bordering on black. I tried to increase the HAB's to 36 to delay main circuit and no change. I lowered the IFR from 31 to 30 and increased the IAB's from 72 to 75, had to increase idle screws tro 2.25 turns. Still sputtering but I swear it also started to surge like a lean surge. Plugs still dark, only drove a few miles. Went back to 31 ifr ad 72 iab. The only gasket on the carb not new is the base gasket. Also replaced the 3.5 PV for a 6.5 which helped for crisper throttle responce but no change to the sputter which I knew shouldnt change anything.
Running NGK 8 plugs with probillet dist and blaster 2 coil with 6AL box. As soon as I accelerate slightly, clears up. Once over 40mph or roughly 3k, clears up. Pulls like a train to 7k. Possible that light load is being affected from fouled plugs? Ready to throw some 7's at it and see what that does. I wouldnt expect ignition to be a problem but is it possible??? Could the multispark be a problem?
 

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Discussion Starter #16
Look for a blown PV diaphragm or leaking PV or PV plug gasket.

Hi Mark,

I just installed new power valve and gaskets on both it and the plug before the last drive when i dropped to 30 IFR and 75 IAB. I am going to swap out the plugs tonight and go up in heat range to some 7's. Wondering if they are not just fuel fouled from all the crap that had been going on. At least fresh plugs should be able to give me a more acurate reading as to if the changes are making a differance.
 

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Discussion Starter #17
Figured I would update progress. It defintley helped swapping the plugs out with some fresh ones. Still some light sputter, but not as bad as previous and although I only drove in a cruise mode for about 4-5 miles, plugs are not turning black, but light brown so far. Played a little with trying to lean it out some more, just was not happy so for now sticking with the 31 IFR's and 72 IAB's. Gonna try locking out the distributer this weekend.
 

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I found this old thread while googling my carb problem, I have a 950 with Proform black billet metering blocks. I used the gaskets that came with the blocks and they have the tombstone shape, it runs like crap, misses at high rpm and struggles to idle. I don't know if it's related but I also have fuel dripping off the primary boosters and the vacuum port coming out of the passenger side of the primary metering block is wet with fuel.

I'm gonna try the Quickfuel 8-139 gaskets and hope for the best.
 
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