I have a two fold problem and they are related. The first is when Ishut the car down when it is hot I still get the fuel percolation. You can watch the fuel ooze out of the air bleeds and the squirters after shut down. It is slowly soaking the primary only side of the carb. Makes the car a bear to start after sitting for 5 minutes then when it does start is sputters and chokes until it clears itself. Both float levels primary and secondary are the same. Right at the halfway mark. Should I go lower and if so how low can you go?

I am also getting some sputtering just off idle. It idles fine but when you pull away under normal acc. it will sometimes sputter. Now I put in my LM1 and it shows it dropping down in the high 10s when this happens. It will go back up to high 12s pretty quick but I have had it die on me twice recently. Starts back up but it cranks like it was flooded with fuel. I am running a 6.5 PV and it is idling right around 7 to 8 inches of vacuum. Just crack the throttle and it goes up to 12 inches. I cruise at about 8 to 12 inches and sometimes more vacuum. Should I drop down to a 4.5 PV.

I posted about FI on the car but that is going to have to wait. There is no reason I should not be able to get this carb running decently. Under full throttle is just screams, no missing no hesitation, you hit the loud pedal and all he$$ breaks loose. The AF is running right at 12 to 12.5 under full throttle so it should be jetted good. The dyno run last winter confirmed this also.

 

Dirt preventing the float needle from sitting in the seat?

 

If the float bowl continues to fill up after shutdown, you have one of 2 problems, both of which I have seen on BG carbs:

1. Dirt or debris in the needle/seat. Many of these carbs have metal particles from the factory in the fuel inlets. Remove the needle/seat and clean it up good.
   
2. Porous casting. I have had 2 BG carbs with porous float bowls in the needle/seat area, allowing fuel to enter the bowl around the needle/seat assembly.

If the bowl is not filling up, but it's still spitting fuel out the accel pump discharge nozzles, you are boiling the fuel in the accel pump well. A heat shield under the accel pump will usually solve the problem.

I have found that the BG carbs, when jetted correctly for A/F at WOT, run pretty rich at cruise, resulting in a momentary over-rich condition when you stab the throttle. Most fuel at light cruise is metered through the transition slot - not through the main discharge nozzles. I have had good luck with inserting a small restrictor wire in the IFR oriface to correct this situation. A regular staple out of an office stapler is just about the correct diameter for effective restriction of the IFR and a nice lean-out for proper cruise mixture.

 

May not help, but you may find interesting...
http://www.maliburacing.com/forum/viewtopic.php?t=52837

 

The bowl is not filling up after shut down. You can see the float level and it stays right at the half way point. It sure seems like the fuel is boiling but why would it only do it on the primary side and not the secondary.

I have installed smaller IFR jets. I got some blanks and drilled them smaller. I don't have the exact size on me but it was a few sizes smaller than the stock ones that came in the carb. I do not have a heat sheild but am running a thick 3/8 gasket tha tI thought would solve some of the fuel boiling. I have had this on this carb and the two previous Holleys I ran on the other motors. I guess a heat sheild is the next trick to try.

 

i have a heat shield and have no boiling over problems

 

What kind of sheild or did you make it?

 

Got a pic of the heat shield?

 

Blow out the needle and seat on primary and secondary as stated above and most of your problems are history

 

If you should need to replace the needles and seats. I just got the newest design NASCAR inspired anti-airation needles and seats from the race shop that dyno tuned my 750

It is now using 5 jet sizes smaller on the primary and 3 jet sizes on the secondary just by fine tuning the metering blocks and air bleeds.

When I even went one size smaller on the primaries I was into lean popping. My plugs are cleaner than they ever have been.

 

I got big help from this thread:
http://forums.corvetteforum.com/show....php?t=1461346

 

home made out of stainless steel for use with a phenolic spacer....

you can use a holly version and just cut out the area for the accelerator pump

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Personally I don't believe in carb shields. Phenolic or wood spacers are the best thermal protection.

You might want to replace the little rubber acc pump flapper valves in both pri and sec,

The float level if it is anywhere from the top of the sight to the bottom only causes minor changes in air fuel ratio

 

A spacer made of that material would be a good if you were having a problem with heat conduction. If you think the heat is being transferred from the manifold to the carb, then go with the spacer. Those spacers serve as an insulator between the carb and the manifold. Unfortunately, that is most likely not your problem, the fuel, which has relatively high alcohol levels compared to past fuels is boiling in your bowls and purging out. Putting a spacer in might help because it will create a bigger gap between the float bowl and the manifold, allowing air to circulate, but putting a heat shield in will deflect the hot air from the engine.

Also, another area you might want to look at is your fuel pump. I have a mechanical fuel pump and I noticed that when I shut the car down, the bowls fill with fuel. I installed a return line close to the carb that keeps fuel at the proper level.

My suggestion, before you go for FI, try a heat shield. It takes 15 minutes to make and it costs about $10.

Thanks,
Joe

 

I have always had the return line style mech fuel pump and didn't think about hot fuel above the pump expanding and forcing it up and out.

 

Either make a heat shield of just buy the one for a LT-1 or Z-28.

 

Never thought about the fuel pressure. I'll have to stick my gauge in there and see what it does after a hot shut down. I do have a thick 3/8 gasket under the carb to try and deflect some of the heat but I don't have much more hood clearance for a thicker spacer.

 

I had similar problems to yours when I put an 850 Speed Demon on my car. I ordered the Holley heat shield and a 1/2" phenolic spacer from Summit and it kept the fuel from boiling. I also had a problem with an off-idle stumble that I could not tune out, I tried different squirters, pump cams, and played with my timing all to no avail. I ended up getting rid of the Speed Demon and I got a Mighty Demon with annular boosters, it is perfect for my car. I am a huge fan of annular style boosters now.

 

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I also have heard the annular boosters for the street work very well. I'll have to fab up a heat sheild to see if that helps.