348 Tri-Power Carb Issues

[davidg8991]

I am the new owner of a 1960 Chevrolet Impala. It has a 348 with Tri-Power. I'm asking for your help today because the engine seems to be drowning in gas at idle and low speeds. This situation gets worse as the engine reaches running temperature. I don't know the history of the car, sorry. Adjusting the timing as no effect on the problem. I have done the following. Installed a new electronic distributor, coil and wiring. New "standard pressure" fuel pump. I rebuilt the center carburetor, adjusting the float level and drop to the correct specifications, and a new needle seat. I might add that the center carb is the only one with the idle/air adjustment screws. Installed new spark plugs that are now black. I have adjusted the carburetor linkage so that the front and rear carbs do not begin to open before 50 to 60%. The number 1 and 2 spark plugs seem to be cleaner than the rest which makes me think that raw gas is running down the front inside the engine.
I saw in past history, 2006, that someone else had a similar problem, and he decided to just give up and put a 4-barrel carb on instead. I would like to solve the problem and keep it stock! Thank you for your help.

 

[427John]

Verify the float level in the outboard carbs also,High fuel level in those carbs will dribble fuel just as quickly as the center carb.While you have the tops off look to see if the outboard carbs have power valves in them.

 

[La Hot Rods]

The best way to approach these is by blocking off the front and rear carburetors with a plate under the throttle valves then you can tune the center carburetor and then one at a time remove a block off plate at the end carburetors one at a time tell you get them tuned in, if that makes sense.
The front and rear carburetor throttle plates need to seal off 100 % when at idle.

 

[davidg8991]

The best way to approach these is by blocking off the front and rear carburetors with a plate under the throttle valves then you can tune the center carburetor and then one at a time remove a block off plate at the end carburetors one at a time tell you get them tuned in, if that makes sense.
The front and rear carburetor throttle plates need to seal off 100 % when at idle.

Can I do the same by plugging the fuel line to the front and rear carb?

 

[davidg8991]

Verify the float level in the outboard carbs also,High fuel level in those carbs will dribble fuel just as quickly as the center carb.While you have the tops off look to see if the outboard carbs have power valves in them.

I have not opened the front or rear carbs. Will do that next. Thank you for your suggestion..

 

[La Hot Rods]

No, you need to block the air off.

 

[davidg8991]

No, you need to block the air off.

Great. Will try it. Thanks.

 

[427John]

If you have to pull the carbs off inspect them really well to make sure there not standard 2g's that have been converted(idle screws removed and pugged)true tripower outboard carbs will have thicker throttle blades and throttle shafts with wider slots for the thicker throttle blades,these are necessary to get a reliable 100% seal when closed.Its not required on the center carb because some air is required for idle and the throttle blades are normally open a little from the idle adjustment screw.As La says you need to get a 100% closed seal on the outboard carbs to tune them.His recommendation to isolate them so that your dealing with one at a time will simplify troubleshooting and tuning.

 

[davidg8991]

If you have to pull the carbs off inspect them really well to make sure there not standard 2g's that have been converted(idle screws removed and pugged)true tripower outboard carbs will have thicker throttle blades and throttle shafts with wider slots for the thicker throttle blades,these are necessary to get a reliable 100% seal when closed.Its not required on the center carb because some air is required for idle and the throttle blades are normally open a little from the idle adjustment screw.As La says you need to get a 100% closed seal on the outboard carbs to tune them.His recommendation to isolate them so that your dealing with one at a time will simplify troubleshooting and tuning.

They are true "2G" carbs. You have given me some great information and as La has mentioned I will isolate the carbs first thing tomorrow. Thank you for your post.

 

[davidg8991]

What fuel pressure should I be running for 348 Tri-Power? 4.5 psi or 6.5psi? Thank you.

 

[chuckl]

I run 5.0 psi--works well with all throttle positions from idle to WOT.

 

[61BUBBLE348]

I have been running my triples for 10 years, once I set them up, I haven't touched them ( bet I will now that I have said this). running progressive mechanical linkage.
the sealing of the butterflies on the outboard carbs is critical, all the other info is great advice.

 

[La Hot Rods]

I would run on the 4.5 PSI for a street car. The only thing to remember is that if you do not use the outside carburetors often the fuel will stale and when you do use them the it will fall on it's face a few time till you get crap flushed out. Same happens with a 4 Holley if you don't open it up every now and then. Carter's AFB'S don't have that problem.

 

[La Hot Rods]

Can I do the same by plugging the fuel line to the front and rear carb?

You can leave the fuel hook up and watch for fuel running over into the the carburetor's at idle, but if you operate the throttle the accelerator pump should squirt fuel in. It would best to not have the linkage hooked up to the outside carburetor's.

 

[Tim]

I would run on the 4.5 PSI for a street car. The only thing to remember is that if you do not use the outside carburetors often the fuel will stale and when you do use them the it will fall on it's face a few time till you get crap flushed out. Same happens with a 4 Holley if you don't open it up every now and then. Carter's AFB'S don't have that problem.

I think opening it up ALL the time should be mandatory! No fun being a grocery cart! Just sayin....

 

[davidg8991]

You can leave the fuel hook up and watch for fuel running over into the the carburetor's at idle, but if you operate the throttle the accelerator pump should squirt fuel in. It would best to not have the linkage hooked up to the outside carburetor's.

How do I get to 4.5 psi fuel pressure using a standard fuel pump? Thank all of you for your time and information.

 

[La Hot Rods]

Have you checked your fuel pressure, you can use a regulator or find a pump with the correct pressure.
Show Cars shows a pump # 4278 that is rebuild able so one could adjust the pressure with some playing with the pressure spring if needed.

 

[davidg8991]

I'm running around 6.5psi fuel pressure. I was just talking with Bruce at Show Cars yesterday. Will call and ask about a low psi fuel pump. Thank you!

 

[chuckl]

I have been running my triples for 10 years, once I set them up, I haven't touched them ( bet I will now that I have said this). running progressive mechanical linkage.
the sealing of the butterflies on the outboard carbs is critical, all the other info is great advice.

Sealing the outboard carbs IS critical for minimizing unwanted airflow (and fuel with it) through them. The butterfly plates can not only be loosened to slide back and forth, but can be rotated (slightly) for stopping almost all airflow. Dressing the carb throat walls and plates for an exact fit is another step, if necessary. There are commercial compounds that can be used, but are temporary.

I am using 93 non-ethanol gas and running progressive linkage and regularly go to WOT to keep fresh fuel in carbs 1 and 3. I have no problem with sticky needles and seats or crud in the bowls. With a little patience and "watch tinkering," Tri-Power setups are very economical and responsive. Besides, they are pretty cool! Chuckl

 

[427John]

A lack of activity on the outer carbs has been an issue with tripowers since they were new,as early as the olds J-2 back in the late 50's.Oldsmobile actually released a service kit to their dealers that consisted of a pair of block off plates and appropriate linkage to eliminate the outer carbs.