KNIPPS

I have a couple questions about my 3x2 set up. First after initial start up and even idling for a few minutes I put it in first and the hit the gas and the car bucks violently but settles after about 10-15 seconds or if you get on it..why?? Second, I have had issues with an erratic engine idle. What baffles me is that it doesn't happen all the time. At some stops the car RPM will dip to 500 and below and stall out (but will perk up a bit if you hit the gas a little) and at times it will idle at about 1200 and be fine. I know neither should happen but dont know why it only happens some times. I am still learning this 3x2 set up and would like to keep it but I have had others already suggest ditching it for a single 4bbl carb. Thanks Jim

67coupe w/rebuilt 427

Nowhere Man

When's the last time you rebuilt the dist

DansYellow66

It could be a lot of things. Have the carbs been gone through? Have you checked for vacuum leaks around the intake and carbs? What is the timing set at? Does it have a stock cam? Is it the stock TI iginition system. Are the heat passages blocked on the intake? Choke set correctly per Holley/GM

Any of the mechanical lifter motors is going to take a little warming up before it will behave in a fairly civilized manner. Some bucking and low speed surging is not unusual due to the cam lope, related fluctuating of the centrifugal advance, intake not warmed up yet, etc.

The fluctuating idle sounds like the carb needs going though. It could have vacuum leaks at the throttle shaft from wear. The accelerator return spring may not be strong enough to return the throttle to fully closed reliably. If you have a longer duration camshaft the idle position on the carb may be too great in order to get idle speed up, uncovering the idle fuel transfer slots in the bores and negating idle fuel/air adjustments.

We probably need more information on your car and what you have checked out on it.

KNIPPS

Thanks for the reply fellas and please go easy on me as I am slowly and I mean slowly learning. To answer a few questions first. The Dist is an MSD complete system with the coil and the ign. box. The carbs were re-built a few years ago and have sat on the engine for maybe a year or so until the engine was put into the car and actually run (they were run on the engine at the engine shop when the engine was rebuilt 2 years ago). Total timing was set at 36, it is not a stock cam it is bumped up a bit and I plan on checking for leaks today if I get a chance. I just am trying to figure out why the problem is so intermittent. As I said before I am learning about this engine especially the 3x2 set up and am trying to avoid scrapping it all together. I did pull a couple of the plugs to see what they looked like and here is what I found

Sniper168gr

The 3x2 setup is such an icon on a big block Corvette that, to me, are worth the hassle to maintain.

I have had 5 cars with that setup over the years and it seems to me the car is leaning out (i.e., vacuum leak). Couple of things to check...

1) Make sure the carbs are tightened down correctly on the intake manifold and that the gaskets are not split. These are the gaskets between the carb base and the intake. During idle the two outside carbs are not actuated and the car is running on the center carb. A way to check this is to spray WD40 around the carb base while the car is running and see if the engine stumbles.

2) Disconnect the vacuum advance on the distributor and stick a screw/plug in the hose. See of the engine smooths out or stumbles.

3) If you have power brakes, repeat the same steps as step to for the vacuum line to the power brake booster unit.

4) Make sure your intake manifold bolts are tightened to the correct torque spec. You can spray WD40 where the ports mate up to the head to determine if the engine stumbles.

5) Make sure you don't have an open vacuum port on the base of the carbs.

6) Check the secondary carbs vacuum diaphragm. You can remove the small rubber hose and plug the ends.

One other thing... If you have the big block ignition shield and stock wires on the engine, remove the cover since the ignition wires could be grounding out.

DansYellow66

It may be that your choke is simply set up too rich. A rich choke will cause stumbling and possibly even some black smoke from the tailpipes. Try opening up the choke gap a little and see if that helps. But if this is a 435 hp engine, expecting to drive off immediately from a cold start without a lot of throttle blipping and other drama is not realistic. Your plug photo indicates a possible rich condition but depending on how you have been driving the car that could be misleading. At least that one looks dry.

On the erratic idle, I would make sure the throttle return spring is strong enough to positively pull the thottle closed. Too light a spring will cause the idle to be all over the place. A lot of guys run dual springs to guard against a run away engine in case of a spring break.

Dan

babbah

Please Remember - The 3x2 setup works like a "six barrel" carb. The middle carb (Primary) idles and does all the work, the outboards (Secondaries x 2) basically stay closed until you romp on it then they open, but you really have to be moving!!

Tri-Powers Rule!!

JohnZ

The stock 3x2 setup uses "ported" vacuum for the distributor vacuum advance, so all the engine sees at idle is your initial (base) timing, which is too retarded for efficient operation. Cap off the existing vacuum port on the base of the center carb and tee into the rubber hose from the front of the carb baseplate to the choke pull-off diaphragm (full manifold vacuum source) to the distributor. That will add 15* of timing at idle - noticeable improvement in driveability and throttle response.

What plugs are you using? Try AC R45XLS. If you're using 43's, throw them in the trash.

Tom454

Lot of good suggestions above.
Also: (as mentioned above)
Check the condition of the center carb throttle body and shaft. Wiggle the shaft forward and up/down (not sideways). The soft throttle body very often wears "egg shaped". When this happens, air leaks past the shaft causing a lean condition which stalls the engine. It is intermittent because the shaft does not always return to the same position under normal driving conditions. The OEM design has the shaft pivoting on the soft aluminum against the spring. Using a stiffer return spring only causes the problem to get worse geometrically. I have encountered this so many times that I have engineered a return spring bracket that mounts on the front carb instead of behind the throttle shaft.
The repair is to use a special piloted reamer to ream out the throttle body and press in a bronze bushing. This tool is available in a kit. I have done many of these. Just did another one last week.

63Corvette

I agree with all the posts above. Three duces look cool, but give three times as many opportunities for vacuum leaks.

KNIPPS

Ok, well I do not have a vacuum canister on my dist. I checked the throttle body shaft on the center carb and it looks/feels pretty secure--no play. I took some picks of my set up and plan to check for leaks in the next day or 2. Also if I understand this right there should only be vacuum from the center carb at idle right? I have vacuum at the cnter and rear carb is this right. Again I an still learning this 3 carb thing so go easy. I appreciate all the posts and ideas. I might just take everything apart while trouble shooting and photo along the way to help others that might come across the 3x2 issues. Thanks again..Jim

6T7L71CPE

Here are a few 3X2 posts I have saved:

http://forums.corvetteforum.com/c3-t...ation-bog.html

http://www.corvetteactioncenter.com/...r-jetting.html

http://www.corvettefaq.com/c3/tripower/

http://forums.corvetteforum.com/c3-t...496-build.html

http://www.corvetteactioncenter.com/...wer-carbs.html

DansYellow66

A vacuum advance distributor would definately help tame your motor and possibly cure most of your complaints. I'm looking at these photos on a small tablet - but I have one question on your vacuum connections.? I see the two end carb vacuum canisters and the center carb choke pulloff all tied together - but I don't see a line run to the vacuum source?? Maybe I just can't see it.

Tom454

Looks right to me... pic#3... the vac tee connects the two end carb pots to the center carb main body port. The choke is connected to the base of the center carb.

??

JohnZ

This will help you understand how your 3x2 system is supposed to operate.

KNIPPS

Thanks John, I had gotten that from another forum member and was just reading it.
Ok I backed the car out of the garage today...fired right up...ran great and the throttle response was excellent. I checked for leaks and couldnt find anything. After running for a bit I shut it off...fixed a sidepipe cover and went to fire it back up again and this---poor idle and poor throttle response

View My Video

6T7L71CPE

Possibly percolation. Do you see large drops of fuel coming off the boosters in the carb? Have you checked your float levels?

KNIPPS

Ok was messing around with this thing again and yes it seems like at times its just bigger drops of fuel being dumped into the carb as opposed to a spray or jet. Next I was playing around with the choke and after is ran for a while and was shut off it looks as thought the choke would stay open--rough idle..I manually closed the choke valve a bit and the idle smoothed out or revved depending on the position. Now these are aftermarket heads so I am not sure if the choke will work the same as stock heads. Still smells really rich and still have the fouled plugs??? I am thinking about changing over to the vacuum advance on the dist. The original Dist. came with a canister but is there a different size or type I should run?? These are the heads I am running---Again thanks for all the help fellas!!!

Material: Superior cast iron alloy
Combustion Chambers: 119cc
Intake Valve Dia: 2.25''
Intake Port Dim: Int Port Location: Stock
Intake Gasket: Mr. Gasket 720-121
Exhaust Valve Dia: 1.88''
Exh Port Volume: 129cc
Exh Port Dim: 1.670'' x 1.800''
Exh Port Location: Raised .300'' Standard Bolt Pattern
Exhaust Gasket: Fel-Pro 375-1412
Exhaust Bolts: 1'' longer than stock (4) ea.
Flow, Intake: 363cfm @ .800'' lift
Flow, Exhaust: 266cfm @ .800'' Lift
Manifold: Rectangular Port Dart (opening - 1.575'' x 2.250'') 6.25'' tall
Milling: Min 110cc (.005'' = 1cc) Flat Mill only
Pistons: Most aftermarket pistons
Push Rod Length: Avg +.200'' / +.250'' Should always measure; Stock Length Int = 8.275'' / Exh = 9.250''
Retainers: 1.625'' - Titanium / 1.550'' - Moly Steel 10°
Spark Plug: .750'' reach, gasketed, Champion C59C / C59YC Street RC12YC
Spring Cups: 1.625'' - .035 cup / 1.550'' - ID locator
Springs: 1.550'' Single Springs, 145lbs @ 1.900'', .660'' Max Lift Comp Cams 249-936
Stud Girdle: Must use Dart design 301-64110001
Valve Angles: Int - 24°, Exh - 15° (Rolled 2°) 4° cant
Valve Length: Int - 5.475'' (+.250'') Exh - 5.435'' (Stock)
Valve Stem Dia: 11/32''
Valve Train: Standard BBC 7/16'' stud mount
Valve Guides: 1/2'' OD Mag-bronze Cut for .530'' PC seals (.002'' press)
Valve Guide Length: Int - 2.100'' Exh - 2.250''
Valve Guide Clearance: .0014'' - .0021'' (with 11/32'' dia valve stem)
Valve Guide Spacing: Exh moved .045'' away from Int
Valve Seats: Exh = Tungsten hardened alloy, .006'' press (TIR .004)
Valve Seat Dim: Exh - 2.000'' x 1.600'' x .375''
Valve Seat Angles: Int = 32° - 45° - 60° - 70°, Exh = 37° - 45° - radius
Valve Covers: 301-68000040
Torque: Head Bolts = 70 ft/lbs; Rocker Studs = 45 ft/lbs; Manifold = 35 ft/lbs
Block Use: Mark IV, Gen V, Gen VI with proper head gasket
Weight: 70 lbs

KNIPPS

ok guys I appreciate everything...I have read a bunch today and as crazy willie goes at the start so do I. I will re rear the thread again tomorrow...Thanks again for helping this newbie. Ha and to think when I joined this forum 7 plus years ago owning a C2 was a pipe dream and I knew very little about them---I know a little more now...maybe another 7 years I will know more about these tri carbs. Fellas!!

DansYellow66

OK, I can see it better on my home computer - two separate vacuum lines hooked to the carb.