Spring could be cracked or broken but have enough strength to close valve but not hold it closed when operating.

 

The configuration of the lower gasket isn't exactly original, but it appears there is enough material to do what needs to be done.

If the car ran acceptably before the carb overhaul, and you haven't done anything to the motor, which you seem to not have done, then quit trying to make a mountain out of a mole-hill. Remember the KISS principle?? Be methodical and meticulous and the problem will surface.

 

but I don't like how large the holes are,I would match the gasket to the bottom of the carb and see if the oversize holes are causing a problem.

 

...."Car ran a little rough before, stalling when making a hard right turn, & minor misfire".....

You have to go back to square one on this. You had issues before this change and now they seem to be the same but worse? Your change has exacerbated the original condition. Seems you've leaned it out even more.

The hard right turn stalling indicates a fuel level issue in the bowls to me or not enough fuel pressure or bad floats as the fuel is fed in on drivers side.

I am not familiar with a tri-pwr setup. Maybe someone else on the forum with a stock six pack can tell you what their vacuum is at idle with the choke open.

Hows your fuel pressure and fuel level in the bowls? Is your pcv stuck open or leaking, plug it?

Whats your dwell reading and is the miss all over or just particular cylinders. Hows does your timing look just want to make sure its not retarded.

Guys... is it possible to temporarily switch the center carb for one of the outers just to see how it runs.. after its warmed up of course?

 

I don't think the outers have an idle circuit.

 

That makes sense.

 

no............but I might try and find another center carb, just for a test, as this thing has been on & off now 10 x's & should be able to do it in my sleep. I'll know more after the bench flow test this eve / i'll keep you posted. Thanks for all of your input, the MYSTERY is killing me

 

Why do this? It makes no sense. You've already said you don't know where your timing is, you've got a questionable base gasket. Have you determined if the secondaries are closed at idle or properly set your timing??

 

My L71 manifold is off the car and I just looked at it. The Holley base gaskets are the same for all three carbs, therefore, the holes are the same size for all of them and you will see a difference between the gasket hole and the manifold hole on the center carb. The gaskets say "For Center Carb this side up" on one side and "For End Carb this side up" on the other side.

 

The center carb has an adjustable idle circuit. The end carbs do have an idle circuit in the metering plate in the fuel bowl, but it is fixed. The end carb idle circuit keeps the fuel from "getting stale".

 

Idle smooths out when hand over center carb. Firing off in the exhaust. Misfiring. Right turn stalling. All these things sound like a lean mixture, as being pointed to above. A lean mixture is harder to light-off than a normal or slightly rich mixture. Want to burn your shoe laces off (literally), lean out your sidepiped car enough, stand by the exhaust outlet and I assure you it can happen. (True story.)

You want the timing right, but if it's off some, that won't make the car stall making a turn. If the timing is way off: retarded - the engine temp will come up very fast when starting from cold, advanced - trouble starting and cranking over. I don't think that either of those is being reported.

If PCV valve stuck open you should see oily mess on intake floor I would think. Oil consumption up for sure. Seems like there's no vacuum leak at the carb bases, based on carb cleaner test that was done. Have you removed the hose and plugged the manifold tap that feeds the vacuum system. That should positively isolate the engine from any vacuum leaks in the system. If you are running ported vacuum to the distributor, it would not matter if the vacuum hose to the distributor, or the vacuum canister itself, had a hole in it, but if you're on full manifold vacuum (like you should be), then those 2 places are a source for a leak.

Have you checked your float levels? Maybe temporarily swap out the brass sight plugs with the clear plastic ones and see what is going on in a safe way (no spilling gas on the manifold). When they have served their purpose, remove the clear plugs and put the brass back in, as the plastic ones get brittle and break-off when you try to remove them after extended use. That won't happen in a few days.

Not sure how slightly open secondary throttle would cause a problem, if anything it would add fuel and increase idle speed, tending to smooth out engine if the float is right. Of course, if that particular secondary carb is lean (low float) that speed effect would be less, but the problem would still be an incorrect fuel level.

The gaskets appear to be OK, not original Holley brand stuff. The center one seems to be on correctly and covering the PCV valley.

The brass sintered filters in the carb inlets? If clogged or fuel delivery issue, the car probably wouldn't run better at higher rpm. Probably runs better at rpm because the car is accelerating and the fuel level is increasing at the back of the bowl, acting to temporarily richen up the mixture.

 

Great to know,its been awhile since I've worked on any tri carbs,the last had bases from 3 center carbs that had been modified.
I was thinking there was some kind of bleed built in to keep the fuel from becoming stale but couldn't remember.Thanks for the info.

 

1. 67-69 tri-powers do not have metering plates in the secondaries. Fuel is "refreshed" via slots in the secondary throttle plates.

7. This would occur only if the float level was too high. With a properly set float level the fuel in the bowl can not "richen up the mixture". Clogged filters or low fuel pressure, would result in a stall under heavy acceleration due to fuel starvation and the motor would attempt to die.

Lastly, who did your carb overhaul? I've had them come back from Holley with major problems, bad cores are still bad even with new parts. What was checked? What was replaced? Did you insure the primary throttle plate was adjusted properly before you installed the center carb?
You've already said one secondary did not set flush on the manifold, how thoroughly were you able to check all around the carb bases for leaks? Where are the primary idle circuit screws set, 1 turn out, 4 turns out? Are they set evenly on each side? As mentioned previously, have you yet checked the valve train for issues?

Until the OP follows a simple, logical process with each step in proper order, he is shooting in the dark, and will continue to be frustrated by a very simple, easy to operate system.

Dan

 

Dan,

Please see my comments sprinkled above amongst your questions and some below. As I write this I can't see the original message, so give me a little slack.

Yes, you're right, you don't need the clear plugs for sure. It's a little safer I think and that's not a bad thing when the fuel level doesn't change instantaneously and it's easy to make too big of an adjustment and slop some on the manifold.

Dan, I hit the quote button but not everything showed up here for me to reply to. What I wrote about burning my laces off is true, it happened on a 427 sidepipe car. I wasn't happy with the idle and I had leaned it out significantly. I went to the 4" exhaust outlet and a fn flame fired out of there just as a set my foot down by the pipe and burnt the part of the laces off that weren't in my sneaker. The solution to that problem was to richen the mixture. My understanding of what happens is that the mixture is too lean to light off in the cylinder or too lean to burn completely (you need more advance for a lean mixture), and you end up with unburned fuel and air in the exhaust which lights off when the next cylinder in the same bank dumps it's 1300 or so degree charge into the exhaust pipe.

 

Dan,

I printed out the other points you brought up.

Dan - "2. There must be unburned fuel in the exhaust to cause a "light-off in the exhaust. This means a rich mixture with overly high temperatures in the exhaust system, not lean."

Super6 - I think I explained what I thought was happening in the post above and that a lean mixture was the cause. This car seems to have a lean condition. A rich mixture generally makes for a cooler exhaust, obviously "cooler" is a relative term. There might be unburned fuel in the exhaust, but you need air in there too, not just fuel. Where's the air coming from? Maybe this car has the AIR system hooked up. Not sure if the pictures show that.

Dan - "3. With float levels high resulting in spillage into the venturis and timing incorrect the car will car stall."

Super 6 - This comment was in part in response to my writing "Idle smoothes out when hand over carb". This is a classic test to see if the engine is running lean, you cup the carb making it difficult for air to get in, so the engine draws in more fuel. If the rpm picks up the engine is running lean. Could be low float setting or a vacuum leak. So, I'm not sure how we get from this test, which shows a lean condition, to you writing above that the floats are high and the timing is incorrect. I don't necessarily disagree with you statement in general as a stand alone statement. But I don't see it in connection with what was reported on this car. Maybe I'm putting too much into the OP's statement that the car ran well/better at high rpm. That has factored into a lot of my thinking. I don't see extreme timing isues here either.

Dan - "4. The OP claims the vacuum system is set up per OEM. A leaking chassis vacuum system will result in a lean mixture."

Super6 - I'm not sure why you commented on this. I wrote that if the distributor vacuum advance was hooked up to ported vacuum (OEM) then a hole in the diaphragm or the hose wouldn't make a difference. It would not cause a vacuum leak at idle. I also wrote to isolate the engine from the whole vacuum system by plugging the manifold tap. I don't get your comment.

Dan - "Secondary butterflys open at idle will result in a higher idle speed that can be masked by having the idle speed screw on the primary set too low. This condition does not result in "smoothing out the engine". A much deeper bass sound from the exhaust, like a hard hitting pro-stock car, is heard as the cylinders attempt to compress a very wet mixture in the cylinders. Additionally, the idle will become rougher and the car will lope. Plus, the fumes will kill you. Further, there is no "lean" or "rich" secondary mixture. They are what they are. Fully closed until the vacuum demand is there to open them, unless of course the mechanical linkage is not correctly installed and they are open all the time, which results in most of the problems reported."

Super6 - My statement was based on someone's thought that the linkage might be adjusted incorrectly and the throttles not closing all the way. In my mind, that's open a little and not some gross amount where you could have the idle speed screw out all the way and the engine is still running at 1500 rpm. How does a secondary being open slighly differ from opening the primary a little more than normal? Is someone reporting a deep bass sound in the exhaust, like a pro-stock car on this car? I didn't read that into this. You're making the argument that this isn't happening, which I agree on. What do you mean by "as the cylinders attempt to compress a very wet mixture in the cylinders". Sounds like you're thinking a secondary being open is only adding fuel to the engine and not air. Why would the mixture be "very wet"? Why would the engine get rougher and why would there be fumes now? You say there is no lean or rich secondary mixture. When you adjust the secondary float up or down you are affecting the mixture. When it's said that the idle circult on the secondary carbs is fixed, that means that you can't adjust it with idle mixture screws. That doesn't mean that the float level doesn't affect the mixture. (BTW, you can also adjust the secondary idle mixture if you change the metering plate, there a whole set of them with various primary and idle metering holes sizes.) At the end of your comments above, you say "unless of course the mechanical linkage is not correctly installed and they are open all the time, which results in most of the problems reported." How does this explain everything reported, particularly the result of cupping your hand over the center carb which results in the engine smoothing out and indicates a lean condition? I don't understand your logic here when you have repeatedly made reference to rich conditions. Maybe I'm missing the clues, I just keep seeing lean.

I'm afraid that these last 2 exchanges on this thread are turning into something other than something to help the OP, which I know we both want. I am very sure of what I wrote about the idle circuit and the metering plates and lot of other stuff, but I'm don't claim to be an expert on all this. I have had 427/435s for sometime. I too am promoting a logical investigation of this problem. I think you can see I'm a logical thinker. And yes, it is frustrating trying to do troubleshooting by remote control without all the facts or all the tests that you or I might do. Anyway, I hope I have contributed something positive here.

 

Did you always have this problem since you owned the car? Or was some change made that initiated the problem? Could you give more history? At what rpm does the engine operation improve? Does it feel normal at that point and after, or does it still seem not quite right?

Where are the sources for a vacuum leak? If you plug the manifold source, that leaves the engine itself. You said you checked the carb bases and the intake runners. Could there be an internal leak from the intake to the engine valley? Are any of the plugs oily? You said there's no smoke, so I'd say there's no blue smoke from oil consumption either. So an internal leak doesn't sound likely.

I have two sets of six pack carbs. I replaced the ones that were on the engine when I bought it, but shortly after that I had the engine rebuilt by a pro engine builder and the engine was out of the car for quite awhile. When it was back in the car, it didn't run quite right, so I took the carbs off and looked them over. On one end carb, the thin metal plate between the carb-to-bowl gasket and the metering plate gasket was missing. This carb was a new Holley carb and it came that way from the factory. So even Holley can screw up when putting one of these together.

There is a gasket between the throttle plate and the carb body, the parts diagrams I am looking at shows that the gasket is generally symmetrical so you could physically put it on either side up, but the hole pattern is not symmetrical. If the gasket is put on upside down, it might be blocking a passage that needs to be open. It's a shot in the dark and it may not matter how the gasket goes on.

On one center carb I had from another Vette, the previous owner had tightened the air cleaner lid so much that the material of the carb body under the stud for the air cleaner parted between the 2 barrels. I think you would have found this by now, but you could take a look.

Was the engine warmed up when you did the compression test? You want everything hot and expanded and the oil at temperature when you do the test.

I don't have any experience with a broken valve spring on a running engine, so I don't know what that would do exactly. With a broken spring on the intake and the valve not closing, I'd think it would backfire out the carb the first time that cylinder was fired. If an exhaust valve was open you would get a dead mixture pumped back into the intake on the exhaust stroke, that would make things lean I would think. Does that sound right? If that were happening, how would rebuilding the carbs make it worse?

Sorry, just throwing out some thoughts.

 

U P D A T E

Ok the mystery might be solved, i'll know more possibly on weds.

FROM THE CARB GUY:

The carb was flow tested on a bench, the same run like crap situation existed. It appears that there was / is an internal vaccume leak. The fuel flow was very weak & it was having a hard time pulling fuel ( running lean ) the leak appeared to be at the base of the metering block. After putting a double gasket in place due to the fact it was not completely flat / slightly warped, it seemed to smooth out exactly as it did while placed my hand previously over the center carb.

SO................it was sent out to the machine shop to be milled. It should be back hopefully in the next day or so & I can try once again.
If that is not the complete / entire remedy, I will then uplug everything vaccume related from the manifold, plug it up & check line by line for other leaks, off the PCV - DISTRIBUTER, headlamp & wiper door, etc....


So for now, i'm crossing my fingers & hoping that the "CARB" man.....nailed it !!!

S T A N D - B Y !!!

 

Still at the machine shop.............

 

OK - the good news is that milling & metering block cleared 90 % of the vaccume / lean issue. The other lies here at the green arrow. After unplugging the line off the back of the manifold & putting my finger over it / then plugging it up & blocking it off it ran 110% / like NEW god, I really dont recall what I was actually missing, it's been so long.

So where to go from here ? what would you check one by one to rule out. I'm thinking it's up front near the headlights, I hear a hissing noise when I shut off the car with the lights in the off position. When I turn them in they dont snap to attention, but not to slow either......
i guess spray around all the other lines ? ideas ? thoughts ? suggestions ? I'm going to have the timing checked over the weekend at work, since my timing light is M.I.A ........ any thing else ???

I'm so happy it wasn't anything internal or really too major !!!

Thanks !

 

Congratulations! Glad you found the problem.

I wouldn't unplug anything at this point, since you already have a hissing noise identified. I would start chasing the noise first, maybe using a small plastic funnel as a hearing aid.. If that proves hard to track down, I would then get a vacuum system diagram and start disconnecting using the diagram as a guide. More than one way to skin a cat though.

Good luck.