Tri-Power Manifold Function

[Thomas S.]

I pulled the 3 carbs off my 67 427/400 to replace the fuel bowl screw gaskets that had developed leaks. I've hade the carbs off before when I replaced the center carb that had been re-stamped. I opted to buy a set of all three from a well known rebuilder in central NJ. Car runs fine an no issues per se. I'm running Sunoco GTX 98 (no ethanol) and it is tinted green.

When I pulled the carbs, the center void underneath the center carb was clean (no residue). I know this is the mostly active carb. The areas in the voids under the front and rear carbs had a lot of green dye residue. I don't beat the car and highway driving 65-70 MPH maybe once or twice a week over several miles. I've never gone full throttle.

The mechanical linkage that ties the three together is correct as per Chevy manual and the vacuum lines are tight. With only 45K on the OD, and low historical use based on prior owner data, I only drive it less than 1,500 miles per year.

Are the outboard carbs active at all under normal street driving conditions? I would think not. I would also assume that there has to be some level of activity or the fuel would never leave the bowls and gum up.

Comments

[Mark F.]

Tom,

Although for the life of me I can't find a reference to this, but my "recollection" is the secondaries don't start to open until roughly 2,000 rpm...so "normal" street driving would not have them opening up all the time...

Duke Williams will know for sure...hopefully he'll see your post and have a better answer.

[Thomas S.]

That would imply that there would be fuel through both outboards which makes sense. A hard pull in 1st or 2nd would trigger that. I left the background which led up to this so here it goes.

Changed the rear spring end bushing over the weekend. The last I did this was on my 63 and on my back under the car. This time around I have 2 different lifts, a Mohawk and a 4-post with a sliding jack. I used the 4-post and jacked up the rear using the rolling jacks. then a pole jack under the spring. Real easy job compared to the last time.

I finished up and wanted to take the car our to settle the suspension and it wouldn't start. It always starts right up. Tried again, same. each time a short fire-up and stall. Pop the hood, gas leaking from the lower 2 screw bolts on the center and front carbs?

I then remembered that there was potential issue with Tri-Powers if they were parked on a hill. Having the rear up in the air for about 2-hours would replicate that scenario and having a full tank of gas would probably make it worse.

Anyway, that's what got me to the point of removing the carbs and seeing the gas residue in the manifold. However, That doesn't explain why the area below the center carb was clean. At that point I was also concerned about fuel lock and pull all the plugs and checked the oil for fuel. Oil was good but the plugs were really bad which of course would not be related to any short-term flooding.

All of this is very strange as the car runs great, no smoke at startup, no excessive oil usage, etc. To be continued.

[Rich G.]

What I remember being told (or read) when I had my 427/435 1968 was that the vacuum secondaries open at around 3500 rpm and WOT. Performance seemed to confirm that. The mechanical linkage is there to pull the carbs closed when you get off it.

Maybe it’s different on the 400 hp set up. I defer to Duke.

Rich

[Thomas S.]

3500 would make sense. My question was if normal air flow through those carbs (above idle) cause any fuel to make its way into the manifold. In theory the vacuum mechanism controls those butterflies so the answer would be no, but does that translate into the fuel in those bowls just sitting around or venting when you turn off the ignition?

I suppose the same question would apply to the secondaries on a 4-barrel although I don't want to go off topic.

[Duke W.]

The end carb butterfly valves BEGIN to open when the vacuum motors get enough VENTURI vacuum signal to begin opening them. With the OE vacuum motor springs they probably begin to open, WOT, at about 2000, but may not be fully open until much higher revs if they reach fully open at all.

A simple test to determine the approximate revs when the end carb butterflies begin to open at WOT is to find a safe place and get the speed down to 1000 revs or less in 3rd or 4th gear. Then floor the throttle, and when the end carb butterflies begin to open you will notice a significant increase in induction roar.

A good test would be to remove the vacuum motor signal hose of one of the end carbs, connect a Mighty Vac and start pumping down. Watch the throttle plate and note the vacuum value when the butterflies begin to open and the vacuum value when they are fully open.

Then do a road test. Tee in a long enough vacuum hose to reach the cockpit and connect a vacuum gage. Then do the above road test and note the revs when the vacuum motors see enough vacuum to start opening the butterflies, and rev it as high as you want and check the vacuum. Is it enough to fully open the butterfly valves? Given the low miles I see no problem going to 5000, but you might want to do that test in second gear.

The end carb opening mechanism is essentially the same as on a Holley four-barrels. LIke the four-barrel secondaries, the end carbs don't have accelerator pumps, so if they open too quickly the engine could bog due to a transient lean condition because the heavier fuel can't keep up with the lighter air's sudden increase in flow. You can adjust the rate of opening by using different springs that I expect Holley still sells, but keep in mind that the Chevrolet calibration engineers spent plenty of time experimenting with different springs at the proving ground in order to select what they considered to be the best spring.

I recall reading the introductory article for the new '67 Corvette models in Corvette News back in the day, and it described the 3x2 system as "very fuel injection like"... well, opinions may differ.

Like the secondaries of a four barrel, the end carbs always have a little air flowing through them which draws in a little fuel from the bowls in order to keep fuel in the end carb bowls from getting "stale". The end carbs, like the secondaries on a Holley four-barrel have an adjustable throttle stop. It's very tough to reach with the carbs installed, and it should always be checked when the carbs are off. Most of the carb adjustments are in the CSM, but you need a COM too, and I believe the secondary throttle stop adjustment is in the COM, so you need both manuals.

Back in the day there was something called an "Italian tuneup". Back then with the typically rich mixtures, big engines that rarely saw WOT could foul plugs and develop carb deposits if the secondaries of a four barrel or the end carbs on trips hardly ever got opened, So even with the slight air and fuel flow at less than WOT, evaporating fuel could leave residues, and that's probably the source of the green stain, which is the dye from dried up fuel, but it probably does no harm.

So my recommendation is to do an occasional "Italian Tuneup". Find a safe place to take it through at least the first two gears at WOT to at least 5000. Given the low mileage I don't think you can do any harm. The con rods probably have not even reached 20 percent of their fatigue life. Every time I drive one of my vintage cars I try to find a safe place in the drive to rev it to or near the redline and get the rear to slide out a bit with some throttle induced oversteer. Otherwise, it hasn't been a fun drive.

Duke

[Thomas S.]

Always good news when Duke replies and thank you. Lots of good suggestions as usual.

BTW, don't let my last name fool you. I'm 98% Italian (DNA tests) so an Italian tuneup should be second nature.

Tom

[Danny P.]

That would imply that there would be fuel through both outboards which makes sense. A hard pull in 1st or 2nd would trigger that. I left the background which led up to this so here it goes.

Changed the rear spring end bushing over the weekend. The last I did this was on my 63 and on my back under the car. This time around I have 2 different lifts, a Mohawk and a 4-post with a sliding jack. I used the 4-post and jacked up the rear using the rolling jacks. then a pole jack under the spring. Real easy job compared to the last time.

I finished up and wanted to take the car our to settle the suspension and it wouldn't start. It always starts right up. Tried again, same. each time a short fire-up and stall. Pop the hood, gas leaking from the lower 2 screw bolts on the center and front carbs?

I then remembered that there was potential issue with Tri-Powers if they were parked on a hill. Having the rear up in the air for about 2-hours would replicate that scenario and having a full tank of gas would probably make it worse.

Anyway, that's what got me to the point of removing the carbs and seeing the gas residue in the manifold. However, That doesn't explain why the area below the center carb was clean. At that point I was also concerned about fuel lock and pull all the plugs and checked the oil for fuel. Oil was good but the plugs were really bad which of course would not be related to any short-term flooding.

All of this is very strange as the car runs great, no smoke at startup, no excessive oil usage, etc. To be continued.

Don't know what your trying to do , have you had any work done to the Carbs. have you every setup the 3 carbs floats levels and air mixture, there should be no leaks from any of the Carb screws and not overtighten them ether there are gaskets on every fuel bowl screw there also Nylon gaskets you can use, by the sound of the spark plugs that are foul you have a run rich setup and maybe flooding needle and seat issue, attach are my Tri power set up on my 67 L71 Duntov.

[Thomas S.]

Original carbs were in good shape and had past Top-Flight in 2023 right before I bought the car. The center carb had a restamp and I don't know why that got past the judges. In any event, I opted to get a second full set that was restored and test by Custom Rebuilt Carb in Middlesex, a great and reputable shop. I went though all of the correct procedures setting up the linkage, checking the float levels with the site plugs removed. Also changed the fuel lines because the brass fittings did not have the correct shape or marks and the lines were not the correct material. So, yes the setup is correct and the car has been running very well since then which was summer of 2024. One change I made was to go with manifold vacuum instead of ported and that was based on Duke's writings.

As I originally wrote, all was well until I had the rear jacked up for the spring bushing replacement. Maybe a 15-degree angle for 2-hours. It never occurred to me that the head pressure from a full tank of gas could cause flooding past the needle valves but according to some posts, parking on a hill, or in my case elevating the rear, can indeed cause that problem. The fact that the car would not start led me to pull the air cleaner and that was when I notices gas pooling on the top surface of the manifold. Using a flexible bore scope, I was able to determine that the leak was from the bottom two bowl screws. I could actually see them developed a drip. The same was occurring on the front carb but much slower. Tightening the screws did not help so I pulled the carbs.

All 3 carbs now had evidence of leaks from the bottom 2 bowl screws. The condition of the inside of the manifold under each carb was as I previously noted. Not knowing if any fuel had leaked into the cylinder, I pulled the plugs and blew compressed air into each cylinder. I noted the conditions of the plugs in my post.

At this point, I will put in a new set of plugs, replace the fuel-bowl screw gaskets only, and tighten them to the recommended torque. Fire it up and see what happens. As I said the car runs great and I will double check the plugs after a full tank of gas and see what happens.

The only other issue I would bring up, which has been a problem, is the tendency to stall if I come to a real sudden stop. I thought that might be related to the float levels but every thing is to spec.

Thanks for writing.

[Timothy B.]

Thomas, the secondary carbs should have very small constant feed holes under the transfer slots which allow a very small amount of fuel to pass under normal driving conditions. I would replace the plugs and go up in heat range for the soot problems. If the bowl screw gaskets solve the leak try to lower the floats some and see if that helps the stalling at stop lights. I have found the nylon gaskets can leak so use the original type screw gaskets on the bottom screws and keep the nylon for the top screws.

[Danny P.]

Original carbs were in good shape and had past Top-Flight in 2023 right before I bought the car. The center carb had a restamp and I don't know why that got past the judges. In any event, I opted to get a second full set that was restored and test by Custom Rebuilt Carb in Middlesex, a great and reputable shop. I went though all of the correct procedures setting up the linkage, checking the float levels with the site plugs removed. Also changed the fuel lines because the brass fittings did not have the correct shape or marks and the lines were not the correct material. So, yes the setup is correct and the car has been running very well since then which was summer of 2024. One change I made was to go with manifold vacuum instead of ported and that was based on Duke's writings.

As I originally wrote, all was well until I had the rear jacked up for the spring bushing replacement. Maybe a 15-degree angle for 2-hours. It never occurred to me that the head pressure from a full tank of gas could cause flooding past the needle valves but according to some posts, parking on a hill, or in my case elevating the rear, can indeed cause that problem. The fact that the car would not start led me to pull the air cleaner and that was when I notices gas pooling on the top surface of the manifold. Using a flexible bore scope, I was able to determine that the leak was from the bottom two bowl screws. I could actually see them developed a drip. The same was occurring on the front carb but much slower. Tightening the screws did not help so I pulled the carbs.

All 3 carbs now had evidence of leaks from the bottom 2 bowl screws. The condition of the inside of the manifold under each carb was as I previously noted. Not knowing if any fuel had leaked into the cylinder, I pulled the plugs and blew compressed air into each cylinder. I noted the conditions of the plugs in my post.

At this point, I will put in a new set of plugs, replace the fuel-bowl screw gaskets only, and tighten them to the recommended torque. Fire it up and see what happens. As I said the car runs great and I will double check the plugs after a full tank of gas and see what happens.

The only other issue I would bring up, which has been a problem, is the tendency to stall if I come to a real sudden stop. I thought that might be related to the float levels but every thing is to spec.

Thanks for writing.

First of all the secondaries don't start to open until roughly 3,000 rpm and you can control that RPM setting by changing the secondaries spring they come in colours codes , I find what works best for my carbs. setup and to pass my NCRS PV test were the yellow springs, you should replace all the screws bowel with proper gaskets or even Nylon on all Carb screw they will never leak and set up the floats up would also check your Center carb. accelerate pump for leaks and if it still leaking you have a warp metering plate or housing body, would like to see a photo of your Tri power set up .

[Thomas S.]

Photo to follow, but right now, everything is on the bench. I filled each carb with fuel and let the sit overnight on a paper towel and the only leaks are occurring are from the two bottom fuel bowl screws of each carb. The center carb is the most significant. While the problem showed up after the rear of the car was up for about 2-hours, I think I can now chalk that up to coincidence.

Since all the plugs are out, I'm going to do a leak down test on all the cylinders because the plugs were very badly fouled, a puzzle, because the car great.

[Thomas S.]

First of all the secondaries don't start to open until roughly 3,000 rpm and you can control that RPM setting by changing the secondaries spring they come in colours codes , I find what works best for my carbs. setup and to pass my NCRS PV test were the yellow springs, you should replace all the screws bowel with proper gaskets or even Nylon on all Carb screw they will never leak and set up the floats up would also check your Center carb. accelerate pump for leaks and if it still leaking you have a warp metering plate or housing body, would like to see a photo of your Tri power set up .

Images showing linkage setup prior to clips, retainers, etc. Not running ported vacuum.

[Thomas S.]

Found this in the archives. I was wondering why the Tri-Power adjustment was missing from 67 chassis manual - now I know.

Does anyone know the source of the information shown halfway down in the old post. It appears to be out of some GM publication. Hats off the the forum in general this information is nowhere else to be found.


https://www.forums.ncrs.org/forum/te...-idle-circuits

[Grahame M.]

I have a 69 L68 and was told to get a copy of this booklet. This one is for 68-69 Tri Power cars. Some of the pages look like they've come from this booklet. It has come in handy but the secondary rod adjustment for the rear rod still doesn't make sense to me, i thought it should go past the hole to force closure.

Cheers
Grahame