Tri-power Carb Tuning Tricks on My 650HP 496 Build
03-14-2010, 12:34 PM
#1
Racer
Thread Starter
Tri-power Carb Tuning Tricks on My 650HP 496 Build
It has seemed like an eternity but my engine finally made it from the dyno to engine bay. I took the time to detail everything under the hood so it looks like a restored car under there now and I am really happy with the finished product.
I can't say as much for the tuning process to get this thing to run right on the street. I always find the carb tuning on the dyno is way off from what it wants on the street. This could not be more true than with a tripower where the dyno air horn doesn't fit, the air cleaners aren't on during testing, and the air temp and carb heat are far from what happens under the hood of a running machine.
I have been through hell and back getting it right and am just about there but I learned so many interesting and valuable idiosyncrasies about this set up that I thought I would pass them on to save some other enthusiasts alot of time. There aren't many of these on the street any more and alot of myth and false information surrounds them. Hope to clarify a few points and give a reasonable blue print to making a tripower work well on the more and more common big inch (500 ci+) big blocks trolling the street. Keep in mind my car is a 69 and the 67 version tripower may exhibit some other characteristics due to a different manifold and higher hoodline.
1. Documentation errors - in anything you read from holley or chevrolet it will say the factory jetting is a 62, 63, or 64 on the center carb. Most say 62 manual and 64 auto. Most new versions of this center carb come with a 63 jet as a compromise. So it will depend on whether you have an original carb or one of the new ones holley makes now. The BIG problem is on the end carbs. ALL the documentation I have ever seen states an equivalent 76 jet size for the metering plate in the end carbs. THIS IS NOT CORRECT. What THEY MEAN is the orifice size is .076 in diameter. This corresponds to a 70 or 71 jet approximately. A 76 jet is actually .082 in area and WAY to large, period. This becomes very important if you convert the original jet plate to an aftermarket plate that uses regular jets ( I HIGHLY RECOMMEND). If you use the aftermarked jet plates, check float clearance to the plate VERY carefully as sometimes they will catch on the jets and flood everything.
2. Many people do not realize that there is an idle circuit in the end carbs. It is a fixed circuit and is not adjustable. BUT if you change to end plates that have replacable jets, MAKE SURE AND CHECK the size of the idle transfer hole in the new plate. The factory hole in the metering plate is .036. Most aftermarket plates are smaller. OPEN this hole to the factory size or you will have idle circuit problems (too lean).
3. DO NOT attempt to increase idle circuit fuel by opening the hole in the BASE PLATE of the end carbs. Trust me, I KNOW. The factory hole is .022. LEAVE IT ALONE as if you open it at all the increase in fuel flow goes up DRAMATICALLY and will over richen the off idle fuel mixture too much to control with the idle mixture screws on the center carb. Part throttle cruise will be very rich and you will wonder why.
4. If you modifiy the carbs by a good shop like AED pay particular attention to the idle transfer slot in the end carbs. I had my end carb base plates modified for thin blades and shafts to help carb flow. These are MUCH thinner than the factory blades. You will find afterward that the transfer slot in the base plate is much farther away from the blade. This is the acclerator pump for the end carbs when they come on. To smooth the transition of the end carbs activating and prevent bogs, lengthen the transfer slot with a small drill bit until it is close to the top of the new thinner blade when it is closed. The end carbs will come in more smoothly and with less of a lean spot on transition.
5. Despite this carb setup being used on a pretty hot factory motor back in the day, I had a big problem getting it enough idle fuel. 500 inches and a solid roller are a far cry from 427 cubes and a flat tappet solid cam. The air bleeds and throttle blades are fine. Just slowy open up the idle feed restrictions .001 at a time until it is right. Factory IFR is .034-.035. Mine needed about .039 to get good idle control with 1-1.5 turns on the screw. You should have no more than .040 of the idle transfer slot showing at idle speed.
6. I STRONGLY recommend you set WOT fuel mixture before anything else. The reason is because the minute you change end carb jet size the fuel flow to the end carb idle circuit will change. If you have already adjusted the center carb for what you think it wants and then jet down on the end carbs for WOT you will be back to square one on the idle circuit and may have already drilled passages you can't go back on.
7. FLOAT LEVEL IS VERY, VERY IMPORTANT on this set up. Any change in float level will move the idle circuit and part throttle fuel mixture around significantly. This is because of 6 sets of barrels instead of just two. I recommend you set float level to the LOW side of acceptable for several reasons. One, the front carb gets alot of heat from the fan. If the float level is too high it will percolate on shutdown and create hardstarting problems. More importantly, you can use an increase in float level to compensate for about .5 in AFR if you need to. This comes in really handy if you are going to the track or weather changes and you need to fatten it up a bit. You can compensate for it with a two second float adjustment rather than rejetting the carbs which takes ALOT more time. Once the floats are set DO NOT change them until you have completed ALL other calibration of carb circuits.
8. I highly recommend you modify your carb air horns with weather stripping to raise the air cleaner base up about a half inch. I did another thread on this. This GREATLY improves fuel level stability and removes a very pesky problem of bowl vent interference that will send the AFR all over the place.
9. Getting the fuel lines to seat is another real pain. I recommend you get the small lines that connect to the carb bowls tight and dry and then DO NOT loosen them. Disconnect the carbs at the fuel blocks side and remove the carbs with the shorty fuel lines attached. Do not loosen the fuel line that connects the two fuel blocks together either. When you remove the carbs work from the back to front. Disconnect the back carb fuel line at the block, then the center, then the front. Pull the fuel line away from the carbs and leave it there. When you reconnect, set the front carb on then hand thread to the fuel block, THEN tighen the carb base bolts, same for center carb. Then when you put the back one on undo the front two hand tighten lines and thread it in. Then reconnect the center and front carbs lines and tighten up evenly as you go. The idea here is very small variations in the carb location on the manifold can create leaks and real aggravation in getting the lines to thread. If you do it this way it is fast and leak free.
10. Make sure you make final idle speed and mixture adjustment with air cleaner on. It changes the air flow considerably and will richen the mixture quite a bit.
11. Believe it or not most of the time the factory jetting will be VERY close to optimum with the engine in the car, even if the dyno is much richer. My final set up actually went down a main jet from factory and up one size on the ends.
12. I STRONGLY, STRONGLY recommend you don't even attempt this task without a good wideband O2 sensor setup like the portable Innovate LM2. There are just too many variables in this systems to short cut it and you will go insane trying to get it right.
13. If you are running a big inch motor like mine you will want one of the middle strength springs on the secondary vaccum pods. THe brown spring is way to stiff. I think silver or yellow is a good selection and compromise.
14. I recommend you jet the main circuit a little lean for cool weather (14.3-14.7 AFR) and power mixture about 12.5 AFR. THen when temps warm up it will be right on and if you need a little more or little less fuel then adjust it through the float level without having to get into the whole system.
Lastly, if you can believe it, with everything dialed in this monster logged 17.5 MPG cruising at 60-65 with the tripower. This really is a great system after you get it right but it takes patience and alot of time.
Good luck.
I can't say as much for the tuning process to get this thing to run right on the street. I always find the carb tuning on the dyno is way off from what it wants on the street. This could not be more true than with a tripower where the dyno air horn doesn't fit, the air cleaners aren't on during testing, and the air temp and carb heat are far from what happens under the hood of a running machine.
I have been through hell and back getting it right and am just about there but I learned so many interesting and valuable idiosyncrasies about this set up that I thought I would pass them on to save some other enthusiasts alot of time. There aren't many of these on the street any more and alot of myth and false information surrounds them. Hope to clarify a few points and give a reasonable blue print to making a tripower work well on the more and more common big inch (500 ci+) big blocks trolling the street. Keep in mind my car is a 69 and the 67 version tripower may exhibit some other characteristics due to a different manifold and higher hoodline.
1. Documentation errors - in anything you read from holley or chevrolet it will say the factory jetting is a 62, 63, or 64 on the center carb. Most say 62 manual and 64 auto. Most new versions of this center carb come with a 63 jet as a compromise. So it will depend on whether you have an original carb or one of the new ones holley makes now. The BIG problem is on the end carbs. ALL the documentation I have ever seen states an equivalent 76 jet size for the metering plate in the end carbs. THIS IS NOT CORRECT. What THEY MEAN is the orifice size is .076 in diameter. This corresponds to a 70 or 71 jet approximately. A 76 jet is actually .082 in area and WAY to large, period. This becomes very important if you convert the original jet plate to an aftermarket plate that uses regular jets ( I HIGHLY RECOMMEND). If you use the aftermarked jet plates, check float clearance to the plate VERY carefully as sometimes they will catch on the jets and flood everything.
2. Many people do not realize that there is an idle circuit in the end carbs. It is a fixed circuit and is not adjustable. BUT if you change to end plates that have replacable jets, MAKE SURE AND CHECK the size of the idle transfer hole in the new plate. The factory hole in the metering plate is .036. Most aftermarket plates are smaller. OPEN this hole to the factory size or you will have idle circuit problems (too lean).
3. DO NOT attempt to increase idle circuit fuel by opening the hole in the BASE PLATE of the end carbs. Trust me, I KNOW. The factory hole is .022. LEAVE IT ALONE as if you open it at all the increase in fuel flow goes up DRAMATICALLY and will over richen the off idle fuel mixture too much to control with the idle mixture screws on the center carb. Part throttle cruise will be very rich and you will wonder why.
4. If you modifiy the carbs by a good shop like AED pay particular attention to the idle transfer slot in the end carbs. I had my end carb base plates modified for thin blades and shafts to help carb flow. These are MUCH thinner than the factory blades. You will find afterward that the transfer slot in the base plate is much farther away from the blade. This is the acclerator pump for the end carbs when they come on. To smooth the transition of the end carbs activating and prevent bogs, lengthen the transfer slot with a small drill bit until it is close to the top of the new thinner blade when it is closed. The end carbs will come in more smoothly and with less of a lean spot on transition.
5. Despite this carb setup being used on a pretty hot factory motor back in the day, I had a big problem getting it enough idle fuel. 500 inches and a solid roller are a far cry from 427 cubes and a flat tappet solid cam. The air bleeds and throttle blades are fine. Just slowy open up the idle feed restrictions .001 at a time until it is right. Factory IFR is .034-.035. Mine needed about .039 to get good idle control with 1-1.5 turns on the screw. You should have no more than .040 of the idle transfer slot showing at idle speed.
6. I STRONGLY recommend you set WOT fuel mixture before anything else. The reason is because the minute you change end carb jet size the fuel flow to the end carb idle circuit will change. If you have already adjusted the center carb for what you think it wants and then jet down on the end carbs for WOT you will be back to square one on the idle circuit and may have already drilled passages you can't go back on.
7. FLOAT LEVEL IS VERY, VERY IMPORTANT on this set up. Any change in float level will move the idle circuit and part throttle fuel mixture around significantly. This is because of 6 sets of barrels instead of just two. I recommend you set float level to the LOW side of acceptable for several reasons. One, the front carb gets alot of heat from the fan. If the float level is too high it will percolate on shutdown and create hardstarting problems. More importantly, you can use an increase in float level to compensate for about .5 in AFR if you need to. This comes in really handy if you are going to the track or weather changes and you need to fatten it up a bit. You can compensate for it with a two second float adjustment rather than rejetting the carbs which takes ALOT more time. Once the floats are set DO NOT change them until you have completed ALL other calibration of carb circuits.
8. I highly recommend you modify your carb air horns with weather stripping to raise the air cleaner base up about a half inch. I did another thread on this. This GREATLY improves fuel level stability and removes a very pesky problem of bowl vent interference that will send the AFR all over the place.
9. Getting the fuel lines to seat is another real pain. I recommend you get the small lines that connect to the carb bowls tight and dry and then DO NOT loosen them. Disconnect the carbs at the fuel blocks side and remove the carbs with the shorty fuel lines attached. Do not loosen the fuel line that connects the two fuel blocks together either. When you remove the carbs work from the back to front. Disconnect the back carb fuel line at the block, then the center, then the front. Pull the fuel line away from the carbs and leave it there. When you reconnect, set the front carb on then hand thread to the fuel block, THEN tighen the carb base bolts, same for center carb. Then when you put the back one on undo the front two hand tighten lines and thread it in. Then reconnect the center and front carbs lines and tighten up evenly as you go. The idea here is very small variations in the carb location on the manifold can create leaks and real aggravation in getting the lines to thread. If you do it this way it is fast and leak free.
10. Make sure you make final idle speed and mixture adjustment with air cleaner on. It changes the air flow considerably and will richen the mixture quite a bit.
11. Believe it or not most of the time the factory jetting will be VERY close to optimum with the engine in the car, even if the dyno is much richer. My final set up actually went down a main jet from factory and up one size on the ends.
12. I STRONGLY, STRONGLY recommend you don't even attempt this task without a good wideband O2 sensor setup like the portable Innovate LM2. There are just too many variables in this systems to short cut it and you will go insane trying to get it right.
13. If you are running a big inch motor like mine you will want one of the middle strength springs on the secondary vaccum pods. THe brown spring is way to stiff. I think silver or yellow is a good selection and compromise.
14. I recommend you jet the main circuit a little lean for cool weather (14.3-14.7 AFR) and power mixture about 12.5 AFR. THen when temps warm up it will be right on and if you need a little more or little less fuel then adjust it through the float level without having to get into the whole system.
Lastly, if you can believe it, with everything dialed in this monster logged 17.5 MPG cruising at 60-65 with the tripower. This really is a great system after you get it right but it takes patience and alot of time.
Good luck.
03-15-2010, 12:24 AM
#2
Team Owner
Member Since: Jul 2004
Location: Redondo Beach, California
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Thanks for this posting. I'm planning on putting a 502 BB in my 70. I'd like to install the 3X2 carbs since I think they are really a part of that era. The AIM manual for the 70 even shows the 3X2 installation so, GM came really close to installing them in the 70's. I have a 67 3X2 rectangular port manifold that I plan to use. This will mean I'll have to go with a L88 hood, since the 67 manifold is a "high rise."
I think for street use, the 3X2 is a good performance carb, since at low throttle settings, you're just cruising on the center 250 cfm carburator.
I think for street use, the 3X2 is a good performance carb, since at low throttle settings, you're just cruising on the center 250 cfm carburator.
