Thread: Tri-Power Carbs build.

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I got interested in Tri-Power setups about 30 years ago at a time when they were pretty much out of fashion (at that time except for the hardcore performance itself was pretty much out of style). Real Tri-Power carbs in buildable condition were pretty scarce so through trial and error I eventually figured out how to convert the common 2 Barrel Rochester’s for use on Tri-power intakes.

I’ve built several sets of Tri-Power carbs over the years mostly using small and large base Rochester 2 bbls and currently have 3 motors with the setups on them. As I tend to prefer the Rochester carbs I have even converted a couple of manifolds from Stromberg and Holley carbs for use with the 2 G/GC carbs.

With a renewed interest in nostalgia engine pieces there are now a few outfits that make bolt on carburetors however I still enjoy building my own. Over a couple of evenings last week I built the carbs that are going to be used on the Hemi so I thought I’d share what I do to convert the carbs for Tri-Power use. I’m only going to cover the mods required, not how to rebuild a Rochester, most kit’s come with pretty good instructions, and if you’re brave enough to try to convert the carbs chances are you’ve rebuilt a carb or 2 before anyway.

I appoligise for the quality of some of the pictures, the camera seems to be acting up a bit.

This would be a very welcome thread. I for one would love to have this info.

Thanks,

Don

Carb Selection:

I’ve mentioned before that what I have found in the past is that a relatively stock engine under 325 cubic inches will work well with 3 small base Rochesters, but once over that displacement engines tend to prefer a large base center Carb.

If you building a setup with 3 small base carbs, the center carb gets a standard rebuild with no mods other than possibly jet and power valve change. All jets and power valves from large and small base carbs interchange so having an abundance of “junk carbs around to rob the jets from can be a blessing as I know of no readily available aftermarket source for Rochester jets.

If I’m using a large base center carb I generally tent to try to find one off an engine of about the same displacement as the tri-power motor I’m building so the initial jetting will at least be in the ball park. I also like to have an electric choke so in a lot of cases I will pick up Pontiac Olds and Buick carbs when I come across them (I’m particularly fond of 400 CI Pontiac 2 bbls.

Up until I started rebuilding the carbs for the Hemi I would have said that all large Base Rochesters used the same size throttle plates….and I would have been wrong. The large base carb I had set aside for the Hemi turned out to have throttle plates that were the size normally found in the small base carbs (needless to say I pick out another carb to build). The picture below shows the difference in the in the throttle plate size.

As far as the end carbs I try to match them as close as possible especially as far as linkage and jetting is concerned.

As the center carb is the one that the engine starts, idles and runs on for the majority of the time no internal mods are necessary or even desired.

If your using a large base center carb one of the things you will note is the fuel inlet is in the front of the carb rather than on the side as is most common on the small base Rochesters. While this may not cause a problem on some intakes on others there is no room to get a 90 degree fitting in.

When this happens I convert the carb to a side inlet. The first step is reasonably simple…. drill and tap the side of the fuel inlet/filter housing for 1/8” pipe thread. You will want the hole as close to the protrusion as you can get it so that the fitting you use will not cover up the screw that holds the top of the carb on. This makes it a lot easier to remove the top of the carb in car for jet changes while you’re dialing the carbs in. Simply install the fitting using a little red locktite and that part of the job is done.

The final step is to make the plug for the original fuel inlet. There are two common styles of inlet fitting a thick one and a thin one with a small nipple. Total thickness of either is the same. Normally I start with the thinner fitting and grind the nipple off. After that I drill the center hole and tap it for a 1/8” pipe plug which I install from the inside, then to insure there is no chance of fuel seepage I fill the recess on the outside with solder. Install the plug with the new gasket from the carb kit and you now have a side feed large base carb.

It should be noted that you can no longer use the stock in carb filter, but if you’re running a good inline filter it won’t make any difference.

The outboard carbs when setup progressively normally only supply fuel/air after the primary passes a certain point (like a typical 4 barrel operates). It is very hard to get a decent idle using the idle circuits on all three carbs and even with the adjustment screw turned all the way in they will still pass a dribble of fuel and air also making it difficult to adjust the idle. Because of this the major mods to the end carbs are in the base plate, which is the elimination of the idle circuits.

The first step is to simply remove the idle screws and fill those holes with solder.

In addition to this you also need to fill the 4 holes in the top of the base plate (in the picture it looks like 5, but one is only a dark spot on the base plate from a hole in the gasket.

Just a note….. The holes you have just filled are the ones that are not even drilled on OE tri-power out board Rochester carbs.

The final step on the base plate is to insure the throttle plates are centered in the bores and close completely to prevent what is in effect a vacuum leak from the end carbs. In many of these carbs the plates need to be re-centered, simply loosen the screws and adjust the plates until they are right. Once that’s done the base plate can go into the carb cleaner in preparation for reassembly.

When you reassemble the end carbs you can delete the choke mechanism by simply removing the choke plate, linkage and fast idle cam…… while you’re at it the idle adjustment screw can also be removed. In order to prevent unfiltered air from entering the holes from the choke shaft, I normally tap the holes with a 5/16 NC tap, install a couple of bolts with a little red locktite and then use a cut off wheel to cut them flush with the housing.

DO NOT REMOVE any of the accelerator mechanism. The accelerator pump shot is required when you smash the gas to prevent a lean out condition.

Power valve:

The final step which is sometimes optional is disabling the power valve. On OE outboard tri-power carbs there was no provision for a power valve. The location for it wasn’t even drilled or tapped.

Personally I normally just jet the end cabs down a little bit and leave the power valve intact however you may find that the car still runs too rich with the end carbs opened and wish to disable it.

The power valve is located in the bottom of the fuel bowl and is normally spring-loaded closed with an internal spring. When the carburator is assembled the spring loaded rod in the top of the carb sits above the power valve. Under normal conditions the when the engine is running the rod with the spring is pulled up with vacuum. When manifold vacuum drops the spring pushes the rod down opening the fuel passages to enrich the fuel mixture.

As you can see from the picture of the power valve, the threads are located on the bottom of the valve so drilling and tapping the top of the hole is the only way to actually plug the hole. The alternative that I normally use is I have to eliminate the power valve is to leave the valve in place and cut the rod in the top of the carb off and remove the spring. Do not remove the upper portion of the rod that sits the top of the carb as this will create an internal vacuum leak.


That’s it for the mods, progressive linkage is pretty straight forward and I’ll get some pictures up of that tomorrow. The final step is just dialing the jets in and that is all pretty straight forward just a little time consuming.

Wow. thanks Mike. I just found out Edelbrock is making a tripower manifold for the Vortec Chevy, and I have a few of these small base rochesters I have been saving to do this with. So the T will be gettin trips.

Great, timely info.


Don

"......So the T will be gettin trips........"

If you run nto any questions when you get into it give me a shout.

When I get to work tomorrow I am going to print this thread (my home printer took a dump). This is really good info. I bought a bunch of small based Rochesters over time on Ebay and am going to use one set on the Olds in my '39 and now I can do a set for the T. I didn't know what the procedure was to modify them, so I was either going to buy the base kits from Speedway (expensive) or just screw in the idle screws on the end carbs. Like you said in your thread, that isn't the best solution though.

This info will really help a lot of guys, especially with the popularity of multiple carbs coming back as strong as it has. You can still pick up these carbs cheap (I paid between $ 15.00 and $25.00 each) so a guy can put a tripower set together pretty reasonably.

For economics I am going to buy one of the Eastwood kits (I think I will) to dye the carbs. I would like to send the carbs to the place my Son sent his, but they charge $ 450.00 for the 3 to rebuild them and dye them. Not a bad deal when you think about it, but it doesn't fit my budget goals on the T.


Mike, thanks again for this very good information.

Don

Thought I'd also post a couple of pictures of linkage that I've built. Personally I'm not to impressed with the aftermarket linkage I've seen, JMO but I think it looks too big and bulky.

The first is probably the simplest, A rod from the primary carb to the front carb and then linkage from the front to open the rear. You want an adjustable end on the outboard carbs to fine tune it. It's right when both carbs are full closed and there is no slack when they open.

The second picture is what I'm using on the Hemi, and just consists of a seperate linkage to each carb. It's adjust properly when both carbs open at the same time.

The couple of things that really need any fine tuning is the ratio between the linkage on the primary carb and that on the secondary carb(s). You want the end carbs linkage pivot point closer to the throttle shaft than the point on the primary carb. THis allows the end carbs to open faster and wider (relatively speaking) than the primary carb.

Normally on a street driven motor, I set the outboard carbs to start opening at 80 MPH on a level strech of road, and have them come in fairly quickly.

As far as Idle adjustment, you adust the center carb just like you would any 2 bbl.

GREAT info!!!..thanks a million..printing this one out,,ive been thinking of doing something different with a 69 elky i have & now this thread has my gears turning in my head to make it different...

Sometimes the idle mixture screws on the center carb can be a real bear to get a screwdriver into. The intake setup on the Hemi is a really good example of this. It can be particularly true if the engine is running a bit rough and requires adjustment.

Rather than go through the self-abuse I've done in the past, I decided to make an adjustment tool from an old flexable carb adjusting screwdriver I had laying around.

I found an old 1/4" drive socket that was the right size (after the screwdriver head on the adjuster had been gound off) was shortened and tack welded to the end of the screwdriver shaft.

The drivers side adjuster was a really tight squeeze so I also shortened the the adjuster screw in addition to removing the screwdriver head.

Works great and will be kept in the tool box in the truck of the 57.

Great thread Mike!

Couldn't have come at a better time, I can now see a set of trips in my future!

Thanks for your time and trouble,

Tom

I have an old tool made by Snap -On does the same thing. My kid got it for me probably 20 years ago it has a 1/4" drive on the end and is on about a 45deg. / Not a lot of use for it but when you need it its the best. I think it has like a speed-o cable down thru the center and a knob to turn. Nessity the mother on invention.