# Tri-Power Question



## Chris-Austria (Dec 23, 2010)

Hi!

I finally got my engine running. But there are still some problems.
The car will idle very fast if it's cold.. about 1500-1700rpm. After the engine is warm enough, it will idle at 900-1000rpm. 
I push the pedal 1-2 times before I start it cold and I was surprised that the engine did rev up to about 4500-5000rpm the first time I started. I shut it off as fast as I could and tried again.. after that the idle was like I said above.. 1500-1700rpm until it was warm.

Also the mixture is too lean at low rpm's and gets better with more rpm. (0,2% CO first, at about 2000rpm it's 1,3%)

I probalby need to get more fuel into the manifold, but what exactly should I try now? New power valve spring because of my bigger camshaft might be needed.. will this give me more fuel at more rpm? And how can I get it to idle better and with more fuel? Pic's of the parts that need to be changed would be really nice! I'll give the car to a shop where they will try to make my tri-power work with the new engine. Any information what they should try is appreciated!!

Thank you!!

Chris


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## BearGFR (Aug 25, 2008)

There are passages in the idle circuit that can be enlarged (both for fuel and air bleed) to make it pass more fuel for idle. This is an area where you need to work in very small increments and be careful - it's "difficult" to un-drill a hole once you make it too large 

Bear


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## Rukee (Feb 8, 2007)

Didn't we just cover this in another thread??
Check the out board carbs, the throttle plates need to seal 100% to the bores, if they leak the motor will run lean as there are no idle circuits in those carbs. Mine were rusted on the inside and I had to replace the bases. Also using the wrong gaskets between the carb base and carb will cause a vacuum leak causing a lean condition. You could make a couple block off plates to put under the out board carbs to eliminate those as the problem.


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## Chris-Austria (Dec 23, 2010)

You're right, there is another thread. But the title isn't good because I wasn't sure of what kind of problem I have. I hope to hear many different opinions and helpfull information in this thread.´

To enlarge the idle tubes I will need a specialist I guess, if I would have a picture of the part that needs to be enlagered it would help a lot. In the throttle plate of the center carb are already 2 small holes drilled to provide more air. They look stock.

The outers have been rebuild and checked, but the block off plates are a good idea to verify if they seal or not. Thank you!


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## geeteeohguy (Feb 2, 2008)

Also sounds like you need to adjust your fast idle speed screw and make sure the cam is not hanging up. It's supposed to idle at 12-1500 rpm briefly, but if you're like me, you cringe at the prospect of a stone cold engine racing to the moon.


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## Rukee (Feb 8, 2007)

Eliminate everything thing else before drilling out any passages!!
I would start with the block off plates and then adjust the idle mixture screws. What your doing there is trying to acheave the highest RPM's with the lowest idle screw setting. Adjust the screws, then turn the idle screw down, then adjust the mixture screws and keep repeating untill it's right.


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## BearGFR (Aug 25, 2008)

Well, I'll tell you what.

Chris sent me via pm a link to a video of it running, and it sounds pretty healthy to me!

Could be, Chris, that you and your mechanic just aren't accustomed to what a cam with some attitude sounds like 

(I won't steal your thunder Chris by posting your link - I'll save that for you 

Bear


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## Chris-Austria (Dec 23, 2010)

If that is the sound it will have I'm happy with that 
But when it's cold it won't run good and if warm there should be a bit more fuel in the mixture at idle and maybe even at higher rpms. 1,2% CO is not very much but the mechanic said it's okay.. the 0,3% at idle is too low.

The video isn't very good but if it helps.. I can share it with you!





Thats how it idles when it's hot.. On the street it isn't very easy to start&stop.. I hope this will also get better after everything is adjusted.


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## Chris-Austria (Dec 23, 2010)

When do the main jets start to work? (rpm)
I compared the old 56 main jets vs the new 64 jets and the 56 are bigger!! Maybe someone did enlarge them.. I guess I need bigger main jets, maybe 68.
That probably won't fix my idle problem.. but smaller jets than before with the more radical engine now might be another problem. 

How does the idle system work? It sucks fuel with it's vacuum from the manifold? If there is not enough vacuum will a vacuum canister help? Drilling bigger holes is not what I want to try first because like Bear said.. it's hard to undrill them and I do not want to destroy my center carb (it's almost impossible to get a new one).


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## Chris-Austria (Dec 23, 2010)

Are the "idle tubes" in the venturi cluster? If anyone could show me a pic of them I'd really appreciate that


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## BearGFR (Aug 25, 2008)

Hi Chris,
You can approximate by thinking about "rpm", but what actually makes all the various circuits in the carb work is a combination of engine vacuum and air flow velocity through the carburetor.
Engine vacuum comes from the action of the pistons "pumping", trying to "pull" against the (mostly) closed throttle plates in the carburetor. How hard it "pulls" is a combination of rpm, engine displacement, and cam timing (duration) --- all the things that go into determining how big the "pump" is and how efficient it is. When you snap the throttle open, vacuum immediately drops to very near zero (and stays there) because you just removed the restriction - the engine is now able to breathe in all the air it wants. As rpm starts to increase, the velocity of air moving through the carburetor(s) also increases. This velocity creates a different "kind" of vacuum in the carb. At idle with the plates closed, the carb "sees" vacuum only below the throttle plates - this is where the fuel outlets for the idle circuit are and the amount of fuel and air available there is regulated by the idle screws (not the main jets) - below the plates. With the plates open, the venturi effect due to flowing air creates a "vacuum" higher up in the carb - where the shaped venturis are - and this pulls fuel through the main metering circuit via the main jets. The main jets begin controlling the amount of fuel that gets fed.
(The purpose of the accelerator pump is to provide a squirt of fuel to help "carry" the engine through the brief transition period that happens immediately after the throttle is opened but the air flow velocity hasn't picked up enough yet to cause the main circuits to start feeding fuel.) The "power valve" system is also connected so that it "sees" engine manifold vacuum. Under normal conditions, there's enough engine vacuum (even at cruise) to hold the power valve closed, but under heavy load (like wide open throttle, or climing a hill, etc) when manifold vacuum drops below "a certain point" there's a spring inside the power valve that will push it open and allow the main circuit to feed more fuel. The power valve is sort of like another main jet that can be turned on and off according to load on the engine.

I haven't found a great diagram of the parts yet, but if you'll look at this page:
Carburetor Factory - Rochester 2G - 2GC - 2GV Exploded View

The idle tubes are the two small, "outer" tubes that are pointing downwards from the underneath side of the venturis (part number 59 in the picture).
It's hard to explain and I haven't found a good visual reference yet, but the carb, venturis, tubes, jets, and fuel bowl are all "arranged" so that at idle fuel + air is being fed only through the idle tubes and out the idle ports below the throttle plates. The amount of fuel and air combined is controlled only by the idle screws. The ratio of air to fuel is controlled by the combination of the orifice size at the bottom of the idle tubes (the tapered part in the diagram) and the size of the idle system air bleed holes (also located on part 59 but you can't see them in the diagram). As air flow increases and the main venturis start to work and feed fuel, it's designed so that the idle tubes "turn off" and fuel is fed via the larger two inboard tubes to the outlets in the venturis, and the amount is contolled by the main jets (and the power valve).

Bear


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## Chris-Austria (Dec 23, 2010)

I start to understand how this works! Great explanation!!
Do you think I could get a new venturi cluster if something happens when we try to enlarge the idle tubes? 

I also looked at the old main jets some hours ago and I found something interesting. The center used 56 jets before. I put in 63's. I also have 64's and compared the 56 to the 64's and the 56 seem to have a larger hole! Maybe someone did enlarge them in the past? I assumed that I now need bigger main jets than before but if the 63's are smaller than the 56's before either the old ones have been way too big or the new ones are too small. I guess we will have to find out which main jet size will provide the best air/fuel ratio and if I need another spring for the power valve. Otherwise it won't open at the correct time or won't open at all?


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## BearGFR (Aug 25, 2008)

Close.
On the power valve it's the spring that OPENS it. Manifold vacuum is what holds it closed.

When looking at jets, looks can be very deceiving - you must actually measure them. One of the best ways is to use a set of good drill bits in the appropriate size range (about .040 to .070 inches) and use the shank end of the bits to measure. When you find the largest one that will fit through the jet, you're there.

Please be careful with the drilling if you decide to try that. You'll need a set of wire drills for that (something like these: Twist Drill Bits | Wire Gauge Drill Bits - Twist Drill Bits | Wire Gauge Drill Bits) and also a pin vise to hold them. With drills this small, you always want to drill by hand with a pin vise - don't try to use them with any kind of power tool. 
Pin Vise - Amazon.com

Once again, find the largest bit that will fit into the existing opening, then move up SLOWLY. Unless you've got a recommendation from a real expert who knows all the ins and outs of modifying these carbs to work on larger engines like yours, you'll be in trial and error mode. Enlarge the tubes maybe one or two bit sizes at a time, put it back together, run the engine --- until you find the "happy place".

I don't know if replacement venturi sets are available or not, but even if they are they probably won't be of the same quality as the originals. I'd look at them as a "last resort only" option.

Bear


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## Rukee (Feb 8, 2007)

Idle circuits could be clogged, again, use block off plates on the outboard carbs and then adjust the idle mixture screws. If you get no response from the screws or even just one of them, the carb may need to be disassembled and boiled out. When I rebuild them I leave them in the carb boil out at least over night, then spray carb cleaner through the little tube attached to the can through every passage followed by compressed air. I've seen some realy messed up Tri-Power carbs in my days, nothing suprises me anymore.


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## Chris-Austria (Dec 23, 2010)

I'll search for such little wire gauge drill bits over here and measure everything. Before I'll try to enlarge the idle tubes I want to make sure everything else is correct like the end carb throttle plates. They have been checken, but I'll check them again just to make sure I didn't miss something. Thank you for that links!
The venturi clusters looked like new when I got them back and the carbs have bee reassambled, but maybe they are clogged. I will check this again too.
Hope I can get my tri-power to work with the rebuild engine, would be really nice to keep it


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