[Chris-Austria]

Hi!

I finally got my engine running. It's a 435cui, 4 speed, E-heads, 10.5:1 CR, 041cam, headers, msd ignition set to 17° initial and 38° total all in at 2800rpm.

My problem is that the car will idle very fast if it's cold.. about 1500-1700rpm. I cannot go back too much because it will stall. After the engine is warm enough, it will idle at 900-1000rpm and sound pretty good. Start & Stop isn't real fun because the car sounds like it will stall soon when I slowly start moving.

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 probably 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 read about the idle tubes and idle air bleeds. I have a 66 tri-power and the center carb has already little holes in the throttle plate. If I need to enlarge the idle tubes, how should I do it and can someone show me a picture of the tubes that need to be drilled?

At the moment I use 73-63-73 main jets. Doesn't seem too big because the mixture is rather lean than rich at more rpm. Maybe I'll try it with 75-66-75 later, but the idle problem is what I want to solve first.

Chris

[pfilean]

First things first. Solve the idle questions. Yes, most of the tri powers and a lot of other carbs have the little holes in the throttle plates. It helps get enough air while still keeping the throttle plates closed enough to get a good vacuum at the idle fuel ports.

Your cold idle speed may be a triffle fast but it is supposed to be about that or else it will stall. The nature of carbureters. But since you have a tri power the first thing to be sure of is that the end carbs have the throttle plates closed at idle. Otherwise the idle mixture will be lean which is someting you suspect anyway. Pull the carbs and look through the bores with the throrrle closed. If you can see light around the throttle plates you will need to get any stops (not likely) adjusted or get throttle plates that have not been nicked or bent by previous attempts to get them to close. You may have to get some DAG 213 to seal around the throttle plates. That was the factory soloution to idle problems. Get that solved first tnd then see what other problems there may be.

[Chris-Austria]

The carbs have been rebuild while the engine was at the shop. The outer carb throttle plates seem to be closed without leaks.
I can check this again and I will.. but I suspect the idle tubes to be to small. How do I enlarge them and where are they located? A pic would be really nice!

What do you think if I let the engine idle and try to spray some flammable fluid (I have a spray that over here is called "starting help spray" if I translate it directly) and if the throttle plates are not closed 100% the rpm should increase as soon as I spray into the outer carbs I guess.

[Hurst65]

Check all of your basics, first. Float levels, End carbs sealing closed, Accelerator pump rod adjustments. Etc.. Probably wouldn't hurt to double check your valve adjustment, too.

63s in the center are a great starting point. You ends are probably too rich. I'd go down to 70s.

Carefully remove the power valve spring, until you get your idle sorted out. Then you can reinstall it and go from there.

[Chris-Austria]

Will the power valve without the spring be closed or open all the time? I was told that I might need a weaker spring because of my bigger than stock cam.

Float levels are good, the mechanic double checked them and accelerator pump rods too.

I didn't rev it up really high until now, the jets in the outer carbs might be to big, that's what I will fix after the idle works.

[Old Man Taylor]

The weaker the spring the later that the power valve opens. I have a combo very much like yours. My jets are 73-66-73. I drilled out the idle tubes to 0.035" (which is stock for a 421). It probably wants to go further, but I didn't want to go too far with this stock cluster. My holes in the throttle plates are 0.120". I'm going to drill them out until it idles better, or until I get to 0.180". I have not had a chance to do that yet.

[Tom Vaught]

I have opened up the Idle tubes to .038" with excellent success on modded engine (with .150" holes in the center throttle plates. 4 bbl Holley carbs can use smaller holes because they drill 4 blades. .150" holes is a good number on a 2bbl with a decent camshaft. The .038" IFRs (Holley) (Idle Tube Restrictions on Rochester stuff) will emulate a 850 Holley primary idle circuit exactly. We are talking physics here. Given engine needs a given fuel orifice diameter and a given "air by-pass hole" diameter to idle. Be it a Holley or a Rochester carb the physics does not change to get a given proper idle air/fuel ratio or idle rpm.

Tom Vaught

[Chris-Austria]

This is very helpfull, I'll give that information to my mechanic so he can probably fix my idle problems.
If I would have used for example a Holley Street Avenger this carb would have worked well on my modded engine?

I'm curious of what main jets will be needed finally.. some have success with 60s in the center and I saw some that needed 68s.... But first I have to fix the idle

[pfilean]

Spraying some flammable fluid like starting fluid around is an old trick to find all manner of vacuum leaks. Manifold to heads or carb base gaskets. But spraying in the end carb bores to see if the idle increases may be misleading as the end carbs probably have the small holes also. My 1964 carbs did. Actually I fillled mine with DAG213 when I redid that seal and my idle is now very good. Although my engine is not built to where yours is. A lot of carb rebuilders will remove the DAG when they rebuild as they don't know why it is there and it does look like a crude attempt to fix something but the rebuilder thinks it is not needed. But it works.

[Jim Doran]

Might want to make some block off plates for the end carbs until you get things sorted out. This way you eliminate many variables for a more controlled approach to tuning the idle.
Here is what I did:

[Chris-Austria]

The block off plates are a good idea. My outer carbs don't have the small holes for air, I checked that when they were rebuild.. only the center carb has them.
We sprayed the fluid around the carb to manifold gasket and into the outer carbs.. no reaction.
It did react with the center carb to manifold gasket.. so we pulled the carb, put in another gasket and after that there was no reaction any more with the same test.

I'm pretty sure my setup was build for a 389AT because of the old jetting. Chances are pretty good that I need to enlarge the idle tubes I guess. I'll check their diameter and I want to use an air/fuel meter.. need to search for a shop that has one.

[old66tiger]

Originally Posted by Jim Doran

Might want to make some block off plates for the end carbs until you get things sorted out. This way you eliminate many variables for a more controlled approach to tuning the idle.
Here is what I did:

X2. Also, get a chassis service manual. Lots of questions answered within the pages of this book.