Can't get my rebuilt '78 400 to idle; runs great, but surges about 100 rpm

[corellian corvette]

Hey everyone! Hoping the experts here can help me diagnose a problem.

Recently had the motor rebuilt in my '78 Trans Am. It was basically a mild/stock 400 rebuild, with flat-top pistons and a Comp Cams XE262 cam. We'd struggled with some bad cam choices (XE 274) but could not get the car to idle properly and had no low end.

After the installation of the XE262 things are much better, but not perfect. We can now at least get the car to idle, but there is a distinct surge at idle of ~ 100RPM, with a pretty steady and rhythmic drop of 5 - 7 hg of vacuum (hovering around 15, going between 13 - 17). It drops down ~ 100rpm then picks right back up.

Although the car does idle OK, I have to turn the idle screw pretty high until it almost starts to drip through the ventruis. Right now I have it at about 850 RPM in neutral, with the idle-mixture screws about 5 turns out, and the APT about 4 turns up. I can't get the car to idle faster than 850 rpm without starting to open the primaries too much and making them drip. Which of course, causes another type of surging.

Carb is the original 17058278 Pontiac Carb, originally jetted with 74 jets and 45K rods, now jetted with 77 gets 45K rods to increase the idle mixture.

It feels like I've done everything right, but I can't figure how to eliminate this surge. With the 74/45 combination, if I started to cover the choke idle would increase. With the 77/45 combination that's much less, but it still seems like I really have to run out the idle mixture screws and the APT just to keep the car running, and I still have this surge. 

I'm running about 19deg initial timing (mechanical) and about 40 with full-manifold vacuum just to smooth things out. If I remove the manifold vacuum (which I am doing for testing purposes) and idle drops to ~ 700 rpm.

I'm really confused how to fix this problem. I feel like I know my way around Q-jets, but I can't figure out this surging. 13-15hg of vacuum should be providing enough signal to the idle circuit - but man I have to crank this carb to the extremes just to get idle but it still surges. 

I should point out this carb was running like a CHAMP before the engine rebuild. Of course, I had a worn and rounded cam (hence the rebuild) so I was pulling 20+hg at idle. I had no POWER, but an awesome idle

Any thoughts?

[Shark Racer]

It sounds like you need more idle fuel to me. You could have a plugged passage or the idle circuit is just too small for the new cam.

Changing jetting won't have a huge effect on idle if at all. When I change jets I frequently don't even have to touch the idle screws.

Now, when you did the rebuild did you run a wire through the idle tubes to make sure they weren't plugged?

[corellian corvette]

How would you suggest fixing that?

Thanks!

[Shark Racer]

Run some thin wire through the idle tubes to make sure they're clear. If they are clear and you still need to upsize, you'll need to pull the tubes and size them up. I would actually just get a replacement set of tubes from Cliff.

My 78 350 has a similar cam, and I actually sized the idle circuit up quite a lot to get rid of off-idle surging.

[corellian corvette]

Thanks Shark Racer

I have Cliffs book, too, but a little more specifics would make me feel better about potentially ruining my carb

I've done a ton of rebuilds but never messed with the internal passages.

Which tubes are the idle tubes again?
What size are they stock?
How do I think about re-sizing them? What size upgrade should I order?

Thanks!

[Shark Racer]

Typically from the factory they run .32-.35ish.

Mine were about .34, and I started at .36 and went to .38.

The idle tubes are in the main body, one on each side of the float well. There is a "path" that leads to the down channel restrictions. Basically two brass wells, ~1/8" diameter. The one that is mostly flush with the top of the main body casting is the idle tube, the restriction is about 2" deep and you'll likely have to remove them to change the size.

The down channel is the larger hole, about 1/4" below the top surface of the main casting.



The tubes are next to the corners of the float, the down channels just slightly forward.

[corellian corvette]

Thanks again. I'm going to ask a bunch of Q's so I don't bugger this thing up.

I was reading an article on HPP that talked about doing this, but the condition they were dealing with was the car was running too RICH.

In my case, it appears to be running too LEAN. So wouldn't opening the idle air bleeds make the carb leaner?

Again, I *really* appreciate all the help and guidance here. I don't mind tackling this at home, but I don't want to start drilling into things without knowing exactly the outcome.

As it stands now, it appears I need to somehow improve the amount of fuel the engine gets at idle, as it's running lean. How would you order the steps of drilling out the tubes to analyze this problem?

Thanks SO much!

[corellian corvette]

I should also point out, after reading several other tuning articles, it seems I need to start by upsizing my IFR (Idle Fuel Restrictors). Are those the ones in the body of the carb, or the ones that are pressed into the carb top?

[Shark Racer]

They are basically a pair.

Opening up the idle air bleeds will add emulsion air and tip the scales more lean. I can honestly admit that I don't understand why a different sized bleed has different characteristics from another. IE, you can get the same mixture with bleed A and tube A, or bleed B and tube B.

At any rate, the down channel restriction may also be called an idle fuel restrictor. It works together with the idle tubes. I would up them both in pairs. There are some recipes in the book that explain different sizes. I'd start by sizing what you have and then go up a thousandth at a time, maybe more. I ended up having to go up QUITE a lot on mine.

Considering you are over 5 turns out and still seem to be running lean, I would think you need to go up a couple steps. However, it's not easy to "shrink" a hole, so ...

[corellian corvette]

OK using Cliff's book, I went ahead and pulled the measurements for my car. My harbor freight drill set doesn't have some of the smaller size increments, so I'm going to get a better set to get more specific

Idle Tube: .046 fit, but not .052 (my next size up)
Idle Down Channel:  .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .055
Idle Bypass Air: None.

The upper air bleed seems really small based on the specs I've seen.

Any thoughts on this?

[Shark Racer]

The restriction in the idle tube is at the very bottom of the idle tube - I doubt it's .046. That's HUGE. You need to be measuring at the necked down section which is ~2" below the surface.



Pic above is from the parts Cliff offers: http://www.cliffshighperformance.dreamhosters.com/parts.html#a

[corellian corvette]

How long are those tubes? The smallest bit I have is .040, but they are super short so I want to make sure I'm getting them all the way down (note - I have not removed the tubes)

Given that I need to make the idle RICHER - what would be the best bet? Drill out the IFR (Idle down channel)?

What about blocking the Air bleeds? I notice my system has 2 - one at the top (.070), and a smaller one at the ventrui (I believe this is called the main air bleed or lower air bleed) which is very small (.055).

Could it be as simple as just blocking the upper idle air bleed which would enrichen the fuel mixture?

[Shark Racer]

You really need to size them as close as possible to determine where to go. Like I said in the last post, the tubes are about 2" long.

Personally, I'd open the IFR and tube up as matched pairs.

Cliff sells screw in idle tubes, which would probably help with speeding the process up. Prying the tubes out is kind of a pain.

You'll need a 61-80 drill bit set, and pulling the tubes will require a drywall screw, .090 punch and a pair of pliers. Probably some heat too.



I took that photo that shows all the tools I used to enlarge the idle tube restriction...

[corellian corvette]

OK thanks for that note - I measured again and the idle tubes are exactly .042. I have a .043 and it doesn't fit.

I also realized that I was measuring the MAIN idle bleed, not the LOWER idle bleed. Since I don't have the baseplate off, I can't measure it but I'm assuming it's the larger size.

These would be my new measurements:

Idle Tube: .042
Idle Down Channel:  .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .070 (assuming)
Idle Bypass Air: None.

Given that I need to make the idle circuit RICHER, it seems like I only have a few options.

1. Trying going up a size or 2 on the Idle Down Channel.
2. Try blocking the upper idle air bleed. Doesn't this mix with the fuel going down the IDC to lean it out?

It already looks like my idle tube is plenty large based on some of the "recipes" in Cliff's book - it appears it's the down-channel that severely undersized.

Make sense?

[Shark Racer]

I'm still really surprised your idle tubes are that large. I would go up, VERY slowly on the down channel restriction until you solve your problem. I might consider pulling the idle tubes anyways and necking them down to 036.

Do you know the static compression ratio of your engine?