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

[corellian corvette]

We used flat-top pistons so I believe I'm in the 8:1 ratio, with the XE262 cam which was recommended after we couldn't get the XE274 to idle AT ALL.

For giggles, I also measured the idle mixture screw hole, which is also VERY small, .055

My guess is to move up .010 on each hole - .065 on the idle mixture screw, and .56 on the idle down channel. Those numbers look more like what I see in some of the upgrades in the book.

I've also heard when drilling the idle mixture screw, you should look at the hole in the baseplate and make sure it's not also restricted.

Does that seem like a logical idea?

[Shark Racer]

You should open it up a little bit, and you may well try that first; it's the easiest of the mods to do. I'm running mine at .090.

So my recommendation would be to start with the mixture screw holes, try that, then the DCR. I would definitely sneak up on the DCR, and not go straight to the 056, as your idle tubes are quite large.

As for the restriction - once you've drilled the hole out, it's pretty much guaranteed not to be blocked. Just clear any chips out of the way...

[blazer74]

Just a note, are u sure your measuring all the way to the idle tube restriction. unless you have drill bits 2+ inches long u may not be getting accurate measurements like shark mentioned. U can get a set of small wire drill gauges from a decent tool  store that are long enough to reach the restrictions that are like 1 21/32 down the idle tube. Taper length drill bits are longer but pricey, precision makes them.

[corellian corvette]

WE HAVE SOME PROGRESS!

There has been some dialog on this on the PY forums, and I should first correct that I went back and re-took all my measurements by doing a more thorough tear-down. I'm still not 100% sure about the idle tube, but I practiced removing one from a junk q-jet, and used that to gage whether my bit was long enough. The idle tube in the carb was .032, and obviously I could not get my .040 bit down that tube. When I put the same bit in the tube in the carb, it went down MUCH further - almost flush enough I had to pick it out. So I'm still feeling like that tube is ~ .040.

Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: None.
Mixture Screw Hole: .065"

I took a small step tonight and used a pin vice to open the idle mixture screws up to .082, and I used the same drill to put .082 bypass air holes. This felt like a good mid-step. I put the 74 jets back in, lowered the APT, and BOY did it make a difference. Timing is back down to 15 deg, idle speed screw has more adjustment, and the rhythmic vacuum and rpm drop is much reduced. I was able to get it to idle in drive, no MVA, pretty steady at 750 rpm. I still have to have the mixture screws about 5.5 turns out, and I still get a VERY mild RPM increase when I close the choke horn, but nothing like it was before. Lean idle burn smell is much reduced. Vacuum in drive is about 13hg.

New settings are

Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: .082
Mixture Screw Hole: .082


If a little is good, more must be better, right?

I think I'm going to take ONE more stab at increasing the idle speed and bypass air holes, to bring them closer to the spec others have listed; maybe use the .89 bit on both idle mixture and bypass air to see if that gives me a little bit more mixture screw adjustment and a tad more enrichment.

Frankly I could live with it right now. Would be totally fine, but I just made a HUGE improvement so it seems like one more small step and it might get me there. I'll be very happy if I don't need to mess with the idle tubes and fuel restrictors.

[Shark Racer]

I'd up the idle mixture screw size to the .089 bit and if you have a very slight up for the DCR I'd open it up just a bit.

I'd leave bypass air alone for now. How far are you opening the throttle blades to maintain the idle you want?

You probably could have gotten that XE274 cam to run at this point.

[corellian corvette]

Thanks Shark Racer!

I'll do that then, first. It's hard because on the PY string people think opening up the idle bypass is also a good idea. So hard to parse the advice.

I'm getting more freedom with the idle screw then I had before. I can get an easy 1000 + RPM on the idle screw without venturi drip (before Max I could get was ~ 850).

But idle screws are still out quite a bit, with a very light lean condition as I stated.

So you think going to .089 on mixture screw, and a very small opening of the down-channel would be better?

The PY forum recommended going to .055 on the Down-Channel, but I'm going to look at my drill-bit options and was thinking of shooting for something more in the .049/.050 range...

RE: XE274. I probably could have made it run, but it was MUCH worse than what I was dealing with on the XE262. I was also told by Lars Grimsrud and other Pontiac guys that the 274 was not a good match for my engine - sacrificing low-end torque for high-end HP, but my stock iron heads and low compression would die out before I was in the sweet spot of the cam. I already can tell this cam is better matched to my engine as stoplight performance is much better. I really had to get into it before to feel any power...

[Shark Racer]

It's much easier to go bigger than smaller, so that's why I'm recommending stepping it up a little at a time.

I went from 55 to 59 on my DCR and that took me from a little too lean to a little too rich. If I did it over again, I'd go with a 1.45mm bit which is about 57. 2 years away until my next smog test, I'll leave it alone for a bit.

As your idle tubes are already quite large, I'd be very cautious going up on the DCR.

[corellian corvette]

I think we're in good shape.

The next size drill bit I had was .052, so I opened up the DCR to .052.

To compensate, I opened the Bypass Air from .082 to .086.

Finally, I went to .089 on the mixture screws.

I think we've nailed it. Idle mixture screws are perfect at 4 turns out. I did need to crank the idle screw in just a bit to bring it up to ~ 900 rpm in Neutral, but I'm still below transfer slot so no venturi drip.

Idle holds steady in drive at 700 RPM, Vaccum is pretty steady at 14hg.

So I think I'm done at this point. I'm going to drive it around and let the engine break in, and I have a rebuild kit coming from Cliff that I'll put on when it arrives.

Thanks for your help!

[Shark Racer]

No problem at all... glad to be of help. Now come take care of mine.