Inconsistant idle- what to look for?

Inconsistant idle- what to look for? - 03/18/12 05:11 PM

After fixing a 'up-until-recently-unknown' vacuum leak, I tried readjusting my idle mixture screw adjustments and I had a question.....

This is on a rebuilt stock 383 (low compression - no more than 9.5/1 and probably a little lower) with a Edlebrock 1405 that had been jetted down 2 sizes (before I did that the exhaust was pretty rich with unburned gas) Previously I had noticed that at around idle the idle was never very consistant- it would vary about 40-80 rpm randomly. I had hoped/thought that the repaired vacuum leak would remedy this but no dice, and with this going on it's a bit of a challenge to get the idle mixture adjusted.

Is there something else going on that I need to look for, or is this 'as good as it's going to get' for my setup?

Thanks!
David

Posted By: sam64

Re: Inconsistant idle- what to look for? - 03/18/12 05:14 PM

is the timing jumping around?

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 05:15 PM

good question- let me check on that and post the results here in a bit...

Posted By: roe

Re: Inconsistant idle- what to look for? - 03/18/12 05:15 PM

I had the exact same thing happen a couple of times. Once is was a really dirty carb. Just took it apart and cleaned out all the crap in it. Worked like new.

The first time it was hooked to the ported vacuum, and switching to manifold vacuum was a huge improvement.

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 05:23 PM

Quote:

is the timing jumping around?

No. It's rock steady while the idle moves around.

One thing I did notice- when the engine's started up hot, for about a half-minute the idle is pretty rock steady. When I blip the throttle though and the speed settles back to idle then it starts up again with the inconsistant idle....

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 05:26 PM

Quote:

I had the exact same thing happen a couple of times. Once is was a really dirty carb. Just took it apart and cleaned out all the crap in it. Worked like new.

The first time it was hooked to the ported vacuum, and switching to manifold vacuum was a huge improvement.

When I rejetted the carb I first cleaned the beejebus out of that carb (I had bought it new but after a year or so it built up some crud in the bottom of the bowls). I could try cleaning it again since it's sat since I cleaned it (early last fall) but I'd rather save that a last-resort ....

I had been reading elsewhere on the board about going directly to manifold vacuum- I might try that too but again I'll save it for the last resort thing (probably before the carb cleaning...)

Edit: if it makes any difference, I've got a manual trans....

Posted By: biginchmotor

Re: Inconsistant idle- what to look for? - 03/18/12 05:31 PM

What intake is the carb on? Make sure you do not have a sealing issue at the base gasket. John.

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 05:38 PM

It's a stock, cast-iron intake that was used on '69 383s. I could probably shoot some ether around the gasket flange to make sure I'm not getting a leak there....

BTW - again in case it makes a difference, I'm running a Pertronix setup (electronic distributor and ignition coil, no ballast resistor).

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 06:00 PM

Well I shot around the base of the carb with ether. I got what I think might be a reaction when I hit the back flange of the carb -I thought I saw the RPMs go up around 50rpm for a brief period. But with the idle unevenness that I'm dealing with going on I'm not even sure if what I saw is evidence of a vacuum leak or just a coincidence....

Posted By: biginchmotor

Re: Inconsistant idle- what to look for? - 03/18/12 06:12 PM

Does the manifold have the same shape/design as the base of the carb? Sometimes those spread bore carbs need that thin steel adapter plate to seal properly at the base. Had this happen on a Weiand Manifold when a 1406 Edelbrock was installed on it. John.

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 06:30 PM

Quote:

Does the manifold have the same shape/design as the base of the carb? Sometimes those spread bore carbs need that thin steel adapter plate to seal properly at the base. Had this happen on a Weiand Manifold when a 1406 Edelbrock was installed on it. John.

Good question.... I'm trying to remember what the intake manifold looked like and the more I think about it, the more I seem to think that it might be a square bore.

Can you (or anyone else out there) confirm whether the original Carters back in '69 were square or spreadbore? If they were square then it sounds like that I do need that adapter...

Edit: to double-check, with the engine off I manually opened the throttle to WOT to see if I could detect any interference with the secondaries (I'd assume there could be interference if I had a spread-bore carb on a square-bore manifold), but I didn't notice any....

Posted By: RapidRobert

Re: Inconsistant idle- what to look for? - 03/18/12 09:16 PM

Quote:

Can you (or anyone else out there) confirm whether the original Carters back in '69 were square or spreadbore?

square bore.

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/18/12 10:31 PM

Quote:

Quote:

Can you (or anyone else out there) confirm whether the original Carters back in '69 were square or spreadbore?

square bore.

I'll pull the carb to be sure but it sounds like I'm running a square peg in a round hole

Has anyone else running a Eddie 1405/6 on a stock BB intake found that they've had to get an adapter, too? If so, any brands to recommend or just any adapter will do?

Posted By: pushbutton

Re: Inconsistant idle- what to look for? - 03/19/12 01:57 AM

Quote:

Quote:

Quote:

Can you (or anyone else out there) confirm whether the original Carters back in '69 were square or spreadbore?

square bore.

I'll pull the carb to be sure but it sounds like I'm running a square peg in a round hole Has anyone else running a Eddie 1405/6 on a stock BB intake found that they've had to get an adapter, too? If so, any brands to recommend or just any adapter will do?

69 4 bbl RR intake is 4 hole dual plane. No need for adapter to bolt up Eddy.

Posted By: 70runner

Re: Inconsistant idle- what to look for? - 03/19/12 04:03 AM

sounds like an intake-crankcase leak

check your plugs to see if you have oil residue

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/20/12 02:29 AM

Wierd.... I posted a reply a while ago but I don't see it posted :/ I'll try again...

Quote:

69 4 bbl RR intake is 4 hole dual plane. No need for adapter to bolt up Eddy.

Ah ok..... well the only other thing I can think of to do on this is to pull the carb and look at the intake gasket to see if I can spot anything on the gasket that would indicate that I'm not getting a good seal somewhere....

Quote:

sounds like an intake-crankcase leak

check your plugs to see if you have oil residue

Well when I pulled the carb and cleaned/rejetted it back early last fall, I had also changed out the spark plugs since the ones I had in there had gotten rather carbon-fouled because of the rich mixture. Miracle of miracles I still had those plugs sitting around, so a while ago I got them out and inspected them. They were rather dark in color and slightly sooty, but there wasn't any oil residue that I could find on any of them....

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/25/12 05:16 PM

An update....

I hadn't been able to look at the car until this weekend. When I did, I started the car and let it warm up then expected to see the tach register the idle RPM swings that I had been reporting earlier in this thread- but it never happened. I can't say it was rock-steady but compared to what I had been experiencing previously, it was. I was still getting a variation of around 20rpms but that's a heck of a lot better than it was before. And the wierd thing is that I hadn't done anything to the car since then.

Should I call this good enough? I'm not sure if I still have a problem or not...

Posted By: David_Trimble

Re: Inconsistant idle- what to look for? - 03/25/12 05:56 PM

Ah I just found something that might be most (if not all) of the cause of this- my tach is apparently taking a dump on me. I was just out working on the car and I noticed that the tach suddenly and momentarily jumped up to over 1200 rpm - but the engine didn't though. I thought I just had a bad connection so I checked the coil and power connections and figured that was the problem, but a few minutes later it did it again.

So either my tach is dying on me or my Pertronix is. I tend to believe that it's the tach since the engine didn't vary it's speed not one bit when the tach acted up. So it looks like I might be in the market for a tach. Any suggestions for a readily-available points/dwell/tach (digital or analog) that plays well with both points and electronic systems?