67 L-71 Idle Woes

[Everett Ogilvie]

Re: SHP Big block idle problems

Duke and I have been corresponding recently about vacuum and idle issues on my own L72 (single carb, 3247 Holley). I thought (assumed) the 3247 used full vacuum for the advance, but mine almost behaves like it is ported. At idle the vacuum is 2" - at the distributor advance line. As soon as you move the throttle, the vacuum starts to come up. I still don't have the definitive answer yet whether this carb is intended to be ported or full vacuum. I am going to measure the vaccum at another source - the choke pull-off line.

Another item which can add to poor idle is the PCV valve - a "bad" one can cause rough idle, which contibutes to poor vaccum, which contributes to more PCV valve (and advance can) dithering - the same feedback loop Duke mentioned previously. On my other L72 car, when I replaced the PCV valve, the car idled beautifully, even with the stock vacuum can, so I am going to replace the PCV on this car to see the contribution to idle quality. The service manual says to remove the PCV valve from the line and to plug the line and note the RPM drop - if less than a 50 RPM drop, the valve is supposed to be bad.

[Bill Becker]

Feedback...Cams

Jerry,

When researching for the proper cam,(is that what you meant by biting the bullet)? I found specs all over the place depending on what book I was looking at, so I couldn't tell you exactly what the specs were in '67. After considerable effort decided on the 143. If I remember correctly (which is rare these days), I think Crane is manufacturing these for GM.

I'm going to try Duke's recommendation inbetween soda's this weekend, and I'll post any improvements, if any. If I have any questions, you don't mind if I call you collect, do you?

Bill

[Duke W.]

Re: Feedback...Cams

I always find that my tuneups yield better results when I drink beer.

Duke

[John M.]

Re: SHP Big block idle problems

I checked my L71 last night and it is full manifold vacuum although very low at idle, 5-7" at 750rpm. But it comes right up to 15 at 1000rpm. I guess I mixed up tripower feed with vacuum advance. Thanks, Duke for your input.

[Mark Ring]

Re: Feedback...Cams

I thought you might like to hear my experience with this same unstable hot idle issue on my new 69 DZ 302. I did what Duke recomends with the vaccum advance can (VC1810) hooked up to manifold vaccum and I recurved my 69 smog calibrated distributor for a more performance oriented quicker distributor curve with more initial advance built into it. I went from a factory 4 degrees at idle and 36 at 4400 RPM to 16 degrees at idle and the engine seems to tolerate it. I still have 36 degrees total at 2500 RPM from a sum of 16 initial plus 16 centrifugal. I have an additional 16 in the vacuum can. I had to restrict the slot in the autocam plate by brazing it shut a bit and dressing it up to remove about 1/3 of the travel as the distributor was "idling on the curve" with the softer centrifugal advance springs meaning the centrifugal was already advancing at idle which also contributes to an unstable idle. This is not the problem you are having as it would be happening even with a cold engine. If your initial timing backs off as the idle is reduced below the factory setting, you are "idling on the curve" and you should address that if it backs off more than a few degrees.

I suspect you may have fuel percolation. My car also do not have a heat riser valve and I found that fully blocking the intake manifold heat riser passage will help. I put a stainless steel shim on one side (drivers) and restricted the passenger side of the intake manifold gasket at the exhaust crossover passage. This mod combined with the recurve helped lower temps and reduce the fuel percolation problem I was having. The result was a stable idle at 950 RPM per factory specs. This is a good mod for aluminum intakes but cast iron takes a long time to warm up so if you have one of those, you might want to use an under carb heat insulator setup. It can also come from fuel additives so keep that in mind if you are using lots of it.

-Mark.

[Duke W.]

Re: SHP Big block idle problems

You're both candidates for the 8" vacuum can. Your engines are not pulling any idle vacuum because they are idling on just the initial timing. More idle advance will pull more vacuum and reduce any tendency to run hot.

Try the Echlin/NAPA VC1810 vacuum can. I think I've read that some have been able to swap the can without removing and disassembling the mag pulse dist.

The more aggressive vacuum can will cause the engine to bootstrap itself up to more idle vacuum and hold it. I'd also recommend an idle speed of about 900, but in any case, idle it high enough so it pulls at least 10" as this will cause the 8" can to be pulled to the stop at idle and stay there. Set the initial at about 10-12, and if the engine doesn't generate any WOT detonation, you have your new igntion map.

Duke

[John M.]

Re: SHP Big block idle problems

Thanks, Duke I'm headed for NAPA

[Duke W.]

Re: SHP Big block idle problems

One other issue I need to mention. By significantly changing the idle vacuum you will need to retune the carburetor.

At idle and off idle, fuel is metered by manifold vacuum, so once you have installed the 8" can and upped the initial timing, go through the idle speed and mixture adjustment procedure. The idle mixture screws should allow you to achieve a suitable idle mixture and you have to experiement to see if the off-idle mixture is suitable by driving the car.

One test I perform is to bring the revs up and down in 100 RPM increments from idle to about 2000 revs via my foot on the throttle. If you can precisely control revs with the throttle, the idle/off idle mixture is not too lean, but it might be too rich.

If the engine exhibits a bit of roughness or you can't control the revs precisely with your foot on the throttle, then it is likely too lean. Sometimes you have to end up with a compromise and run the idle a bit rich to keep the off-idle mixture out of leaness.

Duke