1967 SB Idle Problem

[Tom F.]

Probably this has been addressed many times before, but here goes. My 67 300 HP is essentially unrestored, no major engine work, 44Kmiles. Everything runs fine. Original Holly rebuilt to factory specs by reputable shop about 4 years ago, low mileage since. Has been running great

I drove to the MI Regional from Ohio and after 200 miles when I got to Novi, engine would not idle. It was late so when I looked at it the next morning (cold) everything ran fine. During operations later that morning the idle was rough again (500 RPM) and cost me 5 points

A little adjustment on the idle screw brought the idle up when hot, but now a little too fast when cold. It seems when the engine reaches normal opertating temp, the idle drops off.

Any ideas? I have not yet checked vacuum when hot, cold all is well

[Jim T.]

Re: 1967 SB Idle Problem

Tom easy check for a failure of Holley carbs that does occur. The power valve is a constant moving part inside your Holley carb and when the flexibile material ruptures/fails it will not function in that engine vacuum will not close and hold the valve closed.
Anyway a quick way to check the valve is simple, remove the top of the air cleaner, then start the car and place a finger over the vertical vent tube.
If your 67 dies immediately with the finger over the vent, the power valve will need replacement.
Changing the power valve is easy to do, just buy a new fuel bowl gasket, metering block gasket, and a new power valve. I buy my replacements from Auto Zone, the individual pieces come in blister packs packaged by Holley.
You might buy some carb cleaner and spray the inside of the fuel bowl to clean it and direct some cleaner in the metering block to clean it as well.
The Holley power valve for your 327 that should work fine is 6.5. Look for numbers on the old power valve and you can use them as well to determine the power valve.

[Gerard F.]

Re: 1967 SB Idle Problem

Tom,

I have the same engine with a manual transmission, only 20k more miles than you.

I find it very hard to get it to idle down smoothly to 500 with today's gas, so I set it at 600-650 rpm. Idle and mixture should be set with engine warm.

You should be pulling about 17" vacuum. If not, check your timing. should be 6, although I set mine to 8. (non-AIR, manual trans, AIR /auto is different)

Also check the operation of the heat riser, that it is opening properly. I wire mine open, except for judging.

Another thing that causes erratic idle when hot is the exhaust crossover under the carb in the intake manifold. It tends to cook the carburetor.
If you pull the carb, you ought to plug the holes at the ends of the dogbone in the manifold under the carb.

I did mine with a couple of freeze plugs, tapped in place with a bolt:



Hope this helps,

[Tom F.]

Re: 1967 SB Idle Problem

Thanks Gang

Looks like some weekend garage therapy coming up

Tom

[Dave K.]

Re: 1967 SB Idle Problem

Probably this has been addressed many times before, but here goes. My 67 300 HP is essentially unrestored, no major engine work, 44Kmiles. Everything runs fine. Original Holly rebuilt to factory specs by reputable shop about 4 years ago, low mileage since. Has been running great

I drove to the MI Regional from Ohio and after 200 miles when I got to Novi, engine would not idle. It was late so when I looked at it the next morning (cold) everything ran fine. During operations later that morning the idle was rough again (500 RPM) and cost me 5 points

A little adjustment on the idle screw brought the idle up when hot, but now a little too fast when cold. It seems when the engine reaches normal opertating temp, the idle drops off.

Any ideas? I have not yet checked vacuum when hot, cold all is well

Tom,

I had exactly the same problem with my 66 327/300hp engine. I tied the heat riser open, blocked the manifold crossover holes. Still poor idle at below 600 rpm. Ran racing fuel which helped but not completely.

More than one person has said that even the SB's do not idle smoothly below 600 rpm with todays fuel, especially in hot weather. Much discussion in the archives on this.

Made two recent changes -- found the Holley carb floats were both too low and corrected the level and found what appeared to be sticking distributor advance weights, cleaned and relubed. Set idle around 650 rpm and last run was good with consistent and smooth idle.

Good luck.

Dave K.

[Jeff S.]

Re: 1967 SB Idle Problem

Tom

Are you aware that there is an idle fuel transfer passage that discharges idle fuel + air on the secondaries? There is a second idle set screw for the aft throttle blades located UNDER the carb on the RH side. You may have an imbalance between the primary and secondary side idle stops.

[Tom F.]

Re: 1967 SB Idle Problem

Thanks again, More things to check

[Timothy B.]

Re: 1967 SB Idle Problem

My vote is for a small internal leak that has formed probably in the secondary metering block or base because the gaskets get dry from sitting. Over tightening will only distort more, try new gaskets.

[Brian M.]

Re: 1967 SB Idle Problem

I set the idle on my 67 327/300 4 spd at 725 rpm's.

[Duke W.]

Re: 1967 SB Idle Problem

A 327/300 (ex. K-19) with a correct OE or OE equivalent replacement camshaft with either the AFB or Holley should idle butter smooth (manual trans) in neutral at 500 pulling 18-19" Hg. manifold vacuum, and 450-475 in Drive with PG at a few inches less vacuum.

If the above conditions cannot be achieved, then something is amiss!

Duke

[Dave K.]

Re: 1967 SB Idle Problem

A 327/300 (ex. K-19) with a correct OE or OE equivalent replacement camshaft with either the AFB or Holley should idle butter smooth (manual trans) in neutral at 500 pulling 18-19" Hg. manifold vacuum, and 450-475 in Drive with PG at a few inches less vacuum.

If the above conditions cannot be achieved, then something is amiss!

Duke

Duke,

This problem has popped up a dozen times over the past several years. Why do so many guys with SB base engine cars with Holley carbs keep reporting that they cannot get the idle to smooth out under 600 rpm when they report a stock cam and all other characteristics that point to a stock engine? This problem is exacerbated in hot weather. Changes such as blocking the heat riser open, plugging the cast iron crossover and even running racing fuel does not always help.

The 66 Chevy Service Manual calls for a hot ilde speed of 400-600 rpm on a base SB base engine w/4 speed but this criteria is genre 1966.

Regards,
Dave K.

[Clem Z.]

Re: 1967 SB Idle Problem

A 327/300 (ex. K-19) with a correct OE or OE equivalent replacement camshaft with either the AFB or Holley should idle butter smooth (manual trans) in neutral at 500 pulling 18-19" Hg. manifold vacuum, and 450-475 in Drive with PG at a few inches less vacuum.

If the above conditions cannot be achieved, then something is amiss!

Duke

i bet the density of the gasoline has changed and the IFR, idle fuel restriction, size may need to be changes

[Duke W.]

Re: 1967 SB Idle Problem

Duke,

This problem has popped up a dozen times over the past several years. Why do so many guys with SB base engine cars with Holley carbs keep reporting that they cannot get the idle to smooth out under 600 rpm when they report a stock cam and all other characteristics that point to a stock engine? This problem is exacerbated in hot weather. Changes such as blocking the heat riser open, plugging the cast iron crossover and even running racing fuel does not always help.

The 66 Chevy Service Manual calls for a hot ilde speed of 400-600 rpm on a base SB base engine w/4 speed but this criteria is genre 1966.

Regards,
Dave K.

It does seem to be more of a problem with the Holley than AFB. The late Dale Pearman always had choice comments on this subject. Wiring the heat riser open and plugging the two small passages that allow exhaust gas to impinge directly on the carb base seems to help in most cases.

I'm not a fan of completely blocking the heat riser - tried that on my 340 HP and the engine stumbled badly on takeoff until it had been operating 20-30 minutes in warm weather.

Another idea is to block just one side of the heat riser. This creates a blind passage that exhaust gas can migrate into, but there will be no net flow, so the amount of heat delivered to the carburator base should be less than if the passage is completely open side to side and, hopefully, enough to provide sufficient fuel vaporazation at idle and low revs to not cause stumble, but I don't know of anyone who has tried this. On applications with the choke coil mounted on the heat riser runner, the opposite side should be blocked to ensure that the choke coil gets heat.

There was some variation in recommended idle speed over the years. For '63 manual trans/PG in Drive it's 500/475, 500/500 in '67 ,and 700/600 with K-19.

Current gasoline has less low range volatility (less than 150F) than sixties gasoline, but probably more mid range volatility (150-250F, and ethanol boils at about 170F), which likely contributes to the problem.

I'm glad I've never owned a car with a Holley and had to deal with all the Holley problems. AFBs are a lot less finnicky.

Duke

[Joe C.]

Re: 1967 SB Idle Problem

I'm glad I've never owned a car with a Holley and had to deal with all the Holley problems. AFBs are a lot less finnicky.


One of the posters above, sez he runs a holly carb, but normally only around Christmastime, I would think.

Hollies(sic) /Holleys are used exclusively for racing, and I cannot find any high performance Edelbrock/Carter carburetors sans choke horn. Granted, the AFB style carb is basically a "set it and forget it" carb, but not versatile enough for most racers.

If you think Holleys are finicky, then try checking out the Demon carbs. Holy karaoke, talk about "touchy". I've looked at the Edelbrock carbs, and agree that they're stable and easily adjusted, but I'm sold on a Holley HP Series 0-80528 carb for my "next generation" L76 with Z28 intake.

By the way, my Holley 2818, rebuilt a few years ago by one of our most reputable rebuilders, can idle down my L76 to about 750 RPM. That's with the valves set "tight" @ .025/.025, and running about 9.5 in-hg. I use AC 44 plugs, which have never fouled. Carb has never "loaded up", either. I must say, that if you have Luck with your rebuild, then you can expect reliable service from your hollies (sic).

Merry Christmas,
Joe

[Tom F.]

Re: 1967 SB Idle Problem

Hi Gang

I did check some of my settings, Idle vacuum ~20mg at 700RPM idle nice and steady. A luke warm engine will idle down to 600RPM smoothly. This is a bone stock 300HP without any work done to it in it's 44Kmi. When hot, like the other weekend at a local cruise, it runs rough and has to be taken back up to around 700RPM to run smooth enough without stalling. I beginning to suspect the gas mix these days as someone mentioned. I'm planning a PV and sure would like to solve this before then