67 L71 idle problem

[Tom R.]

I need help determining why I can't get a smooth idle during driving. She idles fine in the garage during and after warmup but stalls when I come to a stop after driving for a while. I have initial timing at 10 degrees @ 750 rpms but at that rev. the idle is really rough & non-steady. I set that with the car at normal temperature and after running for about 20 minutes. Currently, I have bumped up the idle to 850 and she still dies when I come to a stoplight. When I came home, I checked the initial timing just for my curiosity @ 850 rpms and it's about 14 degrees. I'm certainly a novice and need some input. In case this helps, I have about 15" vacuum @ 850 rpms. The engine is a fresh rebuild with only about 100 miles on it.

Thanks

[John H.]

Re: 67 L71 idle problem

Tom -

Your L-71 has "ported" vacuum to the distributor (no vacuum advance at idle); idle stability (and idle cooling, throttle response, and fuel economy) will be improved if you cap off the existing (ported) distributor vacuum source on the carb base and instead tee the line to the distributor into the rubber hose feeding the choke pull-off diaphragm - that's a full manifold vacuum source. You'll need to set the idle speed down a bit after doing this, as it'll add advance at idle you didn't have before.

[Duke W.]

Re: 67 L71 idle problem

If you convert a L-71 to full time vacuum advance you also need to change the VAC to a NAPA/Echlin VC-1765 or equivalent (stamped B20 or B26). The OE 201 15 VAC (or equivalent) may not pull the to the limit at typical L-71 idle vacuum.

However, it should have a stable idle with the OE ported vacuum advance.

Since the engine is a fresh rebuild, it could be a cam, timing map, or carb problem.

What manufacturer/part number camshaft was installed for the rebuild?

What is the ID data stamped on the VAC bracket?

Was anything done to the center carburetor during the rebuild, and is it the correct OE carburetor?

Have you conducted the idle speed/mixture setting procedure in the CSM, and what how far out are the mixture screws from the seats?

It could be your idle speed increase slightly exposed the "ported" vacuum source in the carburetor, which is adding a little vacuum advance or the centrifugal curve starts below 850, which is adding centrifugal advance.

You should test the spark advance map to see what you have. The archives contain explanations of this and how to do it.

The primary benefit of converting to full time vacuum advance is better low speed driving fuel economy and less tendency to run hot/overheat in low speed driving. Full time vauum advance might also result in a slightly smoother idle at the same idle speed, but that's a secondary effect.

Duke

[Tim S.]

Re: 67 L71 idle problem

Tom,
Duke is right on with the items he suggested. The tri power is a little finiky. The only thing I would add to Duke's response is a simple check if you have not tried it already. When the car is warming, take you hand and cover the carbs to see if there is an improvement in idle quality. Take note as the car gets warmer. Perhaps, you may have a vacuum leak on one of or both of the end carbs. If you have a vacuum leak, the choke may help cover the problem until it is fully warm and at that point, you could be very lean. Hence.........no idle.
One other thought. Does it die just when going to stop? Or, is it a warm problem period.

Tim

[Tom R.]

Re: 67 L71 idle problem

Wow, this may be a bit out of my league. Perhaps I need to share additonal information to see if this has any bearing on my problem. I assumed I was already running on full time manifold vacuum since I have the line from the VAC at the distributor connected to the lower center carb nipple. When I pull this line from the distributor the idle doesn't change. If that's not manifold vacuum then maybe I have a problem with the VAC? I also replaced all three carbs during the rebuild new from Holley. Without pulling all of the plug wires from the distributor cap I can see a B20 stamped into the VAC. If I need to pull the wires and cap completely let me know. Before the rebuild, there was no choke spring, rod, etc., simply missing. I installed a new one but it doesn't function very well I believe due to the new center carb - the choke plate seems to have too much resistance for the choke spring to activate. I'm hoping it'll "free" itself a bit with wear as I don't see any adjustment there. Other than the idle problem, I'm very satisfied with the way it runs. It simply won't idle when I come to a stop.

[Duke W.]

Re: 67 L71 idle problem

The B20 is okay for full time vacuum advance as I stated in a previous post. The VAC port on the OE carb is ported. Is the replacement carb the exact same Holley list number as OE?

Did you feel any suction on the VAC hose when you removed it. It's best to check with a vacuum gage. You can also test the B20 VAC with a vacuum pump. It should start to pull at 6" and be fully deployed at 12". Do this with the cap off and watch the pull rod.

Where did you take your idle vacuum readings?

None of the above may be relevent to the idle problem, but without all the nitty gritty details, all anyone can do is guess. The more information you provide, the better the analysis you will receive.

What about the replacement camshaft?

Duke

[Steven C.]

Re: 67 L71 idle problem

Tom,
Duke is right on with the items he suggested. The tri power is a little finiky. The only thing I would add to Duke's response is a simple check if you have not tried it already. When the car is warming, take you hand and cover the carbs to see if there is an improvement in idle quality. Take note as the car gets warmer. Perhaps, you may have a vacuum leak on one of or both of the end carbs. If you have a vacuum leak, the choke may help cover the problem until it is fully warm and at that point, you could be very lean. Hence.........no idle.
One other thought. Does it die just when going to stop? Or, is it a warm problem period.

Tim

Tim, I am experiencing the same problem as Tom. The car starts fine, idles well when cold/warming, but when it gets hot the idle drops off and the engine is sluggish at low RPM and difficult to keep running. When it is shut off while hot, it is very difficult to get started again. Interested to hear the responses, but it may be a front/rear carb vacuum leak around the gasket.

[Tim S.]

Re: 67 L71 idle problem

Tim, I am experiencing the same problem as Tom. The car starts fine, idles well when cold/warming, but when it gets hot the idle drops off and the engine is sluggish at low RPM and difficult to keep running. When it is shut off while hot, it is very difficult to get started again. Interested to hear the responses, but it may be a front/rear carb vacuum leak around the gasket.

Have you tried to cover any of the carbs when the problem surfaces? If so, does it help? Manytimes, aftermarket base gaskets for the end carbs in particular do not fit properly and will actually hold the blades of the carb open a little (vacuum leak). Also, my experience has been that 3x2 set ups are sensitive to timing (affects vacuum). As Duke led on to, there are many variables that can affect idle quality. Items such as camshaft selection, cam timing, ignition timing, proper sealing of intake manifold to cylinder head, etc. all have an influence on idle quality and low to midrange drivability. As far as you hot start issue, does the car act lean (pump the throttle) or rich ( hold the throttle open)? this may provide some direction. If you PM me your phone number, I can give you a call. I've been here before. Anyone else care to chime in?
Tim

[Steven C.]

Re: 67 L71 idle problem

Tim, I am having trouble believing that these cars ran this poorly out of the factory or I would think the client complaints would be numerous. Perhaps they were, not sure. On the hot start problem, it is getting plenty of fuel, just will not fire. Will send a PM.

[Tom R.]

Re: 67 L71 idle problem

Duke,

To answer your questions:
All 3 Holleys are the same part numbers as original. The gaskets that were included are identical to the gaskets that I removed.
The new cam is from Crane and the one that you have recommended on this site for years. That, I paid attention to. I do have suction at the hose when I remove it from the distributor - I can feel and hear it. I measured the vacuum reading by teeing into the line from the upper carb nipple to the choke. The reading was 15" @ 850 rpm.
As for changing to full manifold vac as you advise, how do I reroute? I thought that's what I had since when I remove the hose from the distributor the idle doesn't change. My VAC is connected to the right side carb nipple facing 90 degrees to the right. The choke diaphram? is connected to the lowest nipple at the base of the carb which points about 45 degrees towards the right (say to the right headlight). Finally, the 2 outer carb Vac pods are connected to the center carb nipple just behind or inboard of the choke diaphram. If that makes sense, is that the proper way?
Thanks for your continued advice and patience.

[Duke W.]

Re: 67 L71 idle problem

I not sure I 've ever recommended a Crane cam for SHP big blocks. IIRC they list the L-88 cam as "435 HP", which is dead a... wrong! I and others have contacted them, but they refuse to correct the listing.

The idle vacuum sounds okay, but what is the data listed on the cam card?

If you want full time vacuum advance buy a 1/8" tee and a couple of feet of 1/8" vacuum tubing and make a new harness with the VAC teed into a new choke vacuum break line. That is full time manifold vacuum.

I can't give you a detailed rundown of the OE vacuum line routings. See your CSM and AIM.

On other thing that has not been mentioned is the heat riser. As a minimum it should be wired open. Percolation could be the culprit. The next step is to block the LH side, which will still allow exhaust heat to go up the RH side dead end passage to operate the choke coil, but it will significantly reduce the amount of heat at the base of the carb.

Be sure you check that the installed VAC meets the B20 spec.

Duke

[John H.]

Re: 67 L71 idle problem

As for changing to full manifold vac as you advise, how do I reroute? I thought that's what I had since when I remove the hose from the distributor the idle doesn't change. My VAC is connected to the right side carb nipple facing 90 degrees to the right. The choke diaphram? is connected to the lowest nipple at the base of the carb which points about 45 degrees towards the right (say to the right headlight).

Tom -

The nipple you presently have the VAC connected to at the base of the carb is a "ported" source (that's why your idle doesn't change when you connect/disconnect it); the choke pull-off diaphragm source (the angled nipple at the front of the baseplate) is full manifold vacuum - that's the hose you want to tee into for the VAC source.