72 LT-1 Idle bog down

[Gary S.]

Good evening to all. Here is the car - 1972 LT-1 with 40+k miles and all original engine compartment and here are the symptoms - while sitting at a stoplight waiting for a light change, the idle will slowly drop off to an eventual stall. The car runs extremely well in traffic, cruising, slow speed and high speed cruising. When the idle starts to drop off, a quick "blip" of the throttle and it is back idling at around 900 +/-. I ask here because I really don't know where to start because the car runs so well. Fuel? Ignition? Vacuum? ??? I have no basis to start looking anywhere since I don't know the inherent cause. A month or so ago, several of you correctly diagnosed my poor slow speed and decelerating performance as a blown A.I.R. manifold and I have plugged or neutered that as described in the archives. The only thing I haven't gotten around to is to have the A.I.R pump vanes removed by Bill Hodel - too many significant family matters to get that done.

Can anyone point me in the right direction while I think about prepping my car for the 4 hour cruise to Carlisle?

Thanks in advance,

Gary Schisler
grumpa72@gmail.com

[Joe R.]

Re: 72 LT-1 Idle bog down

Gary,

This is a shot in the dark BUT I would start with the carb. You don't have many miles BUT some later Holley had a plastic bearing in the throttle shafts. With wear this will displace and ruin the idle. It can be fixed or the carb can be re-bushed so the throttle goes back to the same position every time.
If you find this is the trouble go to one of the better known carb rebuilders and they can help you with this problem.

JR

[Gary S.]

Re: 72 LT-1 Idle bog down

Thanks Joe, but my carb was rebuilt by a reputable shop a few years ago and the car sat for all but about 500 miles per year (perhaps a couple with less than 200?) for 10 because of work constraints.

I did just find this link https://www.forums.ncrs.org/showthre...highlight=idle and it has some similarities to my situation with the exception that I am sitting at the light when it begins to bog down and stall. Not like the OP whose car does it when he begins to stop. I like the idea of dropping the fuel level a little bit as Timoth Barbieri suggests but I will do some testing tomorrow.

Any other ideas from the crowd?

Gary

[Joe C.]

Re: 72 LT-1 Idle bog down

Good evening to all. Here is the car - 1972 LT-1 with 40+k miles and all original engine compartment and here are the symptoms - while sitting at a stoplight waiting for a light change, the idle will slowly drop off to an eventual stall. The car runs extremely well in traffic, cruising, slow speed and high speed cruising. When the idle starts to drop off, a quick "blip" of the throttle and it is back idling at around 900 +/-. I ask here because I really don't know where to start because the car runs so well. Fuel? Ignition? Vacuum? ??? I have no basis to start looking anywhere since I don't know the inherent cause. A month or so ago, several of you correctly diagnosed my poor slow speed and decelerating performance as a blown A.I.R. manifold and I have plugged or neutered that as described in the archives. The only thing I haven't gotten around to is to have the A.I.R pump vanes removed by Bill Hodel - too many significant family matters to get that done.

Can anyone point me in the right direction while I think about prepping my car for the 4 hour cruise to Carlisle?

Thanks in advance,

Gary Schisler
grumpa72@gmail.com

Since the condition happens while you are sitting, and not during braking, then I wouldn't blame the float(s). I'd check the advance weights and autocam and make sure everything is clean and not sticking. Also make sure the springs and weights are not binding anywhere. Lubricate all pivot and sliding points with a very small amount of lithium grease. I'm not sure when the weights were backed with teflon sliding surfaces, but if you have this, then lube is not necessary on the weights.

If you don't have the proper vacuum advance can installed, then the vac advance might be "dropping out" on you due to too small margin between idle vacuum and deployment vacuum for the diaphragm. There should be a 2 in-hg margin between advance can deployment start point and engine idle manifold vacuum.

A better solution than the above paragraph, would be to convert the vacuum advance to full time using manifold rather than ported vacuum, and if installed bypass the TCS and run initial timing so that the total initial plus centrifugal is 36-38 degrees. You will also need to install a NAPA VC1810 vacuum advance can or equivalent in another brand (stamped B28).

[Gary S.]

Re: 72 LT-1 Idle bog down

Joe,
I do have the original distributor, also rebuilt 10 years ago (5000 miles maybe?) by a reputable shop, and original vacuum can. In addition, the TCS is running, as originally designed, and, like Duke, you suggested running with manifold vacuum and changing out the can. Let me ponder that a while! Right now, I am reading the distributor calibration report and I have:
1. dwell 30-32
2. centrifugal advance curve 26 degrees, total advance in at 3000 rpm
3. total timing 36 degrees, vacuum line disconnected, 3000 rpm

All of this set on a Sun machine. Refresh me please on what manifold vacuum gets me over the installed ported vacuum. Too lazy to search?

[Duke W.]

Re: 72 LT-1 Idle bog down

The problem you describe will occur if you switch a LT-1 to full time vacuum advance, but do not swap out the OE VAC for a B28.

Duke

[Gary S.]

Re: 72 LT-1 Idle bog down

The problem you describe will occur if you switch a LT-1 to full time vacuum advance, but do not swap out the OE VAC for a B28.

Duke

Duke,
I am reading now on ported vs manifold but my setup is bone stock from TCS to distributor to vacuum can.

Out to the garage to check distributor guts and make sure that they are clean, well lubed and in good shape.

Thank you all for the comments.

Gary

[Timothy B.]

Re: 72 LT-1 Idle bog down

Gary,

Another thing to think about is this new ethonal fuel has a lower evaporation rate and while at idle for a extended time the air cleaner sucks the fuel vapor into the carburetor throat creating a rich A/F ratio then the engine stalls.

If the LT-1 has heat riser try to wire it open and then test to see if it's any better.

The factory ported vacuum advance can be tuned to work much better than the factory settings but it will require changing the centrifugal advance so the total WOT advance stays at 36-38*. A example would be if the initial advance is set at 18* then the centrifugal set at 18-20* and leave the ported advance alone.

Another thing to think about is the primary throttle blade position at idle, make sure the throttle blade at hot idle does not expose the ported feed hole. Set the primary blade so it's JUST showing the transfer slot and crack the secondary blade to make up the air to idle. Always reset the emulsion screws on the hot engine after adjustments.

[Joe C.]

Re: 72 LT-1 Idle bog down

Joe,
I do have the original distributor, also rebuilt 10 years ago (5000 miles maybe?) by a reputable shop, and original vacuum can. In addition, the TCS is running, as originally designed, and, like Duke, you suggested running with manifold vacuum and changing out the can. Let me ponder that a while! Right now, I am reading the distributor calibration report and I have:
1. dwell 30-32
2. centrifugal advance curve 26 degrees, total advance in at 3000 rpm
3. total timing 36 degrees, vacuum line disconnected, 3000 rpm

All of this set on a Sun machine. Refresh me please on what manifold vacuum gets me over the installed ported vacuum. Too lazy to search?

Your dist specs are correct, and means that initial is set to 10 degrees. If your idle is 900 RPM as you stated above, then your manifold vacuum at idle should be around 10-11 in-hg if your lifters are set at factory specs. If they are set at the tighter lash recommended by Jonh Hinkley and another poster here, then it will be about 1 in-hg lower than stock.

Before you change anything, first make sure that everything inside the mechanical advance mechanism is free, lubed and not binding. I don't know the proper, stock vacuum can for a 1972 LT1 but I suspect it is a VC1802/B22. Check its operation as well as the operation of the TCS. THAT WILL ESTABLISH A BASELINE FOR YOU AND ELIMINATE THE IGNITION/TIMING FLUCTUATION AS THE CAUSE OF YOUR PROBLEM.

Timothy speculates that heat is causing the problem, although I don't think that the short period that you're stopped at a traffic light would be long enough to cause the fuel in the bowl(s) to volatilize. One thing is for sure.......................your engine will be much more responsive in everyday local driving and will idle cooler (if idling for long periods) by changing to full time vacuum advance. If your problem is fuel percolation/volatilization, then the change to manifold vacuum might cure it.

[Gary S.]

Re: 72 LT-1 Idle bog down

Added information:
The vacuum is around 11-12 degrees, iirc, TCS was checked this spring and is operating normally, and the heat riser used to be wired open. This spring I removed the butterfly plate completely but it still looks stock. Here is what I am finding online - NAPA VC1810 discontinued, Autozone DC1810 unknown (calling them shortly), AC Delco D1312C disontinued, and GM 88924985 is discontinued.

So, in searching for a B28 type can, the list gets smaller. Any other sources or substitutes?

Still working on the other thoughts and ideas plus I cleaned and re-greased the advance weights even though they were free moving and well greased.

Thank you as always,
Gary

[Joe C.]

Re: 72 LT-1 Idle bog down

Added information:
The vacuum is around 11-12 degrees, iirc, TCS was checked this spring and is operating normally, and the heat riser used to be wired open. This spring I removed the butterfly plate completely but it still looks stock. Here is what I am finding online - NAPA VC1810 discontinued, Autozone DC1810 unknown (calling them shortly), AC Delco D1312C disontinued, and GM 88924985 is discontinued.

So, in searching for a B28 type can, the list gets smaller. Any other sources or substitutes?

Still working on the other thoughts and ideas plus I cleaned and re-greased the advance weights even though they were free moving and well greased.

Thank you as always,
Gary

Long discontinued. You can use this "236" reproduction although it might not be of as good quality as original:

https://www.paragoncorvette.com/p-34...e-control.aspx

and, per the question you asked in the post which you deleted, it shows that it advances the timing by 16 crankshaft degrees when fully deployed.

You can also call Lars Grimsrud who will be able to help you find what you need. I do not have nor can I find his number, but he is a well respected engine tuner/engineer who lives in Colorado. Someone on the CF can give you his contact info if you want it.

The smartest thing would be to do like I did long ago, and install one of these:

http://www.jegs.com/i/Accel/110/3103...oductId=747725

which you can find for 20 bucks on ebay.

[Gary S.]

Re: 72 LT-1 Idle bog down

Joe, I hit delete when I meant to "edit" and add the information that I posted. Fat fingers and in a hurry with the "honey do" list. I have started scanning the various Corvette parts magazines looking for repros and may go that way. I am heading to Carlisle on Friday, gimpy leg and all, and will see what is offered there. Reference the adjustable vacuum can - I looked at that to as it came up in my search for a suitable VC1810 substitute. In addition, I am looking at Duke's list of B28 substitutes and scanning Amazon, eBay and other locations.

I have Lars' contact and may give him a call depending on what Carlisle yields.

Thank you for the help.

[Duke W.]

Re: 72 LT-1 Idle bog down

The B28 in the Airtex brand has been available lately: 4V1053.

Your centrifugal curve is not OE, and you did not say where it starts. If it starts below idle speed, that could be your problem. Observe the timing with a light while it loses idle RPM. If it looses advance too, that is at least part of the problem.

I do not recommend any type of lubrication on the centrifugal mechanism. Those parts are hardened steel and lightly loaded. Grease dries out over time and becomes gummy and sticky. I only recommend wiping down the parts with WD-40 to provide a little corrosion protection, and of course, the mechanism should be very clean. Delco did not use a lubricant on the centrifugal mechanism - only on the point rubbing block/lubricator wick and upper bushing grease well.

Also, go through the idle speed/mixture adustment procedure using the pre-emission method - set the mixture screws to achieve highest vacuum and idle speed. The CSM should have an initial setting. Turn both screws and equal amount with each adjustment. Target an idle speed of about 900 and use an accurate test tach.

Duke

[Gary S.]

Re: 72 LT-1 Idle bog down

Duke,
Thank you for the Airtex vacuum link. It shows as available and very affordable. I was unaware of the distributor lube issue but it is easy enough to remove it. With reference to the distributor curve, when i talked with Jerry Macneish 11 or so years ago, I didn't specify anything more than a strong runnig, good cruising engine, as I recall. I left the specifics up to him and, once i have sorted out these new issues, it has run really well. I have been through the carb beacuse of the issue I had in my previous post that turned out to be the AIR diverter. I can certainly look at it again if I don't resolve the stalling. I hope to get to it tomorrow.

Thank you for the input and assistance.