72 LS5 missing / stumble on acceleration

[Randall K.]

Hoping for a few pointers, as I'm not an ignition/fuel expert...

My engine misses and stumbles badly on hard acceleration, but will run with few stumbles up to higher RPMs if accelerated slowly and smoothly. Running engine with transmission in park to high RPMs doesn't reproduce this problem, so this issue seems to be associated with engine loading.

I'm assuming there could be multiple causes for a problem like this. How should I prioritize my troubleshooting?

Thanks in advance,
Randy

[Patrick H.]

Re: 72 LS5 missing / stumble on acceleration

What, if anything, have you done so far? What can you tell us about the state of your carb and ignition system?

[Duke W.]

Re: 72 LS5 missing / stumble on acceleration

The first thing I would look at is the ignition.

Dwell angle should not drop more than about 2 degrees from idle to max revs. How much end play in the distributor shaft? Measure plug wire resistance - should be no more than 5K ohms per foot on OE type RFI wires. Plugs should be AC heat range 5 or equivalent in any other brand. Borrow another coil or buy a ten buck generic replacement coil and see if it makes any difference. Check that all primary and secondary wiring connections are clean and tight.

The last distributor I overhauled had a bad breaker plate ground wire. It broke off as I removed the breaker plate. NAPA has a repair kit. Look at this thread:

https://www.forums.ncrs.org/showthread.php?97360-L-79-distributor-blueprint-overhaul

One way to test this is disonnect and plug the VAC signal line. Connect a test lead with an alligator clip through the widow to the breaker plate and a good external ground. Then test to see if the engine will rev smoothly under load.

Duke

[Tom L.]

Re: 72 LS5 missing / stumble on acceleration

I had an off idle stumble on my LS-5 when I first Bought the car. I fiddled with the carb quite a bit until I remembered what an old timer once told me, "90% of carburator problems are electrical" (meaning the ignition system) before I almost sent the carb out for a rebuild. I investigated the ignition system and found that the VAC was leaking. Replaced it, and rebuilt the distributor at the same time, it's been running great ever since. Good luck!!

[Rich C.]

Re: 72 LS5 missing / stumble on acceleration

Do you have a steady vacuum reading at idle? My '73 454 is doing that on wide open throttle. I cannot get a steady vacuum reading! I was leaning towards a valve guide wear issue. Maybe valve guide wear wouldn't contribute to this, but I'm running out of ideas. My carb had been rebuilt (quite a few years ago actually) and I replaced virtually EVERYTHING in the distributor.

1973 LS-4 454 Coupe owned 24 years
1996 LT-4 CE Coupe owned 8 months

[Roy V.]

Re: 72 LS5 missing / stumble on acceleration

Don't forget to look below in the "similar threads" box for a few more threads that might be helpful
Regards
Roy

[Randall K.]

Re: 72 LS5 missing / stumble on acceleration

Thanks to all who responded! As it turns out, my point gap was way low, and more important, timing was way off (I had never set it myself). Initial value at idle, no vacuum advance, was 12 degrees BTDC. I took it down to 8, and the performance got even worse. So... I'm thinking that a past owner may have reinstalled the distributor one tooth off.

In the end, for now it got a tuning by ear - running in that sweet spot between missing/stumble and pre-ignition ping, with good power.

What's the easiest way to confirm that the distributor is indeed one tooth off?

Randy

[Tom L.]

Re: 72 LS5 missing / stumble on acceleration

If you were able to adjust it to 8* with the vacuum line disconnected from the VAC, the distributor is likely in the correct location provided it doesn't interfere with the shielding brackets. The AIM shows where the VAC should point and where the #1 plug should be located on the cap, that might help. Others will likely give you more specific info than this but on my couch it's the best I've got. Good luck!!

[Duke W.]

Re: 72 LS5 missing / stumble on acceleration

You have provided no evidence that your distributor is one tooth off.

The first thing you should do is determine what the OE spark advance map is from the CSM or AMA specs.

Then test to see if it conforms. If it does then the engine should run satisfactorily.

However, the OE spark advance map is optimized for emissions, not performance or fuel economy.

Optimization of emission controlled spark advanced maps have been extensively discussed on the TDB.

Duke

[Charles D.]

Re: 72 LS5 missing / stumble on acceleration

A cracked spark plug is possible. Have seen this happen several times. Cheap way to start

[Randall K.]

Re: 72 LS5 missing / stumble on acceleration

Hello all

I've been away from my car for a while, getting ready for a move.

I finally had a chance to (re)start this activity, and here's what I've found.

Following the Hinckley article from CA, I used my new adjustable timing light to set full centrifugal advance at 2500 RPM to 36 degrees. This was after disconnecting the vacuum advance and plugging the vacuum line. I did end up replacing the stock springs with the silver springs from the spring kit to get the centrifugal advance to go full-in.

Took it for a test drive, without reconnecting the vacuum advance. TERRIBLE! No power, sounded funny - NOT knocking.

So... since I had my distributor wrench with me, I reconnected vacuum and slowly retarded the timing up to the point where I was getting knocking - and then backed it off some.

There was a LOT more power this way, and it didn't "lug" on acceleration. This is the good news!

Back in my garage, I checked the total centrifugal advance without vacuum, and it was 43 degrees at 2500 RPM! I then reconnected the vacuum, and the advance went up to 65 degrees at 2500 RPM.

I know - based on the numbers from the Hinckley and Grimsrud articles, these numbers are WAAAAY off.

My first issue is - why did I need to change the timing so radically from what was the "correct" setting to get it to run correctly?

The second issue is the amount of vacuum advance, which I will correct per Grimsrud. And, before anyone makes the comment, I know I need to check my vacuum. My gauge is somewhere in a box in a storage locker...

I really need help answering the first issue...

Thanks
Randy

[Terry M.]

Re: 72 LS5 missing / stumble on acceleration

The first thought that comes to mind is that your timing mark on the harmonic balancer is not accurate.

I believe, while uncommon, the outer ring can slip on the balancer. Perhaps the tab on the timing cover is not placed correctly. I am not sure if that can be done, but verifying that the timing mark on the balancer is at 0 when the number one piston is at top dead center would IMO be a worthwhile exercise.

[Duke W.]

Re: 72 LS5 missing / stumble on acceleration

There's this misunderstanding out there that you should set your total WOT spark advance at 2500. But this only works if the centrifugal advance is fully deployed at no more than 2500, and this is not the case with most OE distributors.

If the max centrifugal advance isn't achieved until well over 2500 and you set total WOT advance in the correct range at 2500 then the engine will be operating with excess advance, which will cause poor performance and possibly detonation.

Here's what you need to do. With the vacuum advance disconnected and plugged use the timing light to determine the point where the advance reaches the maximum point, which is the RPM above which it will no longer advance.

With OE centrifugal weights and springs this may be as high as 5100 RPM. Of course, if you have lighter than OE centrifugal springs, the RPM at max advance should be lower, but not necessarily 2500.

Once you have determined the max centrifugal engine speed, blip the throttle and verify that the advance does not increase at higher revs. Now you set total WOT advance a few hundred revs above the speed at with maximum centrifugal is achieved, but there's a problem doing this with your particular engine. See the last paragraph.

All emission controlled engines have spark advance maps that are retarded below optimum at virtually all operating conditions including lazy centrifugal curves, ported vacuum advance, and insufficient initial timing to achieve optimum total WOT advance, which is in the range of 36-40 for both small and big blocks.

Lighter centrifugal springs, conversion to full time vacuum advance with a VAC that meets the Two-Inch-Rule, and sufficient initial advance to achieve the above total WOT advance range will yield noticeably better performance and fuel economy.

Recommendations and all the why's and wherefore's are in the following pdf:

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

The OE distributor specs for your engine are in the service manual and the AMA specs that are a free download from the GM Heritage Web site. You should determine and post those specs here. Include the number stamped on the VAC and the RPM that centrifugal achieves maximum with the currently installed springs.

I can tell you right now that the OE distributor has insufficient centrifugal advance to achieve optimum total WOT advance without excessive initial timing, and the VAC has too much advance.

You can just set everything to OE spec and the engine will run okay, but you've left a lot of performance and fuel economy on the table that you can get back by optimizing the spark advance map.

Duke