Pontiac 389 engine with a Impco 425 - high idle

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Thu Aug 22, 2013 3:02 am

Im on my 3rd radiator in the camaro.. the original factory got replaced in the early 80s, and I replaced that earlier this year.. all with factory replacements Shipped in air freight to NZ ... and they where still cheaper than getting re cored here... because the cores where going to cost more, and not readily available as damn near every thing is now metric off the self.

The older guys.. who have been messing with these GM engines for decades even find 3 cores run well on BBs pro stock 11 sec cars... NPs
The fact is we change our oil grease the ball joint, change the brake fluid (well hope u guys do) and dont bother with the annual radiator flush
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storm
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by storm » Thu Aug 22, 2013 5:14 am

Steptoe wrote:Im on my 3rd radiator in the camaro.. the original factory got replaced in the early 80s, and I replaced that earlier this year.. all with factory replacements Shipped in air freight to NZ ... and they where still cheaper than getting re cored here... because the cores where going to cost more, and not readily available as damn near every thing is now metric off the self.
Ah we still make our own stuff here in Australia.
Steptoe wrote:The older guys.. who have been messing with these GM engines for decades even find 3 cores run well on BBs pro stock 11 sec cars... NPs
The fact is we change our oil grease the ball joint, change the brake fluid (well hope u guys do) and dont bother with the annual radiator flush
If you follow the OEM service schedule then things should be relatively ok. The problem with mine was a severe corrosion issue that probably originated before I bought the car.
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Thu Aug 22, 2013 5:21 pm

Ah we still make our own stuff here in Australia.
Which is fortunate because a hell of a lot of GM stuff for HK , HT, HQ and HZ holdens is identical to the American items, from wheel bearings, chassis mounts, door locks/window winders, stub axles, discs (but not calipars), brake MC slaves etc... and a lot of the American is now Chinese made crap...Aussie parts are also Korean made which is also of excellent quality.
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liteon
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by liteon » Sat Aug 31, 2013 6:32 am

Well, time for an update. Yesterday I solved my high idle problem.
It was a very sneaky one; the secondaries linkage to the primaries where the issue. The linkage is under some sort of angle that does not allow to close both primaries and secondaries.
When pulling the linkage, both primaries and secondaries could be closed a little bit more. Enough to let the engine speed drop to 400rpms.
Customized the linkage, and it's now running smooth - never had it like this in the past 1,5 years :-)
You might think, why didn't check the linkage before??, well I did, but when the engine was not running the linkage was loose / not blocking anything. It was maybe 1mm to short to let the engine idle properly.
A couple cars guys did have a look to it, but nobody saw this too, that confirms it was a sneaky one :oops:

I can now let the engine run 420rpm in Drive without any problems - oil pressure still 20-25 psi with a warm engine. It's currently idling 500rpms in park and 470rpms in drive, otherwise I can''t hear if the engine was still running :mrgreen:

Found also that my ignition was acting up strange. Found out that one of the weights was sometimes stuck. Fixed that already :-) The curve was not good at all (21 degrees max at 2200rpms), so changed that too.
Had increased the mechanical advance a bit, so it had a little bit more total timing, but noticed it
First run was like this (initial + mech combined. VA disconnected):
450 rpm: 12
500 rpm: 12
750 rpm: 12
1000 rpm:21
1500 rpm: 22
2000 rpm: 25
2500 rpm: 26
3000 rpm: 26
3500 rpm: 26
4000 rpm: 26/27 (hard to read, mark was moving constantly moving about 2 degrees, which it starts doing about 3.5k rpms)
(i know this curve is quite crappy, but can't get it any better with the springs I have at this moment)

With the curve above it was pinging/rattling when doing a 2nd gear pull from 3k rpms to 4/4.5k rpms, but not much, it was very very hard to hear. To confirm that it was indeed pinging, i increased the initial from 12 to 15 (so it had about 29 degrees total) it was immediately pinging mad when flooring it around 2500-3000 rpms... of course I stopped immediately, and gave it less advance and went back to home. Both WOT runs in second gear where done with the VA plugged.
I've now set the initial around 10 degrees, so the max advance is about 23-24 degrees - will do a test run later today.

What i don't understand is why this engine pings that quick. Unfortunately I don't have the original ignition anymore to compare.
I made my own piston stop a week ago, and found out that my TDC mark was exactly right. So the mark on the balancer hasn't been shifted.

Since day I one got this car (still on petrol and points distributor), I had all my power on about 50% throttle. SInce I got this holley plate with Impco 425 I still have all my power on the primaries (so about 50% throttle), when I floor it the engine just sounds stronger but isn't much quicker. The engine has tons and tons of torque, that's not the problem (the 389 engine is known of it's torque).
This engine is still stock and hasn't been opened - so the original cam would be still in there, maybe the cam explains why it doesn't like that much advance? :roll:

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Sat Aug 31, 2013 6:21 pm

With the curve above it was pinging/rattling when doing a 2nd gear pull from 3k rpms to 4/4.5k rpms, but not much, it was very very hard to hear.
ANY ping/ detonation is a death rattle to an engine
In the mean time I strongly suggest dropping even stock springs or even just one in and for get about performance for now .. or at least till u are able to sort the advance out.
Basically what u have is the counter weights are holding in , then suddenly release, going full advance the 5 degs on top of that will just be stuff like wear in bushes...


With 12 initial and about 20 total (assume VA not connected) u only have about 10 degs in the centfifical.

There is nothing wrong with a points dizzy... IF your mixtures are good AND everything .. points , HT leads condenser, plugs etc are in 100% condition......I do strongly recommend going HEI eventually, even a aftermarket Chinese unit.....I dropped one of these in about a yrs ago... daily driver, and so far proving reliable and consistent.... but keep in mind...aftermarket dizzies are generically set up for petrol/ EGR type engines... they do need re curving...
putting more degs in the cent is done by reshaping the tail of the counter weights.... covered many times in older posts

Glad um sorted those butterfly, I pretty well knew it would be those due to your accurate description....I have set mine up so that runs on primaries only and the secondaries start opening about 2/3rd / 3/4..... but set so still get both fully open at WOT.
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liteon
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by liteon » Wed Sep 04, 2013 8:54 am

Yeah, I know it's a death rattle so I immediate stopped accelerating after hearing "gremlins :mrgreen: " in my Engine.

I don't have a points distributor anymore. It's being replaced with a HEI (1,5 years old). The mech. advance on this HEI is limited (stop screw). The mechanical parts of the HEI are in perfect condition, so that couldn't be any problem.
I know the engine ran on a lot more advance in the first day, about 32 degrees if my memory is good. So it's quite mysterious that the engine now starts pinging at 26 degrees @ WOT 3k+ rpms.
I use 10.5mm leads.

Could the spark plugs cause the quick pinging? I run AC Delco 43's (orignally 45, so these are 2 degrees colder). These bougies have a wider gap then they orginally should have. Some guy told me that LPG likes wider gaps, so I increased the gap (from my memory) to about .045 or 050.
The engine with points distributor has originally a sparkplug gap of .035

As the engine was pinging at 26 degrees, i have it less advance for now. It has currently the following curve:
Below are the total degrees @ Initial + mech advance:
450 rpm: 10 degrees (initial, no advance)
500 rpm: 10
1000 rpm: 18
1500 rpm: 19
2000 rpm: 22
2500 rpm: 23
3000+ rpm: 23
(so the total mech. advance is about 13 as the initial is 10).

I have the VA attached with does about 8 degrees idle. Haven't metered the VA advance yet. I know the VA is not the problems that leads to pinging, as I did the test runs without VA.
I must say the car drives pretty good with this crappy curve. :roll:

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by C3H8 » Wed Sep 04, 2013 12:31 pm

There is a lot of engineering and discussion on this. The bottom line is each engine reacts different and once it is no longer stock other parameters come into play. I can note two items you have referred to. The reverse is true on the plug gap. First the choice of heat range you have is good. The 43's were a popular choice in GM's. The gap is usually reduced on LPG. LPG and CNG are dry fuels. It takes more voltage to jump the gap on a dry fuel vs a wet fuel like gasoline. On a points system we typically reduced the gap about 20%. On the HEI system you have we use the standard .035 gap since the HEI has such a high reserve available.

Why has your timing changed. I don't think anyone can say for sure. The only true way to find out would be by using a cylinder pressure test while running on a dyno. Kind of expensive and requires special instrumentation to do it. your mechanical of 23 is very low. One change I prefer and this is strictly personal choice. I prefer to attach the vac for the distributor above the throttle plates, I don't like using vac at idle due to some past idle and emission issues in early 80's vehicles. That being said it unlikely to have any impact on your current situation. The only other thing I can refer to is whether you are still operating an EGR. EGR's were used to cool combustion temps and helped to prevent detonation besides lowering Nox

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Wed Sep 04, 2013 4:40 pm

To detonate at such low advance is abnormal, espec on a basically stock (cam/ compression) engine
Ba;; park I would have started with about the 10 to 14 degs mark intial...coming off around 650/750 rpms and around 32/ 34 degs all in around the 2600/ 2700 that gives around 20 degs in the Cent.... and is about as light a springs one can go without them bouncing at idle.
I can understand a detonation at part throttle with the VA partly in, thats not uncommon and just means changing the VA curve to suit.... but yours is WOT (correct) and with stuff all advance.
This suggests to me something like hot spots in the chamber...maybe a casting dag, or a build up of carbon pre igniting fuel.. or could be a combo with carbon build up which increases combustion pressures + a hot spot
If an engine over a period of time is continually WOT acceleration, and deposits on plugs, chamber tend to glaze...Which is why if on a trip normally takes 50 / 70 miles and the engine seems to go into a nice cruise mode....by acceleration , increasing speed slowly when one hits the highway, this burns deposits off and the engine goes into a 'cruise mode' early...

So how to clean chambers without taking heads off? simple.. old school... with the engine on fast idle a 2 or 3 cups of water very slowly tricked down the carb and not allowing the engine to stall... not much different to a blown head gasket and that cylinder/ plug is very clean. Then new plugs (can use old ones) and a oil / filter change.

Yep LPG likes colder plugs, but this also means higher chamber deposits, and if the engine is a bit worn, just a little... rings...more deposits... the colder plugs may not be helping things...
I have always, regardless of fuel type, run plugs as cold as possible but still remain clean... this often results in a coupl;e of the cnr cylinders running a plug hotter....the cnr cylinders is usually due to the effect on longer runners... in particular on petrol carbed engines

So maybe this engine needs a de coke????
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by storm » Wed Sep 04, 2013 6:01 pm

Steptoe wrote:So how to clean chambers without taking heads off? simple.. old school... with the engine on fast idle a 2 or 3 cups of water very slowly tricked down the carb and not allowing the engine to stall... not much different to a blown head gasket and that cylinder/ plug is very clean. Then new plugs (can use old ones) and a oil / filter change.
Coke bottle full of water, vacumm line down to the bottom of the bottle anc connected to a vacumm port that takes from both levles of the manifold. Run the engine at 1500 RPM and let is suck the water up, doing it this way means you don't accidently dump alot of water and also there is no chance it can get into the float bowl. After the water has been used up run the engine for another 5-10 minutes to ensure it has all gone.
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by liteon » Tue Oct 22, 2013 12:52 pm

Hi all :)
Today I finally had time again (plus it was a great sunny day) to do some work on the Pontiac.

I decided to start with regap all 8 sparkplugs. I ran them at +/- .045, and lowered it back to the original .035 specs.
After that I advanced the timing again by +/- 4 degrees, and did a test run. Engine was still pinging just a little bit @ WOT, starting at about 2500k.
During this test it had the following curve:
450 rpm: 12 degrees (initial, no advance)
500 rpm: 12
1000 rpm: 20
1500 rpm: 21
2000 rpm: 24
2500 rpm: +/- 26
3000+ rpm: +/- 26.

So changing the gap didn't had any effect. I was actually hoping that the gap was causing my problems.
A week ago something came in a my mind that I drove the car with about 32 degrees... 2 years ago.
So why doesn't like the advance these days :roll: :?

The only thing that have been changed since that moment is a Impco 300 -> 425 upgrade, and 2 degrees colder spark plugs with wider gaps.
The 2 degrees colder plus that i'm now using are actually listed as "high performance / heavy duty" in my shop manual, so then I assume tht they would work fine with this engine (i like to floor it quite often :mrgreen: )

The car is running dedicated on LPG for 2,5 years, so how can carbon build up of I don't run petrol anymore. :roll:
I haven't tried a de-coke yet, just because I still think there is pretty much no way of carbon build up @ LPG.

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by BigBlockMopar » Tue Oct 22, 2013 3:19 pm

Balancer ring moved?
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Tue Oct 22, 2013 4:45 pm

450 rpm: 12 degrees (initial, no advance)
500 rpm: 12
1000 rpm: 20
1500 rpm: 21
2000 rpm: 24
2500 rpm: +/- 26
3000+ rpm: +/- 26.
basically what is happening is the springs are too light, the curve coming in way too low .. in effect just above 2000 and the rest is simply 'stretch' taking up any play in the components, bushes , including the main shaft.
increase the weight of 1 of the springs....
so u get a ball park sorta like this
500 12
700 12to14
1500 15to16
2000 around 20/22
2500 25/27
2800 30/32

u will note that u have currently only 25/26 minus 12 = 13/14 degs in the cent
my suggested ball park has 30/32 minus 12 = 20 degs in the cent
And u may very well find that u may need as much as a total of 34 degs in the cent.

I have said this many times before modern dizzies, post early mid 70s factory or aftermarket are machined for modern EGR/ pollution engines with low cent and heaps of cent.
To get more advance degs in the cent u need to file a small round on the tail of the counterweight, approx ,25 to 1 mm off.
you can also make the same adjustment by changing the shape of the anvil the counter weight rubs against but thats harder to get out.
IF u take too much off... there are 2 holes in the top plate ... tap them, take a grub screw and file the head into a cam shape... this becomes an adjustable cent degs stop for the counter weights.. only need to do 1 side.

increasing the cent degs and slowing and extending the curve to higher rpms will REMOVE all detonation issues.....and it will be unlikely to have detonation with the VA attached at low loads also...

Do NOT have the total (initial+cent) plus VA exceed 40/ 42 degrees unless dialing in with a knock sensor.
u can change the starting Vaccuum and finish vaccuum AND the number degrees in the VA ... including adjustable and non adjustable VAs by making small tags that mount under the VA mounting screws... and extending the slot the arm works in if need be.
U can get even more fancy with little effort by cutting the stop end of these tags at an angle and filing 3 or 4 notches in for different settings.

You will then have a fully adjustable cent and VA simply by turning a grub screw / loosen and move a VA stop tag and changing springs.. it is that easy.

Oh the grub screw, once u have established its position ,( IF u have taken too much off the counter weight tails AND filed a cam shape on the grub screw head) mark its position, unscrew it a drop of locktite and screw back in and leave for an hr or so... a couple drops of RTV silicone sealer will do the same thing.
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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by gottago » Sat Oct 26, 2013 11:09 am

The only thing that have been changed since that moment is a Impco 300 -> 425 upgrade, and 2 degrees colder spark
It ran fine with more advance using smaller mixer before the change? mixtures been checked?

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by liteon » Tue Apr 01, 2014 1:28 pm

Hi all - the weather starts getting better and better over here again, so time to take the car for a nice drive 8)
Well, that means also time to do some tune ups etc.
After those issues past year, I haven't spend much time on it this winter. Ignition etc. is still where is was in 2013.

Today I decided to open my Pontiac Shop manual again, and found a "hidden" page that I was looking for. This page contains come interesting info.
I took some pics of that page, as it's not completely clear for me.
I guess this could explain why my car doesn't like much total advance.

On this page under the distributor section, they are talking about "cam angle" I really have no idea what this would have to do with the distributor (i assume this has something to do with the camshaft?).
The "centrifugal advance (dist. degrees)" is very low. The details within the red lines are related to my car and engine combination.
Original on petrol I need to set the initial timing to 6 degrees BTDC. So would this mean the total timing without vacuum was back in the days 6+10 = 16 degrees???
Image

And here are some other pics from the pages that are talking about the initial 6 degree + a picture of the marks on the balancer.
Image

Image

The current curve i'm using is still unchanged:
450 rpm: 10 degrees (initial, no advance)
500 rpm: 10
1000 rpm: 18
1500 rpm: 19
2000 rpm: 22
2500 rpm: 23
3000+ rpm: 23
(so the total mechanical advance is about 13 as the initial is 10).

Vacuum @ manifold is doing 8 degrees at idle (high vacuum, if I remember correctly about 20Hg stable).
For the rest I have no idea what it is doing under different loads.

I can't increase the total advance any further as 23 degrees, otherwise i can hear it pinging at about 26 degrees @ WOT 2500rpm+.
Pulling the vacuum advance doesn't make any difference in that.

What do the LPG guys over here think about that page in my shop manual regarding the advance. Sounds a bit low to me, but maybe it's really an engine that likes low advance.
The engine is a stock 389 cui, with 10.25:1 compression, good for 303HP - 330HP with 430 lb.ft (583nm) of torque. 98 octane petrol was recommended

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Re: Pontiac 389 engine with a Impco 425 - high idle

Post by Steptoe » Tue Apr 01, 2014 4:22 pm

I have not read your post all the way thru....heading out the door on urgent job
The dregree in the tune up specs are in cam degree NOT crank degrees
times them by 2
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