Doing a compression test on my 1949 olds 303 because I suspect a valve issue. Anyone know what the correct cylinder psi is suppose to be? I saw 120 psi at cranking speed on a random post but can’t find the spec in my service manual.

 

My 1958 Motor's Auto Repair Manual shows that a 1949 Olds 303 had a 7.25 to 1 compression ratio.
The minimum compression pressure at cranking speed was 120 PSI.

 

Each of my 8 cylinders ran between 145-155 both hot and cold. Higher than normal. Perhaps due to oil blow by? Plugs look good. I’m still chasing an odd issue. The engine runs great until it reaches operating temp. Then it begins to miss just a bit and stumbles on acceleration. I’ve gone over every system imaginable other than the valve train and the intake manifold. I did a smoke test but found no leaks. I was hoping for evidence of some valve issue on the compression test. No such luck. I suppose I have to do a leak down test next.

 

The 120 PSI quoted in Motor's is a minimum'
145 to 155 is good.
I think your hesitation may be due to a carburetor problem, specifically the accelerator pump.
When was the last time the carb was overhauled?

 

A warm ignition miss can be a failing ignition coil or condenser amongst many other things. What ignition components have been replaced?

 

Those are good compression numbers. A cylinder leak down test isn't indicated. x2 on looking at your ignition or carburetor.

 

Have a look at this thread if you plan to evaluate the IGN coil >>>Electrical advice please. This was a no start condition but contains valuable information for testing the IGN coil. It's a lot to expect a 75 year old IGN coil &/or capacitor hasn't degraded/leaked. I tend to side w/ Sugar Bear. Measuring resistance going into the coil & resistance going out of the coil provides good information regarding both primary & secondary windings of the IGN coil state of health.

Considering age of vehicle I'd change both the IGN coil & capacitor (condenser) w/o skipping a heart beat w/ no need to diagnose each separately. Perhaps not the absolute best troubleshooting/diagnostics but that old beast will surely thank you. IGN coil is ~$16 the condenser ~$6.

I'd likely change the capacitor (condenser) 1st then drive it several times. See if the issue remains. If the issue resolves you've found the capacitor (condenser) was faulty (which is where I'll bet the issue resides). If the issue isn't resolved via R&R of the capacitor, next replace the IGN coil.
Out-of-pocket expense $25 - $30.

BTW, these old beasts had a tune-up interval of about every 10,000 miles. During tune-up the condenser (capacitor) was supposed to be changed.

 

Thanks friends. I’m sure it’s not carb or ignition. Those systems have been thoroughly tested and even replaced to confirm. To fix/find this issue I have:
* rebuilt the carb to factory specs.
then just to confirm I…
* replaced the carb with one rebuilt by carburetor center in California.
* replaced all ignition components with stock replacements.
then to confirm I…
* replaced stock ignition parts with Petronix.
* replaced coil with oil filled perteonix
*replaced oil filled patronix with epoxy filled (customer support told me an oil filled coil will overheat if on its side).
* sent distributor to a professional for rebuild thinking there was internal wear.
* plugs, wires and the like making sure each component was compatible with the system.

the olds starts and runs smooth as butter till it gets to temp and then it develops a miss. That miss barely affects drivability at low speeds. Most would never know there is a problem because I know how to baby the throttle when it’s at temp. But accelerate to what I’d guess is 3000 rpm and the miss becomes an issue. Mat the gas peddle and you’ll pay for it with a BANG out the tail pipe.

yesterday I considered exhaust restriction as a possibility on the exhaust manifold heat riser valve but after pinning it in the open position I can confirm that’s not a he issue. Th exhaust is new and has no restrictions. It’s a beautiful car but something is off and it’s stumped myself (and I’m no dummy) and my local experts at full service shops and restoration shops. For the cost of an intake manifold gasket I’m going to replace it and hope for the best.

 

Speaks to a very rich fuel-to-air ratio. Fuel not being burned exists exhaust manifold & ignites the fuel - BANG!. Hook up a vacuum gauge & dial in your vacuum. Not 100% what vacuum (Hg) a 303 should be but most V8 engines are w/in the range of 15"Hg - 21"Hg. The key point is to have a rock steady needle which does not waiver. Nominally you should expect a rock steady vacuum needle ~17.5"Hg.

You must pass the initial test (Upper LH corner) of this diagram. Then, review the diagram. You may have a vacuum leak or the carburetor is not fine-tuned appropriately - Air:Fuel ratio is too rich.

 

Thank you but vacuum has been confirmed. I pull 22inch lbs at idle both cold and warm. Snap the throttle and it drops about 5 but then returns. I suspect a leak in the manifold once it gets to temp. Sadly I can’t confirm that with a smoke machine because the manifold has a hole in the casting that is there from the factory. Its purpose is to draw fresh air into the heat riser tube. I’ll try to post a link to a previous post where we learned about that:


That’s not a pic of my personal manifold, just one I found online while researching. If it were mine I’d have already replaced the gasket!
In anticipation of other suggestions, the choke has been properly adjusted as per the manual. I’ve also manipulated it manually with no change. Can’t be carb. Just not possible. Could it be ignition? It shouldn’t be with new parts professionally rebuilt. Especially since the issue remained the same from old part to new part. I’m curious about the timing changing at higher rpm. I’m exploring getting a dynamic timing light to see how many degrees off it is at 3000rpm. BUT THEY SHOULDNT MATTER since the miss starts to come at idle once the engine gets heated up to normal range.
that leads me to suspect a valve spring is collapsing. To confirm that I’ll have to do a leak down test. All those woll
come if the manifold gasket is a strike.
bur PLEASE keep the ideas coming. I’m happy to hear all help, even if it’s been considered and resolved.

 

Here's the thread where we mentioned that.

https://classicoldsmobile.com/forums...6/#post1581274

If you have good vacuum and the carburetor has been rebuilt I'd be suspicious of that distributor. Especially if your mechanic thinks that a coil shouldn't be mounted on its side.

 

It's late in the afternoon & I'm exhausted so I'm going to copy/paste this (which I stole) anyways rather than create my own dialogue/diatribe. Just a couple points to consider.

 

It had better drop down to 5"Hg. You stated it drops about 5. This would be a LARGE difference. It MUST drop down to 5"Hg.
Additionally, the needle needs to be rock solid - no variances.

 

Another thing to check when looking for distributor problems is the ground wire for the distributor points plate. ( red arrow in illustration above)
It should be a bare copper wire that goes between the points plate ( blue arrow) and the distributor housing .
It provides a reliable ground for the points.
Because the points plate rotates inside the housing on 3 ball bearings. (yellow arrow)
This wire is a special wire that is made to flex without breaking . An ordinary coated wire won't hold up.

Another thing to check is the distributor for shaft bushing wear.
Simply grab the rotor and try to move it side to side.
If you feel any appreciable play, then it's time to have the distributor bushings replaced.
I had the bushings in my distributor replaced by this outfit. With good results.
Everyday Performance Distributor Services (everyday-performance.com)

Another quick thing to check is the centrifugal advance mechanism.
You should be able to rotate the rotor backwards a few degrees under spring tension.
And have it snap back when released.



.

 

Please remember, I’m not currently using points. I’m using Petronix and the distributor is the third unit I’ve had in the car. I suspected the distributor when I got the car a couple years ago, then rebuilt it myself. Then after assuming I was missing something I sent it in for a professional rebuild.
also remember the car runs 100% perfect with no hesitation and no starting issues until it reaches full operating temp, about a 10 minute drive. Then it develops a slight miss even at idle.

That’s why I suspected coil issue. An overheating coil will act like this. But it’s not that. Three coils prove that. And yes, they are all matched to what ever ignition system I’m using (I’ve used points with some and Petronix with others). No change.

 

(1) Any chance you have any (only two) plug wires in the incorrect firing order?
(2) After 10’ ride pull at least one plug preferably more - wet or carbon fouled?
(3) Ensure plug wires are separated from each other to eliminate possible cross-firing.
(4) Double check timing ensure it isn’t retarded. What is the timing set to?

 

these are the plugs after about 1500 miles. Cyl 2 and 3 look the lost “wet” but none are alarming to me. None are fouled. Plug 2 looks worse in that pic than in real life. I took a video of the plugs and snapped a screen shot of the video for this post.
the distributor has been set as per the service manual. I’ll post that tomorrow. Thanks for all the help everyone.
i forgot to mention I’m sure there is no cross firing because I was worried about that and covered every plug wire in a jacket of fuel line to eliminate the possibility. Also remember, NO ISSUES till at full operational temp.

 

What spark plugs and heat range are you using ?

Does the 303 use a valley pan (cover) like the 324, 371 and 394 ?

I would suggest taking a pic and not doing a screenshot from a movie clip. The clarity of the plug tips is not great.

Your plugs should be a light tan color. Yours look more black. Firing out through the exhaust system leads "me" back toward a carburetion problem.

 

I dug into the intake manifold today to check for leaks. I know… a tail pipe backfire indicates a carb problem and an intake issue usually presents itself with a backfire through the carb.
the gaskets looked ok, not great but I did find another curious issue.
the fresh air tube that passes through the manifold and feeds the warm air tube attached to the choke has a crack in it. I was surprised to see that. I can’t figure how that’s the root of my problem since I’ve attempted to run with the choke disconnected, but it’s a problem none the less. Thankfully I have another intake manifold from a 1952 that is in good shape. Small differences with the casting but it looks like it will bolt up the same. I figured I’d keep you all posted. It will be a couple days till I can reassemble since I’m going to clean up the 52 manifold and paint it before install. Here’s a pic of the broken tube. Maybe it’s somehow causing a restriction in the exhaust manifold port? Long shot but who knows!

 

Did you find the answer to this as I have the exact same problem - starts, runs lovely then after 10 minutes it hesitates over 30 mph and I get nothing on kick-down. 1950 303 all original

 

Sorry the followup is so late on this issue. My problems are fully resolved but to help others I figured I should post here. I posted multiple threads all on this same root issue. I wish I could update all of them but here goes the summery:
When I got my 49 olds with the 303 SO many things were "off" or adjusted incorrectly. It's a miracle the car ran at all (kind of a testament to how robust these systems are). Still, after going through the fuel system and the ignition the car would often hesitate once it reached operating temps. Some times it was easy to overlook the issues because Id baby the car and never really give it the beans! But knowing there was an issue and needing to trust the car to pull out into traffic I kept exploring issues. I replaced the fuel pump, changed fuel filters, rebuilt the carb (found problems with the accelerator pump. It was binding up on a spring and not squirting fuel as it should). These fixes helped but didnt cure the issue. I adjusted ignition timing, checked for vacuum leaks and found a crack in a heat riser tube that fed the choke (which was set to rich because the heat supplied to the choke was coming from a cracked tube). I fixed those things and it helped but did not fix the issue. I changed spark plugs, wires, and coils. Even went from an oil filled coil to an epoxy coil because the tech at Pertronix told me an oil coil cant be mounted on its side. No change there. When I first got the car I replaced the old points and condenser with a the correct pertronix unit for my 6volt system. I did that because ive had great results from pertronix on rebuilds 15+ years ago. Still, even with the Pertronix I wondered if the distributor was functioning as it should or if it was worn out in its mechanical parts so I sent it to North Star Auto Electric for a rebuild. It came back looking good but with no change.
After a year of double and triple checking work id already done I finally came to the conclusion that it HAD to be the distributor. Hard to believe with new parts and a professional rebuild, but I was right!! Turns out Id hired the WRONG people and the Pertronix of today isn't the quality of Pertronix of the past!
My ultimate solution came in the form of a rebuild for the distributor but this time I hired Jeff from Advance Distributor of Shakopee MN. He was super knowledgeable about the 303 and distributors in general. He returned my rebuilt distributor with points and condenser, and fixed what North Star had misaligned. Amazing results!!
I now run 7 degrees of advance at idle with no vacuum as my static timing. I run 16-18 degree of advance at 2500 RPM with no vacuum advance. Be sure to disconnect vacuum at the intake manifold and plug it before setting. While it's not "necessary" to do so on an Olds as it is a Chevy, it wont hurt!! I have the dwell set at 25 degrees and points set at .015 on a warm engine.
With about 1000 miles under my belt with this new set up and I haven't had a single issue. It runs AMAZING!! Every little "fix" and adjustment I did helped but the real issue was spark cutting out and timing being off once warm.

 

Thanks for not giving up, and posting a positive outcome to CO!

 

Glad you got your problem resolved.
I see you have discovered that pertronix isn't the panacea that many think it is