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Olds 303 engine tuning advice needed

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I finished my Rocket 88 restoration last spring and have been driving the car to work several times a week. Since I had no experience with any car this old I had no real idea how it should perform. Since my commute involves driving on 2 major highways I need to keep up with the pace which is about 70mph. (NJ)

I have noticed the engine did not like to rev much and assumed it was the nature of the over square, high torque motor but thought I would poke around to see if there was anything out of adjustment. When I checked the timing it was at least 6 degrees before the mark at idle so I moved it back to the correct timing marks. Vacuum port off the carb plate has no vacuum at idle but I removed it before checking the timing anyway. When I took the car for a ride it made the problem exponentially worse. Car would cough and had zero power so I pulled the distributer and found the centrifugal advance gummed up and some small pieces of plastic insulator had broken off and were rolling around inside. I believe it may have been stuck in "centrifugal advanced" mode. I cleaned it all up, made sure everything was free, sucked on the vacuum advance to see it move and hold, re-gapped the points and put it all back together. I reset the timing back to the marks and it drove ok but still seemed flat. I did verify that the timing started to change at around 750 rpm and then changed even more as I slowly rev'd the motor so it would seem that both "advance" mechanisms were working. I also notice that driving at higher speeds (60+) the car has a hesitation even when just maintaining speed, not accelerating.

As a test, I reset the timing at idle to several degrees before the timing mark. The car feels like it has 30 more hp and rev's much more freely. Acceleration is excellent, shifts are smooth and I only need about half of the gas pedal travel that I was using. When pushed to the floor, the trans downshifts and the engine revs freely. All is great accept I still have the hesitation at 60+ mph. Below 60mph I would say it is perfect.

I feel like it is a timing issue if for no other reason that the car runs much better when I set the timing off the correct marks. What I don't understand is why the engine would develop a hesitation at a certain rpm without trying to accelerate. The same rpm moving up through the gears does not show any sign of hesitation, only in fourth gear around 60mph. The car will still accelerate, but the engine stumbles a little. The faster I go, the more it stumbles. I can even feel the stumble coasting and decelerating although it is much harder to notice. I have cleaned all the jets in the carb (Rochester AA), made sure the accelerator pump is working fine, have no restriction to the carb intake. ( I have run with and without the air cleaner)

I really don't want to through parts at it so any advice from the gurus would be greatly appreciated. The questions I am asking myself..

  • Why would setting the timing at idle several degrees before the correct marks make it run better? Is advancing the timing making up for some other problem? Could I be loosing vacuum at higher rpm and my timing fluctuate causing the stumble?
  • Why would I get great acceleration and free rev'ing of the motor up to 60mph but only then start to feel a "stumble"? Rpm going up through the gears goes higher than what I have at 60mph but engine seems fine.
  • Why would I feel the hesitation just maintaing speed and not trying to accelerate at all?

If you made it this far through the dissertation I'm hoping you have a nugget or two of advice for me.


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Go back to the start...Remember when a person would take their car in for a "Tune-up"? POINTS...PLUGS and CONDENSER.....

Set you dwell first...28-32@, I set mine at 30@. Then set your timing....5@ BTDC..... One degree + will equal one degree - or retard timing......or in other words, as your dwell increases, your timing will retard...31 degrees of dwell, will equal 4 degrees of timing...and so forth.

Chances are your distributor block is original and is a little worn or may even have a bit of wobble in the distributor shaft....

When you set your dwell to the spec's, leave your dwell meter connected. Next set your timing, with your timing light....(Preferably you have the adjustable light that you can zero back)

Start you car and bring up to operating temps....Using the Dwell meter, the RPM side bring up to around 3000 RPM. This is about what the RPM's would be at 60 MPH....Using your dwell meter, switch back to dwell and see if you are still reading the 30@....Using your adjustable timing light, see if your timing mark has remained, or has it advanced or retarded?

Those will tell you if your vacuum advance or mech advance is working as they should. You mention about your initial timing mark...A INCREASE in timing is required for a increase in altitude.. Initial factory settings are at sea level and of a given standard day....29.92 and 59 degrees temp.

Next, I also use a vacuum gauge for my final cross check....Using the vacuum gauge connected to a good vacuum source. What is the vacuum reading at idle? Preferably around 17 inches. This will vary depending on the altitude you live.....Increase your rpm's and see a vacuum reading...Although you will not be under load, you should see a increase in vacuum as you stabalize your rpm's...The vacuum gauge readings will also give you a indication of what your valve timing is doing as well as the condition of your intake and/or exhaust valves. If one or more is leaking, that will in turn affect on whether or not your engine is stumbling at a higher rpm. The vacuum gauge will also indicate if you have advance or retarded timing. (In my opinion, the vacuum gauge is the most important gauge to use in finding the correct timing for a engine)

You did not state how many miles your engine has, but as a engine and all the components change as they wear, so allowance has to be made for those things.

Best wishes in finding that "Sweet Spot".

Old Codger

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Thank you for that info. Since my car is a bit older than me I have little experience with anything pre electronic ignition. I have the original shop manual for the car and there is no mention of "Dwell". The tune up specs say to set the points gap at .015" and set timing with a strobe on the crankshaft mark. The only adjustment I see on the points is gap. I don't see any ability to move the points themselves. I'm sure the distributor has it's wear but no idea if it is within tolerances. The best I could do was to make sure the breaker plate moved freely and that the weights and springs underneath did not show signs of binding.

The vacuum comes off the carb above the valve body so I have 0 vacuum at idle. The engine had a light rebuild last year (Rings/bearing) The motor has <100k miles as far as I can tell. Very little ridge in the bore. Compression is around 115# +/- 5# across all cylinders which is within the factory spec.

I'm wondering if my mechanical advance is still not functioning correctly and my new setting is making up for it but at higher rpm the vacuum advance might be taking it too far causing the stumble. I just hate to drop $250 on a distributor based on a guess.

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The specifications for the Dwell can be found...probably in the back of your book....under Distributor Specifications....Although my engine is a 52...it too is a 303 so the specifications would be the same...

It lists the cam angle range of 26* to 33*...This is the Dwell....Also it goes on to list the Mechanical Advance 1000 RPM 2.5* to -6.5*......2000 RPM 15* to -19* and all in at 3000 RPM 28* to -32* (This is when a adjustable timing light comes in handy. At 3000 rpm..if all is going well....you have initial timing of say 5*...plus all in mechanical advance 28* = 33*. It will remain at 33* anything past 3000 rpm...By using a adjustable timing light, you can dial it back, till you get a zero and read what the total timing is. (The difficulty is, I don't know of any 6 volt adjustable timing lights...They might be out there, but I don't know of one)

Using the Feeler gauge to get a setting of .016 is only a starting point....You can also set your points with a match book cover...the Old Skool way to get you started......Unfortunately this method is sort of a hit and miss...A Dwell reading is obtained, as the engine is running...It will tell you if you need to open the points or close them...So you have to shut the car off, adjust the points as needed..further open, or close, Button up the distributor, start the car, and look at your reading. If need be, do it again till you obtain the dwell you're looking for....Be sure to get the lock down screw tight on the points, or after a few miles, the points will work closed on you. Unfortunately, the early Oldsmobile distributors are not like the mid 50's Chevy V8's that had a little window on the side of the distributor, and all you needed was a allen wrench to set the dwell as the engine was running.

As far as getting a vacuum reading....You have vacuum activated wipers...You can "T" into this line, and hook your vacuum gauge there...Be sure to cap off the "T" after you've used it. A vacuum leak will definitely cause you grief. You can buy a vacuum gauge at about any auto store. They'll come with instructions on what the readings will mean...Although you didn't mention it, but if you need to adjust your fuel/air mixtures, a vacuum gauge can be used for that too. While you're at the store, they will have a tac/dwell meter there too, I'm sure. If not Sears will have them.

For me, using a vacuum gauge to adjust the timing...

1. Set the dwell first

2. Set the initial timing using the timing light.

3. Using the vacuum gauge, loosen the distributor hold down nut, and turn the distributor to see what the highest vacuum reading is just before it starts to stumble. Turn the distributor the other direction to see what the lowest reading is, before it starts to stumble. Go back to the highest reading and turn the distributor back 1" under the highest reading. Lock down the distributor....Using this method will also set your timing for the altitude you're be driving at most of the time....

As another benefit for using a vacuum gauge, if you leave the gauge hooked up to the "T" and use a long hose to run into the inside of the car, you can observe the reading to see how you're doing on your fuel usage....Like it was done in the old days...There were vacuum gauges or some times called motor minder gauges...If you've got your foot into it, the vacuum reading is lower, or going up a hill, the reading will be lower...If you're going down hill, and coasting, the vacuum will rise....Sort of like those vacuum wipers... slow down going up hill, and speed up going down, or stay steady if not under a load. It will give you a reading of how much vacuum is being produced at any given speed. As you're driving at the higher speeds, you'll be under a load, and you'll also be able to observe what your timing is doing as you're driving...

Vacuum gauge = a mechanic's best friend.

You'll get it!!!!

Old Codger

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  • 2 months later...

I followed all your advice and things are much better now. At idle I am pulling 20" vacuum and when I rev the motor to about 2k rpm I am still pulling nearly the same after it recovers. By using your method to set the timing I ended up going from 2.5 degrees (the recommended) to closer to 8 degrees by my estimation. Hard to tell since there are no markings for degrees. I am not getting any pinging under load and the engine rev's easily. Best part is my vacuum wipers work better now :-)

Thanks for all the advice. This has been quite a learning experience and it's always nice when your efforts are rewarded. My next quest is to find an under dash tachometer and vacuum gauge that look appropriate for the age of the car.

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If the timing mark is on a solid metal "hub" attached to the crankshaft or a mark on the flywheel, then you should be getting an accurate timing situation. IF it's located on a rubb-insulated "damper" (as modern engines have, with the outer ring insulated from the inner part by a rubber sheet), then the outer ring CAN move with time and age, which can give your "factory-correct" settings a degree of inaccuracy.

Most engines don't have vacuum at the distributor's vacuum port on the carb at base idle. This is "ported vacuum", but as soon as the throttle blades are openned, THEN vacuum happens there. On some engines, there can be a residual vacuum there at base idle, so that's why you're supposed to unplug that vacuum source to the distributor vacuum advance when setting base idle timing at BASE idle speed.

The fact that the trans is acting better, and the car appears "happier" with your current settings, then things are getting better for all involved! It was not uncommon to advance the base timing over specs by hot rodders back when your car was newer. As long as everything acts "right" and there is no pinging upon hard acceleration, then you might be able to kick things up a few more degrees of base timing.

As for the distributor, when you run into a situation where the point gap is set correctly and the dwell is not like it should be, that will indicate wear on the distributor's "cam" where the "lobes" are which open and close the ignition points. One reason that dwell is preferred is that EACH lobe of that cam can have different wear on them. The ONE you use to set the gap might be representative, or it could be a few .001" off from the others. If you try to set the points with a dial indicator, you'll see this situation, big time, as I did. ALSO remember to use genuine "point grease" to lube the rubbing block on the points! Many point sets have deleted that little vial of grease, over the years, and nobody knew to notice!

Now that you've got the engine tuning pretty much nailed-down, you might ensure the tires are inflated to about 30frt/28rr to help reduce rolling resistance and sharpen steering response.

Happy Holidays!


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