Acceleration bog
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Joined: Nov 2024
Posts: 238
From: Langton, Ontario
Acceleration bog
Hey Gang...
Recently finished my 4 barrel conversion on my 64 Ninety Eight. I have been trying to chase down an acceleration bog from idle, and I was wondering if you guys could add your two cents.
I had the 4Jet rebuilt, AND have properly restored the accelerator pump. It works like it should with two strong streams when activated.
I have chased down all the vacuum leaks
Spark plugs, wires, cap, coil and rotor are all new
The only thing i can think of that I haven't tried is timing. Right now, it's set to factory specs of 2.5 degrees. Can anyone confirm if that is correct for this application?
Thanks!!
Ty
Recently finished my 4 barrel conversion on my 64 Ninety Eight. I have been trying to chase down an acceleration bog from idle, and I was wondering if you guys could add your two cents.
I had the 4Jet rebuilt, AND have properly restored the accelerator pump. It works like it should with two strong streams when activated.
I have chased down all the vacuum leaks
Spark plugs, wires, cap, coil and rotor are all new
The only thing i can think of that I haven't tried is timing. Right now, it's set to factory specs of 2.5 degrees. Can anyone confirm if that is correct for this application?
Thanks!!
Ty
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Joined: Nov 2024
Posts: 238
From: Langton, Ontario
Running On Empty
Joined: Jul 2018
Posts: 26,195
From: Earth
Thread Starter
Registered User
Joined: Nov 2024
Posts: 238
From: Langton, Ontario
Okay! Thanks for the info! I always get the info I'm looking for on this forum. I have NOT set my dwell, HOWEVER, I just so happen to have an old-school Realistic brand dwell meter that I snagged at a garage sale in the 90's!
So, I will set the dwell and bump up the timing. I'll post my results when I test it.
Ty
So, I will set the dwell and bump up the timing. I'll post my results when I test it.
Ty
Running On Empty
Joined: Jul 2018
Posts: 26,195
From: Earth
Always in this order:
(1) Set dwell at correct RPM;
(2) Estabish timing;
(3) Set A/F idle mixture screws.
Disconnect dist. vacuum advance (plug both ends) when establishing timing, reconnect prior to adjusting A/F idle mixture screws one at a time - preferably w/ a vacuum gauge to achieve highest vacuum possible- somewhere between 15”Hg - 22”Hg usually ~ 17”Hg for a stock cam.
(1) Set dwell at correct RPM;
(2) Estabish timing;
(3) Set A/F idle mixture screws.
Disconnect dist. vacuum advance (plug both ends) when establishing timing, reconnect prior to adjusting A/F idle mixture screws one at a time - preferably w/ a vacuum gauge to achieve highest vacuum possible- somewhere between 15”Hg - 22”Hg usually ~ 17”Hg for a stock cam.
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Joined: Jul 2019
Posts: 363
always in this order:
(1) set dwell at correct rpm;
(2) estabish timing;
(3) set a/f idle mixture screws.
Disconnect dist. Vacuum advance (plug both ends) when establishing timing, reconnect prior to adjusting a/f idle mixture screws one at a time - preferably w/ a vacuum gauge to achieve highest vacuum possible- somewhere between 15”hg - 22”hg usually ~ 17”hg for a stock cam.
(1) set dwell at correct rpm;
(2) estabish timing;
(3) set a/f idle mixture screws.
Disconnect dist. Vacuum advance (plug both ends) when establishing timing, reconnect prior to adjusting a/f idle mixture screws one at a time - preferably w/ a vacuum gauge to achieve highest vacuum possible- somewhere between 15”hg - 22”hg usually ~ 17”hg for a stock cam.
Last edited by Dynoking; Oct 16, 2025 at 09:58 AM.
CH3NO2 LEARN IT BURN IT
Joined: Jul 2010
Posts: 5,643
From: Land of Taxes
Once the dewll is set at 30°, use a vacuum gauge connected to a live manifold vacuum source, adjust the ignition timing and air/fuel mixture simultaneously to achieve the highest possible and most steady vacuum reading on a fully warmed engine, choke off.
Do this at curb, idle ~650-750 RPM. If it has working AC, then maybe 800 RPM.
You want to give the engine as much timing as it will tolerate without preignition under warm load or wide open throttle (WOT).
Verify the vacuum advance is working at idle. I recommend hooking that up to a live(not ported) vacuum nipple.
Then use a timing light to see where the timing is. 2.5 to 5° is lame.
The only time I use a light is to see where the timing is after I find the sweet spots. And to map the vacuum & centrifugal curves.
A wide-band air fuel meter is your best friend.
It will tell you which side of stoichiometric the engine is running (lean or rich).
Of course, reading the spark plugs and tail pipe color after several miles will give you an idea.
Just because the carb was rebuilt doesn't mean you are finished tuning it.
Do this at curb, idle ~650-750 RPM. If it has working AC, then maybe 800 RPM.
You want to give the engine as much timing as it will tolerate without preignition under warm load or wide open throttle (WOT).
Verify the vacuum advance is working at idle. I recommend hooking that up to a live(not ported) vacuum nipple.
Then use a timing light to see where the timing is. 2.5 to 5° is lame.
The only time I use a light is to see where the timing is after I find the sweet spots. And to map the vacuum & centrifugal curves.
A wide-band air fuel meter is your best friend.
It will tell you which side of stoichiometric the engine is running (lean or rich).
Of course, reading the spark plugs and tail pipe color after several miles will give you an idea.
Just because the carb was rebuilt doesn't mean you are finished tuning it.
Registered User
Joined: May 2009
Posts: 2,269
From: Northern California
X2 on the Wideband oxygen sensor. Probably the best tuning tool I ever got for carbs. They tell you in real time at idle, part throttle and wide open operation whether you’re lean or rich.
If you’re going to get into 4 jet tuning, buy some rods & jets slightly different from what your car has. You can get these either in unrebuilt original carbs (maybe cheap, maybe not) or perhaps from the classic carburetor houses. I don’t know if 4 jet and Qjet primary rods and jets swap since they’re different generation, but if they do, you’ve got a giant world of parts to test/tune with.
Point 2 is that if you’re getting into tuning on these, get 3-5 sets of gaskets so you can experiment and get the car running again in case of gasket tear or gasket sticking to the metal from disassembly and parts swapping.
If the bog you’re referring to is the transition from idle mixture (controlled by the idle screws) to part throttle operation, this is exactly when the rods and jets are invoked (i.e. rods start moving upward) to meter the fuel through the main jets. Usually if it’s a bog, that means too much fuel. Usually if it’s a miss, that’s not enough fuel.
In the case of a bog, and you believe it’s too rich, you need to reduce the square metering area as the rods move upward out of the jets. You can either use a thicker (larger) rod, or a smaller jet. Usually rod changes are smaller incremental changes than jet changes.
It could also be that the rods & jets are right, but the rods are moving up out of the jets too fast due to a spring that’s too stiff. I don’t know about 4 jets enough to say, but if a spring moves the rods up, consider getting a softer spring (assuming you can find it…) which leaves the rods seated in the jets for a little longer, leaning out the part throttle mixture.
Sorry I can’t be more helpful, I did Holley’s 40 years ago and have focused on quadrajets since then.
Cheers
Chris
If you’re going to get into 4 jet tuning, buy some rods & jets slightly different from what your car has. You can get these either in unrebuilt original carbs (maybe cheap, maybe not) or perhaps from the classic carburetor houses. I don’t know if 4 jet and Qjet primary rods and jets swap since they’re different generation, but if they do, you’ve got a giant world of parts to test/tune with.
Point 2 is that if you’re getting into tuning on these, get 3-5 sets of gaskets so you can experiment and get the car running again in case of gasket tear or gasket sticking to the metal from disassembly and parts swapping.
If the bog you’re referring to is the transition from idle mixture (controlled by the idle screws) to part throttle operation, this is exactly when the rods and jets are invoked (i.e. rods start moving upward) to meter the fuel through the main jets. Usually if it’s a bog, that means too much fuel. Usually if it’s a miss, that’s not enough fuel.
In the case of a bog, and you believe it’s too rich, you need to reduce the square metering area as the rods move upward out of the jets. You can either use a thicker (larger) rod, or a smaller jet. Usually rod changes are smaller incremental changes than jet changes.
It could also be that the rods & jets are right, but the rods are moving up out of the jets too fast due to a spring that’s too stiff. I don’t know about 4 jets enough to say, but if a spring moves the rods up, consider getting a softer spring (assuming you can find it…) which leaves the rods seated in the jets for a little longer, leaning out the part throttle mixture.
Sorry I can’t be more helpful, I did Holley’s 40 years ago and have focused on quadrajets since then.
Cheers
Chris
Phantom Phixer
Joined: Dec 2011
Posts: 5,534
From: Apopka, FL
X2 on the Wideband oxygen sensor. Probably the best tuning tool I ever got for carbs. They tell you in real time at idle, part throttle and wide open operation whether you’re lean or rich.
If you’re going to get into 4 jet tuning, buy some rods & jets slightly different from what your car has. You can get these either in unrebuilt original carbs (maybe cheap, maybe not) or perhaps from the classic carburetor houses. I don’t know if 4 jet and Qjet primary rods and jets swap since they’re different generation, but if they do, you’ve got a giant world of parts to test/tune with.
Point 2 is that if you’re getting into tuning on these, get 3-5 sets of gaskets so you can experiment and get the car running again in case of gasket tear or gasket sticking to the metal from disassembly and parts swapping.
If the bog you’re referring to is the transition from idle mixture (controlled by the idle screws) to part throttle operation, this is exactly when the rods and jets are invoked (i.e. rods start moving upward) to meter the fuel through the main jets. Usually if it’s a bog, that means too much fuel. Usually if it’s a miss, that’s not enough fuel.
In the case of a bog, and you believe it’s too rich, you need to reduce the square metering area as the rods move upward out of the jets. You can either use a thicker (larger) rod, or a smaller jet. Usually rod changes are smaller incremental changes than jet changes.
It could also be that the rods & jets are right, but the rods are moving up out of the jets too fast due to a spring that’s too stiff. I don’t know about 4 jets enough to say, but if a spring moves the rods up, consider getting a softer spring (assuming you can find it…) which leaves the rods seated in the jets for a little longer, leaning out the part throttle mixture.
Sorry I can’t be more helpful, I did Holley’s 40 years ago and have focused on quadrajets since then.
Cheers
Chris
If you’re going to get into 4 jet tuning, buy some rods & jets slightly different from what your car has. You can get these either in unrebuilt original carbs (maybe cheap, maybe not) or perhaps from the classic carburetor houses. I don’t know if 4 jet and Qjet primary rods and jets swap since they’re different generation, but if they do, you’ve got a giant world of parts to test/tune with.
Point 2 is that if you’re getting into tuning on these, get 3-5 sets of gaskets so you can experiment and get the car running again in case of gasket tear or gasket sticking to the metal from disassembly and parts swapping.
If the bog you’re referring to is the transition from idle mixture (controlled by the idle screws) to part throttle operation, this is exactly when the rods and jets are invoked (i.e. rods start moving upward) to meter the fuel through the main jets. Usually if it’s a bog, that means too much fuel. Usually if it’s a miss, that’s not enough fuel.
In the case of a bog, and you believe it’s too rich, you need to reduce the square metering area as the rods move upward out of the jets. You can either use a thicker (larger) rod, or a smaller jet. Usually rod changes are smaller incremental changes than jet changes.
It could also be that the rods & jets are right, but the rods are moving up out of the jets too fast due to a spring that’s too stiff. I don’t know about 4 jets enough to say, but if a spring moves the rods up, consider getting a softer spring (assuming you can find it…) which leaves the rods seated in the jets for a little longer, leaning out the part throttle mixture.
Sorry I can’t be more helpful, I did Holley’s 40 years ago and have focused on quadrajets since then.
Cheers
Chris
This is a completely different carb than a Quadrajet.
There are no "rods", just jets.
To get an idea of the workings of a 4GC carb, read this;
How to ; Rebuilding a Rochester 4GC carb - ClassicOldsmobile.com
Registered User
Joined: May 2009
Posts: 2,269
From: Northern California
Charlie,
Many thanks for the education with that link. Happy to stand corrected on an area outside my experience.
Hope I didn't derail anything with my comments. Just trying to be helpful with what little I know about mechanical air/fuel metering. I've burnt days/weeks on qjet metering and was hoping that experience might serve here. Wrong I was, beyond theory.
Some days, it's more fun getting it _wrong_ and learning rather than getting it right!
Cheers
Chris
Many thanks for the education with that link. Happy to stand corrected on an area outside my experience.
Hope I didn't derail anything with my comments. Just trying to be helpful with what little I know about mechanical air/fuel metering. I've burnt days/weeks on qjet metering and was hoping that experience might serve here. Wrong I was, beyond theory.
Some days, it's more fun getting it _wrong_ and learning rather than getting it right!
Cheers
Chris
Registered User
Joined: May 2009
Posts: 2,269
From: Northern California
Charlie,
All good on my end. In this case I was hoping that the lessons I’d learnt on the qjet applied to the 4GC. Turns out they don’t.
At 61, I’m well past being embarrassed about being wrong. 100% happy to stand corrected that the 4GC and Quadrajet are different designs..
Upside for anyone who hits this thread is they’ll get the fact that qjet logic doesn’t apply to the 4GC carbs. Just ruling out bad advice helps too.
40+ years on with these cars, still happy to be learning.
Cheers
Chris
All good on my end. In this case I was hoping that the lessons I’d learnt on the qjet applied to the 4GC. Turns out they don’t.
At 61, I’m well past being embarrassed about being wrong. 100% happy to stand corrected that the 4GC and Quadrajet are different designs..
Upside for anyone who hits this thread is they’ll get the fact that qjet logic doesn’t apply to the 4GC carbs. Just ruling out bad advice helps too.
40+ years on with these cars, still happy to be learning.
Cheers
Chris
Thread Starter
Registered User
Joined: Nov 2024
Posts: 238
From: Langton, Ontario
Well, I finally got around to setting the timing on the Olds. It was quite the ordeal! I don't think that it's been touched for a very long time, as the distributor post was seized HARD in place. It still had the little metal band tag on it. It took some heat, penetrating oil, and finally a cold chisel and hammer to break it loose. It was at 2.5 degrees, so I bumped it up to 7.5 degrees, and immediately noticed that it was running better. The throttle response when I blip the throttle lever is immediate and strong. Took it for a spin and was shocked at the power improvement, especially with the accelerator mashed to the floor!
I set the air/fuel mixture with a vacuum gauge and have it running smooth at idle.
BUT...it still has an acceleration bog off the line. So, I guess I'm back to it being a fuel issue. Too much or too little.
I set the air/fuel mixture with a vacuum gauge and have it running smooth at idle.
BUT...it still has an acceleration bog off the line. So, I guess I'm back to it being a fuel issue. Too much or too little.
Thread Starter
Registered User
Joined: Nov 2024
Posts: 238
From: Langton, Ontario
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Posts: 2,092
It was at 2.5 degrees, so I bumped it up to 7.5 degrees, and immediately noticed that it was running better. The throttle response when I blip the throttle lever is immediate and strong. Took it for a spin and was shocked at the power improvement, especially with the accelerator mashed to the floor!
Looking over some of your previous threads paints a specific picture.
The 260 hp 394s factory timing for 1962 was 2.5 degrees as yours was. The high compression 280 hp 2bbl 394s factory timing was 5 degrees.
2bbls both low and high compression shared the same cam. But 4bbls did not, they have more aggressive cams. In other words the 4bbl swap will not have a performance impact. But if it matches the 2bbl performance and is in better working condition, great.
Also the spark plugs are not the same. The low compression 2bbl is supposed to be a AC45. The high compression 2bbl and 4bbls are AC44.
For reference.
https://over-drive-magazine.com/2024...ar-fact-sheet/
https://xr793.com/wp-content/uploads...-Full-Line.pdf
http://carnut.com/specs/gen/olds60.html
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