Engine wants too much advance timing?
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Engine wants too much advance timing?
I have just rebuilt my 350 with forged internals, AFR 195 heads and a Comp Cams xfi 268 cam part #08-466-8. I have a aftermarket fuel injection unit for it but before I got into the whole fuel injection thing I was going to run it carbureted to work out any bugs. I put a brand new Chevy HEI distributor with vacuum advance and a new Holley 750 carburetor on it. The engine starts and runs fine. The timing marks and the cam chain are correct. However when you put the car in gear it just dies. Engine wants way more than the normal amount of total timing when run. We are using 8 degrees initial timing, and with vacuum advance on ported vacuum total timing shows 40 degrees. Just wondering if anyone has any ideas or experience in running this particular cam with a carburetor.
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Car: 91 Camaro RS
Engine: 305 (L03)
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Re: Engine wants too much advance timing?
While you are at it, test the fuel pressure and post what you get here.
Last edited by alexisgreene; Aug 21, 2010 at 05:45 PM.
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Re: Engine wants too much advance timing?
It runs fine with 8 degrees? Thats a perfectly normal amount of timing.
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Re: Engine wants too much advance timing?
hows the fueling look and what rpm is it trying to idle at?
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Re: Engine wants too much advance timing?
Engines don't read service manuals or timing lights. If the engine wants it, the engine is right. Give it what it wants, no matter what you think of the "numbers".
Most likely, you have an aftermarket timing pointer, and a stock crank damper. Given that, it's no wonder you're getting screwy results with "numbers".
Not that the "numbers" really matter; but, if you want to know what they are, you must first find #1 TDC ACCURATELY by some means independent of the timing pointer. In an assembled engine, the easiest way to do that is to put something light and flexible in the #1 plug hole, and turn the engine SLOWLY by some means other than the crank damper bolt, until it just touches the object (as determined by watching for it to move); then turning the engine the opposite way until it touches the object again. TDC is halfway in between. Find TDC, mark your damper to match your pointer, now you have an ACCURATE reference. If you have not done this, or you did not do it when the engine was built, then YOU DON'T KNOW WHERE TDC REALLY IS, no matter what you think. Then, make additional marks on the damper every 10° BTDC, so you have ACCURATE knowledge of where those points (which would otherwise occur when your new ACCURATE timing mark is off the pointer) REALLY are.
Then, given what heads you have, your engine will want somewhere real close to 36° of "total" timing. Measure this with the vac adv disconnected and the engine RPM high enough that it no longer advances any farther. DO NOT use a "dial-back" timing light; they are absolutely not dependably accurate at high RPMs. Use a light that flashes EXACTLY when the spark occurs, without allowing it to make some kind of internal "guess" at how far different from that to light. This is your "reference" timing if you will, the TRUE measure of "what the engine wants". The "static" timing, i.e. what you set by twiddling the distributor, should be set to give this timing, and then the weights & springs should be tuned to provide the curve you want, starting from this point.
With the cam you have, most likely your dist has too much centrifugal advance (the kind that takes it from "static" to "total" timing). Most likely, your car will run best with the "total" timing set to about 36°, as described above; about 18° of centrifugal retard at low RPMs; the mechanical advance curve starting just off idle (NO LATER THAN 1200 RPMs), and "all in" by about 2800 RPMs. All of these numbers are with the vac adv disconnected.
Then, once you get that curve, your vac adv will need to be about 12 - 15° ON TOP OF the "total" timing. Yes, that's right: you'll want something around 50° of advance at high RPMs and light load (high vacuum), otherwise known as "cruising" conditions. Typical symptoms of inadequate cruise advance include running hot on the highway, very hot exhaust system on the highway, poor gas mileage, and sluggish throttle response. Vac adv should be set up to give the 15° as stated, and should drop out as soon as the pedal is signifcantly pushed (thereby lowering the vacuum). Best way to tune it is to leave it active down to as low a vacuum as possible, until it starts to ping with light throttle movement at cruising conditions. Adjust the drop-out point to a just high enough vavcuum to make it stop pinging. This will be highly dependent on your converter, car weight, and gears. Odds are, with the cam you have, you'll want the vac adv to remain active down to about 12 - 14" of vacuum, and completely drop out by 10" of vacuum.
As said, first thing you have to do though, is verify your timing marks ACCURATELY, and the 2nd thing is to get a non-"dial-back" timing light (or, turn off the "dial-back" feature so you can get an ACCURATE 0° flsah from it).
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. Then after you've assured yourself of an ACCURATE measurement setup, create the timing curve as described.
You will also probably find that the car runs best with the vac adv hooked to "hard" manifold vacuum instead of the post-emissions "ported" source usually used. Experiment and see after all else is settled.
Most likely, you have an aftermarket timing pointer, and a stock crank damper. Given that, it's no wonder you're getting screwy results with "numbers".
Not that the "numbers" really matter; but, if you want to know what they are, you must first find #1 TDC ACCURATELY by some means independent of the timing pointer. In an assembled engine, the easiest way to do that is to put something light and flexible in the #1 plug hole, and turn the engine SLOWLY by some means other than the crank damper bolt, until it just touches the object (as determined by watching for it to move); then turning the engine the opposite way until it touches the object again. TDC is halfway in between. Find TDC, mark your damper to match your pointer, now you have an ACCURATE reference. If you have not done this, or you did not do it when the engine was built, then YOU DON'T KNOW WHERE TDC REALLY IS, no matter what you think. Then, make additional marks on the damper every 10° BTDC, so you have ACCURATE knowledge of where those points (which would otherwise occur when your new ACCURATE timing mark is off the pointer) REALLY are.
Then, given what heads you have, your engine will want somewhere real close to 36° of "total" timing. Measure this with the vac adv disconnected and the engine RPM high enough that it no longer advances any farther. DO NOT use a "dial-back" timing light; they are absolutely not dependably accurate at high RPMs. Use a light that flashes EXACTLY when the spark occurs, without allowing it to make some kind of internal "guess" at how far different from that to light. This is your "reference" timing if you will, the TRUE measure of "what the engine wants". The "static" timing, i.e. what you set by twiddling the distributor, should be set to give this timing, and then the weights & springs should be tuned to provide the curve you want, starting from this point.
With the cam you have, most likely your dist has too much centrifugal advance (the kind that takes it from "static" to "total" timing). Most likely, your car will run best with the "total" timing set to about 36°, as described above; about 18° of centrifugal retard at low RPMs; the mechanical advance curve starting just off idle (NO LATER THAN 1200 RPMs), and "all in" by about 2800 RPMs. All of these numbers are with the vac adv disconnected.
Then, once you get that curve, your vac adv will need to be about 12 - 15° ON TOP OF the "total" timing. Yes, that's right: you'll want something around 50° of advance at high RPMs and light load (high vacuum), otherwise known as "cruising" conditions. Typical symptoms of inadequate cruise advance include running hot on the highway, very hot exhaust system on the highway, poor gas mileage, and sluggish throttle response. Vac adv should be set up to give the 15° as stated, and should drop out as soon as the pedal is signifcantly pushed (thereby lowering the vacuum). Best way to tune it is to leave it active down to as low a vacuum as possible, until it starts to ping with light throttle movement at cruising conditions. Adjust the drop-out point to a just high enough vavcuum to make it stop pinging. This will be highly dependent on your converter, car weight, and gears. Odds are, with the cam you have, you'll want the vac adv to remain active down to about 12 - 14" of vacuum, and completely drop out by 10" of vacuum.
As said, first thing you have to do though, is verify your timing marks ACCURATELY, and the 2nd thing is to get a non-"dial-back" timing light (or, turn off the "dial-back" feature so you can get an ACCURATE 0° flsah from it).
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. Then after you've assured yourself of an ACCURATE measurement setup, create the timing curve as described.
You will also probably find that the car runs best with the vac adv hooked to "hard" manifold vacuum instead of the post-emissions "ported" source usually used. Experiment and see after all else is settled.
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Re: Engine wants too much advance timing?
[SNIP=sofakingdom;4649076]
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. .[/SNIP]
Sure glad to see someone else is also advocating Accuracy and Attention to Detail. Seems we're a dying breed though, LOL
There's a member on a few other Forums, Marv02, who has been trying to get his new engine to start for the longest, during which he ended up bending (I don't recall how many) pushrods. I wrote basically the same thing to him during my attempts to help out. Of course he simply ignored it. That's when I decided to bow out and leave him to his own devices.
Jake
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. .[/SNIP]
Sure glad to see someone else is also advocating Accuracy and Attention to Detail. Seems we're a dying breed though, LOL
There's a member on a few other Forums, Marv02, who has been trying to get his new engine to start for the longest, during which he ended up bending (I don't recall how many) pushrods. I wrote basically the same thing to him during my attempts to help out. Of course he simply ignored it. That's when I decided to bow out and leave him to his own devices.
Jake
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Joined: Apr 2010
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Car: 84 Firebird
Engine: currently 1968 307
Transmission: 700r4
Axle/Gears: 3.23 lim slip
Re: Engine wants too much advance timing?
Engines don't read service manuals or timing lights. If the engine wants it, the engine is right. Give it what it wants, no matter what you think of the "numbers".
Most likely, you have an aftermarket timing pointer, and a stock crank damper. Given that, it's no wonder you're getting screwy results with "numbers".
Not that the "numbers" really matter; but, if you want to know what they are, you must first find #1 TDC ACCURATELY by some means independent of the timing pointer. In an assembled engine, the easiest way to do that is to put something light and flexible in the #1 plug hole, and turn the engine SLOWLY by some means other than the crank damper bolt, until it just touches the object (as determined by watching for it to move); then turning the engine the opposite way until it touches the object again. TDC is halfway in between. Find TDC, mark your damper to match your pointer, now you have an ACCURATE reference. If you have not done this, or you did not do it when the engine was built, then YOU DON'T KNOW WHERE TDC REALLY IS, no matter what you think. Then, make additional marks on the damper every 10° BTDC, so you have ACCURATE knowledge of where those points (which would otherwise occur when your new ACCURATE timing mark is off the pointer) REALLY are.
Then, given what heads you have, your engine will want somewhere real close to 36° of "total" timing. Measure this with the vac adv disconnected and the engine RPM high enough that it no longer advances any farther. DO NOT use a "dial-back" timing light; they are absolutely not dependably accurate at high RPMs. Use a light that flashes EXACTLY when the spark occurs, without allowing it to make some kind of internal "guess" at how far different from that to light. This is your "reference" timing if you will, the TRUE measure of "what the engine wants". The "static" timing, i.e. what you set by twiddling the distributor, should be set to give this timing, and then the weights & springs should be tuned to provide the curve you want, starting from this point.
With the cam you have, most likely your dist has too much centrifugal advance (the kind that takes it from "static" to "total" timing). Most likely, your car will run best with the "total" timing set to about 36°, as described above; about 18° of centrifugal retard at low RPMs; the mechanical advance curve starting just off idle (NO LATER THAN 1200 RPMs), and "all in" by about 2800 RPMs. All of these numbers are with the vac adv disconnected.
Then, once you get that curve, your vac adv will need to be about 12 - 15° ON TOP OF the "total" timing. Yes, that's right: you'll want something around 50° of advance at high RPMs and light load (high vacuum), otherwise known as "cruising" conditions. Typical symptoms of inadequate cruise advance include running hot on the highway, very hot exhaust system on the highway, poor gas mileage, and sluggish throttle response. Vac adv should be set up to give the 15° as stated, and should drop out as soon as the pedal is signifcantly pushed (thereby lowering the vacuum). Best way to tune it is to leave it active down to as low a vacuum as possible, until it starts to ping with light throttle movement at cruising conditions. Adjust the drop-out point to a just high enough vavcuum to make it stop pinging. This will be highly dependent on your converter, car weight, and gears. Odds are, with the cam you have, you'll want the vac adv to remain active down to about 12 - 14" of vacuum, and completely drop out by 10" of vacuum.
As said, first thing you have to do though, is verify your timing marks ACCURATELY, and the 2nd thing is to get a non-"dial-back" timing light (or, turn off the "dial-back" feature so you can get an ACCURATE 0° flsah from it).
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. Then after you've assured yourself of an ACCURATE measurement setup, create the timing curve as described.
You will also probably find that the car runs best with the vac adv hooked to "hard" manifold vacuum instead of the post-emissions "ported" source usually used. Experiment and see after all else is settled.
Most likely, you have an aftermarket timing pointer, and a stock crank damper. Given that, it's no wonder you're getting screwy results with "numbers".
Not that the "numbers" really matter; but, if you want to know what they are, you must first find #1 TDC ACCURATELY by some means independent of the timing pointer. In an assembled engine, the easiest way to do that is to put something light and flexible in the #1 plug hole, and turn the engine SLOWLY by some means other than the crank damper bolt, until it just touches the object (as determined by watching for it to move); then turning the engine the opposite way until it touches the object again. TDC is halfway in between. Find TDC, mark your damper to match your pointer, now you have an ACCURATE reference. If you have not done this, or you did not do it when the engine was built, then YOU DON'T KNOW WHERE TDC REALLY IS, no matter what you think. Then, make additional marks on the damper every 10° BTDC, so you have ACCURATE knowledge of where those points (which would otherwise occur when your new ACCURATE timing mark is off the pointer) REALLY are.
Then, given what heads you have, your engine will want somewhere real close to 36° of "total" timing. Measure this with the vac adv disconnected and the engine RPM high enough that it no longer advances any farther. DO NOT use a "dial-back" timing light; they are absolutely not dependably accurate at high RPMs. Use a light that flashes EXACTLY when the spark occurs, without allowing it to make some kind of internal "guess" at how far different from that to light. This is your "reference" timing if you will, the TRUE measure of "what the engine wants". The "static" timing, i.e. what you set by twiddling the distributor, should be set to give this timing, and then the weights & springs should be tuned to provide the curve you want, starting from this point.
With the cam you have, most likely your dist has too much centrifugal advance (the kind that takes it from "static" to "total" timing). Most likely, your car will run best with the "total" timing set to about 36°, as described above; about 18° of centrifugal retard at low RPMs; the mechanical advance curve starting just off idle (NO LATER THAN 1200 RPMs), and "all in" by about 2800 RPMs. All of these numbers are with the vac adv disconnected.
Then, once you get that curve, your vac adv will need to be about 12 - 15° ON TOP OF the "total" timing. Yes, that's right: you'll want something around 50° of advance at high RPMs and light load (high vacuum), otherwise known as "cruising" conditions. Typical symptoms of inadequate cruise advance include running hot on the highway, very hot exhaust system on the highway, poor gas mileage, and sluggish throttle response. Vac adv should be set up to give the 15° as stated, and should drop out as soon as the pedal is signifcantly pushed (thereby lowering the vacuum). Best way to tune it is to leave it active down to as low a vacuum as possible, until it starts to ping with light throttle movement at cruising conditions. Adjust the drop-out point to a just high enough vavcuum to make it stop pinging. This will be highly dependent on your converter, car weight, and gears. Odds are, with the cam you have, you'll want the vac adv to remain active down to about 12 - 14" of vacuum, and completely drop out by 10" of vacuum.
As said, first thing you have to do though, is verify your timing marks ACCURATELY, and the 2nd thing is to get a non-"dial-back" timing light (or, turn off the "dial-back" feature so you can get an ACCURATE 0° flsah from it).
Did I mention that ACCURACY is the most important thing here? In case I didn't, then let me introduce the word ACCURATE now. There must be NO guessing, NO assuming, NO "thinking", NO anything except MEASURING, accurately. Then after you've assured yourself of an ACCURATE measurement setup, create the timing curve as described.
You will also probably find that the car runs best with the vac adv hooked to "hard" manifold vacuum instead of the post-emissions "ported" source usually used. Experiment and see after all else is settled.
So I'm confused as to the TDC part now. If you put something in the #1 plug hole and move the engine til that thing moves, say a piece of vacuum hose, the piston is already at the top then right? And then moving it back the other way til it moves again then the piston is at the top again right? And you said that halfway inbetween is TDC? Wouldn't it be BDC? Oh, but halfway is only one turn of the crank and you have to have 2 turns of the balancer to make the piston go down and back up right? I think?
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From: Raleigh, NC
Car: 91 Camaro RS
Engine: 305 (L03)
Transmission: T5
Axle/Gears: Stock
Re: Engine wants too much advance timing?
What he means is there are a few degrees of crank rotation where the piston APPEARS to be TDC. So if you put a plastic straw in the #1 plug hole for instance, and turn the crank until the straw rises right until the point where the straw begins to move downward, this is your first mark. Now slowly turn the crank (just a few degrees) the opposite way until the straw appears to begins moving downward again. This is your second mark. True TDC is between these two points.
Member
Joined: Apr 2010
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Car: 84 Firebird
Engine: currently 1968 307
Transmission: 700r4
Axle/Gears: 3.23 lim slip
Re: Engine wants too much advance timing?
oh. yeah I knew that. kinda. Now, once you find that sweet spot inbetween, how do you know that it is TDC on the firing and not exhaust?
Member
Joined: Apr 2010
Posts: 144
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Car: 84 Firebird
Engine: currently 1968 307
Transmission: 700r4
Axle/Gears: 3.23 lim slip
Re: Engine wants too much advance timing?
here is another question I am debating. I have a cloyes double roller timing set on my 302/5/7 and I can adjust it either 6 degrees advance or 6 degrees retarded. Right now I have it on the regular setting but I am looking at advancing it so I can get more power out of it? How would I set the timing once I move the cam gear? I believe my timing is about it right now because the timing mark on the balancer is at 8 or 10 and the distributer was pointing towards the #1 when I pulled it out.
Member
Joined: Jan 2008
Posts: 137
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From: Raleigh, NC
Car: 91 Camaro RS
Engine: 305 (L03)
Transmission: T5
Axle/Gears: Stock
Re: Engine wants too much advance timing?
Just have a look at the rotor in your distributor. As for the cam advance/retard, you wont get MORE power. Advancing or retarding will simply shift your power band around a little. Advancing will move torque curve to a lower RPM range, retarding will move power curve to higher RPM. You set your timing as normal after you make the adjustment with most kits and offset bushings. I would follow the instructions that come with your kit though.
Last edited by alexisgreene; Aug 22, 2010 at 08:09 PM. Reason: spelling
Joined: Sep 2003
Posts: 25,895
Likes: 429
From: Pittsburgh PA
Car: 89 Iroc-z
Engine: 555 BBC Turbo
Transmission: TH400
Axle/Gears: MWC 9” 3.00
Re: Engine wants too much advance timing?
oh. yeah I knew that. kinda. Now, once you find that sweet spot inbetween, how do you know that it is TDC on the firing and not exhaust?
The whole point in finding TDC is to align the distributor, so you cant really watch the pointer on that because the dizzy may be out of alignment with TDC.
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Car: 84 Firebird
Engine: currently 1968 307
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Re: Engine wants too much advance timing?
Gotcha. Didn't mean more power, just meant what you said, moving the powerband. The cam I have is .470/.470 lift and the rpm range is 1800-5800. Hate when I stomp on the gas and it just kinda sits there til 1800 and then takes off. Then to make things more irritating, my tranny is set to shift out at 4000 rpm. So if advancing the timing moves the powerband, then I should be able to get it down to 1500? Maybe 1200 for where it starts?
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