I don’t know that. It depends on what the engine wants and how much the vacuum can adds and when it adds the extra timing.
Fine Tuning a 360 Magnum with a Brawler "Double Pumper"
- Thread starter jmanhoff
- Start date
-
I don’t know that. It depends on what the engine wants and how much the vacuum can adds and when it adds the extra timing.
TT5.9mag
Two atmospheres are better than one
There is almost no such thing as too much decel timing. Cruise yes but decel no. If you data log a modern vehicle some of the timing numbers would shock most people.
Probably a stupid question but when a cylinder is at TDC the valve springs shouldn’t be compressed at all correct?
I just went to go measure my pushrods to get the right length and noticed with the current pushrods at TDC the springs are slightly compressed. I checked this with multiple cylinders as well. I also checked at the point where the intake valve closes, where I would imagine both lifters are on the back of the cam lobes.
The current pushrods according to Melling are 6.831” (OEM replacement) on my caliper they measured 6.900”, potential discrepancy there but I figured the ticking sound I’m chasing was due to the pushrods being too short, but now, even with a pushrod measuring tool that bottoms out at 6.800” it’s still compressing the spring and I can’t even adjust it. Is it possible my pushrods are too long?
EDIT: I am running Harland Sharp Non-Adjustable 1.6 Ratio Pedestal Mount Rockers.
I just went to go measure my pushrods to get the right length and noticed with the current pushrods at TDC the springs are slightly compressed. I checked this with multiple cylinders as well. I also checked at the point where the intake valve closes, where I would imagine both lifters are on the back of the cam lobes.
The current pushrods according to Melling are 6.831” (OEM replacement) on my caliper they measured 6.900”, potential discrepancy there but I figured the ticking sound I’m chasing was due to the pushrods being too short, but now, even with a pushrod measuring tool that bottoms out at 6.800” it’s still compressing the spring and I can’t even adjust it. Is it possible my pushrods are too long?
EDIT: I am running Harland Sharp Non-Adjustable 1.6 Ratio Pedestal Mount Rockers.
Last edited:
Yes, but modern vehicles almost run the engines out of fuel on decel, most hotrod carbs go rich......so there's that....
The springs are slightly compressed during overlap at TDC EXHAUST stroke, NOT at all on TDC Compression stroke, where the ign. is set to fire.
So does it sound like I’m on the right track that my pushrods are too long? I did have the block decked, unfortunately I don’t have a number on how much it was, but I do know it was not decked for compression it was decked for a smooth surface.
I did verify on every rocker I checked that said cylinder was on the compression stroke, I watched the intake valve open and close and went a little further once the intake valve was closed.
I know another factor that changes the pushrod length is the base circle size on the cam, annoyingly I could not find any specs about that online which would probably be useful to me. It’s just odd that if all of my valves are slightly open that it still maintains compression, I feel like if all 16 valves were slightly open it would run like crap
You would have little to no compression if the valves aren't seating, what was the cranking compression? I've seen bargain cams in the past that failed to close the valve all the way, only "touching down" ~15° before it was time to lift it again, that was a long time ago.....but it idled & ran terrible, stalling & coughing the whole way. We thought We'd done something wrong, (Pontiac 400), nope....pulled it & stuck the stock stick in it & it was fine. I put a dial gauge on it & discovered the grinding error, this was ~'86, not saying it's Your issue, but something to check if You're running out of options for a solution.
I haven’t checked the compression though it would be easy to check, I’ll have to see if I can rent a tool as my compression kit is at my dad’s house. I can hear the compression though, keeping the plugs in and turning the motor over by hand there is hissing
So I just measured my pushrods with an adjuster tool, and at zero lash (I lengthened the tool until it contacted the rocker, but was still able to spin with some resistance) and it measured out to 6.760", so I ordered 6.800" pushrods, both because it's a round number and it gives me .040" of preload, which seems like it's right in the middle for preload for what my engine is built for. I'm hoping this will be the reason why my valve train is all noisy, and once I get the new rods in hopefully Thursday, I'm going to drive around, see if the current carb tune will still work, and then read the plugs and continue tuning from there.
I'm not sure if any of you guys have messed with ChatGPT at all, but I've been communicating with it about my current engine build, specs, etc and asking it for advice and comparing it to what was said in this forum. (I gave it the same situation/questions that I posted here, and it actually gave almost the same answer), and I'm very impressed with the responses its providing me. I've actually been experimenting with its responses to increasing my fuel economy, and while obviously valve hang due to too long of pushrods is the number one thing its saying to fix to gain mpg, its also been saying that I want to be closer to a .060" TSR, and potentially a 64PMJ, but it thinks a 65PMJ would be better, which is actually very close to what has been said here. Of course the number one way of figuring out what jets and restrictors I need is to read the plugs, but I figured I'd share how interesting it is that ChatGPT seems to have the ability to read forum posts and such as well as just simple google searches as it has answers that you can't find on the surface of google.
I'm not sure if any of you guys have messed with ChatGPT at all, but I've been communicating with it about my current engine build, specs, etc and asking it for advice and comparing it to what was said in this forum. (I gave it the same situation/questions that I posted here, and it actually gave almost the same answer), and I'm very impressed with the responses its providing me. I've actually been experimenting with its responses to increasing my fuel economy, and while obviously valve hang due to too long of pushrods is the number one thing its saying to fix to gain mpg, its also been saying that I want to be closer to a .060" TSR, and potentially a 64PMJ, but it thinks a 65PMJ would be better, which is actually very close to what has been said here. Of course the number one way of figuring out what jets and restrictors I need is to read the plugs, but I figured I'd share how interesting it is that ChatGPT seems to have the ability to read forum posts and such as well as just simple google searches as it has answers that you can't find on the surface of google.
Alright yall. An update actually carb/tuning related.
This is my timing chart, with mechanical and vacuum advance (ported).
Vacuum advance seems to be all over the place in terms of ported vacuum. It’s odd that it goes up and down, while my gauge is showing significant vacuum at the manifold.
This is also one of my plugs. This is from low speed driving, some coasting, but not into the main jets, on the transition/idle circuit (what we’re currently working on).
This is my timing chart, with mechanical and vacuum advance (ported).
| RPM | Initial Timing (°) | Mechanical Advance (°) | Vacuum Advance (°) | Total Timing (°) |
| 700 (Idle) | 16° | 0° | 0° | 16° |
| 1000 | 16° | 0° | 2° | 18° |
| 1500 | 16° | 12° | 6° | 34° |
| 2000 | 16° | 16° | 2° (confirmed) | 34° |
| 2500 | 16° | 20° | 2° | 38° |
| 3000 | 16° | 20° | 0° | 36° |
Vacuum advance seems to be all over the place in terms of ported vacuum. It’s odd that it goes up and down, while my gauge is showing significant vacuum at the manifold.
This is also one of my plugs. This is from low speed driving, some coasting, but not into the main jets, on the transition/idle circuit (what we’re currently working on).
36 at 3000 is probably too much. You need more initial and you need to slow the curve down. Leave the VA OFF until you get the curve sorted out.
I don’t remember your combination but I doubt you are making peak torque at 2500. As it is you’ve got 26 at 1500. That’s a TON.
Again, more initial, slow the curve down. You may have to modify the weights and get the heaviest springs you can find to control the curve.
If you are shifting at 5500 you’re probably making peak torque at 3800. That means you need to figure out how to get the timing at 3500-3800 about 24-26 degrees. Hen make the curve gain 1.5-2.0 degrees per 1000 rpm after that.
It matters where peak torque occurs. A wheel dyno is your friend. Better yet an engine dyno where you can load the engine but I suspect it’s too late for that.
do you mean combo as in what cam and stuff I’m running? I do know that peak torque is lower, I don’t think it’s 2500 low though. What if I retard the initial and use vacuum advance instead?
Or do I want more timing initially and limit the mechanical? I’m ideally trying to maximize mpg at cruise, but obviously a properly timed engine is the most efficient it can get.
When I’m getting on it, I shift at or just below 5k. Stock springs, so going over 5k isn’t exactly ideal for them. I can feel at around 4000-4500 the torque starts to drop off. Probably expected given the 650, dual plane, and torque focused cam.
Then you are running close to the power curve of a 340. I’ll look when I get home but I’m pretty sure peak torque was 3800 or a little higher.
It’s known I’m not a fan of using vacuum advance to make an engine idle. That’s a last resort and I can’t see you needing it.
Tuning a distributor is more than just changing springs and limiting advance.
You will probably need to change the shape of the weights to get what you want.
What distributor are you using?
It’s an eBay HEI clone, GM style obviously, with the build in coil. I can provide the link if it would help but it looks like just some generic clone.
I did use vacuum advance for a period of time on manifold, but it idled so high that I needed to completely cover the t slots and then some to get it to idle where I wanted it, so I went to ported.
I probably am going to get a true MSD dizzy at some point. I already plan to get a real Holley as the brawler is having fuel bowl issues (feeding), and if I go with Holley style dual feed bowls, I’ll already be halfway to getting a new carb, so might as well spend the extra bit and have a carb with better quality and such.
Keep what you have.
The GM advance mechanism IMO is about as cheap assed junk as you can get.
The reason it’s used is because it’s cheap to build. And guys love it because they claim they don’t have to pull the distributor to work on it. Plus it has several hundred million man hours of tuning on it.
There is no way to know which “football” it has because there were so many of them. The football is the football shaped part that the weights move around. It has a major effect on the curve.
At any rate, never use two different springs on the weights. The mechanism will wobble (there’s a better term but I can’t think of it now” and it will drive you nuts. Always use the same springs on both sides.
There was a time when you could heat the weights and move the nose closer to the body of the weight. This is a far better way to limit mechanical advance than using the bushings but the last 10 distributors I’ve done and all the aftermarket weight kits I’ve bought are using powdered metal weights.
You can’t just heat and bend them. If you do it will just fracture the weight.
To get what you want you’ll probably need the heaviest springs you can get and see where you end up and tune from there.
I do have a weight kit for this distributor that has new footballs and springs and bushings. Probably should start by just replacing the weights first so I know what those specific weights do to the curve, then put in the heaviest? I’m assuming I want to be all in closer to 3000, not 2500
A quick search says that engine made peak torque at 3250.
That’s the best we have to work with. I don’t think you did anything to lower peak torque so it is what it is.
That means to me that whatever you do, you do not want the timing all in before 3250. Probably not before 3500.
That means you need to figure out how to get the initial up to 20 degrees at 1000 rpm. Then you need to keep the timing at 3500 no more than 26 degrees. After that you need the curve to gain about 2 degrees per 1000 rpm until peak power.
26 may be too much at 3500. You have to test it.
I would do whatever I had to do to not have the curve all in before or even at peak torque.
Edit: a heavier weight overcomes the springs easier. You will need lighter weight to delay the curve.
TT5.9mag
Two atmospheres are better than one
I’d recommend using the vacuum advance for what it’s good for, economy. Disconnect it for now, get the curve sorted out for driving, and power, then add the vacuum advance in later on ported vacuum and tune it for mileage. Your numbers do seem erratic for the vacuum advance so there might be some work that needs to be done to the actual unit itself when you get to that point.
Yeah t
Yeah that’s my goal at the end of the day. I have some like duralast canister on it right now as the one that came on the eBay dizzy started leaking, so I have no idea what the curve is or what it’s supposed to do. I’m looking to get an actual good quality one at some point soon, hoping for it to give me a large amount of advance at cruise for economy.
Yeah that’s my goal at the end of the day. I have some like duralast canister on it right now as the one that came on the eBay dizzy started leaking, so I have no idea what the curve is or what it’s supposed to do. I’m looking to get an actual good quality one at some point soon, hoping for it to give me a large amount of advance at cruise for economy.
Just wanted to poke in again for a quick sanity check. I’ve been reading up a bit on how all of the circuits work in a carb and wanted some corrections/clarifications.
IFR/IAB:
The IFR/IAB work hand in hand, they are what supply the fuel for idle and the majority of low throttle high vacuum operation, like cruising. They feed both the idle and transition circuits. They also get into timing of fuel delivery, for example if you went up in IFR and down in IAB, it delivers fuel sooner and can help with a cruise big.
TSR:
Also related to the IAB/IFR, but controls how much fuel is allowed through the transition circuit. Doing most of the job when cruising down the highway for example.
HSAB:
Controls the timing of when the boosters kick in as well as how strong. Smaller is faster kick in, and richer delivery, larger is the opposite.
PV/PVCR:
Designed to allow us to run a much smaller primary jet so we aren’t gulping fuel while cruising. Opens at X inches of vacuum, and with the PVCR, controls how much fuel is allowed into the boosters and when.
Emulsion Stack (3 in my case):
The top port is for high rpm, middle is medium rpm, bottom is low. All being when the boosters are engaged. The smaller the hole, the less air is mixed with the fuel before reaching the boosters, and larger is the opposite.
So if I’m chasing lean cruise, I would want to optimize my IFR/IAB so I don’t have a big tip in dip, run as small of a PMJ as I can safely and do the same with the TSR.
If I wanted to be able to still stomp on it and get good power, I could potentially run a smaller lower emulsion port to allow the mixture to be a tad more rich before the PV opens, and then let the primary run off the PV? Obviously squirter has a relationship in this too.
I think I have this all correct?
IFR/IAB:
The IFR/IAB work hand in hand, they are what supply the fuel for idle and the majority of low throttle high vacuum operation, like cruising. They feed both the idle and transition circuits. They also get into timing of fuel delivery, for example if you went up in IFR and down in IAB, it delivers fuel sooner and can help with a cruise big.
TSR:
Also related to the IAB/IFR, but controls how much fuel is allowed through the transition circuit. Doing most of the job when cruising down the highway for example.
HSAB:
Controls the timing of when the boosters kick in as well as how strong. Smaller is faster kick in, and richer delivery, larger is the opposite.
PV/PVCR:
Designed to allow us to run a much smaller primary jet so we aren’t gulping fuel while cruising. Opens at X inches of vacuum, and with the PVCR, controls how much fuel is allowed into the boosters and when.
Emulsion Stack (3 in my case):
The top port is for high rpm, middle is medium rpm, bottom is low. All being when the boosters are engaged. The smaller the hole, the less air is mixed with the fuel before reaching the boosters, and larger is the opposite.
So if I’m chasing lean cruise, I would want to optimize my IFR/IAB so I don’t have a big tip in dip, run as small of a PMJ as I can safely and do the same with the TSR.
If I wanted to be able to still stomp on it and get good power, I could potentially run a smaller lower emulsion port to allow the mixture to be a tad more rich before the PV opens, and then let the primary run off the PV? Obviously squirter has a relationship in this too.
I think I have this all correct?
A smaller MAB starts the booster LATER.
A bigger MAB starts the booster SOONER.
A smaller MAB makes the fuel curve RICHER at higher air flow through the carb.
A bigger MAB makes the fuel curve LEANER at higher air flow through the carb.
A bigger MAB starts the booster SOONER.
A smaller MAB makes the fuel curve RICHER at higher air flow through the carb.
A bigger MAB makes the fuel curve LEANER at higher air flow through the carb.
So smaller MAB is a slower, richer booster and larger MAB is faster and leaner essentially?
No, the MAB affects both ends of the fuel curve differently.
At low air flow, the MAB acts like an emulsion hole. The smaller air bleed, with its smaller hole lets less air into the main well so the fuel column is heavier and harder to move to the boosters at low air flow. So the booster starts later.
At high air flow, the same thing happens. The smaller MAB bleeds off less signal so the fuel curve gets richer.
A bigger MAB at lower air flows is letting more air into the main well so the fuel column in the main well is lighter and it starts the booster sooner. At higher air flow, the bigger MAB bleeds off more signal so the fuel curve gets leaner.
Smaller MAB is leaner at low air flow because the booster starts later. The smaller MAB is richer at high air flow because the smaller bleed doesnt “bleed” off the signal.
The bigger main air bleed does the opposite.
You are affecting both ends of the fuel curve by changing MAB’s. The thing to remember is whatever the MAB does at high air flow it does the opposite at low air flow.
Starting the booster sooner makes the fuel curve richer at low air flow. Starting the booster later makes the fuel curve leaner at low air flow.
Most guys use a MAB that is way too big. That starts the booster sooner. Since you are now on the booster and it’s rich, guys start delaying the opening of the power valve.
That makes its own mess.
It also makes the fuel curve richer because the T slot is still delivering fuel.
And the big MAB is now even richer down low.
So if I set it up properly, if I went from a 28 to a 26 MAB, it would delay the boosters from coming in, potentially stopping them from coming in while at highway cruise, I just have to make sure I’m not stupid rich at WOT high rpm?
-
Similar threads
