Am I running on my primary jets!

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Clelan

Clelan

Inferno Red Duster
FABO Gold Member
Joined
May 11, 2006
Messages
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Location
Martensville Sask. Can.
74 Duster, full weight cruiser
318/A500(.69 OD)/4.10 gear. 28" tall tire
Quickfuel 750DP.
2400 rpm at 70mph cruise
Question for you all is, am I cruising on my primary jets or the idle circuit? I'm pretty sure it's primary jets but need another opinion.

Cley
 
Agree.
Even though the rpm is down compared to a more typical gearing, I'd bet the throttle is open enough due to wind resistance.
Start jetting them down and you'll find out.
->It's too lean at "cruise" when the engine surges (revs and dies) driving steady at higher speeds like that.
Be sure you have a place to slow down and drive back at lower speed (or change jets on the spot).
 
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Clay, yes primary jets if you foot has moved the throttle open at all. So yes at 2400 rpm it’s primary jets.

Do you have a problem you are trying to fix? Leaning out the jetting for fun could be a bad thing for your engine.
 
I am tuning the cruise by the plugs. Just optimizing. I went from 72 to 69 jets and they are still darker than I think is good. Now for 67s when I get back from the lake. I checked them after a 70 mile cruise at 70mph.

Cley
 
89on35s said:
I am tuning the cruise by the plugs. Just optimizing. I went from 72 to 69 jets and they are still darker than I think is good. Now for 67s when I get back from the lake. I checked them after a 70 mile cruise at 70mph.

Cley
Click to expand...
I'm not sure how long it will take to change the coloring on the plugs when cruising, especially if they are somewhat carboned to begin with.
Too lean at cruise will result in the performance deterioration as I described. Just let off the throttle and it will generally come back to life.
When right on the edge I can sometimes hear it. [If you've ever run an engine out of gas, any engine - a lawnmower or snow throw, you've observed what happens at too lean under light throttle. The only difference in this case will be that its not actually out of gas, just the access to it is too restricted. It can be a little startling when the engine starts to rev without you adding throttle.)

The caution after leaning out the primaries will be when you go to test for Wide Open Throttle.
Jet back up at the track and then test for best mph in the 1/4 mile. That's when its best being cautious going toward the lean side. If going leaner gets better mph, then that's what it wants. Adjust for any differences between the primary main jets between cruise and WOT using the power valve channel restrictions.

If you want to get fancy, datalog a wideband oxygen sensor to make sure the AFR at WOT is flat from shift though the redline. Then adjust main airbleed and emulsion. That's a bunch more work. What you are doing is already more work than most owners put into their projects. Keep it up.
thumbs_up-gif.gif
 
413 said:
Clay, yes primary jets if you foot has moved the throttle open at all. So yes at 2400 rpm it’s primary jets.

Do you have a problem you are trying to fix? Leaning out the jetting for fun could be a bad thing for your engine.
Click to expand...
Actually the "idle circuit" continues to provide the fuel when the throttle opens from idle. The difference is portion coming through the transition ports versus the idle port increases.
The main circuit starts to dominate when the velocity through the boosters is high enough. To have that much velocity takes both the throttle to be open enough and the rpm to be high enough. Even at highway speeds, during very light throttle (slight downhills) the idle circuit comes into play again.

Sometimes Chrysler called the "idle circuit" the "low speed circuit" which is a somewhat better description.
For example here: Carburetion Fundamentals & Facts, 1970, (Session 273) Chrysler Master Technician's Service Conference
 
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89on35s said:
I am tuning the cruise by the plugs. Just optimizing. I went from 72 to 69 jets and they are still darker than I think is good. Now for 67s when I get back from the lake. I checked them after a 70 mile cruise at 70mph.

Cley
Click to expand...
There's a method where you plug the power valve 'sometimes even lean out the pump shot' and just don't do any stabbing of the throttle or brisk part throttle operation and simply Cruz like you're trying to do and lightly roll the throttle on for any acceleration. I'm mentioning it because you're down V jet sizes or whatever at 69 and you say the plugs are still dark. You're either experiencing power valve operation/open because it's rated wrong...or possibly your plugs are really cold and will go dark anyways because they aren't hot enough to clean themselves the acceptable amount...but check that pv for being open during cruise.
 
I appreciate the tips! I am learning as I go and really enjoy the process. The funny thing is, all that I do this year tuning wise will likely go out the window for next season when I hopefully put my blowed up 360 in it! I guess nobody can take away the knowledge I gain!

Cley
 
89on35s said:
I appreciate the tips! I am learning as I go and really enjoy the process. The funny thing is, all that I do this year tuning wise will likely go out the window for next season when I hopefully put my blowed up 360 in it! I guess nobody can take away the knowledge I gain!

Cley
Click to expand...
You'll be surprised how close the tune will be for the 360.
I'm at 540ft elev. and every carburetor of mine I've tuned in the last 20 yrs while living here has ended up a jet or 2 smaller on the primary than ootb and sometimes 1 size up or down on the secondary.
I use stock heat range, cold is only for trial tune... then I go back to factory spec for 10.5 comp 340 plugs, ngk.
 
For a SBM with a 276?110 cam or smaller,
here is your starting point;
Rev the engine in Neutral to 2400. Leave the VA operating.
Then add timing . When the rpm goes up, close the throttle back to 2400, and add more timing.
Repeat until more timing does not increase the rpm, then back it up 3 degrees. Write down your timing for 2400.
What you have done is started the fire early enough, that peak pressure is being delivered to the crank, at the right time for 2400 rpm. This will give you the smallest throttle setting to cruise at 2400.
Next, fix the throttle at 2400 rpm. Your engine will be running partly on the transfers, partly on the mixture screws, and maaaaybe a lil on the PMJs but I doubt it. So screw in the mixture screws until the engine slows down. Then back them out 1/4 turn. You now have the right amount of fuel to cruise at 2400 regardless of which circuit is operating. Now kick the throttle off and let the engine return to idle.
Then turn the mixture screws in all the way, counting the turns as you go, then put them back where you found them. If the screws are in about the center of their operating range, then you are good to go.
But if not in the center, then the engine may be wanting more or less fuel from another circuit, to satisfy the screws.
To help determine which, start her up, and check the idle-timing. write it down.
Then if it is not in the range of 12 to 16, set it to 14 for now. Next, set the mixture screws to in the middle of their range. Set the rpm to 700/750 in Neutral. This will set the transfer-slot exposure under the primary throttles pretty close to correct.
Make sure you are running a PCV to the front of the carb, to discharge just below the primary throttles. All this assumes that the Float level is correct and stays correct at all times. And if you have a 4-corner idle carb, a cam smaller than 276/110 does not need them so shut them off.
Finally determine if the carb is running rich or lean by first, jamming a rag into the secondaries. If the idle changes, then make sure the secondaries are closing all the way, but not sticking. and remove the rag of course. Reset the idlespeed to 700/750 again. Now, take that rag, bunch it up in your fist a bit, then slowly begin to cover the primarys. If the idle speed goes up,then she's already getting to much air. If the idle speed eventually goes down but never rises first, then she is already short of air.
What to do;
Firstly, at 2400rpm, the engine does NOT care about the size of the Primary MJs. In fact, it will run 2400 in Neutral just fine, even if you take them right out. And I don't think they make MJs small enough to run out of fuel at 2400 in Neutral. So forget about tuning your cruise fuel with MJs.
At 2400 in Neutral, your engine can only be AFR tuned by fine tuning the fuel deliveries from the Idle-circuit against the transfers, and let the MJ fuel be what it will be.
So then, to adjust the Afr keeping the idle screws fixed, the only way to do it, is to alter the fuel coming from the transfers, by using the speedscrew. And if the idlespeed goes too high or too low, then the only way to change it is with Idle-Timing.
So do what you gotta do.
If the engine develops a tip-in hesitation, the transfers are too lean.
If the exhaust starts to burn your eyes, the engine wants some bypass-air, which you need to supply on the primary side of the carb.

Now, lets go back to cruising at 2400.
From wherever your Idle-Timing ends up at, you need to get to whatever your engine wanted earlier.
And you still have to satisfy the power-Timing.
Lets say the idleTiming comes in at 16*. and
the power timing needs to be 34*. and
the required cruise timing was 56*
How are you gonna marry all that?
Well first, you gotta install the right advance cam, which is 34 less 16=18*, so you build or buy one. Then you gotta get the rate of advance set, as quickly as possible, that does not put the engine into detonation.
Suppose you are able to install some springs that get you 34* at 3200rpm and the advancing begins at 1000 rpm. That would be a rate of advance of 18 degrees over 2200rpm, right? Which is 18/2200 x100= .818* per 100 rpm. So then, at 2400 the mechanical will be
(2400 less 1000/100) x.818= 11.5*, which you then add to the 18 initial=29.5 at 2400... but you need 56 so you need 26.5 from the Vcan. Your Vcan does not make that; in fact, the most I have heard of one being modified to get is 24*. I modded mine to get 22*. Say yours goes to 22* as well; now you are still short 4.5*.
Your options are to leave it at that, or to increase the Idle-timing by whatever you need, or to install a faster rate of advance. But honestly .818 is pretty fast. Furthermore, in my experiments, that last 3* is almost not worth chasing. And adding 4.5 degrees of idle timing will likely drive the idle-rpm up higher than you want.
BTW; 67s in the front for a hot 360, is guaranteed to be too small at sealevel. 70s with a 12.5PV is about right with a 230*@ .050 cam, depending on your elevation.. 68s at the smallest, If you try and cover the transition with pumpshot, your city fuel-mileage will suffer.
 
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What the heck are you guys doing!??!!
A complete sidetrack.
shake_head-gif.gif

Headed right off a cliff too

Let the guy tune his carb.
One change at a time.
wink-gif.gif
 
Can't argue with that!
Wanted to be sure the benefits of vac advance were being utilized.
It'll never happen again, I promise.
 
AJ/FormS said:
For a SBM with a 276?110 cam or smaller,
here is your starting point;
Rev the engine in Neutral to 2400. Leave the VA operating.
Then add timing . When the rpm goes up, close the throttle back to 2400, and add more timing.
Repeat until more timing does not increase the rpm, then back it up 3 degrees. Write down your timing for 2400.
What you have done is started the fire early enough, that peak pressure is being delivered to the crank, at the right time for 2400 rpm. This will give you the smallest throttle setting to cruise at 2400.
Next, fix the throttle at 2400 rpm. Your engine will be running partly on the transfers, partly on the mixture screws, and maaaaybe a lil on the PMJs but I doubt it. So screw in the mixture screws until the engine slows down. Then back them out 1/4 turn. You now have the right amount of fuel to cruise at 2400 regardless of which circuit is operating. Now kick the throttle off and let the engine return to idle.
Then turn the mixture screws in all the way, counting the turns as you go, then put them back where you found them. If the screws are in about the center of their operating range, then you are good to go.
But if not in the center, then the engine may be wanting more or less fuel from another circuit, to satisfy the screws.
To help determine which, start her up, and check the idle-timing. write it down.
Then if it is not in the range of 12 to 16, set it to 14 for now. Next, set the mixture screws to in the middle of their range. Set the rpm to 700/750 in Neutral. This will set the transfer-slot exposure under the primary throttles pretty close to correct.
Make sure you are running a PCV to the front of the carb, to discharge just below the primary throttles. All this assumes that the Float level is correct and stays correct at all times. And if you have a 4-corner idle carb, a cam smaller than 276/110 does not need them so shut them off.
Finally determine if the carb is running rich or lean by first, jamming a rag into the secondaries. If the idle changes, then make sure the secondaries are closing all the way, but not sticking. and remove the rag of course. Reset the idlespeed to 700/750 again. Now, take that rag, bunch it up in your fist a bit, then slowly begin to cover the primarys. If the idle speed goes up,then she's already getting to much air. If the idle speed eventually goes down but never rises first, then she is already short of air.
What to do;
Firstly, at 2400rpm, the engine does NOT care about the size of the Primary MJs. In fact, it will run 2400 in Neutral just fine, even if you take them right out. And I don't think they make MJs small enough to run out of fuel at 2400 in Neutral. So forget about tuning your cruise fuel with MJs.
At 2400 in Neutral, your engine can only be AFR tuned by fine tuning the fuel deliveries from the Idle-circuit against the transfers, and let the MJ fuel be what it will be.
So then, to adjust the Afr keeping the idle screws fixed, the only way to do it, is to alter the fuel coming from the transfers, by using the speedscrew. And if the idlespeed goes too high or too low, then the only way to change it is with Idle-Timing.
So do what you gotta do.
If the engine develops a tip-in hesitation, the transfers are too lean.
If the exhaust starts to burn your eyes, the engine wants some bypass-air, which you need to supply on the primary side of the carb.

Now, lets go back to cruising at 2400.
From wherever your Idle-Timing ends up at, you need to get to whatever your engine wanted earlier.
And you still have to satisfy the power-Timing.
Lets say the idleTiming comes in at 16*. and
the power timing needs to be 34*. and
the required cruise timing was 56*
How are you gonna marry all that?
Well first, you gotta install the right advance cam, which is 34 less 16=18*, so you build or buy one. Then you gotta get the rate of advance set, as quickly as possible, that does not put the engine into detonation.
Suppose you are able to install some springs that get you 34* at 3200rpm and the advancing begins at 1000 rpm. That would be a rate of advance of 18 degrees over 2200rpm, right? Which is 18/2200 x100= .818* per 100 rpm. So then, at 2400 the mechanical will be
(2400 less 1000/100) x.818= 11.5*, which you then add to the 18 initial=29.5 at 2400... but you need 56 so you need 26.5 from the Vcan. Your Vcan does not make that; in fact, the most I have heard of one being modified to get is 24*. I modded mine to get 22*. Say yours goes to 22* as well; now you are still short 4.5*.
Your options are to leave it at that, or to increase the Idle-timing by whatever you need, or to install a faster rate of advance. But honestly .818 is pretty fast. Furthermore, in my experiments, that last 3* is almost not worth chasing. And adding 4.5 degrees of idle timing will likely drive the idle-rpm up higher than you want.
BTW; 67s in the front for a hot 360, is guaranteed to be too small at sealevel. 70s with a 12.5PV is about right with a 230*@ .050 cam, depending on your elevation.. 68s at the smallest, If you try and cover the transition with pumpshot, your city fuel-mileage will suffer.
Click to expand...

What the effin eff?
 
Mattax said:
What the heck are you guys doing!??!!
A complete sidetrack. View attachment 1715777783
Headed right off a cliff too

Let the guy tune his carb.
One change at a time. View attachment 1715777784
Click to expand...

Well, because of this;

89on35s said:
74 Duster, full weight cruiser
318/A500(.69 OD)/4.10 gear. 28" tall tire
Quickfuel 750DP.
2400 rpm at 70mph cruise
Question for you all is, am I cruising on my primary jets or the idle circuit? I'm pretty sure it's primary jets but need another opinion.
Cley
Click to expand...
and this
89on35s said:
I am tuning the cruise by the plugs. Just optimizing. I went from 72 to 69 jets and they are still darker than I think is good. Now for 67s when I get back from the lake. I checked them after a 70 mile cruise at 70mph.
Cley
Click to expand...
and this
89on35s said:
Yes,, 20 initial, 34 total with the V can on top of that.
Cley
Click to expand...

So;
I assumed Cley was tuning for economy.
And as you all know, as for an already-existing engine, tuning for economy begins with tuning the slow-speed circuit, and optimizing the cruise timing.
The mains are for after the slow-speed circuit is all used up, and for before the power valve opens.
You can throw 20 different MJs in there, and until they are really really tiny, and too small, they will have little to no effect on cruise-economy. In fact, on a Holley-style carb,if it were possible to pull them out while cruising, Ops combo would continue to cruise just fine.
And the assessment of throwing all these jets in there could take more than an entire summer!
And so, I was/am, just trying to get this project jump started, by offering a proven method.
But if you all think to to drag him along at your speed, ok fine; I'm tucking tail and scurrying back to my corner.

OP
BTW
cruising at 2400 rpm your engine will want a lot more timing than the factory parts can provide. Think 56 degrees plus or minus a couple.
If you do not provide it, peak cylinder-pressure cannot ever be achieved. and whatever pressure it does make is chasing after a piston that is being yanked to the bottom by the previously fired cylinder. Too much later than that, and the piston has to work to push out it's own exhaust. That's work that could have been used to propel the Duster.
with 20/34 timing, that means 14 in the D. Assuming all in by 3400 and starting at 1000, that makes the advance range to be 2400 rpm. And so a rate of advance to be;
(14/2400)x 100=.58* per 100 rpm
Therefore your timing at 2400 is expected to be (.58 x 14) plus 20= 28*. and so, your Vcan is looking to be 56 less 28=28*. So if you currently have something like 12, then the total is just 40 degrees. And that will cost you gas.
The most I have ever heard of, is a guy named @TrailBeast , who modded his to 24* . I got mine to 22*. Even if you get yours to 22*, that's a possibility of being 50* and still not enough.
You gotta fix this first.

How to figure your cruise timing;..
just rev it to 2400 and hold it at 2400, then advance the timing. As the rpm goes up, drop it back to 2400 and add more timing. Repeat until the engine rpm no longer increases; then read the timing and subtract three degrees for load compensation. That's all there is to it. It could be 48, it could be 58. Whatever it is, that's what it is.
Later after you get the cruise AFR dialed in, then you can revisit your cruise timing to see if it has changed.
Just do as instructed and you combo will be 90% cruise-tuned in a couple of days. or
just do what the "other guys" say; it will just take longer. The end will be similar, of course I think my way is the best. But only because I used to make a pretty good living doing things my way, when the "other guys" , doing it their way, were making a better living., lol. But I still ended up fixing those other cars when their way took too long.
I could give you some tips to make it even faster, but, I gave you easier .

How to mod your Vcan;
Just cut the stops down.
I can't tell you how far cuz I don't remember; that was 2 decades ago. But you'll figure it out. While the can is on the bench, loosen the set screw that is in the lil hose-nipple. Turn it fully Clockwise, until it meets resistance, and then stop. All you are doing is removing the preload on the spring inside there, to make it easy for engine vacuum to pull in all the advance possible as soon as possible. Later, you might have to slow it down, but maybe not.
 
Some is true and some is not.
As far as methods, I think at least some is irresponsible to be suggesting.
Not going to argue this with you point by point.
 
To whomever might be taking stock in all this madness from AJ. Keep in mind all this is coming from someone with enough insulation burnt off his wires to think the Earth is flat. Nuff said.
 
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