My 2 cents, I see you are a "ported vacuum" distributor guy. Try to find the Vaccuum diagram for your truck to verify it's supposed to be that way...for example my truck (1979 F350 with 460 and non-CATALYST) calibration (vacuum diagram) *clearly* shows dizzy is connected directly to manifold vacuum. And that's how I have mine hooked up right now. IMO it will idle much smoother, and run cooler at idle if you do so. Just my opinion. Maybe the CATALYST engines used ported for emissions or something.

There's the never ending debate on the internet "Ported vs manifold" well the factory calibration sheet will show you what was originally intended, and to hell with people's opinions after that.

Find the calibration diagram for your truck to see which way it really was set up factory especially if you are using an "original" style carb

 

Thanks! I'll take a look. I've been debating with myself on which vacuum to run

 

Here is the diagram I've been using:

It does show it hooked up to ported. I'll do some reading on using manifold though. Everything is good with the idle except the ported leaks a little. It increases the advance by 3 degrees at 700 rpm idle. From your experience, how much does the manifold increase advance at idle? Right now I've got the advance plugged off. My initial is at 12 and the mechanical is 26 so that's 38 total. I usually keep the initial at 10 but I wanted to experiment by having it at 12 to see if the idle speed would increase enough for me to close off the throttle plates more.

The only way I can close the throttle plates enough to block the ported vacuum is to have the initial at 20 degrees. I know I can't drive it like that right now, but I just wanted to see how far I would need to go.

 

Just to make sure I'm not being bullheaded for no reason, I put the screws out 2 1/2 turns (AS A BASE TO RE-TEST). I went 1/4 turn in at a time watching both the vacuum gauge and the rpm. It seems I hit max between 1 1/4 - 1 1/2 turns. Just like before. I'm only telling you the numbers to show where it's at, I didn't put them there and just hope it works. I have NEVER done that. Now, can we please move past the mixture screws?

Just as a test, I increased the initial timing until the rpm was high enough that I could shut the throttle plates enough to close off vacuum to the ported "nipple". I had to go all the way to 20 degrees initial for the idle to be high enough that I could back the speed screw down. I've set it back to 12 degrees at 650-700rpm right now (plus 3 degrees from the vacuum port). Should I leave things alone and not worry about the ported having a small vacuum at idle, or should I limit my mechanical to 16 degrees and leave the initial at 20 so the port has no vacuum? The starter was kind of sluggish at 20 degrees but not awful. Maybe a happy medium like 16 initial, 20 mechanical, and 1.5 degrees vacuum at idle?

Edit: I should add that the current mechanical advance is 26 degrees

 

Set the carb for max Engine RMP or vacuum this is the procedure for Autolite/Motorcrarft (check the manual) and Holley and basically every one else. (see below) There is no need to go x turns richer. All that does is burn more fuel contribute to cylinder washing when the engine is cold and can lead to plug fouling.

Since you have throttle shaft leaks you are never going to be able to properly tune the carbs idle mix. So set your base idle you should be somewhere around 32-36 total and do not worry about a bit of vacuum on the ported port since your throttle shaft is worn you will likly never be able to get it to zero as the position of the throttle plates have changed and you are getting shaft leaks. Also as soon as you hit the throttle it goes to manifold vacuum.

 

I's like to see your vacuum diagram cause I bet it is not directly hooked to manifold vac.....

 

I appreciate the help but you really should go back a few pages before you and I continue. There is probably half a page or more dedicated to me buying, installing, and tuning the NEW carburetor. (No more throttle shaft leak or any other vacuum leaks for that matter).

All I was asking about was the slight vacuum coming from the timed port at idle. Nothing else related to tuning. I only asked about timing and mixture to see if I could increase rpm anymore than I already had. The answer is NO when it comes to the mixture screws because they're already there. As I said before, I can get down to 0 vacuum on the port but the initial has to be 20 degrees advanced. I don't want to run that high even if my mechanical was only at 16-18 to avoid starter drag.

Most likely, I'll work on setting the mechanical to 24 degrees and run 14 initial to almost completely cut off vacuum to the timed port, but not risk starter dragging. It will still have about 1 1/2 degrees at idle but that's better than 3 and certainly better than 15 or whatever manifold vacuum would cause. That should be 38 total and eventually I'll find an adjustable vacuum advance can to possibly limit the advance a bit so there is less risk of detonation.

 

looks like you bought one of those Chinese knocks-offs, if that is the case then all bets are off. You likely never will be able to get it spot on or to ever really run properly.. You should be able to have the timing set to basically 0° and still have no vacuum on the ported vacuum. Those knock-offs carbs have multiple engineering issues in them not to mention casting quality can be questionable. More than one member here has had to finally pitch one and install a quality rebuilt unit.

Since you really have no experience with carbs just be aware a carbed vehicle should run, idle, and start as good as any fuel-injected engine.

Also there is no setting the mech timing in the Ford dist it is fixed. The only way to change it is to change the reluctor. What you set is the base timing. Do you even know how much mech advance you have? Vac advance is only used during cruise and de-acceleration. Again if you can not get zero vac on the carb at idle the carb is not correct for the application as was stated in post #69.


Also on stock engines, there is a very good reason to run ported. Running manifold vac can result in surge stalling unless you are running high idle speeds (near 1000 RPM).

 

I told you in one of the last posts that it is 26 degrees of mechanical. It WAS at 36 degrees (18L) (probably for the egr) now it is at 26 degrees (13L). Here is a picture so you don't think I'm just guessing again...


I took the "sleeve" off the stop post and flipped the arms around so the 13L side is being using instead of the 18L.

Regarding the carburetor, I'm not going throw this one out just because it has a TINY amount of vacuum getting to the timed port. If it really is that big of a deal, I can just set the idle down to 550 and advance the initial a bit more and lower the mechanical. Problem solved.

Compared to the mess I had with the other one (undersized venturi, undersized jets, throttle shaft leak, etc...) the "knock-off" isn't looking so bad.

I'm not going to run manifold vacuum to the distributor and never have. Nor have I said anything about it not starting, not idling, or not running right. In fact, with this new carb, it starts faster than my fuel injected cars (almost as soon as I turn the key, probably because of the hotter ignition).

I think we can just about move past this. While having 2 degrees or so of vacuum advance at idle (700 rpm) might not be ideal, it's not necessarily bad, especially compared to the 15 or whatever that would come from manifold. That was my only question, nothing about starting or idling etc...

In my opinion, 24-26 degrees mechanical, 12-14 degrees initial, and early available vacuum advance (2 degrees at idle) doesn't sound bad at all... It should even run cooler than stock. Having the vacuum advance kick in before 1000 rpm can't hurt because mechanical doesn't have any advance at that point.

 

Again you should be 32-38° total 38° is pushing it ideally 36° total should be your limit anything more than that you risk at speed detonation that usually is not detectable and will result in broken bits. With 26° mech that leaves you with a maximum of 10° base.

The vac advance is solely load-dependent so saying it kicks in at X rpm is meaningless. As soon as you load up the engine like accelerating even lightly you will have no vac advance. Again vac advance is only applied during cruise and de-acceleration, times of high manifold vacuum.
Ignore the advance added by the vac advance at idel it does not factor into total timing in any manner.
The mech advance should start to come in just above idle and be in fully by 2500 RPM or so.

The issue with having the vac advance applied during idle is, as soon as you crack the throttle ignition timing drops and engine rpm drops this can lead to a stumble under certain circumstances yes even 2° can do that.

This carb is obviously not calibrated for an engine this size more and than likely is calibrated for an early-mid 80's 351 EGR application if it is one of the 2150 knock-offs, if so this is what it will be calibrated for. And chances are you going to have fuel curve issues. Not a huge deal but you will leaving power and economy off the table.

 

Idle speed in park SHOULD be around 1000.. The idle speed that it in the calibration chart is the speed that it should be when engaged in drive , for an automatic anyway. My calibration indicates 650 rpm in drive. Which will cause it to go up to 1000 in park.

And yes to the user who asked if my vacuum diagram shows manifold yes it does I located the exact calibration sheet. Perhaps mine is on manifold because it's 8900 GVER non Catalyst. As I said you can talk all day about the argument but the factory diagram will indicate the original intent..

Mr engine runs at 1000 in park and 650 in drive, it's manifold vacuum and send to idle nicely. But yes I'm not an expert but rather than debate this like everyone wants to do, just check your original emissions calibration diagram. Mine shows manifold

 

Hopefully someone else will weigh in but I don't necessarily doubt that the 2 degrees from vacuum "could" hurt. It does however, seem like the argument could be made in reverse because once the throttle is opened, the vacuum decreases, both for ported or manifold. The only difference between the two is at idle.

I understand how losing 15 degrees right off acceleration could be troublesome when using manifold (stumbling, etc), but only dropping 2 degrees is "probably" not going to be noticeable since that equates to no more than a couple dozen rpms or so I'd imagine (which is essentially immediately made up for as you push in the accelerator). I'd say it seems like it's worth the risk regardless, because the higher the advance at idle, the smoother this truck seems to run, even just 2 degrees more is noticeable in terms of the engines "happiness".

Regarding the carburetor, I'm sure it's not necessarily ideal for a 390, but the carb that came with it was from a 302. I'd say what I've got now is a major improvement. I didn't want to spend any more on the carb right now than I had to because this winter, a rv cam, new intake, and 4 bbl carb will likely be swapped in. Right now, the goal is to get it running as well as possible under the cheapest circumstances and drive it until I have all the new parts collected (likely this winter).

Instead of bringing the initial down, I will work on getting the mechanical down 2 degrees from 26 to 24 for a total of 36 degrees instead of the 38 it's at right now.

 

I have the NP435 4 speed in mine, so I've got the idle at 700 right now. I think that "should" be about right. A 1000 rpm idle would probably be more fun during take off, but just to save a little gas, I'll keep it at or below 700 for now.

 

Losing 2° or advance at idle is not just a couple of dozen RPM. When you mash the accelerator the very first thing that will happen is timing will fall off and the carb's pump shot or transfer slot will be fighting dropping engine RPM which will lead to a stumble or hesitation or reduced acceleration. There will be a penalty no matter how you look at it.

The only ways to lose another 2° of mech advance is by changing the reluctor ( Ford reluctors are long obsolete) or welding it up,or by bending the stop, Since it is just 2° you may get away with bending the stop. But be careful, break the stop off and you will be looking for a new dist shaft. or a new dist

And to be honest you are much better off bringing your initial down and making sure all your adance is in by 2500 rpm or so, this will yield far more gains in terms of power and economy, than pulling back the mech advance. Ideally, you want to run as much mech advance as you can get away with and dial back the initial within reason. This will give you a much better timing curve with stronger more efficient acceleration.
There is no real advantage to running high initial anyway if the cam does not need it. All it really is, is a bodge fix for poorly tuned distributors.

The risk with that knock of carb is lean out at midrange due to it being an EGR application.
And 1.08 venturi carbs are not exclusive to 302's but were used on 351Ws also so your old carb may have been 351W application and depending on the calibration/model may have been a safer choice, if it is a 2100 there would be no risk of mid-range lean out. Regardless keep an eye on your plugs.

Niko's calibration uses a vac res to mitigate rapid timing advance loss when the throttle is opened to prevent stalling or surging and can be assisted with an orifice restrictor in the vac advance to slow its response time.. This was done on applications that could not meet idle emission standards. When manifold vac is run there has to be measures put in place to prevent surging, stumbling, and stalling on factory and mild cams.

 

I'm still not sure about this since my truck is a manual. I can see where off idle comes into play with an automatic but isn't standard practice when driving a stick to revv up to 1200-1500 rpm then let out the clutch? The throttle plates would be open enough to create a vacuum on the timed port, not just on my carb, but all carbs. (I was taught how to drive a stick by my dad, so the idea of revving up before letting the clutch out may or may not be standard practice, it's just what I'm used to).

I can tell you that the throttle feels a lot more "reactive" now than before, with and without the vacuum advance hooked up. Not that that really means anything when idling in the driveway, but it's an improvement non the less.

Regarding the mechanical advance, I can lower it by "sleeving" the stop post. I'll play around with different thicknesses to get to down to 24 degrees. The initial is higher because that brings up the rpms enough that I can close the throttle more to cut the vacuum down to 2 degrees. When I had it set at 10 degrees the idle set screw had to be opened more to reach 700 rpm. That caused about 3 degrees advance at idle from the timed vacuum, opposed to the 2 degrees I'm at now.

The previous owner did eventually get back to me and confirmed that the carb and air cleaner are from a mid 70's 302. He just used what he had to get it running well enough for farm duties. So this carb, while not necessarily correct, is no worse spec wise, and a major improvement when it comes to having no throttle shaft leaks. I'll be sticking with it for now so I can save up for a 4 barrel intake/carb.

I am sticking with the ported vacuum source, even if it still pulls a slight vacuum. I'm not knocking manifold vacuum, because I can appreciate both sides of the "argument" but for this stock engine, I don't need 25 degrees of advance at idle for it to be smooth.

The only other thing I can think of to maybe raise rpms enough that I can close the throttle a little more would be bigger jets. Let me know if that would increase the idle speed enough that I could close the throttle off some. It's close enough that 1 full turn should close off the vacuum to the timed port. I don't want to idle at 500 rpms though because it seems like that would be harder to cold start and keep it running.