Tuning for a mopar 318

[toocheaptosmoke]

Looking for any advice on a starting point for tuning a 318, doesn't seem to be a very popular combo.   It's a newer style magnum 318 converted to a carb, the heads flow better than the old style.  It has the stock cam, edelbrock performer rpm intake, headers, and an HEI ignition.  The carb is number 17059247, off a buick I think.  I rebuilt it stock with Cliff's kit, also got the book. 

It fires up and actually idles pretty decent, but I haven't had the car on the road yet.  Was hoping to get a reasonably close setup to get driving, or if you guys think it's close enough as-is I'll just run it and go from there. 

Here's a video of it running, at the moment it doesn't seem to rev out very cleanly. 

http://youtu.be/ImMFDtsFaOo

[Cliff Ruggles]

Always best to do some driving and evaluate engine performance and fuel economy, etc, then go back and address any areas that need help.....Cliff

[toocheaptosmoke]

Ok, have the car on the road now, think I have an idea what it needs. 

Idle quality has always seemed surprisingly well, still have a stock cam so was hoping not to have many problems here.

Main jets were 73's.  Primary rods have stamps that are hard to read, but looked like possibly 6-9, or 6-3, or 9-8?  Anyways, they look to be quite a thick sectioned rod.  The secondary rods were CZ, which again is a very lean rod.

Low and mid range performance was less than spectacular, engine reached operating temp. very quickly, and there was slight surge while cruising.  I figured it was lean, and put in some 75 main jets, and some richer secondary rods from another carb I had.  (not that I've really tried tuning the secondaries yet)

That made a big difference in performance, when the primaries were wide open, acceleration seemed decent, or at least it was much better than before.  There was however still a stumble/hesitation whenever you gave it throttle while cruising.  I turned the APT screw out another turn and that again made a big improvement, throttle response was much better.

Couple days later the temperature has dropped about 25 degrees, stumble/hesitation is back.  Adjusted APT another turn out, didn't really help this time.  Primaries at wide open don't feel quite as strong either. 

I think I need to for sure try a richer set of primary metering rods, then also go up maybe another size on the primary jets, and possibly install a stronger power piston spring?   I think the engine is still running a bit lean, sometimes it will pop a bit when downshifting.  So the question is, how much richer do you think I need to go?  Thanks.

[toocheaptosmoke]

I purchased a set of the primary rods from Cliff's store, they were the ".044" ones.  Anyways, they are smaller on both diameters, my old rods measured .036" dia. on the tip.  I also used a stiffer rate APT spring this time around.

I put them in with the 75 mains and once again, it seemed to have maybe helped a bit, but the problem still persists.  While cruising 4th gear and you get on it it, there is a half second where power is a bit flat, then there is a half second where it has a bit of a stumble/bog/surge, and after that it comes back and accelerates fairly consistently.   Although, I think it should have more grunt than it does.  I think what may be happening is, after the shot from the accelerator pump it goes lean, then catches up.    I've also noticed that sometimes while accelerating hard say in 3rd, as it's pulling up around 3000+ rpm a slight drop off in in sound and power occurs.  Not sure if this something related to my timing or not though?

Yesterday I went to 76 mains and a longer APT spring, again there was not much of a change, still has a stumble when under load.  I am a bit surprised that all the changes I've been doing seem to have very little impact on the way it's running?  I thought it should be running noticeably richer with the changes I've done.  It's not belching black smoke and the plugs look brand new.

[Cliff Ruggles]

75/76 jets would be HUGE for that carburetor number.  73's should have be plenty of jet.  If you get lean symptoms I'd go back to "basics", float level, fuel pressure, check the power piston hanger arms to make sure they are exactly even and not bent up/down, power piston stuck/sticking in the bore, etc......Cliff

[omaha]

I have a 318 in my 1980 Power Wagon. It has a Q jet from a 1985 Dodge van, 3/4 ton. It also has the 360 heads and the factory intake from the dodge van. this can be a very good combination. I can get up to 18 mpg if I keep it at 60 mph or less. Check your intake adaptor. As far as I know the performer Rpm has the square pattern and you need to be careful that this are is sealed up good and does not have any vacuum leaks in this area. That is it, I cant come up with anything more other than 318's are notoriously "cold natured" engines. ( a good working choke is essential) You cant "get on it" until it is warmed up and this takes a while with a 318. By the way, I think with the magnum style heads you should be able to achieve more power and more fuel mileage.

[toocheaptosmoke]

You're right about the rpm intake being the square pattern, I have an edelbrock adapter plate in there which has a bigger mounting surface than the other generic adaptor plates.   

I went back to the 73 main jets, and double checked other stuff in the carb.  Float level at 1/4", spring hangers look good, power piston moves freely.  I drove it and the bog is back pretty bad, but it only happens when you apply throttle.  For example, say you're cruising in 4th gear at 2000 rpm.  Give it some throttle, bogs for a second, then she comes back to life and takes off.  The power is better now than with the larger jets, but the off idle lean spot is worse.  I think it may be a problem with the power piston not having enough spring, this could be causing a lean spot off idle if it is taking too long to raise the primary metering rods?  I used a straw to "feel" the power piston while revving the engine, it never budges from fully seated.  I already have the heaviest spring I got in there, but just ordered a kit from Cliff's to try a heavier one. 

[toocheaptosmoke]

I received the spring kit, and it looks like none of the springs were any heavier than the one I was already running.  As a last resort I took one of the old springs I had and stretched it out, it put quite a bit of preload onto the power piston. After installing that spring I also was finally able to feel the power piston rise (using a straw) while cracking the throttle, this was the first time it has ever done that.  Initial test runs showed a greatly reduced bog, sometimes almost no hesitation at all.  After a few miles the bog started to return a bit, which I think may be from the spring taking a set back towards its original length.  I don't have a vacuum gauge, but I may try to get one just to see how much vacuum this engine is pulling at idle and cruise. 

Also, I've ran the accelerator pump on both settings throughout the tests, it never really makes a noticeable difference what position it's in.

[Cliff Ruggles]

The dark blue spring in our kit is really strong, and will pick the piston up quickly.  I'm still not sure why your set-up is wanting so much fuel from the main system right to start with?  Did you verify that all the main airbleeds are in place and what size are they?......Cliff

[toocheaptosmoke]

Next time I work on the carb I'll try the blue spring.  I have never measured the airbleeds, if I remember right I just visually compared them to make sure they weren't the large size.  (should have kept better notes along the way)  Here's a picture comparing the old primary rods to the .044 ones, I think you can see the air bleeds pretty decent.

[Cliff Ruggles]

Check the power piston hanger arms to make sure they are level and exactly even.  Very critical on that carburetor to have them both at the same depth in the jets.

The big Buick carb also has primary POE, blow some carb cleaner thru the POE tubes and make sure the holes are open behind the choke flap.

That carb has small MAB's, and you should be fine with 72 or 73 main jets for what you are doing.  If it continues to act lean even with the APT raised up 4-5 turns, something is fundamentally wrong someplace. 

I've build hundreds of those units, and it's rare to have to install large main jets in any of them, at least beyond .073's for a "mild" application.....Cliff

[toocheaptosmoke]

Ok, we're back at this. 


I checked the height of the power piston arms, they were out about .020" but are even now.

I also installed an A/F ratio gauge. 

Here's where it's at right now, I've tried a couple different combos and seem to be going in the right direction.  71 mains, and the dark blue spring from your kit.  The stumble I've been having seems to be a lean spot just off idle.  Basic rundown is that it runs decent at idle (~14), off idle/light throttle it gets leaner (15-16), then gets rich during heavy throttle. (10-12)

With a vacuum gauge the engine pulls about 15 "Hg at idle.  If you crack the throttle a little bit  goes up to 20-23 "Hg.  You have to crack it fairly quick to drop the vacuum.  I previously tried stretching out another spring and was able to get the power piston to move pretty much instantly with cracking the throttle.  That seemed to pretty much eliminate the stumble.  The blue spring doesn't move until you hit the throttle hard, but I was able to fix most of the stumble by adjusting the APT screw.   

[toocheaptosmoke]

I went to 70 jets, same results with the stumble a bit more pronounced.  I can see the A/F guage hit 16-18 real quick when it stumbles.  Still on the rich side during WOT.

Adjusting the APT screw will change the mixture during very light throttle, but it doesn't get rid of the lean spot.

I drilled another hole in the accelerator pump lever to give it more of a shot, that definitely helped but hasn't completely eliminated it.

[Cliff Ruggles]

Might want to try closing down the lower idle airbleed some, as they are pretty big on those units from what I can remember. 

The accl pump and PP spring are not the problem if you are going lean on light "tip-in", it needs more fuel from the transition circuit and APT system.....Cliff

[toocheaptosmoke]

I closed down the lower idle air bleeds, first round I decreased them by about 25% but it didn't do much.  Second time I closed them up to a little less than 50% of what they were.  Hard to get a good measure, but they were originally around .085" currently around .050", close them all the way?

Stumble still there, that didn't change much.  I have unscrewed the APT until the spring no longer held it and it still does it, mixture gets richer just off idle, but as soon as you get into it there's a lean spot. 

Here's a video I made over the weekend,
http://youtu.be/vbURvnKEVlM

Really trying to think if there's anything I'm missing here, I appreciate the help and am open to suggestions.  Starting to get frustrated, haha.  Intake is dual plane, but the spread bore adapter leaves a little gap open between the venturis.  I did some research before I put it together and the consensus seemed to be that it was no problem to do that, in some cases even improving hp?