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Need Help With Tuning

10K views 43 replies 8 participants last post by  SPLUHAR 
#1 ·
Hey all. I need some help tuning my engine. It's a 1973 460, 0.40 over and pretty stock build other than an RV cam, mild head porting (D3VE), Stealth intake, and long tubes. Running an Edelbrock 600cfm carb. Other than that, stock spec stuff. Professionally rebuilt this past fall by the most reputable guy around. I got it back in late October and have been very happy. I have just over 3000 miles on it now.

My issue is that I've noticed a fuel economy drop, and maybe a little bit of WOT performance drop. I'm not so much worried about fuel economy other than the fact that I track it almost every fill-up to watch for issues that may be popping up.

Here's the info that I have:
Normal fuel mileage has been between 9.4MPG and 10.5 MPG for me.
The last several fill-ups that I have tracked have been around 6.6-8.4MPG. A significant difference.
I noticed that my vacuum advance (DS2 setup) didn't seem to be operating just right. This is the first place I looked for my issues. I went to adjust it with an allen key through the vacuum port, and it was as if the adjustment mechanism was missing or out of place. I had an extra vacuum advance sitting around, so I through that in. No significant fuel mileage difference.

This thing has always run rich. You can really smell the hydrocarbons in the exhaust, and it gets a real burning smell when you get on it (only from behind, no smell in the vehicle).

So today I used my digital timing light on it. I found it at:
20* initial
34* initial + vacuum advance on manifold vacuum

I adjusted it to:
16* initial
33* initial + vacuum advance on manifold vacuum
45* at 2400 RPM (normally about where I cruise)

How do these timing numbers look? Where should I be with them? It feels sluggish if I back initial timing down to 15 or below. The higher the vacuum advance is set the better throttle response feel I get. What's a good highway cruise timing advance number?

Also, I've been thinking about going to a 750cfm carburetor (probably Edelbrock with vacuum secondaries and electric choke) for quite a while now. How may this affect power, response, fuel economy, etc...

Could my current fuel economy decrease be caused by junk getting in the carburetor? I may end up pulling it off and cleaning it out.
 
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#2 · (Edited)
Hello BigBlockFord;

I see 2 areas of immediate concern, too little timing and too much intake manifold. But first, lets talk compression. Stock is under 8 to 1. What was done to raise it? Was the block decked? Were the heads milled? Were pistons used with a smaller than stock dish? The reason I ask is because low compression engines don't like much cam and you might be lucky if you are at 8 to 1. Believe it or not, a stock cam and stock intake manifold 460 engine, if prepared properly, is a real low to mid range torque snorter. We are talking everyday street driving in a heavy vehicle.

Low compression, low RPM engines want timing and not that Stealth manifold. That manifold has too large of a plenum and is designed for more of a performance application. I am handicaped here because I do not know how you use the vehicle for mid-range and higher RPM use. I am going to assume that it is your everyday use vehicle that you cruise around in and let it eat once in a while to pass or just to feel the torque. At any rate, the stock manifold is hard to beat when you are operating your engine in the low 3,000 RPM range and under. If you want an aluminum manifold, use a performer which is slanted a little more toward midrange RPM usage.

As for a carburetor, look at Demon's new 750 CFM "Street Demon".

I would not be a bit supprised if your engine liked 40 degrees timing under acceleration. Using that as a base and using your information that it was already running around with 20 degrees initial, I would want a 10 degree plate in the distributor. That would give you 40 degrees total. Have the distributor timing all in by about 2200RPM-2400 RPM. I would want about 12 degrees out of the vacuum advance. That would give you about 52 degrees advance in a cruise situation. By your figures, your vacuum advance is inconsistent. Get a distributor set up for your combination from "The Mad Porter". That little ol distributor can make or break your engine's performance. And in your case, I like drawing your vacuum from the manifold.

Best to you and yours; Dan
 
#5 ·
Thanks for the info. I couldn't tell you off the top of my head everything about it, but my builder said it's at about 8:1 compression. The cam is mild. It makes tons of low end and midrange torque when the timing is set halfway decent. No issue lighting up my 37s while rolling.

I'll try to set the timing at cruise to a little over 50 degrees.

I know I could get someone to do my distributor work for me, but I really want to figure it out myself.

The biggest thing I'm wondering is how I can set the timing where it should be and get my fuel economy and performance back to where it was before I started adjusting. Then I'll go from there and modify to get the most out of my setup.

Also, when I tried setting the timing very much higher than it is now it pinged under load, and it was hard to start. I have a mini high torque starter that's only a few months old. It's supposed to be rated for up to 12:1 applications. It get hot by the headers though...
 
#3 ·
In addition to what was already said, have they recently added ethanol to the gas where you buy it ? Up north, during the winter, more ethanol is added to the fuel, and it does affect your mileage. Also, if you do take the carb. apart, you may want to install or at least check your power valve. With that build it should be a 65 probably.
 
#6 ·
No matter what you do, you are not going to cure the anemia of that gargantuan intake manifold with a band-aid such as more ignition timing. Find a Performer, (not the RPM), and 1" thick four hole spacer; preferably tapered.
 
#7 · (Edited)
What's the issue with it? I'm not doubting you, but I'd like to learn the theory behind it. Seems to make good low torque, and I've seen summer mpg as high as 12mpg. I've used it for years and been happy with performance and economy. I had pretty much the same exact build before this past rebuild, so no real difference there.

Also, I have a 1" 4 hole spacer on it

And like I said, I'd like to get back to square 1 where a was before modifying my setup. I don't want to pass off whatever isn't just right on it currently.
 
#8 ·
Okay, looking in the dizzy it seems that there's a light spring and a heavy spring. I'm guessing that the light spring lets some timing come in quickly, and the heavy one slows it down some at some point. I don't want to swap springs yet, but would I be able to run more total mechanical advance by replacing the heavy spring with a lighter one? If so, I have a lighter one handy. I'm still going to try to get it timed like I had it before I go changing stuff. That way I have a better idea of what performance, economy, and timing changes are going on.
 
#9 ·
First place to start here is with a proper distributor timing curve.

14 degrees initial with 20 degrees mechanical to start. Add timing if performance increases. Best way to tell is with a timed or MPH acceleration run or chassis dyno.

Vacuum advance start out at 12 degrees and add timing until you hear a light ping under light accel up a small grade. DS2 distributors have a dual stage advance mechanism and far too much vacuum advance for use with out EGR.

With out knowing your vehicle, weight or gearing I can not tell you when total advance should be all in.

Vacuum advance is in addition to total advance as it only occurs under part throttle cruise conditions when flame front propagation speeds are slower.

The stealth manifolds have very large runner cross section and lower velocity at part throttle cruise and acceleration. Poor tip in response in my experience vs a performer or RPM. They are great WOT intakes on performance oriented combos. Follow Randy's advice !!!

Your fuel economy will be dictated primarily by the idle and transition circuits in your carb. Out of the box universal carbs are pig rich at idle and transition. Leaning out the idle feed restrictor and or enlarging the idle air bleeds can help with fuel economy on a holley. Eddy carbs are tuned with metering rod changes.


S
 
#11 ·
My Edelbrock 1407 (750) was veeeeeeeeeeeery lean. I'm still trying to get it rich enough to pull hard. I bought the largest jets available (.119) for the primary side, and they helped but were still showing lean on the plugs. I then drilled them out to 1/8" (.125) and were just right with the stock metering rods. The accelerator pump nozzles were .024 stock. Edelbrock's kit has them in 3 sizes (.024,.033 & .043). I drilled mine out to .043, but that wasn't enough. I just drilled them to .055, and the rod still has to be in the hole closest to the pivot. It still feels like there could be more to gain. I'm actually out of material on the last 1/8" of the nozzle, and I may eliminate that 1/8" and drill them to 1/16" (.0625). I still haven't messed with the secondary side much, although I did open the jets up to about .119. I'm going to pull the manual choke partially closed and run it to see if there's something to gain by richening it up. It's all trial and error and I don't want to spend any money on this carb. It's temporary. I'm switching to a quadrajet in the spring.
 
#14 ·
I'm starting to lean towards ordering up a 750 Edelbrock carb. I've been wanting one for a while to hopefully increase WOT performance. It may be a better option than modifying my 600cfm.
 
#16 ·
Looking at the Edelbrock 750cfm carbs it seems to come down between the 1407 and the 1411. At first glance I thought the only difference was the electric choke on the 1411 vs manual on the 1407, but it also looks that the primary jets and rods are slightly different.
110 primary jets for the 1411, & 113 for the 1407
Metering tip 0.075 for the 1411, & 0.071 for the 1407

My first thought was to get the 1411, but now I'm not sure which one to go with... I'd like to do as little carb modifying as possible once it's on there since this is a daily driver and I would prefer not to have it down at all.
 
#24 ·
I've pretty much narrowed it to the 1407 or 1411. Not looking to spend $650 on a carb.

I may end up getting the 1411 today. Autozone can order it for me, give me 15% off, and it'll be in on Friday.
 
#26 ·
I guess this thread comes under the heading "You can lead a horse to water, but you can't make him drink". You asked for advice to tune your engine, but you ignore the advice.

Randy, Scott and I immediatly told you that your stealth was a mismatch to your combo. Your reply was that you have had the manifold for quite a while and was satisfied with if. The truth is that you have had the wrong manifold for quite a while and don't know the difference! The performer would give noticeably better economy and performance.

Randy and I have both told you that the newly engineered "STREET DEMON' is the superior carb for your combo. I have bought two of them in the recent past and I paid $387.93 per carb when I bought them direct from Demon. The top of the carb comes off like an Edelbrock, it has metering rods and a secondary air valve like an Edelbrock, it has 2 small primarys with "tripple stack boosters" and one large secondary and is quite tunable. As you would say, "a carb for dummies". Did you research the STREET DEMON or are you again going with what you know?

All you are doing is re-enforcing the knowledge you came in with, why bother?
 
#25 ·
In the spring, I'm replacing the edelbrock w/ a quadrajet. It was $25 shipped off ebay, and I spent $100 on a kit put together by Cliff Ruggles. I also have a 650 double pumper that I got in a garage sale for $5. It needs a few things but if I get board, I'm going to mess w/ it.
 
#38 ·
Okay, I got the carburetor on the engine today. Started up right out of the box, adjusted the idle screws and the throttle and got it running smooth.then I adjusted the choke because it was stuck closed. I went out and drove it and it threw black smoke when I stepped in it. I adjusted the accelerator pump to the lowest setting (least fuel) and fixed the smoking. It doesn't seem to pull as hard on WOT as with the 600cfm Edelbrock 1406. Throttle response is pretty good. I'm going to track fuel mileage the next couple of days and see what happens. I'm not extremely happy with it currently.
 
#39 ·
Hey guys. Drove about 90 miles today, just normal driving. Only like 7.2MPG... I've talked to a couple of people and it sounds like I made my mistake ordering a 750cfm carb... Seeing as I don't rev this thing really high, the 750 isn't really a great choice for my application. Shouldn't have listened to the local guys telling me a 750 was the only good way to go... Should've gone with the Demon 625 or just settled on modifying my Eddelbrock 600 like you guys were talking about. Learned a lesson there.

Also, I'm still considering what you guys were saying about the intake manifolds. I don't feel really great about dropping a couple hundred bucks or so on an intake in the next couple of weeks, but I'm definitely open to trying something other than the Stealth. I've got a D0VE intake manifold sitting in the garage if you all think it may be a better choice for lower RPM street driving. I found some HP/torque numbers from tests on a 460 running different manifolds, and the D0VE made more torque lower in the RPM range than the Stealth and Performer (not RPM, just Performer). In the particular test it made about 30 lb/ft more torque than the Stealth and about 15 more than the Performer. Plus the D0VE peaked in torque at 3250 RPM where the others peaked at 3500. It made about 30HP less than the Stealth at 5,000 RPM, but I don't rev mine that high, so...
 
#42 ·
Make sure that your air cleaner is not setting on the accelerator pump linkage. If it is, Demon makes a spacer (as do many others) to raise the air cleaner above the accelerator pump linkage. Make sure that the secondarys mechanically open all the way and are not hitting the manifold/spacer. After that, loosen the secondary air valve til the engine bogs upon sudden application of full throtle. Then tighten the secondary air valve spring till the bog just goes away.
 
#40 ·
Under light throttle cruising conditions where you are using the transition circuits the size of a carb has noting to do with its economy potential.

The tuning of the idle restrictor feed and air bleeds to maintain stoic of 14.7 to 1 AF determines economy under those conditions. Depending on cruise rpm your main well may not even come into play.

I get the same part throttle economy with my custom built and self tuned (with a fast wide band) HP 950 as I did with a 750 Holley.

What is your initial timing?
Total timing at what rpm

Amount of vacuum advance?

Timing plays a critical role in both power production and part throttle economy via the vacuum advance.



S
 
#41 ·
Under light throttle cruising conditions where you are using the transition circuits the size of a carb has noting to do with its economy potential.

The tuning of the idle restrictor feed and air bleeds to maintain stoic of 14.7 to 1 AF determines economy under those conditions. Depending on cruise rpm your main well may not even come into play.
I get what you're saying here. This 750 carb can probably achieve the same economy as my 600 when tuned right. What I'm more concerned about is WOT performance. It seems that the smaller carburetor is a better match for a lower revving application like mine. I'm feeling like I just jumped the gun by going to a 750. If you do the math it makes sense too; (468 • 4500) ÷ 3456 = 609.375 at 100% efficiency. Am I confused here?

What is your initial timing?
Total timing at what rpm

Amount of vacuum advance?

Timing plays a critical role in both power production and part throttle economy via the vacuum advance.
13°
40° all in @3400
~10° vacuum advance

Also, I haven't touched timing since the carb swap.
 
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