Well, a few of the recent postings have started to motivate me..Its too hot for me to work on my car, so I might as well start thinking about it.

My car currently runs pretty good, but I feel that its a bit flat right off idle..by this i mean that if I am going along slow in 1st gear and nail it, it takes off strong, but doesnt spin/smoke the tires and get all kinds of scary like I thought it would/should. It has the stinky idle issue so commonly discussed on TC. The other thing I noticed is that it kind of surges a bit when moving along slowly in first gear, like cruising around a parking lot. Also, when the car is hot and I come home from a cruise, it seems to idle a bit higher than when it first warms up, like its racing a bit..not sure what that is about.

I probably have about 1000 miles on the motor since it was rebuilt. It was broken in on the dyno and had 425hp/440 tq, so its pretty healthy. M20 Autogear with 3.31 rear. Carb was a brand new Holley 750HP (4150) with mechanical secondaries. Distributor is currently a GM HEI that was re-curved to match the motor when it was done. From what I recall, when it was on the dyno, all they really did was set the float levels, and change the mains (I will have to check to see what they are currently). I have not touched the carb since, other than to adjust idle speed as necessary (timing, etc). Vacuum is approx. 12-13" @ idle (900 rpm). Cam is Lunati 02005 (Merc Marine). Timing is approx. 17 advanced at idle, 36-38 total all in @ 2800 (at least that was how it was curved.) If it gets too far advanced past this at idle, it gets hard to crank. Also, only running mechanical advance right now, no vacuum advance is hooked up (per the shop that did the motor).

I want to try and clean up the idle smell, and at the same time get it to be a little more snappy off idle. A while back it seemed like my timing was moving around on me, so I bought a brand new MSD Digital E curve distributor and new MSD Blaster 2 coil that I want to install. I dont plan on an MSD box right now. The HP carb has no choke tower, 4 corner idle, and I would assume whatever the default Holley pump cam/shooter and 65 PV (front and rear). Elevation here is basically sea level.

I think I am going to drop in the distributor first, and see if I can go with a slightly more aggressive timing curve, maybe one that is all in a bit earlier. Also I am going to try to set it up with vacuum advance. Once I get that complete, then I am going to turn to the carb and play with it. I can provide all the other details on current jetting as it sits now, but my first inclination is to get the new distributor and timing set where I like them, then do the carb..any thoughts? I have no idea what pump cam/shooter, etc. I should be using for my application, and if changing any of that will make any difference in response/feel. I tend to think the main jet sizes are ok, as the motor made max power with those in place-but it does seem fat.

Any guidance from carb and timing gurus on this site would help..

Thanks.

John

John,
what are the cam specs ?

More info:
396 +.060" over for 408CI.
9.25:1 compression
Lunati 02005 hydraulic flat tappet cam
Speed pro forged pistons, file fit rings
1969 -063 cc heads with 2.19/1.88 manley stainless valves

Cam specs:

advertised duration int/exh: 302/308 degrees
Duration @ .050" int/exh: 224/234 degrees
Lift @ valve int/exh: .534/.559"
Lift @ lobe int/exh: .311/.324"
LSA 114 degrees. intake CL=110
opens/closes @.050": 2 deg BTDC, 42 deg ABC/ 55 deg BBC, -1 deg ATC

I bet they fattened the mains up for the dyno - the surging is dirty plugs, smelly idle from the rich mixture. I had to down jet my mains after the dyno runs. Works fine now!

Well, I know they did indeed increase the mains, I will report back on where they are currently, as they wrote the final size on each float bowl. I agree on the plugs also, as I may have pulled them 1x and checked/gapped them but that was quite some time ago. Cap and rotor were new, and plug wires were new taylor spiro pro's.

I bet they fattened the mains up for the dyno - the surging is dirty plugs, smelly idle from the rich mixture. I had to down jet my mains after the dyno runs. Works fine now!
The size of the main jets has nothing to do with idle quality!

I totally agree - but when the car is stationary (usually it's also idling) you will noticed the increased odor of the rich mixture more so then when you are running at 60mph. I was running rich, it would eventually foul the plugs, giving me the surge at low rpm cruising, and when sitting at a light, the fuel smell was very strong.

Personally before I swapped distributors I would hook up the vacuum advance and get it setup correctly. Best case it might be all you need, worst case you will learn a bit more about how your setup works.

With a 12" vacuum at idle you want a can that is all in by about 10" and 12* crankshaft advance. Try it hooked up to manifold vacuum. This will allow you to crank at the 18* you have now (makes the starter happy) and idle at 30* to make the motor happier. It will also allow you to cruise at 52* which should help clean up the plugs.

Note that the surging may just be that your combination does not want to go that slow ;-) Got a problem like myself with a 402 running a 236/242 @ .050 hydraulic roller cam.

My ZZ454 surged badly at light throttle settings (cruise, going up overpasses over the freeway, etc.).

Normally surging is caused by lean mixture, a fat mixture makes the engine lazy but smoother than a lean engine. It'll also make your eyes water while you're waiting in line at the drive-thru.

I cured my lean surge by using the 4-corner idle that both my Holley and Barry Grant carbs have. It made a very big difference in how the engine performed on the primary side of the carb, and made the car much more drivable.

Using the 4-corner idle allowed my to richen the idle and off-idle without actually richening the mixture itself. I simply was able to add more fuel without changing the mix ratio.

By providing more fuel at idle and off-idle, the whole primary side performance cleaned up quite a lot, not just the idle quality alone.

One holdup with my Holley was that it had drilled primary butterflies, not necessary with a mildly-cammed engine such as the ZZ454. The BG Demon (850 w/annular boosters) has not only a 4-corner idle, but an adjustable idle-air bypass.... kind of like the equivalent of adjustable hole sizes in the primary butterflies, from zero to about .125" . These drillings work ok when your cam is so huge that to get the engine to idle you have to open the primary butterflies way past the idle slots on the baseplate, and in effect you are idling on the main circuit and transition circuit, i.e. the main jets are now metering the idle in this situation, and the needle valves have zero effect. The holes allow more air to pass to maintain idle speed without having to open the primary butterflies so far that the idle circuit becomes "buried" mixture-wise by the main and transition circuits.

Remember, with a 4-corner idle you don't necessarily burn more fuel, since the butterflies are closed more.... the total open area remains the same as with the 2 primary butterflies open only, and the total amount of fuel going into the engine from the idle circuits does not have to increase.

The old way was to open up the primary circuits in the metering block to flow more fuel, such as with the needle valves on the sides of the Holley metering block, but that is not a very elegant solution these days.

The real way to make an idle circuit flow more without opening the butterflies was to enlarge the passages with drilling, which is the realm of a custom carb, and once a carb is drilled the metering block is pretty much dedicated to that engine combo only.

If you are having surging, and your idle is higher when you come back from a drive, the solutions are likely to be found in the idle circuit mixture strength. Once that is taken care of, the next thing to look would be the primary circuit, not just the primary jets. Jets are for gross tuning to get ballpark, and fine tuning is accomplished by the air bleeds and the passage drilling sizes.

Air bleeds are very easy to understand; think of it like this- you just got a large coke, you open the straw, stick it in the coke, and just get foam instead of liquid drink, because the straw is cracked.... that's an emulsion of coke and air... this is an air bleed. The larger the bleed, the more air and therefore the leaner the 'foam' mixture.

Also, liquid gasoline must be accellerated and decellerated within the carb passages as the engine changes its fuel requirements in different situations. Liquid fuel is heavy, where an amulsion is very light, and can be accellerated and decellerated much more quickly, making the carb's mixture changes much faster in response.
This is why replaceable air bleeds are such a good feature of high performance carbs. Without them, the only way to change your bleed size is to drill, and then of course it stays drilled. Get it wrong, it stays wrong.

I'm not suggesting that you run out and buy a 4-corner idle carb, but I just wanted to show you how some things work so that while you're sitting out the heat wave you have some more things to think about.

All in all, IME surging is almost always caused by a lean condition. The idle circuit contributes much more to the overall primary performance of the carb than generally recognized.
Welcome to carb tuning... with some patience and some advice, you will have the crisp tire fryer that you thought your big block would deliver.

One last thing..... not to dis the above, but an old saying around shops is that a lot of carburetor problems are solved in the distributor.... I'm not saying that's your issue here, but it is a good thing to remember and store away for future remembering.

Eric

My ZZ454 surged badly at light throttle settings (cruise, going up overpasses over the freeway, etc.).

Normally surging is caused by lean mixture, a fat mixture makes the engine lazy but smoother than a lean engine.

Air bleeds are very easy to understand; think of it like this- you just got a large coke, you open the straw, stick it in the coke, and just get foam instead of liquid drink, because the straw is cracked.... that's an emulsion of coke and air... this is an air bleed. The larger the bleed, the more air and therefore the leaner the 'foam' mixture.

I'm not suggesting that you run out and buy a 4-corner idle carb, but I just wanted to show you how some things work so that while you're sitting out the heat wave you have some more things to think about.

Welcome to carb tuning... with some patience and some advice, you will have the crisp tire fryer that you thought your big block would deliver.

One last thing..... not to dis the above, but an old saying around shops is that a lot of carburetor problems are solved in the distributor.... I'm not saying that's your issue here, but it is a good thing to remember and store away for future remembering.

Eric

Eric:

Lots of good things for me to think about in your post, and they are in agreement with what I feel I understand about how to tune a motor. As far as the surge vs. stink goes...my car is way more stinky than surge-ey..it stinks all the time, my wife complains when I come in from driving it, and I no longer let it idle in my attached garage. The surge was just recently noticed at VERY low speed in a parking lot, but does not happen at cruising speed at all from what I can tell.

The other thing I have NOT noticed is any big noticeable "KICK" when the secondaries come in..which I understand to be a good sign that jetting is close.

My Holley manual for the carb kind of glosses over the whole air bleed/PV/pump cam/squirter issue..so if anyone can speak to that, great..at this point I dont even know where these parts are on the carb, need to go familiarize myself with the setup.

It is my opinion that this motor should absolutely punish the 14" BFG's that are on it when throttle is applied-but right now it doesnt and I won't be truly happy until it does!! Dont get me wrong, it will light them up, but it wants more rpm and some clutch dumping to do it.

The good news is my carb has the 4 corner idle setup and adjustable air bleeds (I guess all the extra $$ I got hit for is now supposed to start paying off for me?!). My Holley book warns you not to screw with the air bleeds, but I could probably figure out what to do with some help from TC!

Too Vroom's point above..I know I am dumping my HEI for 3 reasons..it was used, and the shop told me when they re-curved it that it was getting tired, the timing settings have since been inconsistent for me, and I want make the car look more 'factory' under the hood. I already spent the $$ on the MSD unit..otherwise I would probably invest in some tuning on that HEI distributor..but I am in agreement that I would probably be better off with vacuum advance for how I use/drive the car (mainly cruises, shows, and occasional strip).

I think I am going to do the following:
A) BUY a book on how to set up and tune Holley carbs! (recommendations welcome!).
1) Re-verify timing and re-set if necessary. Record all current carb jets/settings.
2) pull/check/gap/reinstall or replace plugs (noting condition).
3) Test drive and wait for cooler weather!!
4) Set up the advance curve and rev limit on my MSD distributor.
5) Replace Hei with new MSD. Re-time as required. Test drive again.
6) Once I get it where it feels good, start adjusting the carb, starting with float level check and then on to 4 corner idle screws.

Sound like a halfway decent start? I am a big fan of only changing one thing at a time when it comes to stuff like this.

You sound like you have a pretty good plan of attack. You seem to have the important concept to work the ignition side of the puzzle first and get it right.

One area that cause me some grief with changing timing was the distributor moving. I think I finally got the darn thing tight enough to not self-retard.

I would recommend the David Vizard book on intake design published by SA whatever they are (Steve Allen?). Much better more in depth explanation of air bleeds and their operation.

You might also be a good candidate for a wide band O2 setup. Put one of these in along with a vacuum gauge and you will learn a lot. Really complements the stuff you read in the books with empirical data.

Note that while the prior air bleed description was generally correct, it omitted the very important aspect that air bleeds are used to tune circuit turn on and turn off. The on/off then controls the overlap of idle and main circuits. You have a fairly serious performance carb already so the timing is likely pretty close. I have a bit more cam and have found the circuit timing leaves a bit to be desired with more basic carbs like Edelbrock Performers. I stuck some wire in the secondary air bleeds to get them to suck more fuel (already jetted up to just about the max) and picked up a bit more wheel hop B)

One area that cause me some grief with changing timing was the distributor moving. I think I finally got the darn thing tight enough to not self-retard.

B)


Steve:

I hear you on this problem..This was definitely part of my problem in the past, I replaced the hold down and bolt and it seems to be hanging in there better, but I havent driven it as much as its been so GD hot this summer! Its funny, last year it was self retarding on me (it would start to diesel on shutdown), so it was easy to diagnose. Then late last year I stopped to get gas and upon re-start, it would barely turn over..I have since replaced the battery and found my timing had advanced REALLY far, cant believe it would even run at all..All is well now though. The HEI has been pretty good, but I am pretty sure its not fully optimized hence the MSD (cap and coil have been painted black for some time now!!).

Id love to have an O2 sensor/setup..that kind of info makes this stuff way more of a snap, and I did enjoy and learn a bit from watching them dyno my motor with respect to fuel/air ratios while it was on the stand. Thanks for the Vizard book recommendation, I will check it out. Can you tell I like to immerse myself in something before turning any wrenches? My previous carb fiddling goes back to my motorcycle/dirt bike days, but I know that a little bit of educated fiddling usually goes a long way.

I saw a pic of my carb from when I dropped my motor in..main jets are 78 front and rear..I assume that this means it is jetted "square"(?).

First off, try manifold vacuum advance - this will allow you to close your primary blades thus less exposing the tranfer slot. This is a big deal. RIght now you probably have the idle screw cranked in so far that the transfers are gushing.

Secondly, double pumpers have richer idle/cruise calibrations then do vac secs. The DP are race carbs basically. With the LM-1 that I had, I confirmed this: my 750 and 800 DP's both idled and cruised at ~12afr, no matter what you did. And they BOTH stunk no matter what I did. My 770 and 870 Street Avengers both idle and cruise in the 13-14 afr range. There is very little stink.
Being that your carb is an HP, I bet its even richer. It may be more adjustable though (air bleeds and such).

In the end, I think you'll never get it "clean", but you can probably get it better!

BTW, w/a PV in the back, I'd bet your a little rich w/78's. PV's are good for what, another 6-10 jetsizes. Most guys here w/bigger motors are in the 80-86 w/no PV in the back.

Vince:

I hear ya..the motor did respond on the dyno to the main jet changes ( I think the mains were bumped up 4 or possibly 6 sizes over whatever was in the carb to start with, winding up with the 78's). Doing so picked up at least 15 hp from when this carb was first bolted on. I wonder if I should also be thinking about tweaking the PV at some point too, if the Holley info is correct, PV should be a 65 in there right now but I havent verified.

I need to go get a vacuum gauge, and I will be hooking the new distributor up to manifold vacuum at some point when the time comes.

JTM, sounds like you have 2 issues: rich idle, and a little laziness.

I wonder if the recent heat is causing these engines to run a little too rich. You should try dropping the jetting all around a little and see if it picks up a bit. Maybe it was dyno'd with cooler air.

BTW - you don't live to far from me. I'm near the RT.1 and RT.202 intersection in Concordville PA if you know where thats at.

Vince..

Yeah, my office is about 15 minutes from there (Aston). We should plan on running into each other some time!

I know the heat plays a part in this, and the jetting was done in the fall on a cooler/less humid day..so im sure its going to be softened up because of this when its hot and humid...but i was noticing the issues prior to the heat though too..it just took me a while as I was getting used to the car, etc..

The stink has always been there, the laziness I have noticed since driving the car a bit harder and getting used to it. Some of it is probably just a quick tuneup away, but once that is done, I have a feeling there is still more to be done.

Thanks for all of the motivation guys, I havent driven or touched my car lately and this little exercise has peaked my interest again!~ I will post back after I have dug into it a bit and let you know how Im making out. Vince PM me your # and email address so we can get in touch R/T..maybe I can head down to Cecil and watch you run sometime or maybe make a pass or two someday.

JTM,

I find that books are a lot cheaper than the wrong parts :)

You will find that auto carbs are a very different beasty than motorcycle carbs. I can say that I have played around with both and prefer the motorcycle carbs for their range of adjustments and relative straight forward adjustment. I should have also added that Dave Emmanuel has a few books out... these are ok but oriented more towards rebuild than theory of how they work.

There seem to be a couple of schools on carb setup: the full power and darn the rest school versus the more sophisticated make it run well *and* fast school. IMHO square jetting falls into the former. What you really want is for the primaries to run lean and give you that 14.5 or so AFR you want for best economy and least emissions on the street. Then you want the combination of the power valve and the secondaries to drive that down to 12.5 or so AFR for those full power boogey put a smile on your face sessions. This almost certainly requires, or at least suggests, staggered jets with smaller in primary and larger in secondary. A simple max pull dyno tune will only get you the max power setup and that will likely be a bit different than an optimal street setup.

That said, AFR also depends on air bleeds and a smaller air bleed can cause more fuel to be sucked from a smaller jet. It really is the balance between air bleed, jet size, and vacuum (either generated by airflow through the venturis or manifold) that determines AFR.

To really tune it well you are probably going to have to get into the idle jetting and idle air bleeds. Expect to spend a fair amount of time playing around with it if you don't get the wide band O2. I have mine setup with RPM and MAP/vacuum inputs so I can log onto a laptop PC and review things graphically after the fact. You can see spikes from accelerator pumps and vacuum/AFR change relationships very quickly this way.

That's funny - I grew up in Aston!

I have 3 Holley book if you want to borrow them.

Eric:


I think I am going to do the following:
A) BUY a book on how to set up and tune Holley carbs! (recommendations welcome!).
1) Re-verify timing and re-set if necessary. Record all current carb jets/settings.
2) pull/check/gap/reinstall or replace plugs (noting condition).
3) Test drive and wait for cooler weather!!
4) Set up the advance curve and rev limit on my MSD distributor.
5) Replace Hei with new MSD. Re-time as required. Test drive again.
6) Once I get it where it feels good, start adjusting the carb, starting with float level check and then on to 4 corner idle screws.


Well finally getting around to this again! I pulled the plugs and replaced with new. I set the gaps at about .040" if I remember correctly (did this several weeks ago). A few of the old plugs looked to be a little fouled, but nothing really really bad.

I have driven the car 2x since this..the day I changed the plugs and tonite. Car started and drove well, but seemed reluctant to idle 'cold', even though it was warm and humid out-which it normally does...I know that when I left it last, i had it set to idle at 800 rpm. When I start the car, I need to keep my foot on the gas until its warm or it will stall (carb has no choke). As it gets warm, it will idle at about 500-600 rpm, but its rough and almost stalls. After I drive it and its fully warm/hot, it will idle back at 800 where I set it-its like a different car when I get home from a drive.

After filling up with a fresh tank of 93 octance sunoco, i ran her really good through the gears a couple times, got her nice and hot. Came home and checked the timing: with idle at 800 rpm, it was about 16 degrees advanced at 800rpm (which means I had it set at 12 initial). Total timing was like 30 or maybe 32 degrees at 3000rpm. It should be 34. Hmm...Again, this is why I am dropping a new distributor in it. Seems to shut off a little rough..I think it needs some more initial.

I set my rev limit at 6000 rpm on the digital e curve. I selected a timing curve that is going to provide 20 degrees of mechanical advance, all in by 2500rpm. I also selected a vacuum advance curve that will provide 10 degrees of vacuum advance at 10". I have some other options for both mechanical and vacuum..that will be some trial and error. After I get the distributor dropped in and play around with it, I am going to add the vacuum advance..havent had that up to this point, just mechanical on the old HEI. I have to say this is a pretty slick invention by MSD..hope it performs as well as it appears to be designed and looks.

Tomorrow I plan to run it some more and record vacuum readings at warm idle. Also will verify as many of the initial carb jet sizes as possible, as well as linkage/cam position to make sure that is all working as needed. Then drop in the distributor and new blaster 2 coil. Just figured I'd post back for anyone who might have been or is interested.