Cool bean

I have an issue with my build and i considering a solution that may or may not make sense: fix the issue with carb, or go with fuel injection (FITech Go EFI 4 PA).
here's my build: 355, cast crank, stock pistons, pump up lifters, aluminum heads (64cc, 190 runner, 2.02 intake, 1.6 exhaust), 1 3/4 headers to 2 1/2 inch out the back exhaust, DUI HEI distributior, rpm intake, comp 12-601-4 cam, electric fuel pump, dual electric fans, 700r4 trans in a 74 base coupe. It should be close to 400 hp at crank.

the problem is the plugs are fuel fouling. The carb is a holley 4777-7 650 DP, manual choke. Whenever i run the car, idle, or limited driving and then let it sit a few days, the plugs foul. If I put a fresh set in....repeat.

after reading a number of posts about chasing fuel fouling, I'm wondering if i want to deal with that. My build goal is to upgrade the car, so i dont mind considering fuel injection. I dont mind the cost of fuel injection. Theres a post recently by someone who made a very nice post with video about adding fitech to their later 70's c3. GordonM had some nice things to say about fuel injection too. I'm intrigued, but I'd like to make the carb work.

my application is spirited driving, trips to the track, the goal is a 12 second car that can also be used for road trips / cruising.

what would you do? Does anyone regret adding fuel injection? Maybe i chase down fuel fouling issue and punt Fuel injection to next year. Whaddaya think?

Phil

dmruschell

I daily drive my 1979 Corvette with a built up engine. After trying a 650cfm Edelbrock carb, a 770 CFM Holley Ultra Street Avenger, and FiTech fuel injection, I settled on an 800cfm Edelbrock carb. (I may try out a purpose-built Quadrajet soon, but not because anything is wrong with the Edelbrock). You can search posts I've started for my FiTech experience (it lasted 3 days on my car before I switched back to carb), though it may not affect you since you have an automatic. The car did start up and idle nicely, just like a modern car though.

I've since switched to manifolds (I don't know if that would have affected anything though), but I had long tube headers when I installed the Holley in October of whatever year I had it. It was fine over the winter, but when the weather got warm, the Holley percolated so bad, fuel would boil over into the engine while it was running and stall the car out. It would also boil every time I parked it. That's just something to consider if you're tracking down the problem.

The easiest solution would be to try a different carb. I haven't had any real percolation with the Edelbrock carbs, and if you're running a Holley, they'll fit your intake. Between the 650 and 800cfm carbs, I've put 40,000 miles on my 79 Corvette with Edelbrock carbs and settled on the 800.

BLUE1972

Two club members and I discussed this issue carb or injection. We all went with carbs.

If you cruse and get far from home you will always get home with a carb.
The one thing we did find as an almost requirement was an insulator plate for the carb. A 1/2 in insulator works fine.

Ralph does power tour and other mega mile trips and was concerned about the repair on the road issue. We all felt it's a no brainer to get a carb if you had to in some small town, but fixing an aftermarket fuel injection may not be possible.
He has a rebuild kit in the storage compartment - he has a 383.

I've put a lot of miles on my rebuilt 71 and have no issues with the carb.

Just our opinions..

dmruschell

Most auto parts stores even sell new carburetors. AutoZone stocks Edelbrock performers. Pep Boys usually stocks Edelbrocks, Holleys, and Quadrajets.

lionelhutz

I would never use a carb again. I use GM based FI systems and they work great. Parts are easily available too, probably even easier to get then carb parts. I will also only ever use an EFI system that will also run an electronic transmission. If you're putting the effort into EFI, then also adding the electronic transmission capability simply allows way more control over the automatic shift points and TCC lockup then what is possible with a valve body transmission. I'm way lagging others, but still I'm finally doing a project with a 6L80e transmission so I can play with the extra gears and paddle shifting. Really looking forward to driving that one.

Still, on a smaller scale as you asked, almost everyone who's used it has loved their FiTech EFI system. I wouldn't put much faith in the opinion of someone who had the FiTech system installed for 3 days. That is not enough time to actually troubleshoot what it's doing or why it's acting up and then get it to run correctly. It's basically just giving up when it doesn't immediately work right.

If you're concerned about the aftermarket EFI parts situation then you should ask them what the parts are. Most likely, they are using a mix of OEM parts to create the kit. So, the only part that relies on the EFI company is the electronics.

cardo0

My experience working with the OEM EFI in my '94 camaro is EFI has a large learning curve as is and upgrading from a carb makes the learning curve even larger. Upgrading the fuel system is another headache. I believe the newer EFI swaps are more plug and play these days but you have to ask yourself the question whether you drive the car enough to justify the effort.

Just my 2 cents here.

Gordonm

Nothing wrong with a carb setup but getting it set up correctly and keep it running correctly is where most of the problems start. There are not to many good carb adjusters out there left. I have tinkered for years with them and go OK at it but they still seemed to go away with time. I have converted a bunch of cars over to FI and used a couple of different systems. Most need a little effort to get them tuned in correctly but once they are setup they seem to keep in tune. Today's fuels are also a challenge for most carbs also. The cost of FI is coming down also which makes it almost competitive with a carb. I've run a Mass. Flo system, a GM system currently on my LS3 and currently have an EZ efi 2.0 on my Cobra. Installing a FItech on a forum members car in the next few weeks. I have tossed all my carb stuff out and won't go back to carbs for my cars. They start right up and run no matter where or weather conditions. As I said nothing wrong with a well setup carb but it's all in the setup and if you start changing parts the carb will need to be reset.

AirBusPilot

I'm having the same issue with my Holley 950. Fuel fouling, terrible mileage and when it gets hot after it sits for 30 minutes, I usually have to sit there and let it run at 3000 rpm to clear it out. God help me if I screw that up, then I have to fight to start it.

I'm getting sick of it.

I'm contemplating FITech fuel injection. I will have to replace the fuel tank with one from Tanks, inc, which has internal baffling, along with an in tank pump, along with routing a return line and necessary wiring. This adds about $500 to do it myself.

But to have it start and run reliably every time will make it worth it. And better fuel mileage and longer plug life.

johnt365

Before dropping 1k-2k on an FI system, a compromise may be to install a wideband O2 gauge so that you can read the A/F ratio of each circuit.

My thinking is you would need to install the O2 sensor for the FI anyhow and, having the gauge first just might save your carb.

lionelhutz

What does the amount it's driven really have to do with it?

MotorHead

So your opinion is to go with fuel injection on a fuel injected C5 ?

drwet

I put an air/fuel ratio gauge on my '79 a few years ago and it was the best thing I have done in terms of carb tuning. I've been tuning carbs all my life, but I now realize I didn't really know what I was doing until I had the feedback from an O2 sensor.

BLUE1972

My 1980 Chevy truck has a carb, SBC. I had to replace the carb after 400k miles and 35 years as the metal was starting to disintegrate, AL in the gas and age. It was factory.

I put an Edelbrock carb and Weiand manifold on it 4 years ago.
I set it up as per the instructions and have not touched it since.


Most carb issues I find are due to people using carbs way too large for the engine. If the cab is too large the mix and adjustments will not properly work and the engine runs poorly.

When I set up my 71 on the dyno the engine builder had the same comments as I showed up with a 650 carb and he was amaized that it was not a 900 or some other crazy carb... Two + years and no issues .

Just my thoughts and his..

MotorHead

100% you can't tune what you can't see. With a wideband O2 sensor you can get your carb dialed in for idle, part throttle, cruising and WOT.

MelWff

1. With a clean set of plugs have you measured vacuum at idle? Your power valve on a 47777 is 6.5 you may need a lower number.
2. What manufacturer and part number plugs are you running? They may be too cold.
3. Changing a power valve or a plug is a lot cheaper than going FI.

AirBusPilot

Btw, I have a wide band O2 gauge. A/F is fine at WOT. A little rich during cruise, but that's a Holley DP.

ddawson

It doesn't have to be. I saved this from a great source and it allowed me to dial in the idle, transition and cruise. I had to move down two sizes in the IFR and about 10 sizes in the main jets.


Carb Tuning with Wideband and Vacuum

This is not an all about carbs guide. I’m not going to show off knowledge of fluid dynamics and venturi effect, or talk about pressure differential. I’m not going to talk about annular vs downleg boosters, or explain all the emulsion functions, internal passages, or what they do. I’m also not going to show you where the idle speed or mixture screws are. In fact, I’m not going to show you where anything is.

This guide assumes you know a bit about carbs, and are serious about really tuning it in. If you weren’t, you wouldn’t have a wideband and vacuum gauge hooked up. I will explain how to change each circuit for its intended operating range, and how to avoid common tuning errors of fixing symptoms, with the wrong circuits. Following these steps will give you the best balance of economy, and full power potential.

Base timing, and advance curve must be set prior to tuning. Refinements can be made later on, but it must be close to start. To set base/initial timing, start the engine, run until warm, then set a quick idle mixture. Set the idle RPM to the lowest idle that won’t stall. Advance or retard the timing until the highest vacuum at the chosen rpm is achieved. The idle speed screw will need to be turned with timing adjustments to keep the rpm the same, so this could take a while.

After this initial timing is set, check the amount of advance with a timing light. Write this down in a tuning log. You will then need to limit the total advance either with bushings, or in some cases, the slot must be partially welded, and filed to allow the correct amount of advance. If your initial timing was set at something high, like over 20* you might want to pull the initial back until you know how much mechanical advance you’re getting. Once the total amount of advance is proper, you can return the initial to the setting you wrote down. Choose a set of weights and springs that gives full advance at an RPM that is typically recommended for your engine. Most like around 2500.

Make sure you have the float level and fuel pressure set for what is recommended by the carb manufacturer, not your buddy, or that guy who solved his lean condition by running 12psi at idle.

This will get you close enough to dive into the carb circuits. If you didn’t already do this when installing the carb, pull it off the intake (I know, how annoying) and see where the transfer slot exposure is on the primary side. It should look like a square. You can put a jam nut on and lock it down which will prevent you from screwing with it later on, pun intended. This is important to isolate the idle mixture from the off idle/cruise mixture. With the carb back on, readjust idle speed if needed using the secondary screw, or if equipped, “idle ease” adjustment. Note that carbs with the idle ease, or 4 corner idle will need the secondary transfer slot set as the primary.

Now, to begin some baseline settings. Because of differences in combustion efficiency, compression, overlap etc, engines will “like” different AFR’s (air to fuel ratios). Acceptance of this is critical to optimizing your engine’s manners. This is why we will be using the trusty vacuum gauge once again before using any wideband data.



Idle Mixture:

This is a basic one, but it will greatly impact how you go about tuning your carb’s off idle/cruise circuit. More on that later. Warm the engine up to typical operating range.

Bottom out the mixture screws, then back them out 1-1 ½ turns. Working in equal amounts for driver’s and passenger’s sides, turn them in ¼ turn at a time, maybe even less, until the engine wants to die, but still stumbles along without doing so. Note the AFR.



This is the absolute leanest it will run at, but poorly at that. Now do the opposite, but do it for the highest vacuum reading possible. IGNORE the wideband at this point! Be sure not to go above the highest vacuum reading. Only open them up enough to hold the highest steady vacuum reading. Do this like you did for base timing, and readjust idle speed so you are getting your vacuum readings at the same engine speed. Only use the secondary idle speed screw, or if equipped, idle ease for speed adjustments. Don’t change the primary setting. Okay, now you can look at your wideband, and note the number.



This is where it “likes” to be, but will quite possibly be a very rich figure. Mine for example idles buttery smooth at a 12.0, but will tend to foul plugs quicker. It also idles as lean as 16 but sounds like it has a huge cam. At this point, lean it out until the vacuum, or rpm drops just a little bit, but doesn’t produce the choppy effects seen at the absolute lean mixture. This is going to be your best compromise of smooth idle, and clean plugs. Note this AFR.



Main Jets:

“But what about IFR (idle feed restrictors) and IAB’s (idle air bleeds)” you shout!? We will get back to them later. The selection of the main jet is a tough one to get your head around, but here is the real basic concept. The main jet needs only to be large enough to support cruise and light acceleration at engine speeds of approximately 2000-4000rpm. 4000 is really pushing it too. Who cruises at 4 grand!?



Warmed up to normal temp, drive the car on a flat level road at 2500rpm. Hold the throttle steady, and observe the wideband. You should be able to get away with 14’s for AFR at this range, even with a fairly radical engine because the combustion process is better than at idle, the advance is likely”all in” and there is very little load. If the engine is too rich, jet it down attempting to get it as lean as possible. This is where it doesn’t matter what your buddy’s engine likes, or some Honda tuner. Go lean until it starts surging or develops a steady miss. Go back to the leanest setting that doesn’t surge or miss, and note the AFR. Keep in mind that a steady miss could also be an indicator of a vacuum leak, ignition problems, or poor distribution. The vacuum gauge would be really jumpy if you had a leak though.



Going to a best lean cruise will likely create some drivability symptoms. This is where most guys say, “it needs bigger mains”. WRONG! Those problems will be cured with other circuits. Now we’re going to go back and flatten the fuel curve from idle through the 2500 main jet selection.



Idle Circuit Refinement:

To get a nice smooth fuel injection quality fuel curve, you’re likely going to have to modify some passages in the carb. Make a chart of idle, 1000rpm, 1200, 1500, 1800, 2000, 2200, 2500. With the engine warm, run the car at those speeds in neutral, just sitting in the driveway. At each of those speeds, carefully hold the throttle steady, and watch the wideband. Throttle movement will cause the readings to jump, so take your time, and let it find a steady number. Don’t just jot down the first steady number you see. Wall wetting from accelerator pump, leanout from slowly throttling down, etc can take a good 20 seconds or more to “clear out”.



There are 2 things we’re really interested in. Remember that good balanced idle AFR we achieved earlier? Is the number at 1200 richer or leaner? Is the number at 1800 richer or leaner than that? If the 1200 number is richer, the IFR (idle feed restrictor) needs to be smaller. If leaner, the opposite. If the value at 1800 is richer than at 1200, the IAB (idle air bleed) needs to be bigger. If leaner, the opposite. This is the area where most in town cruising occurs. This is where you can either get max economy, and clean plugs, or a swilling fuel drunk neanderthal that needs the plugs removed and cleaned every week.



This is also an area that requires a lot of time. Since the air bleed can affect the entire curve, you should change it with the restrictor. It should be a good match from the factory, but if the feed restrictor needs to go down, the bleed will seem to have a much more pronounced effect on the curve. As a general rule, if the restrictor goes smaller, the bleed will likely need to go smaller with it to maintain a flat curve. Change one, then the other, a little at a time until your fuel curve is smooth. Also, keep in mind that most engines will want a slightly richer AFR at, and just off idle. If that is your case, the curve should smoothly lean out from idle by around 1500rpm to what the mains tolerated, then stay there into the main circuit.



Carbs with screw in bleeds have it easy. If yours doesn’t, you will either need to experiment with wire to reduce the diameter, or best bet is to drill and tap the stock ones for 8-32 brass set screws. You’ll use the set screws later on too, so it is worth pulling the carb apart for this, and the ones mentioned later. If you read this whole thing first, you’ll know which ones to do. Same is the case for the IFR’s. If you don’t have screw in ones, drill and tap, but the screws will likely need to be smaller.



Another thing to consider is that air bleed changes in the idle circuit will tend to have the opposite effect on the signal to the mains. This is because by reducing the air going through the idle air bleed, more of it then pulls through the main. There could be some changes needed in the main jet after refining the idle circuit, but should be minimal.



Time for a test drive! Drive the car at varied speed, and terrains. With the idle circuit and mains smooth and lean, it could result in poor throttle response, and falling on its face under moderate part throttle acceleration. Why would we tune it like this then!? We will now begin curing drivability symptoms with the proper circuits!



Get the car up to a steady speed, say 25mph, then smoothly push down to accelerate. Don’t romp on it, but don’t baby pedal it. Think of trying to go up a slight hill, or smoothly pass someone. If the car’s AFR plunges lean, watch the vacuum gauge. Look for the lowest vacuum reading that it runs nice at, then the point that it stumbles badly at. Note those vacuum readings.



Accelerator Pump:

Next, romp on the throttle from a dead stop. Do it both to ½ throttle, and full throttle. It needs to be an immediate stab of the gas, not a roll into the throttle. Watch the AFR, and behavior of the car. If it goes lean, and recovers, but doesn’t stumble during that lean spike, you’re probably okay. More likely, is that it will pop through the intake, or stall, or come close then recover. Occasionally, it will go really rich and bog, then clear out.



If the first situation occurs, enlarge the primary squirter until the ½ throttle stabs are responsive, with no stumble or stutters. If the second situation occurs, go smaller until it smooths out. Do the same on the secondary to cover the full throttle stabs. I have found that with the double pumpers, equal squirters front and rear works best, but you might experiment with a smaller one on the primary for economy, and a bigger one on the secondary for full throttle hits.



Power Valve:

Once you can romp on it from a dead stop, go for another test drive. Most of the drivability issues should be gone now. The next step is to optimize the power valve, and PVCR’s (power valve channel restrictions). If you roll into the throttle, you will see the vacuum gauge drop. You want to watch the AFR for a lean out above light throttle, or a rich bog. If it goes rich too soon, a lower power valve needs to be used. To get close, use the lowest steady cruise vacuum reading you got from the first test drive. Go1-2 numbers lower for the PV. For example, if you have a big cam, and only pull 8” of vacuum at cruise, a #6 power valve should be used.



In most mild street and strip combos, the car will starve out for gas before the power valve opens because of lots of vacuum, and the lean main jets. This is where most folks put in bigger mains to cover it, but the best bet is to put the right power valve in, and “jet” the power valve circuit via the PVCR’s. Like with the low vacuum combo, pick a PV that opens just below the lowest part throttle vacuum reading. In some cases it may require a really early opening valve. The most aggressive one Holley makes is a 10.5.



After changing the PV watch the AFR’s as it comes in. You’ll know when it opens by watching the vacuum gauge. When you are under a part throttle, but not full throttle load, and the vacuum is below your PV opening point, look at the AFR. If it is lean, the PVCR’s need to be bigger. If it goes rich, they need to be smaller. If lean, use a pin vise set, or if you have steady hands, a cordless drill, to enlarge the passages a few thousandths at a time until the AFR under load is around a 12.5-13. For max economy, favor the 13. For max power, it will probably pull harder around 12.5. Experimentation will be required to find what your combo likes best. If it goes too rich, you have to reduce the size of the hole. Some folks have gotten away with JB weld, and redrilling, but the best bet is to drill and tap the holes to accept 8-32 brass set screws. This will allow you to keep a few sets to go back and forth with.



Secondary Jets:

Now for the fun part, full throttle blasts! Take the car somewhere you can run it out for a couple gears, while watching the AFR. Obviously a track is best, but do what you can and be realistic about the fact you will be distracted by gauges and need nobody else driving near you. I suggest running it out moderate throttle in first gear, opening it to full as you hit second, staying in it through redline, and a little bit into third. If the car goes lean, or pops or does ANYTHING weird, don’t stay on the throttle. As with the power valve, tune it for max pull. It is probably going to be around 12.5 that it pulls the hardest, but some are stronger a little lean, some like a fatter mix.



The last bit is to watch for a difference in AFR as it approaches your shift point. If it starts to lean out, the high speed air bleeds need to be a little smaller. If it gets richer, they need to be bigger. Again, carbs with screw in bleeds have this easy. If not, drill and tap for the same 8-32’s you used on the IAB’s and PVCR’s. This can take a lot of time and effort, but having an awesome running car is worth it!



The wideband can be a great tuning tool, but it can also become a sort of car guy illness. You can get real finicky and watch the wideband more than the road. Remember a carb isn’t fuel injection, but can be tuned dang close! At some point you have to decide when the car runs well enough, and just enjoy it. Also note that the aggressive initial timing, and lean mixtures can cause some pinging at light throttle. An adjustable vacuum advance can be used to cure that. If the pinging cannot be cured with timing, you might need to slightly enrich that portion of the fuel curve and compromise some economy.









Adjustable power valve:

Sometimes when tuning for economy, and your engine pulls a lot of vacuum it can create a lean spot. If your main jets are tuned small for cruise, you need something to cover the gap between primary and secondary fuel. That’s the power valve’s job. Holley only makes up to a 10″ vacuum valve as far as I know. I had a car that pulled 22″ of vacuum at light throttle cruise. Off idle acceleration was nice and crisp, but above that when I would go for more throttle it would nose over until the vacuum drop was enough to open the PVCR’s, even with the 10″ valve.

I decided to watch my AFR and vacuum to see when it was dropping lean. The car would surge at around 14″ of vacuum. I took a 10″ power valve and very carefully took it apart. I turned a new center shaft on the lathe, with extra threaded length. Once assembled, I was able to tighten the 2 knurled nuts for an earlier opening valve. After a few runs, turning the adjuster **** a 1/4 turn each drive, I was able to clear up the lean spot, without giving up the economical main jetting. The one in the picture used an aluminum shaft. I later made one with a brass shaft, and the high flow type valve. If there is interest I may look into production for these.

ajrothm

This ^^^..

No reason you can't make a carb run well...

How did these 50-60 year old cars ever travel hundreds of thousands of miles?

A wideband is the key to getting a good driveability tune with the carb.

The AEM Wego is a great wideband, doesnt require calibration and is only about $180....


I use an old school Innovate LM1 with the LMA3 aux box on my 71' so I can log everything and its much easier then having to eye ball a wideband gauge.

Remember though, when tuning, give it what it wants, not what you THINK it wants.... Odds are, the engine is not going to want to run smoothly near stoich like a new fuel injection engine..

On mine, it still likes to be a little rich, so I idle it at 13.5-14.0 afr... Idling this rich still allows a decent cold start with no choke and an acceptable rpm drop once it goes into gear.

You'll have to experiment and see what works for your engine.


Or spend $1k on FiTech and hope for the best.

dmruschell

I had this user and another user "helping" me troubleshoot. I diagnosed the problem, contacted FiTech, and was told there was no way to fix the issue I was having, all while hearing people spout on about how EFI is amazing and I didn't know what I was doing and how I was just giving up. We all have personal preferences and quirks we are and aren't willing to live with. I was more willing to live with carburetor quirks than FiTech's (unfixable by their own admission) quirks.

EFI isn't the godsend some people make it out to be. It has its advantages, but carbs are still perfectly fine, too. Get as much information as you can and make the decision that is right for you.

lionelhutz

You claimed a 0.5% throttle movement made the engine go from an idle to almost running away and going above 3000rpm. That means there was a problem. You really didn't put any effort into trying to find it. You had basically decided to pull the system instead of putting any effort into figuring out what was going on.

But then you also claimed it was partially because the car was a standard so the wheels were always accelerating the engine or some oddball claim similar to that???