Any Edelbrock Carburetor experts? Trying my hand at tuning

[Hazieview]

Ok Carb experts. I have a sb400 with a mild Cam. Currently I have a factory 600 CFM Edelbrock Carb. After reviewing the Edelbrock calibration sheet I'm thinking about increasing my metering rods to the 1458s. According to the calibration chart this should put me at a richer (%8) fuel mixture under heavy loading and still maintain an even base mixture while cruising.

I'm not worried about fuel efficiency during WOT however I would like to have decent fuel economy while cruising. The 1458 metering appear to be the option I'm looking for with out having to swap out jets. I also plan on adding heavy springs that are recommended for the vacuum I'm currently running.

Any thoughts or suggestions? Thanks guys I'm new to tuning carbs.

[Desert1957]

Originally Posted by Hazieview

Ok Carb experts. I have a sb400 with a mild Cam. Currently I have a factory 600 CFM Edelbrock Carb. After reviewing the Edelbrock calibration sheet I'm thinking about increasing my metering rods to the 1458s. According to the calibration chart this should put me at a richer (%8) fuel mixture under heavy loading and still maintain an even base mixture while cruising.

I'm not worried about fuel efficiency during WOT however I would like to have decent fuel economy while cruising. The 1458 metering appear to be the option I'm looking for with out having to swap out jets. I also plan on adding heavy springs that are recommended for the vacuum I'm currently running.

Any thoughts or suggestions? Thanks guys I'm new to tuning carbs.

IMO , a 600 CFM carb is just too small for a 400. The problem I see is even if you can get the fuel part close, it still will not move enough air for a big cube small block to get the air fuel ration correct.

Remember years ago 396 (Big Block) engines had Holley's or Q jets that were rated 750 CFM or more. Your cubic inch is already above that.

Here is a useful link.

http://www.superchevy.com/how-to/eng...-combinations/

[wilkin250r]

I would agree and disagree with Desert. I think the 600cfm is a little small, but I don't think it's a deal-breaker. By the same calculations in the article of CFM=(displacementxRPM)/3456, but solving for RPM instead of CFM, you'll get 5200rpm. I think you're still okay, just not ideal.

Now, I've rebuilt a couple Edelbrock carbs, but I'm not a master tuner. However, I AM a master tuner on motorcycle carbs, and in my experience, smaller carbs are easier to tune. The increased intake velocity means all your venturis get plenty of vacuum, plenty of airflow, everything works exactly like it's supposed to and it's much easier to dial in. Big carbs equal big peak HP numbers, smaller carbs have better mid-range and economy.

[Hazieview]

Excellent replies fellas. Y'all definitely go me thinking for sure so Thank y'all! Lol.

I have heard that the smaller 600 cfm Carb might be easier to tune for the reasons suggested by Wilkin250r. I also agree with Desert1957 in the since that I will probably never make the peak HP that my motor is capable of if I choose to use the smaller 600 CFM carb.

So my sb400 isn't anything special. I never intended to push more than 5800 RPM. I know more power can be made but my pockets aren't that deep and the gains would be minimal. For now I'm just plan on having cruising efficiency and maxing out the 600 cfm Carb at the same time. Lol. My motor isn't broke in yet so maybe I will step it up to a 750 cfm Carb after I get it broke in.

[cadillac_al]

If you have stock heads and a mild cam you won't really see 5800 rpm anyway. I have to kill mine to get 5000 rpm, it stops pulling at 4000. I think you are on the right track to richen up that carb. I would just tune the carb so it runs good and be happy with it. Lean carbs suck. FWIW I ran a Eddy 600 marine carb on a few of my vehicles and always said I would tune future carbs to those specs.

[Hazieview]

Here are my sb400 specs.

Head Specs
76cc
2.02/1.60 170cc int

Cam Specs
280°/290° Advertised Cam Duration
214°/224° Cam Duration @ .050"
.473/.497 Gross Valve Lift

I actually gave a rev limiter on my distributor that is set at 5600 RPM.

[cadillac_al]

OK, that's not as mild as I thought. The 600 might just be too small for max power but should run fine cruising around. The marine carbs are jetted richer than the other carbs which is why I like it so much.

[Hazieview]

You know... I haven't heard any noticeable pinging just yet... Of course I also haven't driven it that much. After talking to you guys I may just friggin Jet this 600 and put new rods in it as well... Judt to see what happens.

Then again I would like to get decent mileage while cruising...

I don't know. The new metering rods are on their way. If I'm not happy I'll buy a 750 Carb. Lol

[gmachinz]

Hopefully you have those springs coming too-too low of a vacuum signal andbthe secondaries want to open thinking you're at part throttle when you may just be idling-not changing those springs will create a stumbling/choppy idle until engine RPM gets up there-which 400s dont like to do quickly (stock crank/rods anyway) and if you fatten up the mixture with smaller metering rods it'll make that small problem a bigger one. At the end of the day, theres only a small number of things you can adjust on an Eddy carb whereas on a Holley....but thats another topic.

[Hazieview]

gmachinz, Currently im not suffering from any of the systems you mentioned but I will keep it in mind. Thank you sir.

I ended swapping the metering rods to richen up the fuel during the 'power' curve and I also upgraded the springs to 5 hg. I'm currently pulling about 10 hg while at full throttle so the 5 hg springs should be right.

I can say that I haven't seen any negative impacts for sure. It seems to respond better at WOT than it did but I would probably have to put it on a dyno for true HP accuracy. The awesome news is that these same metering rods and springs will achieve the same results if I reuse them when I upgrade to a 750 cfm Carburetor.

Still no pinging or noticeable power losses so maybe I'm on the right track in regards to my Carburetor tuning adventure.

[geezer#99]

Pulling 10 hg at wot, eh!
Is that a typo?
You did mean zero didn't you!

[gmachinz]

I thought the same thing but was like well-ok. Maybe he means cracking the throttle open with no loading.

[Hazieview]

My vacuum gauge is under the hood. I honestly don't know what my Vacuum Is when I'm driving. I just know when I rev the throttle under the hood the vacuum goes below 10hg. Never have held it until it goes to 0hg. Maybe I'm doing something wrong. I'm glad y'all asked!

My vacuum gauge is hooked in line with the hose on the back of the Carb going to the brake booster. It also may not be 100% accurate.

I can crank it up and tell you where my vacuum is at operating temp if that helps.

[gmachinz]

Well on that split cam 290 duration on the exhaust side combined with that relatively big lift (for a street flat tappet cam) is not going to provide a lot of vacuum @ idle. The 5hg rating simply means once manifold vacuum reaches near that 5hg, the secondary enrichment circuit begins to dump fuel even if it isnt needed by the engine. If you have power brakes, TH350 kickdown line, climate control vacuum feed line-basically anything requiring vacuum is going to use a little bit of it so you really want to maintain a good manifold vacuum-say 16-18hg @ idle is pretty good for an old school SB with a decent cam.

So use a vacuum gauge and put it on a good manifold vacuum source with all your vacuum lines connected and @ idle what do you get? Cracking the throttle does drop the vacuum signal a little but it will drop more under a load (acceleration). So if you have 10hg @ idle before cracking the throttle Im willing to bet just off idle theres a pretty good shot of fuel getting dumped which can cause a momentary bog before "coming alive" so to speak.

A stiffer secondary spring rate serves to delay this secondary enrichment fuel dump so it should help a little by letting the engine speed get up there a little bit off idle so the bog doesnt happen as much or at all. I would like to point out at this time you might want to upgrade your ignition a bit to help light off the extra fuel you're using too-most people think more fuel is better when imo a stronger ignition hit is what most engines can use. If running an HEI think about a set of 40-50 ohm performance wires such as Taylor 409, Magnecor, Moroso Racing, ect. Then Id suggest a matching module/coil from somebody like DUI or Performance distributors. This gives you increased dwell time (which should help with the slightly long duration specs on your cam) and a 60K ignition coil should really light it all off after that!

[Hazieview]

WOW gmmachinz!!! That was an excellent write up and an even better explanation. It actually makes a lot of since. Thank you sir I appreciate you going the extra mile to help me out!

So I narrowed it down to this.
After my engine is up to temp, I have a steady 9 hg of vacuum during my 'slow idle'. I went ahead and Full throttled it a few good times and it definitely went to 0 hg.

The accelerator pump in my Carb was definitely bad. Immediate difference after I changed it out. Only a Small amount of bog at full throttle. I then added new 'richer power mode' metering rods, and orange 5 hg springs. The truck runs like a champ! It now has Zero bog during full throttle! I do have a DUI distributor, new spark plugs, and new Taylor wires. I'm not sure about the specs on the distributor but I can look it up.

Im pretty happy with it now but I'm wondering if I should rejet my Carburetor next. I don't really see the need because it's running so good right now but if it will run better I would like to try everything I can to make it happen! I still haven't adjusted the two springs on the front of the Carburetor. I've read about adjusting them to richen up the Carburetor but I'm not exactly sure what they do to be honest.

Thanks for all the help guys! Learning all this stuff is pretty cool!

[gmachinz]

Those 2 screws are your idle speed mixture screws. Typical adjustment is to seat them in fullybthen back each out 3 full turns-with your vacuum gauge hooked up to manifold vacuum, take each one in a half turn at a time until you see a slight drop in vacuum then turn it back out a half turn-do this for both screws. Besides than, you should check your plugs for a nice tan color on the porcelain-this is an ideal fuel mixture-too black/wet/sooty means mixture is too rich, clean white indicates too lean.

Also, on the pump arm there are 3 holes for the linkage-they come out of the box in the middle hole-the hole furthest away gives you a longer duration fuel squirt while the innermost hole gives you the shortest fuel squirt. The middle hole is more of a universal "economy" setting. Some guys run these on dual quad intakes and thats about the only time I could see running a short fuel squirt setting.

[Hazieview]

Awesome! Thanks again for the info! I'll tinker with the idle screws and see what happens. Then I'll pull a couple spark plugs. If they are bad I'll clean them and make some adjustments. I may try to move the accelerator linkage up to the top hole just to see what it does.

After I reinstall the plugs I'll pull them again. If I'm still too lean I'll either jet the Carb I have or get a 750. Thanks again for everyone's help!!!

[geezer#99]

The mixture screws won't show you anything on the plugs. They're for the idle circuit. They don't have anything to do with the jets(power circuit).
Reading plugs is best done after a full throttle blast with power cut then plugs pulled for a look.

[Hazieview]

Originally Posted by geezer#99

The mixture screws won't show you anything on the plugs. They're for the idle circuit. They don't have anything to do with the jets(power circuit).
Reading plugs is best done after a full throttle blast with power cut then plugs pulled for a look.

Gotcha! I'm wondering if I should jet the 600 cfm Carb I have or just upgrade to the 750...? Still unsure.

[geezer#99]

With your lack of vacuum at idle from your cam I'd leave the carb alone and maximize the timing first.
What's your timing like?
Initial?
Mechanical?

FWIW your carb isn't too small. 1968 Chrysler 440 had a 465 cfm holley which worked perfect.

[Hazieview]

Sorry it took so long to reply back. Busy at work.

I'll check my timing tomorrw but I believe it idles around 10 and maxes out at 35 but I could be wrong. I'll have to throw a timing light on it tomorrow. Are you thinking I need to advance my timing?

[geezer#99]

Likely!!
I run 14 on stock motors at sea level.

[Dead Parrot]

Don't worry about hitting exact timing numbers. Unless you know that your timing mark, indicator tab and true TDC are perfectly synced up, trying to match what someone else has will be an exercise in frustration.

Check where you are at now, then try advancing it a couple of degrees at a time. If you reach a point where it is hard to start when hot, back off until it starts easy and call your initial timing good. If it starts pinging on acceleration before it gets hard to start, back off a couple and consider playing with the timing curves both mechanical and vacuum.

When your timing is good, then go back and play with your carb some more.

Don't be so eager to spend money on a larger carb. The only time it will help is when you are at WOT and near max RPM. And you get to start over with jets, rods and adjustments as it won't be correct out of the box.

[Hazieview]

Thanks for all the info guys.

Here is what's weird... I'm running on 87 octane and I hear zero pinging. Now, as you can see I'm no motor builder. I had a few gear heads help me out a few years ago and I'm just getting my truck put together and on the road. They estimated I would be running a pretty high compression. At least 9:1. Yet, I'm still running 87 octane fuel with no pinging so I guess I'm setup ok.