Carb with better street manners for my BBC 540

[atl69camaro]

I’m looking for a carb with better street manners for my BBC 540. It’s currently running a BIGS 950 Stage V, which has no choke. I understand that these are good carbs for race or semi-race applications, but again, I’m looking for more of a street application for my 69 Camaro convertible.

Engine Specs:
540 ci with single plane intake
Fairly aggressive cam (not sure of specs)
5 speed
Dyno sheet from previous owner shows flywheel hp of 630 at 5500 rpms and 668 ftlbs at 4400 rpms.

Of course, I would prefer not to lose power but would be willing sacrifice a bit for better drivability and a slightly better mileage. And, yes, I know 540ci and mileage are not usually used in the same sentence but double digit mpgs would be nice for this weekend driver.

Thanks in advanced for your help.

[kryptik]

Carburetion is always a compromise when it comes to flow numbers. 950CFM allows for a little extra breathing room on the top end...but it is going to detract from your low end torque, throttle response, and fuel mileage (so much so that the few extra ponies on top may not be worth it to you).

I would recommend an 830-880CFM carburetor with vacuum secondaries.

P.S. If you are not already using a distributor vacuum advance you may want to look into that.

[Shaker455]

I’m looking for a carb with better street manners for my BBC 540. It’s currently running a BIGS 950 Stage V, which has no choke. I understand that these are good carbs for race or semi-race applications, but again, I’m looking for more of a street application for my 69 Camaro convertible.

Engine Specs:
540 ci with single plane intake
Fairly aggressive cam (not sure of specs)
5 speed
Dyno sheet from previous owner shows flywheel hp of 630 at 5500 rpms and 668 ftlbs at 4400 rpms.

Of course, I would prefer not to lose power but would be willing sacrifice a bit for better drivability and a slightly better mileage. And, yes, I know 540ci and mileage are not usually used in the same sentence but double digit mpgs would be nice for this weekend driver.

Thanks in advanced for your help.

I think your going to hurt they're feelings now that they added to you on to there web site as a happy customer!

Just kidding, the 950 has the same venturi size as a 750 (1-3/8" X 1-11/16") but the bottom is the size of the 850 of 1-3/4" as well as the base.

Sorry, carb Co's do play a numbers game with different flow rates. 20.4"Hg VS 28"Hg.

Your effective flow is really about 820 CFM depending on booster used.

Best bet is to tune it better or go with another mainbody to suite your needs better.

[atl69camaro]

I appreicate your input and would value any specific carb models that may work well.

One of my main goals is to simply have a choke.

[Shaker455]

99% of the models I build and tune do not include a choke and the reason is tuning.

If the carb is tuned properly the choke will not be nessary.

In a cold start you should be able to give it "ONE" pump then fire it off.

You may have to repeat but that is okay.

Feather it for about a min the let the temp and RPM's gain on there own.

Is this not happening for you?

[atl69camaro]

Shaker

No, this is not happening for me. BTW, I see from other threads that you build custom carbs yourself and have a number of satisfied customers on this site.

I got in touch with Jesse at BIGS and he seems to think that the carb is not tuned right and that I'd actually be better with one of his street carbs with vacuum secondaries. He also mentioned that I should not even need a choke if it's tuned right, just as you mentioned above.

He seemed like a nice enough guy. What kind of reputation does BIGS have when it comes to building a value/quality product?

[Shaker455]

Yours was the first I heard of BIGS carbs and I did a google search to check out there site as well as some of customers post on various sites.

If I were you I would tackle what you don't like about this carb before moving on as you may engcounter the same thing on the next.

Are you driving your car now?

Do you have all your specs to this carb? jetting, airbleeds etc?

Also need complete engine specs.

Tell me in detail what problems you are encountering?

[atl69camaro]

Shaker,

The car is drivable but not being driven much due to the cold weather. I have not driven the car that much since being built but my main problems are starting related (very cold natured and takes about 5 minutes or more before it will idle), stumbles a bit when taking off, and what seems to be very poor mileage.

The carb specs are:
BIGS 950 Stage V
Primary Jet - 82
Secondary Jet - 87
Primary Power Valve - 4.5
Secondary Power Valve - none
Primary Shooter - .033
Secondary Shooter - .035
Idle Feeds - .038
Bleeds - .071 Idle and .032 main

My engine specs as I know are:
BBC 540 (Dart Block and Heads)
Compression - ???
Manifold - Victor Jr
Cam - Comp Cams #249-11-444-8
Xtreme Energy Camshafts
Retro-Fit Hydraulic Roller Tappet
Lift .560"/.580"
Duration 300/306
RPM Range 3200-6200
Drivetrain: 5-speed with a 3.42 rear gear

According to the dyno sheet I got with the car, it made top hp of 630 at 5500 rpms and torque of 667 at 4400 rpms.

Just let me know if you need more info.

[Shaker455]

Okay you have the XR300HR

I need to know how many turns out is all four mixture screws?

What color plastic cams are under your pump arms?

What do you have for a dist and where the initinal and total set?

[opnwide]

You're just over the edge of using the new EZ EFI by FAST. Those systems are rated for 600hp, but you might be OK with your app if you bumped the fuel pres up a tad. That would solve almost all your problems. I'm waiting for a a 2bar version to come out so I can put one on my TA (but would need bigger inj).

[atl69camaro]

Shaker,

The front two screws are two turns out and the back two screws are one turn out. I was not familiar with the plastic cams under the pump arms but think I found them. I would describe them as a translucent or faded pink color.

Greg,

I actually purchased and later returned two different Fast EZ EFI systems. Although I REALLY wanted it to work, we never could get it to run right. Between my mechanic and myself, we had around 50 hours in trying to tune alone (between the two units), and this is after probably a dozen calls to their tech support. It was incredibly frustrating. It would appear that we had tuned correctly, and then out of the blue, it would start running terrible. Then at times it would not even crank. Again, I really wanted it to work but just too many gremlins and I ran out of patience. FAST claims that their system will work for my hp if you raise the fuel pressure to around 50 psi.

[Shaker455]

Did you read the rest of my post?

It's clear the engine is not properly tuned

Again what dist you running and where's it set??????

If you do not know how to tune the engine then I suggest you take it back to the dyno shop or where you had the motor built and have someone set it up properly.

You need to post:

Type Dist

Initinal Timming

Total timming

Part of the reason your carb is not right is the carb needs to come off and inverted so the throttle blades can be set evenly.

All four mix screws have to be out equal amount of turns in all four corners.

1 turn out is best place to start.

Post some pics of the bottom of your carb so we can see how far off the front is from the back.

If your running an MSD Dist then you should have 18-20 degrees Initinal and use the Black 18 degree bushing and the 2- light blue spring to start.

[atl69camaro]

Shaker,

Yes, I missed your last question regarding timing. I bought the car with current engine already in place so I don't have access to the person who built the engine. However, I set an appointment to have it dyno tuned with someone who has build engines for me before. Hopefully they can get me straightened out.

I appreciate the time you've spent trying to help me.

[Shaker455]

A chasis dyno is not the place to resolve drivability issues but rather to get it's best HP & TQ.

Good luck and post your results

[Zanie]

Not to muddy the water, that cam has a good amount of duration, but is pretty small in the lift department for a 540.

[Shaker455]

Not to muddy the water, that cam has a good amount of duration, but is pretty small in the lift department for a 540.

Your right it does have a good amount of duration and in a 454 it would be radical.

But when you increase cubic inches to 540 the cam becomes more mild due to the large displacement.

[atl69camaro]

I was at Lamar Walden Automotive for most of the day today and ended up replacing my carb with a Holley 800 cfm double pumper with an electric choke. Evidently, it was just too much carb for my build. He also re-curved my distributer, replaced the plugs and made a few other fine tune adjustments. The car drives quite a bit better, at least on my 20 mile trip home. It now has much better throttle response and we actually picked up around 20 hp on the dyno.

[68sixspeed]

What is you idle vacuum like? Seems like a 4.5pv may be openning a bit late for street use, if I go below a 5.5 the manners on mine goes down hill as it just opens too late. (and I run a lot more cam on less cubes).

I wouldn't be afraid to try a smaller carb though, for the street you may find a 750 a lot more responsive. I swap back and forth between a 750 and 950 myself. -Dan

[Shaker455]

I was at Lamar Walden Automotive for most of the day today and ended up replacing my carb with a Holley 800 cfm double pumper with an electric choke. Evidently, it was just too much carb for my build. He also re-curved my distributer, replaced the plugs and made a few other fine tune adjustments. The car drives quite a bit better, at least on my 20 mile trip home. It now has much better throttle response and we actually picked up around 20 hp on the dyno.

Glad you had good results and the 800 actually has a bit larger secondary (1-7/16"X 1-11/16") than your old 950 as I posted eariler.

[Windycity1]

I have a Pro Systems 1000HP on my 540 which is in my primarily street driven 65 Impala SS and I love it.
Ordered another one today for my 468 in my 69 Camaro. Give Patrick a call you won't be sorry!

[atl69camaro]

I forgot to mention that they also brought to my attention that my fuel pump (Magnaflow mp4550) is probably not the best for street driving and continuous use. Does anyone have suggestions on a good replacement given my current engine and carb setup?