carbs and fouled plugs on the street big solid roller cam...

[nastyrs]

how come a carb cant be manipulated to act like fuel injection on the street .. buying sparkplugs every other week sucks.. the street carbs beit a 4150 or 4500 with powervalves only really run lean at idle.. seems the low speed cruising is where the rich a/f ratio is horrible.. .. i want to have sparkplugs last... this is 2007 where is the technology for such matters .. comon !!!!

[Unkl Ian]

The "technology" is in running a carb the correct size
for the motor AND application.

If the carb is too big,you don't have enough velocity
through the venturi for proper atomization.

A single plane intake manifold won't help wither.

[PackardV8]

greetings, nastrys,

with enough knowlege, experience and persistance, a carburetor can be made to function essentially perfectly on a given engine combination at a given elevation, temperature and humidity.

if you want an engine to run perfectly everywhere, all the time, then you are correct in that EFI is preferrable. know, however, that to get an aftermarket EFI to function to its ultimate capability, it takes as much knowlege, experience and persistance as does a carburetor in its more limited application. there ain't no easy horses(power)

[Troy Patterson]

With today's plethora of highly modified carburetors, it is no longer accurate to stereo type all (performance) carburetors as though they were equal.

It is worth differentiating between "box stock" and custom built or modified carburetors. Granted, we're usually talking about a significant difference in cost nevertheless, the difference exists.

The mistake is thinking or believing a mass produced carburetor is calibrated / tuned exactly right for your application, whatever it may be. Even if you believe it runs pretty good, even better than some previous carburetor, there is room for improvement.

As PackardV8 pointed out, there is no free horses(power) be it carburetor or fuel injection.

Find a good tuner and let him fix it, or change the cylinder heads.

Troy Patterson TMPCarbs.net TMP Carbs

[bigjoe1]

Make the idle speed high (1500 or so) keep it up to at least 2200 when you first start it up, and run the HOTTEST spark plugs you can find. If the carb is right , these little things should really help out.Never let it idle slow when it is cold. Plug fouling is almost always from improper startup proceedures.
JOE SHERMAN RACING

[Tuner]

Greetings nastryrs, gentlemen,

According to “modern technology” most of the dual carb engines of the 50’s and 60’s are over carbureted, yet they ran (and still run) great, mileage and everything. By today’s “technology” 409’s, 427 Ford’s and Street Hemi’s should have a single four barrel? Sure, those engine’s cams weren’t as radical as yours probably is, but I have a pal with two 1150’s, a 272 @. 050” x .700 lift roller BBC that gets 11 MPG if he drives it right and the plugs stay reasonably clean.

The solution for you is available but it requires patience because each engine has it’s own needs. The key is in tuning the low speed circuit, specifically the transition slot. In the production carbs of the same size, 600 CFM Holleys for example, for a 430 Lincoln (1850) and a 365 HP 327 Chevy (2818), the MJ and PVCR are the same but they’re miles apart in the idle circuit. The Chevy carb would foul the Lincoln’s plugs but it would get good mileage on the Chevy.

When hundreds of identical engines were produced a single calibration could serve them all. Now, there are more combinations of cam, manifold, header and ???? for almost any engine then there were Street Hemi’s manufactured. Just changing header tube size can make a good running engine completely stupid. If it’s never happened to you, you’re fairly new at this. For this reason, there is no such thing as a “universal carburetor” any more than there is a universal fuel map that will make all EFI engines run like they should. There is a magic jet size for every situation you just have to look through the haystack until you find it, right next to the needle. Current pump gas doesn’t help.

For inspiration, look at this thread. http://www.theturboforums.com/smf/index ... ic=82049.0
The application is a blow-through carb but the problem and the solution in this case are similar to yours. Follow the links to the Innovate website for numerous examples of guys with exactly the problem you have and detailed dialog with calibrations that were successful for them. If you load Innovate’s software in your computer you can read the data logs and follow the progress of the tuning. Don’t be shy to use the search function, there’s a lot in there. There might be someone with a combination close enough to yours it will work for you.

[Doug Schriefer]

nastyrs,

As others have already stated carburetors can be made to due just about what ever you want them to do, but you have to tune them to do this. As Tuner has stated you can work with the different circuits of the carburetor (whichever brand you’re using) to get the results you’re looking for. If you’ll let us know what you’re working on, we may be able to point you in the right direction.

[automotive breath]

how come a carb cant be manipulated to act like fuel injection on the
street .. buying sparkplugs every other week sucks.. the street carbs
beit a 4150 or 4500 with powervalves only really run lean at idle.. seems
the low speed cruising is where the rich a/f ratio is horrible.. .. i want to
have sparkplugs last... this is 2007 where is the technology for such
matters .. comon !!!!

Hi Nastyrs,

First it would be great if you posted more information about the
combination so that people would have a better understanding of
what you are working with. The title of this thread stated a “big
solid roller cam" ; please describe big.

From the information provided it sounds like you have a specific
problem. The engine runs clean at idle yet it’s rich at cruse. Others
described the individual circuits that can be tuned, first you need
to understand exactly what the problem is. Are the power valves
open at cruse?

Years ago the carburetor was blamed when the ignition delivered a
weak spark. Poor ignition and intermittent misfires caused havoc
during overlap disrupting signal to the carburetor. With capacitive
discharge ignition most of these ignition related carburetor problems
went away. Check to make sure the ignition system is working
correctly. What ignition system are you running? How much ignition
advance are you running at cruse?

Another factor is combustion chamber design and flame speed at the
RPM of the problem. Even if the ignition is up to par, the expanding
gases from slow burn can disrupt carburetor signal. What cylinder
head are you running? Pull the carb. What color are the runners in
the intake? Are they black?

The people that don’t understand will smirk when they read this: It is
possible to improve burn speed at idle and low RPM with a “big solid
roller” to a point where the engine will idle below 800 RPM cold with
out fouling the plugs. Once you confirm all problems are resolved, the
key is to improve burn speed to reduce pressure pulses in the intake
track during overlap. Speed up the burn rate and spark plugs will last
so long you’ll replace them because you are tired of looking at them.

[Ed-vancedEngines]

Maybe it is not the 2007 technology that you wish or need.

Way back, in my own street cars I have always had huge cams, healthy carburetors and bunches of compression, and plug fouling from fuel richness was the least of my concerns. I just never had those problems. I have had plug fouling issues one time when trying to tune and get to idle a pair of the Predator carbs on a race engine and ust finally gave up and put my Holley's back on.

I also did experience plug fouling issues before ever using my fisrt MSD box back in 1977. Since then just about everything that was modified I tried street driving has had MSD boxes on them.

I have daily driven engine with as much as 266 I and 272 E @ .050 with .660 and .620 lift in a solid flat tappet cam in a 468 BB Chevy with 12.00 -1 compression using 4.56 -1 gears with a 4 speed in a 4,200 lb car using the old 780 cfm Holley 3310 way back in the late 1970's and the only plug fouling issues I encountered was when using even colder plugs and different carb setting at the race track from idling too rich. On that car my normal street plugs were usually one heat range below factory. I agree that was a ragedly on the edge for strettability but it was my only car and both me and y girlfriend did drive it.

Since then I have donbe a lot of wierd stuff too, but plug fouling from a carburator is jsut not usually an issue, noit even with customer's cars I set up that are street driven. I do have a problem from time to time with customer high performance street cars with plug fouling but they have already been running for months.

IN my opinion you will seldom get a carb that is tuned just right for an engine right out of the box. Any Holley or BG/Demon type carb I get needs to be set up and tweeked for the application. I do prefer those carbs becuase they have lots of tweeking possibilities built in. I also am a firm user of power valves and hate them to be blocked off in anything but the most serious of race cars. Believe this or not, Since 1968 neither me or any customer have ever knocked out a power valve. I do not understand why people had so much problems with them. I know it happened though becuase new customers have come with busted power valves.

Usually a rich condition will be more experienced becuase of huge cams when idling not when cruising. This is even with EFI with big cams.

If you are not up on carb tweeling of Holley type carbs, maybe you will like the Edlebrock (AFB) style of carbs better. There is not much you can tweek or tune on them. You can change the jetting, acellerator pump settings, metering rod sizes and metring rod springs. That is all you need to adjust or change with those.

Ed

[nastyrs]

400ci... ,4340 3.75 crank,billet oliver rods,probe flat top 10.8:1 comp,all arp, ,brodix 11x full cnc port , flowin 330cfm with 2.10 int valve 1.60 ex... .. th350 3500-4000 stahl,3.73 gear for now.with a 275/60/15 mt drag radoa; ,o its in a street driven 3300lb 69 camaro as well.. 2 kits one is 100 shot the 2nd 250 shot both on plates...282/396@ 50 - 697/663 lift
msd 7 igntion hvc coil..
now the carburetor .. i have about 5 carbs to use i have laying around... one is a prosystems 1000 which patrick built for a 383 , he said it would work fine on the new setup ..
alsi i have a BG 750 suppost to flow 925 ,, but i dunno bout that .. its on the car now .. does ok ...
also i have kicking around is a demon speed demon 850
a aed 750
and an old holley 850 stock double pumper with choke horn and all...

i tried a 3 circuit 1050 dominator with adapter plate and the car felt so much better mid and low range.. never really got good run on a full pass to see about up top ....

[bill jones]

-The way I see it---with the loose converter stall you are well into the main circuit so the primary barrels are slobbering fuel---where in a closer to stock situation you could cruise soley on the transfer slot and the corner idle discharge holes.

-If the application was mine----first thing I'd do is fix up a two stage fuel pressure system so you can street drive with about 3psi---then drop the float level so the engine has a harder time pulling fuel from the boosters while cruising.

-I would also use a two stage Holley power valve---I think it's a #125-210---where the first stage (which is restricted) opens at 9.5" and the second stage opens 2.5".

-These two stage power valves are NOT usually recommended with serious power making making engines because of the internal restriction)----but I have used'm for years on predominately street driven engines and some pretty serious engines.
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-One other thing that I think is a BIG deal is having a distributor or ignition system with a vacuum advance.

-The vacuum advance can make a stinky gas guzzeling engine into a treat to drive----tho it takes some experimentation.

-Typically I'd set up the vacuum advance to have about 15degrees advance at all points above about 6" of vacuum---and have all the vacuum advance gone by about 3".

-Another thing that might help would be to drill & tap & restrict the main body entry (to an 8-32 or maybe a 10-32 setscrew) of the transfer slot and then drill out the setsrews to restrict the total square inch area of the slot.

-The slots are about .040" wide by say .250" tall so that is .0100 square inch area which is about the same as a .036" diameter hole if my math is right.

-Another thing to lean out the transfer slots is---you can go directly above the slots----in the main body----and drill a small hole sized about .025" or so to act as an air bleed.

-This hole HAS to be just barely above the forward front edge of the primary throttle blades-----at the throttle position you are wanting to cruise at.
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-No mention of altitude---but any people who live and have issues at 4000 or higher this information I know for sure works because I've done for years.
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-Personally I would get an LM1 or some other data gathering source---get educated about how to use it----figure out a throttle position sensor, a map sensor---figure out how get a tach signal to the LM1---and run the oxygen sensor.

-A deal like that is a neat education if you REALLY want to know how to fix the problems you have created.

-The problems that you have created with the huge cam vs the low compression ratio----and the loose stall-----are far from what a carburetor was ever designed to do---so you have to learn and understand it all.
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-I seriously doubt that it would be EASY to get any EFI to behave decent with your combination.

[PackardV8]

Having the specs helps define the combination. The dynamic compression is pretty low. As has been mentioned, you can run a vacuum advance to help fill the hole at cruise.

I really like a crank-triggered programmable ignition on an application such as this one. Some think the Electromotive is a bit pricey, but add up a MSD billet distributor, 7A box and coil and you are spending more than the Electromotive and the MSD is not nearly as flexible as the Electromotive.

thnx, jv.

[MaxFlow]

I agree totally with Bill and Packard V8. A combination like you have is, well unfriendly to say the least. Why so much cam in a car with those heads and that compression? I'm lost here. Was it something a friend gave you or surely you didn't get that speced by any cam company? You aren't spraying this thing going down Courthouse Blvd or anything are we? Sounds very much like a cam change and something ignition to take some timing away when on the spray..........

[nastyrs]

race cams inc.. designed the cam for pump gas 400 ci .. lots of spray...
i am a sole street racer sometimes i have to drive farr and no way i can affor to do it with c 16 .. and these guys with the ls1 and ls2 motors do it with pump gas so i have to do it to or hear them cry .. and also cuz i want to do this on pump gas.....but i do have a fuel cell up front with c 16 for the nitrous.. i already run a msd 7 ,hvc coil ,with a locked out distributer .. i have been thinkin about a crank trigger sould i get one ?
i also live in florida... and also use an aem air fuel set up .. i havent quite figured out how to get it to log with rpm ... just logs what is showed on the gauge when hooked up to log program..

[Tuner]

Cam sounds OK to me. Torque converter might be a little tight, but with enough nitrous it’s probably about right. It seems easier to me to tune a carburetor to a combination than throw it all away and build something that suits a carburetor. This is why the calibration orifices are threaded. With that much cam it needs two carbs. Better distribution with the throttles closer to the intake ports makes an engine with that much valve timing easier to tune.