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

[Doug Schriefer]

If it were mine, there first thing I do would be to concentrate on getting your AF system to monitor and record properly. We use a lot of the Innovate™ stuff, and it is extremely easy to use. With their DL-32 kit, and some add ons you can not only monitor your AFR, but also throttle position, engine vacuum, RPM and other vitals. Getting this type of information is going to help solving your problems much simpler. You can actually see when your engine is running rich, determine how much throttle you’re using to see where you are in the circuitry, how much vacuum (checking for your PV sizing) and do repeatable testing at that point. You have the technology to do this, it’s just a matter of taking the time to do it.

[automotive breath]

I like the idea of using instrumentation to monitor the A/F mixture but I’m
thinking the some of the problems need to be resolved before going that
route. With the current conditions in the header, sensors would likely foul
rather quickly.

I also like the idea of vacuum advance to get the ignition timing up at
cruse to help build heat in the chamber.

The first thing I would do is to get a good vacuum gauge and record
vacuum at idle and cruse RPM. Then make sure the correct power valve
is in place with the available vacuum. Sometimes you will find the power
valve is on the edge and it’s opening and closing at idle or cruse. If that
the case get one that will fully close.

Leaning the idle circuits and the fuel transfer slots as mentioned before
are critical. In addition make sure you don’t foul the plugs at cold start
with a over rich mixture and excessive idling. As soon as the plugs begin
to foul it’s a downward spiral from there. A quick blast of WOT can build
some heat in the chambers to help clean soot off the plugs.

Once the tune up is close, the focus is eliminating fuel separation in the
chamber at cruse. With that much overlap combined with low cylinder
pressure and very low port velocity, fuel separation is a major concern.
Your top ring is pushing too muck liquid fuel. In addition the amount of
exhaust gas in the fresh charge is huge. If you were to measure exhaust
emissions you would find very high HC. I have fixed numerous engines
with the exact same problems. One groove in the squish area works
wonders to purge the excess liquid.

Basically what you are doing is driving a car at low RPM with a high RPM
race engine that has been detuned to run on pump gas. Drivability in
these conditions can be a real PITA but it doesn’t have to be that way.
Tuning with these conditions is a very difficult challenge but it can be
done. Numerous changes will be required to get where you need to be.
Focus on one thing at a time looking for many small improvements.

[GREG K]

Bill Jones and automotive breath have covered the most important things...
you need more compression in that engine,why is it so low? how is that going to aid the compression energy? you need to build heat,and more heat...Make sure that you use a fuel that has a lower distillation value...when you are running around at those lower rpm's,cylinder efficiency is at its lowest,and its where you have exhaust gasses diluting the mixture...the worse that is,the more you have to be on the money with,not only the ignition energy and when you introduce it,but also with the A/F ratio...as the dilited mix will slow the burn rate down even more that it is already...so concentrate on using a fuel that vaporizes easier,build some more compression into the engine,and start with a 750 size holley with a double step down leg booster,and make sure that the mains dont start to flow any earlier than 2000 rpm...the double step greatly aids in the fuel coming out of the booster with out blobbing...set up your idle and transition slot correctly(or at least as correctly as you can with a production holley),and check your A/F ratios at different rpms,and make sure that you cross referance with the manifold vacuum...if your engine is not producing good cylinder heat,then you will need to run richer at the lower rpm,so fix this problem...remember,vaporization is the key,if its a battle to get your comb to do this,then the battle will be a tough one to get the lower rpm running conditions correct...

[automotive breath]

This picture from moparts gives a good visualization of the amount of
energy required to vaporize the fuel in the cylinder. Pay particular
attention to the amount of liquid on the spark plug and on the cylinder
wall especially below the bottom squish pad.

[Troy Patterson]

Nastrys, how much horsepower you making? You are running more cam than most, if not all "Cup" engines making upwards of 800 + hp.

That thing must be a real dog ALLLLLLLLLLLLL through the mid range! At what rpm does it "come on the cam?" what are you spinning that motor to? and what's the lca on the cam?

I realize you are running nitrous, but it seems to me with all that squeeze, all you'd be doing is getting back from the engine what you lost to overlap in the mid-range.

Troy Patterson TMPCarbs.net TMP Carbs

[Cammer]

A crank trigger ignition may not be best idea for the street.

Water, sticks, rocks, oil, sand, mud, and the like can cause problems for a crank trigger.

Thank you Bill Jones for stepping up on the vacuum advance idea!

I often spec vacuum advance distributors and vacuum secondary carbs for certain street driven cars. These are just added tuning tools!

Engine builders must plan before choosing parts for an engine. It is all about the correct combination. This planning extends to the total vehicle setup.

When driveability issues are indicated the total package needs to be examined.

____________________

[Tuner]

Greetings nastyrs, Gentlemen,

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 !!!!

This was nastyrs’ question that started this thread. The technology is in the AFR data and interpretation, just like tuning EFI.

I don’t mean to imply the things you guys are bringing to this thread don’t have validity, because they do, but the only positive post in this thread that doesn’t call for tearing it down and starting over is Doug Schriefer’s suggestion to get a WBO2 data system like Innovate sells and tune it. To point out the obvious, built-in data gathering is a huge advantage EFI systems have over carbs. Does anybody even consider using EFI without WBO2? I’m not familiar with the AEM system, will it record TPS and MAP too, or just AFR and RPM?

Why do you guys want to tell the gentleman he’s doing it all wrong? Are you worried you’re going to have to race against him? His engine is what it is so tune it. With 10.8/1 C/R it will swallow a ton of nitrous and not be so fussy about needing some $15 a gallon rocket fuel. The valve timing is similar to those listed for superstock and supercomp drag racing in any major cam grinder’s catalog with some extra exhaust timing because it’s for nitrous use. It’s a maximum effort drag racing cam. Cup or any other oval track engines need a wider power band so the cams have less timing. That kind of racing is won getting off the corner, not with maximum power. Does anyone have a positive suggestion that doesn’t require replacing everything from the hood ornament back?

[bill jones]

-I don't see where the guy asked about fixing the problem himself----he just basically asked why a carburetor can't be made to act like an EFI.

-He didn't ask anything about fixing anything else or changing anything else on his engine

-I take that to mean that he's interested in a magical CARBURETOR only----that is capable of doing everything perfect with no other effort necessary----to make the engine at the least capable of not fouling plugs.

-And it sounds to me like he'd probably like to actually race with that same magical carburetor and the same plugs that he had in while driving on the street.

-comon

[automotive breath]

Some call it dual purpose. Drag races are designed around this concept,
drive it so many miles and race it on the same tune up. Actually what
nastyrs has built is very common, I see it often. An engine designed for
one purpose is relatively easy to build and tune, a dual purpose engine
can be more than twice as difficult.

Some have suggested changing the entire combination; others have
suggested small changes that can make what he has work better. I’m
sure it has the potential to be a strong runner with the NOS valves open
but will it idle and cruse on the cold plugs he needs for WOT? I know it can!

[Tuner]

Hi bill jones. I should have said “some of you guys”. Your post on the first page has good suggestions and you mentioned the data gathering as a solution too.

[nastyrs]

-I don't see where the guy asked about fixing the problem himself----he just basically asked why a carburetor can't be made to act like an EFI.

-He didn't ask anything about fixing anything else or changing anything else on his engine

-I take that to mean that he's interested in a magical CARBURETOR only----that is capable of doing everything perfect with no other effort necessary----to make the engine at the least capable of not fouling plugs.

-And it sounds to me like he'd probably like to actually race with that same magical carburetor and the same plugs that he had in while driving on the street.

-comon

yes i want to be able to run like an efi car on the street . is that asking too much in this day and age ?

[GREG K]

I dont think its about changing the entire engine...its more about getting a better understanding of some good fundamentals on what it take to get more of the fuel that you put in the tank,to burn over a wider rpm band,starting at the lower rpm points...
i understand that this is an engine,that will use some nitrous from time to time...all the more reason to concentrate on how well the cylinder is burning...The fact that nitrous is so cold that it nearly freezes the fuel,so you want to pay particular attention to making sure that you get some extra heat into the fuel(compression energy),and there are many ways that you can go about this...getting the carb to perform well at all rpm points is very acheivable,but is not the only way to go about it...it certainly is a very big help,but an engine doesnt start with the carb,at least i dont think it does...what a good carb will do,is show up some of the faults the the engine under it has...

It will be in your best interest to get control of the heat in the engine...you need to understand what wet fuel can do to the ring land...how it does not work in your engines favour,because the majority of it will vaporize at some stage,but you need it to happen at the right stage...and that is not so much a reflection on the amount of static compression the engine has,but rather on how well the fuel load burns when it should...
I was just trying to bring across the importance of fuel evaporization,thats all...

[Doug Schriefer]

yes i want to be able to run like an efi car on the street . is that asking too much in this day and age ?

Can it be done yes, without a doubt. Can you just bolt something on to your specific combination (Carb or FI) and just have it do this no...

It will take some time, effort, and trial and error testing to do what you want, but it can be done.

Your combination (good or bad) is what is, so if you want to take the time, and put in the effort you will be able to make it better.

[Procision-Auto]

I have to agree with most of these guys. Until I understood the basics
of each carb circuit, I ended up banging my head over the poor throttle
response, bucking, bogging, etc.

It has been about ten years that I have been practicing these new
techniques, since having professional training. I wish I knew this stuff
14 years ago...it would have saved me a few headaches.

As the others have mentioned, a properly tuned carburetor is like night
and day for engine performance. If even one circuit is out of balance,
it can really mess up your opinion of whether you have selected the
proper carburetor.

I have been guilty of this starting out, and I have seen numerous cases
when setting up carburetors for customers in their street/strip cars:

- Air bleeds not matched for application
- Idle screw (throttle blades) opened up too much, idle mix too fat
- Accelerator pump circuit not tuned properly (bog off the line)
- Incorrect power valve

9/10 I have found that once a bigger camshaft has been installed, people
forget to change the power valve, and/or the air bleed sizes. Dropping
manifold vacuum by 8 or more points (in./hg.) requires some attention
to carb tuning.

The first instinct is to open the primary throttle blades beyond the transfer
slot. This pretty much renders the idle circuit useless as the pressure
across the transfer slot is too high to draw fuel.

In return, the next logical step is to back out the idle mix screws to
compensate. At this point, the engine may idle, but it's likely surging
and excessively rich.

When you hammer the throttle from a dead stop, the engine tends to
bog/sputter because the transition from idle to main circuit is lost.

To complicate matters, the stock power valve is opening too soon, and
the accelerator pump is not timing the fuel shot/duration properly.

If I may offer some tips:

- Leave the idle speed screw alone for now. About 0.040" of transfer slot
should be in view.
- Install a power valve that is about 2.0 in./hg. less than the idle manifold
pressure.
- With a bigger camshaft, installing smaller idle air bleeds helps to tune the
idle circuit fuel curve to flow sooner.
- The idle mix screws can start at about 1.5 turns out (or best vacuum)

As long as timing is set well, and the carburetor is not grossly oversized
for the RPM/Engine CID, the engine should idle smoothly.

If the engine bogs/sputters from a dead stop when accelerating hard,
my money is on the pump shot timing/duration.

Do yourself a favour and buy a tuning kit! Try different pump cams and
squirter sizes. You will be amazed at how little change in pump cam
profile
can be the difference between a falling flat on your face, or lighting up the
rubber (or hooking hard )

It may take you a few tries to get it right, so be patient. You can even
try to disable the secondary throttle blades and tune the primary side first,
then mix in the secondary side.

Pump cam > when the fuel is pumped
Squiter size > duration of fuel shot. Smaller hole = longer spray

Thanks to everyone else in this thread offering tuning help. I really have
learned from you!

[Cammer]

The EFI vs. Carb subject has been vigorously discussed on this forum.

IMO, at wide open throttle a carb and EFI are essentially equals.

IMO, EFI works better under part throttle conditions.

You will be hard pressed to get a carb to function like EFI under all conditions! Magnify this statement for heavily modified engines!

Please do not over analyze the simple function of a carburetor. Most applications require only proper sizing and basic tuning.

Many jump on the change this, drill that, machine everything bandwagon, only to discover they have a carb that is better off in the garbage!


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