Need advice! 351C fouls plugs....

'69Stang · Aug 26, 2007

The problem is that my engine seems to foul plugs and I am running out of ideas. I have a 950 Holley HP and I think it might be too big for my application. With a new set of plugs the engine runs great, but within 1/2 an hour it begins to lose performance and chug. I was running Motorcraft ASF 32c plugs and when I pulled them the base ring is covered with dry, black soot. The electrode strap looked clean but gave no indication as to whether the combustion was running hot or cold, though I suspect cold. I decided to move to a hotter plug, Motorcraft ASF 42C's, and I got the same results. I dropped the jets from 73 in the front to 71 and now I just changed them to 68, but no difference - the plugs get sooty and fouled and the performance suffers.

The weird thing is that a couple of the spark plugs - cylinders 3 and 6 - are wet and covered with a black gas smelling substance. My thought is that the carb is just too big and that I should switch down to a 750 Holley.

You can see my engine combo in the sig below. I am running 15 degrees advance with and MSD 6AL.

Any advice would be greatly appreciated

rbohm · Aug 26, 2007

sounds like you nailed it yourself. the carb you have is too large for the engine combo. i agree that stepping down to a 750 is the best choice.

speedytang · Aug 26, 2007

I put what you have listed in my engine software for the heck of it and it showed you need a 680cfm at 6500 rpm so using those numbers it shows your 950cfm is way to large.

'69Stang · Aug 27, 2007

Wilson carbs...

I just spoke with the company that sold me the 950 HP, Wilson carbs, and they seem adament that the current carb should work. They told me to focus on the following:
1. Idle screws
2. accelerator pump squirters
3. Power valve
4. Ignition curve

I will check all of the following and see what happens.

ratio411 · Aug 27, 2007

The facts are that you may very well be overcarbed, HOWEVER, even too large a carb won't foul and load up if tuned properly.
Most folks figure the carb is huge, so it is dumping fuel. NOT SO.

You can tune a friggin 1200 cfm Dominator to work on a 289 without fouling the plugs. You just have a ton of carb that you can't use in your power range is all. NOT WET PLUGS.
The carb company is correct.

I put what you have listed in my engine software for the heck of it and it showed you need a 680cfm at 6500 rpm so using those numbers it shows your 950cfm is way to large.

BS, he doesn't tell you enough about his combo in this post to determine what carb he needs.
I don't know what "software" you used to tell you that, but if it's based on the old chart of CID vs RPM that has been circulated for decades (many decades), it is based on a minimum carb required at around 75% VE.
It is NOT a maximum and most cars today run in the 90s% VE, while a good street machine can make 110-115%.

Edit:
Did some quick math:
If 680 cfm minimum is required for 6500 rpm at 75% VE, then 906 cfm minimum is required for 6500 rpm at 100% VE.
With modern cam, intake, headers, mufflers, ignition, and other valvetrain parts that were exotic just a few years ago (ex: roller rockers) an engine can make 100% without trying.

ratio411 · Aug 27, 2007

Have you checked you float level?
They may be high enough to do what's called pullover, but not high enough to flood out of the vents.

Soaring1 · Aug 27, 2007

ratio411 said:
The facts are that you may very well be overcarbed, HOWEVER, even too large a carb won't foul and load up if tuned properly.
Most folks figure the carb is huge, so it is dumping fuel. NOT SO.

You can tune a friggin 1200 cfm Dominator to work on a 289 without fouling the plugs. You just have a ton of carb that you can't use in your power range is all. NOT WET PLUGS.
The carb company is correct.

BS, he doesn't tell you enough about his combo in this post to determine what carb he needs.
I don't know what "software" you used to tell you that, but if it's based on the old chart of CID vs RPM that has been circulated for decades (many decades), it is based on a minimum carb required at around 75% VE.
It is NOT a maximum and most cars today run in the 90s% VE, while a good street machine can make 110-115%.

Edit:
Did some quick math:
If 680 cfm minimum is required for 6500 rpm at 75% VE, then 906 cfm minimum is required for 6500 rpm at 100% VE.
With modern cam, intake, headers, mufflers, ignition, and other valvetrain parts that were exotic just a few years ago (ex: roller rockers) an engine can make 100% without trying.

You guys have honed in on the carb being too big, and it really is, but that in itself won't make the plugs clog. That will just wash down the cylinders and waste gas. That fouling is caused by burning oil. And in this case, a lot of oil. Look at those clogged plugs and see what color they are. If black, then you are burning oil my friend, and time for an overhaul. Check that dipstick a lot before you tear into the engine. Chances are the oil smells like gas, and the level is going down.

jcode68 · Aug 27, 2007

I had similar problem recently and turned out my fuel pump was putting about 11 psi to the carb. I installed a regulator and tweaked it to 5.5 psi. If these look familar:

Then this could be your problem. These were my plugs before installing my regulator.

69gmachine · Aug 27, 2007

ratio411 said:
Have you checked you float level?
They may be high enough to do what's called pullover, but not high enough to flood out of the vents.

And along that same line, have you checked your fuel pressure? What kind of pump are you running? Do you have a pressure regulator?

When you run a bigger carb than needed, the risk is in not getting enough signal to pull gas through the venturi, making you run lean, not rich. However, if you have too much fuel pressure for the needle and seat, gas will pour in regardless of the signal. Keep looking for the real culprit.

My .02 follows:
You're not going to bolt together an engine with greater than 100% VE by just assembling a bunch of name brand parts, even very good parts. You might get in the low 90s if you're lucky or very good. Just like going faster in the 1/4 mile, it gets increasingly harder to make gains as you go faster (approach 100%). To go over 100% on a NA engine, you have to understand the dynamics of pulse tuning both the intake and exhaust, and that doesn't just happen by accident. The entire engine is designed around a narrow RPM band with the bore, stroke, the length and diameter of the intake runners, the flow of the heads (an extension of the intake runner), the length and diameter of the exhaust tubes, and the cam timing events all designed to coincide to generate a wave of energy at the precise time to pull more air in than can be drawn by the piston alone. That's not a street engine. However, there are so many really good products out now, if you do your homework, anyone can follow a "recipe" and get extremely good results (better than 80% over a broad RPM range).

Soaring1 · Aug 27, 2007

That cut an paste makes no sense whatsoever. The man is burning oil.

'69Stang · Aug 27, 2007

Some adjustments...

My idle screws were improperly adjusted. they were about 1 3/4's turns out in front and about 1 1/2 in back. Wilson carbs recommended 1/2 to 1 turn. When I got done adjusting them they were all about 3/4's of a turn and the idle speed increased. I also reduced the primary squirter size from 31 to 28 per Wilson carbs recommendation. I took it for a ride and the engine seemed crisper and more responsive at lower speeds.

I didn't have time to check the PV but I think it is a 5.5, which should be fine.

My ignition curve looks good with 15 initial and 21 centrifugal all in by 3000 RPM's for a totsl of 36. I may want to go up to 18 and 39 total, but I'll probably need a dyno to confirm that.

The plugs are cleaning up, also. The electrode is clean, the porcelain is now very white and the base circle is turning brown instead of black with no soot build-up. The only concern I have is with cylinder 3 and 6 which seem to have a different type of coating on the spark plugs. They were definitely cleaner, but not right. Is it possible the rocker has gotten out of spec with the valve? Too loose or too tight? It is a solid cam and may need adjustment.

Additionally, I may re-jet the front jets back up to the original 72 since they have probably played no role in the spark plug fouling.

Thanks for all of your comments. I'll keep you guys informed with the progress.

ratio411 · Aug 28, 2007

'69Stang said:
My idle screws were improperly adjusted. they were about 1 3/4's turns out in front and about 1 1/2 in back. Wilson carbs recommended 1/2 to 1 turn. When I got done adjusting them they were all about 3/4's of a turn and the idle speed increased. I also reduced the primary squirter size from 31 to 28 per Wilson carbs recommendation. I took it for a ride and the engine seemed crisper and more responsive at lower speeds.

I didn't have time to check the PV but I think it is a 5.5, which should be fine.

My ignition curve looks good with 15 initial and 21 centrifugal all in by 3000 RPM's for a totsl of 36. I may want to go up to 18 and 39 total, but I'll probably need a dyno to confirm that.

The plugs are cleaning up, also. The electrode is clean, the porcelain is now very white and the base circle is turning brown instead of black with no soot build-up. The only concern I have is with cylinder 3 and 6 which seem to have a different type of coating on the spark plugs. They were definitely cleaner, but not right. Is it possible the rocker has gotten out of spec with the valve? Too loose or too tight? It is a solid cam and may need adjustment.

Additionally, I may re-jet the front jets back up to the original 72 since they have probably played no role in the spark plug fouling.

Thanks for all of your comments. I'll keep you guys informed with the progress.

Cool!

ratio411 · Aug 28, 2007

69gmachine said:
My .02 follows:
You're not going to bolt together an engine with greater than 100% VE by just assembling a bunch of name brand parts, even very good parts. You might get in the low 90s if you're lucky or very good. Just like going faster in the 1/4 mile, it gets increasingly harder to make gains as you go faster (approach 100%). To go over 100% on a NA engine, you have to understand the dynamics of pulse tuning both the intake and exhaust, and that doesn't just happen by accident. The entire engine is designed around a narrow RPM band with the bore, stroke, the length and diameter of the intake runners, the flow of the heads (an extension of the intake runner), the length and diameter of the exhaust tubes, and the cam timing events all designed to coincide to generate a wave of energy at the precise time to pull more air in than can be drawn by the piston alone. That's not a street engine. However, there are so many really good products out now, if you do your homework, anyone can follow a "recipe" and get extremely good results (better than 80% over a broad RPM range).

My .02 is that it is not voodoo to get over 100%.
Most modern parts are built to take advantage of pulse dynamics and scavenging.
Especially modern heads, cams, headers, and intakes.

Anyway, charts that suggest cfm for a cid at rpm are all based on a minimum needed, not maximum. And no matter what the facts are on VE, we all know that those charts come from a time where VE was much lower.

'69Stang · Aug 30, 2007

Darn Holleys!!

The engine was running great and the plugs were cleaning off, BUT NO, I couldn't leave well enough alone and had to jet the carb back up to the original 72's and change the power valve to a 4.5. In the process I must have crushed the PV gasket or got some debris in the float bowl because the engine will not start and I hear the electric fuel pump running, probably flooding the combustion chambers and cylinders.

What an idiot I am!

Darn Holleys! (Dumb owners)

I'll try to get the car running tonight.

'69Stang · Aug 31, 2007

Found the problem

Operator error. The power valve had no gasket. new gasket eliminated the problem.

ratio411 · Sep 1, 2007

'69Stang said:
Operator error. The power valve had no gasket. new gasket eliminated the problem.

Yeah, we all have brain farts like that sometimes.
And it's so simple, you don't think of it!

ratio411 · Sep 1, 2007

Soaring1 said:
That cut an paste makes no sense whatsoever. The man is burning oil.

That's funny...

Need advice! 351C fouls plugs....

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