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I have a Chevy small block, 383 stroker crate motor with less then 150 miles on it. I am having a problem with a few of the plugs fouling. #'s 1, 2, and 3 to be exact. When I pulled them out, they were jet black fouled. They appear dry but black. I was told to try a hotter plug. I went from the Autolite 3923 to the Autolite 3924. I also found that while the motor was ideling in low light, I saw a few sparks jumping around the spark plug wires. So I changed all the wires, put the new set of hotter plugs in, drove it for about 60 miles, pulled the plugs and the same thing. # 1 and 2 were jet black, but # 3 wasn't to bad. What would cause only a few cylinders to do this. Is it to rich of fuel or would they all be black. Can anybody give me some advice as to what it could be or what to check for. Thanks. Below is a little info on the motor.

The motor has a SL 750 Quick fuel carb, Howards Cam: Valve lift with 1.5 ROCKERS : INT .530 / EXT .545
DURATION IN DEGREES : ADVERTISED - INT .286 / EXT .294
DURATION @0.050" : INT .233 / EXT .241 LOBE SEPARATION angle 110, NBK 200 heads. (Its actually a skipwhite performance 383 turn key )http://www.skipwhiteperformance.com/detail.aspx?Item=383_STG2_0
 

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What are your idle mixture screws set at? Initial timing and total timing? What jets are in the Quick Fuel carb?
 

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What brand of electric fuel pump are you using. Do you have a regulator and what is it set at? Have you checked the engine vacuum with a vacuum gauge? Set the idle mixture screws at the highest reading with the vacuum gauge. Your timing should be set at 32*- 34* total. Make sure the reading is at 3500-4000 rpm. Which octane gas are you using? Check these things and post back with answers.
 

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Discussion Starter #6
According to the local speed shop the timing was set at 12* initial and 38* total at about 3500rpm. The distributer is a Petronix Flamethrower II, no vacuum advance. I am running a mechanical fuel pump set at 6.5. The local speed shop also is the one that set the idle mixture screws with a vacuum. Don't know those results. The float levels with the car idleing look to be about the center of the windows between the arrows as recommended by Quickfuel. After this was done it appeared to be okay and ran like a raped ape, but only lasted for about 40-50 miles or so then started running like crap. Checked the plugs fouled again. (#1-2-3) and also noticed the spark plug wires jumping spark from each other on drivers side. So I did the plug change and wire change. However as of now after running it another 50-60 miles, it still seems to be running good, but with a slight hesitation on acceleration during high speed cruising. It also idles like its going to stall in gear (automatic) but runs pretty steady in park 1000-1100rpms. With the two or three plugs fouling again, the speed shop also mentioned that I may be getting oil sucked from the pcv valve causing the plug fouling issue. Again just 2-3 plugs or would all be fouled? Could this be a problem? I'm pretty sure its only a matter of time before it starts running like crap again. Not sure what the jets are as asked. But I assume the engine builder knew what they should be as the carb came with the complete turn key build. I have also been studying more about carb tuning and stuff and got a vacuum guage now so that I can check some of this stuff on my own now. Paid $200 bucks labor to have it all checked and tuned and I'm back to square one.
 

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Mark,

If the idle mixture screws were set while you had plug wires that were arcing between each other, those idle mixtures settings very well may no longer be the best now that you've replaced the bad wires. Setting idle mixture is not rocket science so go ahead and give it a try. But remember, idle mixture settings only effect the fuel mixture when at idle and cruising with foot off the throttle. As soon as you put your foot into the pedal and the butterflies open 1/8" or more, the carburetor transitions from the idle circuit to the primary circuit and your primary jets are determining the mixture. If you really stomp on the pedal then the power valve plays a role as well. You'll need to get you primary jetting dialed in as well.

If the carburetor has vacuum secondaries and you really step on it, the secondary jets come into play once the vacuum diaphragm opens the secondary butterflies.

If the carburetor has mechanical secondaries, the secondary jets are playing a mixture role as soon as the secondary butterflies open.

Once the idle mixture and primary jets are dialed in, the secondary jets are next on the list. You should also verify the power valve is both sized properly and doesn't have a failed or leaking diaphragm. Either of those can cause an excessivly rich mixture that will foul plugs as well. And you can have oil fouled and/or mixture fouled plugs. If you can post a couple of good quality close up photos of your fouled plug we may be able to suggest which cause they look like?

A wide band AFR setup is very helpful with carburetor tuning, but you can get fairly close without one.
 

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Try another known good carb, maybe smaller in size.
Black plugs up on the longest runners, 1 & 2, read like a poor fuel distribution problem.
 

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It is a good learning experience to do some of the maintenance and tuning on your own car. It will also keep some money in your wallet. Look at my post before as the questions I asked about your engine's timing settings are recommended by the engine builder. They recommend 32-34* total timing at 3500- 4000 rpms not to worry about the initial timing. Please read your Warranty Info; Tuning, Maintenance and Breakin, Carburetion; Timing, and Spark plugs. The engine builder recommends a electric fuel pump with a regulator instead of a mechanical pump. What type vehicle is this motor in car or truck? How much does it weigh? Are you running a PVC system or Exhaust Evac System as the latter is prefered by the engine builder. Which intake do you have the Hurricane or the Crosswinds. On the fouled spark plugs from 1 & 3 cylinders was there any oil on the spark plug threads? If plug fouling continues to be a problem after you take care of the tuning of the carburetor you might want to purchase an Edelbrock Performer RPM intake manifold.
 

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Discussion Starter #10
Its a mechanical fuel pump, Holly with a factory setting at 6.5. Timing was set at 38* at about 3500 and only using 93 octane.
 

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Discussion Starter #11
It is a good learning experience to do some of the maintenance and tuning on your own car. It will also keep some money in your wallet. Look at my post before as the questions I asked about your engine's timing settings are recommended by the engine builder. They recommend 32-34* total timing at 3500- 4000 rpms not to worry about the initial timing. Please read your Warranty Info; Tuning, Maintenance and Breakin, Carburetion; Timing, and Spark plugs. The engine builder recommends a electric fuel pump with a regulator instead of a mechanical pump. What type vehicle is this motor in car or truck? How much does it weigh? Are you running a PVC system or Exhaust Evac System as the latter is prefered by the engine builder. Which intake do you have the Hurricane or the Crosswinds. On the fouled spark plugs from 1 & 3 cylinders was there any oil on the spark plug threads? If plug fouling continues to be a problem after you take care of the tuning of the carburetor you might want to purchase an Edelbrock Performer RPM intake manifold.
I used a mechanical fuel pump. Builder says electric because of pushrod. I called Howard Cams and they said with my cam there is no issue with a mechanical pump running a regular pushrod. The car its in is a 1967 Camaro Convertible. I am running a PCV system with a crosswind intake. The plugs appear to be just black, didn't really notice any wet oil, but not positive. When the local engine shop looked at it the first time it fouled out he said they did appear to have a little oil on them. What would be different about the Edelbrock intake vs the Crosswind intake?
 

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Discussion Starter #12
Mark,

If the idle mixture screws were set while you had plug wires that were arcing between each other, those idle mixtures settings very well may no longer be the best now that you've replaced the bad wires. Setting idle mixture is not rocket science so go ahead and give it a try. But remember, idle mixture settings only effect the fuel mixture when at idle and cruising with foot off the throttle. As soon as you put your foot into the pedal and the butterflies open 1/8" or more, the carburetor transitions from the idle circuit to the primary circuit and your primary jets are determining the mixture. If you really stomp on the pedal then the power valve plays a role as well. You'll need to get you primary jetting dialed in as well.

If the carburetor has vacuum secondaries and you really step on it, the secondary jets come into play once the vacuum diaphragm opens the secondary butterflies.

If the carburetor has mechanical secondaries, the secondary jets are playing a mixture role as soon as the secondary butterflies open.

Once the idle mixture and primary jets are dialed in, the secondary jets are next on the list. You should also verify the power valve is both sized properly and doesn't have a failed or leaking diaphragm. Either of those can cause an excessivly rich mixture that will foul plugs as well. And you can have oil fouled and/or mixture fouled plugs. If you can post a couple of good quality close up photos of your fouled plug we may be able to suggest which cause they look like?

A wide band AFR setup is very helpful with carburetor tuning, but you can get fairly close without one.
Right I will double check the idle mixture settings with a vac guage again. Not sure about all the other things with the power valve and jetting stuff is done. I will try and see about getting a good picture of the fouled plugs if the weather stays good enough to drive it this weekend.
 

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Discussion Starter #13
Sounds rich. Put a vacuum gauge on it and I bet you'll need to change the power valves and jet it down.
Why? If I get a very low reading this will tell me I need to change them. What is considered a low reading to change the jets and power valve.
 

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Why? If I get a very low reading this will tell me I need to change them. What is considered a low reading to change the jets and power valve.
The power valve(s) operates off of vacuum while the engine is under load, such as going up a hill with your for in the pedal. It only opens when the vacuum drops below the rating of the valve. If you have the incorrect PW, it could be opening and adding fuel to the main circuit without the need for that added fuel causing a rich condition. Personally, I don't believe that this would be the issue since it would not only foul a couple of plugs.

Have you noticed this happening more during city cruising, or higher speed such as the highway?

Although you can eventually get the tune correct manually, as someone else suggested, if you can get an air fuel ratio meter you will get to the root of the problem in no time and you will also get the most HP out of your motor. :yes:

As for the jetting, changing air bleeds, fiddling with mixture and even the PWs, it doesn't make much sense unless you have some basic diagnostic tools.

Lastly, and just my opinion, I have had performance cars with and without vacuum advance. I am a firm believer that every street car benefits from VA. Better running, better mileage and more consistent performance. Again, JMO :thumbsup:
 

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Discussion Starter #15
The power valve(s) operates off of vacuum while the engine is under load, such as going up a hill with your for in the pedal. It only opens when the vacuum drops below the rating of the valve. If you have the incorrect PW, it could be opening and adding fuel to the main circuit without the need for that added fuel causing a rich condition. Personally, I don't believe that this would be the issue since it would not only foul a couple of plugs.

Have you noticed this happening more during city cruising, or higher speed such as the highway?

Although you can eventually get the tune correct manually, as someone else suggested, if you can get an air fuel ratio meter you will get to the root of the problem in no time and you will also get the most HP out of your motor. :yes:

As for the jetting, changing air bleeds, fiddling with mixture and even the PWs, it doesn't make much sense unless you have some basic diagnostic tools.

Lastly, and just my opinion, I have had performance cars with and without vacuum advance. I am a firm believer that every street car benefits from VA. Better running, better mileage and more consistent performance. Again, JMO :thumbsup:
Seems to run worse at idle to city low speeds. Seems if you put your foot into it on and off, it seems to run a little better. The gas mileage is awful too. Of course I know it will be putting my foot in it, but don't think it should be as bad as it is. Also another symptom. At cold start it leaves a little dark spot under the tailpipes on the floor, but not after it heats up.
 

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With the repeated plug wires arcing and not resolved with changing wires may want to be sure engine is well grounded to the frame and battery. Just one more thing to check in addition to all above
 

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With the repeated plug wires arcing and not resolved with changing wires may want to be sure engine is well grounded to the frame and battery. Just one more thing to check in addition to all above
Always a good thing to confirm. Good point..
 

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Seems to run worse at idle to city low speeds. Seems if you put your foot into it on and off, it seems to run a little better. The gas mileage is awful too. Of course I know it will be putting my foot in it, but don't think it should be as bad as it is. Also another symptom. At cold start it leaves a little dark spot under the tailpipes on the floor, but not after it heats up.
Mark,

It could be many things causing the issue or issues so depending upon your willingness and patience you will get there. From my experience, there are a couple of options.

A. If you want to get some tools/equipment (or borrow/rent) you will learn, resolve and get satisfaction but it will take time and money. In the end you will gain a lot of knowledge.

B. Alternatively, get the car on a Dynamometer in a good tune shop, spend $150 an hour and get it resolved in a day.

On a dyno, they will set the timing with a knock sensor, set fuel levels, read the exhaust with a digital AFR, properly jet the mains, air bleeds, accelerator pump ramp and idle mixture. This scenario will get everything set correctly but probably take 5-6 hours. As a result, you can also learn the output HP and TQ of your setup.

With all of that said, is the engine lopy with a rough idle? You may want to try lowering the fuel levels in the bowls a bit below the recommended as it may be overflowing and causing the rich mix. Again, odd that it is only a couple of plugs.

You may want to follow the advice of Everett. :yes:

If you proceed with option A, please tell us more about the QF carb and engine build.
 

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Discussion Starter #19
With the repeated plug wires arcing and not resolved with changing wires may want to be sure engine is well grounded to the frame and battery. Just one more thing to check in addition to all above
You must have misunderstood me. The arching quit with the new wires. The fouling plugs didn't is what I meant. I have good ground straps in three different places.
 

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Discussion Starter #20
Mark,

It could be many things causing the issue or issues so depending upon your willingness and patience you will get there. From my experience, there are a couple of options.

A. If you want to get some tools/equipment (or borrow/rent) you will learn, resolve and get satisfaction but it will take time and money. In the end you will gain a lot of knowledge.

B. Alternatively, get the car on a Dynamometer in a good tune shop, spend $150 an hour and get it resolved in a day.

On a dyno, they will set the timing with a knock sensor, set fuel levels, read the exhaust with a digital AFR, properly jet the mains, air bleeds, accelerator pump ramp and idle mixture. This scenario will get everything set correctly but probably take 5-6 hours. As a result, you can also learn the output HP and TQ of your setup.

With all of that said, is the engine lopy with a rough idle? You may want to try lowering the fuel levels in the bowls a bit below the recommended as it may be overflowing and causing the rich mix. Again, odd that it is only a couple of plugs.

You may want to follow the advice of Everett. :yes:

If you proceed with option A, please tell us more about the QF carb and engine build.
Ok thanks for all the help. I may try a different carb first and see what happens. That could tell me a lot.
 
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