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Discussion Starter #1
Hi guys,
My 420 SHP engine is fouling sparkplugs like crazy and I really dont know what to do to solve this issue...

I put in new sparkplugs and it works like a charm, after a few starts it starts to backfire trough carb and exhaust.

420 SHP around 10:1 comp ratio, AFR 220 heads, mech roller 244/248 around 650 lift, Holley 950 from pro systems, HEI with high voltage module and coil, 8.5 accel wires, heads are grounded to the chassi, 12 Voltage to the dist.

Exhaust gases was smelling gasoline so drilled out the 4 idle jets and lowered the float levels, have the idle screws out 1.5 turn. 35 degree total advance. Tried ac delco R45, Ngks and champions. Plugs get black and sooty and dies really fast!

Any ideas?

Thanks
 

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Did Pro systems build this carb for this engine? Why did you immediately drill out the idle jets instead of changing the main jets first? What are the jets sizes, front and rear?
 

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Discussion Starter #5
68bbnova,

Carb is built by Prosystem for this engine. Pri jet:76 Sec jet: 86. Idle air baseline was 70.

I drilled out the jets because I consulted Patrick and followed his advice step by step. I should drill them .010" steps until the idle was not too rich anymore. It worked because the sticky gasoline smell almost vanished but still the engine kills plugs. Sooty but no oil on them.
 

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What power valve is in it, surely not a 6.5? Front power valve only I'm assuming? Front and back idle screws even? Might try closing the back idle and raising the front.

I have a 255/263*@.050" in mine with 26* initial and 10* mechanical. My buddies cam has about the same duration as yours and his works great at 18* and 18*.
 

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1100 rpm, TH 350, 3500 stall, float levels at the bottom of the sight glasses, initial timing with plugged vaccum can 20 degrees
Initial timing is 20° with vac can plugged.
Are you using vac advance?

If yes, what is timing at idle with vac advance connected?

How much mechanical advance?

At what RPM does mechanical advance start coming in?

At what RPM does mechanical advance reach its max?

Gerry
 

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420 fouling plugs

No ones mentioned the 1st thing you should check---what's your fuel pressure ?? Are you high enough to overpower the needle & seat ?
Start there----try another carb if you have access to one.
 

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Why did you drill the main jets for an idle problem?
My idle was to fat also and I closed up my idle restrictions several times until I got it.

The way I'm reading between the lines on this post is that the idle restrictions are fat!
I'm taking it Patrick informed you to drill out the air idle jets .010 to allow more air on the idle circuit, hence leaning the idle out!
My idle restrictions were interchangeable so I leaned mine up there.
I've seen people put little strands of wire and bend each end over to prevent it from falling out through the idle restriction hole on carbs that they were not interchangeable to lean it up.
 

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Correct myself, you didn't drill the main jets. Sorry!
The main jets are NOT in play on the idle circuit, so playing with main jets will change nothing!
Idle restrictions are to fat!
That is why Patrick is telling you to drill out the idle air jets. :yes:
 

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Discussion Starter #12
Guys,
I didnt drill the main jets, I drilled the idle airbleeds.

Today I noticed my fuel pressure has increased. it was between 9-10 PSI. Wonder how that is possible with a mechanical Carter street and strip fuel pump that not requires a regulator.

I guess it can be high enough to overpower the needle & seat with pressure above 8 PSI. Prosystems says this carb want around 7.25 PSI.

I dont know, but I guess the high fuelpressure can really be an issue here! Its hard to get good reading with a mechanical pump coz of the fluctuations on the needle, but anyway.
 

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Make sure your engine is not idling off of the transfer slots on the primary side. this will make your engine very rich at idle.
Look at your transfer slots. they need to be "square" in shape as you look down at them with the blades closed. If they appear to be rectangular that means the idle screw is set too high exposing too much of the transfer slot. In other words what you can see of the transfer slot sticking above the closed throttle blade needs to be only as tall as it is wide.
If the mixture screws are 1.5 turns off the seats and it still won't idle, open the rear blades a bit using the secondary idle adjustment screw.
With the backfiring you mentioned you might want to look at the condition of your power valve as well.
 

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Another thing to check is fire. Make sure your ignition system is doing it's job. You need to make sure your fuel pressure isn't too high. If your fuel pressure isn't too high I would start backing down my main jets on the primary side. Of course I would make sure the float levels and needle and seats were right first.
 

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No ones mentioned the 1st thing you should check---what's your fuel pressure ?? Are you high enough to overpower the needle & seat ?
Start there----try another carb if you have access to one.
Never mentioned fuel pressure because it won't idle if it's bypassing the needle/seat and you should see that issue real quick by looking down the throat of the carb.
 

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Discussion Starter #16
Everybody, thanks for your input! Appreciate it!

The thing is that the engine runs really good for a short while when installing new plugs. Idle around 1100 RPM, pulls strong and no bog.

After a few starts it starts to pop trough the carb and the exhaust, then I know it has been fouling some plugs. Last time it was popping trough right exhaust, swapped the plugs from right to left and started it, then it popped trough the left exhaust, so its the plugs, no question about that. Fouled and smell of gasoline, NO oil on them.

I really dont know what could be wrong with the NEW HEI dist or coil that would kill the sparkplugs this fast when everything works with new plugs. I bet it is fuelrelated. If its not too high fuelpressure, I really dont know! I will get a regulator and see if I can get the pressure down between 6-7 PSI and see whats happening. If that doesnt help, I will be clueless...

And honestly, I dont really understand how a mechanical fuel pump (Carter strip 120 gph) can start to increase the fuel pressure all of a sudden. I have a new aeromotive gauge, that one could be the issue aswell, but I doubt it.
 

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Try another carburetor. My experience with Patrick is that it will NEVER be the carburetor as the problem. So asking him for help is a waste of time. I went through ignition boxes, coils, a tach, valve springs, re wiring the whole car, fuel pump, filters, changing valve spring heights, and sent the carb back to Patrick. After ALL that, do you know what fixed it?? Borrowing a Bob Book carburetor.
 
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