[F-BIRD'88]

Spark plug gap should be .035` inches. You do the math.

The 02 sensor only trims the AFR at part throttle. The ECM does not read it @ WOT.
When the ECM is reading the 02 sensor "closed loop" the voltage will swing hi low hi low, rapidly.

But @ WOT the 02 sensor voltage will stabilize and should be high. Higher than .700MV DC.

But you can read it yourself @ WOT with a volt meter. Should stay around .800MV @ WOT.
Less than .700MV is way lean @ WOT.. Lack of fuel flow volume.
.800 to .850MV is a normal 02 reading @ WOT. It does not tell you the exact AFR. But tells you its not starved for gas @ WOT.

spark Misfire reads "lean"-- lower voltage @ WOT.

Glowing headers when you rev it up is severe leanout AFR (lack of fuel flow volume) and or severe retarded timing. Its not advancing with rpm. TPS fault. fuel pump, fuel pressure regulator. flow restriction.

[jeepers creepers]

Originally Posted by F-BIRD'88

Spark plug gap should be .035` inches. You do the math.

The 02 sensor only trims the AFR at part throttle. The ECM does not read it @ WOT.
When the ECM is reading the 02 sensor "closed loop" the voltage will swing hi low hi low, rapidly.

But @ WOT the 02 sensor voltage will stabilize and should be high. Higher than .700MV DC.

But you can read it yourself @ WOT with a volt meter. Should stay around .800MV @ WOT.
Less than .700MV is way lean @ WOT.. Lack of fuel flow volume.
.800 to .850MV is a normal 02 reading @ WOT. It does not tell you the exact AFR. But tells you its not starved for gas @ WOT.

spark Misfire reads "lean"-- lower voltage @ WOT.

Glowing headers when you rev it up is severe leanout AFR (lack of fuel flow volume) and or severe retarded timing. Its not advancing with rpm. TPS fault. fuel pump, fuel pressure regulator. flow restriction.

Thank you for the precious advices, headers are wrapped up with fiberglass to keep a decent temp under the hood and prevent spark plug wires to get burnt, i will check timing again and o2 sensor voltage but i want to make sure also spark plugs wires arent' leaking in somewhere.

[cobalt327]

Originally Posted by jeepers creepers

Plugs look all like that, they have a white spot on electrode and ceramic surrounded by a dark layer down below.
What do you think about them? Mixture is ok but too hot plugs?
Any help will be greatly appreciated.

New plugs needed, it's all but impossible to read a plug that is old or has been idled after a WOT run. Your plugs were hot at some point, though. Besides the other suggestions, be sure the heat range isn't too hot. Be sure the timing is all in at or before 3000 rpm and that the timing advance curve is set up correctly.

Reading plugs

[cadillacdave1]

Sounds like the distributor may have moved a little and the timing is retarded. Check timing and re-tighten bolt to hold distributor clamp down a little tighter. Also your spark plug gap measurements are in mm and most folks on her use inch measurements as in .035 or .045 in. Try gapping the plugs to .045 = 45 thousandths With EFi and a good coil you can usually gap even a little larger. Newer cars use gaps of .060.

1) Checka and re-set timing
2) re-gap plugs to .045

Drive car and recheck plugs after 20 miles or so. Good luck

[BuzzLOL]

.. Ignore the red hot headers for now, since they're wrapped/insulated, of course they're going to run hotter than bare ones that can loose heat/energy/power...

.. Sounds like you have a severe lean misfire at high RPMs... and plugs verify it... those look like old used plugs that were sooted up a bit, then you put them back in the new engine build and they're burning white/lean/too hot on the ground strap/outer electrode end...

.. As others noted, is the timing advancing as the RPMs go up... are all the EFI sensors good?

.. What did you do to make a 355"? Was it a 350/355 to start with? Was the EFI/TBI for a 350/355 or for a 305?

.. Is this an edelbrock 2-bbl. TBI intake or a 4-bbl. one with a 2bbl. adapter? Are the cylinders sealed/separated 4 and 4 from each other under the TBI, or is there some common plenum opening feeding all 8 cylinders? If there is common plenum for all 8 cylinders, it usually requires a richer TBI or carb. secondaries to avoid going too lean at WOT...

[jeepers creepers]

Originally Posted by BuzzLOL

.. Ignore the red hot headers for now, since they're wrapped/insulated, of course they're going to run hotter than bare ones that can loose heat/energy/power...

.. Sounds like you have a severe lean misfire at high RPMs... and plugs verify it... those look like old used plugs that were sooted up a bit, then you put them back in the new engine build and they're burning white/lean/too hot on the ground strap/outer electrode end...

.. As others noted, is the timing advancing as the RPMs go up... are all the EFI sensors good?

.. What did you do to make a 355"? Was it a 350/355 to start with? Was the EFI/TBI for a 350/355 or for a 305?

.. Is this an edelbrock 2-bbl. TBI intake or a 4-bbl. one with a 2bbl. adapter? Are the cylinders sealed/separated 4 and 4 from each other under the TBI, or is there some common plenum opening feeding all 8 cylinders? If there is common plenum for all 8 cylinders, it usually requires a richer TBI or carb. secondaries to avoid going too lean at WOT...

First of all, thank you for helping me out, so, the EFI is brand new and comes in a kit made by Howell-EFI using GM 2BBL throttle body and ECM, sensors are new too, there's also a new fuel pump that i mounted below fuel tank, i gave to Howell all the specs of the engine and they made a custom prom for me.
Engine is a common 350 first gen bored over .030 with alu heads, a mild cam, Edelbrock performer intake and Hedman headers.

I noticed today looking at the spark plug wires that one of the caps was seriously cracked by beeing too close to the headers and i'm thinking that the misfire may be caused by this problem.

The spark plugs are new too, i put them after the rebuilt so they are like 2 hours old.
I found here on the forum shortys like the Accel i'm using made by Champion, NGK and others so i will tru to put a new set soon, the only thing i'm not sure about is the heat range, the ones i'm using are the hottest available, when cold the engine shakes a little but just for a minute than runs pretty good, don't know if a colder plug would be a good idea.
I'd like to try Champion RV92YC, looks like they are supershort like Accell but cheaper, hope the quality isn't cheaper too.
About the timing, this is a bit more complicated because of the water pump clearance i have in my motor, is really hard to see the mark on the damper, i found on the net that the ground strap shows the advance too, mine is about in the strap curve, closer to the thread means it is more advanced, any experience on this?
My only concern is to damage pistons running too lean but the plug "circle" is quite dark...
I'll keep you on the loop.

[496CHEVY3100]

You did not say if it was 1 cyl or all ,if 1 cylinder check for vacumn leak at base gasket or intake ,if more than one check vacumn hose ,or LEAN condition at higher rpm not rich,if it were rich it would be black,but by headers turning red i would go with lean and timing .also check harmonic balancer to make sure is us marked for your application ,some are marked at 12 o clock some at 2 oclock

[jeepers creepers]

Originally Posted by 496CHEVY3100

You did not say if it was 1 cyl or all ,if 1 cylinder check for vacumn leak at base gasket or intake ,if more than one check vacumn hose ,or LEAN condition at higher rpm not rich,if it were rich it would be black,but by headers turning red i would go with lean and timing .also check harmonic balancer to make sure is us marked for your application ,some are marked at 12 o clock some at 2 oclock

i will check timing again, i had to machine a new groove on the balancer because the mark on the timing cover wasn't matching, found a great "tutorial" about what to do that properly and now seems to be ok, my feeling is that i can give her a bit more advance.

[jeepers creepers]

today i replaced three spark plug caps, one of them was cracked quite a bit, fixed a wire, gapped the spark plugs to .035, advanced an hair the timing and problem still, motor now has a superfast throttle response, no knocking or detonating, a friend of mine borrowed my timing light so i'll check it tomorrow, i know there are tons of variables but what' s the common total timing for the 350? are 35 degrees ok?
I want to measure the o2 sensor voltage as well, hope to do that tomorrow.

[cdminter59]

Originally Posted by jeepers creepers

today i replaced three spark plug caps, one of them was cracked quite a bit, fixed a wire, gapped the spark plugs to .035, advanced an hair the timing and problem still, motor now has a superfast throttle response, no knocking or detonating, a friend of mine borrowed my timing light so i'll check it tomorrow, i know there are tons of variables but what' s the common total timing for the 350? are 35 degrees ok?
I want to measure the o2 sensor voltage as well, hope to do that tomorrow.

Try about 10-12 initial timing, and the total timing of 36* all in by 3000 rpms. You might have to get an advance curve kit for the distributor. Here is another article on tuning your HEI and setting your timing. http://www.crankshaftcoalition.com/w...EI_distributor

[jeepers creepers]

Hi guys, i should check timing and o2 sensor voltage within a few minutes, said that, i found on the web an interesting thing about Throttle body spring pressure mod... it's nothing more than an adjustable screw against the spring to achieve more fuel volume at WOT when lean conditions or lack of fuel is detected like it's more that probable in my case.
My only concern is to ruin TBI gasket while doing that...

[jeepers creepers]

ok, o2 sensor reached 900 MV at about 3000 RPM with a peak of 1000 at WOT, idle and off idle is running in between of 800-900 MV.

Timing, this wasn't very good, at idle was close to zero and went at about 40 of total, tried to set 10-12 at idle and got almost 60 of total timing!
Tried to set total timing of 35 degrees total and got the engine hot after 5 minutes so that means is running retarded and also didn't get any better while driving it.

I checked the springs on weights of the mechanical advance ,they where the lightest so i put heavier springs, i also want to set properly the vacuum one tomorrow.

[64nailhead]

Curious as to why you changed only 3 plugs?

If you put in heavier springs, then the centrifugal advance will come in later, which is not what your after. As cdminter said, your want all of the advance in by 3000.

In terms of the timing, IMO, I'd change the remaining plugs and clean off the 3 you already changed, then unhook the vacuum advance and plug the line. Set the total timing between 34 & 36 and find out exactly what rpm it is all in by, and then drive a WOT throttle session and recheck the plugs immediately and see what they look like.

It will not hurt the motor to run it with no vacuum advance. And, oh yes, MAKE sure your balancer is marked accurately for #1 TDC.

I didn't see where you checked you vacuum, where is it at idle? Did you check for leaks?

Good luck.

[71nova355]

You said you built the motor? DId you degree the cam properly? did you use the correct balancer and timing marks? You also need to gap your plugs all around .40. The mixture is way way lean and this is not good at all! If its not fixed pronto, you will be re building very soon. Your plugs should never be that white at all. Just make sure when checking your timing you take all the necessary steps because it will determine whether or not you can calibrate your set up correctly or not. as stated earlier 36 total is a good place to start. unless you have a kit to calibrate your distributor then you wont be able to set initial but if you have a gm distributor with stock settings, then where ever 36 total leaves you will work. At least untill you get everything dialed in a lot closer and then you can experiment with a couple degrees or so one way or another. If you bump up your initial you will have to stop your total exceeding the 36.

I hope you can get her figured out before you melt a piston.

[jeepers creepers]

Originally Posted by 64nailhead

Curious as to why you changed only 3 plugs?

If you put in heavier springs, then the centrifugal advance will come in later, which is not what your after. As cdminter said, your want all of the advance in by 3000.

In terms of the timing, IMO, I'd change the remaining plugs and clean off the 3 you already changed, then unhook the vacuum advance and plug the line. Set the total timing between 34 & 36 and find out exactly what rpm it is all in by, and then drive a WOT throttle session and recheck the plugs immediately and see what they look like.

It will not hurt the motor to run it with no vacuum advance. And, oh yes, MAKE sure your balancer is marked accurately for #1 TDC.

I didn't see where you checked you vacuum, where is it at idle? Did you check for leaks?

Good luck.

i didn't change any spark plug but just 3 caps of the wires because on of them was cracked by the heat and the others were going to, i put heavier spring because advance was all in just over 2000 RPM, today i will try to disconnect the vacuum advance and see what happens.

As i said before the mark on the balancer wasn't matching the tab on the timing cover so i fixed the damper by machining a new groove in the right position, i didn't degree the cam, i thought was an overkill for a mild motor like mine but now i realize it's a smart thing, i will for sure next time.
I can take off valve cover and check the TDC.

By measuring the voltage coming out from the O2 sensor shouldn't be lean , 800 to 850 MV should be the ball park and mine is going from 800 to 900 MV with peaks of 1000, picture of the spark plug doesn't make justice to me.