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Discussion Starter · #1 ·
Hi guys, i just finished the build up of my 355 GM first gen, it has an Howell TBI using stock GM components, engine is mildly modified with the pathetic common components like Edelbrock performer intake and cam, headers ,nothing fancy, it runs ok except for something that i will call misfiring and loss of power at WOT under acceleration, weird thing is that when doing burn out or just revving the engine in neutral it has great throttle response and runs great, apparently the issue happens just under load and from 3000 to approx 5000 rpm on the tach...

Set timing with the timing light, put foam in the fuel tank (trying to keep pick up wet at all times) because i thought was a fuel related problem, than i took off spark plugs this morning and i've seen this:

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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.
 

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Discussion Starter · #2 ·
i forgot to mention about gapping, those in the picture i posted before are Accel 576S and they're gapped 0.81 mm, the stock ones used by GM are AC R45 TS and gapped 1mm, the only reason i went for Accels is because they are very short and great for my stupid headers clearance.

thank you.
 

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gap

The HEI and others like MSD have more than enough voltage to jump .080. Problem is;as the plug wears,and other resistances go up,the wires develop leaks. The spark can jump to the V/Cs,headers or inside the cap.When the leak is really bad its easy to find. Stock HEI cap runs out of voltage after 5k,if your engine revs then you will need an external coil.
 

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Discussion Starter · #6 ·
The HEI and others like MSD have more than enough voltage to jump .080. Problem is;as the plug wears,and other resistances go up,the wires develop leaks. The spark can jump to the V/Cs,headers or inside the cap.When the leak is really bad its easy to find. Stock HEI cap runs out of voltage after 5k,if your engine revs then you will need an external coil.
Got it, i'll try to see if there's any leak, any suggestion about wires to use?
My problem are those stupid headers, they're so close to the plugs that i had to wrap them and the wires too to avoid to burn them up.

I prefer to buy universal wires and cut them to match the right lenght...
 

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put heat socks on the ends of the wires.I use aurora wires,but many brands are very good.Make sure you crimp the ends when you shorten the wires. Don t run 5 and 7 parallel.
 

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I have read the responses in this thread concerning plug gap and am unsure whether you are both using the same units of measurement. My understanding is that .8mm is approximately .031 inches. Can the original poster confirm that the measurement is indeed .8mm?

Regards,
Meldrum
 

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Discussion Starter · #10 ·
I have read the responses in this thread concerning plug gap and am unsure whether you are both using the same units of measurement. My understanding is that .8mm is approximately .031 inches. Can the original poster confirm that the measurement is indeed .8mm?

Regards,
Meldrum
Thank you for this, i was thinking about the same thing, i just converted dimension from inches to mm, i usually use mm because i'm more familiar with...
 

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.

Unfortunately i've no A/F meter here.
:confused:

I'm not familiar with a 'Howell EFI' system, but I would find it hard to believe that it wasn't designed to be used with either a narrow or wide band 02 sensor (A/F meter same as AirFuelRatio gauge.) I recommend an Innovate LC1, but there are others are out there that work also.

EFI without AFR gauge is like marriage without sex - what's the point - :D:D

Can you datalog with the Howell system?
 

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Discussion Starter · #14 ·
:confused:

I'm not familiar with a 'Howell EFI' system, but I would find it hard to believe that it wasn't designed to be used with either a narrow or wide band 02 sensor (A/F meter same as AirFuelRatio gauge.) I recommend an Innovate LC1, but there are others are out there that work also.

EFI without AFR gauge is like marriage without sex - what's the point - :D:D

Can you datalog with the Howell system?
The TBI kit comes with the oxygen sensor so it should manage the air/fuel mixture a little.
 

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Discussion Starter · #16 ·
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.
 

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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
 

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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
 

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.. 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...
 

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Discussion Starter · #20 ·
.. 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.
 
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