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lean mixture?

14K views 41 replies 11 participants last post by  jeepers creepers 
#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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#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.
 
#12 ·
.

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?
 
#5 ·
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.
 
#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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#17 ·
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
 
#18 ·
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
 
#19 ·
.. 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...
 
#20 ·
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.
 
#21 · (Edited)
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
 
#23 · (Edited)
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.
 
#25 ·
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...
 
#26 ·
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.
 
#27 ·
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.
 
#29 ·
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.
 
#28 ·
You must fix the timing!

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.:nono::nono:
 
#30 ·
its lean or it wouldnt be white like that. sensors are prone to be inaccurate, especially if its not a high dollar piece.(guage). either way, just was trying to throw some ideas to help get you headed in the right direction.

maybe if you just red line it for 20 minuts itl fix itself?:evil:
 
#31 ·
#32 ·
i printed out the article yesterday night! thank you.
I checked timing again and did some work on it, by setting the initial advance to 10-12 degrees i get a total of 44 with bith mechanical and vacuum,i thought was too much so i tried to disconnect the vacuum one...result is that mechanical only advances 16-18 degrees so i'm reaching 30ish at 4000 RPM wich is too much.

Reading the article on cranckshaft coalition, they say advance can go up to 45-50 during light throttle cruise with no problem getting a better fuel mileage and a cooler engine, i'm not considering mileage like the goal, i just want to solve this problem and a healthy engine, fuel mileage is a bonus.
I' ve got new spark plugs 10 minutes ago, they're colder than the previous Accel, i hope will help.

Another thing i noticed driving the car is the misfiring happens just if i push down the pedal all the way suddenly but not if i rev up the engine gently, guessing is a good clue for you, i'm thinking about too much fuel and a denser mixture in the combustion chamber coupled with a too hot plug that cause preignition?
Just wondering...
 
#36 ·
Where do you pull these spark plug numbers from? Anyway Edelbrock recommends Champion RC12YC plugs for their heads. Now if you want to crossreference that to a NKG plug it is BKR5E. Another thing about the timing. Always unplug the vacuum line from the distributor while checking or adjusting timing. Loosen the distributor lockdown and set the knob on the back of the timing light to zero. Adjust the initial timing to 10*-12* BTDC. Have someone rev the engine to 3000 rpms and point the light at the scale. If the line on the balancer is past the 0 mark on the scale adjust the knob on the back of the timing until the line aligns with 0 on the scale. What is the reading? Do this again at 3500 rpms to see if it changes or stays the same as for 3000 rpms. The reading should have been close to 35*-37*. This is your total (initial + mechanical advance). Now unplug the vacuum hose and connect it to the distributor. That's it, you can check to see how much vaccum is added by revving the engine to 3500 rpms, zero the timing light and check it by turning the knob on the light until the balancer line aligns with 0. What is the reading? 49*-51*. Good. Take the foam out of the tank. If you have enough fuel in it the pickup will stay wet.
 
#37 ·
Thank you for the answer, i spoke with the tech support of the EFI kit and i 've been told to try a little thing to see if is a rich mixture problem related or not just by reducing fuel flow using a clamp or vise grip on the pressure hose.

I noticed the 3 bolts of the exhaust flange in between to pipe and headers a little loose, i guess that air can go in and mess up the o2 sensor wich is 2inches above, i guess that is air go in the o2 sensor would make mixture richer by sending a lean signal to the ECM, i need to test the car better.

Timing is now set about 12* (initial), but i will try to get it more advance like you said, tried to disconnect the vacuum hose and there was no difference at all at idle, any suggestion?
thank you.
 
#38 ·
Timing is now set about 12* (initial), but i will try to get it more advance like you said, tried to disconnect the vacuum hose and there was no difference at all at idle, any suggestion?
thank you.
If you add initial, be sure to recheck the total timing (w/o vacuum advance, vacuum line plugged). Keep the total under 36 degrees for non fast burn chamber heads, 30-32 degrees for fast burn.

If you have no change when the vacuum advance is disconnected, you're either using ported vacuum or the vacuum advance unit is bad or the advance plate is hanging up. Try hooking it up to a vacuum port that has vacuum at idle (manifold vacuum).
 
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