Ignition or Fuel?

-
Green Hornet

Green Hornet

Active Member
Joined
Oct 24, 2008
Messages
42
Reaction score
0
Location
Maryland
I've been having a fair share of issues with a new 408 build in my Dart. There is a miss and pop out of the exhaust starting at around 1800 RPM and continues up around 3200 and tapers off some. I pull the spark plugs and find that plugs 1-2 look like brand new, 3-4 have a little carbon on them and the porcelain is light brown, 5-6 are black, and 7-8 look more similiar to 3-4. Ignition is all new MP electronic with chrome box and new MSD Blaster coil. The carb is a fresh rebuilt Thermoquad. Any help would certainly be appreciated. Thanks!
 
Check the rotor and reluctor phasing of the distributor. You can read the burn marks on the cap to see if there is an issue. How are the wires? Did you have to make them(terminate them) yourself? Are you certain they are good?Sounds more like ignition than anything else.
 
will it pop in N or does it have to be in gear?..........Full throttle only?
 
The wires are all brand new Taylor 8mm. It pops in neutral and when in gear and driving. It actually seems to clear up some above 3,000 rpm when you get on it, but it never totally goes away. Thanks for taking the time to help me out. I will check the rotor cap for marks of it being out of phasing.
 
Did you put ends on the Taylors? Also check the reluctor to pickup gap (the 8 pointed star shaped doohicky under the rotor and the magnetic pick up it rotates past). It should be .010" and should be the same for all points. If it's off that wil also change individual cylinder firings.
 
I did not put ends on the plug wires. They were a pre-made set. I checked the rotor cap for burn marks on the plastic of the cap and I did not see any off of the contacts for each cylinder. So from that I guess the phasing isn't too far off. I will have to recheck the clearance on the pick-up. I adjusted it off of one point when I initially set it up. I will pull it and check all the points to make sure they are with in tolerance.
 
the reluctor gap should be set to .008". their pretty fussy. carbon tracking in the cap will not show up looking like a burn mark. more of a pencil line which is sometimes hard to see. change the cap with a new one. if that dosn't cure it then start looking for an intake gasket leak.
PS
Green Hornet.. cool name.. somebody here thought I was you lol.
 
I had fought an intake gasket leak before, and I am not sure that it is cured. I had problems getting the port surface between the head and the intake to seal. The first intake I had for this new motor was brand new, but it had a high spot in the port surface and it wasn't square from front to back. The new intake seems correct that way but I still fought a leak. I had some oil showing up about half way up the gasket surface from the lifter valley side. The new intake came back off agian after having a lifter go bad and wiping out a lobe on the cam. This time I put silicone down lightly on both sides of the intake gasket and went back to using an all fiber gasket (no metal laminent). The plugs are not wet with oil, but more of a dry black film. That is why I believe it's not oil fouling. I'm not saying I know for sure though. The intake sealing issue has everyone I talk to puzzled. I get a good crush pattern on the side gaskets, even when I knew it was leaking.

I've had my Dart since college. Since she's F5 green with a white stripe we started calling her the Green Hornet.
 
Did the holes in the intake line up with the holes in the heads when you set it in place? You didnt mention the intake issue. From the sound of it you have not solved the problem. If the heas or deck surfaces were milled there is a very good chance the intake needs to be cut to fit properly and that is why the lower edge wont seal.
 
Well that part of the puzling thing. The bolt holes line up perfectly. You can start the bolts easily by hand. I bought the short block from a machine shop (not my best decision) and they can't tell me how much was taken off the heads to zero deck the pistons. Since the intake bolt holes line up and I get a good crush pattern from top to bottom on the gasket I would think it should be sealing.
 
and black plugs means its running too rich.... that means you may be ok on the intake vac leak but need to adjust the carb and resolve whatever else is causing it.
 
That's the thing. If the plugs were oil fouled wouldn't they be kind of moist and sooty? They are new plugs and have only been in for the new cam's break-in period.
 
Green Hornet said:
That's the thing. If the plugs were oil fouled wouldn't they be kind of moist and sooty? They are new plugs and have only been in for the new cam's break-in period.
Click to expand...

Closely look at the porcelain part between the hex and the tip where the wire goes on. If it has a slight crack, it can cause a misfire.
 
Here is a few pictures of my spark plugs. The top row is from the right bank and the front of the engine starts on the left side of the box. The next image with four plugs is #5-8. The last two images are from the blackest one, #6.

Thanks for your help. I have change out at least three sets of plugs with the exact same result.

Spark plugs 001.jpg


Spark plugs 002.jpg


Spark plugs 003.jpg


Spark plugs 004.jpg
 
No, I have triple checked the plug wires. Thanks anyway. Its easy to overlook the obvious sometimes.
 
Another issue I did'nt think to mention is that whenever I hook the vacuum advance up on the distributor it stalls the engine out very quickly.
 
Who rebuilt teh carb? What model Thermoquad is it? What cam is in the engine?
 
Green Hornet said:
Another issue I did'nt think to mention is that whenever I hook the vacuum advance up on the distributor it stalls the engine out very quickly.
Click to expand...

To which side, ported or timed? I would check the timing to see if it is advancing. Maybe the plates are stuck. Let us know what yout initial and total is and what rpm your total is at.
 
The carb was a street strip build by Demon Sizzler. I would have to check on the model, but if I remember right it is a 1972. The vacuum is hooked in to the ported side I believe (front left next to PCV in center) Initial timing is set at 16 with total in at 33 by 2800. I will double check the rpm for total advance. As far as the cam I am using Comp XE 275 with .525 lift and duration at .050 at 231 Int. and 237 Exh. I have 1.6 Crane roller rockers, but for right now still have the factory 1.5 rockers in for the cam break in. I'm using a M1 single plane intake with Eddy heads. MP distributor with chrome box and a MSD Blaster coil.

From the plugs pictures can you determine whether the black fouling is from carbon (fuel) or oil?
 
Looks like fuel but the mix doesnt look bad. Misfiring cylinders look like that too. What rpm is it idling at? Is there vacuum present at the dist vacuum port with the carb at idle?
 
I am running a 4 speed so I set the idle up around a 1,000, right or wrong. There is vacuum at the port, but I don't own a vacuum guage at this point to give you a value. It certainly doesn't seem overly strong.

Thanks for sticking with me here.
 
If you have a different carb, throw it on and see what happens.

Demon Sizzle knows what he is doing!..........But hay sh*t happens.

Also try a different coil. Even the stock one, if you still have it.

Look at the fire wall loom and see if there is any wires going through there that look like they have been hot(insulation blackened or melted).

If so wiggle that wire and the same wire under the dash. If it fixes you problem, you have some work ahead of ya...............
 
Your idle is way too high. You're probably already into the mechanical advance and you're not on the idle circuit of the carb. That will cause mixture issues and the adjustment screws are not as effective if at all. There should be no vacuum present at the ported vacuum port. My impression is the timing is way off. With your parts that engine should idle smooth as glass under 700 and be able to pull away from a stop just fine. You dont raise idle speed to make a stick easier to drive if that was the impetus. I would verify the TDC mark on the balancer for starters. Then make sure the distibutor drive is in properly, then set the idle speed for 600 and then set initial timing at 18°. Adjust the carb for best idle vacuum (gages are less than $10 anywhere) at 600rpm. Then raise the idle to 700 and test drive it. Thermoquads are very good carbs when properly set, and Demonsizzler is one of the best. But when things are off they can be very finicky and MP distributors usually need some sort of tuning. Which leads me to believe it's a simple tuning/setup deal.
 
I am not doubting you at all moper, so don't take my questioning the wrong way. Would a simple tuning issue cause the plugs to be so uneven?
 
You must log in or register to reply here.
-

Back
Top