MACDAD260

I now have 8 hours on the 502 since the rebuild. I am using the Weber 750cfm carb from my 7.4L. I am using a square bore Edelbrock Performer RPM Rectangle Port Intake. I started out with the stock jetting. The first time I got on the throttle hard (just a 20 second blast after an hour of break-in) it seemed to hesitate a little and then pulled hard. Before the next time out I increased the jets 1 size on the primarys and 3 sizes on the secondaries. It ran better after that, no hesitation. I did a plug check on a couple of plugs after a 30 second WOT run. They had just a hint of brown. Seemed too lean so I went up 1 more size on each of the jets. Next time out it was too rough to do a sustained WOT run for a plug check. When I got home I pulled all the plugs just to see what they looked like. They were all an OK color of light brown except the #2 & #3. They were dark brown. Those cyclinders are both fed by the same runner on the intake. I swapped those plugs with the two lightest colored ones. I then went up again one more size on all the jets. I ran it again today and when I got home I pulled the plugs again. Same results as before. #2 & #3 very dark brown. I did a compression check and every cylinder was between 155 psi and 160psi. The motor seems to be broken in now. Good compression and very little blow-by coming out of the vent hoses.

Both times before I pulled the plugs the motor had idled for about 20 minutes while the oil cooled down. I had to wait for an opening at the launch ramp anyway. I know I need to do a WOT run and then pull the plugs to get a true picture of the carb settings.

My question is; What is causing just those two plugs to run rich? Would a 2" carb spacer help mix the fuel-air mixture a little more before it is sucked into the runners? So far this is the only problem I have other than my oil cooler is too small and the oil gets too warm (250-260* after about 10 minutes at 4000 rpm. I don't think that has anything to do with the plugs getting dark brown.

I appreciate any ideas you can give. Thanks, Doug.

Mbam

Doug, reading plugs (at least for me) is a black art. The only real way to get an accurate reading involves shutting the engine down at full throttle. Almost any part throttle operation or idling will change the reading. With today's fuels you are looking for a soot ring way down at the base of the porcelain, the plug will actually be white. There were a couple of good articles a number of years ago by a guy from Champion. I don't want you to burn your engine down based on my explanation, see if you can find a site with some pictures. Just to complicate things a blower motor can need extra fuel to help cool the pistons, so plug readings are pretty useless.

Madmax

So where are you at right now with your jetting? Primary jets/rods, secondary jets?

Edelbrock RPM is a very good manifold for fuel distribution, I would be suprised if it was the cause. Spacers on a good intake usually only bandaid problems with fuel distribution elsewhere....

MKast

My first though was, check resistance in the # 2 and 3 wires, are they the same as the other wires? Next, can you borrow a different carb for testing? With a different carb, is the problem still there?

MACDAD260

Thanks for the help, guys.

First of all, I know I am not doing the correct procedure for getting a plug reading without shutting down at full throttle. I plan on doing that as soon as water conditions will allow it.


My carb settings are as follows;

Primaries- .116

Secondaries- .113

Rods- 6542

Spring- Silver

The largest jets offered are .119 so I can't go up much more.

My timing is set at 35* at 3000 rpm.


I thought of plug wires too but have not checked them yet. Good idea. They are new but, they that doesn't mean they are good. The carb might be the problem. The starboard primary feeds the runner that feeds the #2 & #3 cylinders. Maybe it is dripping too much fuel at an idle and causing the plugs to soot up. I guess I have some work to do the next time out to check these things out.

Thanks again. I will keep posting my results as they occur.

See ya, Doug

jdnca1

MACDAD;

I run a 632 with a dart tunnell ram and 2 775 race demons. My carbs are 99.5% dialed in and I still get uneven plug readings with this set-up. In my opinion the only way to get all of your plugs to read "exactly" the same is to run one of the MSD "pro-stock" ignition boxes that allow you to run different ignition timing on each cylinder...or go EFI. Your plugs sound fine to me, I always look for a "medium brown" shade on the porcelin. I would err on the darker side to be safe, if you were drag racing then maybe it would be worth it to err on the lean side.

MKast

One thing popped in mind, thirty years ago I ran into Holley carbs had a projection of metal, a square, stuck on the flow disrupter on one primary and the kitty-corner secondary barrel. I was told they were there because of the strange breathing characteristics of the BBC. I know that your using a Weber, but it's still something to think about. Maybe your engine is going to run rich on those two cylinders. I wonder if this is helping or confusing.

turbo2256

Some of the problem can be fixed by adding steam llines

MACDAD260

Steam Lines? Dude, what you smokin?

Actually, the more I think about it, I believe the problem is excess fuel dripping into that runner from the mixture screw during a long idle period while waiting at the ramp. I was adjusting the mixture a couple of weeks ago and had a problem with the screw on that side. The tip broke off in the carb. I don't know how that happened except that maybe the screw was turned in too tight and the tip jammed and then broke off when turning the screw out. I had to take the carb off and push the broken tip out from the inside. The seat may be distorted now and allowing extra fuel to get by the new adjuster tip. I am going to check that out next.

Thanks, Doug