EGT Low on two Cylinders

Looking for a little help here with my EGT readings, this is a problem that just cropped up.

BBC 468, 11.65-1, 8-71, Bird, 58 nozzles, 33% over, 24LBS boost, MSD 10.

I have two cylinders (#1 & #6) that go cold every time I’m on the throttle whether it’s in the burnout or going down the track. The two cold cylinders at click off are #1 422 degrees and #6 at 540 degrees. The other six vary from from 1137 to 1304 at the click off. I’m running 8 nozzles just in the hat and none in the port. Don’t think this is my problem. Have changed plugs, swapped plug wires, taken compression checks and all seems fine. No fancy cylinder to cylinder timing changes. EGT’s read the same on all the six at an idle or light load, the other two seem to be a little lower and just go cold on a heavy load and we are down on power and run about 3 tenths off normal. Changed EGT probes too. Only thing that comes to mind is that #1 and #6 are 180 degrees out? Lost on this.

 

birds may be 180 out, but i would pull the nozzles and lines
and flow check them with air and or fluid to see at what psi
the check ball and spring are starting to dribble(bleed) and at what psi
they actually open. compare them 2 cylinders to the others.

 

Sounds like a ignition problem. MSD boxs do fail. Replace the box or put a mag in it & the problem will go away with 99% certanity.

 

Might want to swap those EGT's with one from a cylinder that is not cold and make sure the problem is not with the EGT sensor itself. I have seen this happen before.

 

Thanks for the ideas. I have swapped EGT's and that is not the issue. I've changed distributors including the cap and rotor (in addition to all of the above) and that was no help. I'd think all cylinders would be bad if the MSD box would be bad but not sure. The one thing I haven't checked that is dynamic is the valve springs. Not sure if weak exhaust springs would cause it but without swapping out the MSD box or coils, it looks like my next course unless somebody has a solution for this. Really appreciate the help.

Tom

 

Do the plug readings from these two cylinders confirm that they are cold? (No cad being burnt or blue line)

 

I would swap the box and/or change the coil!!!

 

Bad magnet on the crank trigger?

Try closing up the gap on those two cylinders. If it pickes them up, change the coil ,but my guess is a weak magnet.

 

I agree with Bob on this one. When an ignition box has issues, it will pick on either two or three cylinders, and it will always be the same cylinders. Another interesting fact is that when using a two step with an MSD box, two or three cylinders will go colder than the rest (while on the two step), and it will always be the same two cylinders in that case, too! Put that ignition box on another car, and it will be different cylinders that go cold. I have no clue why this phenomena exists, but it is a truth that I have seen time and again. After re-reading, I agree that it could possibly be a weak magnet as well, 1 and 6 use the same magnet.

 

Mike,
On a friends dyno the MSD box went bad, a couple of cylinders were down on egt's & the engine wouldn't make any power. Found out the MSD box was not working right under a load & compression. Changed boxs & the problem went away.

 

Thanks to all,

After these posts, I went back and looked at some historical EGT data. Up to the end of last year we ran a locked distributor with an Allison opto-electric trigger that fired the MSD 10. It's similar to the Mallory Unilite and had been very reliable. I bought a new MSD distributor (Magnet pickup, not a Mag or a crank trigger) in the winter to replace this unit as Allison isn’t around and Crane had acquired their ignition systems. I didn’t want to be at a race with this setup and have no source for spare parts if it failed.

In the beginning of the year we noticed that we had a couple of low cylinder temps (can’t remember which ones and can’t find my EGT data for that time-frame). At the same time we were testing the MSD programmable computer that lets you retard on boost, RPM, time, launch and by each cylinder. Since this was a new setup and the temps were off, we disconnected this and changed the distributor and trigger back to what we had last year to get back to square one. After we put in the old unit and looking at the EGT data with this older trigger setup, #8, #5 and #6 were a little lower than the rest of the other cylinders. In the heat of a burnout this setup failed and we changed the distributor back to the new MSD distributor. At the same time we were swapping pulleys for some boost checks and I totally forgot about the lower readings.

Since we put the MSD distributor back in #1 and #6 have consistently have been low. Based on what I’ve seen, Travis’ and Bob’s explanations, while I don’t understand the why, is the only thing that makes sense. Looks like we will send the whole MSD system off to MSD for a check and I will seriously consider changing over to a Mag.

Tom

 

Tom,
Call Dave Leahy I'm sure he can explain the why part.
(740) 362-0251

 

Thanks Bob for the advice, I appreciate it.

While this probably (?) doesn't apply to my issue, I thought this may be of interest to folks that run a MSD rev control. This is from the MSD Pro-Mag site:

"What happens when the engine hits the Rev Limiter?
The Rev Limiter will randomly drop cylinders at the specific RPM and in most cases seen by a data recorder the limiter can lock-into a specific cylinder sequence. Meaning that it will drop or choose certain cylinders at that specific RPM. The EGT in those cylinders can remain cooler than other cylinders.
MSD Rev limiters have been around since the early 1980s in all forms of racing and since then we have not resulted in a damaged engine. If anything the MSD Rev Limiters have saved engines from damage due to countless broken drivelines and axles."

Tom

 

I don't know if I'm missing some part of this story, but it is common knowledge that using only hat nozzles will leave the complete fuel distribution to the manifold and the blower alone . Therefore some cylinders will run cooler than others.

I you are satisfied with your setup as far as top end GPM just divide your are in a 70% hat area and 30 % port nozzle and then go up and down the individual port size to get the right temp. Only thing to remember is to keep all nozzle area always the same. the 70% and 30% is just and arbitrary number I used to divide mine as it will keep more fuel on the blower ( cooler and better boost) so pump as much fuel as you can to thru the blower without loosing control of the cylinder temps.

Just my two cents!

 

Just because MSD says it's ok doesn't necessarily mean it's ok. You need to beg, borrow, or steal someone else's box to make sure before you go on another wild goose chase. I have sent several boxes back to MSD only to be told that they were working fine, then swapped to a new (or borrowed) box and the problems go away. That's why Dave Leahy is in business and most send brand new MSD mags to him before they ever see the light of day.