#7 Cylinder Much Cooler

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73Swinger18

73Swinger18

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Joined
Aug 15, 2016
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Location
Camarillo, CA
The engine...
  • Original 318 bored 0.020
  • Original heads with larger valves (1.88/1.60)
  • Sealed Power flat top pistons
  • Holley 600
  • Weiand Stealth intake manifold
  • Comp Cam 20-223-3 (XE268H)
  • Hedman long tube headers with dual Thrush mufflers
  • Autolite 63 Spark Plugs
The issue...

My #8 intake valve stem tip mushroomed. Not sure why it did, there was no other signs of damage to the rocker or other components. I took both heads back to the machine shop under warranty and they redid the valve seats and measured the stem heights. After I installed the heads and during the tune up, I put my IR heat gun on each header tube. The #7 tube measured around 220F. The rest measured around 450F (in drive, idling in the driveway). The #7 spark plug looks brand new except for the timing mark on the ground electrode. There is absolutely no sign of fuel on the base ring. The insulator is bright white. I checked for spark with a gap spark tester and it had a nice bright spark. I put a new plug in and no difference. Compression checked good. I pulled the valve cover off and marked my push rods and cranked the engine. They all spun and the rockers went up and down as usual. With my finger over the spark plug hole while cranking, it produced strong suction/pressure. I'm worried that there is not enough fuel getting to this cylinder. I pulled the carb and made sure the transition slot was square. I'm out 1.5 turns on the mixture screws and idling in drive around 700 RPM. Vacuum is 8" in drive. The only thing I changed from my previous setup was that I did not block the heat crossovers this time. I can't recall measuring the temp on that tube before though either. Should I be concerned with a 200 deg split on this cylinder? Thanks!
 
I am not at all convinced that 230F difference is OK for this level of cam....I have not seen anything like that amount on a Crane Z268 cam.

OP, I would 1st go back to the the exhausts and listen closely to each on individually at idle.... you may be able to hear the left bank have a bit of a miss. If so, then look at the #7 lifters; one may be not holding up properly and not opening the valve fully.

This item is catching my eye: 8" manifold vacuum for that cam at that idle speed looks low. It should be easy to get up to 11-12" for those conditions. Perhaps there is a vacuum leak....or the secondary idle stop screw is opened too far.
 
mech1nxh said:
block casting #? would be a great help, Thx !!!
Click to expand...
IMG_20170811_065832.jpg
 
nm9stheham said:
I am not at all convinced that 230F difference is OK for this level of cam....I have not seen anything like that amount on a Crane Z268 cam.

OP, I would 1st go back to the the exhausts and listen closely to each on individually at idle.... you may be able to hear the left bank have a bit of a miss. If so, then look at the #7 lifters; one may be not holding up properly and not opening the valve fully.

This item is catching my eye: 8" manifold vacuum for that cam at that idle speed looks low. It should be easy to get up to 11-12" for those conditions. Perhaps there is a vacuum leak....or the secondary idle stop screw is opened too far.
Click to expand...
I'll listen to it today real good. I didn't notice any miss last night. I inspected each lifter and cam lobe when it was disassembled. No defects were noted. With the intake I have, I did notice that not all the plugs were colored the same, I assume due to runner length/design. I do open my secondary throttle plates a bit to get the RPM up and keep the primary transition slots square. Do you see any benefit of drilling the primary plates and closing the secondaries up? I wonder if it would help mix the fuel/air better.
 
There is nothing sacred about the slots being square; it is a good starting point; each engine is different so it gets adjusted from there. The idea is that you may be getting too much air through the secondaries at idle and that is making things leaner in the back and lowering the idle vacuum. I would close it up a bit and then readjust the primary idle speed and mixture settings to see where things were. You don't have a big engine or cam so I would think this would adjust out quite well with no added primary holes.

As for the low vacuum, what PCV do you have? A stock type is going to be opened up to the cruise flow opening at that vacuum level and letting too much air into the intake manifold. If you have a stock type PCV, then that needs to be changed before you do anything else.
 
nm9stheham said:
There is nothing sacred about the slots being square; it is a good starting point; each engine is different so it gets adjusted from there. The idea is that you may be getting too much air through the secondaries at idle and that is making things leaner in the back and lowering the idle vacuum. I would close it up a bit and then readjust the primary idle speed and mixture settings to see where things were. You don't have a big engine or cam so I would think this would adjust out quite well with no added primary holes.

As for the low vacuum, what PCV do you have? A stock type is going to be opened up to the cruise flow opening at that vacuum level and letting too much air into the intake manifold. If you have a stock type PCV, then that needs to be changed before you do anything else.
Click to expand...
I did notice that with the secondaries closed, I could get it to idle nice and smooth real low in RPM. I will try some adjustments on the carb again. I will also try removing the PCV all together and see if that helps. Below is the NAPA PCV valve I have. It crossed to the Fram PCV valve a lot of people recommended, I just can't remember that PN.
IMG_20170811_072706.jpg
 
I wonder if the brake booster which is plugged in right near that cylinder on the manifold is attributing to this.
 

73Swinger18 said:
It's a Holley 4150.
Click to expand...


Like an 1850 Holley or a different number?
Is this a new carb, or a proven one?

I ask because I've seen carbs get through with an idle feed restriction drilled too small, or not drilled at all. I've also seen a few where the idle air bleed was bigger on on Venturi than the other three.

Check everything.
 
yellow rose said:
Like an 1850 Holley or a different number?
Is this a new carb, or a proven one?

I ask because I've seen carbs get through with an idle feed restriction drilled too small, or not drilled at all. I've also seen a few where the idle air bleed was bigger on on Venturi than the other three.

Check everything.
Click to expand...
With the brake booster being plugged in right on top of the #7 cylinder, I pulled and plugged the line. Fixed it. The temp of that cylinder climbed up to match the others. Plug the booster back up, the temp dropped. I thought I had crappy brakes the whole time due to low vacuum. She runs so much better now and is much easier to make adjustments. I'll be changing the booster this weekend. Thanks everyone for your help.
 
73Swinger18 said:
With the brake booster being plugged in right on top of the #7 cylinder, I pulled and plugged the line. Fixed it. The temp of that cylinder climbed up to match the others. Plug the booster back up, the temp dropped. I thought I had crappy brakes the whole time due to low vacuum. She runs so much better now and is much easier to make adjustments. I'll be changing the booster this weekend. Thanks everyone for your help.
Click to expand...


Good catch. I haven't run power brakes sine 1981. It's very easy to convert to manual brakes. If you have to fix it, I'd consider converting it to manual brakes.
 
Good story. This goes to show some of the things that an IR gun can do for you. A valuable tool that is pretty cheap to buy today.
 
73Swinger18 said:
With the brake booster being plugged in right on top of the #7 cylinder, I pulled and plugged the line. Fixed it. The temp of that cylinder climbed up to match the others. Plug the booster back up, the temp dropped. I thought I had crappy brakes the whole time due to low vacuum. She runs so much better now and is much easier to make adjustments. I'll be changing the booster this weekend. Thanks everyone for your help.
Click to expand...
Awesome.... let us know where your vacuum level is now!

BTW, your PCV crosses over to a Fram FV191; that valve changes from cruise to idle opening somewhere in the 10-11-12" range (from what I can tell). So if your idle vacuum is up above that level, then the PCV will be closed down to the idle setting.
 
Okay in the future if anyone suspects a bad cylinder/weak hole...go take a plastic handled screw driver, flip it handle out and with the motor running see if when you touch each header tube nearest the head that the handle slides like melted butter. The ones that don't and kinda stick and or drag...those have a problem.

Glad you found the issue op.
 
nm9stheham said:
Awesome.... let us know where your vacuum level is now!

BTW, your PCV crosses over to a Fram FV191; that valve changes from cruise to idle opening somewhere in the 10-11-12" range (from what I can tell). So if your idle vacuum is up above that level, then the PCV will be closed down to the idle setting.
Click to expand...
I'm not running the PCV anymore. My vacuum is back at 10", where it was before I removed the heads. The booster will be ready for pickup in the morning. Have a great weekend.
 
OK. Just know that your oil will foul faster and internal parts will wear faster. The PCV helps clean out the internal moisture, which is a contributor to acids forming in the oils. Keep an eye on the underside of your oil filler and down through the breather holes in the valve covers and look for any tan or grey gunk.... those are definite signs of excess moisture in the crankcase.

And, 10" vacuum still seems low for that cam and carb. (I'd expect around 12-13" at 850 RPM as a point of reference.)
 
nm9stheham said:
Awesome.... let us know where your vacuum level is now!

BTW, your PCV crosses over to a Fram FV191; that valve changes from cruise to idle opening somewhere in the 10-11-12" range (from what I can tell). So if your idle vacuum is up above that level, then the PCV will be closed down to the idle setting.
Click to expand...


Just curious, what factory type (smaller stem) PCV would you recommend for 8-10" of vacuum at idle?
 
I've been back and forth with the PCV. Didn't have one, used one, back to not using one. I started a thread a ways back talking about all the pros/cons. It's just one less variable for me to worry about when tuning. When it comes to vacuum operated items, I think less is more. If I didn't already order that new brake booster, I would have gone to manual brakes too. Maybe a project later on. As for my vacuum reading...I may buy another gauge. The one I have is from Harbor Freight and I've had 1-2" of variance between their gauges. If I raise my RPM to 850, I get 11". Right now I'm sitting just under 800.
 
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