Reading pistons...

R

revkev6

Senior Member
I have an 8ba with 30 over pistons and small chamber edelbrock heads. I don't remember specifically what it has for pistons but I'm pretty sure they are a stock ford style cast piston. Motor was rebuilt 25 years ago and was well cared for with low miles... I pulled it down this year because the ****** needed some work and I figured now would be a good time to clear up all the leaks and put new gaskets in and run the valves. Well she's clean inside with nice cylinder walls with cross hatch still but... but the pistons look like she's burning oil? Oh she gets 140-152lbs in a compression test

What would you do with it?
 

Attachments

  • 20170225_164352.jpg
    20170225_164352.jpg
    49 KB · Views: 184
  • 20170225_164402.jpg
    20170225_164402.jpg
    49.4 KB · Views: 186
No... it would run rich on startup and burn real black but then run clean except it smelled a bit like coolant which I attribute to low torque on the headbolts
 
Rich for sure.. but the clean piston edges are indications of oil burning correct? I don't really want to pull this motor apart for another 20 years
 
It looks like #6 & #7 are much worse than #5 & #8? i.e., right under the carb? And on the lower side? I'd tend to think that's gas washing.
 
It has a offy regular dual on it running two 97s with .043 main jets. Shouldn't be running rich enough to wash the piston tops? Carbs are getting rebuilt as well during this process. I am running the horrible paper helmet filters that seem to choke it out. Want to tune it without them
 
What distributor are you usning? The stock one worn't work and if it's an aftermarket unit what is the advance curve look like.
 
Ron, it's a mallory dual point that was new when the car was built. I gave it a tune up 10 years or so wit new points condenser and coik. I also got the advance kit and set it up to be all in by around 2k. Forget how much advance though.
 
It is not easy to read the situation. Most performance experts will tell you the clean portion of the pistons indicates the areas where complete, proper combustion is occurring. Your engine looks like several I have disassembled during dyno testing and I would not do anything except put it back together.

Since the specter of over-rich conditions has been raised the investment in a Air/Fuel ratio meter might help you with tuning decisions. I would also encourage you to look more carefully at the total advance being provided by the Mallory.
 
It is really hard to just look at the tops of pistons and determine how it was/is running. We have no idea how long it was idling, how much under load, etc.. It is similar to reading plugs - you can't put them under full-throttle load condition, then drive home and check them . . . the color at that time means nothing. My best guess is that it appears to be rich, so as JWL said, put some O2 sensor bungs about 24 - 36" back from the exit of the exhaust from the block, make sure you have a tightly sealed exhaust system (no leaks) and put a AFR meter on the car that can display your true AFR under different running conditions. Also, make sure you have the correct amount of total advance - and that it is coming in when it should. I run about 24 - 26 degrees total on my flatheads.
 
I know I have carb issues. I'm using the stock model A tank with the 8ba pump and it will gravity feed into the motor if I forget to turn the fuel off. My brother did this once and filled the crankcase! Luckily it was caught the next morning and the oil was changed twice. It's hard starting after it sits for 15 minutes which I suspect is from gas draining into the cylinders. I'm already planning to rebuild the carbs with the Stromberg S jets and a little bit leaner powervalve. Although if I take the air filters off it stumbles badly so it can't be too rich?

JWL if you say the pistons are ok I guess that's good enough for me. I do remember reading something about flat head pistons with this clean edge indicating oil burn.
 
Definitely running rich, I'd also say the center cylinders are getting raw fuel to add to the problem, making a complete burn impossible. I'll also bet your oil has a gasoline smell to it.
 
Little hard to understand how the fuel is draining into the engine with the gravity feed. The needle valves in the carbs should be blocking any fuel, so my guess is there is something wrong with the carbs. Something is allowing the fuel to drain past the float bowls and that's not normal. Not sure how much fuel will flow with this gravity feed, but it may also be fuel starving on the top end.
 
JSeery said:
Little hard to understand how the fuel is draining into the engine with the gravity feed. The needle valves in the carbs should be blocking any fuel, so my guess is there is something wrong with the carbs. Something is allowing the fuel to drain past the float bowls and that's not normal. Not sure how much fuel will flow with this gravity feed, but it may also be fuel starving on the top end.
Click to expand...

Also how does the fuel also get by the valves in the fuel pump??

R
 
Ronnie said:
Also how does the fuel also get by the valves in the fuel pump??

R
Click to expand...

my understanding of they way the fuel pump works is that they are one way valves operated by vacuum diaphram. I've never tried but I would imagine the one way valves could allow slow seepage past them in they direction of flow. if you put an electric pump in line with a mechanical pump wont it push right through if the mechanical is not working??
 
You must log in or register to reply here.
Back
Top