351c Won't RUN after Rebuild- carb backfire - Page 3 - Vintage Mustang Forums
 6Likes
Reply
 
Thread Tools
post #31 of 39 (permalink) Old 09-12-2019, 05:02 PM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
1- 180 5-180
2-180 6-175
3-170 7-160
4-165 8-160

Well, those rear numbers are not what we hoped for. I'm somewhat curious if we could have cooked anything last Friday. It did get to 240, but that wasn't for a very extended period of time.

Leak down test next?

As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
Sponsored Links
Advertisement
 
post #32 of 39 (permalink) Old 09-12-2019, 08:58 PM
Member
 
cheapbastard's Avatar
 
Join Date: Nov 2016
Location: michigan
Posts: 55
I do think the pertronix is compatible with the msd and I don't think that has anything to do with your problem. You ran it long enough to break in the cam. I run a similar setup and I had a consistent miss until I grounded the negative side of the coil to each head. As has been said the next step is a leakdown test or if you don't have a leak down tester rigging something up to push air into the cylinders to see where you get a leak. If you don't have a leak down tester you can take the needle out of a compression tester fitting and just hit it with air and see where air leaks on each cylinder. If you have sticking valves you should be able to hear it.

It sucks to get old but at least the expectations are lower. 65 mustang coupe, 306, 10.4 compression, 180cc homeported Chinese heads, funnel web, t5z, 4.56 gears in a 9 inch locker
cheapbastard is offline  
post #33 of 39 (permalink) Old 09-13-2019, 03:24 PM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
Leakdown test shows all noticeable leakage to be going to the crankcase. My assumption is that the rings have not fully seated yet.

We're going to reassemble and bypass the msd box for now and see how that helps.
Woodchuck likes this.

As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
 
post #34 of 39 (permalink) Old 09-19-2019, 04:35 PM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
So, after checking with the scope the cylinders look fine (well, I only checked 2 for now, including the #4 which had the lowest compression), but I'm trying to tell if I am looking at a crack in the piston or if some crude is just positioned to look at that way.

We bypassed the msd and the backfires went away. That said, it didn't run great. After scoping the cylinder my student told me that the rear plug was only hand tight. So, I"m hoping to tighten it back up and run it again, but I cannot deny that the piston looked like it had a crack. I need to figure out how to get a picture from the scope on to here.

As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
post #35 of 39 (permalink) Old 09-19-2019, 04:50 PM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
Ok, so she idles roughly at 800rpm and after revving the motor a bit the vacuum sits fairly steadily at 13-14 and then after letting it sit a bit, it starts doing that erratic bouncing between 5-15. So, it's like it runs ok for a bit and then begins to have issues.*

As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
post #36 of 39 (permalink) Old 09-19-2019, 06:31 PM
Member
 
cheapbastard's Avatar
 
Join Date: Nov 2016
Location: michigan
Posts: 55
I'll let someone with more experience than me give their opinion but to me that seems to indicate a valve or lifter issue as has already been said. I don't think the difference between cylinders is that great of a deal. When you say all the leakdown is going to the crankcase did you measure it or that is where you can hear the air escaping. Some percentage of leakdown may be normal if the engine is cold or depending on where the pistion is. Does it huff out of the breathers or dipstick tube when running? If noticeable pressure is built up in these areas that is usually a sign of bad rings or bad piston. I don't think that is the issue but I await what others say.
Still I think this is a good trouble shooting exercise for your students. It will help them understand the basics.
Good luck!

It sucks to get old but at least the expectations are lower. 65 mustang coupe, 306, 10.4 compression, 180cc homeported Chinese heads, funnel web, t5z, 4.56 gears in a 9 inch locker
cheapbastard is offline  
post #37 of 39 (permalink) Old 09-20-2019, 01:07 PM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
Ok, here is the youtube video from our scope of cylinder 4. Looks like a crack to me?

What do you all say? Is it time to just pull it?

Truth be told, I've been wanting to build an engine test stand....


As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
post #38 of 39 (permalink) Old 09-20-2019, 10:22 PM
Member
 
Join Date: Jul 2016
Posts: 68
Looks like foreign material, possibly aluminum, embedded in the top of the piston. Went through something similar myself.



I had a '71 351C 4V, that had damage to the piston top, and matching damage to the quench surfaces of one cylinder. Engine ran fine, but sometimes would pop the dipstick after an extended high rpm run. Leakdown testing didn't show any problems, and the cranking pressure was over 200psi in all cylinders.


When i disassembled the engine, i got my first look at the damage, and found both compression rings seized in their grooves from the ring lands collapsing. Was very lucky the cast piston didn't break.


If I am seeing your scope video correctly, I think you need to pull that head and have a better look.
Precision Transmission is offline  
post #39 of 39 (permalink) Old 09-30-2019, 11:51 AM Thread Starter
Senior Member
 
Join Date: Jan 2002
Location: Grandview, WA
Posts: 868
Well, the video from our cam was not very good. We pulled the heads and the pistons look fine. Cylinder walls look fine. Bearings look ok. We didn't pull the came yet. On the one piston we pulled (that had 160 compression) the compression rings were only 20 degrees out of phase, so that maybe caused some of the problem?

As iron sharpens iron, So a man sharpens the countenance of his friend.

See my Christian T Shirts!
gijoe985 is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Vintage Mustang Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome