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Discussion Starter #21
Update-

Ok, so I'm not 100% sure we have a vacuum leak now, but I thought we did.

Our 351c is freshly rebuilt, by high schoolers mind you.



We have a Pertronix Flamethrower Ignitor 1 distributor(D134620) with matching coil.*

Crane cam H-278-2 with matching lifters.*

Stock pushrods, rockers, etc. Stock springs, all set to correct height by our machine shop.*

Currently running the stock intake.*

Right now she is hard to start, we get some backfires out of both ends. I retarded the timing some and it seemed to help. We set base timing at 10 degrees BTDC. We droped a new carb on it it it is getting good fuel now. She doesn't want to idle very well below 800 rpm.*

Once it is a little biy more stable I plan to try a power balance test to see if all the cylinders*are firing equally.*

We have not done a compression test as of yet.*

We did set the adjustable timing gear to 2 degrees BTDC as we were suggested after looking at our cam specs.*



On thing I am concerned about is the valvetrain geometry and pushrod lengths. I don't think they should be a problem, but maybe? If valves weren't seating, that would explain backfires.*

We can do a compression test today...*
 

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Discussion Starter #23
<iframe width="560" height="315" src="https://www.youtube.com/embed/1YOW2qYfs6U" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 

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a) Are you using the correct firing order?

b) If so, I believe you have a valve "issue".

I'd go next to a compression test followed by a leak down test.
 

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Discussion Starter #26
a) Are you using the correct firing order?

b) If so, I believe you have a valve "issue".

I'd go next to a compression test followed by a leak down test.
We're definitely good on the firing order. So I agree that compression test is next. :)
 

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a) Are you using the correct firing order?

b) If so, I believe you have a valve "issue".

I'd go next to a compression test followed by a leak down test.
Agreed, the rapidly swinging needle usually indicates a valve problem. I'd do a compression test, but I'd also check the lifter preload. IIRC, you put in an aftermarket cam with the factory bolt-down rockers. If you have too much preload, the valves might be hanging open. You can fix that temporarily with shims under the rocker stands. A permanent fix would be the Crane retrofit kit and some B351 rocker stands.

https://www.summitracing.com/parts/crn-52655-16/overview/
 

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Discussion Starter #28
So, we just did one rocker and what I've noticed is that once we hit zero lash, I can tighten the the bolt until it goes down another ~.033". But I also noticed that it is the valve that opens, not the push rod going down and the lifted compressing. So, I cannot tell how much the lifter is compressing, but the valve is definitely opening. I calculated by the amount I turned the bolt, but I may go grab a dial indicator now and measure more precisely.*
 

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Discussion Starter #30
A THEORY-
We are using an MSD 6A ignition box with the Pertronix distributor. They are supposed to be compatible. From what I read online, people have had issues with the MSD box arcing inside the cap and firing cylinders prematurely. Thoughts? Once the compression test is done we can bypass the msd box for a bit.

3 Cylinders down-
1=180
2=180
3=170.... so we may com back and investigate this one more. Check the lash/preload.
 

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Discussion Starter #31
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?
 

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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.
 

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Discussion Starter #33
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.
 

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Discussion Starter #34
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.
 

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Discussion Starter #35
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.*
 

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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!
 

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Discussion Starter #37
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....

 

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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.
 

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Discussion Starter #39
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?
 
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