TheAMCForum.com Homepage
Forum Home Forum Home > The Garage > AMC V8 Engine Repair and Modifications
  New Posts New Posts RSS Feed - 1970 AMC 390 oil smoke from RH bank
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Click for TheAMCForum Rules / Click for PDF version of Forum Rules
Your donations help keep this valuable resource free and growing. Thank you.

1970 AMC 390 oil smoke from RH bank

 Post Reply Post Reply Page  <123
Author
Message
billd View Drop Down
Moderator Group
Moderator Group
Avatar
Forum Administrator

Joined: Jun/27/2007
Location: Iowa
Status: Offline
Points: 30191
Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2020 at 7:54pm
Originally posted by Ken_Parkman Ken_Parkman wrote:

What intake is on it?

The good compression would indicate there is not a gross cylinder wall problem. I have seen intakes binding and not sealing correctly, and a consumption problem specifically on adjacent cylinders 6 and 8 may mean some kind of a gasket sealing problem in that corner of the intake, and then the vacuum at idle or other higher vacuum condition is sucking oil from the lifter valley.

If you still have the old intake gasket look for signs of incorrect sealing at 6 and 8 intake runner pair. Maybe the sheet metal is not correctly crushed? Look for the intake binding on the valve cover shoulder and not seating right.


I used plastigage and dry-fit an intake on the 390 I had and found the upper part smashed out flat and the bottom ends of the plastigage was hardly touched. Bingo - there was the issue. Angle was off, the intake hit at the top and hardly at the bottom of the ports. 
I had everything clean, put the plastigage along the ports top to bottom. 
Back to Top
pinkta View Drop Down
AMC Fan
AMC Fan
Avatar

Joined: Jul/26/2014
Location: SASKATCHEWAN
Status: Offline
Points: 12
Post Options Post Options   Thanks (0) Thanks(0)   Quote pinkta Quote  Post ReplyReply Direct Link To This Post Posted: Mar/24/2020 at 8:35pm
Thank you to all who have participated in this query. I have a lot of new direction and will be on this as soon as I can. 

When looking at the old intake gasket there is no obvious area around 6/8 ports the is not compressed to the eye. before i go all the way on this, i will plastigage the manifold fit and see what I get. 

I was going to pull the engine, but i agree, pulling the RH head is a lot easier step to take as far as learning what is going on. that will be step 2 of the "next to happen" sequence. 

Thank you again, i fell I am going in the right direction with guidance from those who know. I was a GM dealer for 30 years, but not a whole lot of engine work, especially on AMC's. My first car was a 70 AMX, the passion has not left town. 

Thank you all one more time, I will keep this thread posted. If there are more relative ideas, i will be happy to act on them.

Glenn
Back to Top
gmonde View Drop Down
AMC Apprentice
AMC Apprentice
Avatar

Joined: Jul/06/2010
Location: unionville
Status: Offline
Points: 87
Post Options Post Options   Thanks (0) Thanks(0)   Quote gmonde Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2020 at 6:03pm
Originally posted by 401MATCOUPE 401MATCOUPE wrote:

Do you have records on rebuild???  1970 390 had huge piston breakage problems from AMC, my 70 AMX broke a piston at 14,000 miles.  you could have great compression with a broken oil control ring.....Was the rebuild with pistons and overbore?  or maybe original pistons with new rings????   I had buying cars with "rebuild" engines, 90% of the time I have to pull it back apart to fix common errors.......patiently trouble shoot this, alot of good folks on here to help you look at a lot of angles.  Do you have the receipts from the previous work????  That would be a great place to start. 

yep my thoughts exactly, most off the shelf pistons for 390 where cast,, if its a actual 70 390
532 intake
529 block
291c heads

the compression is over 10.5-1 and pump gas is not is friends with this engine,, i broke a few sets of cast piston ring lands 
Gmonde

www.gmondeperformance.com

1970 rebel machine G/SA

11.23 @ 116.00

1970 amx 390 E/S
10.96 @ 120.96
Back to Top
ccowx View Drop Down
AMC Addicted
AMC Addicted


Joined: Nov/03/2010
Location: Yukon
Status: Offline
Points: 3012
Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Mar/25/2020 at 10:29pm
I am also one that has broken a piston or two in my day in a 1970 390. Not tolerant of detonation! In your case I am inclined to think that you are looking at broken ring lands. I am a bit puzzled by your even compression but you are losing oil for sure on that cylinder and there is lots of blow by, so I would expect you to have one or two pistons with something broken. 

One thing I wondered in looking over this thread, the static compression in this engine seems to be a bit low. 155 seems to be quite low all round, I would expect that from an 8:1 smog motor, not a high compression engine from the 60's. Mine has around 25 pounds more than that. It is stock oem cast pistons other than being .030 over. Compression is around 10.4:1. 

Do you think you have a ring issue all round, perhaps wrong rings or some other problem with the rebuild? The low compression and excessive blow by might be explained by this. I wonder if you might have a broken, mis installed or even missing oil control ring on #6 also?

Chris 
Back to Top
Boris Badanov View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Dec/14/2013
Location: NH USA
Status: Offline
Points: 3783
Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2020 at 3:52am
1970 390 pistons are at best good paperweigts.
I broke a few dozen.

My guess is broken pistons
or a botched back yard overhaul.

Poorly fitted rings or especially the oil rings
missing or sticking will do this.

Either way you have at least one broken piston.
AMC pistons aint bad but the 1970 390 was
an exception, they were really not up to the task.
Gremlin Dreams
Back to Top
303Y03 View Drop Down
AMC Apprentice
AMC Apprentice
Avatar

Joined: Oct/12/2018
Location: NY
Status: Offline
Points: 53
Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Y03 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/26/2020 at 10:10am
There were some very poor 1970 390 cast piston made by Badger 20-30 years ago.
They were known to break from the top ring up from detonation.
Have some here.
Back to Top
pinkta View Drop Down
AMC Fan
AMC Fan
Avatar

Joined: Jul/26/2014
Location: SASKATCHEWAN
Status: Offline
Points: 12
Post Options Post Options   Thanks (0) Thanks(0)   Quote pinkta Quote  Post ReplyReply Direct Link To This Post Posted: Mar/29/2020 at 3:59pm
Hello to those following this thread....I have decided to go right through the engine this fall (hopefully)and ensure it is done right at least once. This new reality of life has made it so i better not start as God knows when i can finish with all the shops shut down. 

I am in complete agreement that at least one piston is broken. i also suggest the compression is low for a 10.5/1 engine. as someone stated, a lot of things tell me folks that were not sure what they were doing are involved in this rebuild 5 years ago. I will not go that route.

   Can I ask what pistons would be recommended to put in when i do get to this?? I am a fan of forged but? I am also thinking lowering compression might be the way to go? I'll also be going through the heads, anything to ensure I do there? 

Thanks again everyone, I truly appreciate the knowledge on the forum and plan to work with you folks come rebuild time. 
Back to Top
ccowx View Drop Down
AMC Addicted
AMC Addicted


Joined: Nov/03/2010
Location: Yukon
Status: Offline
Points: 3012
Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Mar/29/2020 at 6:07pm
I would not neccesarily do the forged thing, unless you are seriously interested in performance. Cast are generally lighter. They should be fine and will save some money. I would not worry too much about the comments on cracking pistons, just make sure the timing is good. I have run a set of stock cast oem types for a decade now, with numerous trips to the drag strip and no problems. 

As for compression, bear in mind that the camshaft will make a big difference here too. I am running a Group 19 cam which has lots of overlap and may help a bit. All that being said, you might consider getting a set of later large chamber heads and use that to lower compression. It is a lot easier to swap heads than pistons and they flow slightly better out of the box anyway. 

Further thoughts on the compression thing, I have 10.4 to one compression, calculated on the overbore and all that and I have no problems. You can limit the total advance and how long it takes to come in. Many people want to have it all in very fast and as much as possible. AMC v8's like anything from 34-38 total at WOT typically. I run 91 octane no ethanol and I have a total of 36 degrees. I could probably do 38 but I prefer to err on the side of caution. Timing is similarly a bit slow to come in by some standards. I still can turn a 13 second quarter on stock tires like that. Lots of inital and a fast advance curve generally help when it comes to low end and off the line, which is less important for most of us since we are just incinerating tires for the first 50' anyway! Ease off on the timing and buy yourself a margin of error. 

Another thing to consider is that you may not have as much work to do as you think. The bores sound like they are good and the bearings may well be also. If they are not damaged then really all you need to do is pull it apart, check all the clearances, put in new pistons and reasemble it correctly. Balancing is also obviously a good idea but overall you are pretty much able to do much of it yourself at home. 

It really depends on what you want, but if a stock rebuild is the goal, you are probably most of the way there and could still make cruising season, assuming you don't have any more ugly surprises! Now, let's just hope there is a cruising season...

Chris 
Back to Top
pinkta View Drop Down
AMC Fan
AMC Fan
Avatar

Joined: Jul/26/2014
Location: SASKATCHEWAN
Status: Offline
Points: 12
Post Options Post Options   Thanks (0) Thanks(0)   Quote pinkta Quote  Post ReplyReply Direct Link To This Post Posted: Mar/30/2020 at 11:35am
Hi Chris
That is exactly the stuff i need to know. Really appreciate the time taken. I am not a hot rodder and have every intention of going back to stock as the engine is original and all the part numbers line up. My summers are fairly booked up for a retired guy, so I am likely going to make this a fall project. I know the shop i would take it to is closed right now. 

Because I really do not know what I have, I am going to do it completely over again. 

I agree, social distance and cruising are not compatible....

Thank you
Glenn
Back to Top
Boris Badanov View Drop Down
AMC Addicted
AMC Addicted
Avatar

Joined: Dec/14/2013
Location: NH USA
Status: Offline
Points: 3783
Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Mar/30/2020 at 5:41pm
Sucking on a Corona BEER and thinking that cars cant get the dang virus.
So why stop? Just nail it and hang on.
Gremlin Dreams
Back to Top
 Post Reply Post Reply Page  <123
  Share Topic   

Forum Jump Forum Permissions View Drop Down



This page was generated in 0.125 seconds.
All content of this site Copyright © 2018 TheAMCForum unless otherwise noted, all rights reserved.
PROBLEMS LOGGING IN or REGISTERING:
If you have problems logging in or registering, then please contact a Moderator or