Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 04-08-2014, 06:11   #1
Registered User

Join Date: Oct 2013
Location: Kingston, Ontario, Canada
Boat: Columbia, 8.3 and 8.7 (1977)
Posts: 153
MD7A Compression

I recently purchased a used condition unknown md7a to replace my seized md6b. Before I install it, I want to check the compression. Specs call for 284 to 355 lbf/in squared at starter motor rpm. First question is when I do the test do I have the throttle wide open or closed or do I need to concern myself with throttle position for the test. 2nd question is what would be the minimum acceptable compression before I would need to check valves or rings. I assume that when I do the test, I would remove one injector then test then reinstall and remove the 2nd injector. Is that correct. Thx
__________________

__________________
weephee is offline   Reply With Quote
Old 04-08-2014, 06:30   #2
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,051
Re: MD7A Compression

This is a Diesel, so no throttle plate. Leave the throttle off. You are correct for a spark ignition engine to have it at full throttle.
I would pull all injectors so it will spin as fast as possible.
Now depending on how long it's sat, it's normal to be down on compression. If you have a borescope, that can tell a lot, and there are pretty decent ones that are USB powered and use a laptop as the display that are pretty cheap.
If it's down on compression, squirting a little oil in the cylinder can help determine if it's valves or rings.
__________________

__________________
a64pilot is online now   Reply With Quote
Old 04-08-2014, 06:32   #3
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,051
Re: MD7A Compression

Too much oil in a cylinder can hydro lock it, and that can bend a rod if spun with the starter, but if you spin it through a time or two by hand to check for a hydro lock, you'll be fine.
__________________
a64pilot is online now   Reply With Quote
Old 04-08-2014, 06:50   #4
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Re: MD7A Compression

If you find a low cylinder then you have to figure out where the loss is.

Differential compression testers are nice. You enter a reference pressure, say 80 psi, send that across a fixed orifice. With a gauge on the downstream side and "perfect" seal you read 80 psi.

Using a dummy plug and air firing you apply the downstream supply to the cylinder, rotate the piston to TDC compression stroke and read the difference. Number like 80/78 are common.

The best thing about this is with a low cylinder you can hear the leak through the breather - pistons, exhaust - exhaust valve, intake - intake valve.
__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Old 04-08-2014, 15:26   #5
Registered User

Join Date: Oct 2013
Location: Kingston, Ontario, Canada
Boat: Columbia, 8.3 and 8.7 (1977)
Posts: 153
Re: MD7A Compression

Thanks for the replies. I will let you know what results I come up with in a few days
__________________
weephee is offline   Reply With Quote
Old 04-08-2014, 16:16   #6
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
Re: MD7A Compression

Open the decomp lever, turn it over a couple of times. Then same, with the starter. Then close the decomp.

Now grab the wheel and try to turn it over.

You cannot, good compression.

You can, bad compression.

MD7 will start way beyond compression is BAD. With bad comp, it will smoke: a lot when cold, less when hot. Even when hot, it will smear the cooling water with oil.

They have no jackets. Fixing compression problem can be lengthy and may cost $$$.

PS If you have bad ring(s) you may find it next to impossible to buy new ones. If you can, they will be outrageously expensive. TIP they are said to be Nissan ...

Do not ask me how I know ;-)

Cheers,
b.
__________________
barnakiel is offline   Reply With Quote
Old 04-08-2014, 16:52   #7
Senior Cruiser
 
hpeer's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: Philadelphia
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 3,900
Re: MD7A Compression

I do t know if my compression is good or bad BUT..

She starts real easy, REAL easy
Does NOT smoke
Leaves no oil on the water
But...
I can spin her through by hand IF I go slow and easy.

So, just for clarity, I think you mean to say "grab the wheel and turn it over with some determination."

I'm asking as much as saying.
__________________
hpeer is offline   Reply With Quote
Old 04-08-2014, 17:55   #8
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Re: MD7A Compression

I don't like to recommend this but if you are brave put the palm of your hand over the intake and crank.

If it doesn't feel like your hand is gonna get sucked in the intake you likely have low compression. On mine when the valve seats finally gave out there was practically no suction at all.
__________________

__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Reply

Tags
compression

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not 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 Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Servicing an MD7a TrevorR Engines and Propulsion Systems 7 23-11-2015 03:01
md7a gearbox trouble trevorjane Engines and Propulsion Systems 4 05-08-2012 23:40
Alternator for Volvo MD7A 13hp hpeer Electrical: Batteries, Generators & Solar 2 08-05-2012 15:25
volvo MD7A jouet 820 Engines and Propulsion Systems 19 22-12-2008 01:03
volvo md7a, low power ian stratton Engines and Propulsion Systems 5 14-11-2008 16:20



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 21:54.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.