Go Back  Offshoreonly.com > Technical > General Q & A
A Lot Of Questions!! 508 & B&M 250??? >

A Lot Of Questions!! 508 & B&M 250???

Notices

A Lot Of Questions!! 508 & B&M 250???

Old 07-01-2011, 12:27 AM
  #1  
Registered
Thread Starter
 
Join Date: Jun 2011
Posts: 10
Likes: 0
Received 0 Likes on 0 Posts
Default A Lot Of Questions!! 508 & B&M 250???

I am new to working on boats and blowers.. I will try to make it short. This is my dads boat it's a Obsession 25 vortex when he got it it was a stock 502 single 1050 carb with a small cam and a 250sc 4 1/2psi boost and 9:1 compression. About 4 years ago the distributor when bad so he took it in to get it replaced they took it out to test it and they had the timing to high and melted a hole in a piston. He had the motor rebuilt with all the good stuff lowered the compression to 7.5:1 compression converted to all roller and bigger cam with a good set of Gm head that was on a racing boat that been all out ported and the biggest valves they could. After the new motor (508) was installed the boat runs the same mph running same boost and does not pull as good so he was disappointed it didn't run any better so it's been parked for 3 years. I was coming back from Florida last weekend and he call and told me to stop by the shop and pick up the boat and do what ever I had to do to get it right. First thing I saw 3/8 fuel line,3/8 fuel filter and 1 holley blue fuel pump so I will be installing all 1/2 fuel line,1/2 fuel filter and two holley blue fuel pumps to makes sure it has the fuel it need. Then I said we need to turn up the blower speed some to make up for the lost compression. I called The Blower Shop and talk to them to see about a smaller pulley and I already have a 3.35 top pull so I measured the bottom pulley and it's close to 5 9/16 they said with the current pulley's it should be making 8-9 psi of boost but it's not. I called holley and they said the same thing but they called the 5 9/16 bottom pulley a 6" pulley. They both said that the blower should be very hot spinning that fast but after 4-5 mins of wot the intake will be cool and have cold sweat on it. So am just going to start from the from bottom and work my way up to find out what's wrong. So here is my questions.

#1 someone that has a B&M 250 on a BBC measure the bottom pulley and see if it's 5 9/16" or 6"?

#2 What plugs should I be running in this motor?

#3 Where should I start with the timing? 28? with the advance locked out?

#4 1050 dominator.. Where should I start with the jet's?

Ok I have went blank!!! That's A.D.D. for ya... I am sure I will have more questions.. If anyone can help me out or get me started in the right direction it would be greatly appreciated.. O yea the motor will only turn 5000 Rpm's and it has a 28P 4 blade prop that been lab finished

Last edited by AllWaysThinking; 07-01-2011 at 06:38 PM.
AllWaysThinking is offline  
Old 07-04-2011, 09:43 PM
  #2  
Registered
Thread Starter
 
Join Date: Jun 2011
Posts: 10
Likes: 0
Received 0 Likes on 0 Posts
Default

No One?
AllWaysThinking is offline  
Old 07-04-2011, 09:52 PM
  #3  
Ginger or Mary Ann?
Charter Member
iTrader: (1)
 
US1 Fountain's Avatar
 
Join Date: Oct 2000
Location: L
Posts: 11,029
Likes: 0
Received 5 Likes on 5 Posts
Default

Ran a 250 with a 1050 on my HP motor just using the stock Merc fuel pump. Pressure was never a problem. Fuel lines were updated as was seperator and all fittings.
__________________
Some people are like Slinkies - Not really good for anything, but they
bring a smile to your face when pushed down the stairs.
US1 Fountain is offline  
Old 07-04-2011, 10:11 PM
  #4  
Registered
Thread Starter
 
Join Date: Jun 2011
Posts: 10
Likes: 0
Received 0 Likes on 0 Posts
Default

I have the 2 new holley blue's that been sitting in my shop for 5 years I have when EFI on all my project's so I thought I would put them to use.
AllWaysThinking is offline  
Old 07-04-2011, 10:47 PM
  #5  
Registered
iTrader: (3)
 
Join Date: Jul 2004
Location: chicago
Posts: 11,332
Received 70 Likes on 38 Posts
Default

Im running the same pulleys you are, on my 468's with B&M 250 blower. I am getting a max of 5psi. My engines have big roller cams, ported aluminum Dart heads, headers. I think you just need to go to a smaller 3" top pulley to get the boost up. According to all the blower charts i have read, they all say my current pulley setup should make 7psi or more on my engines. However, camshafts, head work, etc, play a huge role in boost. It sounds like when they rebuilt your dad's motor, it has a healthy cam, and the engine flows good. This is a good thing.

I read a article where they stuck a small B&M 174 style blower on a stock small block chevy. With the pulley ratio they had, it produced like 12psi of boost. Then they swapped the stock smog cam for a perf grind. Boost dropped to like 6psi.

Sounds like your on the right track. Now just get a 155192 3" top pulley. Should get you 2 more psi. BTW, my bottom pulley is a 5.5".
MILD THUNDER is offline  
Old 07-04-2011, 10:52 PM
  #6  
Registered
iTrader: (3)
 
Join Date: Jul 2004
Location: chicago
Posts: 11,332
Received 70 Likes on 38 Posts
Default

Depending on the heads, you might want to run MR41T is the heads accept a standard reach taper styled plug. These are 2 steps colder than your basic MR43T delco marine plug.

Jets, If you are running powervalves front and rear, I'd start at #98 jets front and rear, or maybe even #100 front and rear. My buddy ran 96's all the way around in his 509CI, with b&m 250's and single dominator. His plugs looked ok, but a little to clean/white for my taste. He also ran 6.5PV front and rear.
MILD THUNDER is offline  
Old 07-04-2011, 11:15 PM
  #7  
Registered
Thread Starter
 
Join Date: Jun 2011
Posts: 10
Likes: 0
Received 0 Likes on 0 Posts
Default

Thanks for the plug and carb info.. What should i gap the plugs it has a msd box and distributor?? .030 or .035 i what going to guess .035 sense it has a msd box.
AllWaysThinking is offline  

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 On
Pingbacks are On
Refbacks are On


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.