BB sometimes diesels at shutdown
Thread Starter
Racer
Joined: Aug 2001
Posts: 473
Likes: 0
From: Raleigh NC
BB sometimes diesels at shutdown
About every third time, my '68 427 diesels for a few seconds after shut off. Nothing horrible, it may turn over just a few times and then dies.
What causes this?
What causes this?
Safety Car
Joined: Aug 2001
Posts: 3,892
Likes: 13
From: Pettis Performance 565 with two stages of Nitrous Supply nitrous 1.082, 4.61 at 155, 7.17 at 192
Re: BB sometimes diesels at shutdown (Turbo-Jet)
High performance, high compression motors on the verge of detonation due to poor gas will do this all the time. If you are not detonationg it is not a big deal, when it is dieseling the motor is actualy turning backwards. I have a 454 that does this on hot days and it sounds terrible and is embarrasing at shows and stuff. If you have a manual transmission put the car in third gear and let off the cluch untill the rpms start to drop, then turn off the ignition, it will shut off immedialty. :chevy
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: BB sometimes diesels at shutdown (69 N.O.X. RATT)
I have also heard tales of running too hot a plug causing this. Not sure if it is true but thought I would throw it out there. If wive's tale, please correct me.
Moderator
Joined: Jun 1999
Posts: 7,117
Likes: 51
From: Dove Mountain Arizona
Re: BB sometimes diesels at shutdown (hudman)
A mis-adjusted carb, the primarys or secondarys open a little too far at idle will suck air/fuel when shut down and cause this, especially on a hot engine.
Safety Car
Joined: Oct 1999
Posts: 4,101
Likes: 10
From: NorCal
Re: BB sometimes diesels at shutdown (hudman)
I'll assume that it's a Q-jet. You can add an A/C solenoid, again assuming it doesn't have air, to set the base idle. Then back off the idle stop 'till the throttle plates are closed (so it won't idle with the solenoid off). These came on a bunch of later model carbs and you should be able to find one with the mounting bracket. It works especially well on auto trans equipped cars. Hope this helps.
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Thread Starter
Racer
Joined: Aug 2001
Posts: 473
Likes: 0
From: Raleigh NC
Re: BB sometimes diesels at shutdown
Wow-
You guys really nailed this one. I have all of the above problems.
First of all, for those of you that have not seen my recent posts, the car is new to me (1 week).
Lets see. . .
I have an idle problem. Car idles fine at cold start up. Choke engages and car idles at 1300-1500. Tap accelerator and idles settles down at 700-800. After driving the car fully warmed up, idle will not settle down to 700-800 range again. It stays in the 1200 range. Very annoying. This was my guess for the problem. What puzzles me is that the carb is new. Any ideas?
Timing is advanced and car should be running higher octane fuel. Previous owner mixed in leaded 110 racing gas with unleaded premium. Without it he got a lot of spark knocking. I have located 100 octane fuel locally but have not gotten the mixture right. Under heavy acceleration, I noticed some spark knocking tonight. I guess I need more of the expensive stuff.
Obviously, it has been hotter than hell around here and combined with the above, I think I have my problem diagnosed.
Thanks for the suggestions. I will work on the correcting them.
You guys really nailed this one. I have all of the above problems.
First of all, for those of you that have not seen my recent posts, the car is new to me (1 week).
Lets see. . .
I have an idle problem. Car idles fine at cold start up. Choke engages and car idles at 1300-1500. Tap accelerator and idles settles down at 700-800. After driving the car fully warmed up, idle will not settle down to 700-800 range again. It stays in the 1200 range. Very annoying. This was my guess for the problem. What puzzles me is that the carb is new. Any ideas?
Timing is advanced and car should be running higher octane fuel. Previous owner mixed in leaded 110 racing gas with unleaded premium. Without it he got a lot of spark knocking. I have located 100 octane fuel locally but have not gotten the mixture right. Under heavy acceleration, I noticed some spark knocking tonight. I guess I need more of the expensive stuff.
Obviously, it has been hotter than hell around here and combined with the above, I think I have my problem diagnosed.
Thanks for the suggestions. I will work on the correcting them.
Safety Car
Joined: Aug 2001
Posts: 3,892
Likes: 13
From: Pettis Performance 565 with two stages of Nitrous Supply nitrous 1.082, 4.61 at 155, 7.17 at 192
Re: BB sometimes diesels at shutdown (hudman)
Regarding the idle problem, could be as simple as neading the idle adjustment screw adjusted. Let the car get warmed up and adjust your idle to were it should be. The fact the carb is new had nothing to do with it needing an adjustment.
Do you know the compression of the motor, if not find out. You may be able to burn lower octane fuel if you do not havce to high of compression. You do not want the car detonationg, destroys engine fast if it is bad enough. :chevy
Do you know the compression of the motor, if not find out. You may be able to burn lower octane fuel if you do not havce to high of compression. You do not want the car detonationg, destroys engine fast if it is bad enough. :chevy
Burning Brakes
Joined: Mar 2002
Posts: 803
Likes: 0
From: South San Francisco CA
Re: BB sometimes diesels at shutdown (hudman)
With my '69 4 speed I put one foot on the brake, put the car into third gear and slowly let my foot off the clutch. When the engine rpm drops...I turn the key off. This ritual has been going on since the 70's. We're victims of high compression, crappy gas and a world that isn't much interested in BB cars anymore. This method works well for me. I also use plugs that are gapped .010 over and this seems to reduce ping and hasn't hurt anything in the engine so far. Also, Tribotech in the crankcase every 60,000 miles seems to help. The engine has never been out of the car and is approaching 180,000 miles and still runs like a champ. Those morons driving $100,000 Porsches wonder why they spent so much money to get their doors blown off by a 30 something year old car. Ain't life great?
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Re: BB sometimes diesels at shutdown (hudman)
About every third time, my '68 427 diesels for a few seconds after shut off. Nothing horrible, it may turn over just a few times and then dies.
What causes this?
What causes this?
The 327 in my 68' only has 9.5:1 compression but if I use 92 octane instead of 93 it sometimes diesels... Avoid SPEEDWAY 92 octane gas!!!
:cheers:
Olivier
