Guest larkja Posted September 29, 2001 Share Posted September 29, 2001 A couple months ago, I was considering whether to go with a V8 or a race-prepped 3.0L. I went with the 3.0L due to the load I will be putting on the car and cost as well - too many pieces to change with the V8. Here's my problem - I have the stock SUs and when I turn it off after the engine is warm, it diesels for about 5 seconds and then shuts off. The only way to prevent this is put it in gear, put the foot on the brake and ease off the clutch to kill the engine - I don't really like doing this. I spoke with my mechanic who also has a 3.0L with the stock SUs and he indicated that his does the same thing. His comment was the only way to fix this is upgrade the carbs. Anybody ever run into this problem? The injection is stock (new needles in carb), I have an MSD coil, optical distributor, and headers. I would love to fix this problem. Thanks Jim Quote Link to comment Share on other sites More sharing options...
Guest larkja Posted October 1, 2001 Share Posted October 1, 2001 Some assistance please??? Quote Link to comment Share on other sites More sharing options...
Modern Motorsports Ltd Posted October 1, 2001 Share Posted October 1, 2001 Larkja, dieselling is typically due to your mixture being too lean so it's v. hot and preignites itself, dieselling on...mainly with carbs where dieselling with draw fuel in to continue the cycle...or too little ignition timing [given fuel/air mix, ie if one is out it affects rest..](or too much/my brains foggy this am). Perhaps your SU's don't quite flow enough at idle for the larger cubes? Or the stroked combo makes for more detonation/preignition hotspots so they keep lighting off after shutdown? Whenever my ride is tuned for our annual smogging (ie. leaned out and timing backed off) it'll diesel for a minute or more after I shut it off if I let it...I literally have to get out and knock dizzy back to reasonable timing before shutoff. I forgot to do that this year and the unburnt fuel haze in my garage had my eyes watering good as I scrambled out of my car and the garage In school last year I was cheap so ran a leaner idle and just shut it off in gear...mines an auto so no biggie. Now with EFI I can run that lean idle without the dieselling thank goodness. good luck (PS thanks for tagging a follow up..I'm not fond of seeing the exact same post in multiple forums and what you did grabs my attention better ) Quote Link to comment Share on other sites More sharing options...
DavyZ Posted October 1, 2001 Share Posted October 1, 2001 You are running high-octane gas, yes? I have found that the better the quality of gas, the better the engine performs, even in shut down. Just a thought. Davy Quote Link to comment Share on other sites More sharing options...
Guest larkja Posted October 3, 2001 Share Posted October 3, 2001 Thanks for the responses. I am running 92 octane gas. When I have the valves adjusted at the 1000 mile mark, I will have the mechanic check the timing and fuel delivery (lean vs. rich). I appreciate the input. Jim Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted October 7, 2001 Share Posted October 7, 2001 Jim, There is nothing wrong with your system. The problem of dieselling in your motor is caused by the fact that SU carbs are constant draft carbs. As long as the engine is turning the carb will continue to deliver fuel. The fix is wonderfully simple. (The British crossed this bridge years ago in the SU equipped Leyland cars) If you can can't stop fuel delivery, then eliminate the vacuum that draws it into the cylinder. Simple. Based on oil pressure which holds for just a bit after the car is shut down, you can arrange a relay to use this dying oil pressure signal to temporarily ground out a relay which will energize a large vacuum solenoid assy. It works neat. When you shut off the key, the relay is energized, but only if oil pressure is present.(Car having been just shut off) The Vacuum solenoid vents outside air pressure directly into the intake manifold and kills all suction through the SU's. This coupled with running the idle mixture as lean as possible, will kill an SU carbed high compression engine almost instantly. Ross, with your obvious inventive talents, why don't you build such a kit and market it? I would be glad to help you with the technical end of it, but I doubt you need it. The solenoid is standard issue from JRT and could easily be plumbed into any one of the many balance tube vents which already exist. I built a prototype which uses a factory Nissan gulp valve instead of an electrically actuated vacuum solenoid. The Datsun standard issue vacuum advance cutout solenoids from the 280 EFI systems will trigger the gulp valve nicely, but a vacuum resevoir is a must. The larger the induced leak, the faster the car shuts off. Food for thought. Kim Quote Link to comment Share on other sites More sharing options...
Guest larkja Posted October 10, 2001 Share Posted October 10, 2001 Thanks for the responses. Quote Link to comment Share on other sites More sharing options...
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