datsun 1/2 dozen SD Posted July 22, 2008 Share Posted July 22, 2008 i was wondering if anyone could help me i got a l28 witha mn47 head dual su carbs every time i turn of the car it still runs deisils out the carbs i run 91 octane with booster its now timed at 0 degrees from preveouis 15 degrees advanced i got colder plugs gaped at 40 degrees and still have the same problem if anyone could help thank you! Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2008 Share Posted July 22, 2008 What is deisil? Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 22, 2008 Share Posted July 22, 2008 Forgive the misspelling Challenger, nobody is perfect... What kind of distributor are you running because it sounds like your timing changes aren't making any effect. Quote Link to comment Share on other sites More sharing options...
1 fast z Posted July 22, 2008 Share Posted July 22, 2008 You will learn with HC setups, you kill it by stepping on the brakes and letting the clutch out, or by having a throttle solinoid that completly kills the air to the motor, so basicly your throttle stop screw rest on a solinoid that is kicked out when the key is on, and when the key is off, the solinoid closes so your butterflys will go 100% closed. Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2008 Share Posted July 22, 2008 Forgive the misspelling Challenger, nobody is perfect... What kind of distributor are you running because it sounds like your timing changes aren't making any effect. Wasnt trying to be a smart ass, but wasnt sure what he meant by it. Is diesel coming out of the carbs, or is it an actual word for something else I dont know about? Is it sound like a diesel??? I could have determined the meaning if the sentence made sense. Quote Link to comment Share on other sites More sharing options...
woldson Posted July 23, 2008 Share Posted July 23, 2008 i was wondering if anyone could help me i got a l28 witha mn47 head dual su carbs every time i turn of the car it still runs deisils out the carbs i run 91 octane with booster its now timed at 0 degrees from preveouis 15 degrees advanced i got colder plugs gaped at 40 degrees and still have the same problem if anyone could help thank you! comeonhiswaspoorlyreditenandwassorunoniiyouhavetogivechanllengersomeredonsuchmaters Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 23, 2008 Share Posted July 23, 2008 HAHAH! I got the jist of that but did not understand it. Quote Link to comment Share on other sites More sharing options...
woldson Posted July 23, 2008 Share Posted July 23, 2008 Short version: I agree with Challenger. He is just backing up the rules. Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 23, 2008 Share Posted July 23, 2008 I still honestly dont understand his problem... rules or no rules I dont know what is wrong. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2008 Share Posted July 23, 2008 I still honestly dont understand his problem... rules or no rules I dont know what is wrong. Dieseling out is when an engine continues to run after the key is shut off and sounds like it's chugging to a stop. Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 What is deisil? yes it come out of the carbs when it finally shuts off Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 Forgive the misspelling Challenger, nobody is perfect... What kind of distributor are you running because it sounds like your timing changes aren't making any effect. iam running a 280zx electronic distributor with a e12-80 Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 Dieseling out is when an engine continues to run after the key is shut off and sounds like it's chugging to a stop. correct Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2008 Share Posted July 23, 2008 What year car is this in? How is this coil and ignition module wired up? Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 You will learn with HC setups, you kill it by stepping on the brakes and letting the clutch out, or by having a throttle solinoid that completly kills the air to the motor, so basicly your throttle stop screw rest on a solinoid that is kicked out when the key is on, and when the key is off, the solinoid closes so your butterflys will go 100% closed. sound great but is that nessesary is there any other way besides doing that like going to a colder plug perhaps? Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 What year car is this in? How is this coil and ignition module wired up? it is ia a 71 240z . from the e12-80 i got the neg - going to neg- on the coil i got the pos+ going to the ballast resistor to the pos+ post on the coil. Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 23, 2008 Share Posted July 23, 2008 Where is the positive feed coming from? Quote Link to comment Share on other sites More sharing options...
datsun 1/2 dozen SD Posted July 23, 2008 Author Share Posted July 23, 2008 Where is the positive feed coming from? positive is coming from b on the e12-80 to the positive on the coil Quote Link to comment Share on other sites More sharing options...
Zmanco Posted July 23, 2008 Share Posted July 23, 2008 Sounds like you have enough carbon deposits in the head creating hot spots that the engine self ignites (diesels) even after you turn off the ignition. Not surprising with a high compression head and carbs. A simple fix is to turn the engine off with the clutch. Put it in 4th, set the parking brake and step on the service brake (just to be safe) and let the clutch out. It's probably not the long term solution you want, but it's easy and really won't hurt anything. Remember to turn the ignition off afterwards. Quote Link to comment Share on other sites More sharing options...
zcarnut Posted July 23, 2008 Share Posted July 23, 2008 “Dieseling†is when a gasoline engine continues to run (albeit coughing and sputtering) even after the ignition has been turned off. This is also known as “run-onâ€. Dieseling only occurs on engines with carburetors, because fuel injection systems shut off the injectors when the ignition is switched off. Dieseling is caused by a combination of factors: low octane fuel, high engine operating temperatures, and a high compression ratio. Additional factors include possible carbon deposits in the combustion chamber (which will glow white hot), and using gasoline that contains some alcohol content. The effect is created by fuel in the engine cylinders being compressed and becoming hot enough to self ignite without the aid of a spark plug. This is similar to how a diesel engine works, so it sounds similar and hence the name. As long as the engine is turning over and drawing air through a carburetor, the carburetor will continue to deliver fuel. It is not possible for ignition timing to cause dieseling directly. With the key turned off all power to the ignition system (and sparkplugs) is disabled and there is no spark. Thus you might conclude that the ignition timing is irrelevant. However, since ignition timing has a noticeable effect on the combustion chamber pressure and therefore its temperature when the engine is running, ignition timing has an indirect effect on dieseling. So, now that we know the cause, how do we fix the problem? Well, there are several ways to combat the problem. Roughly, in order of complexity, you can try: 1. Use the highest octane fuel you can find. Also add a can of octane booster. But, you stated you are already doing this, so I will mention this just for the sake of “completenessâ€. Also, avoid using gasoline that contains any alcohol (methanol). The alcohol vaporizes at a much lower temperature than the rest of the fuel and the result is alcohol vapor in the combustion chamber. 2. Reduce the air flow through the carburetors at turn off. Back in the 1970’s (when dieseling was a big problem) carburetors had an “anti-dieseling†dashpot which was a solenoid that completely closed the throttle blade at engine turn off. With no air flow through the carburetor throat, then no fuel flow into the engine. Since the SU’s do not have a dashpot your recourse is to reduce the engine idle speed as low as possible. You should be able to idle at 600rpm unless you have an aftermarket cam with a lot of valve overlap. You can also play around with the SU choke lever. The SU chokes have a “fast idle†position at about ¼ of their operating range before the fuel enrichment comes in. You can adjust the choke lever into this fast idle region and then set the engine idle speed. Then at engine turn off you fully move the choke lever to its off position which will reduce the idle speed. 3. Reduce the fuel flow at turn off. Although dashpots worked, they were not completely effective because there were other (minor) air paths into the intake manifold. So, in the 1980’s automakers began using “fuel shut-off solenoids†in their carburetors. This was basically a needle valve that blocked fuel flow out of the float bowel at engine turn off. Again, since the SU’s do not have this feature you will have to resort to leaning out the air-fuel idle mixture. However, on the SU’s when you lean the idle mixture you also are leaning out the mixture across the whole range of engine speeds. This can result in increased combustion chamber temperatures. So, do not get carried away with the mixture adjustment. A little bit (1/8 turn) goes along way. What you really want are SU needles with a different taper. 4. Reduce the engine operating temperature. Although L-series engines prefer to run around 185 degrees, you can get 180, 170 and even 160 degree thermostats. 5. Reduce the engine combustion chamber temperature. Spark timing that is too far advanced will increase combustion chamber temperatures as will not having enough EGR flow. Too lean of a mixture will also raise combustion temperatures. 6. Reduce combustion chamber “hot spotsâ€. Again, I do not think this is a concern for you, but high mileage engines tend to have carbon build up in the combustion chamber which retains heat and the extremely hot bits of carbon can ignite the fuel after shutdown. In addition, such deposits will increase the compression ratio of the engine. There are additives you can both pour into the engine and put into the gas tank to dissolve carbon, but for heavy deposits the only solution is removal of the cylinder head and scraping away the deposits. Sharp edges in the combustion chamber can get red hot while the engine is running. The hot edges can also ignite the fuel after shutdown. The cure for this is the physical removal of the sharp edges bits with a hand grinder. Sharp edges can occur on the valves, especially if they have just been re-ground. Even spark plugs of the wrong heat range can run hot enough at the tip to cause dieseling. This can also lead to premature failure of the ceramic insulator near the tip of the plug. Always use the coldest plus you can that don’t misfire (due to carbon build up). 7. Reduce the compression ratio. This is probably the major cause of your dieseling. The MN47 head has only a 39cc combustion chamber volume. If you combine this with a L28 block that has flat-top pistons, then the (static) compression ratio is almost 11:1. Using “dished†(by 10cc) pistons would reduce that to around 9:1, which is much more practical ratio with 91 octane gas. Another option is to use a thicker (2mm) head gasket or perhaps to use two stock gaskets stacked, but I have seen this setup fail before. However, using dished pistons or a thicker head gasket reduces the desired “quelch†effects of the MN-47 head. 8. Switch over to fuel injection (join the rest of us living in the 1980’s!). Seriously, this is the preferred solution to dieseling. It just costs more than carburetors and the complexity of the fuel system increases. 9. Add a “gulp†valve. A gulp valve is a solenoid controlled valve that allows outside air to flow into the intake manifold at engine turn off. This induced air bypasses the carburetors and leans out the intake mixture. To be effective, the vent line and valve diameter should be fairly large, at least 10mm. 10. Increase the engine loading at shut off. Turn on the ac compressor (if you have one). Do not “dump the clutch†while holding the brakes on. Unless you have an automatic transmission, this creates severe loading on the driveline parts and you will eventually break something (usually the U-joints). Some people insist that turning on your lights will help, but the electrical loads are supplied first by the battery and not the alternator. Quote Link to comment Share on other sites More sharing options...
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