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Thread: Fuel Mixture Problems - Car won't start when mixture set properly

  1. #1
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    Unhappy Fuel Mixture Problems - Car won't start when mixture set properly

    Ok, I've got a serious start problem with my engine. I think it has to do with the aftermarket cams.

    I was able to start my car this morning in the garage, cold it seemed to be idling great after it started:


    see next post for what happened next.
    Last edited by spikeygg; 08-28-2021 at 06:06 PM.

  2. #2
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    After about 10 minutes when it warmed up, it sunk into this crappy idle:


    see next post for what happened next.
    Last edited by spikeygg; 08-28-2021 at 06:07 PM.

  3. #3
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    I busted out the dwell device and when the engine RPMs were bouncing around the dwell was stuck very low I can't remember the actual value but maybe 10 or something. I took out my Allen hex key and tweaked the mixture screw and the dwell came back up, when it did, the engine idle improved smoothing out drastically:


    I let the car idle for about 5-10 more minutes and shut it down with the mixture screw set so that dwell is good hot.

    Now when I try to start the engine it won't fire up (hot or cold, I tried firing it up right after I shut it down and it won't even start up with the CSV "plug swap"). It's like I need a significantly enriched mixture for start AND cold idle and a leaned out mixture for hot idle. Is it possible that the cams won't allow the car to have a single fuel mixture balance point across temperature? If I can have a single fuel mixture balance point where should I start? If the accumulator is bad will the car be able to idle/run? What should I try to gather data and troubleshoot?

    I've had lots of problems starting with this car because I can't seem to get it started and running without manually enriching the fuel mixture significantly.

  4. #4
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    Do you know if the warm up regulator is working?

  5. #5
    Member Azar's Avatar
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    You have to operate the motor with the plugged hole where the CO adjuster is. Means you adjust it very slightly, then plug it and wait for 20 seconds untill the RPM settles. If not good, open, turn slightly again and plug it, wait 20 seconds, and so on.
    Keep the plug always there.It sucks air through it.

  6. #6
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    Quote Originally Posted by Helirich View Post
    Do you know if the warm up regulator is working?
    How can I tell? Not sure how to verify it's functional.

    Quote Originally Posted by Azar View Post
    You have to operate the motor with the plugged hole where the CO adjuster is. Means you adjust it very slightly, then plug it and wait for 20 seconds untill the RPM settles. If not good, open, turn slightly again and plug it, wait 20 seconds, and so on.
    Keep the plug always there.It sucks air through it.
    Yes, I did that to get the car idling correctly. How can I get it to start (without having to adjust the fuel mixture)?

  7. #7
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    Quote Originally Posted by spikeygg View Post
    How can I tell? Not sure how to verify it's functional.
    There are several ways. I would say the best way is with a fuel pressure guage. You check the "control pressure" cold and then again when it's hot. The WUR changes the control pressure as it warms up. It warms up via an electric element, but it will also warm up just from heat off the valve cover. (Just takes longer.)

    I hope I got this straight in my head, but I believe when the control pressure is low, it runs richer. Then as the WUR gets hot, it raises the control pressure which makes the engine run leaner. It does this by closing a valve that is bleeding off pressure.

    In any case, if that's your problem, you can get rebuilt ones or a rebuild kit. I got one off eBay. I wasn't very confident when I got done, but it works great.

  8. #8
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    Quote Originally Posted by Helirich View Post
    There are several ways. I would say the best way is with a fuel pressure guage. You check the "control pressure" cold and then again when it's hot. The WUR changes the control pressure as it warms up. It warms up via an electric element, but it will also warm up just from heat off the valve cover. (Just takes longer.)

    I hope I got this straight in my head, but I believe when the control pressure is low, it runs richer. Then as the WUR gets hot, it raises the control pressure which makes the engine run leaner. It does this by closing a valve that is bleeding off pressure.

    In any case, if that's your problem, you can get rebuilt ones or a rebuild kit. I got one off eBay. I wasn't very confident when I got done, but it works great.
    Cool, I just bought this kit: https://www.amazon.com/BETOOLL-Injec...dp/B01L17TPT2/

    I'm going to give it a try. Hopefully it's compatible, someone in the Q&A said they bought this kit for their DeLorean and it worked so ... fingers crossed.

  9. #9
    Motors about after dark Michael's Avatar
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    Cammed cars can be tricky to set the mixture on but even set right, your car should not be behaving this way. Since it started before, and all you did was adjust mixture, I would look into the possibility you set the mixture wrong. When these cars idle for a long time, the probe can get some contamination on it because the EGT is not high enough. Dial the mixture back a little (opposite of what you did until it starts). After you set the mixture, take it out for a nice little drive. Get into the RPM's a bit. No road trip, just clear it's throat for a few miles to burn off any residue from the O2 probe.

    Immediately after, check the Dwell again and zero in your mixture. Of course you will look for vacuum leaks just to make sure you are not compensating for unmetered air. A lot of the "old guard" will tell you idle surge is something that can't be corrected. On a properly running D, idle surge is non existent, even on a cammed car.

    Once you are "back where you started" with the mixture, you can start diagnosing the fuel system. Primary pressure o rings in the FD come to mind, but that's just one of many possibilities(easy to check though).
    Last edited by Michael; 08-28-2021 at 11:05 PM.
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  10. #10
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    Quote Originally Posted by Michael View Post
    Cammed cars can be tricky to set the mixture on but even set right, your car should not be behaving this way. Since it started before, and all you did was adjust mixture, I would look into the possibility you set the mixture wrong. When these cars idle for a long time, the probe can get some contamination on it because the EGT is not high enough. Dial the mixture back a little (opposite of what you did until it starts). After you set the mixture, take it out for a nice little drive. Get into the RPM's a bit. No road trip, just clear it's throat for a few miles to burn off any residue from the O2 probe.

    Immediately after, check the Dwell again and zero in your mixture. Of course you will look for vacuum leaks just to make sure you are not compensating for unmetered air. A lot of the "old guard" will tell you idle surge is something that can't be corrected. On a properly running D, idle surge is non existent, even on a cammed car.

    Once you are "back where you started" with the mixture, you can start diagnosing the fuel system. Primary pressure o rings in the FD come to mind, but that's just one of many possibilities(easy to check though).
    I've been battling this starting/fuel problem since I got the car (around 2016). Years ago right after I got it I determined that the WOT microswitch was stuck on and the previous mechanic(s) that worked on it dialed back the mixture to compensate for the WOT switch being on all the time, I don't think they realized that was the root cause. I've since removed the WOT microswitch and the engine will only start if I dial up the mixture sometimes I have to use the CSV plug swap to make it start even with the enriched mixture. However, if I leave it like that and drive around, once the car is warm it bogs badly when taking off and stumbles a lot in low RPMs and idles similar to the 2nd video I posted: hunting around and never settling, nearly stalling out every 5-10 seconds. If I hop out and lean out the mixture, the car runs great from then on until I shut it down. It's a terrible loop and I'd like to fix it once and for all.

    Also, I'm not sure you read the part about the dwell being pegged on the low end (too rich) and not cycling when the engine was hunting... it only started to cycle normally when I leaned it out which made the idle/throttle response much more normal. You're supposed to set the dwell while the engine is warmed up, right?

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