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Thread: 40DCOE-18s installed on 3.1 stroker tends to die upon deceleration

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    Registered User dawg7's Avatar
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    Default 40DCOE-18s installed on 3.1 stroker tends to die upon deceleration

    Hello Gents,

    As the title states I am experiencing some issues during deceleration when coming to a stop using the gears. I can put the car in neutral and it stays running. But when I use the gears to help with the stop especially when coming in hot and fast the engine just wants to die. It always dips to 500 RPM and most of the time just stops running. It idles good, it runs good going thru the gears, sounds freaking awesome! but the damn stop.....

    Been tuning the carbs for a few weeks and I think it is real close. Idles at 1000. Fuel PSI is 3.5. Fuel return line is capped. I barely have the 2-3K rpm stumble. I can tell it pulls very nice all the way to 5500 but that is as far as my butt is calibrated....anyway I think the cam tops out of 6400???

    Timing set to 12 degrees at 1000 rpm and 34 degrees advance at 3000 rpm ( no creep when rpm increased to 4000)

    I have read many Weber tuning manuals and haven't come across the "dies upon decel" troubleshooting steps. The carbs were good when they came out of a L24 and have been sitting for a few years. I cleaned them up like a rookie would and did a minor overhaul using the Peirce Manifold kit. Change the chokes and the idle and main jets per Peirce Manifold tech reps guidance.

    I don't have a clue what my next step should be other than checking for some restrictions in the carbs.

    Here is the specs of the engine:

    CARB SPECS
    CHOKE 34
    AIR CORRECTION 180
    MAIN JET 140
    IDLE JET 50F9
    EMULSION F11
    AUX VENT 4.5
    PUMP JET 1.75
    NEEDLE SEAT 1.75
    INLET VALVE .55

    ENGINE SPECS
    N42 block overbore 3mm
    KA24E PISTONS ITM RY6660 CUT .5mm
    N42 head port polished gasket matched
    ISKY L480 Cam
    Cosmetic head gasket 1.2mm
    71 240z connecting rods
    MSA 6-2-1 header
    2 “ Magnaflow glass pack and Magna flow exhaust
    Canon Intake
    Vacuum plugged
    Weber DCOE 40-18
    1975 280z distributor
    3 ohm Petronix flamethrower
    Taylor 8mm spark plug wires

    Click image for larger version. 

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    Last edited by dawg7; 01-23-2014 at 04:01 PM. Reason: wrong engine pic

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

  2. #2
    Registered User madkaw's Avatar
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    Default

    I'm guessing no wideband. What do the plugs look like?
    I noticed with my wideband in the conditions you stated the AFRs are rich - as in the engine is trying to burn off the extra fuel from running hard to a fast stop. Maybe a bit rich and she's loading up.
    I run Mikunis so they are a different animal
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User dawg7's Avatar
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    My engine builder looked at them and told me they looked great. The plugs have that milk chocolate appearance of a good burn.

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

  4. #4
    Nova Scotia,Canada,Earth Blue's Avatar
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    Can you compare immediate deceleration vs slow gentle deceleration? This will give us two data points.


    When you do an immediate deceleration (called overrun), the pistons are going fast and sucking hard (high vacuum in the manifold).

    The only air it can get is what is pulled past the closed throttle plate AND the air + fuel from the idle jet.


    If it is fuel starvation, just open up the idle jets a turn or more to give it more fuel then try testing again.

    If it is too much fuel, you will have to open the throttle plate stops a bit more then re-jet the idle.

    (Thanks Dr 240z for the Passini book!)
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    Nova Scotia,Canada,Earth Blue's Avatar
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    What is your fuel level? (This normally is not affected by forward/backward G's too much but it is another data point).
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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    Nova Scotia,Canada,Earth Blue's Avatar
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    Opps I should clarify in post #4 that I meant "idle enrichment screw" in place of "idle jet"

    Gee I wish the 1hr edit limit could be extended to 24hr or indefinitely.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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    Registered User dawg7's Avatar
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    Thanks for the replies. The fuel tank level was 3/4 full when I started driving that day. Ended up with a 1/4 tank and experienced the same issue through out the drive. I'll open up the idle mixture a little bit and see what happens. My hope is the RPM doesn't change too much. I feel good that the throttle plate to the first progression hole is set correctly. I did it with the carbs removed. To me it was hard to get the hole covered correctly by shining the flashlight thru the choke with the carb installed. Thanks again!

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

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    Registered User siteunseen's Avatar
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    Quote Originally Posted by Blue View Post

    Gee I wish the 1hr edit limit could be extended to 24hr or indefinitely.
    I'd never get any sleep!

    Noun 1. anal personality - (psychoanalysis) a personality characterized by meticulous neatness and suspicion and reserve
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Nova Scotia,Canada,Earth Blue's Avatar
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    Thanks for the data.

    For fuel level it would be good to also note to the level in the float bowls. It should be identical for all 3 carbs.

    The carbs will work better if you first set them up on a bench but fine tune on the car. O2 sensor and air flow tool are highly recommended.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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    Wish I could report that I made good progress but........nope can't say that. I only got a few runs before life got in the way. Didn't get much done at all. This weekend is mine right up to the Sunday at 5pm CST!

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

  11. #11
    Registered User Lazeum's Avatar
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    Are the throttle plate closed completely when you're at idle? Is the linkage ok and free to move within their entire range?

    Timing at idle seems very low to me. I'm running 20 with 9.5:1 CR, Rebello 276 cam and same 40DCOE-18. That would change total timing (so be careful with knock at high engine load) but you should try to increase advance at idle with engine running and see if rpm rise. Best timing at idle would be when rpm are the highest.
    You've got a big overlap cam, you definitely need higher timing.

    Have you checked for leaks? It could also lead to some problem.
    Last edited by Lazeum; 02-02-2014 at 04:36 AM.
    Matt - 72' 240z

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    '72 240Z(G) 3.2L
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    Okay, stupid question(s) time.

    1) fuel pressure at both idle and WOT

    2) why didn't you go to a larger diameter fuel feed line (say 3/8th)

    Okay, not so stupid question time.

    1) You have brakes so why are you using the gears to stop you? That's what they're there for.

    2) Wouldn't a better set of pads, a full on upgrade to the Toyota calipers or Wilwoods, and yes, I'm going there, a better driver make this mostly go away as an issue?
    if a little knowledge can make you dangerous, I'm a little dangerous

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    I drove the car for about 3 hours yesterday adjusting the idle mixture screws and ever so slightly...meaning less than 1/2 turn total the throttle plate screws to get a 1000 rpm idle. During the drive and coming to a stop the engine never died. It did dip down to 600 RPM a few times but never quit. But more importantly I noticed that the more then 3/4 of the time that idle would never come down to 1000 unless I tapped the gas pedal. So there is an intermittent bind somewhere. My tasks for the week: fix the binding; start over on the tuning and then go up on the timing a few degrees at a time. I am more than convinced that there are several issues that is causing the engine to die....

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

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    Nova Scotia,Canada,Earth Blue's Avatar
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    Do you have an air flow meter to set up carbs? Do you have an O2 sensor?

    These make tuning so much easier.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


  15. #15
    Registered User dawg7's Avatar
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    I have the SK airflow meter and I have the carbs sync'd within 0.5. And I am planning on installing the O2 sensor and AFR reader next month. I will be researching those products in the next few weeks. This week and weekend will be spent on fixing the binding.

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

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    Crumudgeon
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    Fix your linkage and make sure the throttle plates are in sync. It sounds like your idle is too lean. If you're idle mixture screw is out more then 1 turn you need to richen the idle jet. The idle circuit in a Weber DCOE is not just for idle. I is critical to engine running up to about 2,000 rpm. If you have a stumble around 2,100 to 2,500, which is about where the carbs transition to the mains, you're idle circuit is also too lean or you have the wrong emulsion tubes in the carb.

  17. #17
    Registered User TBone028's Avatar
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    +1 on ensuring all throttle plates are set the same, sounds like you already had that figured out with the progression hole alignment you mentioned earlier. It looks like you have 3 linkage-rod mounts (I've read sometimes the 3 mounts arent always aligned and causes slight binding)...does your linkage rod rotate freely when the engine is off?

    More importantly, it doesn't look like you have an external throttle return spring? I would DEFINITELY implement that as soon as possible (for safety). I use the set-up sold by piercemanifolds and would recommend it.

    p.s. an O2 meter is a great tool, take the guesswork out!
    Last edited by TBone028; 02-03-2014 at 10:56 AM.
    1972 240Z - 3.1L with some goodies
    1974 260Z - SOLD!

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    A huge percentage of Weber problems are linkage related...problem is when the linkage doesn't work precisely, tuning them is a moot point.
    Dave C, Putnam, NY
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    Registered User dawg7's Avatar
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    First of all I appreciate the feed back and tips. From what I am reading to what I am seeing on my setup it leads me to believe that I have a binding issue. So I am going to replace the linkage. I don't like that look of it anyway... I rushed this project trying to beat some established timelines. Big mistake on my part. So first thing I am going to do is look through this site and check out some setups. I have seen some nice billet sets, (cygnusx1 yours looks amazing) I just can't find them online. I already am using a Lokar throttle cable.

    BTW the PSI is a constant 3.5. I have a 4 psi fuel boost and the mechanical pump installed.

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

  20. #20
    Registered User dawg7's Avatar
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    I have been working the issue for the past days and have made progress. BLUF. The linkage was binding and timing was off (for triple webers). I bought the universal set from Pierce Manifolds and it works great; looks good too! I also replaced the dizzy with one from a 79 280zx. Reading the Weber power tuning manual I increased the timing at idle until the RPM stopped increasing. The idle timing now is 17 and the mechanical advance is 17 for that dizzy. The idle is really smooth; what a difference that dizzy makes and the engine doesn't die at all. Next is the AFR for final tuning. Again thanks for the replies.
    Last edited by dawg7; 02-18-2014 at 12:15 PM.

    74 260Z RLS30-016515 12/73
    3.1 Stroker; Weber 40DCOE; 5 sp
    225/50/15 BFG gForce on Konig Rewinds

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    Nova Scotia,Canada,Earth Blue's Avatar
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    On the timing, you are where I tuned a 240z with triples. Good to have convergence.

    btw Lazeum is spot on when it comes to webers and timing. Read his achived of posts.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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