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  1. #1
    Join Date
    Dec 2002
    Location
    stoke on trent.
    Posts
    342

    One for the ignition timing experts....

    I have a newly built Chevy 383 stroker fitted into a Dax De Dion. The top end package is Edelbrock Performer RPM heads, Doug Herbert cam (similar spec to Edelbrock Performer RPM cam), Edelbrock Victor JNR intake manifold, Holley 750cfm vac sec carb.



    The ignition package is an MSD Pro Billet ready to run vac advance distributor, MSD Blaster SS coil and MSD 6AL control box.



    Since the installation of the engine into the Cobra and the Cobra being completed and running on the road, I am not happy with the performance of the engine. It seems to ‘stumble’ or become ‘nervous’ at approx 4000 rpm in 4th gear (Tremec TKO) and tends to hold back. I can clearly hear a ‘popping’ sound in the sidepipes when this is evident as well as ‘feeling’ that the engine is not running correctly at this time.



    The timing is set to 14 degrees initial, 38 degrees total (without vac). When the vac is operating, I noticed the timing go to about 55 degrees (too much I think). So, I made a small bushing and fitted it over the advance mechanism in the distributor to ‘limit’ the amount of vac advance. This reduced the vac advance to approx 48 degrees max. This made the engine run much better although the problem was still evident. All the testing was done with no air filter fitted. I then made minor adjustments to the throttle cable and fitted a much lighter spring in the vac unit on the carb. This improved things even more but not yet 100%. I drove the car and it was pretty good and certainly driveable and liveable with. I thought at this time I had cured the problem.



    I fitted a new K&N 14” X 4” air cleaner with the filtered top for max air flow. I then took the car for a test run to note the improvement, but the ‘stumbling’ was back again, but this time in second gear only at approx 4000 rpm.



    Removing the vac advance pipe and plugging the distributor and carb seem to cure the problem entirely. Running the engine under no load at 4000 rpm with the vac connected makes it run ‘rough’, pull the pipe off and it rund smooth, very smooth, just how you would expect.



    I have spoken to a couple of engine builders. One says keep the vac on for fuel economy and low speed driveability and try further adjusments of limiting the amount of vac advance, the other one says remove the vac completely as the engine seems not to like the vac advance. Please help. What do you suggest?

    Rob
    Cobra-less What next I wonder

  2. #2
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
    Age
    63
    Posts
    10,382

    Re: One for the ignition timing experts....

    Rob, It sounds to me like what you are describing is light throttle detonation, does it happen under higher throttle application or just on light throttle.
    My engine is similar to yours but I'm running a high output HEI.
    I suffer from a little bit of light throttle det too at around 2000 rpm, bit by bit I'm backing off the vac advance as I have an adjustable can.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  3. #3
    Join Date
    Dec 2002
    Location
    stoke on trent.
    Posts
    342

    Re: One for the ignition timing experts....

    Its more on hard acceleration at about 4000 rpm. I don't think its detonation, its not that kind of 'pinking' noise accociated with detonation, its more of a 'splutter' and lack of performance for a few seconds. Its really wierd. If I disconnect the vac advance, all runs well. Why though, is it also ok with the air filter off but not ok with it on? I can only assume that the air filter (although a high flow jobby) must be restricting the air flow into the carb and hence altering the amount of vac in the carb, hence a different 'signal' to the dizzy vac unit. Problem is, I don't fully understand and as you know, a little knowledge is dangerous!!!!


    Rob
    Cobra-less What next I wonder

  4. #4
    Join Date
    Jul 2001
    Location
    Newton Abbot, Devon, UK. (God's waiting room)
    Age
    72
    Posts
    13,455

    Re: One for the ignition timing experts....

    Rob

    Have you checked the float levels and power valve adjustment ???????

    Also hesitation can be caused by the secondaries coming in to quickley, have a look at the Holley web-site it will tell how to change to a heavier or lighter spring.
    Mike
    Cobraless


    I've often been asked, "What do you old folks do now that you're retired?
    Well, I'm fortunate to have a chemical engineering background, one of the things I enjoy most is turning beer, wine, Scotch and margaritas into urine.

    Life's good, live it.



  5. #5
    Join Date
    Dec 2002
    Location
    stoke on trent.
    Posts
    342

    Re: One for the ignition timing experts....

    Float levels checked (they were ok). Power valvo ok. I don't think this is a fuel thing though Mike, its a vac advance concern. All is OK with the vac disconnected. The problem only appears when the vac is connected up. Unusual though how fitting an air filter made things worse (see my original posting at the top of this thread). Its a real pain this one, i'll tell you. I understand that Brian at Repower sometimes comes across this one when MSD's are fitted. He has some special bushes made up to reduce the amount of vac adjustment in the dizzy. As my original post states, I made bushes up which made things better (got vac adv down from 55 degrees to 48 max) but hasn't cured it. Another strange one is that if I disconnect my MSD 6AL box and run on the dizzy and coil only, the problem is far less worse (but still exists). I think though that the MSD is just 'amplifying' the problem. I could of course run without the 6AL but would prefer to keep it on for obvious reasons


    Rob
    Cobra-less What next I wonder

  6. #6
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
    Age
    63
    Posts
    10,382

    Re: One for the ignition timing experts....

    I agree it's very strange that the problem goes away without the air filter on.
    It would suggest that the filter is restricting the air flow at higher throttle demands.
    Is it dropped down low on top of the carb?
    As you suggest if it's restricting the flow that will as you say increase carb and manifold depression and obviously vac advance.
    That said at high er throttle openings the vac would normally reduce hence why the vac advance doesn't normally do a lot a wide open throttle.
    As the others say it's only there for emissions and economy.
    If you don't mind losing a bit of economy dump it. As for emissions this ain't California. You might need it for MOT time emissions but probably not.
    I wonder if the air filter is being sucked onto the top of the carb and restricting flow under higher loads?
    Do you have a normal air filter with a metal top you can try?
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  7. #7
    Join Date
    Aug 2011
    Location
    East Sussex
    Age
    69
    Posts
    15,858

    Re: One for the ignition timing experts....

    Rob.

    I may be wrong (I frequently am) but surely 55 degrees total advance is way too much I always thought 38 was the maximum recommended. Could explain why it gets better with the vac disconnected
    Chris
    Ex AK427 351Cleveland T5WC

  8. #8
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
    Age
    63
    Posts
    10,382

    Re: One for the ignition timing experts....

    55 degees does sound rather high, which is why I initial thought the noise was detonation.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  9. #9
    Join Date
    Aug 2011
    Location
    East Sussex
    Age
    69
    Posts
    15,858

    Re: One for the ignition timing experts....

    It would also explain why it gets worst with the filter on, as it is making more vacuum
    Chris
    Ex AK427 351Cleveland T5WC

  10. #10
    Join Date
    Feb 2007
    Location
    Nr Biggar
    Posts
    423

    Re: One for the ignition timing experts....

    My 350 was much the same with the rpm cam and a vacuum can connected. Take 2 degrees off your static timing and plug the vacuum can and the carb port - I bet you'll have a new car. 36 should be max advance curved on the bob weights/springs at any time. With too much vacuum induced advance at low revs as you accelerate you'll stumble away. Too much at high revs and you'll get pre-ignition and overheating. The vacuum is for economy on mild cams and is a liability on something lumpy where you've got valve overlap.

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