Page 1 of 2 12 LastLast
Results 1 to 10 of 19
  1. #1
    Join Date
    Dec 2011
    Location
    Rugby,West Midlands.
    Age
    60
    Posts
    594

    Stroked SBC timing advice.

    Hi All, my Huddart built sbc seems to be lacking a bit of power and around one in ten times ,when the engine is hot ,it tries to run on for about 2 seconds after I have switched off. Also now and again ,if i stab the throttle down , i get the odd miss-fire . I must point out this usually happens when i am in too high a gear, so may be my fault So I want to check the timing is ok, as I know that too far advanced can damage the engine or even blow holes through the pistons if left too long. I don’t know whether "running on" is a symptom of too advanced or retarded.

    The timing was last adjusted about two years ago by Troy at Northampton Motorsport on his rolling road. Last time I did this was on my old chevette , but have never done this on my cobra yet. It’s a 5.7, stroked to 6.3, and has patriot cylinder heads.

    What are the correct timing figures, and what rpm for when i do this myself . Do I need to remove the pipe from the vacuum advance capsule and block the tube .Any other advice welcome from people with same setup as me.I am thinking of getting the garage to check this out while my AK is in for MOT tomorrow morning, as his timing light is bound to be way better than my ancient battered old one.
    Paul.

    AK Huddart Chevy 383 Stroker, BMW Royal Grey Metal Metallic , 5 speed Jaguar Getrag , Holley 670cfm 4 barrel street Avenger Carb, 450 BHP at flywheel , 460 Ibs/ft torque, and luvin it !

    " We dont stop playing because we grow old, we grow old because we stop playing" - George Bernard Shaw.

  2. #2
    Join Date
    Sep 2010
    Location
    Cambridge
    Age
    35
    Posts
    1,188
    Hi Paul

    Apologies in advance (ha - timing related joke!) if you already know any of this......

    I've had running on (or dieseling) before, it was caused by throttle idle position set too far open, which itself was out of necessity because my timing was not optimal. My engine sounds similar to yours but my numbers aren't quite as big as whats in your signature. I'm at about an even 425/425 hp and torque from my 383 SBC.

    After much fiddling and a couple of different rolling roads I ended up with my initial timing at 17 degrees and the full "all in" timing at 34 degrees coming in by about 2700rpm. I have a standard chevy HEI distributor so I had to modify the mechanism to limit the total timing. I also had to change the springs to one heavy and one medium to get that curve. On the original lighter springs the advance was coming in too quickly, all in by 2200 and I was getting pinging on initial acceleration from low rpm.

    A standard HEI dizzy will have around 20-22 degrees mechanical advance built in. A fairly regular static timing value for a normal road cam is 12 degrees, then 12 + 22 gives you about 34 degrees full advance. My cam is fairly hot and best idle (approx 900rpm) was acheived after much experimentation at 17 degrees (ignoring vacuum for now). To keep my total timing at around 34 where I know peak power roughly is, I had to modify the dizzy to limit the mechanical advance to 17 degrees (17 base + 17 mech = 34 total). This was done with a screw dremelled to form an adjustable end stop for the bob weights. Pic below is without my vac adjuster. You can see my limiter screw in the second hole from the left of the rectangular metal strip. As the bob weights expand it butts up against this screw. If you dremel the screw head semicircular you have yourself an adjuster. The hole is pre-drilled in my dizzy so I did all of this without having to remove the distributor. All of this needs to be done with vac unplugged and blocked off.

    If you go for 12 static and 34 all in by 3000rpm then you should be safe for pinging assuming your mixture is ok but you'll be a fair bit off optimal depending mainly on cam and with static timing that low you may have to compromise on idle and low rpm characteristics. I spent ages twiddling with my carb to no avail when the real culprit all along was timing.







    I ran my engine when it was new at the basic 12 static and 34 all in, with whatever bob weight springs were standard. This is when I used to get dieseling because I had to wind the throttle idle screw open a lot to keep the hot cam turning over. The low static timing didn't work well with a hot cam because of..... science. When I raised the static value the engine ran a lot cleaner (idle revs rose) so I could close the throttle idle screw down and the running on went away.

    As far as vacuum goes, I ran mine without vac initially because ha, its a cobra, who cares about vacuum.... I've changed my mind since. I have a Crane adjustable vacuum timing mechanism on my dizzy so I can adjust the total value and the vacuum level at which it pulls in. I know from my vacuum gauge that my best idle vac was 11.5in.hg so I set the adjustawhatsit slightly below this so its definitely fully deployed at idle vacuum, and to allow for drops in idle for example if the fan comes on. The settings that work for me are vac timing 9 degrees, fully deployed at 10in.hg and fully off at 7.5in.hg. The fully off figure was arrived at through driving trial and error and some rolling road work so you get the right blend - the vac timing drops off as you put your foot down, but the mechanical timing comes in as the revs rise. This was all done using a vacuum gauge tee'd into the vac line so I could see exactly what was happening and when. With the vac connected this means I get 17 (static) + 9 (vac) = 26 degrees at idle which the engine really likes. I can get a really nice clean idle on my dirty great cam and low revs cruising with small throttle inputs is nice and sharp and responsive. At cruise the extra vac timing helps keep things clean and allows me to lean the mixture off nicely. 5th gear 70mph I'm at about 2400rpm with my getrag box (same as yours) and 3.54 rear end so my mech advance isn't fully deployed. That extra 9 degrees of vac timing makes cruising and low(ish) revs far more tolerable.

    regards

    Dave A
    Attached Images Attached Images
    Last edited by hifihedgehog; 04-04-19 at 12:46 PM.

  3. #3
    Join Date
    Apr 2004
    Location
    on the edge...almost falling off
    Age
    56
    Posts
    8,514
    With a non standard engine there are too many variables to say what your timing should be Paul, best check with troy (he may keep records), ask huddart or go for another rolling road. To just quote figures without fully knowing what cam and set up you have could possibly make things worse.

  4. #4
    Join Date
    Dec 2011
    Location
    Rugby,West Midlands.
    Age
    60
    Posts
    594
    I've had running on (or dieseling) before, it was caused by throttle idle position set too far open, which itself was out of necessity because my timing was not optimal. My engine sounds similar to yours but my numbers aren't quite as big as whats in your signature. I'm at about an even 425/425 hp and torque from my 383 SBC.
    I don’t think my engine “run on” is being caused by too high a tick over setting, as my engine ticks over at roughly 600 rpm, albeit a bit lumpy, but I was told by a vehicle tech once that all SBC V8’s have lumpy tick overs because of their particular firing order, and said there was a name given to the V8 firing order, which I cannot remember now.

    I googled “dieseling” and “run on” and apparently this can be caused by “hot spots “ in the combustion chamber igniting the mixture which is still being drawn in form the carb ,even though the plugs have stopped firing and one common cause for this can be the spark plug tips running too hot . So I will check what type of plugs I am running and see if fitting a set of “colder” plugs might cure the “run on” . I will only go one grade colder as I don’t want to get fouled plugs, due to them not getting hot enough.

    With a non standard engine there are too many variables to say what your timing should be Paul, best check with troy (he may keep records), ask huddart or go for another rolling road. To just quote figures without fully knowing what cam and set up you have could possibly make things worse.
    You are right ,and that's a good idea, I will just get the garage to check what the timing is currently set at, but not make any adjustments, then check with Troy or Mike H what he set it at. Maybe my dizzy has moved since it was set by troy.
    Paul.

    AK Huddart Chevy 383 Stroker, BMW Royal Grey Metal Metallic , 5 speed Jaguar Getrag , Holley 670cfm 4 barrel street Avenger Carb, 450 BHP at flywheel , 460 Ibs/ft torque, and luvin it !

    " We dont stop playing because we grow old, we grow old because we stop playing" - George Bernard Shaw.

  5. #5
    Join Date
    Sep 2009
    Location
    Leeds
    Posts
    2,868
    With regards to run-on/dieseling, my problem with this was due to not using super-unleaded. If i used normal unleaded it would do it when turning of ignition, super unleaded and it would be fine, although following the engine rebuild all of these issues have gone away now

    With regards to the timing etc issues, as it was 2 yrs ago you last had the rolling road session, it maybe worth booking in for another one to go through setting everything up again properly as tonym says


    rgds
    Gary
    Conny - A Carnival Red Metallic (paint code Ref: Jag CGG/1811) Southern Roadcraft V8 ( SRV8 ) with 383ci stroker chevy.

  6. #6
    Join Date
    Nov 2006
    Location
    Shropshire/Wales Border
    Age
    48
    Posts
    5,942
    Agree that rolling road is by far the best way to get your timing spot on. Short term you can set the initial timing by connecting a vacuum gauge to your intake manifold and adjusting the timing for the highest reading. That'll get you pretty much there for your particular engine.
    I found that if you have one of the cheaper dizzy clamps on an SBC they dont always hold the dizzy tight and your timing slips. Might be worth checking how well yours clamps
    Lloyd B
    Current: Crendon #54 in build - 427 Side Oiler/Cobrajet Heads/Dual 600cfm Holleys/4 speed toploader/Vintage cast knock on wheels
    Dedion Dax/Clarkson 383 Chevy with roller 4/7 swap cam, AFR195 heads - SOLD

  7. #7
    Join Date
    Dec 2011
    Location
    Rugby,West Midlands.
    Age
    60
    Posts
    594
    My AK sailed through its MOT this morning and now has fresh oil and filter and as all the grease points are at the back end of the car and my ramps are not wide enough for the rear tyres, I got the master tech to top them all up while it was up on their ramps. God how I would love a 2 post car lift. Unfortunately, he didn’t have time to check the timing, so I will have to buy myself a decent timing light and check it myself.

    I discussed my “run on “ and “miss fire” problem with the Master Tech and he agreed that as my engine is not standard , I cannot rely on the standard static timing figures ,and the carb and timing does need setting up properly on a rolling road or by someone who specialises in tuning Holley carbs . He also agreed that the “run on” could be caused by the plugs getting too hot and igniting the fuel after the engine is switched off , as the engine can still suck in fuel through the carb. However after he had taken the cobra out for a test drive, he no longer thought the timing was the main fault, but rather the fuel delivery, as the car played up big time. When he blipped the throttle while idling ,the engine didn't pick up cleanly and there was a puff of dark smoke each time ,which made him think the mixture was too rich and out on the road when he floored the throttle the engine bogged/hesitated and miss fired and it really held back , and bogged badly for quite a while ,then only when the car really got going ,the engine smoothed out and it picked up , and went reasonably well, but not how it used to pull.

    I was pleased with how the engine was performing after Northampton Motor sport had it on their rolling road a few years ago, but Troy didn’t touch the carb as he said the fuelling was fine at the low end, but a tad rich at WOT. According to the print out from the rolling road runs, it ended up running the best at 7 degrees BTDC at idle and 28 degrees at 4000rpm. So I will check to see if the dizzy is loose and if it is , I will reset it back to 7 degrees at idle ,and if that doesn’t cure the problem, I may have to take it back to them or another place somewhere in the midlands nearer to me. So if anyone can recommend a good rolling road tester who is familiar with Holley carbs let me know.
    Last edited by psh; 05-04-19 at 05:19 PM.
    Paul.

    AK Huddart Chevy 383 Stroker, BMW Royal Grey Metal Metallic , 5 speed Jaguar Getrag , Holley 670cfm 4 barrel street Avenger Carb, 450 BHP at flywheel , 460 Ibs/ft torque, and luvin it !

    " We dont stop playing because we grow old, we grow old because we stop playing" - George Bernard Shaw.

  8. #8
    Join Date
    Jun 2004
    Location
    Northampton
    Posts
    1,576
    Paul, two things worth considering. Firstly fuel. There can be quite a bit of variation especially with supermarket fuel. Might be worth waiting until your empty then putting some better quality stuff in like Shell VPower to see if things improve, especially the running on. Secondly, the electronic ignition system. I had a similar problem last year with my Pertronix which caused poor pick up in higher gear which I suspected to be the coil. Once replaced back to normal! As has been said I would be reluctant to go altering a rolling road set up unless you’re sure the dizzy has slipped. Hope you soon get it sorted.
    Dek

  9. #9
    Join Date
    Mar 2008
    Location
    Coventry
    Posts
    2,308
    Paul,

    use a vacuum gauge, it will tell you want timing your engine wants using that fuel. You can check the coil with a multimeter, simple to do with loads of info on youtube

    Paul
    AK, on the road Oct 2012

  10. #10
    Join Date
    Dec 2011
    Location
    Rugby,West Midlands.
    Age
    60
    Posts
    594
    Quote Originally Posted by Lloyd Barnes View Post
    Agree that rolling road is by far the best way to get your timing spot on. Short term you can set the initial timing by connecting a vacuum gauge to your intake manifold and adjusting the timing for the highest reading. That'll get you pretty much there for your particular engine.
    I found that if you have one of the cheaper dizzy clamps on an SBC they dont always hold the dizzy tight and your timing slips. Might be worth checking how well yours clamps
    I marked the position of dizzy to inlet manifold , with a couple of small indentations on the dizzy and on the inlet manifold ,using a centre punch.Then tried rotating the dizzy and it was rock solid,wouldn't budge. So timing hasn't moved.
    Paul.

    AK Huddart Chevy 383 Stroker, BMW Royal Grey Metal Metallic , 5 speed Jaguar Getrag , Holley 670cfm 4 barrel street Avenger Carb, 450 BHP at flywheel , 460 Ibs/ft torque, and luvin it !

    " We dont stop playing because we grow old, we grow old because we stop playing" - George Bernard Shaw.

Page 1 of 2 12 LastLast

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •