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| Will not idle https://slantsix.org/forum/viewtopic.php?t=60996 |
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| Author: | jhdeval [ Wed Apr 12, 2017 1:52 pm ] |
| Post subject: | Will not idle |
I can not get my slant six to idle once it warms up. I have rebuilt the carburetor I have replaced the fuel filter. I have set the rocker arms. I have adjusted the fuel air ratio and the idle screws. I am out of ideas at this point. I am using the HEI ignition system. The car starts no problem but I have to feather the gas the whole time to keep it going. |
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| Author: | Reed [ Wed Apr 12, 2017 2:05 pm ] |
| Post subject: | |
Have you loosened and retorqued all the mainfold bolts in the proper sequence? Can you get an idle vacuum reading? Have you tried advancing the base timing? Try wiring the choke mostly closed and see if it has an easier time running. If it runs with the choke mostly closed then you likely have a vacuum leak. |
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| Author: | jhdeval [ Wed Apr 12, 2017 3:12 pm ] |
| Post subject: | |
Quote: Have you loosened and retorqued all the mainfold bolts in the proper sequence? Can you get an idle vacuum reading? Have you tried advancing the base timing?
I can give a vacuum reading when first turned over it holds a stead 1k RPM at between 18 and 20 Vacuum once it warms up the idle drops to around 750 and hold 16-17 Vacuum. I am running a pretty aggressive Cam as well. I have set the valves to the .015 and .017 as the cam spec sheet shows.Try wiring the choke mostly closed and see if it has an easier time running. If it runs with the choke mostly closed then you likely have a vacuum leak. Oregon Cam Grinding 791 Rocker Ratio 1.5 Lobe Seperation: 108 Lobe Center Line Intake 104 Exhaust 112 Cam Lobe Lift Intake 326 Exhaust 326 Duration @ .015 Intake 284 Exhaust 284 Duration @ .020 Intake 268 Exhaust 268 Duration @ .026 Intake 257 Exhaust 257 Duration @ .050 Intake 230 Exhaust 230 Duration @ .100 Intake 197 Exhaust 197 Duration @ .200 Intake 142 Exhaust 142 Valve Clearance Intake .015 Exhaust .017 Cam Timing @ .050 Open Intake 11 btdc Exhaust 47 bbdc Close Intake 39 abdc Exhaust 3 atdc Valve Lift 1.50:1 Intake 489 Exhaust 489 |
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| Author: | Reed [ Wed Apr 12, 2017 3:16 pm ] |
| Post subject: | |
With an aggressive cam you may need to increase your idle speed. Try setting your curb idle to 1000-1100 RPM. |
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| Author: | afastcuda1970 [ Wed Apr 12, 2017 4:01 pm ] |
| Post subject: | |
You can also try connecting the vacuum advance to manifold vacuum instead of ported vacuum. |
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| Author: | DusterIdiot [ Wed Apr 12, 2017 4:39 pm ] |
| Post subject: | Yep... |
Quote: With an aggressive cam you may need to increase your idle speed. Try setting your curb idle to 1000-1100 RPM.
With the 791, assuming you picked a 10:1 static compression ratio to support it, and centerline at 104, would prefer to have the rpm higher at idle (and will shift the power band to a higher rpm as well). As Reed has stated set your curb idle to 10-1100 rpm. (You will throw the stock specs out the window and tune to best vacuum reading).This can also be helped by recurving the distributor and getting more initial (10-12 BTDC is desired with the mech advance all in by 2800-3000 rpm limited to 30 total mech + initial). |
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| Author: | Reed [ Wed Apr 12, 2017 5:16 pm ] |
| Post subject: | |
Quote: You can also try connecting the vacuum advance to manifold vacuum instead of ported vacuum.
Bad idea in all circumstances when using a stock Mopar distributor. Do not do this unless the distributor being used is designed to use full manifold for vacuum advance. Slant six distributors are designed to use ported manifold vacuum, not full manifold vacuum. Using full manifold vacuum results in improperly retarded base timing and a loss of proper full advance timing when cruising.
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| Author: | afastcuda1970 [ Wed Apr 12, 2017 6:48 pm ] |
| Post subject: | |
Quote: Quote: You can also try connecting the vacuum advance to manifold vacuum instead of ported vacuum.
Bad idea in all circumstances when using a stock Mopar distributor. Do not do this unless the distributor being used is designed to use full manifold for vacuum advance. Slant six distributors are designed to use ported manifold vacuum, not full manifold vacuum. Using full manifold vacuum results in improperly retarded base timing and a loss of proper full advance timing when cruising.The only thing that moving the vacuum hose to manifold will do is add vacuum advance at idle. This will do 2 things, increase power at idle by getting a more complete burn of the leaner mixture at idle resulting in a better, smother idle. The other is it will decrease engine temp at idle. The ported vacuum was introduced to reduce emissions at idle back in the day. |
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| Author: | DusterIdiot [ Wed Apr 12, 2017 8:54 pm ] |
| Post subject: | Close... |
Quote: The only thing that moving the vacuum hose to manifold will do is add vacuum advance at idle.
Correct... and if the vacuum is high enough it might add between 17 and 22 degrees of advance... if his initial is at 12 and he's running a VC-184/185/239... he'll add 17 more for 29... which would be perfect for main jet metering on the road but not for idle....Putting the control of the vacuum advance at the throttle plates instead of the manifold, allows the advance to adjust based on throttle plate position instead of waiting for mean vacuum in the middle of the runner to change based on demand... If you super tune the carb and timing using a vacuum gauge hooked up at the orifice just below the throttle plates, you will notice that the reading will give you a good indication of the plate position while driving (and will be more accurate if the carb is sized correctly for the application). Quote: This will do 2 things, increase power at idle by getting a more complete burn of the leaner mixture at idle resulting in a better, smother idle. The other is it will decrease engine temp at idle.
Lean mixtures are just that, not much fuel, so advancing the timing will start the burn earlier and burn up the mix sooner which can cause over heating and pinging if it's under load.... advancing the timing in the standard driving rpm range will lean a rich mixture out although a rich mixture will suppress detonation/ping because the extra fuel will carry excess heat out of the combustion chamber (on reason old school racers used to fatten the crap out of the mains.... this can also wash down the cylinder walls if way too rich)... that being said if the A/F charge is lazy and or stratified, then the extra advance would be welcome to get a more complete burn of the charge....With a carb at idle the mix is lean on fuel as compared to the mains, but because the throttle plates are barely open and provide not so much air, they are close to a good air fuel mix for low load use if tuned correctly. As the plates open and the rpm increases they will sweep into the transition circuit since the car is passing from the idle circuit but not quite into the mains yet, they are a little richer (bigger orifice or slot than the idle port... on some carbs this is also fed by the idle circuit)... at some point (depending on calibration or modification to the high speed air bleeds...about 1800 - 2200 rpm), the mains come on line at which point you could be at max init+mech and if high vacuum like highway cruise full vacuum advance, at this point you should also have high turbulence and better mix of air fuel than at lower rpm so the extra timing gets the burn started earlier and you get better fuel economy.... if passing or heavy on the throttle (i.e. low vacuum), the vacuum advance drops out and relies solely on init+ mech that way you are not over advanced... (and if really hauling, the secondaries should open... and if really heavy the economizing valve/circuit will open a really make things rich). Quote: The ported vacuum was introduced to reduce emissions at idle back in the day
Considering that this was added back before DEQ sniffing stations and EPA mandated testing, I would surmise that economy / fuel efficiency was the desire... and I don't think that they got the best of either emissions or economy considering that slant six points distributors only come with 13 crank degrees of vacuum advance, and pretty heavy mechanical advance springs compared to the EI distributors that use staged springs and vacuum advances that could be 17-22... (or Feather Duster cans which are 22 at all but full throttle....)
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| Author: | Reed [ Wed Apr 12, 2017 9:12 pm ] |
| Post subject: | |
Quote: Quote: Quote: You can also try connecting the vacuum advance to manifold vacuum instead of ported vacuum.
Bad idea in all circumstances when using a stock Mopar distributor. Do not do this unless the distributor being used is designed to use full manifold for vacuum advance. Slant six distributors are designed to use ported manifold vacuum, not full manifold vacuum. Using full manifold vacuum results in improperly retarded base timing and a loss of proper full advance timing when cruising.Quote: The only thing that moving the vacuum hose to manifold will do is add vacuum advance at idle.
Right.Quote: This will do 2 things, increase power at idle by getting a more complete burn of the leaner mixture at idle resulting in a better, smother idle. The other is it will decrease engine temp at idle.
Wrong. It will advance the timing at idle causing the combustion event to occur sooner, which, as DI has already pointed out, can exacerbate a lean running condition, especially one caused by a vacuum leak. If you are a little bit clever, you will realize that running 20+ degrees of timing at idle will mess up the curb idle and mixture settings of the carb, which, especially if you are running a stock carb, can lead to misadjustment of the carb across the entire operating RPM. Further, if you run manifold vacuum but then read the insructions and set your timing to the stock setting with the manifold vacuum hook to the distributor, you will end up with your base mechanical timing being improperly retarded and you will, in effect, loose vacuum advance off idle and at cruise. A bad idea all around. Running manifold vacuum advance on a slant six is a bandaid to cover some other problem. At best it has minimal impact, at worse it will lead to poor performance, poor fuel economy, and possibly overheating and engine damage. Quote: The ported vacuum was introduced to reduce emissions at idle back in the day.
Wrong. As has been pointed out already, vacuum advance existed before any emissions regulations.
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| Author: | afastcuda1970 [ Wed Apr 12, 2017 11:04 pm ] |
| Post subject: | |
| Author: | Dart270 [ Thu Apr 13, 2017 3:06 am ] |
| Post subject: | Lou's 2 cents... |
Two things: 1) Your valve lash is almost certainly too tight (cam maker specs are rarely acceptable for Slants). I would back off both I and E by 0.005" and see if it idles better. If yes, then back off another 0.005" and check idle again. It will make some noise at this setting. Some noise is OK. If idle did not improve more on the 2nd "opening up" then readjust them tighter again. Look for best idle with a little noise but not loud chattering. You will not hurt anything. 2) Make sure to run ported vacuum advance, and then narrow the slot widths in your mechanical advance until you have 12-16 deg advance at idle and no more than 30 deg at 3000+ RPM (vac unplugged completely). If you cannot modify the dist yourself, contact DusterIdiot or BigSlant6fan on this site and they can build you one. That will likely fix you up, Lou |
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| Author: | DusterIdiot [ Thu Apr 13, 2017 4:43 pm ] |
| Post subject: | It's an old repeating debate... |
That article is great for GM cars and already has been thrown to the peanut gallery years ago, here are a couple of links and some input and debate from other members and SSD. Initial Timing and Classic Dan Manifold vs. Ported and SSD The article smacks of generic GM vacuum advances available yesterwhen, we now only have a limited amount of mopar cans that are available (and the outsourced 3rd world manufacturer has truncated that by only using 8.5x arms even on models that originally went to 11), but these are very adjustable to move that range up or down the vacc reading scale as desired (unless you have VC-208, which is pretty much all in, in a very narrow vacuum range loose or tight on the adjustment...) His cam is really not that big (it is much like the Erson RV30 and is very manageable if he has the compression to support it, and phased it for a more useable powerband... or got some nice gearing), his vacuum readings are actually very good for the build and should be very streetable if not overcarbed, under secondary sprung, or can't keep his foot off the go pedal (The OCG 549 I ran was 250@.050 and at 1000 rpm idle it was only down to 16 in Hg but a little lumpy). Having run this style of build and depending on tires, gearing, and transmission... it would easily like 12-14 BTDC static, using a 9R governor with a set of light small block springs on both pegs, with the mechanical being all out by 2300 rpm, and depending on DCR can sustain best highway mileage/performance at 47-48 degrees total if DCR is about 8.3 and running 87 regular, or up to 50-52 total if running 92 octane... (as stated results will vary based on build specifics and powertrain selections...) Lou is pretty straight up in his response. I agree that the lash should be about .020 on both lobes and maybe a little looser (.022). |
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