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| ignition time.. https://www.slantsix.org/forum/viewtopic.php?t=57805 |
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| Author: | mgtumwater25 [ Wed Jun 03, 2015 9:45 pm ] |
| Post subject: | ignition time.. |
So I've been driving the slant six for a week now with no problems but have been curious about where others think my ignition timing should be...the truck is an 85 d100 with a holley 1945 , I have converted it to an electronic ignition as well. The underhood tag reads 16° btdc . My question is should I stick with this number even though I've gone away from the lean burn? Also I set my base timing at 16 btdc and when I plug my vacume advance in it shoots to about 26btdc @ idle....it seems to run decent and I don't notice any power gain or loss by adjusting between 12°/22° or 10°/20° I also cannot hear any detonation or motor noise develop between adjustments ...the only thing I notice is the exhaust pops slightly more around 16°/26° Also I do not have a ported vacume tube on my carb they all deliver straight vacume |
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| Author: | Josh P [ Wed Jun 03, 2015 10:22 pm ] |
| Post subject: | |
I'm not at all familiar with the lean burn setups so I won't be much help there. However, most have their initial timing set to something like 50-56° - total advance. So depending on how much vacuum and mechanical advance you have 16-ish° is normal. I find it odd that you have your vacuum advance connected to manifold vacuum rather than ported vacuum. The 1945 should have a ported vacuum at the base of the carb next to the PCV port. Take a look at this diagram https://app.box.com/s/fkunm43723zt01b33pgz Also, remember to set the initial timing with the vacuum advance disconnected and plugged at the source. |
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| Author: | mgtumwater25 [ Wed Jun 03, 2015 11:07 pm ] |
| Post subject: | |
Yeah all the diagrams say this is a ported vacume source but at the "ported vacume tube" I get straight vacume all the time....so I set my ignition time at 16°btdc but when I plug the vscume back in it jumps straight to 26° btdc...it seems to run good here , can't here any detonation going on but at the same time my exhaust is pretty loud so it's hard to play it by ear.... |
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| Author: | mgtumwater25 [ Wed Jun 03, 2015 11:14 pm ] |
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Btw The only vacume line I am running goes from the ported vacume tube on my carb, to the heated air delay valve , then from their it goes to the vac advance on my distributor so I don't get any vacume advance at startup when it's cold |
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| Author: | Reed [ Thu Jun 04, 2015 1:14 am ] |
| Post subject: | |
1980s lean burn systems came in three style- computer controlled carb only,computer controlled ignition only, and computer controlled carb and ignition. All lean burn computers calculated engine load by constantly monitoring manifold vacuum. Any carburetor intended for use with a lean burn computer, even if the carb itself was not computer controlled, will have a vacuum port that provides a full manifold/non-ported vacuum signal. It sounds like you have a Holley 1945 that was originally intended for use on a lean burn motor. It could very well not have a ported vacuum source for the distributor. The only way to fix this problem is to install a carb that has ported vacuum. An older 1945 or 1920 can be adapted. Base timing with a non lean burn ignition system should not be set at more than 12 BTDC. 16 is a bit much initial advance unless you open the distributor up and play with your advance curve. Try starting at 10 BTDC and leaving the distributor vacuum advance disconnected until you install a carb with the right vacuum signal. |
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| Author: | cpslntdchrg [ Thu Jun 04, 2015 6:01 am ] |
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But, I reccomend you to never give your carb as a core. Remanufactured carbs are usually garbage, even when sometimes they work (in my case, 1/3 of the times). You know yours work, even if it does not have ported vacuum. Best ones are new old stock you can find sometimes in the internet or in this forum, but if you roll your dice eith a remanufactured you can put the original carb back if the remanufactured is crap. |
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| Author: | Josh P [ Thu Jun 04, 2015 3:26 pm ] |
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Reed, are some lean burn distributors fully computer controlled (no mechanical or vacuum advance)? |
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| Author: | Reed [ Thu Jun 04, 2015 3:36 pm ] |
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Quote: Reed, are some lean burn distributors fully computer controlled (no mechanical or vacuum advance)?
Yes. By definition, a distributor referred to as a "lean burn distributor" is one where the timing is completely controlled by the computer. A "lean burn distributor" has no mechanical or vacuum advance mechanism, and has two pickups in the distributor. It is easy to spot a "lean burn distributor" because it will have no vacuum advance pod and four wires coming out of the body for the two pickups. If a distributor has a vacuum advance pod, it is not a lean burn distributor.
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| Author: | DusterIdiot [ Thu Jun 04, 2015 7:40 pm ] |
| Post subject: | Depends.... |
Quote: A "lean burn distributor" has no mechanical or vacuum advance mechanism, and has two pickups in the distributor.
I actually have been prowling for these for drag racers running locked out timing and I've seen 3 different types for the late 225 trucks...I have 2 from the early 80's that have no internal mechanisms and only have one fixed pickup on the plate... I have a few from 1983-1985 D-150's that have the dual pickups (10 degree phasing), and the plugs are normal 2 wire units... I have one oddball from a 1986 D-150 that has the dual pickups with typical phasing but the base pick up has 3 wires (looks like the 2 wire EI plug but the 'low' notchout has 2 wires and corresponding bullet receptacles...) Not sure what the change was that year, but the carb on the 225 was a feedback 1945... |
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| Author: | Reed [ Thu Jun 04, 2015 8:41 pm ] |
| Post subject: | |
Interesting! Thanks for sharing that DI. It is always good to learn these details. |
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| Author: | mgtumwater25 [ Thu Jun 04, 2015 10:36 pm ] |
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So is turning the vacume advance back an option? Can I set base time at 12btdc and turn the vacume advance down through the vac pot till it hits 16 which is what the truck said to run ? Kinda set it at a medium point I guess? |
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| Author: | Josh P [ Thu Jun 04, 2015 11:29 pm ] |
| Post subject: | |
The underhood tag's recommendation of 16° is no longer valid since you have converted the ignition system. 16° may still be a good base timing setting but its at the upper limit for even the shortest advancing /6 distributors. Here are a few things to consider if you are concerned with your base timing setting. 1. As Reed mentioned, if your carb's "ported" vacuum was meant to feed a lean burn system rather than a distributor, you will need to replace the carb to get the ported vacuum required for your vacuum advance to function properly. 2. It is apparently common for the timing mark on the vibration dampener to slip as it ages. If this is the case for you it would be very difficult to set the timing accurately. 3. Your base timing with depend on how much mechanical and vacuum advance your distributor is setup to provide. I believe some distributors provide as little as 18° of mechanical advance and others as much as 30° (can someone verify?). With such a wide range it is difficult to recommend a specific base timing. |
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| Author: | Reed [ Thu Jun 04, 2015 11:30 pm ] |
| Post subject: | |
Not really. The best thing you can do right now is set your base timing in the 10-12 BTDC range and not use the vacuum advance. Save up and get a carb that has the correct vacuum source for the vacuum advance. |
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| Author: | Nicademas [ Thu Jun 04, 2015 11:33 pm ] |
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I would try 12 deg then 10. I have a 79 D-100 somewhat modified and the stock emishions sticker says 12 degs. I recently changed it to 10 and can feel more tork at takeoff and climbing hills. Keep in mind I have no EGR or lean burn so yours my differ. |
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| Author: | Sam Powell [ Fri Jun 05, 2015 3:57 am ] |
| Post subject: | |
If you have a carb that works, I would not change it. You can easily add a ported vacuum port to that carb. Simply drill a hole in the side of the carb above the throttle plate, but below the venture, and JB weld a tub into the hole. A small metal tube of almost any kind will work. When I was still running mechanical timing with my EFI I had to do this to the Buick throttle body and it worked fine. It has since been made obsolete by the electronic timing control since added, but it was absolutely fine with the added tube. Set your timing at 10-12 without vacuum advance hooked up, as said many times here, and it should not change when you hook up the vacuum advance to the ported tube. The problem with hooking it up to manifold is that constant manifold vacuum will make it idle fast because it will advance the timing. So, you dial the idle down with the carb adjustments. Then if there is a slight blip of any kind in the idle, vacuum drops, idle drops again, vacuum drops again, and the engine dies. Having the idle depend on vacuum advance being steady is a recipe for poor idle stability. One the throttle opens up, it makes no difference where the vacuum advance is referenced. This is all about getting it to idle well. This is more important for an automatic transmission as there is no load on the engine at idle with a stick shift. I suspect this is one of the primary culprits for those situations where folks complain their engine idles fine until they put it into gear with an auto, and then it dies. Manifold vacuum drops when you pull it into gear. If you are referencing manifold vacuum at that point, timing will drop, and the engine will go into the death spiral. Sam |
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