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  #16 (permalink)  
Old 02-20-2013, 02:34 PM
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Originally Posted by ggevaert View Post
Wow, lots of discussion.


Thanks for the continued assistance;

Cheers
Larger air bleed = leaner idle. If this is QF recommendations, I'd stay w/it unless the engine performance/plugs/a-f meter dictates otherwise.

As has been said already, use as much timing as the engine wants. The upper limit is determined by how it starts up, if it pings at throttle tip in (this will become more evident when the vacuum advance is brought on-line). But using the absolute maximum amount of initial timing isn't necessarily the best way to go about it. It is my opinion that using a medium amount of initial timing (which gives a broader amount of mechanical timing) should give better control of the spark timing over a broader range of engine rpm.

An example of this thinking is seen here, where Demon carbs recommend 14-16 degrees BTDC for your engine/cam/carb size as a safe starting point. I'd recommend you work upwards 2 degrees at a time from there.

When it comes time to set up the vacuum advance, if for some reason you end up using 24-plus degrees initial, you may find using ported vacuum better than manifold vacuum. Generally speaking, you won't need more than 10-12 degrees of vacuum advance.

More on timing is here. The page is on GM HEI, but the timing info is relevant regardless of the distributor.

DO NOT be hesitant to readjust the idle mixture screws! They should be readjusted after just about any change made to the engine perimeters (timing, idle speed, plug heat range, idle air bleeds, etc.), and as long as the engine is fully up to temperature (fully- as in after a drive or a long warm up if still undriveable), you cannot go wrong readjusting them as needed.

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Old 02-20-2013, 09:10 PM
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Timing/Carb

Hi,

F-BIRD'88, Idle timing is rock solid. Interesting about the vacuum, i thought that was odd but attributed it to the cam and low idle RPM; I will have to review the cylinders if any are missing; not sure. (i'm rather hard of hearing so I need to use alternate methods to investigate those types of things, any suggestions?). I did check for vacuum leaks with propane and none were found. I will hookup the wideband AFR this weekend.

Using a Quickfuel SS 680VS with pri and sec idle air bleeds changed from stock 70's to 74's. (as recommended by quickfuel when I provided them with the engine specs). The high speed air bleeds were left alone and are 28/28.
Everything else on Quickfuel was also left alone (primary main jet 68, Secondary jet 78, pri nozzle 31, needle and seat 110, power valve 6.5). As I got through my carb dialin I will be reviewing each of these to ensure they are the correct setup for my application.

I actually bought a wideband AFR electronic setup that I have not yet hooked up. Was planning to get basics on engine done then go back over the setup with it on. Never used one before so need to learn how.

cobalt327, I understand; At the beginning of this timing exercise I started at 18 degrees initial with an advance stop of 18 degrees (which was working well) but I decided to go higher to see how that would look. Hence my original question as I was trying to understand the reasoning for why we set initial timing where we do and why we stop where we do. I did increase the timing 2 degrees at a time from there, every time checking for starter issues or detonation and found none all the way to 24. I stopped there for no reason other than I was worried about going higher. Also, keep in mind that I have not been able to take the truck for a drive as I have no windshield in it yet (and its 20 below out here) so my only load testing has been with my foot on the brake and with a chock block on the wheels.

Vacuum can is the stock MSD one, it has a vacuum advance stop plate installed and set to 13 degrees; with the vacuum hooked up, at idle my timing was 30 (so +6 from initial) and at all in (3000 rpm) it ran at 42. Keep in mind that was at idle and not under load.

Mine is a GM HEI so the article you note is very relevant and, yes, I did read it thorougly.

I do plan on adjusting the idle mixture screws; that is the plan this weekend; I just wanted to get timing done before going there. (but I do understand it sometimes is not a linear exercise; you think everything is set, take it for a drive and end up re-adjusting everything ) but that's the fun part.

Cheers

Last edited by ggevaert; 02-20-2013 at 09:19 PM.
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Old 02-21-2013, 11:56 AM
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Originally Posted by ggevaert View Post
Also, keep in mind that I have not been able to take the truck for a drive as I have no windshield in it yet (and its 20 below out here) so my only load testing has been with my foot on the brake and with a chock block on the wheels.

Vacuum can is the stock MSD one, it has a vacuum advance stop plate installed and set to 13 degrees; with the vacuum hooked up, at idle my timing was 30 (so +6 from initial) and at all in (3000 rpm) it ran at 42. Keep in mind that was at idle and not under load.

Mine is a GM HEI so the article you note is very relevant and, yes, I did read it thorougly.

I do plan on adjusting the idle mixture screws; that is the plan this weekend; I just wanted to get timing done before going there. (but I do understand it sometimes is not a linear exercise; you think everything is set, take it for a drive and end up re-adjusting everything ) but that's the fun part.

Cheers
Good deal. My only comment at this stage is that you cannot judge detonation (or the lack of it) w/o driving the vehicle. You can't duplicate the same load it'll see on the road using the brakes, unfortunately. What you'll want to do is go back to ~18 degrees initial and test it on the road after it's roadworthy. Then readjust from there. If you choose to leave it as-is, just be uber cautious and listen carefully for detonation.
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Old 02-21-2013, 11:12 PM
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Timing

Thanks Gents, I have some work ahead of myself.

F-BIRD'88, Quickfuel sells air bleeds in every size so I will just get one of their assortment kits at summit for $50, they come with every size from 36 up to 75. With items that small and precise I don't really trust myself to get them correct.

Don't know what cam brand is in there; it came with the motor and I'm trying to work with it.

My torque converter is 10", I purchased it new from a local reputable racing transmission shop because I wanted a good one.

How do you know a power valve is fluctuating at idle?

Also, I'm running 94 octane sunoco fuel so that will allow me some detonation headroom.

Cheers
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Old 02-22-2013, 05:24 PM
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Detonation

Cobalt327, are you saying that even though I have a mechanical advance stop that stops it at 36 degrees I can still get detonation?
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Old 02-22-2013, 11:13 PM
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Quote:
Originally Posted by ggevaert View Post
Cobalt327, are you saying that even though I have a mechanical advance stop that stops it at 36 degrees I can still get detonation?
Detonation can come from overadvanced initial timing, as well as too much total timing. You would notice it at the hit of the throttler under load.
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Old 02-23-2013, 11:14 AM
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detonation

Ok, Thanks for the clarification; truck was running on the road great at 18 so I will continue working with 24 and keep an eye out for detonation under load.
Cheers
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Old 02-23-2013, 10:24 PM
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Originally Posted by MouseFink View Post
Engine Timing 101:

Regular production distributors have timing advance that starts at 700-900 RPM. The timing advance in distributors used in high performance engines starts at 1100-1200 RPM.

1.) Engines equipped with some high performance camshafts will not idle below 700 RPM.
2.) Since the centrifugal advance in a stock distributor starts at 700 RPM, you cannot set the initial timing.
3.) In order to lock down the initial timing in a engine e/w a high performance camshaft, you must have a distributor that starts the centrifugal (mechanical) advance at 1100-1200 RPM.

A distributor with a higher RPM centrifugal advance mechanism will allow you to turn the engine idle speed low enough so you can set the initial timing before the centrifugal advance starts. After the initial timing is locked by the distributor bolt, you can then turn the idle speed above 1100 RPM and observe the initial timing plus the centrifugal advance.

If your timing is 22 degrees BTDC, the initial timing is 12 degrees BTDC with 10 degrees timing advance in by 1200 RPM. .
Funny, my timing is set at 22* at idle. Idle is 1000rpms. When I drop the idle to 700rpms the timing stays at 22* When I raise it back to 1000rpms and the rev it, it starts to build after about 1200rpms and is all in by 3000rpms, 36* total. How did I do that? Cam is .660" lift, 253*@0.050" 110LSA 106ICL

Skip White HEI distributor with tuned springs, plates and plate stop. Maybe $50 in it total. Pretty easy to prevent advance below 1200rpms with a spring change and new plates.
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Old 02-24-2013, 01:08 AM
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Bad tidings

Ok, got some problems, something went south and not sure what that is yet. Need some help here.

Started up the engine today to verify my new tach and AFT gauge installation and to describe that it ran terrible is an understatement.

I have made NO changes (other than these gauges) since last time it ran, which was excellent.
* First could not start it at all; could only get it to run for a little bit using ether. tapping the accelerator to give it gas was having no effect and eventually was just causing it to flood.
* We finally got it going, roughly, but then she ran rough and at very high RPM's of 1600-2000 (compared to where I left which was 850). Playing with the idle has eratic results; it either goes really high (2000) or stalls out.
* when it was running really high I dialed down the idle to get it to come down and then I got lots of backfiring and engine would stall around 1100.
* a very heavy fuel smell around the engine
* and the oddest of all, when I look at my timing light, which was hooked up and in RPM mode, it would every once in a while go to all zeros; which means no signal. So I think that means I'm not getting a spark to plug 1 (I only see all zeroes when I turn off the engine so this is definitely odd).
* fuel pressure gauge stayed where expected; between 6 and 8.

The last point makes me think my distributor went south but maybe the cylinder is flooded to the point where the spark plug can't fire?

I verified everything just to be sure (wires correct, no vacuum leaks, etc) and I disconnected all newly installed gauges to make sure there was nothing they were doing to interfere. What gets me is that nothing was changed since the last run otherwise I would focus on that area.

I do have my old distributor so I will be putting that one in to eliminate any potential distributor problem.

Any help and/or troubleshooting tips are much appreciated

Gary

Last edited by ggevaert; 02-24-2013 at 01:16 AM.
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Old 03-09-2013, 11:00 AM
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Bad dizzy

Looks like the Dizzy went bad; dropped in another one I had sitting around and it ran again. Had to clean plugs as they were absolutely black; likely from the constant stop/starts in testing the dizzy. jeez.

Ordered a pertronix unit to replace bad dizzy.
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