I'm not sure what you can be doing to use the timing light incorrectly. I assume you are placing the pickup on the #1 cylinder, and beyond that there's very little to do wrong.

 

This is a aftermarket dist. It certainly can have too much advance.

As an experiment, move the dist with the timing light till it's at 2 degrees. You might have to temporarily set the idle speed up a little bit for it to run. Then see if the bucking is less with the vacuum hose hooked up. You may find this is the case. I would call the DUI people and see if there is an adjustment on their advance, get their advice, maybe they have a different advance unit that has less advance when the vacuum is applied.

I have had this happen with different engines. Too much advance can cause your symptoms. I have also had them buck for the same reason, but only when they got warmed up and cruising. A colder plug fixed that. But your problem seems more severe.

 

Heres what we have after the final trip to the garage.

I can not get any change in timing or RPM by simply sucking on the hose to the advance on the distributor. And no pump to test further.
So I adjusted my idle way up bout 1000 RPM and then retarded my timing to bring the idle back down to 650 RPM.3 good whips on the throttle and she settles back at 650, still pretty smooth but could probably take a little tweaking. Now hook up advance to PORTED vacuum, head out for a drive, If I'm VERY light footed on the gas, I can accelerate with very minimal bucking, basically, if I can bring the vacuum on slow enough with my foot, she'll drive and will only start bucking mildly at the top of the gear.

So I'm obviously getting somewhere.
Something to note.....when I Timmed the motor before, I set the light to 10.0 degrees advanced, then adjusted the dizzy so that the timing marks lined up. This time, after retarding the dizzy quite a bit, I left the light set at zero but dajusted my dizzy so that my timing mark on the balancer lined up with the ten degree mark on the plate, its still painted white (sort of) from before I purchased the vehicle as is the mark on the balancer. Now, if I adjust the light to 10 degrees advanced, the mark on the balancer will move into the hole that marks TDC and 0 degrees.
So I believe I've just taken my motor from roughly 20 degrees advanced down to ten.
I ran out of time to try messing with the timing any more but I'm guessing with some more retarding to the timing I may be able to get the advance to function as it should.

Unless I'm completely wrong in my thinking.

Snapped one pic of the timing plate but didn't turn out well. Not sure how to post 'em yet either. But I've seen a few pics of it here on the forum. Mine does NOT have a deeper V at 10 degrees. But still looks like a sawzall blade with a resebalence to a tape measure stamped the length of the plate.

 

It sounds like you are getting your timing light figured out. When setting initial, keep the **** set to zero. The only time you need to move the **** is when the balancer and timing marks don't go high enough such as when you want to map the entire curve. For example, if you wanted to know what the advance was at 2500 rpm, you would hold the engine at that speed and rotate the **** on the light until the balancer marks showed zero. If you continue to rev the engine higher you can find where the mechanical advance maxes out. If you then add the vacuum advance, you can find the total advance at any rpm.
I don't know because I have never had one, but I would think the DUI would have an adjustable vacuum advance can. If it does, it sounds like you just need to tweak it a little and you will have it.

 

I hadn't thought about you having one of those fancy timing lights. Doh! But, I'm not sure I followed what you did with it, although you are convinced that you now have 10 degrees before TDC. So, if that's the case and you still can't drive it properly your advance must be giving you some huge advance numbers. With that I agree with Dave - call DUI and find out what the deal is with that advance. How much advance does it have? Is it adjustable, and if so, how much? You are going to need to crank in a lot of preload so it won't get to full advance until something higher than 20" of vacuum, and that may not be possible.

 

You can keep fiddling with it, and if you like the results and it runs good, fine. I will warn you though, I have ran into this some with the v8's, and they seem to run better with the static timing set higher than the sticker recommends(this is after they have been modified with different carbs, etc.) In other words, if the sticker on the radiator recommends 8 or 10 degrees BTDC and you set it there, the engine is very doggy on the low end. If it's set at 12-14 degrees, the engine runs much better with just a light touch of the throttle making it go. But when you do that, you can run into the same problems you are having now.

So what I am saying, get it running the best you can, but you may find running the initial higher may make the engine more responsive, and if it does you will have to do something else with the vacuum advance unit.

 

Hope everyone had a good Christmas!!

Few updates on my issues....

Sticker says 8 degrees timing. Truck really likes 14 degrees timing. Seems to run well.

If I apply vacuum to the advance with a mitivac, I get a slitght advance, almost un-measurable, at 5hg. nothing untill 5hg. I get roughly 4 more degrees advance up to 10hg, about 1 degree for every 1hg. I get another 4 degrees up to 16hg, but it comes on at about 14hg and at 15hg I've got 3 of those degrees with the last degree at 16hg. I can pull another 3 hg on the advance unit but I don't get anymore advance after 16hg.

I've been all over the place with the static advance, I can make the engine idle well just about anywhere, but the less timing the rougher it will run and the less power I have.

Hooking up the vacuum advance at any setting produces the same results, runs like crap if at all.

I HAVE NOT hooked the vacuum advance to manifold vacuum but I think I may try that tomorrow.


I've also discovered my muffler is shot. I can spin the can on the inlet/outlet pipes, so its likely collapsed inside, probably causing some issues.

I'll report more tomorrow.

Thanks for all your input!

 

I don't think you will like that, but certainly try it, it might work out in your case.

 

Some updates......And more questions!!

Cut off the muffler assuming it was collapsed inside probably restricting things. Was actually excited about this as I was curious how it would sound. Muffler is shot, but upon removal, I had no change in sound, NONE! I had considered the cats being shot from the overly rich condition for so long, and had planned on replacing eventually. I've also had an air/fuel gauge for a while but with the O2 sensor siezed in the manifold I had yet to install it.
So....New cat installed, O2 sensor left in manifold, not wanting to mess with it, and welded a sensor into the down pipe where the "air tube" port was since I wont be using the air pump, nor had I been using it for quite some time.
Sound??? Interesting since I havent put a muffler on yet, dont care for the current tone with the exception of the tractor tone when I lug it a bit. However it is exiting under the cab, maybe once I get it out from under the truck it may be more desirable.
Idles great, runs a bit better, havent tried the vac advance yet. Its currently disconnected.

My question at the moment, regarding the air/fuel gauge, at idle, gauge shows 10.5 to 11. Under normal load its any where from 14 to 16 and I can peg it at 18 with the pedal if I wanted to. So whats considered "perfect" when it comes to the gauge reading?? I recall reading 14.8 is considered "stoich" I believe was the word. So for the 4.9L, whats a good number for Idle, cruise, and under acceleration?
Also, while considering mufflers, does it matter where I put it? meaning...does it have to be a certain distance from the cat? Is there any point thats more beneficial then another? The replacement muffler that I removed was roughly 18 inches away from the cat and used the OE mount location and hanger.
Once I know I've got acceptable air/fuel mixture, I'll try the vac advance again now that I know its exhaling the way its supposed to.

 

The muffler can be as close or as far from the cat as you want it.

As for the AFR, your idle is too rich. I have Rusty idling at 12.5 and want to lean him out a bit - on the idle only. And the reason for doing that is that idle influences the AFR at very small throttle openings, and I'm seeing a rich condition below about 30 MPH. And when I say "rich" I mean anything less than 14.7, which is the ideal where all the fuel and oxygen are burned.

Having said that, 14.7 isn't ideal for fuel economy. I have Rusty running between 15.0 just above 30 MPH and something like 16.0 at 65 to 70. But my understanding is that something like 16.5 is about the max you want to run under normal circumstances. And with Rusty if it gets to 18.0, like in cold weather when the choke comes off early, the engine is just about to bog.

But, I will say that where you put the O2 sensor as well as the sensor itself and the meter all determine your reading. The closer you get to the exhaust port on the engine the richer the reading. So, if your engine loves to run at 17.0 then your readings are probably a bit lean.

 

Thanks for the quick response. I'll play with the mixture screw and see what happens. As for the location of the sensor, its 21 inches further down stream then the OE location in the manifold. Sounds like my accel and cruise readings are decent. This carb is supposed to be a bit leaner then the carb that was on there. I'd hate to think what the gauge would of read with the other carb. It was rich enough to soak through at the EGR block. It was always damp with fuel.

Thanks again! I'll be sure to report back once I can mess with it more.

 

Some progress...

Air/fuel mixture at idle hangs aroung 12.5-13 which leaves my mixture screw at 1 1/2 turns out, Idles at 650 RPM might increase that a bit maybe smooth it out a bit more. Ended up backing off timing about 2 degrees. Runs well pulls well, I'd like to richen up the the off idle a bit. I thing I read theres a screw atop the needle I can use to fatten up the carb above idle. Or am I mistaken??

As for the vac advance, I can connect it while idleing, and while parked I can increase RPM and get minor bucking. while driving I get more bucking but I can use my foot to minimize it and get down the road. So obviously progress regarding the vac advance. Still have it disconnected though, in order to keep the bucking at bay, I need to accelerate extremly slow.

Probably wont get to mess with it more till this weekend.

 

Different truck; same problem. My guess was that the 36 mechanical plus the huge advance as soon as vacuum is applied are tuning for EGR application and with the EGR blocked/removed that there was just too much total advance. I recall long ago having mechanical of 22-26. I just left the vacuum off and truck is quite drivable.
I think when you get runamuckr fixed I'll know how to fix mine.

Is 36 mechanical too much for non-EGR application? How much total advance is too much at say 3000rpm?

 

I have just read the 3 pages of this thread and might have something.


Gary, I have quoted you as I believe you can better explain to runamuckr what I am about to detail...


First, runamuckr, it looks like the DUI distributor is 2 1/2 years old (?). Did the bucking at higher RPMs start after installing the DUI unit?


Gary, remember when we talked at the OK-GTG and I told you about the bucking problem I previously had with my modified Duraspark update? [By the way, runamuckr, I, too, use a Carter FY from a 1970 F350 on my 1986 4.9L engine.] I have a MSD 6AL box being triggered by the Duraspark [recurved] distributor. Upon initial installation and set @ 14 degrees BTDC, the engine fired up and sounded great @ idle [~700 RPMs]. If the throttle was quickly opened and closed, the engine revved up quite well. But, if the throttle was slowly opened to ~1700-1800 RPMs and held, the engine would start cutting out and sounded like a rev limiter was cutting in. The truck could not be driven without violent bucking.


I installed a restrictor tube in the vacuum line, which somewhat delays the vacuum signal, and this might (?) have delayed the bucking, but it was still there and just as bad. I drove some time with the vacuum disconnected from the canister on the distributor. I played with timing and actually got it to run well, but when timed for the best gas mileage I could obtain, the engine would sometimes drag when starting [too much initial advance].


After much Internet searching, I found that the two wires from the distributor which trigger the MSD box were wired backwards []. This is where Gary can better explain what was going on, but the best I can explain and understand, the signal is a sine wave and the MSD box is triggered by the leading edge [as it rises]. If the wires are reversed, it is as though the electronic wave is flipped and the MSD box is triggered by the trailing edge. However it works, the MSD box advanced the timing by a great deal, so the position of the distributor had been rotated quite a ways clockwise [retard on the 6 cylinder engine] to compensate.


When the two wires were connected correctly, the engine would barely start and idled @ ~350 RPMs [I was amazed that it continued to run!] The distributor was rotated CCW [a lot!] to advance and the idle was adjusted. Anyway, the engine now runs great with the vacuum attached and I get better mileage than ever, before.


Now, I know you have a DUI and are not using MSD, but perhaps from the factory [when you purchased the DUI], the internal wiring in backward?


What do you guys think? Is this possible?

 

David - I do remember that. And, if I remember correctly, the theory about your issue was that by going off the wrong side of the waveform the rotor wasn't pointing where it should have been. At low RPM it worked ok, but as the advance came in the rotor finally got so far away from the terminal for the proper cylinder that the spark started going to the next terminal and, therefore, to another cylinder.

And, while the wiring in the DUI dizzy is all internal, it is possible that its rotor is improperly positioned - either because it is firing off the wrong side of the waveform or because the pickup is improperly positioned. And, that would certainly cause problems like are being experience. Good thinking!

Runamuckr - You should check your dizzy out. One thing to check is that the rotor is pointing at the center of the terminal when the reluctor is in the center of the pickup. And, you could put your timing light on whatever cylinder is second in the firing order and rev the engine up while watching the timing on the damper. If you start seeing it jump around and the timing suddenly start showing up within the marks on the damper, then you may well have the problem we are discussing.