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ALLstrokedOUT
08-04-2008, 04:48 PM
Sorry , its a little lenghty, but its a complicated problem.
We were trying to get the duster out for its first run around the block and we did that, but on the way back it sounded like it was running rough, maybe missing a cylinder. after letting the engine cool down we took out and cleaned all of the spark plugs, most had carbon on them from blow-by (7 miles on the new rebuild)
But after putting them back in after cleaning them, and starting it back up, it still ran just as rough. We stuck the timing light on it to see if anything got bumped out of place, we tried a few things with the ignition module, heres what were using for ignition:
Mallory Hyfire VI-AL ignition box
Mallory Unilite ignition module in the distributor
Mallory Promaster ignition coil

With Switch #4 down (for when not using Accel 3 stage rev limiter -we arent using one) everything was fine timing wise, but when increasing the rpm:
1. The timing light would zone out/ intermittently stop blinking (engine wouldnt increase rpm).
2. The tach would zone out also (not show above 1300rpm).

When Switch #4 was pushed up (intended for use with Accel 3 stage external rev limiter- we dont have one) The engine seemed to respond better, and the tach would read past 1500rpm, but would jump around a little more. The Timing light worked, but was showing 2 readings 20 degrees then 50 degrees 20,50,20,50...

What could be the problem? I soldered all the connections, and checked them all, as far as i can tell everything looks correct, and its still running- it ran perfectly for five miles, then the last two something happened to cause this problem. Please give me some advice of what to look for, anything; anyone, please help, we gotta get this thing running right again. Thanks.

ALLstrokedOUT
08-05-2008, 06:33 AM
Think i figured it out with the help of Mallory, not sure though as i havnt driven it yet. Heres what it was if anyone ever has this problem:

According to them the two wires going to the coil (orange and black) from the box were acting as an antennae picking up interferance when they were running next to each other. so they told me to twist them together, and make sure the box is grounded well (it was). I dont know how something like that can just happen in the middle of a drive, but ill see how it works out.

derekf
08-05-2008, 06:45 AM
Thanks for the followup. Please let us know how the test drive goes.

ALLstrokedOUT
08-09-2008, 11:36 AM
I wish i could say everything is great now, but its not. It ran fine for over 60 miles, and then comming back from getting it aligned it started acting up again- like tuning a switch on (or off). The same symptoms as before- very rough running like missing a cylinder IF at or around 2000rpm; if above 2500 it runs great under all throttle levels.

I checked the plugs, cleaned them, and it still runs like crap. The timing is fine, unless its the vacuum advance not kicking in at the right time; Its in the ballpark though, could that cause this? Im getting about 10inHg direct vacuum at around 1000rpm (dist. is hooked up to ported vac.)

Could it be the carb tuning, its a demon 725cfm out of the box (other than checking it and cleaning it out)? It idles fine runs fine over 2500rpm.

I have no clue what it could be, any ideas anyone?

EDIT: i played with the vacuum advance, and screwed it all the way in so the least amount of vacuum will engage it. Its running a lot better, but still has a little bit of a studder, i may be able to work that out.

ALLstrokedOUT
08-11-2008, 03:37 PM
Well, as i said i screwed the vacuum advance adjustment all the way in so that the least amount of vacuum would actuate it, but its still takes too much vacuum at that rpm to bring it to the proper timing.

I tried to just advance the overall timing as an experiment to see if it helped, it didnt. I tried re calibrating the vacuum advance and to use direct vacuum, but had the same results.

I thought it might not be the ignition, so i changed the jetting on the carb to lean it out, and i also adjusted the idle mixture screws until highest manifold vacuum. Neither of those things helped.

So, heres a video i tried taking while driving (i was watching the road more than the camera, and video quality shows) But as youll notice once i get past 2000rpm it runs fine in 1st gear. 2nd gear theres a bit of a studder and itll linger at 1500rpm before accelerating. When i go into 3rd gear i am trying to accelerate the whole time, but cant go over 1500rpm. When im in 3rd gear im not trying to accelerate hard, im only using maybe 1/4 throttle-more throttle and it only gets worse.
Can anyone help? Ive tried everything i can think of over the last three days and have seemed to hit a brick wall.
https://static1.pt-content.com/images/noimg.gif (http://s179.photobucket.com/albums/w293/63vette77/duster/?action=view&current=dusterproblem.flv)

ALLstrokedOUT
08-13-2008, 07:47 AM
I think i found the problem, maybe. It still has to do with the vacuum advance. I hooked up a vacuum gauge and snaked it into the interior so i could see what its doing when it bogs down.

When it gets into 3rd gear, crusing the vacuum is at about 15-20 inHg depending on the speed im at. But when I try to accelerate more, as it begins to studder, the vacuum decreases at exactly the same time, it goes down to about 3 inHg. When it goes down that low im assuming that the timing is retarding, since the vacuum advance is no longer working with that little of vacuum.

So how do i fix this? obviously as i open the throttle the vacuum is going to go down, but how do i keep the vacuum advance working on only 3 inHg of vacuum? Is there some sort of low operating vacuum advance?

paul67
08-13-2008, 10:38 AM
Are there more than 1 vacum port on the carb ,if so try it in another 1. I had the same problem with SU carbs on a range rover.

paul67
08-13-2008, 10:46 AM
According to there demon website the vac pipe should connected to the outlet closest to the base plate hold down hole.

ALLstrokedOUT
08-13-2008, 10:57 AM
yes, the vac. advance is connected to ported vacuum, the manifold vacuum is right behind it on the base plate. just for fun i tried switching it, made no difference, i have the same carb on my 77' vette, and it runs great, ive been going back and forth to compare things, nothings out of the ordinary.

But for some reason theres a drop in about 12 inHg when i open the throttle 1/8 of the way when crusing. The 77' vettes only decreases by about 5 inHg at any amount of additional throttle. Thats what i need to find out now: Whys it losing that much vacuum, and how to stop it.

paul67
08-13-2008, 11:12 AM
What gearbox are you running ?

paul67
08-13-2008, 11:14 AM
It might pay to doudle check the vac pipes as 1 might be letting in under load. If you are running the same on the vette poss to swap pipes or last resort swap carb.before spend money.

paul67
08-13-2008, 11:16 AM
What about camshaft that can alter vac depending on lift and duration.

ALLstrokedOUT
08-13-2008, 03:12 PM
Im running an A-833 4 speed for a transmission.

There are only three vacuum pipes on the Demon carb and all or them are plugged up, i spreyed some carb cleaner at the base of the carb while it was running to see if there was a leak there, there wasnt.

I swapped the carb with the 77 vette, it ran noticably better, but still had a horrible studder problem under load, it just seemed to be up higher in rpm than before so i swapped carbs back.

The cam is a hydraulic roller Comp Cams thumpr series cam Duration @ 050 - 243/257 and Lift - .533/.519 I would expect the vacuum to be bad, but bad enough to alter the vacuum advance under load? Theres only the brake booster running on vacuum other than the advance, and ive disconnected it.

Can an extremely rich/lean accelerator pump setting or jetting or improper power valve cause this drop in vacuum under load? Ive pretty much eliminated the possibility of this being an electrical issue. I also moved the timing around from 12 deg. initial to 16- no difference. Im running out of ideas.

68nate
08-13-2008, 09:00 PM
Judging by the video it sounds like an ignition problem/ spark scatter. I would try it with the mallory box taken out of the picture. If it still runs poor without the ignition box, I would try a different ignition module. I would think it has nothing to do with the vacuum advance. How does it run with the vacuum advance unhooked? The huge vacuum drop is caused by the engine running poor.

paul67
08-13-2008, 10:28 PM
I would try a new set of plugs if you have had carbon on them, I have found that carbon build up kills them this is a know fault on motorbike engines if it run rich.

ALLstrokedOUT
08-14-2008, 07:02 AM
68Nate, Could an Ignition problem/spark scatter only be a problem under load- not cruising or light load? because this isnt even noticable in 1st and 2nd gear getting up to cruising speed. I called up Mallory about it being an ignition problem before i took everything apart again, they said basically it either runs or it doesnt, and if it starts and idles while using the ignition box, "unless its a huge fluke; something ive never seen before", its not the box, and if it runs at all its not the module- thats what they told me.

I tried running it without the vacuum advance, it ran the same if not worse than with it. I would agree that the vacuum dropping is not a cause of how its running, but a symptom.

Yesturday i must have tried half a dozen different things, most to do with the ignition. I also took the plugs out and looked at them, they seem o.k. But Mallory techs told me to try and gap them to .045 instead of .035, so i changed that. Today i think im going to look into the power valve as possibly resolving this problem, since the idle mixture screws are set perfectly to highest manifold vacuum, The plugs look to be a good color, and it cruises fine.

EDIT: i got a 10.5 power valve and put it in, in place of the 6.5- Its a night and day difference! Threres still al little hesitation when you step on it, but i think that can be worked out with the accelerator pump/ pump cam as it overcomes the hesitation rather quickly.