Ok, so I'm trying to tune my engine to idle better, and I'm seeing something very strange with the timing.
Basically, while looking down on the harmonic balancer, while using a timing light, I see that I have it set at around 4 degrees BTDC at idle. When I rev the engine, the timing mark moves in the direction the engine is spinning...as if it's retarding rather than advancing! I KNOW this is wrong, because I've time this engine before. I know that the timing mark should move the opposite direction that the engine is spinning (engine spinning clockwise, advancing the timing should move it counter-clockwise).
This isn't exactly a complicated setup... 350ci Chevy with a HEI distributor, vacuum advance. What the Hell could be causing this? The vacuum advance module was replaced with a brand-new one.
What's even stranger is, the same thing happens regardless of whether I use ported or manifold vacuum as the source to the advance module. Shouldn't one produce exactly the opposite effect of the other?
FWIW, the only thing that's been changed was, I jumpered the 1st and
3rd pins of the 5-pin module together to bypass the ESC (cheapo ECM-like device that retards spark based on input from a knock sensor). This mod was/is GM approved and was released in a service bulletin years ago.
Your distributor has both mechanical advance and vacuum advance. (plus the now disabled electronic retard) First, check what is happening with the vacuum advance disconnected, (this is how you should be setting base timing anyway) if it is still retarding, then one possibility is that someone has monkeyed with the mechanical advance and put it together backwards. Take the distributor cap off (don't lose any parts this time) give the rotor a twist, it should only move in the clockwise direction. Take the rotor off, check that the bob weights are able to move freely against the springs (there -are- springs aren't there?).
Third possibility is that your timing chain is really, really loose.
Ok... When I had the distributor out, I didn't mess with the mechanical advance. The springs and weights are in place, and should be as functional as they were prior to all this.
I set initial timing with the vacuum advance disconnected and a plug on the vacuum port on the carb. I'll disconnect it again and watch the timing change while I rev it.
The timing chain is the same age as the engine (2 years -/+) and was not disturbed while the truck wasn't running.
I can't imagine that the ignition module would affect this.
Same thing... no difference with the vacuum advance unplugged and the port capped off. Timing was at around 4 degrees BTDC at that point. It was running like crap, and when I revved it up, it backfired a HUGE puff of smoke out of the carb. I plugged a vacuum gauge to the manifold vacuum port, and played with the idle mixture screws to get max vacuum. I advanced timing quite a bit to do the same, and was pulling around 14" Hg. At this point the timing is set to about 10 degrees BTDC.
This'll make you wonder why I bothered posting back... I haven't checked to see what the advance is doing now that I've reset timing at idle. Probably the same though. Just thought I'd update you with the previous results. I've been out there for hours, but I'm also trying to correct last minute wiring issues. I have a few loose harnesses that I'm trying to determine where they went. What's bad is, the factory wiring manual is being little help. The closest thing I can find to my engine in there is a V6 California engine. Hmm...Mine is a V8 Tennessee truck. I'm positive it was sold new in TN, as it was a Police truck prior to me buying it.
I have a nice new puddle of burnt tranny fluid in the parking lot too...the property managers are gonna love that. It keeps dripping on the exhaust while I'm fiddling with the engine, and that makes me understandably paranoid :-)
Oh, I forgot to mention... advancing the timing to get it idling better made it respond much better... but after I eat a bite and let it cool down a bit, I will be out there checking the advance with the timing light while I rev it.
If engine runs okay, you may be making more out this than there is. It could be a latencey problem with your timing light. (the delaty between when it receives the pulse input and triggers that light) which would cause exactly what you see.
Well... it doesn't run ok. I reset the timing at idle up to about 8-10 degrees BTDC. It idles, ok, with a single miss here and there. It's harder starting, so I should probably pull the timing back till it's not.
When I rev it, it backfires. A huge puff of smoke shot through the top of the air cleaner element. (I have one of those open, foam element air cleaners)
As far as timing retarding....no, it's actually doing that. With or without the vacuum advance connected.
Ideas on the timing?? I've never had an engine that backfired until yesterday, so I don't know what causes it.
Well... would that matter as long as timing could still be set and adjusted? I didn't have a problem getting it to go down in there when I reinstalled it. I'd marked the outer (adjustable) and inner (rotating) portions of the pickup coil before I removed it. When I slid it back in, both marks aligned properly--the first time I might add, guess I got lucky.
What kinds of issues could the distro being off by a tooth cause? How does it relate to certain issues?
It never ever goes back in exactly as it comes out because the oil pump drive rotates as the distrib is pulled out, and when it goes back it will be at least one tooth off..
I dont remember what year this rig your working on, but if its a carb, one tooth off shouldn't matter, as it won't have a cam sensor or crank sensor, cam sensor being part of the distrib on many models, as such a tooth or two off wont hurt a thing, as long as you can rotate the distrib enough to get it in time.
Does it back fire through the carb regardless of where timing is set? and if so are you sure you dont have #5 and #7 reversed in the firing order(common mistake)? are you sure you have the vacumm advance connected to the correct vacumm source? Wrong one will cause vacumm advance to work too soon. source has to ported, not manifold.
'86 K5 Blazer, 350 (non-stock, originally had a 305), carbed (600cfm Edelbrock, elec. choke, vac. 2ndaries)
Yep.
I'll double check it tomorrow. I followed the Haynes manual when putting the plug wires on. I put them on in order of firing, starting with piston #1.
I have seen a "corrected" HEI cap, that's wired internally as to let you put the plug wires on in a sensible order--(clockwise from left front) 2-4-6-8-7-5-3-1. I *so* wished I had one, lol...
Yep...ported...
Caught fire and burned up the harness on 12.28.2005. Started for the first time 5.20.2006 (yesterday). So it's been a while.
Ok, I've read the other post; this is what I would do:
Double check that all you spark plug wires are routed correctly.
2.) Yank that thing out (Distrib), and set your motor to TDC, number one pistion.
3.) Insert the distrib, then pull it out again. Turn the rotor 180 degress, and put it back in. Each time you do this, it'll move the rotor alittle bit farter each time. Repeat untill the rotor is facing the number one spark plug connection in the cap. (you dont need to pull the distrib ALL the way out, just enough to disengage the sprockets).
4.) Hook your cap and spark plug wire up, and start it up. Set the timing at TDC, not BTDC.
a lean mix can cause a backfire. set your base to about 10 before TDC. you should see about 25 to 30 deg of advance with the vac and weights when rev'ed up. when you advance timing the idle speed will increase.
I didnot catch the year you are working on, But u mention VACUM ADVANCE...
with the vacum disconnect the TIMING degrees will change if u REV the engine, due to the CENTRIPHCAL force on the Springs that the rotor mounts onto ( OLDER distributors with POINTS.) This is why u Set the timing on those engines at a CERTAIN RPM.
I donnot remember ( SENIOR MOMENT) what we called the advance that works with those springs., this is why the rotor will move if you turn it while the distributor is in place.
265/283/327/350/400 engines
Whitelightn> It never ever goes back in exactly as it comes out because the oil pump
This doesn't even seem mechanically possible. There is no way a vaccum advance could be hooked up wrong. Take the cap off and use your mouth and suck on the end of the vaccum line, it should pull the diaphram in moving the arm in toward the vaccum pod. I don't think it is possible but maybe you ended up with a advance from some odd ball reverse motor. Stuck weights just wouldn't move, no change in timing but make sure they are free. Manifold or ported doesn't make a diffrence on which way your vaccum advance works, its vaccum no matter if its on all the time or not. Distributor position off won't cause this problem, you can put your distributor in any direction just make sure you are pointing at number one @ tdc. This isn't your problem, it is running so you aren't 180 off. Installing small blocks in vehicles not originally equipped with them you often have to turn distrubutors way off for clearance then move your wires to appropriate position. Double check your module wireing, just to make sure. Also double check your plug wires for correct order. To backfire through carb you either have severe advance allowing fire befoe intake valve closes, wrong fireing order or a bad intake valve. let us know what happens.
Well the timing is retarding even with the vacuum advance disconnected. This should STILL be impossible though... All I know is, the timing gauge/tab above the harmonic balancer is marked something like "12 - 8
- 4 - 0 - 4 - 8" and says "before" on the left hand side and "after" on the right. I set timing at idle to be about 8 degress on the "before" side. When I rev it, the mark moves to the right. It's completely asinine. And I KNOW that when I timed it in the past it went the opposite way (advanced) when I revved it.
The problem is occuring even with the vacuum advance disconnected.
I concour, and they are free to move, springs are functioning too.
Again, I agree... whether it's ported or manifold vacuum should only change when the vacuum advance kicks in and whether it stays "advanced". Although, connecting to the manifold vacuum source *could* cause this (high vacuum at idle=max advance...crack open throttle, manifold vacuum drops, timing retards) but that's not the case. Again, it happens regardless of whether that vacuum advance is even connected to a vacuum source. I did try the manifold vacuum as a test--no difference.
Agreed.
Also agreed...
It's a K5 Blazer...originally had a 305. This is a 350 that was a new crate motor, installed 2 years ago.
Here is what's different about the engine POST-fire...
1) The module wiring is a mod/hack that I pulled off these here newsgroups as well as a few websites. It involves disconnecting the rear harness coming out of the HEI although and then jumpering pins 1 and 3 on the pickup-coil side of the ignition module. The pickup is connected to pins 2 and 3. Basically, either pin 1 or 3 is an "output" and the other is an "input" in a manner of speaking. The old ESC (elec. spark control/wannabe ECM) just retarded spark based on input from a knock sensor. It got the signal to spark based on the previously mentioned "output" of the ignition module and sent the signal back to the module's "input". That's the simplified explanation. Either way, by eliminating the ESC, and jumpering pins 1 and 3 together, you just create a loop that allows the engine to run.
It's a Chevy-approved mod/hack. I did this because I could not find a replacement wiring harness to go back to the ESC. The original harness was toasted in the fire. Plus I read that the ESC can cause problems when it goes south, and it's a real performance killer.
2) 5 minutes before the fire, I'd changed the jets to smaller ones. This could cause a lean condition off idle couldn't it?
What year is this engine and ignition system and is it is a like year truck? This is a interesting problem that I have never seen before but I need more data to suggest a course of action.
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