Weird timing issue...

Page 1 of 3  
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.
Help.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In article

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
~jp
aarcuda69062 wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In article

Post back with the results.
If you have a dwell meter, connect it to the coil negative, dwell should equal one degree for each 100RPM of engine speed; i.e., 700 RPM = 7 degrees of dwell. 1500 RPM = 15 degrees of dwell. 2500 RPM = 25 degrees of dwell.

Maybe if the magnet in the pole piece is cracked.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 :-)
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
Valve timing has not changed...
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Almost sounds like your distributor is a tooth off. Just my thoughts. Wayne

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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?
Thanks,
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
How long has this thing been down?
Whitelightning
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
<snip>

'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.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon R. Pickens wrote:

1-8-4-3-6-5-7-2
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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
ShoeSalesman wrote:

That's how I wired it...

This lean mix business has me thinking...the only change to the carb was the smaller jets. I'll find the old ones and reinstall them.
The retarding of the spark is occuring with or without the vacuum advance connected.
Thanks,
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Try checking gap in magnetic pickup or replacing the unit. Also swap of the distributor as the advance mechanism may be damaged.
wrote:

----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Whitelightning wrote:

--
It Doesnot Take Brains To
Get A High Paying Job!
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
1986 K5 Blazer... Originally a 305ci, and I suspect a California emissions package. My factory wiring manual leads me to believe this. It was a Police truck originally; maybe some gov't vehicles are ordered to comply with the strictest emissions standards...?
Currently a 350ci. I ordered the 290hp crate engine from a dealer. It's like the 260hp, but with a slightly hotter cam. I actually would've ordered the 260hp version in retrospect.
The 260hp engine was only warrantied up to a 1985 truck. I could either spend $300 on an extended warranty of sorts, or for the same extra $300 get the 290hp.
The distributor was the stock unit from the 305. It's a HEI with the 5-pin module, modded to work like a 4-pin. Although it's a factory-approved mod, I gotta wonder about it. Especially since the whole purpose of the 5-pin setup is to retard timing based on knock. Now technically, that should be disabled (because of the mod), and the knock sensor and ESC are no longer connected. Still...has me wondering if I shouldn't throw a 4-pin module in there.
I think the pickup coil should be fine. It's obviously doing its job, and it's a fairly passive device. I couldn't imagine that it could "slip" in any way. When I had the distributor out, I didn't disturb it. I only pulled the distributor to clean it out. The fire melted a hole in the cap and there was plenty of garbage in there. The only way I could realistically do a good job of cleaning it out was to remove it. I only used a non-conductive spray cleaner for electronics (motors and alternators) that I got from a parts store.
Engine is grounded off the passenger-side head via a strap to the firewall. Testing for continuity between any metal part of the engine and the negative terminal on the battery shows a good connection.
The last thing I did before the fire was re-jet the carb for smaller jets. It was running extremely rich at the time. I don't believe I under-jetted it. I followed the suggestions of many people and actually only went down like, one size...some folks suggested I try much smaller jets.
I don't think it would've leaned it out THAT much...and with this weird-ass timing thing, I'm leaning toward it.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

At this stage of it, I would try it. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
What are your thoughts on pickup coil polarity? Should it matter?
I'm thinking it shouldn't, however if one side of the pickup is grounded (as it probably has to be for it to work) then it's possible that the wrong side of the module is being grounded.
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.