Idle backfire when hot?

[smurfbus]

79 930 with MSD 6A and blaster coil. Rotor 1.2Kohm no_name with a bit longer contact area. IIRC plug wire resistance 1.4Kohm, BB headers, tial 46, 0.8bar.

This is hard to figure out. Im going to check the header bolts tomorrow but it sounds like it's not running 6 cyl. Plugging out different cylinders won't stop the backfire or even affect the idle much. Spark seems very strong at the plugs and it seems I have always had a lot of advance at idle. I tried different timing but no help. It does not backfire when cold.

This started when I figured my distributor rotor had melted (engine still ran fine) and at the same time I did an oil change and valve adjustment plus installed a new no_name 1.2 Kohm rotor. I first tried original 930 rotor but it did not idle under 2000rpm.

firing order 1 6 2 4 3 5, ccw distributor, closer to distributor vacuum port plugged. boost side connected.

I have removed all the helper devices (only wur left) so there should not be any vacuum leaks and a hand torch (butane?) did not affect idle when I pointed it at several hose connections and IC connections (tried starter help too).

I tried to adjust plug gap (and different plugs too) and I have tried 0.03-0.04 with no help.

Tried different idle COs and rpms, no help. AFR initially goes lean when it starts to act up and richening it won't help. This leads me to beleive its ignition related. Misfires show as lean.

I saw one plug wire at the distributor arch with the turbo oil line (to intake filter) fastener but removing the fastener did not help idle backfire but stopped arching.



1. Does bigger gap help idle?
2. Should I have same resistance at the plug wires as in the rotor?
3. Is a bad rotor different resistance?
4. is the plug wire trash if it arches agains metal?
5. Can I buy american muscle car wires 8.5mm or 7.5mm, should they be resistor ones? And how long?


Almost all the backfire cases on the forum archives are when on decel but my car does not backfire too bad on decel. Only the normal burble unless really hammering it and loooong decel afterwords.

I wan't my smoothness back. And again this started when I changed the melted rotor! I can't order the NAPAonline rotor so I have to use whatever I find. I have one spare 1 Kohm rotor I will try too (maybe even new distributor hat).

TIA

[mark houghton]

Hmmm, weirdness. So, this is backfiring just sitting in the driveway idling?

Sounds like even with pulling plug wires to try to isolate it, no change occurs. So it aint wires or plugs, that would suggest. Did this really start when you changed out the rotor, or could your valve adjustment coincidentally have anything to do with it, since all was well prior (except for melting your resistor rotor). I wonder if your MSD or coil are failing you? How's the dizzy cap look inside...any evidence of carbon tracking? Also, might try hooking up your timing light to see if the timing is dancing around when it backfires, and then hooking up the light to each individual plug wire as close to the plug as you can get, to verify consistant spark to each cylinder.

Also, have you watched the engine bay in complete darkness to look for arching? You know, in the garage with the lights off and an oxygen bottle hooked to your face so you can survive the drill?

[smurfbus]

I tried to adjust the valves snug with the special tool .004

IMO the idle has always been a bit uneven now that I could compare it to my friends 930 engine.

Pulling a plugs is not a good way to try to isolate non firing plug but I tried to pull 2 at one time but it was not good either.

Dizzy cap looks fine and is probably only 8tmiles old just like the wires. No carbon tracking.

Timing is dancing even if it's not backfiring. The mechanical advance moves freely. Hooking strobo up at the end of each plug is a good call.

Earlier I watched the engine in the dark, could not see any arching but will do it again.

Plan now is to try a new 1.2Kohm rotor. Then measure the wires and swap a new cap. Put back the original plugs with the original 0.025 gap unless bigger is better for idle (then 0.04).

[mark houghton]

That dancing timing at idle is trying to say something, but I'm starting to run out of ideas. Hang tight, all the 930 gurus out there are just waiting to swoop in with the fix (meanwhile, having fun watching us stumble around a solution).

I run my NKG plugs, fired by my MSD6AL, at .042 or something close to that anyway. The MSD has plenty of fire to support that gap and supposedly would only be a concern under high boost if there wasn't enough spark energy to keep the spark from "blowing out".

[smurfbus]

Thats why I started to widen the gap at the same time with the valve/rotor job. The 0.024 sounds weak but thats how the car came with and it ran good at the time last year. There were some Bosch 3wc series plugs in and now NKG8 series.

I can also try to solder a jumper wire on the rotor if that 1.2Kohm is too much for longevity. Earlier I melted a 5Kohm rotor in one day/30miles and first it was fine and then started to backfire. This 1.2Kohm that is in there (maybe 150 miles now) still looks fine and still measured the 1.2Kohm so it should be ok.


From MSD site: http://www.msdignition.com/troubleshooting.aspx

Timing Fluctuations

If your timing seems to be jumping all over the place, check the following items:

What kind of timing light are you using? Many lights have trouble reading the MSD's capacitive discharge multiple sparks. Dial back timing lights are not recommended. Try another non-adjustable light and check the timing again. MSD offers a Timing Light, PN 8990.
Make sure you are not using Solid Core wires.
Inspect the routing of the Magnetic Pickup Harness. It should be routed clear of the coil primary wires and plug wires. It is best to route it along a ground plain such as the firewall, frame or engine block to form a shield around it, or replace the Black magnetic pickup cable with a shielded Gray cable PN 8862, which has a ground wire on one end.
Check the polarity of the magnetic pickup wires of the distributor. Reverse the connectors and check the timing again. For Analog ignitions the correct polarity is whichever orientation gives the most retarded timing. For Digital Ignitions the correct polarity is whichever orientation give the most advanced timing.
On Chevrolet engines, if the timing is retarding inspect for cam-walk. This is especially prevalent on engines with a chrome or stock type replacement timing chain cover that is actually bowing forward allowing the cam to "walk", due to thin stamping. To check this, wedge something in-between the water pump and timing chain cover to keep the camp from walking. If this corrects the timing fluctuation, replace the timing chain cover.
Worn distributor gear, inspect the distributor gear for excess wear or incorrect gear mesh if the gear is riding too high or too low on the cam gear. If correct the wear pattern should show the gear mesh in the center of the gear.

Found this from archives:

The MSD instructions are totally backwards. The Purple MSD mag wire goes to the shield instead of the center conductor. The Green goes to the green center conductor instead of the shield.

And this:

The car will run, but out of phase like that will give you a 2,000 rpm idle.

Magnetic Pickup in the Distributor

The magnetic pickup in MSD Distributors can also be checked with an Ohm meter to make sure it is within operating specifications. Once again, connect the Ohm meter's leads to the two terminals of the pickup. The resistance should be within 500 - 700 ohms. If the resistance is out of this specification, inspect the condition of the wires leading to the pickup for abrasion, shorting or opens. If the wiring is okay, the pickup is at fault.

NOTE: The Ignition will check good if the pickup is at fault. Follow the Checking for Spark procedure.

Spark Plug Wires

If you suspect a spark plug wire to be open causing poor performance, check the resistance of each wire. By checking the resistance of each wire you will determine two things: That there is continuity (not open) and if there is too much resistance in the wire. Use an Ohm Meter and connect the terminals to the spark plug wire terminals. Use care to make sure there is good contact or the readings will be inaccurate. On MSD wires the resistance should be:

Heli-Core Wires (Blue):
150 - 1,200 ohms per foot

8.5mm Super Conductor (Red):
40 - 50 ohms per foot

If you do not know the specification for your wires, measure the resistance of each wire. Remember to divide the length of the wire by 12 to get a "per foot" measurement. If one wire is out of that average spec, it is at fault

Solid Core Wires: You cannot run Solid Core Spark Plug Wires with an MSD Ignition Control. Solid Core wires do not suppress any Electro Magnetic Interference (EMI) which will interfere with electronics or rev limiters on your vehicle. A suppression style or spiral wound spark plug wire MUST be used.

[smurfbus]

Almost got to the starting point today hardware wise. Found similar rotor I had for 5000 miles (5Kohm) on the spare parts bin and installed it. Also installed new distributor cap and the old bosch plugs I had when it ran fine BUT tried the 0.04 gap everyone uses.

Started the car and backfire.

Adjusted timing to where it was earlier at around 40BTDC!! and no more backfiring and nice idle but it still does not run good. One or two cylinders are out I think, atleast when driving. When I got back from a small testdrive the 5Kohm rotor had already cracked on the epoxy.




I don't think I have ever had nice consistant non_misfire idle so something might have been done wrong before I got the car?

Strobo cuts atleast on cylinders 1,2 and 3every couple of seconds. I haven't swapped the plug wires yet as I don't have spares. Resistances look ok though.

1 1.86
6 4.86
2 2.07
4 4.01
3 2.5
5 4.36 Kohm

Forgot to measure the coil wire : (

Now the strange part:

Check the polarity of the magnetic pickup wires of the distributor. Reverse the connectors and check the timing again. For Analog ignitions the correct polarity is whichever orientation gives the most retarded timing.

I changed the polarity on the magnetic pic-up wires (had to cut the connector a bit to be able to change the polarity). It does change the timing to more normal values (more retarded and according to the quote above is the correct way to wire those) and it idles ok but sounds like an v8 rumble but also makes some kind of mechanical noises. I did not try to drive it as it makes the engine click/knock slightly on the front of the engine (fuel distributor end)?

Swapped back to the high advance polarity like it has been for one year when I got the car.



I did not check the exhaust leaks as this clearly seems ignition related.


So no more bacfiring on idle (more advance) but it does not run on 6 cylinders.

It almost sounds like the 1 6 2 4 3 5 is not the correct order for this engine but when I adjusted the valves this order had the valves loose.

Tomorrow I will once again strip the IC and stuff and try much smaller spark plug gap. Not much more I can think of doing unless the polarity change makes sense even though it gives the mechanical noise?

I have to check the coil too.

Any more ideas?

TIA

[turbo nut]

When you adjusted the valves how did the cams look? I've heard motors make strange noises and backfires when the cams have gone flat.

[smurfbus]

Cams and oil spray holes looked good. It does not make mechanical noises when the polarity is set the other way around like it has been all the time. But according to MSD instructions it should be set which way is most retarded (no pun intended)

One more possiblity, distributor coil: Copy paste from Ken911's thread.

Originally Posted by wjfk32 View Post
The coil should read 600-700..ohms

so i checked the resistance across the coil and it was OK but the manual says check the ground against the car ground. which was open so the coil needs replaced

Henry Schmidt: Testing continuity to the distributor body will produce no result other than open circuit. If you have continuity the coil in the distributor is bad

A bit mixed info on different threads but since it's a coil I think Henry got it right?

Coil testing. I have IIRC MSD Blaster SS coil so values are primary 0.355ohms and secondary 4.4Kohms

HFR_racer: You properly test the primary side of the coil by placing your meter leads on terminal "A" or "+" and the output to distributor terminal "4". This reading shoud be approx. 0.x ohms.
You properly test the secondry side of the coil by placing your meter leads on terminal "A" or "+" and the "-" terminal of the coil. This reading shoud be approx. xxxx ohms. I used the 20K ohm setting on my multi-meter to test this.

IMPORTANT NOTE: You should fully disconnect your coil from the vehicle prior to testing.

[JBL930]

The cam timing could be out, have you checked this?

[smurfbus]

It was fine all last year and before I changed the rotor plus I'm not yet skilled enough to do cam timing. Valves were loose when at compression TDC. Can they still be out of spec ?

I'm a bit puzzled about the polarity and timing advance and how it still has performed without problems untill now?

If the tighter plug gaps and disributor coil checking do not bring any light to this problem I'm going to check the valve clearance again. Maybe the 'snug' 0.004 was too snug and the valves are too tight when it gets hot? Would't that cause intake backfire though?

[JBL930]

They say to leave the exhaust valve clearances on the large side to allow for heat expansion, maybe that's your problem. It could be pushing fuel past the exhaust valve on the compression stroke

[smurfbus]

Atleast it's nice to know there still are things to check even though it's a bit stressfull.

I hope someone can comment on the polarity issue too.

Thanks for you input.

Current status:

-Intermittend cutting of ingnition on some/maybe all cylinders (found by attaching the HELLA non_adjustable timing light on different plug wires)
-Green wire polarity to the MSD seems to be (has always been) wrong(?) (with about 40BTDC at idle) but the other polarity makes mechanical sounds with about correct advance 10 BTDC

- plug gaps at 0.04 which might be too much as last year it was around 0.028 with the same plugs
- new rotor and cap
- plug wires measured pretty good 1.8Kohm-4.8Kohm
- CIS pressures are fine (I have permanent gauge)
- AFR not sure of the real value as misfiring makes it seem lean but adjusting it doesn't help

[Ken911]

Ok i know this sounds pretty basic and you might have already checked it but are all of the plug wires plugged in? Are they connected to the correct spot on the distributor? Remember this one turns ccw instead of clock wise. The jumping signal is probably just the msd. It fires over 20 degrees of crank shaft rotation. Back in the 80's i had a 455 pontiac with a msd7 and when i put it on a SUN machine i could see that the plugs were each firing 5 times instead of just once. Imagine what that looks like on a timing light. Oh and bosch has stopped making the igniter coils for these distributors. So if yours goes it might be time to just give up and go to EFI.

[smurfbus]

Thanks for your input. All the plugs are snug and can't be pulled out too easily.

The green wire connector was bad but the timing remained the same, maybe a bit more consistant. Have to leave out of town now so I can't test drive. The same 40 degrees BTDC idles best at the moment. Coild was 579 ohm so I think it's ok.

[mark houghton]

Originally Posted by smurfbus

Thanks for your input. All the plugs are snug and can't be pulled out too easily.

The green wire connector was bad but the timing remained the same, maybe a bit more consistant. Have to leave out of town now so I can't test drive. The same 40 degrees BTDC idles best at the moment. Coild was 579 ohm so I think it's ok.

What amazes me is the 40 degrees advanced at idle. That's gotta be an erroneous number somehow. If that's truely where the timing is, you'll have plenty there to detonate once on boost.

[Ken911]

maybe the springs on the centrifugal advance broke. Are the plug wires all in the correct position on the distributor cap? You didnt accidentally plug any of them back in the wrong spot? It's easy to do.

[smurfbus]

I have checked the wires million times. Went for a test drive after some more rerouting and investigating.

Car started misfiring when it got hot so I tried the other polarity and now (maybe the rerouting of green wire?) it idles fine (no mechanical noises and no misfiring) at least when engine hot. Timing is at about 20 BTDC now. It makes boost like mad now but goes lean so it has really changed the behavior of the engine and I'm not sure I like the 3psi at 60 mph!

I need to try more fuel to see how it goes on boost but before that I need to check the timing on boost and higher rpms.


Why does it spool so much earlier now? Is it because it's has more retard. I could get 0.5 bar 7 PSI at 2000rpms (had to stop as it was still at 15AFR)

[Ken911]

did you put a cis gauge on it and check all the pressures? warm control and system pressure? then check fuel delivery?

[smurfbus]

Like I said before I have a CIS gauge installed permanently. Fuel delivery was checked last summer and is in check with two 044 fuel pumps. Will try more fuel later.

[Ken911]

ok missed that