I've started up my 496, and have run into what I think is an ignition problem. The engine runs pretty rough at lower rpms and has a stumble to it. Also, if I snap the throttle from idle, a nice ball of fire shoots out the top. What might the problem be?

Engine specs can be found on my website, but the ignition is a MSD 6AL, MSD Magnetic Pickup Distributor, MSD Blaster Coil and Champion RN14YC spark plugs gapped at .060"

Thanks for the help,

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

What is your base timing at? minus the vacuum advance.

try base = 10 BTDC and see how she response to goose.


try base = 20 and do the same.

you'll see what I mean.

Should that be at idle or at a certain rpm? Without vacuum advance, I set it at 34 degress at 3000 rpm.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

Hey Jeff,

When I hear about a backfire through the carb, I usually think about the possiblilty of an intake valve hanging open slightly. Is there any change the lash is set too tight?

Cam degreed and checked twice? I know stupid question, but better to cover all the bases.

I think .060" might be a little too much gap for the MSD 6AL to handle. What is your compression ratio? I typically stick with .035" plug gap with my 6AL and 12:1 comp.

You may want to try a larger advance bushing to give more initial and limit the mechanical advance. Most Chevy's (performance) seem to like at least 20 degrees initial timing.

BTW, I really like your page...very nicely done as well as the car. That motta should really thump when sorted out http://www.chevelles.com/forum/smile.gif
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Todd Geisler
79 Malibu
11.16@125.8mph
Malibu Muscle
http://www.MalibuMuscle.cjb.net

[This message has been edited by Todd Geisler (edited 12-04-2000).]

Yes 0.060 is much too wide. 0.040 is more correct. You might just as pull them all out, visually check them adn regap. I'd test the wires with a VOM while I was at it.

This probably has noting to do with your timing settings.

You can wipe out a coil by gapping your plugs too wide. Doubt you did with quality components.

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Gene Chaas
Gold Member 62/ACES
67 SS 427 (http://www.chevelles.com/feature/october2000.html)

JeffK,

Where is your initial timing set? Todd is right on the money. Most high performance motors like a LOT (comparatively) of initial timing. My solid roller 383 spit and sputtered at 14 deg. initial(on the initial fire-up). Bumped it just 3 deg. to 17 BTDC and it purred like a kitten and acted like a completely different motor. I run locked out timing at 36 deg. now and it idles better yet.

NICE CAR!!!!

Good Luck,

Jason Gore
AKA DragRacer
jgchevelle@aol.com

Hi Todd, Thanks for the comments on the website.

I double and triple checked the cam when I degreed it. I originally drilled out the gear to use the bushings, but it came in straigt up and I just installed the 0 degree bushing.

I also rechecked the hot lash setting yesterday, and it's set at .020 per compcams recommendation.

Gene, I'll reduce the plug gap to .040 and give it a try tomorrow.

I originally thought my fuel injecttion was giving me troubles, but the fuel injection would have nothing to do with the carb backfire. Si I fugured it must me in the ignition system somewhere.

DragRacer, I plan on doing exactly that when I get the plugs regapped. Trying some diffrerent settings. Just to clarify? Initial timing is at idle with no vac. advance right???

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

[This message has been edited by JeffK (edited 12-04-2000).]

Wait a sec. "Fuel injection" and a "carb" you mean to say throttle body??


With car idling, say 1000RPM and the vaccum advance pulled and plugged you can check your base timing. this low RPM is to elliminate mechanical (spring/weight) advance.


If you tune for a total timing of 36 @ about 3000 RPM you are including about 10 degrees of vacuum advance and about 10 degrees of mechanical advance...... seems to me your base timing may be around 10BTDC. With that base timing my car idles like a top-fuel car. "chop-chop...pause..chop-chop..pause" something no choke and 30 degrees outside wont allow!!

Also, vaccum advance will "fall away" at WOT, so that total timing number your see at cruise (constant 3k RPM) will not be there at WOT. http://www.chevelles.com/forum/wink.gif tricky 'ey

I run 18 base ans 36 total and leave the vacuum advance popd for bugs to live in http://www.chevelles.com/forum/biggrin.gif


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Member of TC Superfreind's society for goodness and camaraderie.
BB414, TH400, 3.73 Eaton posi,
13.16sec @ 105.15 MPH (TH400 needs clutch pack, Radial T/As = zero traction)
" Don't worry about the world coming to an end today. It's already tomorrow in Australia"
Picture of me roasting the tires and other guy stuff (http://www.auburnextremeracing.org/drivers/mike/)
Video of me staging (smoke of course) (http://www.notabusinessracing.org/videos/mike_chevelle_burnout.avi)

Yep, fuel injection (Throttle Body). Saying carb is just a habit.

I'll give my timing another try tomorrow.

I bolted on some cheap glasspacks just for the startup, have my dr.gas and torque tech waiting to be put on, and it sure is loud. It sounds like a top fuel, shaking the ground and all. Sounds kinda neat though http://www.chevelles.com/forum/biggrin.gif
I'm sure the neighbors have a different opinion.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

Well, I still haven't found the problem yet. I've regapped the plugs to .035, and checked the plug wires, which were OK according to MSD. Tried initial timing settings from 10-30 degrees with no noticeable change in idle characteristics.

One thing I did notice, when I pulled the plugs, the drivers side was a lot blacker than the passenger side. The passenger side looked good with a tannish color, but plugs for 1-3-5-7, were all pretty dark.

I have one other idea to try. Currently I have a ground strap from the passenger side head to the frame. I'm going to try ground the other head the same way and see if this affects spark at all. That's the only thing I can think of for now.

Any more ideas?

I've been told this 244 duration cam is mild for this size of engine, so I don't think it cam lope that I'm feeling.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

How about mechanically checking your TDC with a TDC locater. Timing numbers mean nothing unless you are 100% positive that you are at TDC when it says TDC.

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67 Malibu Conv (always under construction)
64 Lemans Conv, Building for my brother-in-law
"If You Don't Drive It-Sell It"

Good idea onabudget, but already took care of locating and marking TDC when I degreed my cam.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

JeffK,

Do the plugs look a little oily on 1-3-5-7? It may me possible that you have a vacuum leak at the bottom of the intake on that side due to an alignment problem. The only reason I say this is because the whole bank is that way. Never seen this just on one side though. Fuel rail/injector problems on that side?

Good Luck,

Jason Gore
AKA DragRacer
jgchevelle@aol.com

Does this setup use dual O2 sensors? What I'm wondering is how does the Puter control the injectors respective duty cycles. Is it per bank, i.e. an O2 sensor on each side's collector. Or just one 02 sensor and it tunes from that.

I hope you folllowed the direction's to the Tee on degreeing the cam.


Did you maybe use a vally shield (heat blocker) under then intake and it prevented the intake gaskets form sealing the two surfaces flush?

--that's it for ideas today http://www.chevelles.com/forum/biggrin.gif , will check back 'morrow

This is an open loop system (No O2 Sensors). It uses throttle position, manifold pressure and engine temp to inject fuel. There are settings that I can adjust for richness, but they are not setup to adjust each side of the engine separately. So if one side injector is firing OK, the the other side should the firing the same amount of fuel.

I'm going to double check my TDC and distributor position again tonight. The backfire through the intake sure makes it seem like an ignition problem.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

Just an update and another question. A buddy of mine came over last night for a look at the problems I've been having. He races a Bigblock dragster, so I figured he could help. I took the dist. cap off and verified TDC and rotor orientation. We were able to get it running decent at 20 degrees of Initial timing at idle. I checked the plugs again and both sides appear to be burning the same now, overly rich. I need to play with the settings on my Injection more.

He did say that the combination of the lopey cam and low 9.5 compression, that the cylinders are loading up with extra fuel at idle and a hotter spark plug would help alleviate the problem. What kind of plugs do you guys think I should run? I have champion RN14YC now.

Also the timing advance has us a little confused. With Initial set a 20 degrees, my MSD book says the dist. has a advance stop bushing that will limit centrifugal advance to 21 degrees. But when we rev it up, it climbs to 55 degrees (according to the marks on the balancer) before leveling off. I'm wondering if the MSD box is creating false readings at the timing light.

Overall it runs good. Sounds like a racercar at idle, but once you get on the gas, it smoothes right out.

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)

[This message has been edited by JeffK (edited 12-07-2000).]

Hey Jeff,

In the past I have used the black advance bushing which is probably what you installed. This will usually give about 20 degrees initial timing. I would unhook your vacuum advance and not use it. I myself prefer alll machanical advance on a performance car. One less variable to mess with. On my car, I simply run 36-38 degrees with the distributor completely locked out...might not be the best on a street car, but might be worth giving it a shot.

Has your backfire gone away? It sounds like you just didn't have enough initial advance to me. From here on out, it sounds like some tinkering with the injection.

good luck

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Todd Geisler
79 Malibu
11.16@125.8mph
Malibu Muscle
http://www.MalibuMuscle.cjb.net

Hi Todd, 20 degrees initial seems to be a good spot, anything less it will backfire. So that issue is taken care of. After I figure out the injection, it should be good to go.

I have the blue advance busing installed, which is supposed to provide 21 degrees. 20 initial + 21 centrifigul should level off at 41 degrees right? Would the vacuum advance cause it to climb to 55?

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JeffK ACES #01510
Team Chevelle Gold #72
496 EFI/Richmond 6-Speed
JeffK@chevelles.com
Jeff's Chevelle Page (http://www.72chevelle.com)