help solve my sputtering problem!

[lnfletcher]

Engine:
87 Roller 302 with B303 Cam, Wieland single plane intake, 9.1 TRW pistons and oversized 194/160 valves. Edelbrock 650 AVS carb. Ford duraspark dist with MSD 6AL box.

With WOT starting at 3000 rpm or below, the engine sputters until it reaches about 3000 and the clears out and runs good. On hard engine braking, it backfires through exhaust.

Running 10 deg base with max 38 deg timing with vacuum advance connected.

I have tried advancing to 12, 14 and 16, no change.
I have used stock ford duraspark dist curve and recurved for faster advance, no change.
I have gone through the carb with a rebuild kit and cleaned just in case of some trash, no change.
Plugs look good with light grey color.
Tried rejeting carb with leaner secondaries, no change.

I am at a loss, nothing seems to effect this problem. Can one of you experts much smarter than me give me an idea or direction to look at?

[Jerry Clayton]

close the plug gap to .035 to see if sputtering goes away

[Z-linkCobra]

Is this a problem that just started recently. I once made a plug wire change on a 302 and had all kinds of problems. Went back to the old plug wires and it was fine. The mallory dist I had did not like the MSD bluemax wires...had to run mallory wires.


I would get rid of the Edelbrock carb and the duraspark.

[lnfletcher]

Thanks for helping,
Tried the plug gap, no change.
I will try the wires tonight because I have puchased a MSD dist and wires as another throw parts at it attempt!
As far as the Edelbrock carb swap, I don't have that kind of money as of just yet!

[Z-linkCobra]

Its generaly spark or fuel related. Have you checked the valve lash? Might want to pull the valve covers and just give them a once over to eliminate that possibility.

If the lash was too tight then that might cause a valve to hang open a little and give you grief with reflux at the lower rpms. If it was to loose then it can retard the valve timing and give you a backfire through the exhaust...Im just thinking out loud really. Just something to look at.

[lnfletcher]

I pulled the covers and checked all of the rockers. These have possitive stop. Also the compression checks good on all cyl.
Here is a link to dyno pull that may help you hear the sputter. (large file so be patient)
http://lnfletcher.homeip.net/66Cobra/MVI_2287.AVI
The guys at this shop couldn't help me either!

[CobraEd]

maybe the electronic pick up module in the dist. Also if you have the duraspark module (looks like a voltage regulator), that may be the culprit.


.

[lnfletcher]

May be pickup, but would think it would be worse at higher rpms.
I will eliminate that tonight with a new dist.
Also, I'm using a MSD box, not the module. (Changed to this after problem occured without change.)

[FWB]

exhaust popping could be as simple as a header flange leak, or a lean condition from WOT.
while i do not condone using single plane intakes on the street, this could be the reason all is good at 3000 rpm and up, this is where the intake is designed to work. the pop could be the edel carb not keeping up with fuel needs.

the duraspark distrib. is a good unit, honestly i prefer them over MSD.
look for black soot marks around the header flange. pull plugs and see how they are burning that willl lead you in the right direction.

fred

[jhv48]

Is the advance curve on the distributor smooth?

Could be a sticking advance, but my bet is on the carb. You've got a single plane manifold which is "better" at higher rpm but that carb is my bet.

You can fart around all day long with that carb and it will run better, but will never run right. Ditch it for a mechanical secondary carb. Even Edelbrock will tell you that this carb is not the best choice for a light 5 speed Cobra.

My first Cobra had that Ed carb. Tuner was able to eliminate the problem you are having but the car would sputter on hard turns and almost die at hard stops. I replaced with a Speed Demon and never looked back.

Backfiring through the exhaust is probably a blown side pipe gasket. Try tightening them first. If doesn't go away, replace both sidepipe gaskets and hope it goes away. If it doesn't, check the exhaust manifold bolts for tightness.

[CobraEd]

you could have an intake manifold leak/crack that affects the way the car runs at low rpm, but as the rpm builds, the leak is too small relative to the total intake flow to affect the way the engine runs.


.

[Tommy]

One inexpensive way to test the carb theory is to borrow one from a friend with a known, properly working Holley carb. It shouldn't be hard to swap carbs long enough to see if it makes a difference. If it does, you at least know what's wrong.

[Jerry Clayton]

OK--watched your tape of the dyno run----

Couple of questions---I only saw one shift so I assume that it was just above an idle in 4th gear???

Does this do it on ordinary street driving????

Your set up will not take a wot at an idle in high gear---not enough timing, not enough fuel and it recovers by 3000rpm and runs----

You can't just jump on the throttle from idle in high gear tied down on a dyno with a stick shift car. an auto trans has a t/c vs your clutch--

Nothing wrong with car, just improper dyno operation

[Rick Parker]

Not a fan of the Edelbrock carbs, HOWEVER Single plane manifold will not work well with vacuum secondary carb or vacuum advance. They need an accelerator pump on the secondary circuit and slightly rich jetting throughout. Grey plugs indicate lean condition. Fatten it up

[madmaxx]

FWIW, I had a simiar noise but much more pronounced, turned out I cracked a ceramic insulator on one of my spark plugs while removing it. Trust me in my weeks of searching i did all the traditional techique, sometimes it would arc sometimes it would not. $1.99 later and it was fixed.

[bobcowan]

I'm with the intake and carb group. The cam, heads, and compression are all for a low rpm torque engine. But the intake is for a high rpm HP engine. I'd start by switching to a dual plane inake.

The Edelbrock carb is another story. Essentially, it's an updated version of the old Chevy quadrajet carb. By all accounts an excellent carb with good tuning capabilities. But, that's where the problem lies. if you don't know how to tune it properly, it's easy to get it so out of shape that it just won't run right. Find some one who knows how to tune them, or switch to a Holley.

But, that's just my opinion. I could be wrong.

[lnfletcher]

Ok, thank for all of the directions!
First, it did run well with this setup at one time, so this is a newer problem.
This weekend, I retorqued the intake, and replaced the dist and wires. Yes it does the same on the street as on the dyno. He was in third at 2000 when he was giving it WOT. The plugs have been replaced and regapped. Also, the ED AVS 650 is a manual secondary carb. I didn't want to go changing intakes until I figured out what caused this to start. I have thrown enough money at it already!
I will check the exhaust bolts when I get home tonight.
I may just replace intake and exhaust gaskets to be sure.?
Would the exhaust cause the stumbling?!

Thanks again guys!

[jhv48]

The Ed AVS (Adjustable Valve Secondary) carb is not a mechanical secondary carb. It is a vacuum secondary carb, thus the AVS moniker.

If you are thinking of going to all the trouble of replacing intake and exhaust maniforld gaskets, why not try another carb and eliminate it from the equation. Much easier than replacing gaskets.

[lnfletcher]

Will do,
Shows my ignorance, I thought if the secondary butterflies were mechanically linked to open, then it was mechanical secondaries and that the AVS refered to the adjustable spring flap on then top of the Thunder series.

[Rick Parker]

You might verify your plug heat range, the original description of a gray coloring is indicative of unfavorable conditions.