This is new: stumbling/missing at idle/cruise speed

[Dangerous Doug]

This just started this afternoon. Heading down to the hardware store I noticed my engine sounded like it was missing a little while at a steady cruise---running rpm's just above idle. When I stop on it, it accelerates fine, but sounds awful at a steady cruise. Never did this before.

Checked the plugs and the cap/rotor, and spotted no issues. I haven't checked the timing yet, as I suspect the carb.

Engine is a 302, mildly pumped up with a Holley "Street Avenger" 670cfm. Checking it at home, it seems to idle okay, but if I start lightly feeding it some pedal it starts to stumble.

I did find a cracked vacumn cap, but I replaced it and there was no change.

Any suggestions?

DD

[PANAVIA]

Could be a vacuum leak - or something is making the street avenger misbehave.

it might have some sort of contamination , or degraded rubber in a power valve area.

The street avenger is not my favorite carb, as they just are not that configurable.

[menace1]

Check for H20 in the gas,clogged filter,and for a possible loose rocker. Maybe the intake manifold gasket leaks??

[patrickt]

Originally Posted by Dangerous Doug

Any suggestions?

DD

Air bleeds! And every time I tell somebody about this they come back saying I'm God's Gift to Motorsports. Read this thread and then take five minutes to clean your air bleeds. Holley Carb Woes - DCDoug Pay Attention (Long Post) Here's a typical example of post-air-bleed sentiment: http://www.clubcobra.com/forums/show...ky#post1095547

[Wbulk]

I think it's debris in the idle transfer circuit. Carb cleaner in the curb idle air bleeds may blow it out. It's the small slot just above the throttle blade. You may have to take the carb off and blow carb cleaner through the slot. Idle is Ok, then as the throttle opens further you have the problem, which is were the idle trasfer circuit is. Then as you open the throttle further you go into main metering were the fuel goes out the discharge nozzel in the venturi and that works fine.

[Bobcat]

X2 on Patrick`s answer . I found out about air bleeds and ethanol based fuels summer before last .... had the exact same problems you did , and did the same tests you did with the same results . About once a month , I just spray the bleeds before going out as a preventive measure . You also might want to start using Star Tron additive in your fuel to negate the negative effects of the ethanol based fuels .

[trularin]

Could you tell us what happened the two or three times BEFORE this last time you took the car out?

Were there any back-fires or pops? Did you decide to turn the wrench in between runs? Does black smoke come out the exhaust? If there are pops or back-fires, are they exhaust or intake?

Give us a bit more diagnostic info and we might be able to narrow this down even further.

[Dangerous Doug]

The odd thing is that my Cobra had been running perfectly fine the last couple of runs. I did have some misbehavior recently, but realized that the vacumn caps were brittle and cracked causing leaks. I've replaced all of the vacumn caps and it had been running fine.

Drove to the gym, then drove down to the market. After lunch, I got in and once I got out onto the blvd and hit cruising speed, I noticed this odd cutting in and out. A little throttle and it comes to life. Maintain a cruising speed, and the problem is back.

I'll try the carb cleaner "air bleeds" trick tonight. I read that post last night, but by the time I saw it the local parts house was closed and I didn't have any carb cleaner on hand.

DD

[patrickt]

Put a nice big bag over your carb when you spray your bleeds. The inner bleeds with the smaller holes will spray back on you from the high pressure coming out of the can.

[vector1]

or just steal one of the wifes sewing needles... otherwise watch your eyes spraying the carb cleaner!

[Dangerous Doug]

Roger that!

DD

[Wbulk]

There may be no long term solution for this zinc/ethanol problem with these carbs other than replacing them with the newer aluminum version. Fuel flows mostly trough the main jet. Debris from whatever source once it makes it through the main jet can get stuck in the idle circuit, the idle transfer slots, or the discharge nozzle in the main venturi. Spraying carb cleaner down the air bleeds may push the debris out the idle transfer slots or back out the main jet and may end up just being a temporary fix. If the debris is pushed back into the fuel bowl you may get to experience this all over again and it may happen at an unfortunate time. Taking the carb off, removing the venturi, the idle screws, and the main jets, and then blowing out the entire circuits will remove all of the debris but then again the zinc corrosion can continue and the cycle can start all over again.

[Dangerous Doug]

Well, patrickt, you ARE the man! Maybe even possibly God's Gift to Motorsports (except for that whole "dual roll hoop thing"...).

It worked. I sprayed carb cleaner into the air bleeds (all eight of them)---using a plastic bag as a sprayback catch, I might add---then gave the carb a clean-out spray.

I took my Cobra out in the chilly 44F night, and cruised down the local strip. No more stumbling at cruise speed. Romped on it a couple of times and got the local authorities to escort be halfway home (oops...).

Back to running nice. What a relief. I have enough to deal with as it is (see "Cobra guy needs a kidney transplant" thread...) and to not have be able to play with my favorite toy, what a bummer!

Thanks everyone for the advice.
DD

[patrickt]

Originally Posted by Dangerous Doug

Well, patrickt, you ARE the man! Maybe even possibly God's Gift to Motorsports (except for that whole "dual roll hoop thing"...).

It worked.

Hmmm, "patrickt, God's Gift to Motorsports" -- you know, that has a nice ring to it.

[Dangerous Doug]

Well, it would be better than "Half-ass Member"...

DD

[trularin]

OMG, there will be no living with him after that comment.

[ACademic]

Originally Posted by patrickt

Hmmm, "patrickt, God's Gift to Motorsports" -- you know, that has a nice ring to it.

I was thinking more along the lines of "The Holley Enema King" or "Carburetor Proctologist". Fits his sub-title of being a half-assed member, too.