340 Idle misfire/rich

[Lustle]

Hey Folks,

I'm new to the forums. And to owning a Mopar A body. I have a 1972 Scamp! With a decent 340/727/8-3/4 in it. Bought it with a bunch of work already done to it... 12/14 years ago. And then it was left to sit. It was basically completely gone through, lug nuts, bushings, torsion bars, ball joints, etc. Transmission and rear end work. Engine work. Guy rebuilt it for his kid, and the kid just let it sit. 5000k on the body. 1000k on the engine when I bought it. I've put about 1000k on it myself.

But with the sitting comes problems. Right now it has a misfire. I've tried searching the forums but haven't come across anything quite like mine. It's had it since I got it. But it wasn't bad. Mainly at idle. More noticeable in gear at stoplights and the like. Barely noticeable, more like a rough idle/big cam than a misfire. Not as noticeable in park, but curb idle was set high. It was running rich, more than likely to help with the misfire. A little hesitation and stumbling on off idle/low rpm acceleration, figured the carb needed tuning. At higher RPMs or once under way with heavy throttle, she gives her. No misses that I can tell. Pulls hard. So the misfire only seems to be at low rpm/idle/in gear.

I'll give you the basics of the engine build.

340
.040 over
Keith Black 243 pistons
Comp 274H cam 260/268 at 0.05
Iron heads, worked, 2.02 and 1.60 valves
Edelbrcok RPM Air gap
Holley Street Avenger 670
Stock distributor
Stock ignition box
Combo should be good for 10:1 static, 7.5 dynamic compression.

So to fix it I started going through the ignition. Working from the plugs back. New plugs. Old ones are dry black, definitely rich. I put new ones in cause I want to colour them myself. See what I get. New wires. New MSD coil, thinking maybe the old coil sat and was giving off weak voltage. But with the new coil. The misfire got much rougher. Almost stalling/dying at stop lights. Which seemed odd to me. It should be a better spark. But somehow the old coil was hiding the misfire? I haven't tried hooking up the old coil to see if it helps yet. Firing order is good. Pull dist cap, cleaned up rotor/points. Same problem. Tried tuning the carb, thinking maybe the rich was the problem. The metering needles were out over 3 turns! Crazy high. I got them back to 1.5 turns. Started tuning appropriately. 1/8 turns. Adjusting curb idle as needed. But now I can't get it into gear without it stalling. The misfire just lets it stall out. I'm sure I could get it into gear, if I turn up the curb idle and pull the metering needles way out again. The more I lean it out, the worse the misfire gets.

So at this point I'm forgetting the rich. And just looking to target the misfire. Why would it be worse with a new coil? Timing? I'm thinking the usual going through the ignition. Doing dist cap and rotor next. Then box, etc. And keep working. I'm going to test for vacuum leak whenever the rain stops. And a compression test maybe if I can't figure it out. But I'm also hoping that maybe some of you guys have come across this.

Any help is appreciated!

 

[jerry6]

What is the timing set at ? and what gas is in there ?

 

[3406pk]

Since the misfire is mostly at idle and low rpm, I'd guess vacuum leak. Check for leaks.
1 Hook up vacuum test gauge to see if that shows problem. There is usually guide sheet with kit to help troubleshoot the readings.
2 Be sure hose for dist vacuum advance is hooked into correct port on carb. I've had similar stumbling stall problems with vacuum advance hose to wrong port. If can't find vacuum leak, pull hose off for vacuum advance and plug port for test drive.
3. Use vacuum gauge to set timing and idle for optimum.

 

[66fyssh]

Since its been sitting so long, I would pull and rebuild the carb. Kits are only like $50. Take your time and clean it up well while you're rebuilding it.

I had the same miss at idle. Ran great otherwise. Rebuilt the carb and now when I slow to a stop I can barely tell the car is idling!!

 

[Lustle]

Not sure of timing. Going to beg/borrow/steal/buy a timing light and check that this weekend. Gas is 94 octane up to 10 percent ethanol. Husky sells it.

Vacuum was my next idea. Gunna try and put my hands over the carb to stall it. Or the old carb cleaner around the manifold/carb base trick. See what happens. If it wasn't raining/windy I'd be out there right now. Vacuum lines should be good. Only got them from vacuum advance port to dist. And full manifold port to the power brakes. I did check. They are on the right ports. When I the vacuum gauge hooked up at idle it bounces a little, maybe 1-2 inches. Which could be the light miss as well as the overlap on the cam. At idle in gear its bounces a good 5 inches up and down pretty rapidly, which originally indicated to me the miss.

Carb rebuild was also an idea of mine even if I get the miss fixed, I'm sure after 12+ years of sitting, it needs it. I've never rebuilt a street avenger. But everything I see about them they look pretty basic. I have a feeling I'm gunna pull it apart and see the internal filters clogged with little bits of dust. On the same note. I'm also gunna run new fuel line from the mech pump and tee it so I can hook up a fuel pressure gauge. Make sure fuel pressure isn't a problem.

I just can't figure out why a coil would hide the problem, and a new coil highlight it. It seems backwards to me. I'm hoping its something simple like this. I hate to think that one of the pistons is losing compression.

Thanks for the idea guys.

 

[Lustle]

So update.

Not vacuum, car will stall if I force it. No leaks anywhere that carb cleaner/fuel shooting out as i try and set the float bowls (which is another story), can detect.

Rebuilt the carb. There was some oxidation. Lots of little floaties as I cleaned it and it soaked. My first time doing it but I think I got her put back together right. They are pretty simple. Everything is set. Its on the car and running. But I'm still having the same problem. I have to turn the meter screws out a good 3 turns before it will run in gear. Under 1k RPM in no gear and she has a noticeable shake/miss. In gear its around 600, even with the idle screws out 3+ turns, and shakes even worse. Reeeeks rich.

So at this point. I'm assuming I rebuilt the carb correctly. And that the problem is timing. I have a feeling initial is set like ****. Way under what the setup actually needs. But would low initial timing cause such rich conditions? My nose is burning and not the good methanol, nitro burning I love. I know with low initial timing it would be a dog engine at low rpm but not how rich it runs. Unless that's just what it takes to keep it running? I'm tempted to just take vacuum off and see how it runs at idle. And I'm going to put a timing light on it tomorrow and let you guys know what the verdict is.

Either way I appreciate your guys help!

 

[txstang84]

So update.

Not vacuum, car will stall if I force it. No leaks anywhere that carb cleaner/fuel shooting out as i try and set the float bowls (which is another story), can detect.

Rebuilt the carb. There was some oxidation. Lots of little floaties as I cleaned it and it soaked. My first time doing it but I think I got her put back together right. They are pretty simple. Everything is set. Its on the car and running. But I'm still having the same problem. I have to turn the meter screws out a good 3 turns before it will run in gear. Under 1k RPM in no gear and she has a noticeable shake/miss. In gear its around 600, even with the idle screws out 3+ turns, and shakes even worse. Reeeeks rich.

So at this point. I'm assuming I rebuilt the carb correctly. And that the problem is timing. I have a feeling initial is set like ****. Way under what the setup actually needs. But would low initial timing cause such rich conditions? My nose is burning and not the good methanol, nitro burning I love. I know with low initial timing it would be a dog engine at low rpm but not how rich it runs. Unless that's just what it takes to keep it running? I'm tempted to just take vacuum off and see how it runs at idle. And I'm going to put a timing light on it tomorrow and let you guys know what the verdict is.

Either way I appreciate your guys help!

Set the timing to about 14 to start with (it may want more), with the vacuum disconnected. ..then start with the carb tinkering. And just because you can stall it out with your hand doesn't mean it doesn't have a vacuum leak somewhere. I'd also put the known good coil back on and check your wires for serviceability if you haven't already. JMO

 

[Lustle]

So i had the timing light on it. Seems to be about 10 at initial. Didn't check total. Because at initial. It really bounces around. Between 9-11 from what I could see. The light doesn't have that "steady" that I think I should see. It's really my first time using a timing light. I had it all hooked up correct (arrow pointing to plug, kept other plug wires away) so I don't think interference was a problem. Vacuum was off. Car still runs not that great. A noticeable shake. Vacuum gauge bouncing.

What could cause the timing light to not be steady like I've seen in video's with guys setting their timing? The rich condition or the misfire? Or should it be that way if the initial is too low for my cam? I'm no expert in this stuff.

 

[crackedback]

Simple test. At idle, twist the distributor a little counter-clockwise, if the engine picked up RPM, it wants the timing at idle. Initial timing is finding the point where the engine is running most efficiently and it will still start when hot.

IMO, your way the f*ck off base at 10* initial. That's stock 318 territory.

No way that cam is 260 at .050 either

 

[Lustle]

I'll try twisting it and seeing if it starts sounding better and running better. I imagine it should, and that most likely timing is the cause of the idle problems I'm having. I just wasn't sure why the light would bounce around so much. It kinda shows 10, a little under a little over, but then for at least one rotation it misses it completely it seems like. But I'll try tuning by rpm and see what happens. I imagine I will end up with much more initial. A lot of SBC guys run this cam and end up anywhere between 16-22. SO It definitely is way off there.

What do you mean that the cam isn't 260 at .050? That's what comp is saying it should run. Why would you think it would be different? I'm just curious.

 

[fishy68]

I had the Comp 274 in a 360 and it liked 22 degrees initial timing if I remember right and Rob is correct about that cam not being 260 at .050. It's 230/236 at .050. Total duration is only 274 on the intake so no way can it be 260 at .050. You either looked up the wrong cam to get them #'s or comps website is all screwed up

 

[Lustle]

Well I tried playing around with it last night. Spun it till the rpm didn't rise anymore. Wouldn't hot start. So I backed it down a bit till it would. Didn't put a light on it. But noticed an interesting second effect. The RPM would not drop below 1500 or so. Even with the curb idle backed all the way out. Metering blocks set at 1.5 turns. Now I didn't play with the metering blocks. SO the problem could be in there. But I find that RPM rather high for the curb idle to be pulled completely out. Is that just more to do with carb tuning than anything?

As for the cam, you guys are right. I messed up on the specs. I knew it was 230/236, that must have been my partsman "hey better screw that number up" training. Oh the joy of having a few hundred numbers in your head all day at work.

 

[cannucky]

make sure the butterflies are closing all the way when the idle screw is backed out , sometimes guys bend the little rod connecting the secondaries and the primaries so they don't actually close all the way , also did you try backing out the secondary idle adjuster screw to reduce your idle ?

 

[crackedback]

Or the throttle linkage is hanging them open.

When you twist the distributor, you reset the idle speed every time you do it.

 

[Lustle]

I'll actually look down it and check the butterflies next time. I didn't try the secondary idle adjuster. I honestly figured it was probably still at factory settings. But given the engines trouble stalling earlier, someone might have opened it earlier. That's a good suggestion, thanks.

Throttle linkage moves freely, everything seems to operate good. I'll check down and look at the butterflies during operation next time. I was adjusting the curb idle as I played with the distributor. I got the distributor where it ran at best idle, but would still hot start. I figured then I should adjust the idle speed? Or should I adjust it every time I move it?

I'm going to play with it more tomorrow. Put the actual light on it and see what timing I get. Hopefully finally get her set. I have a feeling the distributor is the cause of some of my problems.

 

[cannucky]

sounds like you did it backwards , first move the timing then set the idle , repeat until you find best timing then set idle final time so yes every time , the rod bending deal is it can hold the butterflies open but it will still operate freely , the secondary idle underneath will be a little movement does a lot of adjustment deal so do it while it's running and hot .

 

[Mattax]

When you twist the distributor, you reset the idle speed every time you do it.

Exactly. Your combo should have a base timing between 14-18* and idle in N below 1000 rpm. The real test for best timing and idle with an automatic, is that it should not lose much rpm or vac when you put it into gear (engine fully warmed). In N it can be happy with lots of timing and be very lean because there is almost no load - so you must keep checking it D after each adjustment.

I think your best approach is set the timing, Adjust your rpm down with the idle speed screw, then adjust mix screws for best vac (or best rpm in gear).

Keep in mind that the idle speed screw is adjusting throttle position. The carb will not work well if the too much or too little transition slot is exposed at idle. Usually from .020-.040" is about the limit of acceptable. The only way to really know this to take the carb off. (If you want more info on this do a search for transfer slot exposure. Here's a drawing from Crankshaft coalition)

If you end up outside this range, then try setting it in that range and then just adjust timing and mix screws. If that doesn't work, then it needs a bit more air at idle. You can supply that by slightly opening the secondaries or drilling small holes in the primary throttle plates near the idle discharge ports. (This is illustrated in the Urich/Fisher Holley books and probably elsewhere)

 

[Fishing68]

Any resolution or updates?

 

[12many]

Any resolution or updates?

I believe the OP had a change of some sort around 2017 and now identifies as a different member here.

 

[Fishing68]

I believe the OP had a change of some sort around 2017 and now identifies as a different member here.

Interesting

 

[2dusters]

I'll actually look down it and check the butterflies next time. I didn't try the secondary idle adjuster. I honestly figured it was probably still at factory settings. But given the engines trouble stalling earlier, someone might have opened it earlier. That's a good suggestion, thanks.

Throttle linkage moves freely, everything seems to operate good. I'll check down and look at the butterflies during operation next time. I was adjusting the curb idle as I played with the distributor. I got the distributor where it ran at best idle, but would still hot start. I figured then I should adjust the idle speed? Or should I adjust it every time I move it?

I'm going to play with it more tomorrow. Put the actual light on it and see what timing I get. Hopefully finally get her set. I have a feeling the distributor is the cause of some of my problems.

I know this is an old post…did you ever figure out what the problem was?