Rough Idle

[TriodeLuvr]

My SBC 350 started idling a little rough a week ago. The problem suddenly got worse several days later while I was driving the car. The idle speed also dropped to the point where the motor would almost stall, even in neutral. When stopped in drive, I had to feather the throttle to keep her running.

I've decided to pull the carb (Edelbrock 1406) and spray out the passages. However, I rebuilt this carb a few months back (also new rotor, cap, wires and plugs) and she's been running fine until now. For that reason, even though the symptoms point to the carb, maybe someone can tell me whether other problems can do something similar.

Here's what I have so far...

Full throttle acceleration doesn't seem to be affected. In the transition from idle to midrange under load, the motor seems to suddenly "turn on," as though a misfiring cylinder or three has come to life. The idle exhaust look clean and doesn't smell excessively rich. All the plugs look about the same from one to the next, but the insulation isn't evenly colored. Each one is light tan on one side and white on the other. I haven't seen this before, but most of my work has been BBCs and motorcycles.

The reason I think this is the carb is that the right idle screw has almost no effect. I can't hear any difference, even if I pull it out. The left idle screw operates normally; it peaks the idle speed in about the right location, and the motor dies if I screw it all the way in or take it out. If I spray carb cleaner into the left venture at idle, the motor slows and sometimes stalls. If I spray it into the right venturi (where the idle screw isn't working), the motor speeds up and (I think) smooths out a little.

So, is this almost certain to be the carb, or can something else create symptoms like these? I was thinking about internal issues that might somehow dilute the charge going into one or more right-side cylinders. Is that possible?

Jack

 

[2old2fast]

Might be the idle air bleed on the right side picked up some trash ??
dave

 

[roadmonster]

If you rebuilt it recently, it could be fibers from gasket material clogging the idle circuit. Happened to me with my Holley. Just break it down as if for rebuild and look for stuff blocking the jets. A pain, I know, but costs nothing.

 

[TriodeLuvr]

You're both right! Don't know if it was gasket material, but there was a blob of crud in the lower end of the idle jet. Sprayed it out, put the carb back together, and the problem's gone. FWIW, the two needle assemblies each have a small tubular screen filter wrapped around them, but there was no filter between the pump and carb. The only filter was back at the tank. I installed a new Edelbrock inlet tube to clean up the fuel path and added a large see-through filter (plastic) just ahead of it. The filter isn't too pretty, but it should trap any garbage from the lines. I'll make some sort of aluminum cover later.

Thanks!

Jack

 

[Mike]

Get some drive time and make sure everything is OK, then take another look at those plugs. It is almost impossible to get a good reading on a plug from a street driver, simply because of all the time spent idling. But a plug in a street car should never, never, ever be showing you a white insulator. If you are still seeing white, you are going to need to get some more fuel to it.

 

[rcnurd]

I know this probally isn't the problem, but you asked for other symptoms that could cause this. I had a 65 Mustang when I was a kid and it had a 289 with a few miles. I used to drive it like I stole it, and one day the timing chain jumped a couple a teeth. It idled rough and was hard to keep running, but if you stepped on it, stand back. I guess retarding the cam has something to do with a hard acceleration. When I'd leave a light, it stumbled a little then pulled like nothing was wrong. I found it by setting the timing to smooth out the idle, when the vacuum advance hit the thermostat housing, I knew something jumped time.

 

[TriodeLuvr]

Mike, I'm still evaluating the plugs to see how they color and wear in. The car came to me with AC Delco R43TS. I replaced them along with the other maintenance items shortly after I bought the car because it had developed a bad midrange miss. AC isn't my favorite brand of plug, so I replaced them with Champion RV15YC4. It was really difficult to find a cross for the ACs, and I'm still not sure these are exact replacements. So far, the Champions seem to be coloring tan whereas the ACs were light gray. However, I have no history on the AC plugs, and considering the car is originally from a different part of the country, even the gas might have been different.

Here are a few pics of the plugs. They have about 2K miles on them now. I'm a little concerned about no. 3 being so dark, but I don't know if the AC in that position was doing the same thing. I guess the "shadowing" effect is normal for SBCs; the ACs also have it, but it's not so obvious because of the color difference.

Jack

 

[T-Test]

Looks real lean to me. 1406 Edelbrock I think is a 650cfm carb and should be enough for a 350 sbc unless you made changes when you rebuilt it.

 

[Screaming Metal]

Mike, I'm still evaluating the plugs to see how they color and wear in. The car came to me with AC Delco R43TS. I replaced them along with the other maintenance items shortly after I bought the car because it had developed a bad midrange miss. AC isn't my favorite brand of plug, so I replaced them with Champion RV15YC4. It was really difficult to find a cross for the ACs, and I'm still not sure these are exact replacements. So far, the Champions seem to be coloring tan whereas the ACs were light gray. However, I have no history on the AC plugs, and considering the car is originally from a different part of the country, even the gas might have been different.

Here are a few pics of the plugs. They have about 2K miles on them now. I'm a little concerned about no. 3 being so dark, but I don't know if the AC in that position was doing the same thing. I guess the "shadowing" effect is normal for SBCs; the ACs also have it, but it's not so obvious because of the color difference.

Jack

Yea Buddy! Thats Lean! you can see which side of the plug is facing the exhaust valve. That E85 gas can cause problems like that also. A friend filled up a bucket, it started spitting and popping. came into the shop, I disconnected the fuel-line at the carb, and ran the pump till we hit gas. Got 3 pts. of water....
GotGlad you found the culprit. Put some miles on it after you fatten her up some. AC plugs are fine for a bone stock motor, Champions are good but have a tendency to gas foul beyond a certain point. Autolites are pretty good.
Everyone has a different view of plugs....and diff. opinions....

 

[Mike]

Those plugs have been run far too long to get any kind of accurate reading from them. You can get some general idea what is going on, but no real, definite clues.

Heat range on the plugs looks to be pretty close, in general. More so on #1 and #3, than on #8.

As heavily used as these plugs are, that porcelain should be showing a lot of color. I can't really tell, but it looks like #1 and #8 are showing some 'pepper' on the porcelain. You need to get a good magnifying light, or get out in some sunlight with a magnifying glass and really look closely at those specks. Are they all black, or are you seeing some of them looking silver? A bit of black pepper can be some oil splash.. If the pepper is metallic, then you know you are detonating that cylinder and you need to start knocking back the timing

When it comes down to reading plugs, you have to understand that most street motors spend a lot of time idling, which really colors plugs up a lot. And that color makes them nearly impossible to read. We always had a set of starting plugs and a set of run plugs. The starting plugs were an old set of run plugs that we would use to start the motor and build heat. Once the car was ready to run, we would put in the run plugs. The motor was always shut off cleanly, at the other end, so the plug would show us what individual cylinder conditions were like, under power.

SM, I've never really seen anything, between standard plugs like AC, Autolite and Champion, when it came to making power. The 'trick-o-the-week' plugs all come up pretty short (Bosch platinum has a tiny flame kernel, and the plugs with multiple ground straps do a pretty good job of masking the kernel), but with the other 3, as long as heat range was correct, it always seemed like a plug was a plug was a plug. Obviously, the plugs with the cadmium shells are easier to read for heat, but when it came down to actually making power, none of the big 3 seemed to rise to the top. Champion and Autolite seem to have a better coverage of heat ranges, so that was always a plus.

We had one motor combination that used four, different plugs. It was a matter of juggling some heat ranges on standard plugs in four holes and running surface gap plugs in a two others. Whenever there was down time, where we were just sitting around and chin-wagging, I would have a plug index tool out and would be looking for plugs we could use.

 

[roadmonster]

If you run Delco plugs, they should probably be R45s. If you cross-reference to R43s, you might not get the correct range. Not sure what you're running, but R45s work for a wide range of street performance engines.

 

[Mike]

Bearing in mind, of course, that hese plugs are tapered seat and not gasketed plugs, so that number would actually be R45TS., rather than R45S.

Personally, I would avoid the resistor plug entirely, but that is just my personal preference.

 

[roadmonster]

I meant the plural of R45. Good all around plug for small blocks, whether short or extended. Did not mean R45S.

 

[Screaming Metal]

I agree with you 100% Mike....its always a compromise when on the street, thats why I said those. Nothing like sitting around 'Dexing plugs while everyone else is jack-jawing....;-) Like Mike said, aplug is a plug is a plug when doing a street motor.
NGK's /Bosch are the like are best if you spring for the platnium. They clean out better, cadmium shells ARE the easiest to read, like Mike said.
We were over at SpeedTalk and got to yacking about spark plugs....Holy Crappies Bat Man!

Its funny, we can pick up 2.5 to 3 horses or more just from changing oils, or tightening/loosing lash, or messing with ring tension/endgaps. Theres never been any real earth shattering advances in spark plugs. There has been improvements, but nothing magical....

There ARE some really good plugs out there, but they are EXPENSIVE....

 

[Mike]

I never much cared for the Bosch plug, as that wee electrode produced such a tiny flame kernel. We had drilled and tapped a piece of aluminum, so we could thread different plugs into it, then we would run plug wires from a distributor in the distributor machine, so we could actually see what the flame kernels looked like. Obviously, it was in a still room, not in the hurricane of a combustion chamber and was at atmospheric pressure, rather than at cylinder pressure, but we could still see a lot of things happening.

 

[CARS34PHORD]

AC came in all new Chevy s/b. i run NGK in my blowin b/b one run at Bonneville pull them out put in the trash can. Champions never could get the engine to run an was getting them free from a champ rep