idle rich, then leans out on highway?

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fonzy

fonzy

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I need to get in and look at the carb, but the idle seems to run rich (monkeyed with the idle screw and it seems to make no difference, so i am assuming it is in the power phase of the carb?), however on the highway it seems to chug, i am assuming this is leaning out but i am not certain, it idles fine but with peddle it chugs. i have a single barrel carb on my 66 225, we had the carb rebuilt in june-ish, ran fine...i am actually worried that the fuel system is dirty and it took this long to cause the problem again. Just wondering what you guys think. thanks for any responses. I was also wondering if the fuel filter could be clogged slightly and cause similar issues? definately rich at idle, not lean.
 
1. Take the old filter off, turn the INLET side down and tap it on a table. See what comes out. Blow through the inlet. If air passes it's not clogged up.

2. You gotta give better descriptions.

There's an idle speed screw and an idle mixture screw. Which one did you turn back and forth? Do you know how to adjust a mixture screw? Lots of people these days have no idea.

Chug. What do you mean...you give it gas to accelerate and it won't? It accelerates but runs roughly?

3. Is the choke working right? Chokes CAN cause all kinds of issues. They don't cause any when they are working properly and maintained.

4. What kind of carb is it..a Holley or a Carter? The Carter BBS is a better carb than the Holley, which is a troublesome, leaky POS, in my opinion.

Chime back in with more info.
 
sorry, it's the carter bbs, automatic so no dash pot. The idle mixture screw has no effect on the engine, which makes me think it's running on the power side...it pops out of the exhaust, like it's running rich at idle. When you hit the peddle while driving it will run rough while giving it gas, runs but rough under throttle, it has power loss as well....it will idle ok, not perfect but ok. i really need to check the filter, that will clue me in to any fuel starving from that. i believe the choke is working ok, the linkage is bent all to heck, but it seems to open properly at start(which should be about 1/3rd open?). one of the reasons i think it's rich is that if i hit the peddle more than once to start it it will flood out and be hard to start without mashing the peddle to the floor and cranking it hard. plus it will run rough at high idle and put out black crud from the exhaust. as far as the idle mixture screw i thought it was all the way in, lightly in, then back out 1 1/2 to 2 1/2? i hope i clarified enough, i had thought if it was lean it would pop through the carb and rich would pop through the exhaust...but if it's rich under throttle it doesn't clear out with more throttle, it seems worse actually. thanks again.
 
The idle setting you describe is about right; around 1-1/2 half turns out is right. When cold, and you press the throttle to the floor once before starting, the choke should close almost all the way. It will open a very slight amount when the cars is started. Does your choke use a long rod to a 'choke action' bolted into the exhaust manifold? It normally has a couple of bends in it.

If it is rich then remove the spark plugs and see if the plugs are coated with a flat black sooty looking deposit. That would truly indicate a rich condition without having to guess. (A shiny, wet looking balkck is due to oil in the cylinders.)

Since the carb is rebuilt and you have concerns that the fuel system is dirty, then I would first look at the needle and seat in the carb; a tiny crud particle could cause it to not fully close and it can flood the carb.
 
nm9stheham said:
If it is rich then remove the spark plugs and see if the plugs are coated with a flat black sooty looking deposit. That would truly indicate a rich condition without having to guess. (A shiny, wet looking balkck is due to oil in the cylinders.)
Click to expand...

You are pretty good at this. :)

Pull a plug or two would be the very first thing I would do.
That way we know where to start.
 
One thing to keep in mind that ignition timing that is out of adjustment one way or the other can cause problems that MIMIC carburetor problems, but aren't.

So, along with all the other good advice, I would get access to a timing light and ensure the base timing is where it should be before accusing the carb(which may well have it's own issues).
 
Ok...I put a fuel pressure gauge in the line. At idle it's 4.5 psi..which is ok. My question it when you Rev the engine shouldn't the pressure go up? I revved it pretty good and it never went above 5 psi.
 
Tomorrow I'll check the timing and compression. It's finally a warm day. I may also check fuel amount, a quart a min at 500 rpm? Correct?
 
Also..we adjusted the idle mixture screw. It almost died then turned out and idled fine, however, it still "chugs" a little while giving it gas at around 55 mph. Also still get an occasional pop out of the exhaust when letting off the gas at lower speed.
 
Your pump pressure if OK, even with not going up. It should stay in the same range from idle to full out. Just look for a pint in 30 seconds but typically will be more; the hard part is how to run the pump at 500 rpm? It does not crank that fast LOL. Cranking is all you can do; it can still shoot out quite a way at low RPM so be careful.

When you say 'chugs' do mean it pauses or stumbles for a moment when you push down the accelerator at cruising speed? Is this a Holley 1920?

And is the 'pop' a loud, distinct 'pop' or 'bang' or more of a series of low rumbly, popping sounds?
 
Just low rumble pops...not backfiring full out. Chugging is just a barely noticeable intermittent power loss when giving it gas, if I just hold it at a speed it's not noticeable or it stops. Only at higher speeds though...not below 50mph. Yeah I didn't think when I posted the fuel amount check.
 
Well, I would expect the pops are more due to valves (worn guides) or timing variations (worn chain.) Does this engine have some miles on it? (And I see that you posted it is a BBS carb.)
 
Single barrel carter, I ended up doing the brakes on my other driver..I will run through the compression, timing fuel amount tomorrow. Thanks guys.
 
I just checked timing...it's right. So compression and fuel volume tomorrow.
 
Be aware, that ignition timing being spot on does not tell you if the timing chain is worn..... because you compensate out the chain wear with the distributor adjustment. That does not fix a retarded cam or cam timing variations. Replacing a worn /6 chain will surprise you at how doggy the engine was before.

Do you know the engine mileage?
 
59000..replaced the fuel pump. The pressure now reads 6psi..and it's worse. The stammering happens at any speed with acceleration. I can only think it's a carb issue. Apparently these single barrels are finicky. It's the carter single which is supposed to be better. We also noted that the choke has play in it when warm. Is that normal?
 
Also, it idles although it did die and required flooring the pedal to restart. I am inclined towards the carb.
 
My dad held the choke open but I still had to mash the pedal to start it.
 
Spray starter fluid down the carb throat when it is acting up. It should run real smooth on that if the ignition is OK. I once got a VW Jetta running w/ just starter fluid when the fuel pump wasn't working (Bosch Jetronic type). It ran perfectly smooth as long as I kept giving it squirts.
 
All this began after replacing the spark plugs, I went with a gap of .040. I am running the ballast resistor and a 9 volt coil, I was reading up on the gap and found that factory is .035, also noted that the one continuous theme for rich or lean running was an improper spark plug gap...I am going to re gap the plugs (I have to replace the plug sleeve o-ring anyway). Has anyone experienced this before?
 
So let me get this straight, all you did was change the spark plugs and now it runs like crap? If that is the case and nothing else was touched besides the plugs, then I'm wondering if you have the right heat range plug. How did it run before the plug change? Was the plug change just for maintenance? Double check that the plug you put in matched what you pulled out.

Also, I had a slant before. They do not like funky plugs like, platinum or iridium. Good ol fashion champion copper plus plugs works best. Believe me, I've been there and done it.
 
I changed the rotor, cap, wires and plugs...it had ngk r plugs in it, which is exactly what I repleced..I left them at factory .040, I checked the old plugs, they are .035...I am going to work backwards like I should have when this started...I'll start with the gap. I was numb headed and should have done this first. Why I assumed all this other b.s. is beyond me. Live and learn, thanks for all the responses and I'll keep you posted.
 
At least I was smart enough to keep all the old stuff so I can work backwards and replace each piece till I figure it out. Good news..I know the compression, timing and fuel system is good.
 
By the way my pop pulled the card and gave it a once over...clean as a whistle. Like I said I was stupid not to retrace my steps after the "tune up"...I really should have replaced every thing with the original stuff piecemeal and figured it out that way. Dumb dee dumb dumb..on my part.
 
Live and learn. My carb on my dodge d100 had the same problem, came out to be the choke, removed the electric choke and added a cable (manual choke) runs like a champ now.
 
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