Idle Speed and Engine Performance

[ninjazombiemaster]

Hi Everyone,

I'm driving an early '73 240 with the original engine. Its dual carb'd, I'm not positive which brand - but I know they aren't stock. When I bought it in July, it did 0-60mph in about 8.5 seconds, which is a little slow, but that probably had more to do with my driving skill, and got to top speed almost too easily. Now its 0-60 times seem awful and it has a rough time getting past 85mph. I put some high performance injector/carb cleaner in and it seems to run a little better, but not like it did.

Also, I don't know a lot about idle speeds, but I know that most cars run idle at 600-1000rpm, except those that are modified for high performance, and may behave oddly at lower idles. I feel like it idles a little rough even at 1000, and doesn't feel real stable until maybe 1,200. Also, the choke lever has to be pulled all or nearly all the way towards the back of the car for it to maintain a strong idle. I'm pretty new with that sort of thing too, so I'm not sure if that's normal.

I guess, what I'm getting at is:

-Is this a normal idle for an engine like this?

-What could have caused such a drastic change in performance? de-tuning, something worse?

-What is the best methods for finding the ideal idle speed and fuel mixture for my engine?

EDIT: Also, it sounds like it backfires at high RPMs. Usually not till like 6-7,000+. I recall this may mean the engine is running lean?

Edited September 23, 2012 by ninjazombiemaster
Additional info

[Gary in NJ]

Engine idle is called out in the FSM (factory service manual). Do a search and you'll find a few spots do download the entire book. What do you have for carbs. If it's a '73 it's likely that the original carbs have been swapped-out for earlier '70-'72 carbs, typically called "round tops". If this is the case, follow the tune-up guide for an earlier 240Z.

If you were simply complaining about the idle, I'd suggest that you have a vacuum leak, but if the car has lost performance and is now not idling properly, it's a good indication that you are in need of an engine tune-up (plugs, points, rotor, cap, wires (maybe), ignition timing, carb sync and valve clearance).

This "drastic change" is a simple result of driving the car. Modern cars can go 100,000 miles between tune-ups. Cars like a Z have scheduled items at 3,000, 6,000 & 12,000 mile increments.

One final thought. There is a method for setting idle in the FSM. I follow this method, however I set my idle at 800-900. I just like it there. In every Z I've owned I find that this idle speed helps with throttle tip-in and driving smoothly from a full stop.

Edited September 23, 2012 by Gary in NJ

[ninjazombiemaster]

Engine idle is called out in the FSM (factory service manual). Do a search and you'll find a few spots do download the entire book. What do you have for carbs. If it's a '73 it's likely that the original carbs have been swapped-out for earlier '70-'72 carbs, typically called "round tops". If this is the case, follow the tune-up guide for an earlier 240Z.

If you were simply complaining about the idle, I'd suggest that you have a vacuum leak, but if the car has lost performance and is now not idling properly, it's a good indication that you are in need of an engine tune-up (plugs, points, rotor, cap, wires (maybe), ignition timing, carb sync and valve clearance).

This "drastic change" is a simple result of driving the car. Modern cars can go 100,000 miles between tune-ups. Cars like a Z have scheduled items at 3,000, 6,000 & 12,000 mile increments.

One final thought. There is a method for setting idle in the FSM. I follow this method, however I set my idle at 800-900. I just like it there. In every Z I've owned I find that this idle speed helps with throttle tip-in and driving smoothly from a full stop.

Gotcha. I have the FSM, but I did read here in a few places that most people prefer 8-900rpm. I guess I'll bring it into the shop and have my dad show me what I need to be keeping up on.

[Gary in NJ]

Why let your dad have all the fun? Maintaining a Z is half the fun of ownership.

[ninjazombiemaster]

Why let your dad have all the fun? Maintaining a Z is half the fun of ownership.

Haha, he's been doing this thing for decades, I've only been in it a few months. I've got a lot to learn, but I do plan on learning. And trust me, I'm having plenty of fun

[ninjazombiemaster]

Alright.

So I took it into the shop and it looks like the carbs are tuned great. The other day I upped the idle speed, so now it doesn't die with the choke off after its warmed up. Put some new front brakes in (mine were good, but metallic - so it sounded like an 18 wheeler coming to a stop). The rear ones still squeal a bit though, but I'm not real worried about that.

The fuel filter was totally filthy, and may have been responsible for the performance drop. I put a new one in, but it may have pushed some of that gunk farther into the system, and I may not see that performance come back until the cleaner is done with its job. I'll also take a closer look at the timing when I get the chance.

At this point it seems like the issue was mostly the engine starving for fuel but we'll see what happens.

While I had it on the lift, it seems I may have found the problem with my fuel gauge. It appears that fuel is actually leaking through the sending unit itself (not even the seal). I'll have to pick up a new one and see how it effects things. May kill two birds with one stone, there.

There also seems to be a very minor leak in one of the braided coolant hoses, so I'll need to fix that as well, however the engine is maintaining good temperature overall.

Lastly, I got my first real good look at the sub-frame and floor pans. Altogether its rust situation is pretty good, but the pans, especially the passenger side need to be replaced/repaired, as well as the passenger wheel well. Any suggestions on how to handle that? Just cut it off and weld some new metal on? Or are there aftermarket sub-frames that I could just install.

[ninjazombiemaster]

So she seemed to run real great today. Didn't start right up in the morning, but once it did, I gave it a few big revs and its been driving smooth since. Hopefully that means whatever was clogging things up has cleared out - but we'll see. I've still got a lot of driving to do tonight, I'll keep you guys posted. And I keep intending on taking some bay pictures, I know how you all love those, but I just haven't found the time. Soon!

[ninjazombiemaster]

I still feel like there's a problem. Initially I realized that my idle speed was much too low, and that even hot the engine dies with the choke off (that's towards the engine, correct?). There was a lot of sooty residue in the tail pipe, which would indicate running rich - which makes sense since I was driving with the engine choked up.

I upped the idle speed so it was about 800-900 RPM when the engine was at operating temperature and the choke off, and changed a very filthy fuel filter.

Once its warmed up a bit, it seems to run and start real well generally, Its mostly just its cold start in the mornings and the couple of minutes it takes to warm up. It'll run very sluggish, and I have to keep it choked until the temp needle reads about center, ~200 degrees.

It also seems to be having a hard time breaking 70Mph. I guess I shouldn't be too worried about that since the speed limits are never more than 65 here - but they seem like they're related issues.

Either way, I feel like the issue is with the fuel delivery, especially based on the condition of the old filter. As for the poor cold start, I was skimming another thread where someone had an issue that ended up being a fuel leak from the bottom of the carb that emptied the bowl, so he had to crank extra for initial start up until it could refill, so I'll check into that.

As for the sluggishness, could it be a weakening fuel pump or possibly battery or a clog somewhere? I'd like to get this all sorted before the weather gets chilly!

Edit: Also, it tends to backfire at high RPMs still, 6-7k which usually indicates a lean situation or a timing issue, correct? Any thoughts on that?

Thanks!

P.s. here are those engine bay pics.

http://www.classiczcars.com/forums/thread46994-2.html

Edited October 4, 2012 by ninjazombiemaster
additional info and question

[Bonzi Lon]

Were you aware that there is ( should be ) a small brass screen filter in each of the banjo bolts on the lid of each carb?

If not, remove the lid, take the banjo bolt out, there are the screens. Inspect & clean.

Bonzi Lon

[madkaw]

Sure sounds like you are fighting a vacuum leak- maybe your brake booster which would tie the brakes into all of this.

Check your banjo bolts as suggested above

Don't drive the car until you have the brakes figured out- that's a bad scenario !

Disconnect your vacuum line from the balance tube that goes to your brake and note any changes.

What's your timing doing at idle and full advance.?

Condition of battery?

Is your battery receiving full charging voltage ?

[ninjazombiemaster]

I checked the vacuum advance and it all seemed normal. At idle, the RPMs waver between 800-1k and if you rev it sometimes drops clear down to 400 before building back up. I'll take a look at those filters, and find a volt meter laying around somewhere and get back to you guys. The car itself brakes just fine, just occasionally squeals. More likely some gunk on the rear brakes, but I'll check it out! Thanks!

[ninjazombiemaster]

Checked te banjo bolt and filter and they seemed clean. I touched them up anyway. Gotta get cleaned up for work so that's about all I have time to check today.