My SELF-TUNING motor !

[Rob72]

Well, after sitting idle for 10 months, I've completed my install of 4.11 diff and a 82 5-speed into my otherwise stock 72.

After some encouragement, the motor fired up and I setup to set timing, carbs, etc.

Now, I've never really felt I had the car tuned up as it should be. I have no Colortune, and no Uni-syn. Well I bought the Unisyn and figured I could play with mixture by using the info from the various well-known articles.

The results were less than stellar. I got the air-flow balanced pretty good, but the mixture nuts aren't doing their job. That's probably related to the overall condition of the carbs (never re-built). Timing seemed the best at 12 degrees.

HERE'S THE INTERESTING PART:

I re-tuned and test-drove 3 or 4 times, never really feeling I was getting any kind of acceleration or improvement. This is a 4.11 ratio in the rear-end, after all! This thing should be impressive to say the least, and thrilling at the most!

Anyway, during my test drive last night, the usual bog was there, and almost no power (or rpm's) above about 3000.

I pulled up at idle, pulled a u-turn, and suddenly noticed the idle speed was 500-600 rpm's higher than it had been set at (900-1000). I thought screws were loose, etc. I let the clutch out and the engine willingly accelerated, I mean easily up to 4000 rpm's (no higher due to clutch break-in time). But the little Z pushed my little butt back into the seat as we zipped down the road. It did seem to be a little rough, almost like a miss, but the bog was gone, and the engine easily revved up.

Now what little gremlin was twiddling with my motor while I'm driving? Can this be attributed to some increase in valve performance; maybe stuck or restricted? or carbon buildup on the plugs that finally got burned off? Fuel filter? I'm talking about a car that had an entirely different personality here!

It'll be interesting to see what happens tonight. I'm praying for no more bog.

What do you think?

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Rob72

5-spd/4.11/SU's

[norcal z]

Hey, Rob -

Hmmm, this is one of those "good ones". My guess is that over the 10 months it sat, something in one of the carbs got stuck. Either just driving it, or your u-turn (or a combination of both) flushed/shook it loose. I'm not familiar with these carbs (yet), but in other cars I've had, things like stuck needles, floats or check balls can play havoc with you after a car has been parked for awhile. How is it now?

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Gary P

'72 240z - 5 spd., Crane Cams elec. ignition, BFG Comp TAs (needs tlc)

'97 JGC Laredo - K&N filter mod, Don-A-Vee TB spacer, factory skids/front tow hooks

[norcal z]

OK, after hitting the 'send' key a few minutes ago, I decided to check out the Haynes manual. Briefly, it mentions that there are differences in carbs for auto tranny cars and manual tranny cars. I saw in another forum area that you have just swapped in a 5 spd from an automatic. I assume since you didn't say otherwise that these are the auto tranny carbs, right? That might also have something to do with your problem (tuning), although it doesn't seem to answer why it would all of a sudden "come on" as you described. It *could* have an impact on eventualy being able to tune them properly, I suppose.

Someone else who's done the tranny swap should be able to address this...

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Gary P

'72 240z - 5 spd., Crane Cams elec. ignition, BFG Comp TAs (needs tlc)

'97 JGC Laredo - K&N filter mod, Don-A-Vee TB spacer, factory skids/front tow hooks

[Rob72]

Thanks for the ideas Gary. I haven't made much progress yet as it was back stuttering again the next day.

I believe the carbs are the same no matter which tranny you have. There IS a difference in the distributors, the auto's have a dual point system, with one set acting to 'retard' the timing. I just need to get through emissions with it as is. Then the plan is to switch over the electronic ingnition and drop the points.

I've thought about it some, and I think now that I have a Gunson Colortune, (see-thru sparkplug), I'll be able to get the mixture spot on, and I'm going to check my choke cables and make sure they're retracting after the car is warmed up. Could be contributing the seemingly rich conditions.

I'm also quite sure the vacuum advance on the distributor is not working properly, but that shouldn't make it bog down.

I'll post back with results. Thanks...

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Rob72

5-spd/4.11/SU's

[norcal z]

Hey Rob, just curious - what's up with the "self tuning" gremlin?

[Kyle]

Hi guys;

sounds like Gary and Rob are workin thru this okay, but I thought I'd toss in my 2c...

And I hate to be a yutz, but I always say start at the beginning - are your valves adjusted? Points? Cap/rotor/plugs/wires? etc. Start with one thing and keep on keepin on.

That aside, it sounds like you might have a stuck piston in your carb; very easy to check - pull the air cleaner and look inside, see if the piston is sitting on the throat bridge like it's supposed to. Make sure both carbs' pistons slide smoothly in the domes, and that they slap back down at the same rate. If there's anything fishy going on, look at the needles as they rise out of the nozzle; they should be absolutely centered.

If that checks out okay, double check your mixture adjustment (roughly 2.5 turns out) and reset the balance. Kinda doubt that this is the problem, but it's important to have 'em balanced.

After that, make sure all your throttle linkage is smooth and silky, and that your floor mat didn't get on top of the gas pedal (been known to happen!)

My 2c, and please excuse if it's been already addressed; I'm just passin the time;-)

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Kyle Hagemann

Moderator, 240Z list

'69 2000 Roadster

'71 PL521 Pickup

'72 240Z Hotrod