carb settings - engine surging when cold

[bradbig]

Hi gang. I have a 1973 Spider with the 1592 cc engine. I'm still getting to know the car, but there's one peculiar thing I can't figure out yet. The chose and throttle knobs work correctly (I think). The car starts easily when cold, and I ease the choke back towards open as the engine warms up and it idles, although it's a very low idle (maybe too low?). The problem is only apparent when going from standing still to rolling, in first gear. The engine/carb surges and then slows, surges then slows, surges and slows until I get it into second gear, then the problem seems to go away. I haven't driven it more than a few minutes at a time, yet, as I'm still working with the interior. Am I just not letting the engine get hot enough, to normal operating temperature range maybe? If I choke it a bit when it starts doing this, the problem seems to go away. Perhaps it's one of the jets inside the carb? Maybe it's juts not getting hot enough. Any thoughts would be helpful.

[bradartigue]

This is the result of how the throttle assembly is on the early cars. You can replace the bushings in the firewall and it sort of helps, but this is just how they work. My 1438s were horrible about this until I put in a bigger carburetor; in your case you just have to put up with it when cold.

[majicwrench]

The (bad) combo of motor mounts and throttle linkage on those early cars (like my 72) can make them kinda jerky to get moving at times. basicly, as the motor torques, if move the throttle linkage. Certainly, if they don't run right the problem gets worse.
Does it idle well within a couple minutes of starting?? How low is low?

[bradbig]

Thanks for the info. I appreciate it. I don't think it's a matter of motor mounts or throttle linkage. If I pull the choke lever out a bit to make the air/fuel mixture more rich, the problem seems to go away. I read an article last night, online, about "surging" possibly being caused by incorrect clearances in the throttle body and valves, or incorrect carb float position. Are they any Spider carb experts out there? I can't say for certain, but I don't think it's engine torquing and linkage that's to blame, otherwise, why would pulling the choke lever out make the problem disappear? Thanks again.

[majicwrench]

Brad,
Lots of "experts" on the board

Choke out a bit may just be making car run a tad bit better, thus less of that torqe issue. As I said, car has to run right to be smooth. And you're right, it may not be a mount issue, but all these older cars suffer a bit from this malady, which is why fiat went to a different throttle arrangment.
And again, does it idle well?? Can ytou adjust the idle mixutre?? Number one problem w these early carbs is the idle/low speed jet plugs.

[bradbig]

Thanks for the help and opinion. Once it's hot, it idles fine, but VERY low (slow). I'll check the RPM. I haven't had a go, yet, at setting the idle speed screw (I think that's how it's adjusted, based on other older cars). I'll record the rpm while it's idling, and report that to this topic. Thanks again.

[FiatBen]

You will not find a better source of Fiat Spider "experts" than on the forums.
I'm a bit confused as to having a low idle problem and not having at least put a screwdriver to the idle stop screw and idle mixture screw to see if it is simply out of adjustment. Certainly if it is cold and idling low, then jerking when you try to get moving seems a very likely outcome IMHO. Time to do your homework. This will give you guidelines for adjusting your carb.
http://www.artigue.com/fiatcontent/Arti ... 124_MM.pdf

Have to agree with others. Experience says its a linkage/mount design flaw. Also, likely to have jets plugging or the float sticking. But I'd start with adjusting the carb first, just so you have a base point from which to provide detail enough to get a solution from the forums.

At any rate, hang with us, give us information, and together we will solve the problems.

[bradbig]

Again, sincerest thanks for the input. OK - the sputtering (when still) and surging when going from still to rolling in 1st gear happens even when the engine is hot. As it was, the engine, once hot, was set to idle at about 350 rpm, if the tach reading is close to accurate.

Increasing the idle speed makes no difference in this symptom, it's the same cold or hot. Pulling the choke knob out, to enrich the mixture does cure the problem, once the engine is hot. I pulled out the choke know very gradually, locking it into place by turning it. The sweet spot seems to be at the position where the engine rpm picks up to about 1100 rpm. At 1100 rpm with the choke locked in position, I can depress the accelerator pedal both slow and smooth, or sudden (a tromp) and there's no hesitation at all, and no surging when in 1st gear; I get an immediate and near perfect increase in rpm. And, a very satisfying sound from the exhaust! Perhaps I have a hole in the exhaust pipe or muffler than I haven't found yet, or, maybe it's supposed to spound that way.

Thoughts?

[bradartigue]

So far the only problem I see you mentioning is the low idle when hot, so fix that first. The surging is going to happen with that throttle assembly but it is far less apparent when the engine is tuned correctly. I also wouldn't run around with the choke out, you're bathing your pistons with gas and contaminating your oil.

Get the timing, fuel mixture and idle straight - I have a whole process documented: http://www.artigue.com/fiat - or use the shop manual. That will get you idling correctly or at least let you determine if the carburetor jetting is correct. Then move on.

Those cars surge, so don't get too excited about curing the entire problem.

[btoran]

brad is the king of carbs. when in doubt, you'd be wise to follow his advice.

i think your idle is a bit low and should be set higher. my 1975 came from the po with the idle set too low (about what yours is at). i had problems with the car stalling and it took forever to warm up. i bumped up the idle with a quick turn of the idle adjustment screw and it now idles just under 1K rpm when it's warmed. my only issue at this point is it takes a while for it to warm up (more than 5 mins) and te rpms to drop down. once warm, it runs like a dream. it seems to run on the cool side, even when warmed up, so i'm not complaining.

[bradbig]

Boy, that slow idle adjustment screw is in an awkward place. I'm about 6 foot, 3 inches tall, and have relatively large hands. I removed the air cleaner cover, and the crankcase breather pipe and another hose from under the air cleaner, but it was still awkward to reach. I had to use my little snake-type inspection cam to see underneath, to find the correct screw. The slow idle mixture adjustment screw is visible if i lean towards the front of the car, but it's also in an awkward place. With the steel lines from the brake system right beside the screw, and with the screw having such a small head, I was only able to enrich the mixture just a tiny little bit of a turn.

With the isle speed now at about 800, and the slightly richer mixture, it definitely runs better. There's no sputter at all, now, when I move the throttle quickly by hand, and the surging from still to rolling, is less. I think I could still use to turn out the idle mixture screw just a tiny bit more, and pick up the isle speed just a bit more. All the adjustments were made after the engine reached operating temperature - after the cooling fan came on for 2-3 cycles.

All this having been said and done, if I vroom-vroom-vroom the engine to get the rpm up, the surging and sputtering are substantially less, but I don't think I should have to do this. I've observed, over the years, people with sports cars doing just that, vroom-vroom-vroom, as the start to move, and I always thought that was just silliness and enthusiasm. Maybe not?

I'll advise once I've done that to report any improvement.

[btoran]

the idle adjustment screw is in a bad spot, but it should turn freely once you get to it. i had an additional problem with mine in that the post of the screw was actually bent and sometmes it would find the hole it was meant to and sometimes it didn't, so my idle was not only low, but even once adjusted, it was erratic. so i replaced the screw. might be a good idea to check if yours is straight by taking it out (carefully).

[bradartigue]

Your tool kit has a tool that you place on the end of the idle screw so you can turn it. Its a small socket wrench with a fixed end. Of course if your carb no longer has the 10mm hex headed flat head screw then never mind.

[bradbig]

I'll have to check for that tool. The plastic box is in the trunk. The Fiat jack and jack handle are there. I haven't looked in the tool kit box, but it does rattle when shaken, so there is something in it. I do know the idle mixture screw doesn't have a head on it, so it must have been replaced. The slow idle speed screw is original, I believe, and could be in the size range of 10mm. I'll let you know what I find. I did have the car out to my mechanic again, and it was still surging, even with the idle rpm at about 900, and with my slightly richer setting. Pulling the choke out makes it better. I may need to enrich my mixture just a bit more. I was looking underneath yesterday, tightening the bolts on the oil pan and gear box bottom, and I did see that the engine mount bolts appear tight, although the rubber bushings were somewhat cracked, one worse than the other. That may be a job for another day.

[majicwrench]

You gotta get that idle mixture set. You are correct, the MIXTURE screw does not have a hex head on it.

My 72 has a lock nut on the idle mix screw, does yours?? One mine, I have glued a short hunk of vacuum hose to idle mix screw, and I use that hose to turn screw as needed. Dang near impossible to get a screwdriver in there and make an adjustment.

Don't worry about mounts just yet. Get er running right.
Yes, a little of that vroom vroom to get er moving is sometimes neccessary. And it sounds cool