Technical Idle only with choke on, or when mixture screw adjusted to very rich. Help!

[Mj500]

Hi Guys,

New to the classic fiat 500 scene, purchased a lovely little car recently that in my viewing /ownership has only run on choke until today, we got it to idle smoother earlier with the mixture screw a long way unwound (presumably simulating the same very rich conditions it gets when the choke is on). When the mixture screw is wound back in and unscrewed a couple of turns as suggested no matter how much adjustment with the throttle screw, I can't get it to idle without choke on.

Gave it a service inc fuel filter, air filter, oil change & spark plugs (old spark plug ends were covered in black suggesting very rich fuel mix). Didn't help at all. Blasted the old fuel through the car and used some fuel cleaner, didnt help. Stripped the top of the carb off to the point of being able to see the idle jet screw, removed it and cleaned the carb with carb cleaner, no joy.

When the car is idling with choke, after heating up a little, it gets to a point when every now and then it sounds as if a pressure relief valve functions, this usually causes the engine RPM to increase, but it can also sometimes cause stall.

Also it may be worth mentioning that on the test drive we went on before purchasing the car, it was only running on choke at the time but it pretty much ran out of fuel.

I have a video of it happening, but can't post it here because i'm new and need 5 posts before I can post links lol, in the video you can hear the pressure relief (or whatever it is at 50 second, then the engine slows and splutters)

Another way I can get the engine to run without choke is if I constantly blip the throttle, holding on the throttle in one position doesnt work, it revs initially then causes the engine to stall. Only blipping it on and off will deliver enough fuel to keep it running.

Any thoughts or ideas on why this might be occurring is very much appreciated?

 

[the hobbler]

This sounds like a LACK of fuel---have you removed the MAIN jet and cleaned it out? There is also a chance that you have some gunge in the main-jet orifice. Lastly, how many fuel filters have you got? There should only be 3---one in the tank unit (check that isn't blocked or covered in muck), one in the top of the pump and a small filter in the top of the carb. You do NOT require an additional filter between the pump and the carb (if indeed you have one fitted there).
To check the tank-unit filter, look on top of the fuel tank---there will be a round disc with a pipe coming out of it and wires to it. It will have a number of small nuts--undo those and then remove the plate, complete with the tank unit (fuel gauge+ fuel pick-up + small tube filter)---check and clean as neccesary.

 

[Mj500]

Thanks for the reply! I'm going to buy a full carb overhaul kit so next weekend i'll remove it from the car, give all the components a clean including the main jet. The car is back in storage for the week now so again i'll take a look next weekend and clean/replace as necessary. It does indeed have a filter between the pump and carb which I replaced as the old one was very dirty and the Ricambio service kit I purchased had one included with it.

Again thank you for your reply and ideas, i'll let you know my progress next weekend!

 

[Toshi 975]

You need to check for any air leaks between the carb and the cylinder head. The Weber 26IMB carbs are also notorious for the bottom flange bending if people overtighten the nuts. Normally happens when people try to compensate for bad gaskets. The idle screw goes right through the bottom flange causing a weak spot.

 

[the hobbler]

As a matter of safety, please remove the filter between the pump and the carb. I would put money on it that it is a plastic filter. If that should brealk (or leak) it will spill fuel over the dynamo--not a good move! Replace it with E10 resistant fuel pipe--1 piece from pump to carb. You do NOT need that filter.

 

[Mj500]

Thanks Toshi, I watched a good video on Youtube of a complete carb overhaul which detailed the same thing with the flange bent, so when I take mine off i'll see how flat it is and adjust as necessary.

Hobbler, I note the Haynes manual all the diagrams don't feature the additional fuel filter, but almost every car you see does have one fitted, strange.

 

[the hobbler]

I will agree, a lot of people DO fit a filter between the pump and the carb, but I assure you, it is not neccesary. As I mentioned, the factory fitted 3 filters, and in fact the more filters you fit, the more obstruction there is to fuel flow. I am not the only Forum menmber who has a 'thing' about filters between the pump and the carb---it is such a dangerous place to put it. With the speed at which most garages seem to go through their fuel these days, the chance of picking up contamination is slim

 

[Toshi 975]

Thanks Toshi, I watched a good video on Youtube of a complete carb overhaul which detailed the same thing with the flange bent, so when I take mine off i'll see how flat it is and adjust as necessary.

Hobbler, I note the Haynes manual all the diagrams don't feature the additional fuel filter, but almost every car you see does have one fitted, strange.

I know what you mean because it seems like almost an obsession to stick that cheap plastic fuel filter in the easiest place between the pump and carb. I think that it is irresponsible if Ricambio are selling those as part of a refurb kit.
One of the most frightening things I have ever seen was one was fitted to my car when I got it, when the cooling fan exploded and I had no idea what had happened, tried to drive home, within about 3 miles the engine had overheated to the extent that the plastic filter had swollen to about the size of a Jaffa orange, was soft to the touch like a balloon and the petrol inside was boiling. That was a lucky escape and an education.

 

[Mj500]

I'm sure you'll both be glad to hear i've taken your advice on board and have just ordered a new length of fuel hose, so will replace the existing filtered piece next weekend. I'll let you know how I get on with the carb overhaul and will take and post some pictures of the little beauty i'm working on!

 

[Mj500]

Only managed to get the carb cleaned up and overhauled today, didnt manage to get it put back on the car (both studs that fit it to the cylinder head are bent so going to order some new ones) also the rocker cover gasket isnt in the best condition so going to replace that too. Straightened up the flanges on the bottom of the carb as that was a little bent as Toshi suggested it might be. Choke cable snapped off right next to where it gets retained in place as it was folded over and it didnt like being straightened back up for carb removal, so i'll order and change that cable

All in all, a few steps forward but also a few steps back!

 

[the hobbler]

If the choke cable was bent, was there a chance that the choke was 'ON' all the time?

 

[Mj500]

Sorry I explained myself badly, the cable ran smoothly into the choke mechanism and allowed a full range of movement between both the open/close position. It was directly after the cable retaining bolt/screw on the mechanism that it was bent back over on itself presumably to get the excess out of the way, when I straightened it up again to remove the whole carb it snapped

 

[The Whitakers]

When I straightened the base on my carb I flatted it of on sheets of wet and dry laid out on a smooth flat surface to ensure it was totally flat.

 

[Mj500]

Reassembled everything today, adjusted the settings on the carb and success, we have a running engine! i'm still not 100% convinced i'm fully set up as well as can be so will continue to go through service items and check them, but it's a huge step in the right direction and i'm extremely pleased. I drove numerous laps of the block and got up into third gear with the engine seemingly running reliably enough to cover some decent distance. Good days progress all in all.

I've attached some pictures from the last couple of weeks. Please note the full pictures of the car are upon when it was purchased, before I did any work to the engine

 

[Toshi 975]

Looking good there. If you have got them you need to fit the rubber spark plug seals otherwise you loose a lot of the cooling air

 

[Mj500]

Frustrating day of work today, after previously having what I thought was a smooth running idle I noticed a small fuel leak in the carb somewhere so investigated, turned out to be the main jet holder wasn't quite fully tightened, so nipped that up a little tighter, problem solved. Whilst investigating i'd removed the carb again as I initially thought that the issue was the carb to base/drip tray where i'd previously straightened out the base of the carb flange and wasn't sure i'd got it 100% flat so added a second gasket and bolted everything down again.

All appeared to be running okay to start with but it then soon developed an issue which sounds like something is getting stuck and is throwing in lots of fuel as during idle the engine RPM will suddenly speed up significantly on its own, occasionally it corrects itself on its own and idles okay again, if I give it some throttle and it usually sorts itself out and will temporarily idle slower before the same thing happening again.

A fuel delivery problem i'm sure, but any recommended places to start? Link to video below -

Dropbox - File Deleted - Simplify your life

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Also I've now bought and fitted some rubber the rubber spark plug seals

 

[Mj500]

Just as an update, removed the second gasket, reflatened the bottom of the carb (i'd over tightened it) doh. Adjusted the points and it runs pretty sweet now.

Just rewired the car and chasing other problems now, mainly fuse 2 blowing when key turned right in the ignition, but is fine when key is turned left in the ignition. Lights doing all sort of funny things too. Oh the joys!