Problems with idle speed on 1.7

[lentinj]

My B172 runs fine, apart from one annoying problem which is pretty much beyond my abilities. The idle speed is somewhat random. Most of the time it's the high side of the recommended 800rpm, however it can be alot higher. Especially when in stop-start traffic it will eventually rise (once it hit 1500rpm :/ ). It seems to be pulsing the accellerator that does it. The simplistic answer is the return spring on the butterfly, but the pedal itself seems to be rising back up. I wondered if anyone knew what's best to check out.

A few other hints:-

• * It's the replacement webber carburretor, not the solex
  * It's definitely worse in hot weather
  * The crankcase ventilator is probably well overdue a prod and poke
  * There's a few places on the carburretor that look like they want a pipe connected to them, but don't have one

It's annoying, because apart from this, it's running beautifully....

[foggyjames]

Prodding the throttle on a carb car squirts fuel from the accelerator pump, which (I believe) can sometimes sit (especially in high vacuum conditions) in a pool, being gradually picked up and burned off, causing the idle speed to lower slowly.

Is it rising all by itself, or just failing to drop, say after moving a couple of meters in a traffic jam?

It could be worse...at one point soon after fitting the twin carbs my car was idling at 4000!

cheers

James

[5lab]

make sure the valve that turns the air from the engine OFF in hot weather is working correctly

[lentinj]

I think it's the revs failing to drop rather than the revs actually rising on their own. It doesn't drop back over time whilst the car is sitting though, it only sorts itself after you get going again for 5 mins or so.

Yeah, 4000 is a fair bit worse! Even so it's mighty worrying when stuck in a vast M25 jam, crawling past car after car with burst cooling systems.... However those air-vents for demisting the door windows are damn handy for keeping the car cool also.

I'll have a look at the valve. I'm presuming that Mr. Haynes has some handy tips for checking it.

Cheers,

[foggyjames]

I'm 99% sure this is a carb problem. Once you've checked for vacuum leaks (let me know if you don't know the technique), and if you're stuck, it's worth getting the carb looked at by an expert. It might simply be that the Weber wasn't set up quite right from when it was fitted.

cheers

James

[pettaw]

I would say that you're running a bit lean most of the time, that's why the revs are low. Possibly because of a small air leak, or possibly because the carbs are simply not tuned properly.

when they get hot, usually from sitting idling in traffic in the hot weather, carbs overheat and over richen the fuel mixture above and beyond what they were designed/set up to do, thats why the revs suddenly rise in the hot weather and then they drop off when you start again as the fuel gets pumped in, cooling off the carb.

I would look closely for air leaks and then think about getting the idle speed/CO content reset if no air leaks are found.

One of the things about the 1.7s is the Renix is set up in such a way that, from idle, the ignition is extremely retarded until the vacuum advance kicks in, around 1300 RPM and the car revs suddenly jump. In other words there is IMO a slight hole in the ignition mapping that gives this characteristic. I bet when the car does this, if you put the fans on full and the headlights on, it'll drag the revs down on the alternator and then when you turn off everything again the revs will stay down on the ignition curve. Thats just the engine, but you need to get the carb set up right first.

HTH
Andy

[classicswede]

Ive had this problem on 3 1.7 carbs and in each case it was the secondary throttle flap sticking open and holding the revs high. That is why mine is now like this!!




You could try soaking the spindle in WD40 and open and close the throttle to free it up rather than going to the extent I did.

Dai