[benjamin181991]

Since owning my Sierra I have constantly had troubles with rough and erratic idle, bought a Weber and installed that whilst the stock one was being rebuilt. Even with the Weber, which ironed out the rough idle, i could not get it to idle below 1000rpm (sitting at 1100 when warm). Since putting the stock carb back on it idles great but still in the same range, never below 1000, the mixture screw ets to a point where it just stops affecting the rpm. The only way I have managed to get it to idle at 800 is with the lights on and adjust the idle up screw (at the front of the carb). Any input would be greatly appreciated.

[Gwagensteve]

The mixture screw shouldn’t make make any difference to idle speed.

If putting the headlights on lowers the idle then that’s a clue it might be misadjustment of the idle up system that’s holding the idle high. Normally the opposite is true- putting the lights on makes the idle go crazy.

[sideways]

Gwagensteve wrote:

The mixture screw shouldn’t make make any difference to idle speed.

If putting the headlights on lowers the idle then that’s a clue it might be misadjustment of the idle up system that’s holding the idle high. Normally the opposite is true- putting the lights on makes the idle go crazy.

The mixture screw absolutely will make a difference to idle speed but it should be a curve. For example (number of turns will vary between carbs and jetting but you get the idea), 0-1 turns out = low idle. 1-2 turns out = higher idle. 2+ turns low again. And yes, it does get to a point where more turns out wont change anything.

So do both throttles return fully? I've come across a few carbs that didn't have a strong enough return spring to bring it all the way back to idle. Of course, it could be something getting in the way too.

If so I'd look for vacuum leaks. There's quite a few vacuum lines on a G13 and these can be split or loose. Personally I eliminated all the vacuum solenoids and EGR so prevent vacuum leaks, my only vacuum line is to the dizzy (I run a weber). Speaking of which a hidden vacuum leak could be a hole in the vacuum advance diaphragm. In my experience this is an extremely common problem with G13s. Simply pull the hose off at the carby and suck on it, it should hold vacuum and if it doesn't you should replace it. In the meantime, disconnect the hose and block fitting at the carby.

With the motor idling spray carby cleaner around the base of the carburetor, over the intake manifold gasket, vacuum lines and egr gasket. If the revs drop you've found a leak. I've found fly spray also works for this but having the straw on the carby cleaner makes it easy to pinpoint.

[benjamin181991]

Vacuum advance diaphragm is not holding vacuum so I've ordered another. Another issue arose like a day late: car is hesitating when I go to accelerate sometimes even stalling, also it's pinging quite badly. Idles perfectly but. This is doing my head in.

[bumstein]

Might be to do with the vac advance. If it’s not advancing the timing it could do that under acceleration.. it could be any number of other things but given that your vac advance isn’t working I’d sort that before hunting for another cause

[fordem]

benjamin181991 wrote:

Vacuum advance diaphragm is not holding vacuum so I've ordered another. Another issue arose like a day late: car is hesitating when I go to accelerate sometimes even stalling, also it's pinging quite badly. Idles perfectly but. This is doing my head in.

Either the ignition timing is set incorrectly or the wrong grade of fuel is being used.

bumstein wrote:

Might be to do with the vac advance. If it’s not advancing the timing it could do that under acceleration.. it could be any number of other things but given that your vac advance isn’t working I’d sort that before hunting for another cause

I would have to disagree with that.

The engine will ping, or if you prefer, pre-ignition will occur if the ignition timing is too advanced - if the vacuum advance is not working because of a leak the timing will be retarded from the optimum position making pinging less likely to occur.

Vacuum advance is about fuel economy, it advances the ignition timing under cruise conditions - low engine load and small throttle openings produce high manifold vacuum which is then piped to the vacuum advance diaphragm which advances the timing - if the vacuum is low, either because of the throttle position or because of a leak the diaphragm should return (under spring tension) either to it's most retarded position, or to a position of less advance.

I ran a G13 in a Swift for over a decade with the vacuum advance disabled, because of a bad diaphragm - all it needs is to have the manifold connection blanked to prevent the vacuum leak - you won't get the economy advantage, but it will not prevent the idle speed from being correctly set, and the cold idle, hot idle and load idle up mechanisms for a/c & electrical loads will not be affected in any way.

[bumstein]

fordem wrote:

The engine will ping, or if you prefer, pre-ignition will occur if the ignition timing is too advanced - if the vacuum advance is not working because of a leak the timing will be retarded from the optimum position making pinging less likely to occur.

Yep, fair call

[benjamin181991]

Hey thanks for all the input, I really don’t think it’s to do with the vacuum advance because the problem is intermittent (no problem driving at all today) and the diaphram has been stuffed for ages. I was able to make the carby die last night by spraying carby cleaner (somewhere closer to the firewall seems to be the leak and it’s not from the base) which sucks because I literally just had the thing rebuilt. Really curious as to why the problem only occurs sometimes (twice all up now). I can rev freely in N or with clutch compressed but as soon as it’s under load that’s when the problems start.

[Technotron101]

Just a thought, not sure if it lines up exactly with your symptoms, but in another thread someone had issues with the clutch switch for EGR. So when you let the clutch out in gear to take off it dies off, whereas revving in neutral is fine.

Do you have a problem when taking off from a standing start say, or is it while driving, then accelerating?

Just something that came to mind.

[bumstein]

Technotron101 wrote:

issues with the clutch switch for EGR. So when you let the clutch out in gear to take off it dies off, whereas revving in neutral is fine..

Was just about to say the same!! Symptoms line up.

Can remember if you disconnect the switch on the clutch pedal or earth it out. Pretty sure you just disconnect it

[benjamin181991]

Yeah it’s when taking off from stationary. However I can definitely “feel” it whilst driving seems kind of low on power and bunny hops a bit and then when you get the revs up whilst cruising the pinging starts. I’ll try disconnecting the switch on the clutch pedal next time it happens.

[cairns27]

Key way on crank flogging out.


bye