12 replies to this topic

[BaronVonchesto]

Hey guys,

 

I've been trying to sort out a problem thats been eluding me for a few weeks now. A couple of months back, I was driving on B-roads when my throttle cable snapped. Luckily I was able to get the car into the nearest town by setting a really high idle on my carb and just driving it with a dead throttle. Then i got a throttle cable for another car and bodged a repair, to get it home.

 

To be honest i left it like that for the longest time since it works, though it had a problem where the throttle would stick open slightly with the pedal released, causing a high engine idle, which if i tugged the trottle pedal up to close the throttle properly, would go back to normal.

 

Initially I didn't realise that it was the cable itself resisting movement, so I did a full rebuild of the carb, which didn't fix the problem (since the problem was the cable itself).

 

Now I've finally gotten round to replacing the cable with a proper mini one, but I still can't fix the high idle.

 

The problem is, even with the idler screw turned all the way out, and the throttle fully closed, the car idles at around 1400rpm, going up to around 1800rpm once the engine is fully hot. I've checked that the butterfly is fully closed by sticking a finger into the throttle body to check for gaps and I can't feel any. lifting up the carb piston slightly causes the engine rpm to go down a fair bit, suggesting a rich mixture, but I've tried playing around with the mixture to no avail. I also can't hear any air leak like sounds, but for what it's worth, I have a breather hose set up going from the tappet cover to the carb.

 

Could I have fitted the butterfly the wrong way round when rebuilding the carb? possibly, but I checked that when rebuilding the carb, and again i stuck my finger into the throttle body and can feel no gaps. My butterfly is the kind with a small hole in it.

 

Oh yes, I'm using a single HS4 carb with a waxstat. Engine is a standard 998 A+.

Edited by BaronVonchesto, Today, 10:04 AM.

[ADRay]

I have the same setup, and I’d say the next adjustment point is the jet nut.

also, how is the state of the butterfly bushings? I didn’t bother with redoing them when I rebuilt my carb and found it was a pretty big air leak point, and had to them after all.

Edited by ADRay, Today, 10:32 AM.

[Rubbershorts]

Two things I've encountered recently. 1) is the piston dropping fully when you release the throttle (take your foot off the gas), and 2) have you pulled the new cable too tight when you fitted it? It needed a tiny bit of slack on mine to allow the throttle to return fully in all circumstances. Daz.

[BaronVonchesto]

I have the same setup, and I’d say the next adjustment point is the jet nut.

also, how is the state of the butterfly bushings? I didn’t bother with redoing them when I rebuilt my carb and found it was a pretty big air leak point, and had to them after all.

 

The jet nut sets the fuel mixture, but theres no way a rich mixture on its own can have the engine rpm that high at idle unless extra air is getting in somewhere. i could try adjusting the mixture to the point its lean and then see what happens i suppose?

The butterfly spindle had some dark lines on them where they meet the bushing. any suggestion on how to check this? or just go ahead and replace them? Though I did a full carb rebuild with a CRK121 about 5 or 6 years ago, and the car hasnt had that many miles of use since. Its mostly a garage princess nowadays 

 

 

Two things I've encountered recently. 1) is the piston dropping fully when you release the throttle (take your foot off the gas), and 2) have you pulled the new cable too tight when you fitted it? It needed a tiny bit of slack on mine to allow the throttle to return fully in all circumstances. Daz.

The throttle actuator arm is in all the way. Its not the cable tension i physically cannot move the throttle arm closed any further. I will try disconnecting the cable just in case, but i suspect it will be the same.

 

As for the piston, the engine idles that way even from start so im not sure the piston would have gone up at all.

Edited by BaronVonchesto, Today, 12:02 PM.

[Ethel]

I'd be looking for an air leak, breather hose, throttle spindle....

 

What's the plug colour like after a run?

[Rubbershorts]

 

I have the same setup, and I’d say the next adjustment point is the jet nut.

also, how is the state of the butterfly bushings? I didn’t bother with redoing them when I rebuilt my carb and found it was a pretty big air leak point, and had to them after all.

 

The jet nut sets the fuel mixture, but theres no way a rich mixture on its own can have the engine rpm that high at idle unless extra air is getting in somewhere. i could try adjusting the mixture to the point its lean and then see what happens i suppose?

The butterfly spindle had some dark lines on them where they meet the bushing. any suggestion on how to check this? or just go ahead and replace them? Though I did a full carb rebuild with a CRK121 about 5 or 6 years ago, and the car hasnt had that many miles of use since. Its mostly a garage princess nowadays 

 

 

Two things I've encountered recently. 1) is the piston dropping fully when you release the throttle (take your foot off the gas), and 2) have you pulled the new cable too tight when you fitted it? It needed a tiny bit of slack on mine to allow the throttle to return fully in all circumstances. Daz.

The throttle actuator arm is in all the way. Its not the cable tension i physically cannot move the throttle arm closed any further. I will try disconnecting the cable just in case, but i suspect it will be the same.

 

As for the piston, the engine idles that way even from start so im not sure the piston would have gone up at all.

 

How about taking the carb apart and having a proper look at the butterfly. Even if it's to rule it out. I'm not sure reaching in with your finger will tell you as much as shining a light through the carb and assessing if there's a sizable gap or not.  Daz. 

[ADRay]

any suggestion on how to check this?

While it’s idling, carefully spray a little carburetor cleaner near the butterfly bushings. If the fluid gets sucked in, you have an air leak.

Edited by ADRay, Today, 01:39 PM.

[Rubbershorts]

 

I have the same setup, and I’d say the next adjustment point is the jet nut.

also, how is the state of the butterfly bushings? I didn’t bother with redoing them when I rebuilt my carb and found it was a pretty big air leak point, and had to them after all.

 

The jet nut sets the fuel mixture, but theres no way a rich mixture on its own can have the engine rpm that high at idle unless extra air is getting in somewhere. i could try adjusting the mixture to the point its lean and then see what happens i suppose?

The butterfly spindle had some dark lines on them where they meet the bushing. any suggestion on how to check this? or just go ahead and replace them? Though I did a full carb rebuild with a CRK121 about 5 or 6 years ago, and the car hasnt had that many miles of use since. Its mostly a garage princess nowadays 

 

 

Two things I've encountered recently. 1) is the piston dropping fully when you release the throttle (take your foot off the gas), and 2) have you pulled the new cable too tight when you fitted it? It needed a tiny bit of slack on mine to allow the throttle to return fully in all circumstances. Daz.

The throttle actuator arm is in all the way. Its not the cable tension i physically cannot move the throttle arm closed any further. I will try disconnecting the cable just in case, but i suspect it will be the same.

 

As for the piston, the engine idles that way even from start so im not sure the piston would have gone up at all.

 

Have you tested the drop of the piston? A bit of crud in there may mean it never returns to the bottom anyway?

[sonscar]

The edges of the butterfly are tapered and it can be incorrectly fitted.To rev up excessively needs air,it is coming from somewhere.Steve..

[stuart bowes]

if it is the spindle you might be able to mitigate leaks a little with a spindle that has O rings fitted

 

https://www.ebay.co....ical_ms=9046048

(others available may be cheaper..)

 

it's probably not the ideal solution and depends how bad they are (or are not?) but short of rebushing and getting reamed to size, it might help

Edited by stuart bowes, Today, 03:40 PM.

[BaronVonchesto]

cool! Thanks for the suggestions everyone. Guess I'll be taking apart the carburettor the next time im in Kuala Lumpur again... but first as Ethel and AdRay suggest, I'll take a look at the plugs and spray some carb cleaner at the spindle bushings

[BaronVonchesto]

The edges of the butterfly are tapered and it can be incorrectly fitted.To rev up excessively needs air,it is coming from somewhere.Steve..

 

yup I thought of this, and when refitting the carb i made sure to have the tapered end face in the opposite direction that the butterfly rotates, so as to form a good seal against the wall of the carb. Pretty sure its obvious when its facing the wrong way. but i'll check again just the same

[Spider]

I'd say the Throttle Disc isn't 100% centred. You won't be able to feel for it, during a build, I hold them up to the light to check them. It only takes the smallest of gaps. If it's been driven for a while like this, the throttle disc can 'eat' in to the body of the carb around the throttle disc, so check for that when you have it off.

If you suspect it's a worn spindle, there's a few ways to check for that. Spray some carb cleaner (or similar) around the ends of the spindle and see if the speed picks up further, of put some light grease around the ends to see if t slows it down. It would need to be pretty bad though to give a continuous high idle, they usually cause an erratic idle.