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Erratic idle Top 80


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I have a Top 80 with a very erratic idle. It can be anywhere from 1700rpm to 2700rpm for a given idle setting. It also stops if idle is low when I angle it over to put on my back. Apart from this it runs well. It only revs to 9100rpm but I think this is more prop related.

I have just overhauled the carb with new diaphrams and bits (the machine is 5 years old but only 15 hours), and have played with the low mixture setting to no avail.

Any ideas.



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Thanks for replies.

I’m using the poor weather to give the motor a good overhaul.

Throttle cable routing looks ok, and it shuts off cleanly at the carb.

I’ve removed the carb again and put a thin layer of sealant on the manifold gasket.

I’ve changed the plug – I found that the current plug was put in without changing the gap (It was 0.75mm instead of 0.55mm), so may be significant.

I’ve put a new metering spring in the carb (as well as the full rebuild items earlier).

The fuel lines are being replaced with Tygon lines and the tank is being replaced with a new Kart type one (I damaged the old tank).

The arms have been replaced with the alloy ones.

I’d like to check the pop-off pressure but don’t have a gauge.

I’ll see how I get on after that lot.


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Thanks Colin. I'm at Colchester though so a bit of a trek from you.

I've picked up a gauge, tee piece and check valve and I'll put something together.

I've done all the work on the motor but not run it up yet. May have to wait until next weekend now.

I'm hoping that the large spark plug gap was also the reason I never got full rpm as well - probably wishfull thinking!


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No. It's a 8 litre Go-kart tank similar to the one originally fitted.

The only one I could get from PAP was the new 13l tank which was expensive, and I don't really need it to be that big.

I had to drill a hole on the back to insert a bolt - but no big deal.

I've run the motor now after all the work. It is better, but still the idle revs drop from 2500rpm to 1900rpm after blipping throttle, but it slowly picks up again. I think I'll leave it now, as long as it doesn't stall.


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