993 carb slow running

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993 carb slow running

Post by Reid on Wed Dec 14, 2011 8:13 pm

I am still at my wits end to figure this out. After an engine rebuild there was a big hole/ hiccup in initial acceleration - the rest of the range fine. The idling was surging and stalling.
Removed carb, dismantled and cleaned (It didn't really need it!).
Put back on engine - just the same.
I checked the slow run solenoid jet which does work. The carb ran perfectly before the engine rebuild . . .


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Re: 993 carb slow running

Post by rich the mechanic on Wed Dec 14, 2011 8:35 pm

what sort of fuel pump is fitted on you model?
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Re: 993 carb slow running

Post by Reid on Thu Dec 15, 2011 1:13 am

It's in the tank. I have not had it out but I'm sure that's where it is.

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Re: 993 carb slow running

Post by rich the mechanic on Thu Dec 15, 2011 7:56 pm

Have you checked the fuel filter and or pressure? I had my car play up and actually stopped and when I checked the carb for fuel it was bone dry but the filter was full (clear one so I could see) and when I took the fuel pipe off no fuel came out and when removed I had to tip the fuel out the wrong way but it looked clean.
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Re: 993 carb slow running

Post by Reid on Thu Dec 15, 2011 9:15 pm

Thankyou Rich but the flow of petrol to the carb is fine. The medium throttle at higher revs works well and the accelerator jet squirts out well. Indeed at low revs when the slow run jet is required the throttle can be floored to make the carb work - or I can pull out the choke to make it pick up.
Its just not working at gradual pick up from low low revs!

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Re: 993 carb slow running

Post by rich the mechanic on Thu Dec 15, 2011 9:36 pm

Right was the carb rebuilt by someone else or just you if so did they/you check the float height? the accelorator pump picks up at the base of most carbs.

If the fuel height is to low you can get poor pick up and rough running but at higher revs there is more vacuum so will drag the fuel up easier much like when running out of fuel you can rev it but not get enough power to drive normally.

Alternatively fuel to high can flood at lower revs causing rough running but at higher revs atomise better and not be as noticeable and if the timing was done in this state or the other may explain the better running at higher revs.

Is the dizzy at the original location if not put it back and run (low revs off choke) if you can and then check the plug colour for the mix if grey/white to lean, black and sooty to rich it should be a light milk chocolate colour then if driveable at high revs then go round the block and kill the ignition and neutral to a stop and when cool enough to get your hands in recheck the colour.
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Re: 993 carb slow running

Post by Reid on Thu Dec 15, 2011 10:10 pm

Yes Rich I rebuilt the carb. I shall take it off again and check the float height. It did run correctly before it was removed to rebuild the engine. I was wondering whether one of those little tubes or their filters sprouting out from everywhere could have developed a fault.
The dizzy was set back to the mark on the flywheel by timing light through the view hole. I shall try your suggestion.
Thankyou, Andy.

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Re: 993 carb slow running

Post by rich the mechanic on Thu Dec 15, 2011 10:25 pm

Float height makes a huge difference some are adjustable some arn't check the manuel for spec but if it has a one piece float with a clip on valve then look for wear/damage and check for leaks in the float same applies if it has a metal tang but then it is adjustable and easily bent at college we were shown how sticking full airline pressure done the fuel hose could bend the metal float, the book shoud show you where to measure and how I don't know what the carb for a hijet looks like but guess it will be similar to a bike one based on space under the seats.
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Re: 993 carb slow running

Post by Reid on Thu Dec 15, 2011 10:34 pm

The only problem I've just realised Rich is that details for a carb 993 are the most impossible to find. I've got various torque lists and dismantling /rebuilding details for the similar 993 EFi but not carb pics or advice.
I shall dismantle and examine. Thanks.

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Re: 993 carb slow running

Post by rich the mechanic on Thu Dec 15, 2011 10:40 pm

PM me your email adress and I will send you a compressed copy of the relevant section of the manuel I just checked it shows float height measurement and lists assembly order.
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