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January 22, 2018, 04:51:58 AM
 
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Author Topic: Engine races and revs really high on startup  (Read 436 times)
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Steady-On
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« on: August 11, 2016, 04:39:45 PM »

Hi all, I've recently overhauled my rascal and have had a new carb from Derek. I had the wee little thing running, after some head scratching, and it all sounded rather good (perhaps with a little more than the 'occasional plume of smoke' the Haynes mentions). So far so good, until I decide to adjust the idle speed in order to set the dynamic timing...

Where I am now is that the engine fires up nicely, with the choke out, but then after a second starts revving really high. At this point I shut the engine off because it scares me to death, and I'm loath to let this newly overhauled engine race from a cold start.

So far research suggests a vacuum leak as a likely culprit. I've done visual checks of all the hoses and so on but can't see any glaring problems. I also wonder if I've buggered something on the carb when adjusting the screws, but I don't think so.

Any suggestions? Am I missing something blindingly obvious?
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Llywelyn1984
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« Reply #1 on: August 12, 2016, 10:42:53 AM »

Pass.
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Dell Boy
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« Reply #2 on: August 12, 2016, 08:16:45 PM »

Sounds like a bad air leak somewhere or the choke is not opening when you push the knob in.

Derek.
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Dell Boy - The Rascal Specialist
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« Reply #3 on: August 12, 2016, 10:13:19 PM »

Choke appears to be working correctly. Checked compression in the chambers to set my mind at rest, and all seems good there.

Thinking I'll take your advice, Dell, and check further for air leaks.

Another thought: could there be issues with the distributor advance mechanism? Could dodgy timing cause this?
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Llywelyn1984
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« Reply #4 on: August 14, 2016, 06:45:11 PM »

Not that i know but id say stick with the obvious - ie a big air leak.
Remember this happening on my dad's focus and it was a split pipe - it would rev at junctions.
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Dell Boy
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« Reply #5 on: August 14, 2016, 10:45:22 PM »

Try spraying Easy Start around the engine. Where it gets sucked in & the engine speeds up is where your air leak is.

Derek.
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Dell Boy - The Rascal Specialist
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« Reply #6 on: August 15, 2016, 11:42:23 AM »

Not sure I want it to speed up much more! I attached a tachometer and it's going straight up to 3000rpm, at which point I switch it off.

It does look very much like an air leak though; I covered the air filter intake port and that slowed things right down. Now I just need to find the blasted thing. I've tightened all the manifold bolts and things I can find. Does the exhaust section affect this kind of thing?
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Steady-On
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« Reply #7 on: August 15, 2016, 03:13:54 PM »

Well now, after much fruitless searching for a vacuum leak, checking the timing, removing and refitting the carburettor, adjusting valves, tightening various bolts and so on, I decided to refit the accelerator cable at the carb. The little devil wasn't slack after all; it was tugging ever-so-gently, razzing up the engine every time I turned the key.

There's a lesson there somewhere. Anyway, now there's smoke pouring out of the exhaust, so I need to chase that down. Might I have buggered the piston rings, or valve seals or anything like that?
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Llywelyn1984
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« Reply #8 on: August 15, 2016, 03:45:14 PM »

Well done.

I was right to pass it seems!
ive been there...but with the choke cable not releasing.
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Dell Boy
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« Reply #9 on: August 15, 2016, 08:57:16 PM »

Could it be that your exhaust has oil/water residue in it after an engine failure. This is common & takes a few miles to burn out.

Derek.
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Dell Boy - The Rascal Specialist
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« Reply #10 on: August 15, 2016, 09:01:00 PM »

Well it's just been through a full rebuild, so hopefully this is just oil burning off or piston rings bedding in, or something like that. I need to get the brakes working properly then take it for a run, I think!

Thanks all for your help, I'm sure I'll have more questions soon enough.
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