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Old 12-05-2004, 19:47   #1 (permalink)
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Losing power and misfiring

OK here's the story.
My clevo is backfiring through the carby under load and has no power.
The engine is in reasonable nick and the heads were in good shape last time they were pulled 18 months ago.
I had the motor out of the ute for a respray and while it was out replaced the valve stem oil seals using a piece of nylon rope and an overhead valve compressor. I also fitted new spark plugs. Drove the ute into Canberra (60Km ) to have some paint work finished off with no dramas.
When I picked up the ute it had a fuel blockage which I fixed and drove the ute home. I noticed a loss of power and a tendancy to backfire through the carby under load and put it down to some crap in the carby and possibly a blocked power valve.
On the weekend I rebuilt the carby and have no improvement.
It starts and runs ok at idle, however if you rev the engine it backfires through the passengers side barrel of the carby. I have removed plug leads etc while running and it would seem that No 3 cylinder may not be firing consistantly although it is difficult to tell with 100% certainty. I have checked that all leads are on correctly. The timimg has not moved as the dissy is siezed in the block (electronic one so no points involved)
My question is how would you diagnose this further, and have I got a failed valve. I'm open to any suggetsions to try prior to pulling the heads off to have a proper look. Will a compression test help to troubleshoot further?

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Old 12-05-2004, 20:15   #2 (permalink)
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Re: Losing power and misfiring

You could have a dud lifter. Replace the plugs and see what happens. If the plugs solve the problems it's time for a tune. Yes, a compression test will help diagnosis.
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Old 12-05-2004, 21:34   #3 (permalink)
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Re: Losing power and misfiring

More info please.

What carb?

What engine size?

Camshaft?

The panel shop didn't do anything?

What do the plugs look like?

Distributor Cap Condition?

Distributor Condition?

Any thing you've changed?

What vacuum have you got?

What power valve did you put in? What was the old one?

Is it Lean?

What is the Timing?

Accelorator Pump working?

Blocked fuel filter?

Help us out some more
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Old 12-06-2004, 01:10   #4 (permalink)
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Re: Losing power and misfiring

Quote:
Originally Posted by TTNOS8
More info please. Answers below

What carb? Holley 600DP

What engine size? 351

Camshaft? Mild hydraulic

The panel shop didn't do anything? Not to my knowledge

What do the plugs look like? Black and sooty

Distributor Cap Condition? Ok - no corrosion and clean

Distributor Condition? OK

Any thing you've changed? Carby, swapped plugs around, valve stem seals as mentioned

What vacuum have you got? Havent got a meter but sufficient to easily operate vacuum advance

What power valve did you put in? What was the old one? new one supplied with kit, 65 I think

Is it Lean? not at idle

What is the Timing? Timing light died so I can't check at the moment but I would be reasonably sure it hasnt moved as mentioned above

Accelorator Pump working? Yes

Blocked fuel filter? new, plenty of fuel delivered to carb

Help us out some more
Hope this gives some more info, as I said I havent checked the timing but with the vacuum advance disconnected symptoms are similar, mechanical advance mechanism seems to be free but it's a bit hard to tell without a light.
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Old 12-06-2004, 02:37   #5 (permalink)
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Re: Losing power and misfiring

Logic says it can only be the carby. And the problems followed a fuel blockage. What do you mean by this? Was some muck drawn through the carby?

If you have rebuilt it and the same problem persists, it can only be something you missed in the rebuild. Did you use a compressed air to blow through all the passages in the metering and main bodies?

So rather than strip down the Holley again, first try a blast of compressed air through the fuel inlet, the air bleeds on the top of the air-horn and take of the fuel bowls and through the metering plate orifices.
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Old 12-06-2004, 04:30   #6 (permalink)
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Re: Losing power and misfiring

have you checked the Valve timing by any chance, do a compression test that will ensure mechanically should be ok, then check the carb etc. I had problems with my clevo once, it was gutless and kept spitting up the carb and i did everything but ruled out the valve timing and thats what it happened to be, just a thought :)
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Old 12-06-2004, 21:37   #7 (permalink)
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Re: Losing power and misfiring

Quote:
Originally Posted by STROKEXD
Logic says it can only be the carby. And the problems followed a fuel blockage. What do you mean by this? Was some muck drawn through the carby?

If you have rebuilt it and the same problem persists, it can only be something you missed in the rebuild. Did you use a compressed air to blow through all the passages in the metering and main bodies?

So rather than strip down the Holley again, first try a blast of compressed air through the fuel inlet, the air bleeds on the top of the air-horn and take of the fuel bowls and through the metering plate orifices.
Yep, carby was cleaned with carby cleaner and blown out with compressed air. Fuel blockage was in one of the tank pickups or the changeover switch, (dual tanks) havent had time to check yet but there is a fuel filter between there and the carb so I'm thinking the blocked fuel line may have been coincedence.
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Old 12-07-2004, 02:40   #8 (permalink)
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Re: Losing power and misfiring

Hmmm this armchair expert reckons your coil may be the problem. Check if it's getting really hot or swap it for another one and see if that helps.

It's definitely fuel or spark anyway lol
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Old 12-09-2004, 03:30   #9 (permalink)
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Re: Losing power and misfiring

Update...
My pissedoffness subsided enough to let me go out and have a look at the ute tonight without throwing matches at it.
It's not the coil, I replaced it with one that worked last time I used it and have exactly the same symptoms.
Same with the coil lead.
I'm off to buy some new spark plugs to replace the 100Km old NGK ones and see if that makes any difference.
I pulled the drivers side tappet cover and all the rockers seem to be doing their thing, cleaned the rotor and distributor cap, no difference. If the plugs don't fix it I'll buy a compression tester and go from there.
Bloody clevos, maybe a set of those CHI heads would fix my problem.......
Thanks for the suggestions,
Regards,
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Old 12-09-2004, 13:00   #10 (permalink)
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Re: Losing power and misfiring

Sounds like a weak spark to me Tote.
Make sure you have a good earth, straight from the battery to the block.
And I'd still be changing the coil. If it was wired incorrectly or you were supplied with the wrong one it would not last very long.

Good luck, it's probably something simple and put those matches away!
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