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Old 14-08-2006, 01:14 PM   #1
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Poorly mota

Hi All

Wondered if anyone might have any ideas why my motor is not performing as it should. Its a late 80's 200 hp v6 merc (carb). Just been serviced with new plugs and leads.

Compression check was fine on all cylinders but the motor is not running right. It ticks over fine in neutral and throttle response is very good also sounds fine when given plenty of gas in neutral.

In gear, however, throttle response is poor and it sounds rough when running at low revs / speeds (almost like its missing). Only reaching about 45 - 50 at WOT where as we are used to more like 55-60+ and it sounds like its not reving as high as it should (guage knackered so cant be precise).

Also boat stalled totally when running at flat out yesteday but then started up without any probs??.

I thought it could be fuel related and was gonna replace filter. We normally run 45:1 mix with quicksilver oil.

Any ideas or suggestions would be more than appreciated.

Cheers all - the doc

ps attached a pic of the poorly mota in question
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Old 14-08-2006, 03:05 PM   #2
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Have you checked for a spark on all six cylinders?

looks like a clip on fuel line from the picture above, try removing the quick connect and fitting the hose straight to the carbs.
Also check the breather on the fuel tank is clear or open if its a remote tank.

Do you have a primer bulb? if so get someone to pump it whilst you are going along to see if the problem is fuel related, might be the mechanical fuel pump diaphrams.
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Old 14-08-2006, 03:09 PM   #3
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Agree with cye comments. Have had occasions where an air leak in the fuel supply means the motor is sucking air and starving the top cylinders of fuel.
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Old 14-08-2006, 03:22 PM   #4
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Another reason for the above is water in the fuel ......... maybe!
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Old 14-08-2006, 03:26 PM   #5
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Agree with all above, If it was running OK before Plugs & Leads have a look there also test insulation of plug leads with a cheap check tool
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Old 15-08-2006, 08:34 AM   #6
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Hi Cyco, Cookee, Matt and Blufin

Thanks for the replies. The mota started playing up just b4 the service. That was why I insisted on new leads (+ plugs). I havnt checked sparks but assumed all cylinders were firing as it sounds spot on in neutral and thought that this would have been checked as part of service / compression check.

Can you check sparks in same way as car - ie remove plug with lead attached and hold against block??

There is a primer bulb on the fuel line so I will give that a try whilst running. I will also check lead for air leaks and check breather on tank.

Thanks again for all your advice

The doc
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Old 15-08-2006, 08:57 AM   #7
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I understood it was since the plugs & leads were changed. You check the spark the same way but I was thinking more of the leads breaking down and going to earth underload at the higher RPM. Here is a pic of the basic one tiz about 3 quid.
That said as it was before the plugs I'd start with the fuel system, been sitting with any petrol in it for a while?
A nice easy way to check plugs is to make up a plug bar with some older plus that you know are OK and mount them in a metal bar about 2" wide and 7" long fix a wire from the end to a decent spring clip. all you need to do then is connect the sping clip to the block eart point, leads to the plugs on the bar & crank, saves wasting time pulling the plugs
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Old 15-08-2006, 09:11 AM   #8
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A really quick crappy sketch of what I mean simple home build
For a V6 I'd use a longer strip & put 6 plugs in it . Plugs go where the black sploges are Go the fuel route first
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Old 15-08-2006, 09:55 AM   #9
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Blufin,

Where did you get your tester gizmo from? I've never seen one before and need to add it to my tool kit.
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Old 15-08-2006, 10:16 AM   #10
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Good simple cheap effective gizmo to check HT voltages over did the price they are £2.50. had my one years but you can get them From Here Bottom Of The Page
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Old 15-08-2006, 11:20 AM   #11
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Cheers, ordered one already!
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Old 15-08-2006, 11:36 AM   #12
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Hi Blufin

Cheers - I will start with the fuel then move on to the ignition circuit.

Fuel has been sitting in the tank (bow mounted 100 litre) for a good few months. Last time she ran dry was about three months ago. Since then she has been refilled and then topped up a couple of times. I will also check the breather. Would you suggest draining the tank

Also what about changing the fuel filter? could a dirty filter cause such a problem? The thing that I really dont understand is why it seems to run so sweet in neutral - I assume it draws a lot more fuel while in gear (and hence under load)

Cheers

The doc

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Old 15-08-2006, 11:37 AM   #13
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Another mota pic - really hoping to get this sorted by cowes powerboat weekend - hope to see lots of you there!
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Old 15-08-2006, 11:51 AM   #14
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Fuel will go "off" the time frame depends on the ambient conditions and how good it was to start with, filters, Lines, Jets, ect ect clean them out with something that will get rid of the muck/gel
I use Ambersil FE10 or ME20, stings like bugger on cuts & excellent at lighting BBQ's. But you need to practice that a bit
I get mine through work but you can get it from a place in Southampton (click here)
EDIT: They do a load of good stuff have a look round the web site, solvent, glues, putties & all the Devcon stuff but need to route round the site.
Yes no load & loaded conditions make a big difference
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Old 15-08-2006, 12:56 PM   #15
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fuel starts to deteriate after a 1000 hours thats about 41 days. and thats not from when you bought it either thats from when its made.

Jay Smith told me that
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Old 15-08-2006, 01:11 PM   #16
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Dont I know it 3 carb rebuilds
1/ chainsaw
2/ strimmer
3/ hedgecutter
coz I forgot to drain them last year
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Old 15-08-2006, 01:20 PM   #17
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thats not saying its unusabe after that point just it starts to lose its goodness. The main problem is when the fuel and oil mix and then it conjelles ( cant spell) and sticks to everything.
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Old 15-08-2006, 01:29 PM   #18
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Congeals but I cheated
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Old 15-08-2006, 02:30 PM   #19
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I had exactly the same problem with my Suzuki DT140, turned out to be the carbs and crap in them, had them cleaned properly and now it's running like a dream.
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Old 15-08-2006, 06:35 PM   #20
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Quote:
Originally posted by lasermatrix
I had exactly the same problem with my Suzuki DT140, turned out to be the carbs and crap in them, had them cleaned properly and now it's running like a dream.
I had the same problem with my suzuki 90dt after i left the fuel in it one winter so had to have them cleaned out too.
I remember a thread on here from a guy (cant remember who) but he said... unleaded fuel has a shelf life of only 2000 hours.
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