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Flat 350 Mag

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puzzolente

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"Hello all, I started a thread

"Hello all, I started a thread a few weeks ago about the lack of power from my 91 350 mag, I'm really sorry to bother you again I'm really struggling!! I'm not mechanically gifted so I've got my mechanic helping me and now he's completely stuck! As a bit of background we've done the following:

Full Service
New plugs/leads/thunderbolt module/dizzy cap
Full outdrive overhaul (Alpha 1)
Stripped/overhauled carb. (rochester)

The engine runs perfectly, all temps/oil press. are great but as soon as it's under load it has no grunt at all, I can just about get 2800 RPM and at full throttle the revs just die off until I can only get 2000rpm, if i throttle off and open it up again, it will get about 2800rpm then it will die off again!! I've swapped props (19" and 20") and it's made no difference, the boat was laid up for 5 years with quite a bit of fuel, I've since put about 150 litres in it and I'm assuming the old stuff has now gone through as I've used it quite a lot! We've been out and altered the timing at sea advancing it as much as possible and it's made no difference, how sensitive are these engines to timing?? 2 Weeks ago I took the sundeck off (very heavy) just to be sure the weight of it wasn't causing problems and took it out, pulled 3750 rpm with the bigger prop but it was pinking so we retarded the ignition slightly, put the sundeck back on and now we're back to square one!
In the UK really good knowledge of these engines is very hard to come by (at a sensible cost!!) so forgive me once again but any advice would be hugely appreciated.

Regards

Mark"
 
"Mark,
try using a small fue

"Mark,
try using a small fuel tank (portable outboard with primb-bulb) connected to the carb. you will have to keep the carb full with the squeeze-bulb.

or of you dare, use a small squeeze bottle with fuel in it, when the rpms drop, squeeze a small amount of fuel down the carb and note the change.

either way you will need another person to help assist. you might keep a fire extinguisher handy also. Don't be careless with the fuel.
If the rpms gets better with the added fuel, or runs better on the small tank, you will know he problem is with the fuel system"
 
"Thanks AK I'll give that

"Thanks AK I'll give that a try, it really feels more than just bad fuel as there is no erratic running/misfire, out of gear she'll rev freely but I'm ready to look at all options so thanks for taking the time to reply"
 
"Good luck, also make sure yo

"Good luck, also make sure you have reset the timing,8* ATDC."
 
""boat was laid up for 5 y

""boat was laid up for 5 years with quite a bit of fuel...Stripped/overhauled carb. (rochester)...can just about get 2800 RPM and at full throttle the revs just die off until I can only get 2000rpm"

AK gave some good advice on the external tank.

When you rebuilt the carb, did you find brown sand looking debris in the bowl? That is granular varnish. The mechanical fuel pump is full of that crap from the 5 yr. old fuel. The only way to get rid of it is to rebuild or replace the fuel pump. The carb screen catches the heavy stuff but the fine grain varnish gets past it and clogs the metering passages of your rebuilt carb.

If you pull the pump, dismantle it and find the crud in it, you'll need to redo the carb. Make sure to blow out all the passages w/compressed air."
 
"thanks for the advice folks,

"thanks for the advice folks, Guy there was some sandy looking stuff in the bowl now you mention it but very little! Rod, the boat weighs about 2 and half tons dry! (Rinker 236) John, we've changed the fuel filter twice!"
 
"If you don't already have

"If you don't already have a fuel/water separator filter between the fuel tank and the pump, install one because it will catch all of the crud in the bottom of the tank. Change the fuel pump dome filter annually."
 
"Hello all,
Many t

"Hello all,
Many thanks for all of your advice on both my gear selection problem (cured!) and my lack of power (will be confirmed by sea trial at the weekend) we think it ended up being a combination of the timing being way out and the carb in desperate need of an overhaul although the previous mechanic told me he had done it!! Whilst it's been worked on it's been pointed out to me that there's quite a bit of crankcase back pressure and she's breathing quite heavy, oil consumption is fine etc. and bearing in mind it's 17 years old how concerned should I be? As I've said in the past, getting good advice in this part of the world for these engines is not easy!

Many thanks

Mark"
 
If you are getting blowby &#40

If you are getting blowby (crankcase backpressure)from an engine that sat for many years then you have a couple of options.

get some marvel mistery oil and pour it in each cylinder thru the spark plug holes and let sit for a couple of days. maybe you have a frozen/stuck ring.

After that fire it up and run it for a short time and then change oil and filter. Maybe 1/2 hour or so.
just put in some cheap 20 - 50 weight so as not to waste to much money while working the bugs out.

The age is not so much of an issue it is the fact it sat for so long.

Other than that you may need a rebiuld or just purchase a rebiult long block and swap all needed parts.
 
"Thanks Kghost, I'll start

"Thanks Kghost, I'll start that process tonight whilst the boat is on it's trailer! It wouldn't have anything to do with my lack of power would it? It starts and runs perfectly, good temp and hot holds 2.5bar oil pressure, did a compression test a few months ago and all was OK!"
 
"I put a pint of SEA FOAM in t

"I put a pint of SEA FOAM in the oil once a year. It can help remove varnish, sludge and carbon buildup around the rings."
 
"Hello all,
Just a q

"Hello all,
Just a quick note to say thanks for all your tips and advice particularly about having a fire extinguisher to hand when using the portable tank as we had a small fire in the bilge from the vapour! Boat all sorted now, pulling 5000 rpm (19" prop) and going great!

Thanks once again

Mark"
 
"Mark,

What was the problem

"Mark,

What was the problem and how did you solve it???

What was "sorted out".

Let everyone know......."
 
"Hi Kghost,
Ended u

"Hi Kghost,
Ended up being a bit of a combination of things! The timing was way out so that helped. The previous engineer told me that he'd rebuilt the carb......but he hadn't! So we got a kit, tried a portable tank and followed what Guy suggested! Upon stripping the carb it was very black inside one of the chokes and basically full of crap. It also had the wrong HT leads (for a pre electronic ignition model?!) As I said, good quality Merc. engineers are a rare breed in the UK, but I've found one now and learnt alot from you guys on the way!
As soon as I put it back in the water and fired it yesterday, it sounded COMPLETELY different and when we cleared the 4 knot river limit and opened the transom pipes, it went like hell!

Once again many thanks"
 
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"Mark:

The fire extinguishe

"Mark:

The fire extinguisher...we recently had someone ask why he needed a fire extinguisher at hand as instructed when pumping fuel out of his fuel tank. Your small bilge fire could have sent you to your final reward. Glad you are still here to enjoy that boat.
sun.gif
"
 
"Thanks Guy, needed a new set

"Thanks Guy, needed a new set of leads as they melted (they were the wrong ones anyway!) in fact, the contact was so poor on the plug end, we think that might where the igniting spark originated from! The bilge pump and a pipe from the exhaust changeover compressor also got cooked but it was all pretty minor compared to what could have been with 300 litres of fuel on board......lesson learned for me!!"
 
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