Engine drinking way too much fuel for my liking

[Solent Bombard]

From experience from cleaning motorbike carbs best thing to do is strip them off and take to a carb cleaner who will do a hypersonic clean on them as this gets every last bit of crap out and has the carbs looking like new.

Depending on the jets size a air line will only block them further

[Hambleite]

That does 'seem' a bit high. I'd be interested to know what you thought the consumption might be?

By way of a comparison, using the tried and tested 'man math' methodology:

The 6.5m Scorpion, with a Mariner 150hp Optimax burns around 11-13 litres on a round trip from Hamble to Cowes, cruising at about 28-30 knots. I reckon (using man math) that Yarmouth is about twice as far.

So, that would consume around 22-26 litres. Now, the Optimax burns about the same amount of fuel as a 90hp Mariner on carbs did on our previous Avon 5.4m searider.

Some say that a carbed two-stroke will burn around twice as much a Optimax / Etec / 4 stroke. I'm not sure that the difference is that great but it's probably not far off.

So, using all the above info, I reckon that when you're sure your engine is running as fine as it should, the expected fuel burn would be around 35 litres.

[uncle al]

In the old days, (someone swing the light) before the four strokes OBMs came on the market, we used as a rule of thumb, for any OBM over 50hp the fuel consumption was about one third of the hp in litres per hour.

[nugent]

Lol ime lucky to get 2 ltrs per mile on my latest rig

But run many 150 carb mercury motors getting 70 mile on a 90 ltr tank
So most defo some thing a miss

[Will75]

What speed are you getting Nugent with the PAC....our A22 with twin ETEC 150s does about 45kts but is a bit chiney above that. Re fuel we get about 1.7l per nm at 32kts

[nugent]

Originally Posted by Will75

What speed are you getting Nugent with the PAC....our A22 with twin ETEC 150s does about 45kts but is a bit chiney above that. Re fuel we get about 1.7l per nm at 32kts

Witnessed on gps at 63 mph few weeks back Rocknrolla

But on return journey around Anglesey one up very low on fuel hit the magic 58 knots and sat stable as any thing


Not meaning to hijack the thread but Vidio attached

[nugent]

Originally Posted by Will75

What speed are you getting Nugent with the PAC....our A22 with twin ETEC 150s does about 45kts but is a bit chiney above that. Re fuel we get about 1.7l per nm at 32kts

Witnessed on gps at 63 mph few weeks back Rocknrolla

But on return journey around Anglesey one up very low on fuel hit the magic 58 knots and sat stable as any thing


Not meaning to hijack the thread but Vidio attached


My Atlantic 21 chine walked also at first lifted engine up another inch and a half cured it. There really twitchy if weights not right

[spittle123]

hi have you thought about the prop that can make it drink fuel if its set up for ski I changed mine once from a 21 prop to a 19 and when I whent on cruis it drank twice as much fuel just thought I would say hope this helps

[Solent Bombard]

Originally Posted by spittle123

hi have you thought about the prop that can make it drink fuel if its set up for ski I changed mine once from a 21 prop to a 19 and when I whent on cruis it drank twice as much fuel just thought I would say hope this helps

Hi spittle im still waiting for very calm weather so I can get the boat at WOT so I can see the revs and try and work out if my prop is efficient or not.

Also I thinking about getting a 4 bladed prop as well as I have a lot of traits with the engine in that it ventilates a lot on turns and harbour/river speeds of 6 knots we are sat on the plane bubble so pushing a lot of water and drinking the fuel

[Chris]

Engine looks too low on the transom to me. Anti vent plate is buried...

[Jon Tallis]

2L/nm doesn't seem massively over what you might expect if you're driving it hard but still amounts to scary fuel bills. For ref I've now got a 150HPDI which does around 1L/nm even driven hard but I recon my old carb'd mariner 75 drank twice that.

I'd still be checking everything out - worth it given the cost of fuel. As posts above, do check trim / prop / engine height etc, but I'd definitely be checking the fuel system over. Dropping the float bowls off might uncover a significant problem (holed / stuck float, missing main jet), so definitely worth doing, but won't uncover more subtle tuning issues. Checking your plugs will also tell you if you've got a cylinder running 'off' - try to do this as soon as pos after a flat out blast - lots of idling will tend to oil the plugs and cover the problem.

Don't however try to tune it yourself unless you know exactly what you're doing and have the kit to balance a 6-carb engine - you'll almost certainly make it worse. Definitely don't lean-off the mixture to try to save fuel - unless you get it right you'll end up with holed pistons.

Ignition timing will also affect fuel consumption and is easy to check if you've got access to a strobe light.

[jim99bob]

I went from Cowes to Weymouth on Wednesday back yesterday. Used £150 each way...135 Optimax, 5.85 Vipermax. Couple of mates did the same trip as well 135 Optimax and 6m Avon, £90 each way..
Jim

[Chris]

Originally Posted by jim99bob

I went from Cowes to Weymouth on Wednesday back yesterday. Used £150 each way...135 Optimax, 5.85 Vipermax. Couple of mates did the same trip as well 135 Optimax and 6m Avon, £90 each way..
Jim

Cowes to Weymouth is about 55Nautical Miles .... so thats about 2 litres per NM which is really bad.

Our 7 Meter Vipermax with a 250 on the back would do that trip and use half the fuel....

[paddlers]

There seems to be a direct link between helming style and fuel consumption with the bigger engines.
If I have the family in my 135 V6 Mariner drinks roughly half the fuel in spite of the extra weight that it does when it's just me

If it's flat calm you're going to go a lot further/faster than when it's rough so this too has a direct link.

[cptsideways]

What speed does your boat start planing at?

How far did you go at sub planing speeds as this can make a huge difference if you came out of the harbour off the plane for some distance.


Have these got six carbs? as I've done triples many times over, they are easy to sort out & balance, buy yourself the kit to do it properly & its easy enough diy if you read up.

[Solent Bombard]

Well after Friday evening the fuel issue is no longer a problem with this engine.

5 mins into a trip on Friday evening the No6 piston let go started smashing into the cylinder head and has given the cylinder block a good going over as well.

[A1an]

Ouch!

[nugent]

Originally Posted by Solent Bombard

Well after Friday evening the fuel issue is no longer a problem with this engine.

5 mins into a trip on Friday evening the No6 piston let go started smashing into the cylinder head and has given the cylinder block a good going over as well.

Bloody oil pumps. What os your plans now with this engine

[nugent]

Sorry was on phone

Are you going to rebuild it ?

Got lots of bits for these lumps dependant on year

[Solent Bombard]

Well it's a 1999 v6 carb model. (Pre optimax)

And looking at the damage I would need

Cylinder block I guess with heads (if it's the same as motorbikes there mated so need both)

Pistons (might as well change them all)

And I would expect Conrod minimum maybe even whole crank depending on the hammering it's took.

And other stuff I guess but have not took too much apart yet just took the head off to asses damage.

Bloody pissed me off because the last couple of months I have sunk over £1500 into that engine.

Gearbox refurb, New water pump parts, updated voltage regulators etc etc including a service as well.

Bloody typical.