Page 1 of 1

MD7A decoke

Posted: Tue Apr 03, 2018 12:48 pm
by Gangesboy
Due to my engine chuffing exhaust out of the air cleaner,
I decided it was time to take the head off to see what was going on. I suspect a sticky valve, but don’t know till I disassemble it. The head has a lot of carbon on it, and the tips of the injectors look blocked and corroded, almost none existent, yet the engine was running.
So what to do? How do I get all the crud off easily, do I re-grind the valves or attempt to get new ones, and what do I do about the injectors, I think ASAP supplies do some bits for them. I’ve tried some local engine places but as soon as you mention boat they suck there teeth and say we don’t get involved with those! It’s just a cylinder head guys!!! Can any members suggest a plan of action :roll: please?

Re: MD7A decoke

Posted: Wed Apr 04, 2018 8:23 am
by BrightStar
I have just done the same job, I had the injectors serviced by Colchester Injection, a local specialist and the head levelled and the valves skimmed and ground in by a local machine shop which specialised in reconditioning engines. All the carbon was scrapped out of the manifolds and ports. I soaked the head and manifold in phosphoric acid to descale it. The exhaust bend was also scraped clean and descaled.
The water ports in the head and the cylinder block needed to be drilled out and then the water passages in the block were descaled with the phosphoric acid.
A new head, exhaust manifold (2 of them) and a rocker cover gasket and thermostat I got from ASAP
For an old seawater cooled engine I was very surprised at how little internal corrosion there was, after cleaning it looked almost new.
The cost - injectors serviced and reset - £50, head surfacing, including a small bead of weld to fill a slight groove and overhaul of valves - £75, gaskets / jointing etc £60, new thermostat £24

I changed the water inlet arrangement on the exhaust manifold by tapping the ports to take standard elbows and used a combination of threaded and plastic steel to make new connections to do away with those push fit pipes.

I had the alternator serviced with new bearings and brushes and at the same time had a tachometer signal feed fitted, so now have a tacho. I also changed the water temperature switch for a combined switch and sender so have a water temperature gauge, the same with the oil pressure switch, so now can see the oil pressure.

I have only test run on a hose into the water intake filled but started on 3 turn and once the grease and oil had cleared ran with a clear exhaust. I initially had some puffing at the intake manifold - the valve lifters needed adjustment.

Altogether a satisfactory job finished with a new paint job.

Where are you based, I would be pleased to show you mine and give you a hand.

Best of luck

Alan

Re: MD7A decoke

Posted: Wed Apr 04, 2018 11:35 am
by Gangesboy
Thanks Alan, you’ve been a busy boy, it sounds like a new engine! Unfortunately I’m in the West of Scotland so not very handy to take a look at yours. I know of a place in Ayr that does heads so I might nip in there and get them to take a look.
Of the top of your head do you know if both pistons move together as I believe some engines do work on this principle.
Thanks very much for your reply.
Paul.

Re: MD7A decoke

Posted: Wed Apr 04, 2018 7:22 pm
by BrightStar
your welcome

Another interesting point the 7A runs at 2700rpm and puts out 13.7 hp, the 7B runs at 3000 rpm and outputs 18hp, the only difference is the fuel pump setting

What boat do you have?

Re: MD7A decoke

Posted: Thu Apr 05, 2018 5:26 pm
by Gangesboy
Hi again, she’s a Griffon fin keel. By the way I was tinkering with the head yesterday trying to figure out how to remove the injectors, how do you get the three metal tabs off? Do you remove the appropriate stud? Thanks for that info regarding the rpm’s on the two engines, I wonder :roll:
Paul.

Re: MD7A decoke

Posted: Thu Apr 05, 2018 8:28 pm
by BrightStar
If you have the head off, then removed the injector nuts and then gently tap the injector tips, with a piece of wood to protect the tip. Once the injector is out, complete with collar, clean up around the collar, it slides one way and comes off.

If the head is still on remove both studs and then remove the injector complete with collar.

The collar should come off but I have found them always b....dy awkward, once they are off use a file to remove any burrs from the collars before refittng.

Be careful not to damage the injector bleed off nipple - if it is sheared off the injector body is scrap - now how do I know that !!

My MD7 is also in a Griffon - bilge keeler, which is ideal on the East coast

Alan

Re: MD7A decoke

Posted: Thu Apr 05, 2018 8:30 pm
by BrightStar
I didnt reply about the pistons, yes they both move at the same time but not necessarly in the same direction!!

Re: MD7A decoke

Posted: Fri Apr 06, 2018 8:13 am
by Gangesboy
Fantastic Alan, I’ll give that a go, I didn’t think it would be too difficult and I’ll remember the bleed off nipple!
Thanks for the info on the pistons too, busy weekend coming up me’thinks!!
Paul.

Re: MD7A decoke

Posted: Thu Jun 07, 2018 8:34 pm
by BrightStar
Having serviced the fuel pump and adjusted the govenor to allow the MD7A to run at 3,000 rpm, I tested today, at 2,700 rpm I was getting about 5.3knot, at 3,000 the speed went up to 6.5 knot, after an hour and half running at 3,000 there was no rise in temperatures or vibration - looks good.

Not that I will use the extra power normally but nice to know there are some extra horses in the stable, at the cost of fuel consumption