Engine revs won't remain constant, any advice please?

[Quidi Vidi]

Finally afloat after a prolonged refit which involved a top end service amongst other things and ran the engine yesterday (Petter AC1W) it kept losing revs and then picking up again. Father in law pumped the bulb in the fuel line and the revs increased going even faster than before they had dropped, before settling down for awhile and then dropping again.
Do you think this could be an air leak in the fuel line? I can't see any leaks at any of the connections in the line when the bulb is used to pressurise the system except for some drips from the glass bowl on the water separator. It appears to come from where the glass bowl meets the metal top where a flat rubber ring is fitted, I've tightened it up as tight as it will go but it still drips under pressure. As this is on the low pressure side and I can't see any air bubbles enter the bowl, I'm thinking that this is not the problem, is it worth me changing it for a new one anyway (it is probably 25 years old)
I'm thinking of renewing the fuel lines and clips anyway but would welcome your advice first to save unnecessary work.

 

[boatmike]

It sounds as if you have a CAV filter with a glass bowl. You say you have tightened it as much as you can. Frankly that is often the problem. They should never be over tightened as distortion is caused that ensures the bowl never seats properly again. If in good order they only need nipping up a quarter turn which should stop all leakage. Frankly there are several other makes that are better but if you replace with another CAV (because they are cheap) don't over tighten it!

 

[Quidi Vidi]

It sounds as if you have a CAV filter with a glass bowl. You say you have tightened it as much as you can. Frankly that is often the problem. They should never be over tightened as distortion is caused that ensures the bowl never seats properly again. If in good order they only need nipping up a quarter turn which should stop all leakage. Frankly there are several other makes that are better but if you replace with another CAV (because they are cheap) don't over tighten it!

Thanks for the response, I'm not sure it's a CAV filter as its just a simple glass bowl/water separator. I think there is a flat piece of gauze in the housing, the engine has a separate fine fuel filter in its own housing. The glass bowl is held on by a knurled nut underneath which I have only tightened by hand.
Do you think the symptom point towards air getting into the fuel line?

 

[sailorman]

Thanks for the response, I'm not sure it's a CAV filter as its just a simple glass bowl/water separator. I think there is a flat piece of gauze in the housing, the engine has a separate fine fuel filter in its own housing. The glass bowl is held on by a knurled nut underneath which I have only tightened by hand.
Do you think the symptom point towards air getting into the fuel line ?[/ QUOTE]

yes

 

[Quidi Vidi]

Thanks Sailorman, could anyone point me towards where I should start looking for it, ie. high pressure side, low pressure side or should I be checking the lot? I'm about to order some hose and hose clips, should I be ordering a water separator too?

 

[cryan]

Since you have just had a top end overhaul have you bled the fuel lines all the way to injectors at every joint, plug and coupling.

 

[Quidi Vidi]

Since you have just had a top end overhaul have you bled the fuel lines all the way to injectors at every joint, plug and coupling.

Yes we did but perhaps we should do it again, thanks for the pointer. By the way it's the reason I fitted the bulb inline, to make it easier to bleed, I would recommend this to anyone who hasn't fitted one yet.

 

[PCUK]

If I understand your system correctly the air leak in is where the fuel is coming out. This is on the suction side and therefore draws air in. Also is the bulb an outboard primer? I'd think that was suspect as well if you've only just fitted it.

 

[Quidi Vidi]

Thanks for all your replies, it looks like the air leak is the most likely cause. I've just ordered a CAV 296 filter/ water separator unit to replace my old faulty one. I've also ordered some fuel hose and clips so hopefully that should sort it out.

 

[mtb]

you might try popping the large rubber ring from the aglomerator into hot water to soften it. You should not need a bulb, the lift pump on full stroke is more than capable of drawing fuel through, be sure you check the lift pump gauze is clear and the large rubber ring is sealed ok
The bulb will only cause problems well in my opinion that is. Don't forget to check the copper washers on the injection pump if you've done so yet aneal them and see how it goes
Did you use blue hylomar around the head gasket water and oil ways ? modern ones seem to fail I found the ones from uxbridge boat centre were the best available, the **** ones from the dodgy people in York will fail which happened to me twice so I'll never buy their stuff again !

cheers
mick

 

[boatmike]

So do I!

 

[DaveS]

The bulb will only cause problems well in my opinion that is.
mick

Well, like the OP, I fitted a priming bulb to greatly simplify bleeding and getting the fuel up to the primary filter and I find it extremely useful. It also provides a "get you home" fix if the lift pump fails and, as described here, has allowed a suction side air leak to be pin pointed by pressurising the system and watching for diesel. Without the bulb the only solution would have been to undo and remake every joint. What's not to like about having a bulb?

 

[Daydream believer]

I agree fully about the bulb in the system. Bit get a diesel friendly one as not all are.
With the CAV filter i had problems & someone on this forum suggested i had the wrong filter fitted
I disregarded this until after much hand wringing i removed the filter & used a micrometer to measure the seating diameter
I went to Does of Ulting where we got three makes of filter on the counter
Petters, new holland & davidsons
The petters one was 0.8mm larger diameter. Than the others
This means that when tightening the filter compresses the ring & does not force it out of the seating as the smaller ones do
Air leak solved