Honda 15hp 4-Stroke Engine Help

[Captain Beaky]

Hi,

I am after some help with a fault on a Honda 15hp 4stroke. It’s a late 90's model in almost new condition, long-shaft with a pull start.

The best way I can describe the fault is that the engine sounds like it’s misfiring and goes into a slow knocking rhythm either on start up or after a short distance running time.

This fault started to happen prior to the last service and got progressively worse as time went on. The fault seemed to disappear for a while after the service, but has now started up again.

It generally occurs when decelerating down and not necessarily when the engine is first run up. When the engine is accelerated with the fault present, the ‘knocking’ becomes more exaggerated and engine revs are non-existent. (just a heavy knocking)

At present, the fault clears after leaving for 5mins or so in neutral then fires into life ticking over nicely. Before last years service it would take a lot longer to clear – but would still clear eventually and not necessarily happen every time.

Just after any help/ideas/possible causes so I can start to eliminate one by one, as on inspection during the service no fault could be found and the carbs were apparently very clean.

Many Thanks

Dave

[michael1923]

Have you checked gearbox oil. Could be that, when warm it thins.

Maybe low

[michael1923]

One other thing, have you checked the timing could be as simple as it pinking, i.e timing incorrect too far advanced. mind you i dont think that would settle down.

[monkeynuts]

without hearing it it would be hard to say ,
but does it do it when accelerateing in gear?
is it a defo metalic knock or could you say it was a banging/back firing noise???

[Henry]

Apr 14, 2008 18:22:18 GMT monkeynuts said:

without hearing it it would be hard to say ,
but does it do it when accelerating in gear?
is it a defo metalic knock or could you say it was a banging/back firing noise???

Put it in to gear when its doing it and she cuts out....you literally can't give it any revs at all.

It ticks over with a rhythmical jolt, jolt jolt (quiet at tick over) give it any revs and it is more pronounced with a thud thud thud (without being able to rev it ) if you know what I mean.

Just sounds like its a dull repeated thud with the engine kicking each time.

Then all of sudden it will just run perfectly and after a good handful of revs the problem goes away.

It used to happen a lot from cold or when you throttle down from a short trip. I think it seems to happen more when its wet..

[monkeynuts]

with out seeing it its hard

but if it happens when its wet, its normaly HT leads breaking down or a lot of damp getting in on the engine electrics,
next time you go to fire it up try coating the leads, coils and plugs in WD40 first and see if it starts any better
also its a long shot but does it have a auto choke???

[Henry]

Apr 14, 2008 19:39:02 GMT monkeynuts said:

with out seeing it its hard

but if it happens when its wet, its normaly HT leads breaking down or a lot of damp getting in on the engine electrics,
next time you go to fire it up try coating the leads, coils and plugs in WD40 first and see if it starts any better
also its a long shot but does it have a auto choke???

No manual choke...

What would you expect to happen if you had water in the fuel (this shouldn't be the case but just looking at every option)..

[monkeynuts]

if it would start with water in fuel at all, a very bad misfire all the time and soon the carb would gam up

[Dave.S]

Can I suggest to go back to basics

All petrol engines require three things;

1 Fuel/Air (mixture)
2 Spark
3 Compression

Start with the easy ones first
Fresh fuel and getting to the carb ok

Are the plugs black/sooty/wet?
If so fuel/mixture too rich - Should be a whitish gey

Are you getting a good spark at the plug(s)?
Check timming against marks on the flywheel

Is it "very" easy to turn the engine over?
Carry out a compression check
Could be a head gasket failure/cracked head

At least this will hopefuly get you looking in the right area, good luck.

Dave