avon jetrib yamaha J700G 63N engine issues (surging, hunting, wont go WOT)

[vas]

hello all,

got this jetrib a few years back, took me one season to manage to get it up and down the bathing platform, last year it worked fine, but at the end of the season I got the behaviour shown in the video below.

Before the video I've started slowly rebuilding bit by bit, first was the Mikuni carbs with new full rebuilt kits (they were fine, well 20yo fine, so dirty and with aging rubber) and scrapping the chokes and replacing them with a easy start kit (effectively a pump spraying fuel via one new jet per carb before cranking)
Also fitted a new expensive fuel filter that you can open up and clean
Also bought a new exhaust as old one had developed a couple of holes due to corrosion (again 20y for an exhaust in seawater is more than enough apparently). Exhaust not fitted yet.
I feel that this is either overheating or something (video run was with old exhaust which meant slightly blocked) to do with fuel. I think electrics are fine (he says...)

will start and idle fine, work at 1/3 throttle fine, anything over it is a nono, funny behaviour starts at 10sec and then again close to the end (in the meantime I lift off as I wasn't happy)

Compression test shows 155 (IIRC) on both-small variation, so should be fine.
I wonder if I should play with the two screws on each carb (mind it did work at WOT during the season - scaring me sh1tless a few times - so shouldn't be it).

Cannot identify anything I changed since the beggining of the season and anyway only burnt a tank of fuel and did manage to run out of fuel before the end of the season - hm that could be a thing, easy to check!

next week's plan is to remove the engine all together (have it stripped already but stuck at the el.box as I need to remove the fuel tank and exhaust first !), remove cyl head, check waterways, clean, see general condition and refit hoping to find something.

any ideas on what could be the reason for this behaviour welcomed!

cheers

V.

 

[TK Blue MAX]

Hi Vas, possibly sounds more ignition related than fuel, not trying to teach! , but maybe plugs ,plug caps ,leads, coils, and lastly cdi unit, in that order.(cheap to shocking !!!!)
sorry i cant be more help.

blowing exhaust definitely wont help on a 2 stroke, if it does overheat, theirs a strong chance of nipping up( seizing)

 

[jrudge]

to me that moulds like an ingestion breakdown also. Does it have a condenser ?

 

[julians]

Hi Vas, possibly sounds more ignition related than fuel, not trying to teach! , but maybe plugs ,plug caps ,leads, coils, and lastly cdi unit, in that order.(cheap to shocking !!!!)
sorry i cant be more help.

blowing exhaust definitely wont help on a 2 stroke, if it does overheat, theirs a strong chance of nipping up( seizing)

Agree sounds like a misfire, which could be fuel related, but more likely to be cause by spark problems.

Change plugs,leads,coil etc.

 

[vas]

thanks guys my experience on 2T motors is fading, last time was a seagul we had in a 9ft bathtub my father was using for fishing 40yrs ago BTW, still have the seagul somewhere...

well AVON in their infinite wisdom have placed the el.box which has the starter relay, the el.ignition (well I think it does at least!), the coil (?) and few more electricy bits in a way that you have to remove the rear seat (OK), the fuel tank (ffs!) and mostlikely the exhaust silencer... before you can get hold of the screws that keep the lid on. I gave up when I was stripping the engine in situ ready to remove it, will go back (it's in a friend's garage) with better light and try to find a way around it, else, I'll rip the lot and then make sure I put it back in way I wont have to do all that again...

Plugs were new (like 2h new max!)
not sure there's a condenser there
thinking of it haven't bothered with the oil (has separate tank and oil pump thing that all say I should rip out and blank and use ready mixed fuel) wonder if that could cause some issues (unlikely I guess)
It doesn't feel hot tbh, water goes through and out of the pee hole alright. Dash is also fading so I'm having problems see oil/gasoline level
more when I get it all apart I guess!

cheers

V.

 

[julians]

Does it have a distributor? That could be failing or corroded so that it has too much resistance at high engine loadings.

I'd definitely be looking closely at the whole ignition side of the engine.

I've got a Caterham 7 that doesn't get much use, but I took it out for a spin recently and it started to misfire at high rpms when hot,it wouldn't rev past 6000rpm, it should go to 8000rpm. It was well overdue a service on the ignition side so I replaced plugs and leads, and this fixed it, a night and day difference.

 

[vas]

You're talking old school thing with a rotor turning and a breaker inside hooked up on the crank or cam shaft, right? No, definitely no, pretty sure sparkplug cables come straight from this black box I can not open!
problem is that I'm working on a generic manual from avon, and then yamaha jetskis manuals, only this 63N engine is unheard of to all the specialists, I do know it's 700cc but most (read all) are 62something not 3.
anyway I'm in Athens now, hopefully Monday morning I'll take it apart and find out

 

[julians]

You're talking old school thing with a rotor turning and a breaker inside hooked up on the crank or cam shaft, right?

yep - if you've already replaced the plugs, and there's no distributor then the HT leads are probably worth changing. You can sometimes see if the leads are breaking down by starting the engine in a dark room (you'd have to hook up a water supply to stop it overheating) and spray a fine water mist (using a pump action water spray bottle) over the length of the leads, if you see blue sparks then you have a problem with the leads.

 

[vas]

yep - if you've already replaced the plugs, and there's no distributor then the HT leads are probably worth changing. You can sometimes see if the leads are breaking down by starting the engine in a dark room (you'd have to hook up a water supply to stop it overheating) and spray a fine water mist (using a pump action water spray bottle) over the length of the leads, if you see blue sparks then you have a problem with the leads.

lol

julian, that's a bit far fetched on this tiny engine bay! I may just get two decent quality aftermarket leads (or nick from my fiat coupe engine which taking all the garage space atm)

V.

 

[julians]

lol

julian, that's a bit far fetched on this tiny engine bay! I may just get two decent quality aftermarket leads (or nick from my fiat coupe engine which taking all the garage space atm)

V.

I used to have a fiat coupe 20v turbo a long time ago, it snapped a con rod and put a hole in the block.

 

[vas]

I used to have a fiat coupe 20v turbo a long time ago, it snapped a con rod and put a hole in the block.

16vt here, still have it (havent used it in 4yrs now...) fully forged rebuilt engine, way too many things done on it (just before buying the boat) 330whp. Waiting for kid #2 to leave next ac.year so that I can get it out of the garage

 

[Hurricane]

We had exactly that Avon JetRib 15 years ago.
It came with our Sealine T51.
At the time, it was a young JetRib but had been mistreated and thrashed.
When I got it, it did exactly the same as in your video.
Mine would stop when warm and then only run for a few seconds (enough to get to safety).
I took it to a Jetski mechanic who stripped it down and advised re-boring.
So, after some significant expense, it was re-bored and rebuilt.
That did fix the problem but I also had fuel issues when starting.
I eventually fitted a priming bulb in the fuel line to any clear air whilst starting.
Great bit of kit when it worked but I spent so much time (and money) keeping it going.

 

[rafiki_]

Vas always start with what has changed since you last used it. Work forward from then. Start with the fuel.

 

[vas]

thanks guys, Mike, I should know on Mon or Tue what's the condition of the bores, I hope I don't have to go for a full rebuilt, one full rebuilt per two years is enough (did my 2cyl yanmar generator engine last year!)
Installing a priming thingy was the best 20euro spent, now you press a few times till it feels hard (means it's full of fuel and has squirte some in the two carbs) then hit the starter and starts in 2sec max!

Paul, true, but v.little use and various things done, so not really easy. I still do believe it's fuel or overheating due to blocked (salted) waterways.

Even bought a bilge fan for it (previous one swam in water when the exhaust failed and filled the bilge two yrs ago)
we shall see!

cheers

V.

 

[rafiki_]

Is the fuel fresh Vas, or last season’s?

 

[vas]

Is the fuel fresh Vas, or last season’s?

P, it was fresh as I run out of fuel in Sept, then bought some new and added in the tank in Oct. For sure need to open the filter and see if it's clean and not picked up crap from the tank emptying! will do!
Mind the video was end of last season (end of Oct) Jetrib currently in pieces in my mate's workshop

V.

 

[Hurricane]

Jetrib currently in pieces in my mate's workshop

How are your knuckles.
Any work on those JetRibs plays havoc with your hands.
The GRP mouldings are vary close to the mechanics.

 

[vas]

taking apart was at the beginning of the lockdown... Did spent some time to take the seat steering et al apart, sort of hang them from the ceiling of the workshop. Gives decent access. Not sure how long it will take me put it all back together though...

V

 

[rafiki_]

Vas, 6 months is old for petrol. Especially in the mid where it can get warm in spring.

 

[vas]

P, sorry, I've almost confused myself as well.. I'm reporting now on events that happened last Oct. rib run out of fuel on last outing (end of Sept) and was filled with fresh fuel two days before the video was shot. Fuel definitely fresh. I'll probably take the engine off today if weather and other obligations play ball.
will let you know

V.