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Discussion Starter · #1 ·
Hi,

I've been having some prob's with a vire-7 2-stroke petrol inboard and wondered if anyone can offer some advice as my experience with engines amounts to a few months of research / messing around with this one.

Firstly the problems started manifesting with the engine cutting out and back firing through the carb, we eventually managed to solve these problems after discovering a small leak below the carb where a seal had previously been snapped / glued and replacing a pipe connecting to the exhaust. The next problem was a cooling issue, after checking all the obvious we have eventually removed the cylinder and piston, flushed out the cylinder jacket and are relatively sure the cooling problem was due to the banjo outlet getting blocked ... which we will know for sure when we put her back together. However now we have the piston out there is evidence of some scratching and have been told it has quite probably seized at some point.



I have had conflicting advice over whether its worth replacing the piston ... one person saying "that's as bad as it gets" and another saying not to worry about it.

The main concern really is that we are not getting maximum power out of the engine, we know we should be getting a couple more knots out of her. The person saying its as bad as it gets is putting it down to a loss of compression ... however that could be just to sell us some rather costly parts. Another person is saying the power loss is more likely to be caused by timing, which I think is pretty feasible as I know the timing was messed with and would only be roughly set now (ie. told to set it to 5 turns of the screw).

Getting back to the piston ... my initial thinking was a DIY approach, hone the cylinder and replace just the piston rings ... but after some research I'm not entirely convinced we need to change the piston or rings as the loss from this damage would be minimal.

Can anyone shed any light and recommend how best to proceed?

Jamie
 

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Piston is not that bad,I have seen worse,run your fingers round it and if there are any groves fine wet and dry,but if any deep scoring replace them,then what was the cause ? as pistons will only get this if the bore on the block has worn,this allows the piston to SLAP,personally I would check the block carefully for score marks,if scored that will then need the block re`boring,and that then obviously needs oversize pistons and rings replacing,sorry if I am not the bearer of good news.Pat
 

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Discussion Starter · #3 ·
Thanks for the reply, I will check the cylinder. Is this likely to have much effect on power? We haven't heard any knocking noises etc. and I believe the scratches are more from it overheating and causing the piston to expand rather than slapping?
 

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Discussion Starter · #4 ·
Hi Paul,

Yes I've come across that site, there's only two with anything about vire engines that i know of.

I will get some more photo's regarding the timing, I think there's basically two screws on the back of the carb to set it. A mate who part owns the boat with me basically dealt with that bit on the advice of a vire specialist, but i know it was definitely more of a guess rather than being timed properly. He also has the cylinder and piston at home so I've forwarded him a link to this thread ... I'll find out how confident he is about what you've said, but we may well take you up on your kind offer!

Jamie.
 

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Ah the joys of 2 strokes :)

First and foremost don't put abrasive paper of any sort near the piston!! Use a smooth file to remove any high spots on the piston. The reason? aluminium is a soft metal and abrasive particles easily become embedded in the surface, these in turn will wear the cylinder bore when you reassemble it.

Secondly you really do need to find the cause of the partial seizure, likeliest is lean mix IMHO and this can be caused by a fair few things including not enough fuel and air leaks in the inlet tract or possibly even bearing seals.

I ran an F2 sidecar for quite a while and prepped my own engines, when I sold it the new owner managed to seize it first time out and do the same several more times. He was repairing the damage but not looking for the root cause. It turned out to be a small bit of fluff from cleaning rag in the diaphragm fuel pump which was partially blocking the fuel flow and letting it run lean, it was pointless fitting bigger jets.

You will have a similar pump on your engine which uses fluctuation in crankcase pressure to activate the diaphragm, if it is blocked or the diaphragm is damaged that can cause problems. Carb badly seated on the inlet manifold causing air leaks could cause it, bad water flow around the cooling jacket, ignition too far advanced.

These are the likeliest things.

Make sure you get rid of the aluminium that will have picked up in the bore, our engines had plated bores so we used acid to get the aluminium off, if they got too bad the plating had to be stripped and redone. Some marine engines will use that system rather than cast iron liners which can be bored oversize if necessary.


Added>>>>>>>>>>>>

Just had a look at the website Ali G posted the link to, you have a "pumper" carb on that little engine, it is a diaphragm within the carb rather than a separate pump which still works on crankcase pressure pulses. I'd check that out mate!

Interesting little engine, one piece cylinder and head.
 

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Discussion Starter · #6 ·
Secondly you really do need to find the cause of the partial seizure, likeliest is lean mix IMHO and this can be caused by a fair few things including not enough fuel and air leaks in the inlet tract or possibly even bearing seals.
There was an air issue, we have a clear tube between the air filter and carb I think and could see the bubbles going through ... I believe I've sorted this now by fitting a primer pump and tightening some jubilee clips, but I've not run the engine for long since due to the overheating.

I'll have my mate read this over as he's a bit more clued up than me and also knows the history on the engine as he's owned the boat for a lot longer than I've been involved.
 

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There was an air issue, we have a clear tube between the air filter and carb I think and could see the bubbles going through ... I believe I've sorted this now by fitting a primer pump and tightening some jubilee clips, but I've not run the engine for long since due to the overheating.

I'll have my mate read this over as he's a bit more clued up than me and also knows the history on the engine as he's owned the boat for a lot longer than I've been involved.
Are the filter elements OK? If they have been removed or have broken up that can cause lean running, probably unlikely to have resulted in the partial seize though.

At least you haven't burnt a hole through the piston crown! That can happen with over advanced ignition as a localised hot spot burns through. Leads me to think your problem is likeliest to be weak mixture.

When you clean up the piston crown just scrape it rather than using abrasive paper.
 

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Discussion Starter · #14 ·
Right got a few more photo's together ...




The filter was 99% full fuel as when I fitted the primer pump I drained as much air out of it as possible as I was told it should be practically full, however it did take more air on again while diagnosing the cooling over the course of a week or two after and now is less than half full so presumably there was some getting in still.

You are correct on the screws, now you mention it controlling the mix that rings a bell! I will double check they are in the right position as its hard to see from that photo and I guess once we get it cleaned & rebuilt we will need to check the timing then.
 

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Jamie. Looking at the picture of the piston were you have it being removed from the engine block,it shows a gap of around 5mm ?. If that is the case its had it,the piston should just fit tight down the bore,(engine block)also the score mark looks quite deep,and shows it has grooved the ring space,if you are proposing to just fit rings personally I would get a mechanic just to give you an opinion,no doubt I will get told I am wrong again but what do I know.
 

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Jamie. Looking at the picture of the piston were you have it being removed from the engine block,it shows a gap of around 5mm ?. If that is the case its had it,the piston should just fit tight down the bore,(engine block)also the score mark looks quite deep,and shows it has grooved the ring space,if you are proposing to just fit rings personally I would get a mechanic just to give you an opinion,no doubt I will get told I am wrong again but what do I know.
The 5mm gap, is between the piston and the hole in the crankcase, the cylinder has been removed. Cylinder and head are one piece.
 

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The 5mm gap, is between the piston and the hole in the crankcase, the cylinder has been removed. Cylinder and head are one piece.
Spot on! Interesting how the seize is often adjacent to a stud, the castings don't expand as freely at those points so they do tend to be the natural pick up spots.

This isn't a modern high performance motor so I'd tidy it up and put it back together as previously described. Should be fine. Make sure to check the end gaps on any new rings fitted and check that they aren't pinched in the ring grooves around the damage area of the piston.

If you think this one looks bad you should see the state of the old Howard Gem rotovator that I renovated last year! It was seized solid when I got it so had it all apart, skimmed the head and barrel mating faces, stuck a glaze breaker down the bores, ground the valves in and stuck it all back together with new rings. Had to make up new gaskets but that was easy enough.


Works a treat now :) :)..........http://www.youtube.com/watch?v=3TlZuV179so&feature=plcp
 

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So, back to where we started then :laugh:

In pic 3, it looks like the top ring has a bit missing from the right hand end (as viewed in the pic).

The old rings, either snapped in half or bent around, make good groove cleaners.

I'd have had this stripped, cleaned and rebuilt in between races, with time for a cuppa and a sarny. The damage to the piston is insignificant, there is no damage to any critical areas at all.
No, that top ring looks fine, there is a slight angle at the underside edge at both ends if you look closely (beer goggles off!) Might even be worth a good clean up and put back together to see if it works again.

My biggest paddock rebuild between races was when a gearbox shim let go and blew the whole gearbox to bits, locked up the back wheel on the start finish straight at Snetterton. Had to drive into London to get a full set of gearbox internals, strip the engine completely, rebuild it when I got back. Went out the next day and had a couple of 2nd place finishes. :) :)

In all the years I raced 2 strokes we never had one seize (probably cos we never jetted them too aggressively)

Anyway enough of this, you must be wanting to get back to your electrickery!
 

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Enquiry for our 2 forum racers , before mr Lewis puts his engine back together should he be checking for bump clearance .ie is there different thicknesses for head or barrel gaskets on this engine ?
If he finds a head gasket on it I'll be buying the beers!! ;)

I'd replace the base gasket personally, not a difficult one to make if not readily available, if anything use one slightly thicker than original to stop the rings hitting the top of the wear ridge in the bore, as you suggest.
 
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