Mercury 2.5hp 2 stroke modification?

[MINESAPINT2]

I was donated the above engine a couple of years ago not working. Not being a competent mechanic a couple of half hearted attempts failed to get it going, however I have made progress over the last couple of days. It did sort of propel the dinghy yesterday and got me to my boat and back but does not start well and stops soon after getting underway but after a few re starts it eventually keeps going.

It has a new plug - just fitted.

I have previously noticed the spring in the centre of the carb is out of shape but managed to re wind it. I also noticed the outside of the brass barrel has file marks on the outside which I have sanded smooth.

I have just noticed the end of the brass barrel is not quite square which indicates someone has shortened it to increase power. and am wondering whether this modification has caused problems??.

The overall length of the brass barrel is 27.5mm now.

I have inspected the rest of the carb for dirt and blocked jets etc and reckon it is clean.

Judging by the wear on the 2 screws holding the float chamber on this carb has been dismantled many times. Perhaps time for a new one.

Look forward to advice

Mike

 

[MINESAPINT2]

I perhaps should have mentioned when running it runs better on half choke.

Mike

 

[fredrussell]

I did that same modification to a 2.5 tohatsu, but it is important to get that brass barrel slider square at the bottom. It could indeed be new carb time, especially if the spring has been mangled in the past.

 

[Mistroma]

Two things come to mind:

1) Oil mix
My wife's godson runs a marine engineering company. I remember him telling me about the huge research effort required to alter the mix from 50:1 to 100:1. Apparently it involved changing the label and accepting that customers would have engines with shorter bearing life.
Virtually zero cost for the company, result. He said we should stick with 50:1 and ignore the label.

2) Increase power output (mod. you are referring to)
You should find details online to get 3.3HP instead of 2.5HP and the extra 0.8HP does make a big difference in some conditions. I seem the remember that the mod. only involves extending the slot for the throttle lever and trimming a part inside. The slot was actually the correct size on one I looked at and the plastic sticker on the face had a shorter slot. Just a matter of cutting the plastic in that case. The manufacturers obviously fitted a plate with a longer slot on the 3.3HP model.

I don't remember the details of the internal mod. but it did seem to be quite simple when I read about it many years ago and am pretty certain that's the bit you are talking about. It didn't involve changing jets, just allowing the throttle to open further. Sounds as if the internal mod was bodged on your engine.

Look at the throttle lever slot to see if there are signs that it was also extended. I think that the slot in is longer that slot on the faceplate and you can see that by looking at the inside of the cover on an unmodified engine.

 

[[159032]]

Get a new carb. I've had good service from China sellers Marine Engine Carburetor Carb For Tohatsu Nissan 2.5 3.5hp 2-Stroke Outboard | eBay

 

[MINESAPINT2]

Thanks for replies.

100% sure the slot in the face plate has not been extended. It is obvious the sticker allows for a longer slot but the existing slot is original, still has original paint/coating.

I have also noticed there is option for adjustment in the needle held in place with the spring that passes through the brass barrel. There are 4 options for the location of a sur clip which alters the effective length of the needle. This one is located in the 3rd from the pointy end.

China. Thanks for the suggestion and link. I have considered this but am nervous. The new units seem to be around £18 from China which I understand would attract a VAT charge and a Royal Mail handling charge which would add about £12. There would not be any import duty. Am I correct???.

Look forward to replies

Mike

 

[Graham376]

China. Thanks for the suggestion and link. I have considered this but am nervous. The new units seem to be around £18 from China which I understand would attract a VAT charge and a Royal Mail handling charge which would add about £12. There would not be any import duty. Am I correct???.

Worth a try, peanuts compared to what you will be charged if you bung it in for repair and they'll then charge full manufacturer's price for a new carb.

 

[fredrussell]

China. Thanks for the suggestion and link. I have considered this but am nervous. The new units seem to be around £18 from China which I understand would attract a VAT charge and a Royal Mail handling charge which would add about £12. There would not be any import duty. Am I correct???.

Look forward to replies

Mike

You’re reluctant to spend £30 on a NEW carb??? Short arms and long pockets eh?Try getting a price on a U.K. one from a dealer then see if it still seems a lot. Gotta be worth a punt surely?

 

[MINESAPINT2]

No, I am reluctant to get my bank account emptied by someone in China. Additionally I once imported something from USA that ended up costing an arm and a leg in duties and charges.

Mike

 

[peteK]

I have had loads of stuff from china,even a diesel heater never had to pay any extra charges.

 

[CliveF]

before resorting to a new carb make use of the " already there " ability to adjust it.

You say it runs better on choke which means less air for the metered fuel. you can not alter the air , other than by choke.
So alter the fuel instead. As your needle has a cirlip I would move it down one slot so that effectively lifts the needle by one slot thus allowing more fuel through.
Same effect as less air i.e choke.

Its got to be worth a try before any expense is incurred.

Regards Clive

 

[[159032]]

No, I am reluctant to get my bank account emptied by someone in China. Additionally I once imported something from USA that ended up costing an arm and a leg in duties and charges.

Mike

Look at the listing again. You can pay with PayPal.
Bombproof for purchasers.

.

 

[Beneteau381]

No, I am reluctant to get my bank account emptied by someone in China. Additionally I once imported something from USA that ended up costing an arm and a leg in duties and charges.

Mike

Send me a PM with your email address and Ill send you the article I wrote for PBO on how to do it

 

[Kelpie]

There may be a restrictor plate in the carb that needs to be removed- basically a washer with a slot in it- couldn't be easier. All my little outboards are now 3.5hp!

 

[Beneteau381]

There may be a restrictor plate in the carb that needs to be removed- basically a washer with a slot in it- couldn't be easier. All my little outboards are now 3.5hp!

The later ones had a longer throttle slide, this butted against the top of the carb and so only allowed half throttle. I sourced a 3.5 slide but you could cut off about 7mm to achieve the same effect

 

[MINESAPINT2]

before resorting to a new carb make use of the " already there " ability to adjust it.

You say it runs better on choke which means less air for the metered fuel. you can not alter the air , other than by choke.
So alter the fuel instead. As your needle has a cirlip I would move it down one slot so that effectively lifts the needle by one slot thus allowing more fuel through.
Same effect as less air i.e choke.

Its got to be worth a try before any expense is incurred.

Regards Clive

Hi Clive,

You were spot on thanks.

Engine started second pull this morning but would not keep running irrespective of how much or little choke or throttle. I kept re starting it and it kept stalling.

Carb off and into the workshop and dismantled it Carefully removed the Circlip from the needle and replaced it so the needle was on the shortest of its 4 positions (more fuel delivery). Put carb back together.

Re fitted carb pulled starter cord and away it went, running like a dream.

Went back into the workshop and noticed a part on the bench I had omitted to re fit to the carb when I rebuilt it. The brass screw with a hole in its centre which picks up the fuel from the float chamber. I guess the size of the hole as well as the position of the circlip which affects the effective length of the needle determines how much fuel is drawn into the engine. Well with this part on the bench there was certainly plenty of fuel and it was immediately obvious the problems had been caused by fuel starvation.

The engine was running so well and starting first time every time I considered leaving the part on the bench but instead gave it a good clean in Acetone and soaked a piece of braided fishing line in brasso and gave it a good pull through then re fitted it. Still running perfectly and starts first time every time.

 

[Beneteau381]

Hi Clive,

You were spot on thanks.

Engine started second pull this morning but would not keep running irrespective of how much or little choke or throttle. I kept re starting it and it kept stalling.

Carb off and into the workshop and dismantled it Carefully removed the Circlip from the needle and replaced it so the needle was on the shortest of its 4 positions (more fuel delivery). Put carb back together.

Re fitted carb pulled starter cord and away it went, running like a dream.

Went back into the workshop and noticed a part on the bench I had omitted to re fit to the carb when I rebuilt it. The brass screw with a hole in its centre which picks up the fuel from the float chamber. I guess the size of the hole as well as the position of the circlip which affects the effective length of the needle determines how much fuel is drawn into the engine. Well with this part on the bench there was certainly plenty of fuel and it was immediately obvious the problems had been caused by fuel starvation.

The engine was running so well and starting first time every time I considered leaving the part on the bench but instead gave it a good clean in Acetone and soaked a piece of braided fishing line in brasso and gave it a good pull through then re fitted it. Still running perfectly and starts first time every time.

That sounds like the main jet! Logic says that it would have over richened the mixture, ie a massive main jet in effect! However I auspect that the venturi effect with the main jet out was lost and so no fuel was being sucked in. Put the needle back to where it was otherwise you will be using loads of juice!

 

[[159032]]

Went back into the workshop and noticed a part on the bench I had omitted to re fit to the carb when I rebuilt it. The brass screw with a hole in its centre which picks up the fuel from the float chamber.

This shows the problems with advising remotely.