Tohatsu 8 hp 2st twin 2004- hardly runs and won't idle- HT test??

[Heckler]

PS- love the thought of smacking a (v.expensive) crankshaft with a lead hammer! Not for the faint-hearted!

Aimed at rotax too, many years ago when we started with Tiger Cubs, the big ends were fragile. I remember watching Ted Williams from Wrexham change one and line the flywheels up after. He starte off by using a right angle square and scribing a line across the two halves of the flywheel. Then pushed out the crank pin with his press, fitted the new one in near enough, then did what rotrax dir, used a lead hammer to hit the flywheel halves whilst holding in his left hand. He got it spot in with his scribed line. I did several like this too, Not as difficult as it sounds!

 

[rotrax]

Aimed at rotax too, many years ago when we started with Tiger Cubs, the big ends were fragile. I remember watching Ted Williams from Wrexham change one and line the flywheels up after. He starte off by using a right angle square and scribing a line across the two halves of the flywheel. Then pushed out the crank pin with his press, fitted the new one in near enough, then did what rotrax dir, used a lead hammer to hit the flywheel halves whilst holding in his left hand. He got it spot in with his scribed line. I did several like this too, Not as difficult as it sounds!

Not with a Tiger cub crankpin-its only about 24mm diameter in the flywheels. A Jawa or GM Speedway crankpin is a 38mm parallel pin using INA type caged rollers. The pressure gauge on my big Churchill 100 ton press would always exceed 30 tons before they would move. By comparison, you could almost press out a tiger cub pin with a big vice and a socket larger than the pin.

The last one I did was a KTM motocross crank, that required almost 20 tons to shift it. The owner had a special tool which was like a heavy cylinder split for the con rod, but a tight fit on the flywheels. This allowed the flywheels to be lightly connected, adjusted to fit the cylinder of the tool and then pressed together, the tool keeping the alignment almost perfect. It worked well.

I never liked Tiger cubs-a 1961 Sports Cub nearly killed me when the stop lamp switch melted in a massive short circuit, stopping the engine right in front of an accelerating Jag!

A brown trouser moment.

 

[sparkie]

Indeed! All v.interesting. Hope I never have to try it myself- probably just smash my thumb... or the crankshaft... Thanks for the anecdotes. Curious how many boat owners (inc. me) are bike fanatics as well. Cheers!

 

[sparkie]

Further to the above, I've fitted a new (from Singapore) CDI unit, and things are much improved. Only remaining problem is a tendency to misfire and fourstroke in the mid range, especially after a spell of slow running or idling. This can usually be sorted by a brief burst of full throttle, which seems to clear some fouling or other? I'm wondering whether the plugs are a bit too hard, and that maybe a softer pair would help. Presently fitted (as per spec) NGK BPR7HS-10, and running 50:1 mix petroil.

Anyone any experience of fitting different plugs to standard spec? Cheers

 

[Oletimer]

Decent 2 strokes are in demand and fetching good money so it's worth fettling. My old 9.8 had an overheating problem and needed the head off to replace thermostat and I decided to strip to check pistons. Only problem I found was a gritty bottom crankshaft bearing, new one coming from Japan. All other spares, gaskets, seals etc. were off the shelf from Tohatsu via an agent so, don't give up, a new engine will cost you far more. There's a parts list here - https://tohatsuoutboardparts.com/Pa...s Catalog M8B-9.8B (2003) - (00221030-5).pdf

Thank you for this, it may prove useful for my 9.8 Tohatsu. George

 

[Oletimer]

Further to the above, I've fitted a new (from Singapore) CDI unit, and things are much improved. Only remaining problem is a tendency to misfire and fourstroke in the mid range, especially after a spell of slow running or idling. This can usually be sorted by a brief burst of full throttle, which seems to clear some fouling or other? I'm wondering whether the plugs are a bit too hard, and that maybe a softer pair would help. Presently fitted (as per spec) NGK BPR7HS-10, and running 50:1 mix petroil.

Anyone any experience of fitting different plugs to standard spec? Cheers

The plugs sound right to me, I keep 2 new sets onboard, just in case.
My 9.8's ratio mix is 25:1, is yours the recommended ratio, or am I using too much oil?

 

[RichardS]

The plugs sound right to me, I keep 2 new sets onboard, just in case.
My 9.8's ratio mix is 25:1, is yours the recommended ratio, or am I using too much oil?

Crikey .... 25:1. I've not heard that ratio since my days with two strokes in the 1960's.

I think you'll find that 25:1 is only needed for the first few hours running-in period for a new motor. After that I'm sure it will be 50:1 but it will be in the manual.

Richard

 

[Graham376]

The plugs sound right to me, I keep 2 new sets onboard, just in case.
My 9.8's ratio mix is 25:1, is yours the recommended ratio, or am I using too much oil?

Tohatsu 9.8 should be on 50:1 unless it's a new engine running in on 25:1 for first 10 hours.

 

[sparkie]

Yes 50:1 is the manual recommendation, and it still smokes a fair amount with that. I've found some "hotter" plugs in the same NGK range - BPR5HS- they're only £3 or so each so might get a pair and give it a go.

 

[Oletimer]

Yes 50:1 is the manual recommendation, and it still smokes a fair amount with that. I've found some "hotter" plugs in the same NGK range - BPR5HS- they're only £3 or so each so might get a pair and give it a go.

That will explain that letter I received from Brussels then, something along the lines they want to blame me for the sudden increase in blue smokey like fog conditions around Portsmouth on an otherwise clear day?

 

[VicS]

Yes 50:1 is the manual recommendation, and it still smokes a fair amount with that. I've found some "hotter" plugs in the same NGK range - BPR5HS- they're only £3 or so each so might get a pair and give it a go.

The next hotter plugs up from BPR7HS-10 are BPR6HS-10 .... You should not need to go up to a heat rating of 5 if the fuel/oil mix is correct and the carb is adjusted properly

(BTW -10 in an NGK plug number indicates that they are supplied ready gapped at 1mm )

 

[sparkie]

The next hotter plugs up from BPR7HS-10 are BPR6HS-10 .... You should not need to go up to a heat rating of 5 if the fuel/oil mix is correct and the carb is adjusted properly

(BTW -10 in an NGK plug number indicates that they are supplied ready gapped at 1mm )

Thanks for that Vic, wasn't sure whether one grade up would make a difference but I'll do as you suggest. Cheers.

 

[Heckler]

Just the same as motorcycle two stroke twin cranks then. If the bits can be purchased many bike specialists can handle the job easily.

I could, still have the big press and the crankshaft can be trued in a lathe on centres if a trueing jig is not available.

To true a crank, you hit it with a lead hammer. Hitting it in the correct place is the key. More problematical is getting the pressed length correct-often a special jig is used for this. I still have a few somewhere, Jawa and Yamaha IIRC. The three cylinder Kawasaki's added a further dimension to the job...………………….

Balancing should not be required unless rods have been changed-the reciprocating weight should be the same.

Agree, there used to be a repair shop in Liverpool that I used, I took a seized RD350LC crank there, got there about 11.00am, he said go and get some fish and chips and sit in the park for a couple of hours and we will sort it. 3 hours later job done. Similar setup, no keys, all pressed middle bearings and seals