Lack of Power

[Kevin N Bayliss]

Interesting one for some of you.

I have just rebuilt my engine 500 short stroke.

Crank pins replaced, con rods ground and new little end bushes, rings, main and big end bearings, glands put in bin and replaced by modern seals, non return valves fitted in oil lines, boyer ignition, standard carb and jets, tdc found by using piston lock rotating engine one way then the other mid point true tdc, strobe 30deg btdc, max reducing as revs drop, pistons fitted correct way round,

it will start 1st or second kick, exhaust sounds like cracking explosion noise have trouble making it quiet, the engine does not sound like a Scott, full throttle is required when in second or third, you cannot get into 3rd until at least 40mph otherwise the speed drops and you have to drop back into second, Tick over is slow as can be seen via youtube link,

have removed one plug at a time and ran engine and both sides appear to run the same, soapy liquid on all joints, all appears to be ok,

Both cylinders are wet with oil nice amount in the wells, Yamaha positive pressure oil pump, Castrol R oil,

Before some some says its the electronic ignition Yamaha use a waste spark system on there rd350lc 2 stroke but as points are cheap i’ll try them anyway. has anyone got any ideas before i spend more money or put the bike on ebay.

I only had it on the road for a few miles before the left hand big end went due to having two right hand glands fitted so do not have a performance comparison before and after only my Dad riding a bike at Abbottsholme a couple of years ago with this engine fitted but that was just around the arena but seemed to be ok.

The porting does not look to have been messed with, and the pistons are plus 40thou.

yes I know its not original, yes `I know the colour is wrong, but this was for a everyday bike with better brakes that I could enjoy, and I like it. Thanks in advance Kev (click on picture to get video of bike running)

[BRIAN MARSHALL]

Hi Kev, The slow and gentle tickover may be a clue…. I would try increasing the ignition advance to say 36 degrees before TDC on full advance, and see what difference that makes. As it is so very non-standard it is difficult to suggest other check points, but I see that you have changed the exhaust pipe and silencer from the system seen in the photo taken at Abbotsholme. If you still have it try changing back to that. Above all, only change ONE thing at a time, or you will never know what was/is the culprit.
Brian

[Kevin N Bayliss]

Will do Brian, the exhaust that i had on last year was far too loud causing general disturbance so i’m afraid that went in the bucket. I lifted the barrel off just to check pistons, rings, seals etc the only thing noticeable was the residue around the pistons and exhaust which was quiet black as if running far too rich. I’ll check choke diameter and jet sizes etc before reassembly. Thanks Kev

[John Rose]

Hello Kevin,
I agree with Brian’s comment about your ignition timing, have you tried a strobe lamp on a timing mark on the flywheel?
I would be interested to know a little more about your Boyer ignition set-up, which amplifier box are you using, 6 or 12 volts and where is the trigger plate mounted?
My interest is due my own bike fitted with a BSA/Triumph twin four stroke kit, which I am informed by boyer that in this application will produce half the advance at half the revs. This is evident when using a strobe lamp. Their solution is to use an amplifier box from a model where the trigger plate is activated by the crankshaft rather than the camshaft system usually employed. Food for thought. Hope this is useful.
John

[Kevin N Bayliss]

Hi John, I made a new crankcase door for the right hand side, this houses the Boyer system and I also have a gear driven oil pump from the end of this housing. The system is 6 volt. after more investigation into the poor running I have found that both cylinders have a knock / bang when the engine is running,( if you hold your hand on the cylinder wall). After speaking to several people I believe that the wasted spark that the Boyer produces is actually firing the Scott engine when at or close to bottom dead center, this also looks like it’s causing the big end rollers to skid rather than turn as I have slight wear marks on the rollers, (new coming). also it would account for the noisy exhaust. The residue around the pistons and bores and even in the crankcase wells seems to be quite dark/black colour. This would suggest all the fuel not being burnt properly as if very rich and the cylinder firing twice per revolution. Maybe it’s the timing on the Yamaha that stops this happening? So next step once the engine is back together is to remove the Boyer and fit twin points and coils and see what this does then. will keep you posted. Kev

[Dripfeed]

Hello Kevin.
This is probably irrelevant to your problem , I ran my 29 flyer on a 4 cylinder car mag I linked the two unused to the two used H T pickups so had two sparks per rev one at btdc one at bbdc the motor performed as it did with the correct twin mag without any probs . The motor was a 500 s/ stroke firing 32 deg BTDC.thats if the flywheel is correctly marked it goes pretty well anyway. Regards D F .

[John Rose]

Hello Kevin,
Thanks for the details of your ignition set-up.
I have run my bike with a Boyer fitted for 14 years and have noticed no ill effects of the wasted spark. Have you tried timing it on the other hole on the trigger plate ?
John

[Kevin N Bayliss]

Hi John, strange that yours is ok, i’ve only timed it on the clockwise rotation hole, is that where yours is or did you use the opposite rotation hole for timing, Kev

[chstorm]

Hi Kevin,
I presume that you have a standard Amal carburettor. What is the position of the needle? 3 notches from top? Your engine sounded lean in the mid-range.

Kind regards,

Carl

[John Rose]

Hi Kevin,
My ignition trigger plate is mounted in the mag housing and when viewed from the cover end (on the near side of the bike) the magnets turn anti clockwise.
Hope this helps
John

[TedParkin]

😀 😀 😀 Hi Kev! I was stopped by a lady Police Constable on my short stroke Flyer a while ago doing 74 MPH on the standard Scott mag and settings in a 50MPH area in Scotland. She did not prosecute me but told me that if she saw me again at this speed that she would “Do me for cruelty to classic machinery” Standard is the Best!!!!!!!!!!!!!! 😀 😀 😀 Wot Larks Eh!!!!!!!!Regards Ted

[TedParkin]

😀 😀 I should perhaps mention re the above that this was late at night (a summer night IE light till 11 30 Pm in Scotland) on a dual carriageway undergoing repairs where the limit was normally 60MPH but was at the time subject to roadworks!! 😀 😀

[Kevin N Bayliss]

Thanks for all the info,
After rebuilding engine again just to make sure nothing was missed, it started second kick, it ran for just a few seconds before I stopped it.
I then checked the timing by strobe to previously marked flywheel at 6mm btdc max advance which was spot on.
Then I checked the needle as per Carl, it was in the third grove from the top but I raised it another notch, this made the running worse, after several attempts I have now dropped the needle as far as i can, i.e using the uppermost groove, this has made a big difference in the running, sound, and hopefully power, the air in the shed is getting fresher while running, big doors fully open of course.
So as a mere amateur to Scott’s I have come to the conclusion that most of the carb is goosed,(well worn out).
Open to suggestions, new needle, slide, jet, or do I fit something else, any thoughts, Just need to pass Ted when he’s on his favorite duel carriageway. Kev

[BRIAN MARSHALL]

Hi Kev, Short-stroke engines need more advance than 6mm. Try 5/16″ or 8mm on full advance, but no more than that.
Brian

[Kevin N Bayliss]

Ok Brian, didn’t want to go too far until running properly, will advance it when I get carb sorted.

My book states
main jet 170 touring 180 racing
needle jet number 6
slide 6/3

any advances on this, thanks Kev

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