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Sprag 'revisited'

Submitted by mark_chapman2 on Tue, 23/02/2016
Forums
Commando

Hi. I know another sprag thread but this has been an on going issue for me.

I bought my Mk111 and within two days and as a result of the tiniest of backfires I had a disintegrated Sprag.

I had no time so someone whom is a locally known reputable classic restorer fitted a new sprag. Within two hundred yards the sprag grabbed grinding me to a halt. Luckily the starter was fine as I was only doing 15mph. Classic man very embarrassed and convinced it was a faulty sprag fitted another. I had little time to ride the bike but I did a few short trips. It was on these short trips 10 miles on average that the bike, once warm, felt like it was holding back on occasion. I blamed fuel starvation. I cleaned those amal carbs to within an inch of their lives but still you could feel something was not right.Being fed up pulling the carbs on and off I went for the single carb conversion. A2 mile trip around the blockwas all I could manage but all seemed well. This weekend was my first long run out and after six or so miles I once again felt that familiar holding back. I was very windy so I blamed some onto that but all to soon it got worse in fact slowing me up at one point. I worried that the engine was nipping up. But I had checked my oil flow to the tank and my tickoverwas good when I pulled over, no undue noise or smokeso nothing seemed astray in that department. When you felt the resistance I pulled in the clutch and she rolled on smoothly. I had to be honest no idea apart from maybe carburation?? I nursed her home after around 45 miles and on pressing the starter to put her away a few minutes later I once again got that all too familiar whirr of the sprag not engaging. I stripped out the sprag, it didn't look discoloured or overly damaged in any way but the last one didn't either. Today i ran the bike around 15 miles with no sprag fittedand no holding back so am convinced it was the sprag dragging once warm?? Plenty of oil in case and i could see no reason apart from inferior manufacture. I will fit a new sprag, roller bearing and bushjust tosee.

can anyone put any light on this.

Many Thanks.

Mark.

Submitted by tony_ripley on Tue, 23/02/2016

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Hi Mark,

If the sprag was dragging your starter would be wizzing round at such speed that you would soon notice !

I have never heard of a sprag grabbing after the bike has been started by the electric start.

If it had been started by the manual foot and the sprag was in the wrong way round then it will soon grab and drive the starter around.

Is everything standard inside the primary chaincase ? you are not running a belt drive by any chance ?

Regards

Tony

Submitted by norman_lorton on Tue, 23/02/2016

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Hi Mark

I have a lot of sympathy for you because I had strange 'power losses' when my new Mk3 was just a couple of years old. It was after I switched to TQF in the primary and happened at speeds beyond 70mph. Just like the brakes going on, but in the engine, and a de-clutch had you rolling smoothly whilst the engine died. After a five minute cool down all fine again. It happened twice in two years.

But, I have to agree with Tony Ripley that it is not mechanically plausible for a correctly oriented sprag to apply itself and retard the engine. I now think my earlier 'seizures' were just that and that the use of TQF was a coincidence.

I suspect you to might be suffering a 'coincidence'. All you can do is a) fit new parts and see what happens, or b) run without the sprag for a couple of long and very hot runs of 80 miles+, then refit the old sprag items and repeat the 80+ mile runs. If it tightens again check the barrel temperature - if it is a partial seizure then the barrel will be smoking.

It is frustrating because 9 out of 10 new sprags work without hitch I suspect. When they don't people seem to take them out and abandon them.

Norm

Submitted by barry_carson on Tue, 23/02/2016

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Hi. i haven't got a commando. just wondering what effect you would get if the starter was engaging intermittently when the bike was running without pressing the start button, ie faulty switch or wiring ect. Baz

Submitted by mark_chapman2 on Wed, 24/02/2016

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Previously Tony Ripley wrote:

Hi Mark,

If the sprag was dragging your starter would be wizzing round at such speed that you would soon notice !

I have never heard of a sprag grabbing after the bike has been started by the electric start.

If it had been started by the manual foot and the sprag was in the wrong way round then it will soon grab and drive the starter around.

Is everything standard inside the primary chaincase ? you are not running a belt drive by any chance ?

Regards

Tony

Hi Tony and all whom have helped to solve this mystery. My firstbit of confusionwas the fact that the starter was still intact, as you rightly say if that sprag grabs at speed then you are in trouble, yet the starter seems fine. The engine doesn't get over heated and doesn't fully react as if it is trying to seize. Once you feel the 'holding back' if I pull over the engine ticks over perfectly??I only suspected the sprag when for no apparent reason the starter just spun without the spag engaging. By removing just the sprag and going over the same journey I experience no problems.

I will run the bike over a longer distance without the spag fitted just to be sure. After thatI will re fit a new sprag and report back.

Thanks again for all your help

Mark.

Submitted by tony_ripley on Wed, 24/02/2016

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Previously Barry Carson wrote:

Hi. i haven't got a commando. just wondering what effect you would get if the starter was engaging intermittently when the bike was running without pressing the start button, ie faulty switch or wiring ect. Baz

Hi Baz,

You can happily press the starter button when the engine is running as it is all going in the same direction. It is not like a car starter with a bendix drive onto the flywheel, that is always messy if it engages when the engine is running.

Regards

Tony

Submitted by barry_carson on Wed, 24/02/2016

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Hello Tony thanks for that just thought as the starter would be rotating at a pre determined speed and the engine variable just thought there might be some sort of interaction between the two if the starter kicked in on its own causing problems of some sort. Baz

 


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