Author Topic: mk v low power?  (Read 3698 times)

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Offline Evanc

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mk v low power?
« on: 25 May 2016 at 11:02 »
Hi Guys, haven't posted for a while as I thought the mkv was going o.k, but I have some doubts as to wether it is going as it should. The bike starts first kick every time and idles fine, however I struggle to get to 60mph and if I do I have to hold it flat out to maintain it. The engine seems very "tappety" at idle, despite valve clearances being set to .oo5". The engine also "flatspots" badly on anything but gentle acceleration. It also sounds like it is labouring at high throttle openings. Varying the timing makes little difference to the noise or performance.

I have recently put rings in the bike and replaced valves and reconditioned the head. The carburettors have been fully overhauled with new jets and needles and a new float needles. The magneto was overhauled and have spent hours fiddling with the timing, which I feel is perfect.

I removed the timing cover last night as I thought the valve timing may have been out, but it seems fine, but to me there seems to be excessive backlash between crank gear and cam gears. I would say there would be about 5 degrees of movement in the crankshaft before the cams move.

Tinkering last night I feel the rockers may be wrong as the tappets contact the very edge of the valve. It seems the rocker arms are too short? is it possible my bike has wrong rocker arms? if they were a fraction longer they would contact the centre of the valve stem and also open the valves a little further.
The tappet noise at idle could  be related to the backlash in the timing gears. Are they common for wearing and can new gears be purchased? Is it possibly just the idler?
My bike has a waffle box exhaust, could it be blocked? I have had it off and run some solvent through it. To me it seemed to flow o.k.
The left carby seems to be running a lot richer than the right,  despite being jetted the same. I have noticed the carbies are not synched very well the left one being a little retarded. This will no doubt help with the flatspot when corrected.
My last question is do the cams wear? I was hoping to have a look at them with timing cover off, but it seems it is a full stripdown to see them.
Any advice will be appreciated,
Evan from Oz

Offline shuswapkev

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Re: mk v low power?
« Reply #1 on: 25 May 2016 at 11:21 »


  I had similar with a matchless G/80...a cam follower collapsed...120 miles from home...

Offline shuswapkev

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Re: mk v low power?
« Reply #2 on: 25 May 2016 at 11:29 »


  the valves...when reconditioning a  head...when the valve seats get ground the valves spring end should be measured and cut to factory length...(ground off)...as the deeper the valves sink  into the head..  the further out they end up on the spring end..
I really don't think it would affect your motor much but...any lever needs to be at 90 degrees to get max leverage....whether a rocker arm...brake arm or clutch arm...
most times when this happens...well for me....valves sunk into the head... I don't have enough valve lash adjustment to valves to even work at all...
most of my bikes are built up from loose parts

Offline eddie

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Re: mk v low power?
« Reply #3 on: 25 May 2016 at 11:54 »
Evan,
         As you have the timing cover off, check that you have the timing gears on the correct side. For some weird reason, Mr Douglas referred to parts as left or right hand as you sit on the bike - except for the engine, in which case the parts are handed as you look at the timing cover. With this in mind, check that the cam gear marked LH is actually on the right hand side of the bike. The cam gears look the same, but one has the keyway in line with a tooth, and the other has the keyway between two teeth. The common sense solution would have been to make both gears the same and then move the keyway on one of the camshafts - then it couldn't be put together wrong!
 Whilst you may have had the carbs refurbished, you may still need to make more adjustments due to the differences between 1950's petrol and our modern chemical concoctions. Here in the UK, we have to use unleaded fuel, and I have found our bikes respond better with a slightly smaller cutaway on the carb slide, along with an increase in main jet size by 10 - then compensate by lowering the needle if the engine runs rich at half throttle.
  Having said this, if you are new to 'Douglas', the perceived poor performance may be down to your riding style - Douglas engines are not like most other 1950's engines - they are not sloggers, they like to be revved. I made the same mistake when I first got my Dragonfly - it seemed 55mph was it's limit - until experienced Doulas owners told me to rev it more. With it geared the same as a Mark 3 Sports, it will go to about 58 in second, just over 75 in third, but only just manages to maintain 70 in top gear.
 Getting back to the timing gears, providing the teeth are not undercut, I wouldn't worry too much about the backlash - mechanical noise seems to be a feature of the postwar engines. Camshaft wear can be a problem, but seems to be more prevalent on Dragonflies than Marks.
  Moving on to the rocker arms - there were 2 types for the postwar T35 engines. The original T35's (the Mark 1's)had deeper combustion chambers than subsequent Mark 3 -5 engines. The deeper combustion chambers had the valves set at a wider angle, so the rocker arm geometry is different (enough to cause the problem you have mentioned, but difficult to see with the naked eye).

  Hope some of this helps,
        Regards,
                    Eddie.

Offline Dougiethenoo

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Re: mk v low power?
« Reply #4 on: 26 May 2016 at 21:31 »
I had similar noise on my Mk3 Sport. I found the pushrods were worn badly (pointed), the cam lobes are rounded, while the follower faces are worn concave. The tappet adjusters were tightened almost all the way in to maintain valve clearance and were bearing on the outside edge of the valve ends.
The valve seats don't appear to be significantly recessed but then I don't have anything to compare them to.
Certainly they are not recessed as much as the substantial wear on the followers & pushrods.

The cam gears do seem to make a lot of noise at lower revs but I think that's the inescapable nature of cyclicly loaded straight cut gears.

A later solution to this was spring loading a (split) pair of gear wheels on the same shaft, as Honda did on VFR cam gears.

Offline Evanc

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Re: mk v low power?
« Reply #5 on: 02 Jun 2016 at 11:28 »
Hi All, have taken all your advice on-board and had another play, coming up with the following results:
Backing the valve adjustment off to a point where the rocker contacts the valve at a better angle (simulating a longer push rod) the rocker arm still only contacts the edge of the valve. I have basically backed off the tappet adjustment all the way to do this(about 5/16") so my question now is can anyone tell me how much longer the early rocker arms are compared to the later type? It would appear I am only chasing 3 or 4 mm. also is there any difference with push rod lengths or does the geometry of the early rockers change in this regard,as longer push rods will help things also? Any ideas where I might find some older rocker gear or I guess some new heads, preferably in Australia?
Thanks again, Evan from Oz

Offline Daren W Australia

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Re: mk v low power?
« Reply #6 on: 02 Jun 2016 at 11:41 »
Hi Evan I have a heap of rocker gear come and have a look Daren
too many dougli not enough time!

Offline eddie

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Re: mk v low power?
« Reply #7 on: 02 Jun 2016 at 19:12 »
Evan,
         If you suspect that the push rods/ cam followers are severely worn, have a look at the angle the push rods form with the rocker arm - ideally, it should be 90 degrees with the valve at 1/2 lift. If you need new push rods, they can be turned from 1/4" dia silver steel. The dome on each end should be 1/8" radius - giving a full dome on the top end and a truncated dome on the smaller diameter (3/16") lower end. I would suggest hardening just the ends and tempering them back to a dark straw colour for longer life.
 Some of the wear problems on postwar engines can be attributed to Douglas using the same steel for parts that run together under high loads - original cams and followers were made of a carbon steel - changing to an alloy steel for the followers reduces the wear rate considerably. The same is probably the case with the push rod ends in the tops of the cam followers.
Regards,
             Eddie.

 

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