Author Topic: Fun and games!  (Read 2822 times)

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Offline dragonmk5

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Fun and games!
« on: 23 Jul 2014 at 15:53 »
What have I missed?
Having rebuilt the engine and gearbox on the Mk4, it has never run satisfactorily so have only taken a trip round the local area, as I don't trust it. The chief suspect for the problem is the carbs- both appear well worn, hence my recent post about buying a pair of modern units. Before going down that route I thought I'd investigate further to see if anything else is causing me grief, so this is the chain of events after stripping and checking both carbs:
The problem appears to be the LH cylinder (sitting on bike), so I confirmed that there is a good spark on both cylinders under running conditions. Magneto was refurbished prior to engine rebuild.
Placed palm of hand over RH carb mouth - strong suction. Did same on LH carb - intermittent suction, strong to weak. Did compression test on both cyls and they came out the same(?)
Tappet clearances on both valves correct. Removed cyl head. Gasket ok, comb. chamber and valves ok.
Removed valves and springs. All in good condition. No broken springs, valve guides fine.
Removed LH barrel. No score marks on piston or in barrel. Both circlips properly located. Camshaft had new bearings fitted and from what I can see, theyre ok. No play on big end or small end.  Any suggestions welcome.

Offline Clive

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Re: Fun and games!
« Reply #1 on: 23 Jul 2014 at 22:19 »
Hi ,
Sounds like the camshaft on the LH cylinder is not timed correctly [1] or the valves are sticking [2]
Regards
Clive

Offline eddie

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Re: Fun and games!
« Reply #2 on: 24 Jul 2014 at 07:51 »
Hi,
      I doubt that the problem is incorrect valve timing or a sticking valve, as they would affect the pressures on the compression test.
  Presumably, when you checked for suction at the carbs, the engine was running at a moderately fast tick over - in which case, the problem could be something as simple as poor balancing of the carbs. This could cause intermittent firing on one cylinder, which will in turn affect the suction on that cylinder. I would suggest going back to basics by slacking back both throttle cables until there is some free play, then set up each carb individually to get both cylinders pulling equally with a strong, steady tickover. Then readjust the cables so that both cylinders are doing equal work as the throttle is opened.
  Something else you could check is the nipples on the throttle cables - originally they would have been 1/8"dia x x1/8" long - some modern nipples are somewhat longer, in which case the nipple might be landing on the top of the choke block and holding the slide off of the throttle stop (on one carb or the other). Or maybe the inner wire is just sticking through the nipple too far!

  Regards,
               Eddie.

Offline dragonmk5

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Re: Fun and games!
« Reply #3 on: 24 Jul 2014 at 09:04 »
Thanks to Clive and Eddie for their input. I can check the throttle cable nipples and settings etc., now, but will have to re-assemble the LH cylinder, which I will do later today. Will let you know how I get on, and what the results are.  I bet you can't wait :lol:

Offline Douglas52

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Re: Fun and games!
« Reply #4 on: 25 Jul 2014 at 11:31 »
Well I can't wait. :)
My first thought was like Clive that a valve was sticking. Then I saw in an earlier post you were concerned about excessive oil delivered to the LHS valve gear. When you say the spark is good,  is the LHS spark plug clean? Is it possible to swap over the carbs (and cables)? This might narrow down the problem area, and confirm your hunch that the problem is worn carbs.
Cheers
Steve

Offline dragonmk5

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Re: Fun and games!
« Reply #5 on: 25 Jul 2014 at 15:23 »
Oh all right then Steve :lol:  progress to date:
Rebuilt LH side, checked carbs (again), taking care to get cable and stop adjustments as close to identical as possible. Gave it a boot and off it went. It's still not running well, but fiddling with mixture and throttle stop improved it a bit, but still not as it needs to be.  The centre stand on this bike is not a standard item and it lifts the rear wheel about 2" off the floor, so I ran it in gear up to about 40MPH :o during which there was an intermittent misfire. It starts when on full advance, but not when retarded, and the carbs need tickling to get it going again.
Prior to the above, I had swapped the carb from the LH to the RH (no carb on left cyl)
And it ran.
All in all, so far there is nothing absolutely positive to conclude, so a bit more adjustment is required.
Now, look here, Steve, I happen to love NZ, and you sound a knowledgeable bloke, so I reckon I should come there and you should come and fix this piggin thing :mrgreen: