Author Topic: t35 tappet clearences.  (Read 3300 times)

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Offline george sutton

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t35 tappet clearences.
« on: 20 Jun 2010 at 16:00 »
Hi you wizz kids.    with regard to the mark tappet clearences, i've been setting as rec. in the service book, ie, .002" for both valves.  Now, i was taught on very good engine courses all about the theory of valve OVER lap to "Get as much into the head as possible". also the importance of the burn rate for a certain fuel to get the best power Dureing the both valves CLOSED time. but you all know that dont you ?   SO, it seems to me that  .002" is very small, as once engine is hot, the actual time that both valves fully closed is really very small for best power stroke. In other words,  when the rockers are hot, there is'nt hardly any clearence in them. Now if you are useing a racing engine, that can be good with the right petrol.     BUT, what about the plodding old mark1 with guicker and hotter burning unleaded fuel.   what please does the club tecnicals advise for a good clearence on mark tappets.  denis S.

Offline eddie

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Re: t35 tappet clearences.
« Reply #1 on: 20 Jun 2010 at 17:25 »
Denis,
           If you think the bike ought to go better with larger tappet clearances - try it! Set the tappets to .005" (as on later Mks) and see if it performs better - you wont do any damage (other than to people's ears with the extra rattles!). Your last comment about a 'plodding old Mk1' may be a clue to any lack of performance. Post war Douglases never were 'plodders' - they are much happier when allowed to rev. If you are concerned about the rate of burning with modern fuels, then try adjusting the advance/retard to see if there is any improvement. Be careful if you try using the 'super unleaded' as this has a higher viscosity and tends to run weak in engines fed by a carburettor (fuel injection pumps a specific amount each time, so isn't affected by the change in viscosity).
                               Regards,
                                         Eddie.

Offline george sutton

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Re: t35 tappet clearences.
« Reply #2 on: 21 Jun 2010 at 06:38 »
Thanks Eddie,  talking 'super unleaded', no!no. that equates to using an aviation fuel, which tends to explode rather than have a burn rate .used with stelited valve seats ,but not i think in an old  air cooled engine. sorry about the plodders! that was an insult to the memory of my original one, which in 1954 would do c/ London to Taunton in 4hrs (with a pillion pass.) on the 'old' roads. Will reset clearances to .004".Denis s


Quote of previous post removed - Dave, 21st June 2010
« Last Edit: 21 Jun 2010 at 06:57 by Dave »

Offline Ian

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Re: t35 tappet clearences.
« Reply #3 on: 22 Jun 2010 at 06:51 »
George, my understanding is that the "explodes" versus "burns" is the other way round - the higher the octane the more even the burn - which is why you can run more advance and higher compressions. Not the same as the discussion above about fuel density.

Engines that are designed for 100LL avgas will detonate on lower octanes due to the faster burn.

I stand to be corrected here !!

Offline george sutton

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Re: t35 tappet clearences.
« Reply #4 on: 22 Jun 2010 at 20:28 »
Hello Ian,  don't want to get too far into this subject ,except to say ,that aero engines using higher octane is connected to chamber design, and fuel delivery systems.  Octane rating is a MEASUREment of a fuels ability to resist engine knock.   Think of our old leaded petrol, the lead was added not just to help lubricate valves etc. but mainly to slow down the burn rate of the fuel and give a longer punch on the piston as it began its downward movement.. For a std. type of older engine to give that maximum power then yes, advanced ignition is good AT the right time.  The important thing to remember is that all fuel TAKES a given time to burn! And the higher your rpm, the shorter that time is available to do so .   Hence the development of super charging to get MORE IN! and FASTER.   There now, see what you've started.  Denis s

Quote of previous post removed - Dave, 23rd June 2010
« Last Edit: 23 Jun 2010 at 06:40 by Dave »

 

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