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Timeing

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bricky07/08/2021 11:04:41
584 forum posts
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I have finished a 4 stroke single cylinder open crank engine and am having dificulty in starting it.Compression is good and I have had the valves out and reground them .I have looked up valve timeing for 4 stroke model i/c engines and some diagams show the ignition before the input valve opens and this puzzles me,is this correct.Any help on this would be good.

Frank

not done it yet07/08/2021 11:10:49
6889 forum posts
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I would say that could only be true if the ignition system runs at crankshaft speed.

JasonB07/08/2021 13:29:45
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Wants to be aprox one whole crankshaft revolution before the inlet valve opens as you need to get the combustions and exhaust strokes in first.

On something slow like the Rocker block try 5 to 10 degrees before TDC so just before piston comes to the top of the compression stroke.

Dave Halford07/08/2021 14:01:12
2100 forum posts
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Posted by JasonB on 07/08/2021 13:29:45:

Wants to be aprox one whole crankshaft revolution before the inlet valve opens as you need to get the combustions and exhaust strokes in first.

On something slow like the Rocker block try 5 to 10 degrees before TDC so just before piston comes to the top of the compression stroke.

Just like every car did in the 50's, 60's and 70's, the points need to be closed for long enough to energise the coil if it has one.

kevin laxon07/08/2021 14:40:44
16 forum posts

Frank your spark should not be just before the inlet valve opens,it should be one complete flywheel revolution after that point this is assuming the ignition contacts are running from the cam gear.a bit more information on the type of engine would be helpfull.Kevin.

bricky07/08/2021 15:08:12
584 forum posts
72 photos

Kevin,the engine is a Rockerthump By Brian Rupnow.I have built his Rockerblock and that ran a treat .It is stated that the inlet valve opens 20Degrees before TDC.Thanks to all for the input .

Frank

kevin laxon07/08/2021 16:23:37
16 forum posts

Sorry I do not know this engine but have found a video of one running on youtube,20° sounds plenty enough advance I would of thought half that would be good enough to start with or maybe even less,are the contacts on the Crank or cam?? & at what point does the spark occur at the moment.Kevin

JasonB07/08/2021 16:36:50
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have a look at Brian's thread ( last few pages) for the Thumper over on MEM, plenty of talk about ignition timing there , he also took up my suggestion of making the timing adjustable so it's easy to advance and retard it while the engine is running rather than having to stop it and fiddle about with a screwdriver to loosen off the points and refix.

SillyOldDuffer07/08/2021 16:52:30
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8905 forum posts
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Just a thought, but retarding the ignition, to less that 10° of TDC will make the engine easier to start, though it might not do to leave it that way.

It gets harder for a spark to jump inside the cylinder the more compressed the fuel/air mix becomes as the piston approaches TDC. The problem particularly effects old cars with conventional contact breaker ignition. The battery struggles to crank the engine over, causing the volts to drop, and a weak spark. Once the engine is running, there's plenty of electricity and away she goes, but a cold start with a tired battery is likely to fail, particularly if fuel is cold-condensing inside the cylinder as well.

Electronic ignitions are engineered to deliver a fat spark under all conditions, but this can cause another problem which is the spark not happening because of insulation failures. The CD electronic ignition I fitted to my Vauxhall Victor caused a small thunderstorm inside the engine compartment one dark damp morning - 2" sparks jumping randomly between HT leads and to the engine block; a quick squirt of WD40 fixed it.

You can guess how I know, but with electronic ignition it's not smart to pull the lead off a running engine to check for a spark. The insulation on HT leads isn't good enough to stop a nasty shock...

But check the spark on this model engine, too weak and too strong can both cause problems. Try starting the engine in the dark to make sure the insulation is OK. (Safety first!)

Dave

 

Edited By SillyOldDuffer on 07/08/2021 16:52:50

Maurice Taylor07/08/2021 19:52:57
219 forum posts
36 photos

Hi ,It could have wasted spark ignition ,where the plug fires once every rev .This is used in most modern car petrol engines.

Maurice

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