Author Topic: Valve Timing  (Read 2696 times)

Online Brian

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Valve Timing
« on: 15.04. 2009 00:28 »
This is sort of somewhere between a story and a tech topic.

Ever heard the saying never take anything for granted ?

Most of you will know I have not long finished restoring my 1950 Star Twin. When I finished it it started and ran very well, idled nicely and would cruise at around 55mph. All along I have been a bit dissapointed with its top end performance and pulling power, I wanted to get a few hundred k's on it before I made much of an assesment of it however.

Last weekend I went for a ride on a friends 48' A7 and it definitely had more go than my Star Twin which it shouldnt have. I knew mine was in excellent mechanical condition, has new carby's, reconditioned magneto etc etc. After much thinking I decided to check the valve timing, when I assembled the engine I lined up the appropriate marks, dot to dot on the crank gear and dash to dash on the cam gear. This time I put a degree disc on it and found everything was opening and closing 25 degrees too late. The marks on my gears are wrong ! They are one tooth out which equates to about 25 degrees.

I dont know if one of the gears has been replaced during its life or maybe it has been wrong since it was new.

The difference is amazing, it is a real little rocket now.

This bike was a runner when I bought it and I have spoken to two previous owners who both confirmed that 50 -55mph was about its cruising speed. Now it will easily run along at 65-70 mph.

So I guess the moral of the story is to check everything, I wont get caught like this again. I will check with a degree disc every motor I put together from now on. I have always used a degree disc on any engine there was any doubt about but the A7 is straight forward with its dots and dashes, or so I thought !

Brian.


Offline LJ.

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Re: Valve Timing
« Reply #1 on: 15.04. 2009 12:40 »
Thanks Brian... Nice little read, gives me alot more confidence now with my Star Twin - twin carb jobbie. I'm not expecting much more in performance from the duel carb set up but it goes to show that I might also need to check the timings with a disc. Trouble is... I still find the disc setting up and reading difficult to understand. Is it really so very accurate as I'm led to believe it is? I must admit that I have not even tried to use a disc yet.

Just waiting now for the petrol tank to come back from the Chromers, its a week overdue so any day now and after a paint up... and she'll be running!

cheers.
LJ.
Ride Safely Lads! LJ.
**********************
1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
1961 BSA A10  650cc Golden Flash-Blue
1961 BSA A10  650cc Golden Flash-Red

Online trevinoz

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Re: Valve Timing
« Reply #2 on: 15.04. 2009 22:53 »
Brian, you probably have an idler pinion from the later series engines. The timing marks are in different positions.
Physically they are the same.
Long stroke is part number 67-706 and later is 67-335.
    Trev.

Online Brian

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Re: Valve Timing
« Reply #3 on: 15.04. 2009 23:32 »
Trev, I think you are probably right. After I had set up the timing correctly etc I remembered I have a spare idler pinion that I could have used. I have a spare engine that doesnt have a cam or any gears in it but it does have the idler pinion, I had forgotten about it. Next time I have the timing side off for some reason I will check it with the spare gear.

LJ, if your engine was a complete going concern before you should be right. I am sure my experience was fairly uncommon. I have traced the ownership of my bike back to the early seventies when it was bought totally dismantled and then assembled with whatever parts the guy who bought it could get hold off. I am fairly sure thats when it had the wrong part put in it. Its had four owners since then.

Online trevinoz

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Re: Valve Timing
« Reply #4 on: 18.04. 2009 07:19 »
Brian, for reference, there 16 teeth between timing marks in a long stroke engine and 20 in the others.
  Trev.

Offline rio169

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Re: Valve Timing
« Reply #5 on: 13.05. 2009 12:06 »
I recently started a thread titled sooty plugs and one suggestion was incorrect valve timing. I am confident as regards mag and carb condition settings. If the valves were to open a little early one tooth 25 degrees could this result in too much fuel mixture or as the exhaust valve also closes a little early could not all the combustion material not escape resulting in a build up which is sooting up the plugs. Interesting thread which may help me.