Author Topic: Tuning my 1969 750 S fitted with VHB30's.  (Read 814 times)

Offline Piranha Brother 2

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #15 on: October 13, 2017, 00:09 »
I can't take credit for it. It was Red's idea.
That doesn't surprise me  ;D
What ... leave it standard??!!

Offline Simon Yates

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #16 on: October 13, 2017, 18:29 »
Today I adjusted the tappets, cam chain, fitted new plugs and adjusted the points and timing.
The inner set is statically set at about 2-3 ATDC because I cannot advance it any further as the screws have reached the end of the slots. >:(
Still not running well so it is the carbs next.  Although ultrasonically cleaned I did not blow the ports through and neither did I record the jet sizes.
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Offline Ernesto

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #17 on: October 13, 2017, 18:37 »
Quote
I cannot advance it any further as the screws have reached the end of the slots.
Check the contact breaker gap again, has to be 0,4 mm.
Wrong gap influences the timing, too.

hth,
Ernesto

Offline Simon Yates

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #18 on: October 13, 2017, 18:49 »
The contact breaker or points were checked and both were 0.4 mm.
I presume the points cam rotates at 1/2 engine speed ( not a wasted spark design).  Do I have to remove the timing plates and extend the inner slots or is there any other way or retarding the timing slightly?
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180 Jota XGF354W #7284

Offline Ernesto

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #19 on: October 13, 2017, 19:11 »
Quote
I presume the points cam rotates at 1/2 engine speed
Yes, that's right.
You have to take the primary cover off to check if the oil-pump
gear is in the right position in relation to the crankshaft.

Ernesto


Offline Tippie

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #20 on: October 13, 2017, 20:35 »
But first check your timing marks against correct top dead centre.
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Online Paul Marx

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #21 on: October 13, 2017, 22:04 »
Simon,

You can play a bit with the points gap to adjust the timing.. Nothing is going to blow up.
Whip the points plate off and enlarge the slots.

Paul

Offline Dellortoman

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #22 on: October 13, 2017, 23:15 »
is there any other way or retarding the timing slightly?

You don't want to retard the timing, you want to advance it. 2-3 ATDC is already way retarded. Are you sure you have your BTDC and ATDC the right way around? It's easy to get confused sometimes.
Location: Tasmania, Approx 4253S 14723E

Offline Piranha Brother 2

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #23 on: October 14, 2017, 00:13 »
Hope you're not getting bamboozled by all the replies. The crank rotates fwd, the points cam backward (when sitting on the bike!). It's quite common for the oil pump drive to be one tooth out either way when it's been reassembled, but the simplest way to get it right is to just widen the slots a tad (for 2 deg that would only be 1 deg on the backing plate!). Be aware that you obviously adjust the points on the main plate first, then the smaller plate fitted onto that for the other cyl). And once you've adjusted the smaller plate, recheck the points gap, because play in the plate can easily affect points gap).

As Paul said, a tad wider or narrower on the points will have no negative effects on engine running.

For triple heads, don't be alarmed by the timing figures - it is correct to set static timing at 0 deg - ie, TDC.

But no doubt, your problems are carb (includes all air-fuel functions, such as inlet stub sealing) related. As an almost universal rule, I've found with later 36mm PHFs, you will generally get the best pickup off idle with the idle mixture screws about turned 1/8 to 1/4 further out than where smoothest idle occurs - ie a smidge richer.
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Offline Dellortoman

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #24 on: October 14, 2017, 01:14 »
I've found with later 36mm PHFs, you will generally get the best pickup off idle with the idle mixture screws about turned 1/8 to 1/4 further out than where smoothest idle occurs - ie a smidge richer.

I've found the same on most Dellorto equipped bikes. My Jota was like that (when it had Dells). My Ducati has 40mm Dells and they're the same. Gotta have it a bit rich on the idle setting for smooth pick-up.
Location: Tasmania, Approx 4253S 14723E

Offline Simon Yates

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #25 on: October 14, 2017, 07:49 »
Thank you all for the advice.  Back to basics I think.
Firstly, satisfy myself that TDC co-incides with the P/M timing mark.
TDC should equate to the point where the 'saw-cut' mark, P/M and the crank centreline are in a straight line.
If this is all OK, then it is likely that the oil pump drive may be one tooth out of position.
This would explain why the ignition timing screws are at the limit of movement permitted by the slots.
Then either reset the oil pump gears or extend the slots a smidgen.
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Offline phil37

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #26 on: October 14, 2017, 07:56 »
  If you want to do a quick check on the oil pump cog timimg - set motor at TDC and remove the A/R unit - then the bolt and washer holding the A/R carrier to the shaft - the slot for the woodruff key should at 12 o'clock. - regards - Phil

Offline Simon Yates

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Re: Tuning my 1969 750 S fitted with VHB30's.
« Reply #27 on: October 14, 2017, 11:20 »
Thanks Phil, the keyway was indeed at 12:00.  Slots extended and timing done.
Time for another carb check.
750 Bregenze
1200 Anniversary
180 Jota XGF354W #7284