SOHC.co.uk Forums > Project Board

Rolling Road recommendations

<< < (3/4) > >>

K2-K6:
My spell check changed "Degens" to "DeGeneres" and now edited in previous post  ;D

Hollywood obsessive spell checking overlord......what's the world coming to ?

Laverda Dave:

--- Quote from: K2-K6 on July 10, 2023, 02:40:14 PM ---Could it be worth an educated guess start point Dave ?

If so, I'd err on caution with base level ignition advance (if too much it may have contributed to nipping the pistons) based on shorter stroke.

Burn time would stay the same for an equivalent combustion chamber and mix, but the piston linear approach would be slower at any given rpm. The combustion time constant vs shift in timing angle measured in degrees doesnt show the full picture.

Effectively I'd see it as a shorter stroke needs less advance (in degrees) to match the combustion time of mixture burnt to prevent the whole event going off too far before tdc and trying to go backwards against forward inertia of crank etc.
The start point would be sensible at the lowest degrees specification to allow safer margin.
Jetting, some hold the view that excess fuel should be used to "cool" the combustion and compensate for other compromise in design spec, unless quantified its hard to support this.
Worthwhile taking 10% off (from existing Degens spec) main jet size, running conservstive advance (perhaps- 2 from lowest listed conventional spec)  and run on equivalent of NGK 7 heat range plugs to see what it'll give you.

Note:- you'd not use a 7 grade plug on a dyno as it's likely to overheat the ti0 and give you problems.

Getting hot under light load suggests static advance is too high as there's not much other input to it without wide open big load running conditions.  Promotes erratic tickover too.

Also, if the main jets are much above a decent range for that carb it starts to loose the fine metering effect in comparison to the needle taper, making it difficult to resolve apparent errors.

--- End quote ---

Thank for the advice Nigel. Your suggestions do sound like a good starting point. I am at Kempton Park on Saturday collecting some TEC shocks for a 400/4 so I will visit the Amal guy and get some additional jets. I'll knock back the timing to put it between the 650 & 750 setting as a base point. I'll get some plugs as well. I have no idea what CR is but I think it is high considering the hepolite pistons had to be machined to clear the squish band in the combustion chamber as built up by Degens. It really does take some kicking over however, if I take it over compression and turn the ignition off I get a very loud blowback through the carbs and I certainly wouldn't like to suffer a kickback on it, I'd have a broken ankle :-\!
Do you do house calls ;D

K2-K6:
I'd be fine with coming to look at it with you, a couple of heads together to consider a few things further. Let me know if you want to arrange something Dave.

He was an alternative thinker, Mr Degens, with some interesting angles on things that others may have dismissed.

Short stroke, bigger bore, high squish combustion geometry could be described as not that far from the current F1 engines in terms of what they are doing.
I know they are light years apart, but of very close axis in my view.
The current F1 is around 18:1 compression with single point injection to a chamber enclosure around the spark plug, this on first ignition sends out a flame shockwave to periphery of bore which serms to the burn back towards the centre, a little like squish but more advanced in concept. That's loosely speaking.
It's all fascinating stuff to see how others have set about solving some issues.

Sesman:
It may be worth measuring the CR as anything above say 9:1 on these old motors with modern fuels can be a real problem. I may have missed what jet sizes you are currently using, but I’ve been recommended 190s and I’m running twin premier 30’s on a splayed head. Might also be worth checking the valve timing; if DD believed you were performance oriented, he may have got radical?

Sesman:
Incidentally, are you sure both carbs are marked ‘R’? It’s just that they come as handed pairs ‘R’ and ‘L’? You’ll have a devil of a job accessing the idle screw adjustment on a parallel head arrangement. Also make sure the idle jet airways are scrupulously clean. Concentrics are very prone to clogging in this airway and Amal modified the 930 ‘modern’ concentrics to aid cleaning. Good luck and let’s us know how you get on.

Navigation

[0] Message Index

[#] Next page

[*] Previous page