SOHC.co.uk Forums > The Black Bomber Board
cold starting
McCabe-Thiele (Ted):
--- Quote from: Oddjob on March 20, 2025, 11:45:56 AM ---It's a bomber Ted, it's a twin cylinder bike ;) ;)
--- End quote ---
Oops I was getting confused with Mikes 550, my bad.
I'll get my coat....
K2-K6:
Some further points in considering what may be causing this problem.
Try starting as normal BUT with a spare plug connected to one of the leads. Leave the fitted one's in situ though, this to check if there's spark from ignition under cranking load with both cylinder in compression. Its more of a battery check and its capability to supply starter AND ignition circuit simultaneously. Some ignition system are susceptible to voltage "floor" which could be being breeched at cold start attempt.
Digital volt meter on battery to watch the peak drop would help for information too.
Could try connecting it to a car with jump lead to attempt start, that to give higher voltage in regards to above analysis. Is kick starting possible ?
Seems like fuelling is reliably there as the plugs get wet. Possibly too much, possibly spark absent under load as discussed above.
If plugs will run in normal conditions, I'd not have great suspicion of them as faulty.
mo goldie:
Hi All
Well thanks for all the suggestions as I will have to try them out, i did start her up today from cold and it did start...but on one side the right so took the left plug out and done the blow lamp job on it and fired on both, Now I have done the changing over of plugs and tried New plugs as i have 3 sets did Not make any difference so Not plugs that are at fault,even went and changed the coils out at one point still No difference.
getting back to what K20-K6 about the springs I never had springs in there when I got the Bike but as Ashley said about the kit you could get.....or should I say used to get due to the 4,000 flat spot I did get some springs in the end and put them in, but I must say I never had it running without springs so don`t Know if Not having them would be a good thing to try them without them ? so at some point I will have to give that a try. also I will have to get round again to do a compressions test and check that out too.
McCabe-Thiele (Ted):
Reading this thread I've learnt that the Honda CB450 is firstly a two cylinder bike, I had a look at the manual in Ash's dropbox to see what the carburettor design was like.
At first glance it reminded me of the old SU design fitted extensively on British Cars, I'm assuming that the carb here has butterflys that when opened cause a vacuum to raise the needle, Mo Goldie has a springs fitted presumably to dampen the carb piston unit response.
This quote based on Wiki information explains how the SU design works, interestingly it mentions motor cycle applications, none of this is likely to be of any help with the cold start issues. I just found it interesting, others here might too. Back in the day I would tinker with twin carb'd Minis, changing spring colours, using thinner damping fluids plus sometimes fitting different needles.
My quickest ever ADO15 Mini was a 1310 S that was extensively tuned with BL 731 camshaft, mirror finish gas flowed cylinder head etc, etc.
The strange thing was I fitted twin 1.5" Stromberg Carbs rather than the SU ones, it was called an Alexander Conversion. A bit off topic I know but some here will also have /had/owned similar tweaked vehicles. I found the Strombergs superior in my particular engine, certainly not saying they are any better.
SU carburettors feature a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburettor.[20] Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand. The exact dimensions of the taper are tailored during engine development.
The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides lifts the piston. Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising. Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.
If the airflow into the engine is increased - by opening the throttle plate (also known as the "butterfly"), or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is pushed upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the rate of air delivery.
Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.
This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor.
To prevent erratic and sudden movements of the piston it is damped by light oil (20W Grade) in a dashpot, which requires periodic replenishment. The damping is asymmetrical: it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburettors by temporarily increasing the speed of air through the venturi when the throttle is suddenly opened, thus increasing the richness of the mixture.
SU carburettors do not have a conventional choke flap, which in a fixed-jet carburettor enriches the mixture for starting the engine from cold by restricting the air supply upstream of the venturi. Instead a mechanism lowers the jet assembly, which has the same effect as the needle rising in normal operation - increasing the supply of fuel so that the carburettor will deliver an enriched mixture at all engine speeds and throttle positions. The 'choke' mechanism on an SU carburettor usually also incorporates a system for holding the throttle plate slightly open to raise the engine's idling speed and prevent stalling at low speeds due to a rich mixture.
The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine screw (26TPI for most pre-HIF versions). At first sight, the principle appears to bear a similarity to that of the slide carburettor, which was previously used on many motorcycles. The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor. The piston in a slide carburettor is controlled by the operator's demands rather than the demands of the engine. This means that the metering of the fuel can be inaccurate unless the vehicle is travelling at a constant speed at a constant throttle setting - conditions rarely encountered except on motorways. This inaccuracy results in fuel waste, particularly as the carburettor must be set slightly rich to avoid a lean condition (which can cause engine damage). For this reason Japanese motorcycle manufacturers ceased to fit slide carburettors and substituted constant-depression carburettors, which are essentially miniature SUs. It is also possible - indeed, easy - to retrofit an SU carburettor to a bike that was originally manufactured with a slide carburettor, and obtain improved fuel economy and more tractable low-speed behaviour.
One of the downsides of the constant depression carburettor is in high performance applications. Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch. By contrast, the fixed choke design adds extra fuel under these conditions using its accelerator pump.
K2-K6:
Absolutely spot on Ted, they are all but the same principle with only the seal (rubber diaphragm vs SU labyrinth) and air damping vs oil type, but operation is the same for all three mentioned.
One observation in "At first glance it reminded me of the old SU design fitted extensively on British Cars, I'm assuming that the carb here has butterflys that when opened cause a vacuum to raise the needle, Mo Goldie has a springs fitted presumably to dampen the carb piston unit response." this sentence though to clarify, the spring doesn't "damp" as such but changes the resistance against a force.
In this case it changes now far the slide ultimately moves proportional to the vacuum imparted onto it via the slide.
The effect of this is to make it richer if heavier spring is used, or leaner if a reduced spring is used. Thats without changing needle or jet size.
I don't know if they were originally run without a spring and one was specified to adapt OR if it was originally fitted with a different spec spring, then changed to the one Ash advised. Ash would be the source of that detail though in confirming what's what.
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