Hitchcocks Forum

Hello all,
was on a few weeks ago when I was trying to improve my compression and found that I'd a 350cc Inlet valve in a 500cc Head. I've never previously replaced that valve so maybe it came from a previous owner. I did have the larger exhaust valve fitted so perhaps when that was fitted there was some confusion.

Anyhow I've had the proper inlet valve in now a few weeks had to drop down the main jet a size as it was a little black on the plug test. Now I have a bike with acceleration, a much higher top speed and it eats hills

That all sounds good but we all know that's going to come at a price. Well this week I decided I'd work out the cost. Basically my mpg has gone down from the mid 70s to 43mpg. I have to say I didn't expect that much of a hit. It's my regular commute bike, bread and butter, and my happy justification was almost 80mpg, Obviously given the change in performance the 350cc valve, which sort of slipped into the throat of the valve opening was restricting the amount of fuel/air that was making it into the combustion chamber.

So the question how do I restrict the inlet flow to get a few miles back? I've an amal concentric Mk1 carb with I think the 30mm opening. Think it's a 930. Do I get a carb with a smaller opening? Change the cams?

Don't tell me to change my riding style If the juice is there I can't help using it.

Something doesn't sound right here - your fuel consumption should stay close to what it was before, bearing in mind you have made the engine more efficient and it is making [or should be making] better use of the fuel / air mix going into it. A 350 inlet valve is shorter than a 500 one, but if that's what you say you had in there, you certainly don't want another one in there.
Your jetting or float height might be out, or maybe you have a binding brake or something. the fuel consumption has not risen to that extent because you have improved the engine's mechanical condition and efficiency.

Hi JFW ---------- just a "quickie" check for you, bearing in mind you've had the head/carb assy. apart and that is}--- re-visit the air passageways/filter set-up.... always assuming your method of calculating the MPG is accurate and the same as used before....You might have altered or partially blocked the air filter etc OR }---- is there a fuel leak etc..... To paraphrase Admiral Beatty at the WW1 Battle of Jutland }----" There's something wrong with our bloody Bullets to-day !!"...... Just as an aside, this may interest you ??------ http://en.wikipedia.org/wiki/Battle_of_Jutland ---------- Worse things happen at sea, so they say !!

Thanks for the advice lads,

Still working through Jutland. I'm off tomorrow so I'll see how free the wheels are but my train of thought was that as the 350 valve was actually "seating" inside the throat that the cam had to lift the valve say 5mm before it was becoming open. Thus the inlet was much more restrictive to air/fuel flow then the current 500 valve which when the cam lifts it 5mm it's well and truly open.



I'll have to do some things to see if I can't get some miles back.

Hi JFW. You've sort of answered your question yourself. What ever the problems were to cause the low performance they were commensurate with a good fuel economy. Now you have the engine able to take in more fuel and therefore create more power, you say you are using it by using the improved acceleration and flying up hills. This explains the higher fuel consumption. A 30 mm carb is unnecessarily large with STD compression ratio and STD cams so the air speed through it will not be optimum, especially when you give the bike a good handful of throttle and fuel atomisation will be poor. Un-atomised fuel will tend to make a bike run richer but with no extra power, so I reckon you could get enough commuting power even if you drop down to the smaller 350cc size Micarb carb which is just 26mm choke size or maybe just use the std 28mm version. However, with the 26mm carb you will always be getting good high speed air flow through it even if you suddenly wind it wide open, which will often be the case if you accelerate hard from traffic lights on your daily journey. You won't have the same maximum power but jetted correctly it won't be much less but the fuel economy could be some percent better off for most journeys. The carbs can be bought for around £40 but probably loads available second hand for less than 20 quid. Only problem is that your inlet manifold is likely to be for the Amal so you need the Micarb version. If you do get a cheap 26mm Micarb the manifold should be tapered and blended to match the inlet port size which is likely to be a little wider for best results. Just an idea this but if fuel economy is important to you, you might well gets a very good compromise between adequate power and cheaper fuel bills.

Just realised the 350cc Micarb has a 24mm bore and not the 26mm venturi as I mentioned above. Having corrected that I still think the 24mm carb might work very well in producing a very fuel efficient 500 engine and still be sufficient in size to produce over 20 bhp (Still more than the std. 350)...See this interesting Mikuni carb guide and see fig.11 on page 7 ...........http://www.mikunipower.com/Manuals/VM_Manual.pdf

Hi guys-------have just been into the workshop and measured 2 spare 500 heads. Both inlet tracts--although rough cast--- approximate to 30mm dia....so this should match in diameter terms with the 930 Mk1 Concentric.( Is this a conversion kit from our hosts, maybe ?? Their kit uses a 30mm Mk1 for the Classic 500. You could try phoning them to ask the jetting sizes etc ... can't find that stuff in the current catalogue, only the Mikcarb ones)... The standard Mikcarb for the Classic 500 is the VM28 (28mm), which along with the "Miser cams", VERY low CR (6.5:1),LONG)TOM exhaust and restrictive air filtering, gives "rice pudding skin" performance.... During this last year I, & others (RIGGERS for one) have used a modified arrangement of }---- TEA CADDY filter, VM28 carb with uprated jetting (Details upon request), STD. valves and timing, 7.5:1 CR and semi-free flow exhaust sparked by an IRRIDIUM plug and timed by CB points at 0.8mm BTDC........From this I was getting an AVERAGE of 80mpg with daily results of 70mpg.... So you see JFW we need to know the FULL STORY of how your bike is set up to offer any real help.... I agree fully with LES re:-- the 350 valve was artificially restricting the gas-flow due to less opening, but think the 24mm Mikcarb would be over (or under-)doing it. The std. VM28 will give a good account of itself for commuting..........They're not a bad cab for general use and the 500 manifold, with its' rubber tubes, allows you to change jets in situ.

Hi guys------- I apologise in advance but, looking at this thread heading I had to draw you to this !!!! }---------------- http://en.wikipedia.org/wiki/Miles_Aircraft ------------------ My late father was employed there after WW2 building the wings on the Magister, whilst "Mum-to-Be" (once he plucked up the courage to ask her out--) was in the Office..... One of her brothers (Stan) worked in a factory unit just across the Industrial Estate at Woodley Aerodrome as a development Engineer on something called a "Biro"??? ------------- http://aebrain.blogspot.co.uk/2004/05/m ... -time.html --------- ENJOY

43 mpg suggests a excessively rich mixture unless you are riding at huge speeds. The 500 single should be able to get a reliable 65-70 mpg. My Ducati 860 gets 50-55 and it is a very heavy machine.

being a firm believer in not mending things till they are broke the Infamous Bullet 500 has standard valves, cams & piston, VM28 Mikcarb with std tea caddy filter. Non standard pipe & "silencer" (Carburation slightly enriched to compensate), boyer ignition, iridium plug, one up on the final drive sprocket.....around 85 mpg hauling us fully freighted at motorway speeds, up hill, down dale etc...........So, reasonably priced VM 28 carbs are available in fact there must be plenty discarded by contibutors to this illustrious site, why not revert to stadard & start from there