hi again I have fitted a concentric 932 to my a10 it has high compression pistons a spitfire cam was reworked by srm with a bearing upgrade and billeted conrods there oil pump etc. the thing is i cannot seem to get it to do more than 80 miles an hour this seems very low to me surely these were quicker than that? i have now polished out the inlet to match the amal 932 carb spaced it with a tufnell spacer. does anyone have this carb on theres and if so what jets are you running please mine has a short needle and a 220 main it is running incredibly rich and also very lumpy any thoughts on this would be greatly appreciated thanks mark.
If it's any consolation you've just made the bike slower by opening the port out to match the 32mm carb... also why have you fitted a 32mm? A super rocket port is 1 5/32 which is 29.5mm. By making the port bigger you slow the air speed down in the port meaning that by the time air speed gets up the valve is already closing so your getting less into the cylinder.
Before making any permanent adjustments like that.
Check the down pipe doesn't enter the silencer any more than 2-3" (fit the silencer, make a pen mark where the silencer ends at the pipe. Remove the silencer and cut off anything that's greater than 2". If it's an armours pipe then I've found they tend to be on the long side. Made my bike do about 70 mph till i clipped it short, did 95 on soft Compression pistons after that.
Check ignition timing.
Check valve timing, do not tighten the valve clearances. It was recommended to use 0.010 inlet and 0.012 gaps in the interest of a sportier ride at the sake of some more valve Noise (I've used these settings and find no difference in noise)
Check valve timing with a dual gauge (not just the timing marks on the gears)
Also make sure you haven't over geared the bike.
Ensure the crank, timing pinions, cam, gearbox every moving part turns freely with no binding
Hi,
Tappets should be 0.010" inlet and 0.016" exhaust if maximum power is required.
Best wishes...
:) :) :)
Running rich and lumpy???have you got the right throttle in.???
Mike 8) 8) 8)
thanks for all the comments , i am not sure i agree with Allen the vortex or differential in air pressure happens inside the carb body hence the squeeze in the middle opening up the port actually increases the draw from the vacuum caused by the piston,the important bit is surely jetting and needle length i will open the tappet gap as suggested as i am running .008 on both so obviously to tight .thank you all it has given me some ideas and hopefully soon some speed,
Hi Mark,
Before you dismiss Allans comment regarding porting maybe worth reading the Eddie Dow 'Twin Tips' regarding porting. I have also attached the BSA service sheet 70 regarding timing and tappets, 10 and 16 thou for maximum power.
Best wishes...
Thanks for the Back up Bess,
I read the Roland pike memoirs and what he did with the Gold stars about 10 years ago. I have done a lot of research at my own expense and modified several A65 heads, the smaller port works best. Every time. The best head I modified was a small port A65 head but with the ports reduced further by filling the port floor. That thing had so much power and it was hard to jet the Jet the bike lean enough at the same time. Throttle response was excellent.
BSA weren't stupid during the mid 50's and up to mid 60's, they knew that smaller valves (even the big valve super rocket had smaller valves than a post 66 A65) they knew what made power and got good speeds from modest compression bikes. Triumph were the same. Advertisement ruined this by demanding bigger carbs,ports and valves they did and knocked at least 20mph off the same bike.
I'll elaborate on my earlier comment, I was trying to keep it simple as I've made the same post many times over.
When the valve opens, before you place enough "signal" as the jet to draw fuel through the pressure within the port needs to be equal, as does the pressure under the "tea cup" area of the slide. If you have a large port it takes a Long time for this to happen, there fore you have a long period of time before there is enough signal on the carb to draw through the right amount of fuel. The air speed is also very slow.
If you reduce the space within the port, more specifically the slow areas within the port, there is less area for pressure to equalise thus the act upon the carb is much quicker, you also get a much better throttle response.
Another way of looking at this is the cyclonic action on a Dyson vacuum cleaner.
THANKS BESS FOR THE SETTINGS FOR TIMING AND THE INTERESTING READ ON CYLINDER HEADS, ON THAT EDDY DOW ONE HE IS TALKING ABOUT THE THROAT THAT IS AT THE VALVE END WHICH MAKES ABSOLUTE SENSE NOT TO OPEN THAT UP, NOT THE END WHERE THE CARB CONNECTS TO. SORRY FOR THE CAPS I ONLY JUST REALISED I AM NOT SHOUTING AT YOU LOL. WILL LET YOU KNOW WHAT THE OUTCOME IS. At the worst i can always put in a sleeve .
Hi,
You're welcome, a quick read I maybe wrong on your set up :-\
More airflow does not always make a faster bike even if air flow gains are made, the bike may end up being slower on the road.
If a head is made to flow more air it doesn't mean the engine will have that quantity of air movement. The head doesn't move the air, the suction of the engine does. If the engine can't draw the air from the larger port there's no gain, especially at low revs. Its velocity you're after and increasing the diameter will reduce the air speed and effect mixture in the cylinder.
Hopefully the alteration hasn't affected velocity. Let us know how you get on, your issue is probably something simpler.
Best wishes...
carburation is a strange thing, my 5 litre Firebird was fast enough but had its performance improved by a small modification to the intake system ans enlarging the exhaust. modern cars have their cat converters removed.
theory is bigger exhaust means exhaust is let out quicker, the result means you have to put it in faster.
so it seems you need to enlarge the exhaust port and fit a bigger unrestricted exhaust. the downside obviously , is the more you get out, the more you have to put in, this is why sports vehicles use more fuel but go faster.
my v6 3.2 petrol vectra is faster than the 1.9 Insignia diesel, but the insignia if driven economically does twice the distance.. or the vectra takes 4 tanks of fuel to get to France and back, the Insignia can get there and back on a single tank. We have done it once, same route.
:) :) :)
Sorry, I missed a word in my previous post. It should read "do you have the correct throttle valve in"
Mike 8) 8) 8)
Hi,
Thinking about Allan's last post, I thought about a drinking straw. Did the conventional straw bore come about by accident or was it calculated that this was the best for the full range of 'suck' ability. If the straw was a bigger sized tube it would take a lot of effort to draw the fluid up until it would be too big and nothing would be drawn up. And if it was too small a lot of effort would be needed to draw any fluid up. So it looks like this balance is vital to getting the best from our engines.
The engineer I use for doing valve guides/seats etc. also does dragster engines and is also an advocate of smaller ports and valves.
Dave.
That makes sense in that scenario dave but Bsa introduced the lighter alloy cylinder head with much bigger valves on the exhaust and slightly bigger valves on the inlet. so i guess their design engineers came to the conclusion that there was enough suck and blow to make this effective,i have increased the piston ratio and put in a spitfire cam all this to try and increase the blow and suck,i will post my results good or bad in a few weeks all the best mark.
Yes, valves a bit bigger for the Spitfire cam but still relatively small. The A65 having the Spitfire cam profile as standard and bigger valves again. A few more revs needed to get the shorter stroke to work. I am trying to decide if 30mm or 32mm is best for my long stroke 750 A65. With roughly the same bore and stroke, the Triumph TR7 ran a single 30mm so I really need to decide if I want bottom end and mid-range or top end.
The engine ran quite well with a 32mm MK2 conc. but was a bit thirsty.
Dave.
Right at the start you say your carb has a short needle - does this mean a short jet needle?
If yes then the short needle was superceded 50 years ago when the needle, needle jet and jet holder designs were changed. If you have a short needle with long needle parts you will get problems.
See what Amal say;
http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter