Hello all, I have a new challenge, that is, to prepare and race a A65 in classic MX. I would like to concentrate on the engine to start with which according to the engine number, is from a 1968 lightning. The head is for a single carb and is standard with the correct size valves for that year. No carb fitted. Bores are good and on +040" with 10.5:1 pistons. The camshaft lobes have been welded up and reprofiled giving 0.354" lift when rotated on a lathe with a DTI. Think that it is more than standard lift? Don't know the timing. I'm wondering if there will be any gain from a higher lift cam being fed by a single carb through standard ports? Would appreciate any advice on setting up this engine. It is my intention to replace the timing side bush with a needle roller bearing and quill as well. Cheers
If you look at the pre 65 and 72 scrambles in Scotland the courses are mainly small with tight corners and not many long straights, a big A65 will have the power but will lose out badly to smaller bikes as there are few sections you can apply it. Also the extra weight will take its toll and leave you exhausted on the last lap, I would take Alex's advice and focus on brakes and most importantly losing weight. Otherwise like the TR6's etc you will be B44/B50 fodder.
You can reduce the port sizes and advance the cam timing by 1 tooth which will give loads of power at the bottom end and upto about 6000 rpm.
You can safely advance it further by another tooth (providing the cam timing is correct as per the manual in the first place using a standard spitfire cam) this might make the bottom end a little too fierce and will definitely spin the back wheel, especially if using a gearing less than 19:47.
As Alex says, get a cam with stock lift, you won't gain anything! I am fitting an SRM race cam into my motor. If I knew someone that could reduce the height of the lift I would be much happier with it, I bought it because I liked the cam timing/overlap.
Last edited by Allan Gill; 09/11/136:46 am. Reason: added image
Thanks lads, I have already taken care of building the bike a light as possible with lighter forks and wheels ( a huge improvement over the original BSA parts) but will be using the standard frame and swinging arm. I currently race a wee 441 Victor and now need something with a bit more power to pull my 16 stone around the track. Yes, the tracks can be a bit tight but it would be good to have the power when needed. I could have gone the TRIBSA route but I want to be a bit different with the A65. Haven't actually seen one at a race meeting,plenty unit and pre unit Triumphs and a very fast bloke on a A10. Cheers
Here's my 2c. Build it standard. One carb ok. Stock cam ok but make sure of what you have. Check the specs in the book and compute the centers with the cam in motor. Use a degree wheel and a dial indicator off the pushrod. Use .040 or .050 to check openings and closings and then compute the actual centers to make sure they match up with specs. The Web Camshafts site has a great procedure for doing this. You could experiment with opening sooner (advancing) but not a whole tooth. (That's about 14 degrees. Too much. A little is a lot here. Maybe 4 or 5 degrees). In order to do this you must use an offset key or multiple keyways in the cam gear ala Triumph. Significantly, get a set of pipes from a 1969-1970 Firebird Scrambler or build a 2 into 1 system like the ones used on the Triumph offroaders. These are smaller diameter and linked together and will fatten up your torque curve which will be useful in the dirt. This may be helpful. BSA Specs for lobe centers #473 cam. I - 98.5 E - 110.5. Cam lift (my measurement) .342 Duration at .040 262. Good luck. Have fun, PRT
On first reading (last night), I thought an A65 MXer sounded like a pretty crazy idea. But maybe these vintage MX courses are not as aggressive as modern-day MX. If you say people are doing it on TR6's and A10's, then maybe it's not so crazy. But I still wonder how you'll make the bike able to endure the abuse that an MXer must endure. Even with alloy wheels and front end and lightened bodywork, the weight will still be over 300lbs, and that's a lot of weight to drop without incurring damage.
In that vein, a 2-into-1 high exhaust system would be good for durability as well as performance. Running a complete-loss ignition system may also be a good idea; then you can get rid of the alternator and use one of those flat primary covers like they use on flattrackers.
i used to do pre65 , a guy (peter hollinshead)i raced with had a stock thunderbolt engine in a b50 modded frame, power was never a issue so i would leave it standard.he always finnished in the top 3 even against people like roger niel on their mettises.
Hello all, I am now back working on my A65. I have done the Beeza Bill needle roller conversion on the timing side and will be setting up the engine bottom end to be turned by my lathe to see if the oil goes where it should!
I have been thinking about removing cooling fins like the bike on the picture but have been wondering about the lack of cooling, or would this bike be running on methanol? Hope the picture shows. [img]https://www.flickr.com/photos/42573412@N07/5075003218[/img]