Good morning All,
Which is best for low and midrange - two pipes or two in to one? I have tried to get my head around this but there seems to be opposite views. So, a single carb long stroke 750 A65 with, currently, a 32 mm Mk2 Concentric, two in to one with standard looking silencer with no obstructions. Also, is it worth doing anything to the pipes to stop those reverse pulses that are talked about?
Dave.
I think the gains you would receive will be hard to find. The silencer will make a big difference though.
When I first fitted a 2-1 with mine I found I was up and down the box a lot, clubman silencer etc. Sadly like most after market pipes they use the same over size pipe on the exit of the silencer as what they use for the sleeve at the front, instead of using a smaller size. I was constantly having to keep the revs high like 5k up if I wanted the bike to shift.... so I sleeved the outlet pipe, incidentally with 2 pipes. The first was a length of front pipe which had been cut off, the second was the next size down. At the time I was reading one of my tuning books and it was discussing the outlet pipe wants to be around 1/8th smaller than the inlet pipe (I took this as being 1/8 in a proportionate amount and not 1/8") it worked very well and gave a lot of midrange, apply just the right amount of back pressure into the cylinder.
The next one I would do is ADVANCE the cam a tooth. One tooth is about 16.5 degrees, one of the crank pinions I have advanced the crank 20 degrees when the marks are aligned, this also gave a lot of midrange and didn't effect top end (at that time the bike was good for 105) when I started doing cylinder head mods I changed the cam position back to standard which then gave more at top end.
Hi Allan,
Thanks for the reply. Is the silencer outlet only dependant on the pipe size or is cc, bore/stroke relevant? Also, reducing silencer outlet by a 1/8th proportionally on a 1 1/2" pipe is a reduction of 3/16" total so an ID of 1 5/16" is needed. Is that correct? Does this apply to two pipes as well as siamese?
Which way to rotate to advance the cam, clockwise or anti-clockwise?
Sorry for all the questions.
Dave.
Hi Dave, from what I've read the size is relative to the inlet pipe size, 3/16" looks and sounds right from what I calculated at the time. I stated to be more specific that it should
Be the same or smaller up to 1/8 but not any bigger. The wall tube I was using at the time was 1.5mm so I was closer to 1/8"≈ 3mm. It would apply for single or twin silencers, with the Siamese you'd have almost a constant pulse or flow through the one pipe (staggered slightly by the length of the left pipe
Being slightly longer) individual silencers would be more pop pop pop with or without the balance pipe it's location and angle isn't enough to have much effect for balancing flow (my previous job was making stainless exhausts for cars)
For advancing the cam from memory your want to rotate the cam clockwise 1 tooth. Give me
A few mins to get the crayons out and reassure myself. I did mark one pinion up but I think that's the one I'm currently using.
Hi Dave, yes you would turn the cam one tooth forward.
Naturally whip the plugs out and turn the engine over in 4th making sure there's no valves clinking the pistons. The piston speed of the long stroke will be greater than a standard A65 so I don't know how close this will get with your setup. On a standard A65 I've found 2 teeth (33 degrees) still don't catch with high compression pistons.
Thanks Allan.
I think I will set the 750 as it was so that I can reacquaint myself with how it ran and then be able to compare when making changes and probably try some new ideas on the 650 including C/R 2nd pinions in place of STD 3rd.
The most I ever got from my 650 was an indicated 95 mph. That was loaded up with camping gear and sitting upright, low compression and it was incredibly smooth at that speed and sounded great.
Rather than top end I am more interested in low/mid range and economy. With the speed limits abroad and the traffic at home there is little legal opportunity for ton plus riding.
Dave.
I agree Dave, it's nice to have the legs at top end but I'd much prefer more tractable running and better economy. 95 with luggage and low comp pistons is a good output though. I was getting that solo before I fitted the higher compression pistons.
I would have thought the long stroke would have had a decent midrange (over and above a stock A65) for the same reasons as you I have fitted a touring cam (same as what we know as used on the A50's but common placement on all late A10 and early A65) to the 823cc Thunderbolt motor. I'm not bothered about top speed, just want plenty of torque for 2-up with luggage
Hi Allan,
823cc so bored and stroked. What carb and does it start OK on the kick start? The PO of my 750 tried and 850 configuration but found starting it very difficult.
I have some big bore barrels so may try this in future or go the whole hog to 900+ with the Norton crank that I have. I think with this only 3 gears would be needed.
Dave.
It's still in build stage at the moment, bottom end is together almost, just need to save up for the Bob Newby Clutch basket (I already have the front portion as it was sent away for balancing)
The bike was a us import with no nova, so I've had to assemble to to make sure everything fits etc before I strip it again and do all the paint. So hopefully sometime within the next 6 months should
See it on the road.
I plan fitting a 928, but the pair of 928's I have, have gone walk about so it might be a 930. It's got an A10 crank, still using the TS Bush, crank is balanced, but don't ask what factor, I have a load of paper work with all the weights etc but it doesn't state factor, but I assume i can work it out from the numbers on there. Bassett down did the balance work. Pistons are 79mm JE B44 pistons I bought from PES (aka CCM Great Britain) and I'm using a John Hill Alloy barrel. It comes with 2 gaskets so you can lower the compression if you wish with the thicker gasket, as I'm using this as a tourer I want the compression as low as I can get away with, trying to balance a bike with a passenger and luggage isn't easy... I might change my mind if I can get an electric boot for it... but until then.
The starting does concern me, cranking my dads super rocket over with 8.25:1 pistons is harder than my A65 at over 10:1, but I did find even that got easier with time and practice.
My 750 has an A70 crank but with T140 pistons. They are a bit shorter than the A70 pistons so give about 8.5:1 compression which seems to be enough. With today's fuel it is better to err on the side of caution re pinking etc.
I hope to have it ready incase there any camps towards the end of the calendar. If I find that we like it as we did before then I will tidy up the mudguards etc.
Dave.
Fingers crossed there will be some chance to use it. High Compression is cheap torque across the whole range but if your only wanting to target certain points then there are multiple ways around it. However nothing compensates for a poor exhaust (ie the outlet being too large)
I think that I will make a tube to go in the outlet of the standard type silencer and go on a run then fit the tube and re-run the run. An easy way to get a direct comparison. Also, is there any mileage in putting something in the header pipe to counter the reverse pulse?
Dave.
If you fit a sleeve or blank off/weld up a lower portion of the port it will make a big change, however probably not worth it.
I have done this with my custom 2-1 with original Dunstall silencer. Before welding up the lower part the bike would ride quite nice but at a certain rpms the bike would take off. The silencer is about 27" from the valve if measure from the rhs pipe.
With the welded section it does give a bit more torque at the bottom end but you don't get that sudden kick in the pants when it takes off like I did before. The bike still pulls well and it doesn't seem to have effected top end. But it has changed the exhaust note, it also made the mixture much leaner and so far I've had to change from a #3 slide to a #4, otherwise it just fouled plugs. No change required for needle position though.
You could try a sleeve like the 69 on firebird, but just inside the pipe,
Any idea what the sleeve looked like? This could also be an easy thing to try.
Dave.
The sleeve on the firebird was on the outside to allow a 1 3/8" pipe to be fitted to a standard head. You could tack a sleeve to the inside of the pipe about an inch long, this is what I should have done with mine. But it works well enough
Thanks Allan,
I will keep that in my stock of things to try. The tube in the end of the silencer will be first as it is easiest to do and reverse.
Interestingly I have a s/s Goldie type silencer on my 650 and the exit is huge but it sounds great with a distinct burble when shutting off. There must be an exit size that is big enough to almost make the silencer a straight through system.
Dave.