Megacycle 250-40

jay760

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Has anybody else found the 250-40 raise the compression so much it's vertually impossible to turn over let alone start, I have je pistons I've tried this cam in this engine several times now keep hoping the motorcycle fairies have been in and made it better but every time I put it in I can just about kick it between strokes that's before I put the ignition on!
 
Turns over easily with the plugs out, checked clearance during building the top end, got 35 thou between the piston and head around the bottom of the dome, plenty on the valves
I normally run an unknown cam which seems to be almost a cross between a 30 and 20 with higher lift on the inlet but much more than the 20 but it is very rounded so I assume will keep the valves open more for longer.
 
Hey, Jay. Coupla questions.

- What crank/conrod combo you running?
- Did you degree the cam, and confirm IVC?
- And, what is your IVC?
- What intake valve lash?

I have the Megacycles cam charts. Got some numbers for you, will post later...
 

Whups. Sorry azman. I was buried in cam spec sheets, and that blurted out.

IVC = Intake Valve Close, the number of degrees past Bottom Dead Center when the intake valve closes.

The compression stroke is a funny guy. And compression ratio, CR, is another character.

You've got your mechanical, or static CR, based on the ratio (stroke volume + combustion chamber volume) / (combustion chamber volume).

Then you've got your dynamic CRs. Occurring at various rpms with their various volumetric efficiencies (VE).

For Jay's kickstart scenario, a strong influence is when the intake valve actually seats, and seals the cylinder. The cam grinders publish their valve events with some pre-defined valve openings, like 0.050", 0.040", 0.012" (Yamaha value). Those give you a fair idea of engine performance at higher rpms, but don't help much when trying to find the actual valve closure.

Long, slow ramps will hold the valve off the seat well beyond the published spec, making the kickstart stroke short, reducing effort. Influenced by the valve lash setting. Modern fast ramps will have the valve closure closer to the published spec, increasing kickstart CR and effort.

Hot, high-rpm cams usually have longer durations, and will leave the intake valve open quite a ways past BDC, reducing the kickstart effort.

For example, on the XS650, here's the approximate distances the piston travels from BDC for various IVC (Intake Valve Closure) values.

50° ABDC - 10mm
60° ABDC - 15mm
70° ABDC - 20mm
80° ABDC - 26mm

Take those values away from the stock 74mm stroke, and you can see how the residual stroke reduces the effective CR during kickstart. With reduced valve lash, holding the valve off the seat even longer, the kickstart CR really comes down.

Hopefully this helps Jay figure out his kickstart issue. To be sure, need to get the actual IVC from his cams.

Other things that can help reduce kickstart efforts:

- The "256" transmission, with its taller 1st gear, and matching kickstart gear.
- The "high-ratio primary" gearset.
- Idle, low-speed ignition timing set back aways.
 
Megacycle cam profiles 650.PNG
 
@jay760

Did you get at the end the 40 profile to work for you?
If yes what was the culprit/problem?

Kind regards from Italy
Christian

Ps: sorry darn double post, something doesn't seem to work ok on my cell.
 
hi Christian, welcome aboard, it seems like the like the SR500 forum is no longer with us, I always enjoyed your contributions to the other forum, so hope you stick around, all good people here too.
 
@jay760
Just for curiosities sake: did you get at the end the cam to work?

Kind regards


Christian

Did give it another go last year, but was assuming it was somewhere about right as far as timing as it was already timed and marked by the previous owner, I had to take out part of the inlet valve pocket to make it miss, this year I bought some tools to do it and found it was 10 degrees out, lesson to be learnt on assuming the po had done it right still waiting to try it though as all racing has been cancelled so far this year, least I know theres good clearance this time as I can get a small Allen key between the valve and piston.
 
@jay760

Good to hear, I would heavily reccommend relying on megacycles timing recommendations as Jim dour and his crews know their piece of cake. At the most I would see +/- 4degree changes on Ic point in order to accommodate to track.

Btw: 10 degrees off on Ic is a whole lot in terms of engine characteristics.

All the best have a good weekend and kind regards

Christian

Ps: in case the -40 ever ticks you off too many ch please keep me in your rooster as I'm looking for one on the long run.
 
@stinky78

Thanks a lot for the warm welcome, Ive been following this forum already for years but due to workload family etc I always hesitated to enroll. Funny as I own an Xs already for 20years.
I know the fellas here are good people or else I wouldn't have enrolled for chatting.

All the best and kind regards from Italy
Christian
 
You mean the XS Perfrormance billet cam, Joe? The grind on that is a clone of the Megacycle 250-00. I made the mistake of buying one (it should have been obvious to me that the price was too low for quality), so what follows is based on personal experience.

The sprocket has 2 holes and 2 slots and mounts on two 7 mm. screws. If you trust a 2-point mount not to shift (nobody else does, other adjustable sprocket setups use a 3-point mount) you can use the slots to dial in. If you want to play it safe and use the holes, you're forced to tweak the deck height to set valve timing. And that's not the only problem. The sprocket screws mount in a pair of big, thick steel lobes 180* apart on the camshaft, which means they swap places every 360* of crank rotation; if your motor is too smooth and you want it to double as a paint shaker, install that camshaft.

Bottom line: after a little bit of use I pulled that cam and reinstalled a Shell #1. I gave the XS Performance cam away and wouldn't take money for it because I considered it worthless. It just goes to show that you can't get something for nothing, but it's easy to get nothing for something if you buy cheap enough. .
 
After spending ages timing the 250-40 cam up I took it out last weekend haven't gained a bit, in fact I'm going through the end 5 mph slower even tried not changing into 5th to let the engine rev harder still need difference, 12.2 at 105mph think I should have left it where it was and took a bit more out of the valve pockets, much as it sounds stupid, I'll try it again in September if I cant get any more gain that's what will be happening over winter.
 
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