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Sportster Motorcycle Motor - Top End Discuss Sportster Motorcycle Top End issues. Rockerboxes, Valves, Cylinders, Pistons, Rings, Lift Rods, etc...

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  #1  
Old 22nd August 2019
60Gunner's Avatar
60Gunner 60Gunner is offline
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Default Bigger valves

Is there any advantage to using bigger valves than already in the xb heads? Especially on stock 1200 pistons/cylinders? Or even 1250?
I was talking to someone about this the other day when we were discussing cleaning up the heads and blending the seats on the intake to improve flow and he was wondering about bigger valves.
He has stock 1200 pistons/cylinders and I think 551 cams. Maybe N4s.
Sorry for the double threads. Not sure what happened.

Last edited by 60Gunner; 22nd August 2019 at 15:52..
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Old 22nd August 2019
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Default

It will only help to the extent the current valve sizes are constraining the motor.

The stock valve sizes don't constrain the motor when the air demand is low (low to mid range rpm). So don't expect any gain there.

But even at high rpm, they won't help if something else is constraining the motor. Say for example you have an exhaust that chokes off the top end power (several exhausts do this, by the way). In that case, making the valves bigger isn't going to help you a bit. The valve sizes aren't what's holding you back, the exhaust is.

Always work on what holds the motor back. Working on anything else won't help.
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Old 22nd August 2019
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I think he expected more from his cams than he gets. He has CycleShacks and he's pretty sure 551 cams. He bought it last fall and that's what he was told. The guy told him they were big power cams.
He's coming over shortly and I'm pulling his cam cover to see. He's not too mechanical and Im trying to help him out. I didnt think headwork or bigger valves would help him. Im thinking he just needs better cams. He doesn't ride WOT high rpm that much but does like to open it up and seemed to be really impressed with mine so 570s or even 560s for better top end if he wants.
I think he would've been happier with the Ws even.
He does have an aftermarket ICM but didn't know which so I'll look at timing and his carb too.

Last edited by 60Gunner; 22nd August 2019 at 18:36..
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Old 22nd August 2019
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Ok, I just took his bike for a ride and I see what he's talking about. Now while the 551s aren't the greatest imo, (He verified they're 551s) I see people that have them say they pull hard all the way or they pull hard from 3000 all the way.
Honestly, I'm not seeing it. This is seat of the pants compared to mine. It's lacking. This is an 06, 21,000 miles. No smoke and quick compression test of 172/175. Plugs look good.
I wound it out to around 5500 a few times and while it seems to run good, I'm not impressed.
Can the cams intake close and overlap make that much difference? I'm not sure what to tell him or where to look next because it seems to have no noticeable issues.
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Old 22nd August 2019
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Well, I've never been impressed with them, but again, it comes down to what's holding the motor back. You have to figure out the constraint and work on that, because working on anything else won't do anything for you.

What exhaust is on the bike? What air cleaner? Is it tuned properly? Any of those things can easily constrain the motor.

Does his throttle get all the way open? Another common constraint is improperly adjusted throttle cables.
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Old 22nd August 2019
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CycleShacks, Plugs looked good. Jetted 45/180. Carb seems to function well but I didn't verify the throttle opening all the way. Stock air cleaner with the k&n. Stock breather setup. I did tell him a better AC and get the breathers to atmosphere.
It's the stock ICM with the SE Race Ignition Tuner. I'm familiar with it but it's running a canned map for SE cams. Lot of advance and early if I recall but less than the stock cam map. I think 33° by 3500 if I recall. That needs work im sure but damn, my bike stock was better with nothing but drag pipes.
I don't want to tell him better cams if that's not it but I believe its part of it. I just didnt expect it to be so noticeable and maybe part of it is timing and better AC.
Can the early intake close and overlap help that much with low and mid range with a good exhaust?
If i still had my Ws I'd throw them in to see if helped downstairs.

He couldn't stick around long today but will be back most likely Saturday.

Last edited by 60Gunner; 22nd August 2019 at 20:41..
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Old 24th August 2019
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Tuned this thing for him this morning. Shimmed the needle. Throttle opens all the way. Loaded a couple different maps I have for the SERT. Went with one with a max timing of 30 at 4500. He bought an SE AC. Not my choice but better. I also put my breather bolts in venting to atmosphere since I don't need them anymore.
Just doesn't have the get up and go at anything below 4500 rpm in comparison to mine. It will rev to 6500 where the rev limiter is at but even up there it's sure no better than mine. Seat of the pants that is.
Short of a dyno I can't say for sure.
I hate to recomend changing cams but I know he would definitely benefit in the low and midrange by 570s and all the way up with 560s. He's got decent enough exhaust for the overlap to really help. What are the 551s? 34°?
He's going to think it over.
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Old 24th August 2019
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Oh and they noisy as hell. Sounds like something is going to break if I didn't already know it was his cams.
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Old 24th August 2019
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Quote:
Originally Posted by 60Gunner View Post
Just doesn't have the get up and go at anything below 4500 rpm in comparison to mine. It will rev to 6500 where the rev limiter is at but even up there it's sure no better than mine. Seat of the pants that is.
Short of a dyno I can't say for sure.
You didn't run run down the road side by side to get a real comparison? Seems that's what most would do...
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Old 24th August 2019
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Quote:
Originally Posted by Tomcatt View Post
You didn't run run down the road side by side to get a real comparison? Seems that's what most would do...
Yes we did. First thing we did when we went to get him a new AC. And not even close. That's why I also used my butt dyno on both thinking he may not twist it as hard as I do. I also have a more responsive carb and my exhaust does better down there. Not to mention a fresh build 50cc more. But I pulled ahead from the start. Way ahead. And he just got further behind.
I don't want to judge it by the fact I'm more inclined to give it hell than he is. He's pretty green. My bike scares him WOT.
So its really not a fair test drag racing and I'm trying to be objective and find another reason for it. I can only do that using my butt dyno, not his too.
He wants his power early. He's never going to rev it that high. That's why I think even the Ws would be better. He likes the way mine jumps around 3500. His is somewhat better now after the tuning I did but nothing like mine.
I sometimes wish mine pulled a little harder up top but not enough to give up what I have in the middle where it's climbing 10hp in 300rpm. The torque hangs on enough to keep the HP in the mid 80s all the way up to 6500. Good enough for me. I thought about cutting my baffles down to 4" but I'm not sure it would help. It's not pushing back.
I don't wind it out much more than 5800 - 6000 or so anyway.

Last edited by 60Gunner; 24th August 2019 at 20:16..
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