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For anyone that's never heard one in person, and XR750 is just one of the most awesome sounding motorcycles I've ever heard. It just screams "race bike" which is weird coming from an HD.

I heard one at a hill climb that was incredible, twin magnetos, buring nitro etc. Thing sounded like it was reving to 12,000 rpm ...which I assume is impossible for an XR.

Anyone here knowledgable about XR's? Do they have the same crankshaft arrangement as a standard HD? They sound completely different to me, like the firing arrangement is different in some way, or the cylinders don't fire 45 degrees apart like a run of the mill HD.
JohnnyB
 

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For anyone that's never heard one in person, and XR750 is just one of the most awesome sounding motorcycles I've ever heard. It just screams "race bike" which is weird coming from an HD.

I heard one at a hill climb that was incredible, twin magnetos, buring nitro etc. Thing sounded like it was reving to 12,000 rpm ...which I assume is impossible for an XR.

Anyone here knowledgable about XR's? Do they have the same crankshaft arrangement as a standard HD? They sound completely different to me, like the firing arrangement is different in some way, or the cylinders don't fire 45 degrees apart like a run of the mill HD.
JohnnyB
 

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depends on the application, and the year, and the motor.

I don't know them real well, but there are ironhead XR750s and Alloy head XR750s. neither of the parts will swap over to an XL easily. There is also an XLR which is the KR bottom end and an Ironhead top end.

I think the valve angle is something like 68 degrees vs the XL's 90 degrees. Also the front and rear cylinder have their own pipes and carbs and are not sharing a carb like the XL.

I think guys who race flat track rephase them to act as close to big singles as they can. The road race xr750 at TO sounds different from any xr750 tracker I have ever heard. could just be the cams though too.
 

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i want to go to a flat track soon. some guys on the tv can do the drift with no the foot touch that is better skill to watch. gary mccoy, vale rossi, and mick doohan on the pave have the skill. the old kenny roberts flat track movies let us see the skill also. but the sound is music in that harley. the normal harley street bikes, not so much.
-p1j
 

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I heard at one time that an XR750 is a totally different beast from a standard Sportster ...different cases, cylinders, crank, everything.
JohnnyB
 

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Ahh flattrack. Full throttle then pitch it sideways. I did that once at Loudon in the FCB class. If I remember right it was followed by alot of bent parts.
Bob Cam
 

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Discussion Starter #11
quote:Originally posted by bob cam

Ahh flattrack. Full throttle then pitch it sideways. I did that once at Loudon in the FCB class. If I remember right it was followed by alot of bent parts.
Bob Cam
I like the saying as it reminded me of some times on the my old 88 TZ250.
 

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quote:Originally posted by jbranson

I heard at one time that an XR750 is a totally different beast from a standard Sportster ...different cases, cylinders, crank, everything.
JohnnyB
the cases are because the cylinders are too small to fit to a conventional XL bottom end. Not sure if it has a roller crank like the KRs do or if it uses the same crank. XLs are all 900s or 1000, the XR is 750 (and then 1000 for a short while), hence why the cylinders are smaller.

The more I think about it the more I think you might be right that it doesn't share anything in common with the XL other than the engine mounts.
 
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