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Old 09-17-2020, 01:21 AM   #1
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Join Date: Oct 2017
Location: Portland OR
Default Mikuni HSRs worth all the fuss?

So Im running two SU HS6s with DX needles on a slightly built B20 (F head, nice header, slightly hot cam) and Im wondering whats the truth on Mikuni HSRs. Big power gains + efficiency gains? Or just efficiency? 42 vs 45 vs 48? I know they’re a lot easier to tune and have a far better overall design. Just wondering if worth the cost to upgrade, or to just get the old SUs professionally tuned and deal with the quirks.
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Old 09-17-2020, 07:45 AM   #2
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Join Date: Oct 2019
Location: Mississauga, ON (Canada)

If I'm remembering right, the HSRs are for motorcycle and other small engines... I've seen the dyno charts from v-performance on these, and honestly doesn't look like it's worth the money for a proper kit.

This one was a direct comparison between stock SUs (green) and the 42mm mikunis (red). They didn't specify what the blue line was, perhaps larger mikunis.

Wasn't a B20 being dynoed though. I think in their tests it was a B-series BMC engine with a hot cam and a mega-ported head, so I'm not sure you'd see the 13 hp difference in a lightly done-over B20. Also consider that peak power in the 'road test' condition for the dyno was 105 mph, so I'd assume they were revving this engine pretty hard.

EDIT: For what it's worth, assuming you're running a pretty stock head (not super shaved or ported or with big valves), you'd probably be better off sticking with the SUs.
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Last edited by fatcatbestcat; 09-17-2020 at 09:39 PM..
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Old 09-17-2020, 10:22 PM   #3
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Thanks for the advice! Definitely don’t want to spend money if it isn’t worth it. Ive learned to not exactly trust V Performance after some of the stories Ive heard about the shipping on their supercharger or carb setups. Guess Ill put the money into maybe putting a P1800 rear axle and disc rears later.
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Old 09-24-2020, 08:53 PM   #4
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there is a book " tuning bl A series motors" about mini motors by Vizard. There is one chapter just on air cleaners and bellmouths. The important one to you is s chapter just about getting SU's to flow more, almost as much as a weber sidedrafts. There are a lot of little cheap mods, like cut the throttle shaft in half and use buttonhead screws. Most of the data was for 1 1/4 or 1 1/2 sus but would still apply any of them.
He air flowed everything he did to show any improvement.
Well worth the read.
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