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L20b compatible Carbs


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I’m going to toss in my $.02, if it’s your first carbed vehicle stick with simple carbs that are easy to tune and don’t have to be constantly fucked with. My recommendation, as it seems you like the dual side draft look, is 38mm hitachi SU’s. Flat or bell top doesn’t matter, they’re functionally identical. They were made to run on sss engines, the l20b stock cam is basically an sss cam to feed the extra displacement. Factor that with the l20b being less rev happy, these carbs are right at home on the l20b. SU’s are set and forget, they self compensate for altitude, and perform better than stock down drafts. I’ve ran the same set on two l20b engines over 16 years, I’ve synchronized them twice, and didn’t touch them when swapping from one engine to the other with no issues at all. The fuel enrichment circuit works extremely well too, no cold start problems at 20 degrees F. I’ve always ran an AFR gauge with them and they’ve never run lean, from 7,000ft to sea level they run smooth. I am a full on hitachi SU slut, but for good reasons.

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RacerX - if and when that happens - i'll be sure to let you know.

 

Dolomite - Thanks for the suggestion - i'll definitely look into it. im a big fan of very little maintenance. the less I do - the better for me...and the car. lol but ya - these sound pretty awesome. thanks for you suggestion - if I do end finding and going with these, i'll let you know what I think of them. Thanks again.

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MazerRakum If you're still thinking about bike carbs, here's some stuff to pay attention to. Most motorcycle carbs including the R1 Yamaha's are CV (constant velocity). Not to get too complicated, but they have a rubber diaphragm that keeps the slide from opening too fast. CV's prevent a small displacement high revving engine from getting more fuel than it can handle which makes it bog. That means throttle response is much more dependent on engine vacuum than the physical throttle position. Slam the peddle quickly with CVs on a 2.0L, as vacuum builds the slides will open slower and the engine falls on it's face. Works great for little screamer applications with a very specific tune, but not for your big L series. I would stick to 38mm Mikuni Round Slide (VM) carbs. They're much easier to tune, and way more consistent than Flat Slide [TM] carbs. Not to mention you can get a set of them for under $400 and there are TONS of aftermarket parts for dirt cheep, oh and they can be bored to 40mm pretty damn easy too.  :devil:

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Sounds reasonable except this is how all constant velocity carbs work. The slight delay in opening is the carburetor equivalent of an accelerator pump. On SU style carburetors the damper oil viscosity limits how fast the slide rises so that sudden openings are prevented and the mixture goes richer initially. If there is a diaphragm controlling the R1 carbs then surely it can be changed/adjusted. An O2 sensor would help with this.

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True Mike, but while the SU's slide opens at a mechanically constant controlled rate, the diaphragm can literally hold the slide down indefinitely if there's enough vacuum. I had a 1200 Goldwing with CVs and if I hit the throttle WO in 5th to pass, it would go into a vacuum lock. Eventually it would balance out and the slides would open, but if there was a strong head wind or incline it could take literally over 10 seconds to catch up. 

 

in recommending (VM) carbs I'm also thinking in terms of simplicity and reliability. Finding the right needle shape and jetting for a CV carb is way more complex given the variability of a rubber diaphragm under various load conditions. The immediate feedback of simply adjusting the pilot main and needle jets as a linear expression of throttle position is much easier to dial in. Not to mention a much crisper throttle response on the road.

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