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Old 01-22-2013, 10:24 PM   #16
RGregor
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Quote:
Originally Posted by Airhead Wrangler View Post
Maybe not the carbs themselves, but the fact that he went to quite a bit of trouble to polish the hell out of them maybe. TM40s aren't usually that shiny.
Polishing the carbs was done by a company. It did cost ~40 for the pair. Don't remember exactly but it was cheap. I had my Dells polished there, too.

The TM40s work good.
Can't say if they perform better or worse than Dellortos. Not yet.
Currently I'm running 38mm Dells, I wanted to switch to 40mm Dells and now got a new pair of TM40s for a very reasonable price.
I'm planning to do some kind of cross-comparison for the three carbs on the dyno.
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Old 01-23-2013, 08:22 AM   #17
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I'm planning to do some kind of cross-comparison for the three carbs on the dyno.
I like that idea.

Maybe you should test a bing as well. It'd be interesting to see the difference
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Old 01-23-2013, 10:42 AM   #18
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Originally Posted by Airhead Wrangler View Post
I like that idea.

Maybe you should test a bing as well. It'd be interesting to see the difference
Sorry, but I'm not interested in Bings ....
Additionally it would mean quite some work to adapt the spigot (is that the right word? Anyway, mines are welded in) to the Bing with it's 42mm or 44mm throttle diameter.
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Old 01-23-2013, 11:27 AM   #19
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Sounds like a lot of work like we ever cared. At least the Dellorto's will jet damn near if not exactly alike I bet. Be sure and run a sniffer in order to help determine that you are not comparing jetting versus the carbs. As far as that carbs themselves are concerned, it sounds like you are going to compare them with the same spigots? The Mikuni might work best with an entirely different spigot but what works best with your setup is most important to you, of course. I suspect the biggest carb (the Mikuni) will make the most peak power. The tricky part is that peak power is just a small part of the rev range and first you have to GET there. The further you get away from peak power, the trickier getting reliable dyno readings get. Especially with wheel dynos. Carb tuning is SO much about just off idle and midrange. Wheel dyno's don't even do just off idle and midrange readings are so dependent on so many variables most serious tuners use engine dynos for that kind of testing and development. Unless one setup has a huge advantage that you can see on the dyno and feel on the bike, I would be more interested in what your butt dyno tells you about each carb setup. Some comparative lap times would be great too. When it comes to just off idle and midrange, what works best on the dyno very often isn't what works best outside of that room. The dyno contest winner won't be the fastest bike on the road or the track. Even if the track is a drag strip!

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Old 01-24-2013, 05:13 AM   #20
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My butt dyno is the motivation to test different carbs.
The engine is very strong at low and medium revs, but it feels as if the 38mm Dells already may be a bit small for high revs. Regarding the pure numbers from the chart the engine is strong at high revs also, but it doesn't feel aggressive enough for these numbers.
Can't describe it better.
I'll do the test without mechanical change just to get an impression, if that feeling is right.
If there is an improvement I will decide how to get the best mechanical setup for the carb in question.
And yes, I'll spend enough time to get a proper jetting for each carb and check F/A ratio with the sniffer.
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Old 01-24-2013, 12:35 PM   #21
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My setup right now is real strong off idle on up but I do think it could perform better above 7k rpm. I think a lot of it is a restrictive exhaust but I have been thinking lately that I don't want a real loud exhaust. I think my engine needs a shorter, bigger, freer flowing setup but I know that setup is going to be loud. Bigger carbs? The best running and revving airhead I have ever been around had 38mm flat slides on it. That thing had WAY more top end power than my bike with the same size carbs. Of course, it also had a magical port job and long rods in it too!
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Old 01-24-2013, 01:42 PM   #22
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38mm flat slides are not the same size as 38mm round slides. At least not regarding effective size.

I had the opportunity to ride another bike with the same cam, same carbs (but with round filter setup), but less displacement (980 vs. 1070).
This engine felt more aggressive at higher rpm.
My conclusion was: more displacement -> higher velocities in the carb -> earlier peak
Maybe velocities are already too high.
We'll see.
If the bigger carbs perform worse I'll switch back to my 38mm Dells and that's it.
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Old 01-24-2013, 03:27 PM   #23
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Effective size? How's that? Isn't 38mm 38mm?
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Old 01-25-2013, 01:28 AM   #24
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Effective size? How's that? Isn't 38mm 38mm?
38mm is 38mm, that's clear.
But a flat slide (or smooth bore) flows much better than a round slide carb.
So you get the same result with a smaller carb.

In the 70ies Udo Gietl / Todd Schuster converted the Dell into a smooth bore carb for better flow.
They had to stick to the stock carb bodies.
I shot a picture of that setup:
https://picasaweb.google.com/funhous...09464114917266

So maybe the 40mm flat slides will already be too big for my engine.
We'll see.

Edit: a friend had 40mm Dells on his racer. He switched to 39mm Keihin FCRs.
+2 peak HP, +6HP at medium revs.

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Old 01-25-2013, 09:11 AM   #25
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Excellent photos! Thanks for sharing.
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Old 01-25-2013, 12:35 PM   #26
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Smooth bores flow more. Flat slides are mostly all about concentrating the low pressure right over the needle jet for better throttle response. After the slide is all the way up? It's basically the same deal as a round slide.

I am pretty sure I did a valve job on that B+S bike about 15 years ago when I worked at San Jose BMW. It was one of them. It was 'restored' back then if it's the same one. .
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Old 01-26-2013, 02:10 AM   #27
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Originally Posted by supershaft View Post
Smooth bores flow more. Flat slides are mostly all about concentrating the low pressure right over the needle jet for better throttle response. After the slide is all the way up? It's basically the same deal as a round slide.
I'd say a flat slide can be both.
Looking into the Mikuni TM I'd call it a smooth bore.

Edit: looking into the Mikuni manual I see they also call their TM series smooth bores.
The racer you mention probably was from SJBMW? Looking at the pictures at their website I see TMs ...

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Old 01-26-2013, 07:10 AM   #28
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Originally Posted by supershaft View Post
I am pretty sure I did a valve job on that B+S bike about 15 years ago when I worked at San Jose BMW. It was one of them. It was 'restored' back then if it's the same one. .
That's probably around the time that Bruce Armstrong was restoring the bike he got from Johnny's. That bike is also in Rudy's album. Far glossier, and far less correct.
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Old 01-26-2013, 08:01 AM   #29
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That's probably around the time that Bruce Armstrong was restoring the bike he got from Johnny's. That bike is also in Rudy's album. Far glossier, and far less correct.
Bruce told me he never touched the engine.
And he already restored it in the early 80s.

http://www.advrider.com/forums/showthread.php?t=244174
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Old 01-26-2013, 12:50 PM   #30
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I don't know if it was that one or not. I didn't really care back then and I still don't care much. IF that's the bike, Johnny never touched it, the shop did. I can certainly understand why someone would say they had never touched the engine whether they had or not. The one I worked on probably was a different one. It might of even had a different number on it. At the time, it was just another job to me. I was working on a lot of old and current airhead race bikes at the time. I worked on a number of old famous early BMW superbikes. I have read interviews were Gietl says he didn't spend much time on porting and from what I have seen I can believe it. Hodgeson on the hand . . . . Porting and the cam are where the BIG gains are! IMO from what I have seen long rods are next in line concerning big gains airhead wise. All that pretty much sums up why HD XR750's are still kicking ass.

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