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Post by colinjay on Sept 5, 2007 17:24:50 GMT -5
Hi all,
I have just been given a 38mm round slide Mikuni carbie off of a Yamaha IT 400 enduro bike. Can anyone give me a "baseline" for jetting this carbie to suit a stock SR engine so I have a reasonable starting point?
CJ
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Post by wotavidone on Sept 6, 2007 17:27:19 GMT -5
Heck CJ,
I'da thought you would have had a complete list by now. pilot, main, needle, cutaway, etc.
Must be someone out there who has one.
Mick
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Post by StewRoss on Sept 6, 2007 21:55:27 GMT -5
My black SR has a 38 (PJ) from a TZ and the jetting in that carby is as follows: 280 main 30 pilot...possibly too small? Needle 6F8/2 3.0 slide cutaway Perhaps a P4 needle jet... Powerjet is blocked off... I have several lots of 38 settings, so to totally confuse you here are some more: 240-270 main 30 pilot Needle 6F8/2 P4 NJ #2 air jet 1.5 slide cutaway 270 main 35 pilot Needle 6F8/3 P6 NJ #2 air jet 7.0MM slide cutaway (MEASURED) Have fun...heh, heh...a couple of years ago I did one for a guy who was happy to do lots of footwork to the carb place and back...plus Ian Williams (Adelaide) was happy to exchange the parts free of charge as well...it took us a little while but we had that one set up near to absolutely perfect...unfortunately I neglected to write the settings down at the end! Arrgh!  SR  |
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Post by wotavidone on Sept 6, 2007 22:03:25 GMT -5
Far out. Looks like a case of bolt her on and keep fiddling till you are happy with the result.
Mick
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Post by colinjay on Sept 6, 2007 23:02:16 GMT -5
Thanks for the info Stew.
Sad, isn't it, back in the early 80's I had all the details for the jetting of a 38mm Mikuni for a STD 500 engine, and for a engine with the same cam and performance mod as my hot engine for running on 100 octane fuel and on methanol. I had it all written down in a book, but after 25 years of moving around the country, I can't find the book, Bugger!
CJ
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Post by wotavidone on Sept 6, 2007 23:56:04 GMT -5
That is sad. Especially the methanol jetting. I predict that it will not be too long at all before methylated spirits, which is mostly ethanol with methanol and bittering agents added so you can't drink it, will be cheaper and more readily available than petrol.
I have an industrial chemistry book that looks at the different energy densities of petrol and alcohol. I reckon its something like 28g of ethanol gives the same energy as 14g petrol. Or maybe it wa 18g petrol. Lots more needed anyway.
So that suggests to me that you need a much bigger jet?
What was the rationale for running methanol? Cooler? More power? Greater knock resistance?
Mick
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Post by colinjay on Sept 7, 2007 1:02:21 GMT -5
Mick,
Methanol actually contains less heat energy (BTU's) than petrol, weight for weight. From memory, (I can look into my books and give a better answer on Monday), you need a fuel/air ratio of 5.5:1 with metanol verses 14.2:1 with petrol for a Stoichiometric Ratio. Now since you are putting twice as much fuel into the engine, you actually end up with more heat energy, and since a petrol engine is a heat engine you get more power, normally 7 - 10%. You normally need to run a compression ratio of about 12 or more (when I ran my SR on methanol it had a CR of 14.5:1.
For jetting, you need to jet to get about 2.2 times more fuel into the engine, so if you have a 200 mikuni hex main jet (they are flow rated) you start jetting with a 420 main jet, and suck and see from there.
The big down side to metanol as a fuel is the lousy fuel consumption, from memory I got between 20 to 25mpg, and worst if I ran it hard.
CJ
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Post by wotavidone on Sept 7, 2007 1:48:17 GMT -5
Yeah that sort of gels with what I remember - about twice the weight to get the same energy.
So you have to expect only about half t he fuel economy.
May come a time when its all we have.
Mick
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Post by milkman on Sept 7, 2007 5:17:00 GMT -5
Depends on whatthey can make the most money out of.
As soon as the gas looks like running out, the hydrogen cars will roll out, but not until you see the oil/car companies already control that too.
Some fellas went to a work conference in Iceland last year, and said it was amazing the amount of standard looking bmws, VW's etc over there, running on hydrogen, that was delivered normally from fuel stations.
We haven't seen anything like it because we don't have the stringent EU emissions controls.
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Post by caferacercarl on Dec 3, 2007 2:26:31 GMT -5
Pull out the wallet Colin and buy a lovely 40mm Dellorto off Thunderbikes in Perth, Tell Mario you want the flip lever choke and the metal fuel banjo and then ring me, your bikes look to good for rotten old two stroke carbs!! [orange bike finished?] but,as usual Stew is pretty well spot on. the middle settings with stock type cam but K&N/ noisy muffler should be close. regards Carl, oh, I actually had regard for the "plastic maggot cx500" they went well for a 500 in their day,loved the VT more! love the ugliest bike bit.
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Post by colinjay on Dec 3, 2007 16:33:14 GMT -5
Hi Carl, how's thing going?
I was in at Show & Go M/cycles in Adelaide about a month ago and drooling over a monster 40mm Keihn carbie that was in ther display case, problem is that it was something like $900!, and I can find a lot more use for the money than a new carbie.
In regards to methanol as a fuel, on the same trip down to Adelaide as above, I stopped in a a place in Wingfeild that supplies most of the race fuel in Adelaide. They have 105 octane unlead and methanol on pump! Suprisingly the methanol was cheaper than std pump unleaded at $1.20, I can't remember the cost of the 105 unlead, but I an thinking of give my racer a go on it once I have it u and running.
CJ
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Post by wotavidone on Dec 5, 2007 20:10:01 GMT -5
Colin,
Where is this servo? I might be making a trip to Adelaide soon, and a couple of jerry cans of 105 octane would be good.
Mick
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Post by colinjay on Dec 5, 2007 22:13:12 GMT -5
Mick,
I can't remenber the name of the business, but it is a small fuel depot and it is on Senna Rd, in Wingfield. Senna Rd is a little side road on the left between Cormack Rd and Grand Junction Rd as you head into Adelaide. They sell a variety of race fuels and oil.
CJ
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Post by caferacercarl on Dec 7, 2007 2:02:43 GMT -5
Again on the carb thing, Dellorto new is about $400,don't bother with the race fuel,the fastest one I've built runs on BP ultimate,to burn anything else properly you will need a different advance curve and too much compression,its all in the cam,head work and the full exhaust, 10,500rpm plus with max power from 4,500 on all the way to 10,000 is very real. these sound amazing at 9 grand with a 2 inch system and big muffler [ look for the rgvsr pics on my site soon] thanks colin, Carl |
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Post by wotavidone on Dec 7, 2007 19:28:29 GMT -5
Carl,
I think I need to explain the race fuel bit. When I bought my bike, it had had a rebuild in about 1990 according to the previous owner. At thew time he reckoned he couldn't get a new yamaha piston so he put a Wiseco 10:1 in it. This was back in the days of leaded fuel.
The bike runs OK on BP ultimate, but still pings on steepish grades or with big head winds.
Jetting looks nice according to my mechanic mate's reading of the plug, but as he pointed out, it may be only a 10:1 piston, but without cc'ing the head I have no real idea what compression ratio the thing is really running - the PO could have shaved it within an inch of its life.
I put a stepped key in it to retard the flywheel about 4 degrees, this helped a lot actually.
But a while ago I got hold of a little AVGAS. a 50:50 blend of 98 octane and avgas made all the pinging go away and the bike went very well. However, AVGAS ain't exactly easy to get, this was done as a one-off favour to help with diagnosis. This is why I took notice when Colin mentioned he had found a place that sold 105 octane.
When I get lots more time than I have now, I'll probably take the head off and cc it.
Unfortunately time and budget don't currently allow for what it really deserves - a full rebuild with a target CR of 9.5 to 1.
Pity you are so far away, otherwise I'd bring it over to your shop and see what you could do with it.
I have been researching the net with a different idea in mind. I'm sure I will one day find an ignition from a later model bike which will interface with a knock sensor.
Mick
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