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DUAL Mikuni TM38 flat slides Part 2

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DUAL Mikuni TM38 flat slides Part 2

PostAuthor: ivogiev » Thu Apr 13, 2017 1:30 am

Continued from Part 1 viewtopic.php?f=34&t=55832

Dave, sorry if it sounded as if I am trying to find faults in your setup - that is not the case.
I have bought TM38-85s, I have raised the needles to top position and have printed your 3D files for adapters.
I am going 100% after your setup , just noticed something that may have affected you in a way. If that is not true, I am happy to hear it.
But it was just something I wouldn't leave without mentioning , cause it could have mean you are not 100% opening the slides on WOT.
Thank you for all the advice and shared know-how, I am not in any way decrying your efforts and result.

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Re: DUAL Mikuni TM38 flat slides

PostAuthor: ivogiev » Sat Apr 22, 2017 12:07 pm

Dave,
I was wondering how many turns out of soft seat do you run your idle screw on you original set up?
I have read the thread few times, but nowhere is written the exact amount and it would be useful for followers like me with the same set up.
Mine at the moment is 2,5 turns out of soft seat.
Was 2 turns and backed it out 0,5 turn - nothing changed on idle and decided to leave it as is,because before that it was massive rich on idle.
Got my bike home - runs a ton stronger than with the stock set up, but is a bit loud with my new exhaust.

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Re: DUAL Mikuni TM38 flat slides

PostAuthor: Brodie » Sat Apr 22, 2017 4:42 pm

I have installed mine and I am very impressed. Beats my old worn out mikunis. Have not taken her for a ride yet but it idles a lot nicer and is easy to tune the idle.

I need a new exhaust and am still waiting on my throttle cable but I will post again once I have taken her for a ride.
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Re: DUAL Mikuni TM38 flat slides

PostAuthor: DGA » Sat Apr 22, 2017 10:28 pm

Remember, the idle mixture screws are AIR screws, opening them or going anticlockwise LEANS the idle mixture. I am at 1.5 turns open now, but I have different cams now as well. I think it was too lean with the stock settings, you might try 2 turns open, I seem to recall they were set at 2.5 turns open from stock. If you get a high idle cold, and inconsistent idle speed it means they are set too lean. Go out a quarter turn at a time and try. As you know, you need a really long small flat bladed screwdriver to adjust them.
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Re: DUAL Mikuni TM38 flat slides

PostAuthor: cookeal » Fri Apr 28, 2017 2:50 pm

Having been in several circles regarding carb choice for my XV750 83, finally have found this thread and it feels like the right way to go. Was not really comfortable with the single carb approach for lots of reasons, and having spoken to a few of the makers felt like their claims were a bit too good to be true, and I would end up with a pokey-out carb, and be quite a bit poorer!

So am about to order the carbs (TM-38-85) and just have a couple of questions:

1. Would really appreciate the 3D print file for the connectors - will PM my email address
2. Petcocks - remain standard with the vacuum lines to the inlets?
3. Air Filter - Airbox has already gone so would appreciate any recommendations here?
4. The 2 into 1 cable does not appear to be available any more (on ebay) - any other known sources?

Much appreciated

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Re: DUAL Mikuni TM38 flat slides

PostAuthor: DGA » Fri Apr 28, 2017 4:00 pm

1. Answered in PM
2. Standard petcocks work fine.
3. Ditch the entire stock air filter assembly, even the rubber bellmouth in the frame inlet. Leave the two in the frame just before the rubber 90 degree carb bends. I machined an adapter from the frame to mount a 4" clamp size round air filter, but any decent round filter will work, just as long as you start with the frame inlet and all the crap before it is gone.
4. The cable from Niche cycle in this page http://www.ebay.com/itm/Yamaha-XS650-34 ... gg&vxp=mtr
looks like the right one, might contact them and ask. Niche cycle have good prices on the TM38 anyway, they might include it as a deal if you talk to them.
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Re: DUAL Mikuni TM38 flat slides

PostAuthor: Brodie » Wed May 17, 2017 5:18 pm

I have not had much time to work on these but from what I have done I can add some info.

The replacement stock intake boots I bought seem to be just a slight bit to large but do compress down once you tighten the clamps. The stock cars fit snug before they were clamped.

With the needles set to the highest setting I seem to blow black smoke when I rev the engine without load or an intake setup.

The throttle cable from Mikuni Oz does not fit without modification. I had to shorten the main cable and fit the stock end adjuster.
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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: ivogiev » Tue May 30, 2017 5:19 am

Maybe it would be good to post my input on the same topic.
I followed DGA's setup as it was originally, but my bike doesn't like those settings.
I have risen the needles to their max settings and it was way too rich. Couldn't influence it with the AIR idle screw on the inside of the carbs.
The float height was checked just in case and appeared to be OK and equal on both carbs( that was the only thing I didn't check when installed them on the bike).
I have restored the needle settings to their default middle position and the bike stopped throwing flames and banging on deceleration.
I just had the test ride last night, but other issues are arising that affect the performance.
It starts very difficult and on 1 cylinder most of the times, so my next move will be to change the spark plugs again cause they had a hard time working when the bike was way too rich.
I hope this will bring consistency to my test rides.
My air filter is K&N replacement unit without airbox on it - it is sitting on the frame and breathes as much as it can through the frame.
My exhaust is a custom 2-1 short SS system with calculated diameter and length of the runners. The end "can" is very short and basically I doubt a more free flowing exhaust system is possible.

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: Brodie » Fri Jun 09, 2017 7:56 pm

Ok I have had a quick chance to give these carbs a bit of a go. From what I have found on my bike (XV1100 '90) The pilot jets are way to rich. I have given the bike a few runs and each time I have had to change the plugs to make it back home after about 200m. That is with 22.5 jets and 2 turns out.
I am at 3 turns out now and it is still very rich when on idle. I was going to order some 17.5 jets but just wanted to see if any one thought I should try the 20 jets before dropping down so far. I have run BP6ES plugs and have just stepped up to BP5ES to try and combat how rich it is running. Just so people know I am a total novice at tuning but I am willing to give it a go. Here is a quick shot of the bike as she sits. Undecided on the shape of the seat so that is why there is not a proper cover.
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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: tymck7 » Tue Jun 13, 2017 7:44 pm

Everyone:

This topic has been extremely interesting and helpful! As the new owner of a completely stock '83 xv750 (and possibly because I came from riding a pretty fast 2005 WR450F supermoto) I've been looking for some decent power adding mods that are reasonable. I'm pretty stoked to try out the TM38-85's.

DGA:

Thank you so much for being the guinea pig on this! I wanted to ask you, what's your elevation over there in Indiana (saw your license plate in your video)? I'm over here in Idaho (1800' give or take) and I ride up into Utah routinely (4500' or more). Wondering if you're close enough to my elevation to where I won't need to drop the needle or switch out the main jet/pilot jet etc... Also, I don't own a 3D printer, but I think your connectors are pretty sweet. What would you charge me to print a couple and ship them my way (I think you deserve something on top of shipping/materials for your trouble putting this all together)? PM me if that's something you'd be willing to do.

- Tyler
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Done: TR1 tank, brat subframe and seat, R6 front end, dual mikuni TM38-85's.

Plans: XT600 rear shock, 750 heads, rear wheel 17" 900 XJ Diversion complete with disk brakes and anything else cool I read about on here.

Thanks all!
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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: ivogiev » Wed Jul 05, 2017 2:40 pm

Just some more useful info on the table from my side.
My bike is ridden on 2460 ft. elevation or 750 meters above sea level - that is important. Here we have 4 seasons, but since the carb upgrade the temp was approx. 20-35 degrees Celsius ( 68-95 Farenheit).
Same as Brodie my bike doesn't like the 22.5 pilots, but for the moment I ride it on the highest needle slot ( opposite to DGA's recommendation) and my AIR screws are 2 turns out on one cylinder and 2.5 turns out on second. I am now having a good spark plug color , but poor performance on take off and up to 1/4 throttle. From 1/4 and up the bike has a noticeable power increase ( feels like the old B- and H-series Vtec that kicks in :)). Have tried to compensate the poor performance with the AIR screws, but didn't work out well - plugs were fouled and didn't want to start easily, but still ran only a bit better up to 1/4 throttle.
So same as Brodie I will probably go for a 17.5 pilots.The last thing I want to test before buying 17.5 pilots is to raise the needles one step or even two steps and go back to default settings of TM38 ( in the middle slot) and then play again with the AIR screws just to be sure my bike needs those new pilots.
I have managed to crash with the bike and have been playing with Gen I starter fix so couldn't do too much on the carbs. Now the starter works great so could focus back on the carbs.
Brodie, JFYI I don't think 20 pilot jet exists for TM38s. They go up and down the numbers by 5 , therefor the leaner jet after 22.5 is actually 17.5.
17.5 is the default pilot jet in the round VM38 Mikuni, which is also interesting to know.

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: JimL » Wed Jul 05, 2017 3:41 pm

I have been looking at this thread with some confusion, I must admit. I used the TM38, Pro Series 39, and Pro Series 41 for a number of years on 500 to 680 cc four stroke twins. My 38 s and 39s were based on the -68 version which has different bottom shape on the slide (to keep them from flooding at part throttle). Two stroke slides are designed to create high "ported vacuum" over the needle jet, at small throttle openings.

If it is crazy rich at small throttle openings, it sounds like the snowmobile version of these carbs (widely available at reasonable prices).

SUDCO has always been my source for Mikuni stuff and they are expert at putting together the correct setup. They handle all the different slide types, etc. For custom throttle cables, you wll get good work from Barnett (the clutch people). They provded all my cables and throttles for my various Salt Flats bikes that used these carbs.

A note on the Mikuni jet numbers: these are pretty handy for tuning because they reflect flow rate, not orifice diameter. A 250 main jet is about equivalent to a 250cc/min fuel injector flow rate. Fuel measurement, in cc per minute is a good predictor for available horsepower.

For example, back in my "turbo days" we ran a 2000cc 4-cylinder roadster at about 15 pounds of boost (double atmosperic pressure so to speak). The basic rule was that each 500cc cylinder needed 500cc/min fuel rate to make 100 hp per cylinder. Of course, in the efi world we ran 660cc injectors to stay away from 100% duty cycle, but here is the rest of the story. We knew we needed about 430 HP for 200 mph with that car....we never got past about 186-190 mph.

My 680cc Pushrd V-twins were using Mikuni TM38-39-41 carbs always with 230cc man jets at Bonnevilles density altitude of 5000-7500 feet (depending on temp, humidty, etc.). That meant I had about 35-40 HP per cylinder available (no turbos on my bikes) which meant roughly 70-80HP to the rear wheel (using 10-11,000 RPM). My best speeds were in the 160-165 MPH range which is right where we expect that horsepower to put us on the salt.

Those speeds compare well with the current 600cc records for DOHC bikes. They are getting around 170-175 mph on about 90-95 HP to the rear wheel (at those high "density altitudes). Over and over, we see that the math works if you build it right.

I hope this helps you understand the Mikuni jet numbers a little better. If you know about what the realistic power capability is (of your particular engine) you can pretty much tell whats going on with your mainjet size. My XV250 type project is running a Mikuni 250 main jet (TM28 carb) at sea level. On a 250 cc engine, that would be about 20 HP ....IF ....the engine could rev high enough and pump enough air...but it won't and it runs like about 15-16 HP. Pretty simple, isnt it?

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: TR1 » Sat Jul 15, 2017 2:51 pm

I have no experience in carb syncing. But I was thinking that with these carbs it is not possible? With the 2-1 throttle cable that is.

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: nanno » Sat Jul 15, 2017 4:56 pm

TR1 wrote:I have no experience in carb syncing. But I was thinking that with these carbs it is not possible? With the 2-1 throttle cable that is.


You do that with the cable adjusters on top of the carbs.
http://viragotechforum.com/viewtopic.php?f=12&t=43227 - Nanno's accumulated findings (and blatant show-off) - infos on my TR1's, my performance and reliability mods and a bit of show-casing of the stainless steel exhausts I build

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: tymck7 » Mon Sep 11, 2017 11:36 am

I was planning on building a 750, and DGA had some good tips on setting up the carb for that engine, but... I recently swapped out the 750 for a 920. Still wanting to use Mikuni TM-38-85's. I know anything you guys would say would just be a guess (unless someone has set up a 920 with dual Mikuni flatslides), but I plan on setting up the carb like DGA did for his 750 and adjusting from there, so, which direction do you think I should go? Should I pick up the Q-4 needle jet? Do you guys think I will need a larger main or pilot? Or smaller on both for some reason? (sorry, if I sound like I have almost no idea, that's because I don't. First time carb set-up)(I do have a decent understanding of how carbs work though). I'm at 2000 feet altitude, temperate climate (Idaho). Or do you guys think that DGA's setup will work alright on a 920? Any help appreciated.
'83 xv750 converted to '83 xv920..
Done: TR1 tank, brat subframe and seat, R6 front end, dual mikuni TM38-85's.

Plans: XT600 rear shock, 750 heads, rear wheel 17" 900 XJ Diversion complete with disk brakes and anything else cool I read about on here.

Thanks all!
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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: nanno » Mon Sep 11, 2017 1:27 pm

Considering how neglible the difference in displacement is, I'd say go with his 750 settings and expect them to be more or less spot on. (This isn't a chevy small-block which comes in sizes from 265 to 500-ish cubic inches...)
http://viragotechforum.com/viewtopic.php?f=12&t=43227 - Nanno's accumulated findings (and blatant show-off) - infos on my TR1's, my performance and reliability mods and a bit of show-casing of the stainless steel exhausts I build

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: ivogiev » Fri Sep 22, 2017 11:42 am

tymck7, my bike is a '83 920 and runs on the TM38-85s' bought because of this thread.
I do experience problems that are not solved yet although I am now running 17,5 pilot jets in the carbs.
The pilots come in 2,5 increments, not in 5 as I stated earlier in the thread.
I bought a set of 17,5 and a set of 20 , but installed 17,5 first and still haven't managed to tune the machine properly.
At a certain point in Part 1 of this thread I was not sure if the VM34s' or TM38s' was the best choice for our bikes.
And after all the issues I had and still have with my bike, maybe the better way to go is to use the already promised as properly tuned set of VM34s'.
Because although the intake port size of our bikes is the same as the TM38s' , our ports are of very poor design and this probably has a great affect on how these carbs will work.
When I end up with a proper setup on the 38s' on my 920 I will post my settings here for sure, but now the climate is getting worse and in general I have very little time to play with my hobbies - so input will not come soon probably.

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Re: DUAL Mikuni TM38 flat slides Part 2

PostAuthor: tymck7 » Fri Sep 22, 2017 12:28 pm

Thanks ivogev. Yeah I already bought and installed the TM-38-85's, haven't had a chance to try em out yet though because I'm also finishing the engine swap I'm doing from 750 to 920 (pain in the butt if you don't have the right equipment, like a decent jack..or a garage... and if it's raining like it has been in Idaho. I was up till 4 in the morning last night, typing this drowsy at work right now...but that's a whole different story). I raised the needle to the top like DGA advised, and I'm hoping for good things. But I'll let you know if I have similar problems, and when you get yours dialed in, let us know what you had to do.
'83 xv750 converted to '83 xv920..
Done: TR1 tank, brat subframe and seat, R6 front end, dual mikuni TM38-85's.

Plans: XT600 rear shock, 750 heads, rear wheel 17" 900 XJ Diversion complete with disk brakes and anything else cool I read about on here.

Thanks all!
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