Mikuni tm28s

[rd500ian]

Hi has anyone had any experience with fitting issues with mikuni tm28 carbs,idle screws chokes etc clearance issues.any ways around these problems.cheers Ian

[Jeff B]

choke cables
viewtopic.php?f=3&t=16068&p=100318#p100318

You will need a reed block spacer for two of the carbs.

[rd500ian]

Any pictures of these blocks please

[rd500ian]

Any idea of thickness of blocks

[Jeff B]

I did a quick search to find a pic of reed block spacers. I believe they are the same for the 350 and 500.
http://www.rzrd500.com/phpBB3/viewtopic.php?t=9186

I took a look at my bike. I’d guess mine are 1/4 inch thick. Just a little thicker than the reed block. On the upper cylinders.

If you need a throttle cable I think Bill Wilson has those.

[rd500ian]

Thanks for that

[Jeff B]

No problem! Glad I could help!

[rd500ian]

Jeff, any ideas for a base setting for these carbs.cheer’s Ian

[Jeff B]

My project has been on hold for quite a while now. I never did get it properly jetted. Your better off asking someone with a good running bike. Maybe Luther??? I'd recommend post your engine setup as well. Pipes, porting, air filter type, etc.

[rd500ian]

[Luther]

Don't know how I missed this post but yeah I'd like to hear what jets others are useing. Heres mine

Main Jet 180 probably rich if hot and wfo at the track
Needle Jet N-8 or O-0 with hoods cut to 3.5mm Oh boy how did I get there
Needle 5DP39 I think. Taper cut a smidge so its starts same place as stock
Pilot Jet 30
Air Jet 1.1 from stock carbs, more crisp and stronger around town
Slide cutaway 3.5mm to get rid of hang idle down, slides are expensive and you cant go back

It runs really good but I thought they would be closer out of the box. I've replaced stock carbs on other bikes with good results so took the gamble. Was 20 years ago and I knew a lot less, hadn't met Rick Lance. You can put PWK 28's on an RZ350 and it will run better. It will run better with bigger carbs but it brings out the hooligan in the bike and wants to be rode like a dirtbike with no regard for the law. I don't think the same applies on the RZ500 because it has good transfer ports unlike the crap 350 transfers. Bill Wilson has supplied bigger VM Mikunis to a few folks and they are the only ones who say yes it runs way better than stock. I'm ready to try that when i have some money but i'm picturing cocked and crammed in there to fit. I'll loan out or sell the TM28's then

[steveho]

Slide cutaway 3.5mm to get rid of hang idle down, slides are expensive and you cant go back

Luther, I have an idle that hangs after blipping with these 28mm flat slides too.
I assumed the ‘hang’ is due to a lean condition, so am surprised that you have gone from 2.5mm to 3.5mm cutaway, as that is going leaner.
I’m currently on 25 pilot jets and things improved a little with the pilot air screw turned in, so will try 27.5 or 30’s.

Any chance for you to elaborate with your experience on getting rid of the hanging idle issue?

Thanks.

[Luther]

Hi Steve, I'll give it a go but i'm still learning so free advice might not be worth much. I get sidetracked too.
Back in the day racing mx there were a few bikes with hang idle issues. Changing jets and clips didn't help. Then there would be a tech paper at the bike shop or so an so did some nationals and the mechanics said cut the slide and start over. The problem is a strong signal "vacuum" from the motor, carbs a shade on the small size, to make a streatable restricted bike they could sell. Now we put on TM's that are even more efficient than stock. They can work better on motors with a weak signal. So any time your just riding around not accelerating hard the carbs have no problem feeding fuel compared to an inefficient motor. When you close the throttle with too small a cutaway it's going to suck something from somewhere and too much gas is its only solution. It goes temporally rich until it calms down. As far as jetting goes most of my time is spent figuring out what isn't causing the problem. Thats how it works anyway so best to go with the flow. So if you change a jet richer then leaner and get no results it means your either way out of range or an overlapping circuit is masking or overriding your adjustments. I can't tell you how many times on this and other bikes i've changed needle jets needles clip positions to find the pilot jets too big or small or the other way around. Or find out from someone that those dont run good with an air bleed jet just take it out.

So whats the rest of your jets? How's it run other than hang idle?

[Jeff B]

Also a I believe a hanging idle can be caused by carbs that are not properly synchronized.

[steveho]

Luther, thanks for the detailed reply. Appreciated. I thought it was bad enough just trying to figure out rich/lean, never mind signal strengths! Something else to consider.
The bike in question is a YZR replica, and the carbs are VM28-418 (but apparently Mikuni screwed up their part numbering as they are really TM28’s as are flat slide, not round slide). I have only just got it running and haven’t ridden it yet. Just want to get it idling and able to blip it before taking it out. But the hanging idle is clearly an issue.
These carbs came with 5DP39 needles, 175 P-8 needle jets which I’ll keep initially. The 15 pilot jets have been changed to 25’s, but will try 27.5 and maybe 30’s too.
As I will run individual foam filters, the main jets will probably be pretty big, 240+ I am guessing, depending on air jet size.
Got a bit of a steep learning curve ahead!
Thanks for the help.
Rgds,