Advice tuning flat slide Mikuni

mean gene

mean gene

VIP MEMBER
Joined
Dec 21, 2020
Messages
1,273
Country flag
Advice and sanity ck on tuning my Hot Rod 850 with 34mm flat slides. Had it running pretty good in cool weather now it runs shitty. Eletronic ignition, Timing is correct. Main jet is a 240. Going down the road at 55 everything seems good no surging no missing. As soon as I get to 3/4 throttle maybe 65/70mph it goes to missing. The plugs are very clean see pic, with some black around metal. I've been trying to read plugs but doesn't make sense, they are simply to clean after 4 mile run! Today I realized the pipes are grey/black. Should I be reading the pipes and not worry about plugs? My thoughts are I'm a little rich on the needle and the main. The combination of the two are exeragating the missing (rich?) at 3/4 throttle. Or this the untunable area caused by the short intakes that I was forced to use? I just cannot find a suitable place to hold it wide open for any length of time for main jet check.

Advice tuning flat slide MikuniAdvice tuning flat slide MikuniAdvice tuning flat slide Mikuni
 

Attachments

  • Advice tuning flat slide Mikuni
    DSC05652.JPG
    163.5 KB · Views: 50
Unleaded fuel does not color plugs until WAAAAAAAY OFF THE MAP RICH. (Despite herds clinging to 1972 rules.)
Unless you are running leaded gas, plug chops are of no value.

But, there are many other tools to get the job done.

💡it ran better in cool weather, suggests rich. Take the mains down one size, re-test.
Report back.

Take the needle down one clip as well.
 
Last edited:
I'm running Non ethenol and avation gas mix. We are thinking the same. The black rings made me think it was rich on top end. The plugs are soooo clean they scare me. Not yellow which would really scare me! Just wanted to hear it from others. Dropped the needles today, didn't get a chance to run. Cold weather I was two needles clip rings richer and it ran good. If I get really upset I'll buy and install a fuel ratio gage, I put one on my worked Sporty and it told me how to correct my problems. Wish they were cheaper and or portable!
 
mean gene said:
I'm running Non ethenol and avation gas mix. We are thinking the same. The black rings made me think it was rich on top end. The plugs are soooo clean they scare me. Not yellow which would really scare me! Just wanted to hear it from others. Dropped the needles today, didn't get a chance to run. Cold weather I was two needles clip rings richer and it ran good. If I get really upset I'll buy and install a fuel ratio gage, I put one on my worked Sporty and it told me how to correct my problems. Wish they were cheaper and or portable!
Click to expand...
Where would you put the bung hole? :)

In all the years I've been turning a throttle on a set of handlebars, I've never had the change in seasonal weather cause a rich enough condition to cause an ignition misfire. Slightly rich (a 240 main is not particularly over rich on a Norton) usually causes the exhaust note to get lower and the engine start to bog and run out of top end power way too soon. Misfiring is usually a lean condition or something off in the ignition.

The 34mm twin Mikuni VM Norton kits came with 230 mains. No TM kits existed, so I have no idea what they would have come with. The main might be related, but a 240 wouldn't cause a misfire with plugs that clean.

It is harder to read unleaded fuel on plugs, but when rich enough to cause problems, it will show. The black on the base of the threads could be because the rings are not yet fully seated.

Did you take the bowls off to see if anything other than fuel was in there? Coils good?
 
Schwany said:
Where would you put the bung hole? :)

In all the years I've been turning a throttle on a set of handlebars, I've never had the change in seasonal weather cause a rich enough condition to cause an ignition misfire. Slightly rich (a 240 main is not particularly over rich on a Norton) usually causes the exhaust note to get lower and the engine start to bog and run out of top end power way too soon. Misfiring is usually a lean condition or something off in the ignition.

The 34mm twin Mikuni VM Norton kits came with 230 mains. No TM kits existed, so I have no idea what they would have come with. The main might be related, but a 240 wouldn't cause a misfire with plugs that clean.

It is harder to read unleaded fuel on plugs, but when rich enough to cause problems, it will show. The black on the base of the threads could be because the rings are not yet fully seated.

Did you take the bowls off to see if anything other than fuel was in there? Coils good?
Click to expand...
With my afr guage I had to weld the bung in the downpipe within 200mm of the exhaust port
I really wish I'd bought a twin channel one but whilst running a single carb and electronic ignition it should be pretty even from one side to the other with everything else being equal
Valve clearance, compression etc 🤞
 

"Advice tuning flat slide Mikuni"​




"(a 240 main is not particularly over rich on a Norton) "


Not a direct cross to Amal concentrics
 
concours said:

"Advice tuning flat slide Mikuni"​




"(a 240 main is not particularly over rich on a Norton) "


Not a direct cross to Amal concentrics
Click to expand...
Hmmm, Read my post again, or not, doesn't matter.

34mm Mikuni VM kits for the Norton worked fine with 230 mains as I mentioned, and 240 is a little bit richer, but a 240 main worked previously for Gene on his Mikuni flat slides. It is possible although unlikely that a 240 TM main does not flow the same as a VM 240 main. I'd have to go look and my boxes of Mikuni jets to be sure.
 
New coil, new plug wires Not that that means they're good LOL Compression good by how hard it is to kick, Haven't checked valves sinse assemble, usually set a little loose for start. Bowls are clean. Read your post again, honest! Sudco said jetting is basically the same for all Mikuni. One thing that is different is the slide number, there is only one available, it's #4. Jets interchange. New tri spark, checked connections, new Dyna coil.
 
Last edited:
This is a remote possibility but worth the check...shine a flashlight down the mufflers and see if there is any kind of restriction. I ran into this one time after spending a month trying to tune. Some mufflers came with a baffle plate with an exit hole the size of a nickel. The Atlas mufflers have this restriction. I ran the biggest hole saw I could down the rear of the muffler and it had more power, responded to tuning, and sounded better.
 
Thanks but I made the pipes no restrictions. But what I did find when I was changing the main jet, the plastic collar(see Pic) had fallen off the main jet housing. I guess it is to stop fuel from sloshing away from the main jet. It snaps on but from age or previous ethanol, it falls off. Went to my spare carb and got another that fits. Try running it again!


Advice tuning flat slide MikuniAdvice tuning flat slide Mikuni
 
Still kind of surprising that it worked with the 240 mains and now doesn't since the only thing that had changed before you rode it previously is the passage of time.

Not that it matters at all, but I'm not a fan of NGK plugs. They die way to easily. Anyway, I'm gonna STFU because you got this so to speak.
 
Are you cursing me? LOL Put 220 mains, needle in middle runs worse! Plugs clean but piston domes are black/brown. Beginning to think the fleabay Dana coil is breaking down (blowing out spark at higher RPM). Spark looks weak, might have been a asian copy no box or instructions. Need to walk away for a day or two. Contact tri-spark about a dual output coil.
 
mean gene said:
Are you cursing me? LOL Put 220 mains, needle in middle runs worse! Plugs clean but piston domes are black/brown. Beginning to think the fleabay Dana coil is breaking down (blowing out spark at higher RPM). Spark looks weak, might have been a asian copy no box or instructions. Need to walk away for a day or two. Contact tri-spark about a dual output coil.
Click to expand...
Go up a size..... if you have 260's on hand.

It will rule it out
 
Thanks have 260's Need to take a day away. My head hurts LOL Spark is not as crisp as i think it should be. 4 omh dana coil. I have a similar set up on another bike and the electronic tach works fine. This bike the same tach won't settle down, electronic interferance is the reason. Need to add a resistor according to manufacture. Don't know if there is a connection
 
mean gene said:
Thanks have 260's Need to take a day away. My head hurts LOL Spark is not as crisp as i think it should be. 4 omh dana coil. I have a similar set up on another bike and the electronic tach works fine. This bike the same tach won't settle down, electronic interferance is the reason. Need to add a resistor according to manufacture. Don't know if there is a connection
Click to expand...
Spark should be a white-blue viewed at night with a plug resting/clamped against the head to ground it . Good luck .
 
Woke up in the middle of the night thinking of this Norton and Schwany words, carbs don't change that much with temp . So I have a video March 12th of the bike running fine. So it could be electrical/electronics and I'm blaming the carbs. Plan on eliminating possible wiring problem by wiring the coil direct to battery and changing dual coil!
 
Forgive me if I’m stating the blindingly obvious here, but…

You have tried fresh plugs ?

And fresh fuel ??
 
If your issues are at 3/4 throttle it would not surprise me if your air jet was off. However, this will effect your needle. List your jetting including needle and needle jet. My suspicion is that you are lean and when the cam comes on around 4000 rpm you are transitioning lean off the slide onto the needle. Set a tape line at 0, 1/2, and 3/4 to confirm that you are really at 3/4 throttle.

EDIT: Buy a colortune plug. It sounds silly, until you realize your idle is perfect and you are setting everything based off that. It may take a few attempts if you slide is lean as you will need to rejet your pilot. I can cut my tuning time in half with this tool.
 
Last edited:
You must log in or register to reply here.

Similar threads

D
Can you give me advice about tuning/jets etc on my single Mikuni VM34
2 3
Replies
47
Views
1,281
DennisMo
D
D
Tuning the mikuni low speed fuel circuit with air screw
Replies
6
Views
194
acotrel
acotrel
Britstuff
MK II 850 Commando 1974 Amal Carb choice.
Replies
13
Views
508
acotrel
acotrel
Big_Jim59
Tuning Issues
2
Replies
23
Views
971
Big_Jim59
Big_Jim59
W
Amal premier jetting for 4000' elevation?
Replies
4
Views
313
Wireman
W
Back
Top