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Wierd jetting issue Mikuni

Posted By: Hillbilly bike

Wierd jetting issue Mikuni - 06/06/19 9:11 pm

I installed a pair of Mikuni 32mm TM flat side carbs on my T140D.. They are on both my race bikes so I'm familar with them..Easy swap, the slipped right on with no changes to the cables or inlet rubbers...
The engine idles nicely,pulls smooth at part throttle, and wide open is great... BUT,,,at part throttle, various RPM's depending on jetting it runs very rich ,dah, dah,dah like the cold start enricheners were left on, but they are not on...What's wierd is the problem is not load or throttle opening dependent...With a certain needle and jet it does it at 3500 RPM, change the jetting and it does it at 4500 rpm ot 5500...This is always at steady throttle in the lower gears,open the throttle and the engine responds and pulls good to redline...I thought it was maybe reversion since the engine is not stock...I put a "lollypop" in the mufller exit to change exhaust tuning but not really any change...The air filters on or off have no effect on the problem.
Kevin Roberts is sending me an air fuel gauge and sensor, I welded in a bung for 2 into one stock T140D header...Hopefully when ithe problem occurs I will see a change in gauge reading...

[Linked Image]
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/07/19 2:07 pm

that oxygen sensor is in the mail to you. it's set for AF ratio, but there's another face plate and a toggle on the back if you want to make it read lambda instead.

the advantage of the AF gauge is that it removes ambiguity. you'll be able to creep up on the problem while looking at the gauge and then actually see what the mixture is doing right when you enter the problem zone. then you can change stuff and see which direction it's going even if the change is too slight to feel accurately.

the only thing i don't like about the gauge is that it's digital, rather than analog. i realize how hard it is to make a needle gauge respond quickly to a digital input, but what happens is that you have to average changing digital readouts in your head, which for me is harder than gauging an average needle position.

there's an analog color band around the perimeter of the gauge, but i've never found it much better.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/07/19 2:23 pm

Kevin, Thanks....The engine feels good going through the gears at moderate or full throttle...But hold the rpm at about 1/4-1/3 throttle in the lower gears and it starts what guys here call "8 stroking"...At least that's what I think is going on.. If it is, the gauge will show a big change to rich...If it's ignition, I don't think it is, the gauge won't show a lack of oxygen, yes?
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/07/19 2:43 pm

the gauge only reads what passes by the oxygen sensor in the exhaust pipe. if you have an ignition misfire up front that lets unburned fuel run out the pipe, the gauge will read that as rich, even though the problem is ignition.

it isn't intelligent. if you have weird valve timing that gives you best horsepower while shooting fuel out the pipe, it will read that as rich as well.

and if the exhaust pipe is hot and unburned fuel is igniting as it runs along the pipe, then the sensor will read rich up front and lean towards the end, depending on where in the pipe you put it.

i don't believe in using the actual gauge numbers for tuning. but for looking at relative values and changes, it's priceless.
Posted By: pushrod tom

Re: Wierd jetting issue Mikuni - 06/07/19 7:12 pm

Interesting........very interesting. First. If you have an ignition misfire the gauge will show lean. I would guess that if it is rich where the 8 stroke is then the plugs would be sooty. If thats the case then maybe the needle jet is too big?
In order to keep the armchair tuners happy keep us posted. Cheers, PRT
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/07/19 8:37 pm

Sooty plugs....sometimes... If I ride " normally" , the plugs look ok.If ... I've tried three needle jet and two needles...One change at a time...More than anything it feels like reversion...The cam timing and exhaust are not stuff that invites reversion.... Right now it has a 22.5 pilot, 6DH3 needle ans a 175 main...
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/08/19 2:41 am

Originally Posted by pushrod tom
Interesting........very interesting. First. If you have an ignition misfire the gauge will show lean.


i have way little experience with AF gauges. i forgot that the sensor is reading O2 and not fuel.

so if there's a misfire, then the unburned fuel moving past the sensor would be accompanied by unburned O2 that would normally indicate a lean mixture.

thanks for pointing that out.



Posted By: Allan Gill

Re: Wierd jetting issue Mikuni - 06/08/19 6:55 am

I’m curious how the O2 fails to burn, unless it has what been carried through on overlap. Once an ignition is present (even a flame) it will continue to burn all the oxygen available.
Fuel on the other hand is different and more so with a carburettor than F.I. If the fuel isn’t atomised correctly then it will fail to burn, I’m not sure what temperatures reach a combustion chamber from memory but I would say around 600°c (someone will tell me if I’m wrong) it might take a further 500°c to break that carbon chain and so the fuel passes through un-atomised and unburnt. That would give a lean misfire if I’m not mistaken


(...... now open to being shot down)
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/08/19 11:02 am

Oxygen is the oxidizer for the fuel combustion...As far as I understand, the oxygen by itself doesn't burn..............I'm going to take out the bike today for more trials....
Posted By: pushrod tom

Re: Wierd jetting issue Mikuni - 06/08/19 11:36 am

Patience will prevail!! PS On board A/f is a great tool but just keep an eye on the road.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/08/19 12:37 pm

Tom, I live in a rural agricultural area so there's many two lane roads with no traffic and few houses...But at this time of year a huge tractor hauling a 20 foot wide array of cultivating equipment may be over the hill or around the bend..and clumps of mud from tractor tires or the occasional cow....Yes, a bit of caution, lol
Posted By: Allan Gill

Re: Wierd jetting issue Mikuni - 06/08/19 12:46 pm

Originally Posted by Hillbilly bike
Oxygen is the oxidizer for the fuel combustion...As far as I understand, the oxygen by itself doesn't burn..............I'm going to take out the bike today for more trials....


If you light a candle and place a jar over it (sealing it) the candle will continue to burn until all the oxygen has been used up. Basic fire triangle being fuel,oxygen,ignition. Take away anyone of these and there is no flame.

The ratio of oxygen to nitrogen in the atmosphere will also change the combustion delay by even 1%.


I think I’m taking this away from your original question.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/08/19 3:40 pm

Originally Posted by Allan Gill
Originally Posted by Hillbilly bike
Oxygen is the oxidizer for the fuel combustion...As far as I understand, the oxygen by itself doesn't burn..............I'm going to take out the bike today for more trials....


If you light a candle and place a jar over it (sealing it) the candle will continue to burn until all the oxygen has been used up. Basic fire triangle being fuel,oxygen,ignition. Take away anyone of these and there is no flame.

The ratio of oxygen to nitrogen in the atmosphere will also change the combustion delay by even 1%.


I think I’m taking this away from your original question.


We are talking two different things Allan...Oxygen by it's self does not burn , try lighting and oxy acet torch with just the oxy turned on...But it is necessary to support combustion as you say....

I took a few 10 miles rides on the bike..with the jetting mentioned...Riding normally as in using no more than 1/2 throttle or so and below 5500 rpm the engine is a real sweetie, instant response to small changes in the throttle, nice idle and it runs smooth for a Triumph. I did open it through second and third and it pulled good to 6000 rpm..The plugs are dark but not sooty...I raised the needle clip one notch and will take a longer ride to "recolor" the plugs...The plugs are NGK Competition # 7 , one step colder than normal for a T140E...
Sometimes I need to walk aways from this tuning [***] and then it seems better after a day of not fussing with it..
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/08/19 11:44 pm

I took another ride for about 45 minutes ,the engine performance is quite good overall....There's still a bit of funny business during throttle but it's very brief and easy to power right through it...When the O2 gauge is hooked up I may have a better idea...
The right side plug is a little darker than the left..The insulators are dull white the but shell shows signs of a mixture a bit richer than it needs to be,,The ground strap shows only 25% heat discoloration...Usually means the plug is too cold...The plugs are one heat range colder than normal....I was running a .035 plug gap I closed down to .030...It may be the ignition is not happy with a larger gap..Probably should screw in the correct heat range...
Posted By: NickL

Re: Wierd jetting issue Mikuni - 06/09/19 1:20 am

Just thought i'd chime in here,,,,,,,, The Rita has a very short spark time (around 500us) this is considerably
shorter than the normal 1-1.5ms that most ignitions of this type use. It allows a greater dwell at higher rpm but the short
spark time may make it more sensitive to mixture setups.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/10/19 11:54 am

Nick I closed the plug gap to .028.I believe that wider gaps can shorten spark duration,thanks for the reminder....Yesterday I rode for an hour chasing a friend on his 950 Guzzi cafe bike...The engine pulls very nicely at 3/4 throttle to 5500 through the gears,..Full throttle at higher rpm in the upper gears is not what I expect it to be but it will pull 6500 rpm .Not misfiring...I believe it goes rich despite what seems like a small 175 main jet. The spark plug ground strap color looks good,The insulator is dull off white and the shell appears to show decent mixture.No signs of detonation..It was warmer yesterday, 80F...Like all Triumphs, flogging the engine for an extended period brings on all sorts of clattering..
Despite often said statement that the main jet stands alone,it's been my observation doing all the needle, needle jet ,pilot jet and main jet work on this engine, a change in one effects all the other circuits to some degree...
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/10/19 8:59 pm

i don't believe anything in these old carburetors work all by themselves.

it's always a mixed-up head scratcher.

fuzzy logic, we used to call it.
Posted By: Allan Gill

Re: Wierd jetting issue Mikuni - 06/10/19 9:14 pm

Originally Posted by Hillbilly bike

Despite often said statement that the main jet stands alone,it's been my observation doing all the needle, needle jet ,pilot jet and main jet work on this engine, a change in one effects all the other circuits to some degree...


Very much so, every change I’ve done on my bike has resorted to doing the full rejecting sequence from Idle mixture to WOT main jet then finishing with idle mixture again. Each jet will influence the one before it and when you don’t have a great deal of free time it becomes a real chore. I had my lightning setup pretty good, bike would pull 110, when I decided to strobe the motor I found it was well over advanced.... the previous jetting is now too lean for the spark advance so need to go through that sequence again. Although it should rev a little harder with less spark advance from my experience.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/10/19 11:13 pm

Yes, a little less timing can make more top end power......Carburetors as we know it were in form around 1900...It's something from a different place and time, a device that responds to the natual laws of the universe....it needs no power source....it's so simple it's confusing...
Posted By: John Healy

Re: Wierd jetting issue Mikuni - 06/11/19 2:00 am

Many of the Mikuni range carbs the needle never leaves the needle jet orifice. This makes changes at lower throttle openings effect, and what on an Amal would have no effect on the main jet, change the effect of the main jet.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/11/19 11:29 am

Yes, the needle is always in in needle jet orfice.I had marked the needle originally to see what goes on..there is about 8 needle jets and 14 different profile metering rods for this type Mikuni.Today I'l have the air fuel ratio gauge on the bike...The actual readings aren't so important as the change in readings past 2/3 rds throttle. The old standby of listening for 8 stroking or rolling back the throttle a bit to check for too lean doesn't seem to give any feedback..On my race bike with the same but larger Mikuni TM's, on the dyno, power fell off about 2 HP with one jet size leaner or richer than optimal..
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/11/19 3:40 pm

Ok...I have been running the bike around with the air/fuel gauge installed...The O2 sensor is located on the two into one collector about 10 inches past where the two pipes merge and about 32 inches from the head port...It's a bit cool right now, 60F.. I fire up the bike and wait a few minutes to warm up the exhaus system...The bike is very lean with the chokes off, 17-1 and running a bit rough...Pull on one choke and the gauge drops to 13-1 ,so it's working..I take off down the road at a moderate pace to warm the engine...The engine acts lean and the gauge is 16-1...After a few miles the engine is running smoothly at part throttle and the gauge is around 14-1 at 50 mph...I roll open the throttle slowly in high gear , the gauge drops to 10.9 and climbs up to near 12 as the engine winds out...I stop in deserted section and full throttle it through the first three gears to 6500 rpm, the gauge reads 12-1.I did it several more times to confirm....Open the throttle in fourth ,the gauge drops to 10.8 during the longer pull and creeps up near 12 at 6000 rpm....On the way home I mess with shifting up and at part throttle to check the gauge reading...
My conclusion is the air/fuel reading are actually reasonably accurate from the way the engine responds... To sum it up...It's too lean at 1/8 throttle,just a touch lean at 1/4 throttle and the rest ok...the full throttle today felt strong and the air/fuel ratio is where it should be I believe .....The engine going rich when the throttle is rolled open in the higher gears at 4000 rpm may be a bit of reversion, or????
Posted By: Andy Higham

Re: Wierd jetting issue Mikuni - 06/11/19 7:16 pm

Something that happens in high performance 2 stroked is "triple loading". When the engine is out of its resonant range, air is sucked in through the carb picking up fuel, then pushed back through the carb picking up more fuel before being drawn back into the engine picking up yet more fuel on the way.
I wonder if something similar is happening to your bike? have you got wild cams in it?
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/11/19 7:29 pm

that gauge has a lower display limit of 10.0, so it will read the same at 10:1 and richer.

appears that the pilots are lean, the mains are about right.

10.9 seems rich for what you're getting, because you're showing 12s at open throttle.

on my street bike i got best power when everything was at 11.5, more or less all the way up from the slide cutaway.

maybe tom has some thoughts in the numbers.

can you see any fog at the carburetor mouth blipping the throttle?
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/11/19 7:46 pm

No wild cams in this bike..Stock T140 intake timed at 100 degree lobe center..,3134 type exhaust timed at 103...I'm interested in why with the throttle held wide open the air fuel ratio above 4000 rpm in the upper gears is richer than the lower gears...

Kevin thisThis T140 e type head combined with tight piston to head squish might have leaner fueling requrements...so 12-1 might be good..
Posted By: kevin roberts

Re: Wierd jetting issue Mikuni - 06/11/19 8:15 pm

yes, i think.

the reason i have to show 11.5 on the gauge isn't because that's what's burning, it's because my head is inefficient, so to burn at 12 or so i have to waste mixture through the pipes. likerunning at partial choke all the time. i haven't measured fuel mileage in over 30 years.

Quote
I stop in deserted section and full throttle it through the first three gears to 6500 rpm, the gauge reads 12-1.I did it several more times to confirm....Open the throttle in fourth ,the gauge drops to 10.8 during the longer pull and creeps up near 12 at 6000 rpm....


i don't understand ^^^this, unless you do have reversion but over such a narrow rpm range that you're going through it too quickly to matter when you open the throttle quickly.

obviously the solution is to ride at full throttle all the time. you'll have more fun that way anyway.
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/11/19 9:16 pm

Yeah, i don't understand either.... so I can roll open the throttle in third gear at 4000 rpm and the A/F drops from 14 to 12 and stays there up to 6500....I do that in fourth and drops to 10.5 and stays there pretty much, Same for 5th full throttle roll on but I have not pushed it beyond 80 because I see cows near the road or there's another sharp bend coming up....
Posted By: koncretekid

Re: Wierd jetting issue Mikuni - 06/11/19 10:01 pm

Andy,
I like this analogy to the 2- strokes. We think of our old Brit iron as having mild cams, but in fact, they all have some overlap at TDC on the exhaust stroke which will cause some reversion. All of the testing I have done has in fact shown a tendency to rich at some point in the cycle (albeit, only bikes with an A/F gauge can be verified), and it could be due to a similar situation as depending on lengths of intake and exhaust, back flow thru the carb may be causing the apparent over rich mixture. On our race bikes, tuned to take advantage of helpful reversion at high rpm, the problem becomes worse.
Tom
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/12/19 12:44 am

The valve overlap on this engine is 43 degrees... The valve timing in this engine, intake and exhaust lobe centers are pretty much the same as T120 Bonnevilles that were noted for a nice linear power band..Been my experience that this cam timing and having the carbs close to the head doesn't result in reversion issues .. However, the stock two into one T140D exhaust pipes and collector on this bike might create some reversion.......
Posted By: DavidP

Re: Wierd jetting issue Mikuni - 06/12/19 6:08 am

Air correction jets? What size are you using?
Posted By: Hillbilly bike

Re: Wierd jetting issue Mikuni - 06/12/19 10:07 am

Originally Posted by DavidP
Air correction jets? What size are you using?

Good call...but 32-36 MM Mikuni TM's don't have air bleed jets..........I'm going to ride around more today forA/F readings...Once the engine is warmed up the performance from idle to wide open feels flawless to the rider. Engine responds instantly and linear to throttle movement..,better than the Amal Mk2's....But the richness in wide open high gear is puzzling..
What also is pleasing, the modifications I did for tight piston to head squish has worked out...This engine has an actual 9.2 static compression and using 90 octane non ethanol gas there is no signs of detonation....
Posted By: koncretekid

Re: Wierd jetting issue Mikuni - 06/12/19 11:28 am

Don't be afraid to try smaller main jets. I told you on an earlier post that my TR25 was still rich at mid throttle and at WOT at 5-6000 rpm on a 160 mj, but I forgot that I actually bought 4 more jets and actually am now running a 140 mj. It would seem that your 650 would have a stronger vacuum signal that my 250, so you may be able to lean it out more now that you have an A/F gauge to check.
Tom
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