mikuni flatslide flatspot

acotrel said:

jetting at 3/4 throttle is the most difficult thing to get correct. If you get it right, the rest is easy. If you lower the needle one notch and the hesitation gets worse, you know which direction you must go. If it doesn't get worse, your needle jet is too big. If the motor runs OK at full throttle, leave setting the mains by reading plugs, until you have 3/4 throttle right.

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With this larger carb the needle steps are likely less effective...one step probably will not show much change based on what Baz has posted on his changes so far.

baz said:

i feel this is a maybe a needle shape problem rather than main jet.the needle is in the leanest position where it runs the best although it seems a little lean at 3/4 throttle opening hence it gets better the hotter the motor gets if i raise the needle it affects it lower down the range making it run rich at just over 1/2 throttle and still dosent run any better at 3/4 open but after riding the bike today i think i will leave alone and just enjoy it as it is with not much more than a hiccup at around 6500 dosent bother me regards baz

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Do a full throttle plug chop and check your color if you leave it where it is.

Dennis, if you were to size the single Carb for one cylinder on the twin as you say, then you would mount a 30 mm Carb on the 750, that is the optimum twin Carb per cylinder size. But in practice we know that size will be much too small for good performance as the larger 34 mm single Carb already chokes the top end out of these bikes. This has been shown on dyno as well.
According to Pete, and now Baz, the 40 single gives great performance. It would be nice to see the results of a dyno test with a 40mm single carb.

Glen

worntorn said:

Dennis, if you were to size the single Carb for one cylinder on the twin as you say, then you would mount a 30 mm Carb on the 750, that is the optimum twin Carb per cylinder size. But in practice we know that size will be much too small for good performance as the larger 34 mm single Carb already chokes the top end out of these bikes. This has been shown on dyno as well.
According to Pete, and now Baz, the 40 single gives great performance. It would be nice to see the results of a dyno test with a 40mm single carb.

Glen

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yes i would love to put it on a dyno just to see what it power it has i would not expect it to get near a twin carb setup for absolute power but where it counts for me is in the midrange my bike handles extremely well and the way it powers out of bends is something else cheers baz

baz said:

worntorn said:

Dennis, if you were to size the single Carb for one cylinder on the twin as you say, then you would mount a 30 mm Carb on the 750, that is the optimum twin Carb per cylinder size. But in practice we know that size will be much too small for good performance as the larger 34 mm single Carb already chokes the top end out of these bikes. This has been shown on dyno as well.
According to Pete, and now Baz, the 40 single gives great performance. It would be nice to see the results of a dyno test with a 40mm single carb.

Glen

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yes i would love to put it on a dyno just to see what it power it has i would not expect it to get near a twin carb setup for absolute power but where it counts for me is in the midrange my bike handles extremely well and the way it powers out of bends is something else cheers baz

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I have found a Dyno service near my home. My plan is to get some data to report as soon as the weather breaks. Stay tuned.

worntorn said:

Dennis, if you were to size the single Carb for one cylinder on the twin as you say, then you would mount a 30 mm Carb on the 750, that is the optimum twin Carb per cylinder size. But in practice we know that size will be much too small for good performance as the larger 34 mm single Carb already chokes the top end out of these bikes. This has been shown on dyno as well.
According to Pete, and now Baz, the 40 single gives great performance. It would be nice to see the results of a dyno test with a 40mm single carb.

Glen

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Glen,

If you look at the chart I posted (this is in no way absolute), it shows the ranges for each engine based on maximum air flow. If you push RPM or engine CC's up in the calculation it will move the size range accordingly (I used 7500). This is why it is not absolute because if you make engine modifications the calculation should change accordingly. I kept the calc pretty basic because Baz has a stock bike. In the discussion we have talked about the potential to get the 40mm to work. In theory you can put a smaller or a larger carb on the engine and get it to work although as you pass out of the recommended ranges you get poorer results on one end of the scale depending on which way you go and it also makes it more difficult to tune as Baz has experienced. In this case the 40mm probably gives up a little low end response for more top end and likely a small increase in horsepower. Baz feels pretty good about this carb on his bike, but it would be interesting for him to try a smaller carb (34-36mm), because the flat slides improve response and likely a smaller carb would be even more responsive than the 40mm.

The 30mm duals are on the small end of the scale...not sure why they were chosen other than to reduce fuel use or maybe to meet pollution at the time although I'm not sure that was a big deal back then.

One clarification. Figure 10 does not relate to Figure 11 directly. Any carb size that falls within the range would fall on the center line in figure 10 "proper aperature". This along with the multitude of different performance combinations and engine modifications will change the range, so unless talking about a specific bike a "specific" size carb can not be assummed. The fact that a 34mm single carbs seems to run "okay" on most Commandos probably relates to that it falls in the center of the range...but again on some bikes it might not work because of performance changes made. Carb sizing really should be done on each particular bike.

We also were not talking about performance tuning here. That goes way outside of the tuning issue Baz has been dealing with.

One additional thing to consider. Comparing dual carbs to a single is not a fair comparison because there are trade-offs with running a single carb. The "Y" manifold used on a single carb set-up is a major compromise to flow. This is likely why there is a Dyno drop off with single carb set-ups. Again this starts to move into performance tuning, when most people who change to single carb are not as worried about top end performance...or at least they shouldn't be thinking in those terms.

My 34mm Mikuni Flat Slide performs well on my 850. I have not had it on a dyno and really don't care too much about top end performance but more consistent performance through the range and simplicity from a tuning and maintenance standpoint.

The reason I shied away from going to a larger carb was due to comments that said the larger carbs won't fit in the frame...and binding issues with throttle cable. I had a bad Mikuni VM on the bike and needed to replace it. I felt the TM would be an upgrade and it is in terms of response.

'The 30mm duals are on the small end of the scale...not sure why they were chosen other than to reduce fuel use or maybe to meet pollution at the time although I'm not sure that was a big deal back then.'

TORQUE ! The strength of the long stroke commando motor with it's heavy pistons doesn't lie in revving the tits off it. Increase the torque, increase the gearing and go faster. It is not rocket science.

A well tuned single carb on a Y manifold will usually perform better than poorly tuned twin carbs, and because you are only playing with one set of jets, it is easier to get it right. I've tried both configurations on Triumph 650s and my 500cc top end motor, and there is virtually no difference, unless you are very patient jetting the twins. If you are using methanol, the twin carbs are better because you can have errors which don't matter much. When you use petrol, it is much more critical that you get it correct for maximum power, the differences in jet dimensions (flow) have a much greater effect than with methanol unless you are on the limit of leaness. With methanol you usually have twice the margin for error (unless your bike is a two stroke).

acotrel said:

'The 30mm duals are on the small end of the scale...not sure why they were chosen other than to reduce fuel use or maybe to meet pollution at the time although I'm not sure that was a big deal back then.'

TORQUE ! The strength of the long stroke commando motor with it's heavy pistons doesn't lie in revving the tits off it. Increase the torque, increase the gearing and go faster. It is not rocket science.

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Not sure I understand why 30mm would be better than 32, 34 or even 36mm if properly jetted for the given engine. Maybe a very slight improvement on the low end. All of those carbs should be able to be tuned to deliver the fuel/air mixture properly...it's about getting the mixture right and into the engine.

It is about gas flow velocity down the port. If you port out bigger the flow rate drops too much at low revs, so you lose midrange power and that is where you need it the most. Bigger ports shift the torque upwards often to where you cannot use most of it. The standard ports in a Triumph 650 are at about the maximum for that sized motor, and a commando has a lower usable rev range because of the mechanical limitations.
The port size goes hand in hand with balance factor. The first thing to establish is the rev range the motor can withstand. The result affects the way the bike must handle and be geared to get max performance on various circuits.
Big ports might look good, however they are not necessarily better.

acotrel said:

It is about gas flow velocity down the port. If you port out bigger the flow rate drops too much at low revs, so you lose midrange power and that is where you need it the most. Bigger ports shift the torque upwards often to where you cannot use most of it. The standard ports in a Triumph 650 are at about the maximum for that sized motor, and a commando has a lower usable rev range because of the mechanical limitations.
The port size goes hand in hand with balance factor. The first thing to establish is the rev range the motor can withstand. The result affects the way the bike must handle and be geared to get max performance on various circuits.
Big ports might look good, however they are not necessarily better.

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Okay, that makes sense...velocity is higher with the 30mm so smallest carb that is in the range provides the best performance. The same amount of fuel will be delivered but at a faster velocity...sorry it took me a minute to catch up :shock:

Re: mikuni flatslide

I have a basically stock MK2 A 850 motor . I went from a single Mikuni 34 mm VM carb to a single 34 mm Mikuni FLatslide TM carb.

My bike starts great with the old Boyer and runs much more aggresively with the 34 flat slide.
I am pretty thrilled with the response from the single 34 TM flatslide.

I keep seeing offerings for a lengthened intake for the commando saying the response would be better and even a 2 hp gain just from that.
Now I am assuming that this gain would be the case for dual carbs , not a single.





My plugs ...tip and insulator are a brownish tan, not a light tan. The circular end of the plug is sooty black.

I can go one more notch drop on the circlip to the topmost groove ( leaner).

SHould I go for a lighter tan insulator ?

What might I expect from changing the intake to a longer one?

Ed

Some pretty good info here
http://www.empirenet.com/pkelley2/sparkplugreading.html
Pete