mikuni flatslide flatspot
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This is the thread I used as a starting point for my TM34...note the air jet is called out. I had to drop down on pilot jet.
Might not be applicable, but the main jet is much bigger than what we have been discussing here.
jets-and-needles-for-tm34-stk750-t8204.html?hilit=mikuni%20flat%20slide
Might not be applicable, but the main jet is much bigger than what we have been discussing here.
jets-and-needles-for-tm34-stk750-t8204.html?hilit=mikuni%20flat%20slide
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baz said:
Baz,
I was thinking about this and wonder if you might want to try different heat range plugs if you don't sort this. It might tell you if you are dealing with a rich issue if you go up a range and it improves. It wouldn't do anything if it is a starvation problem. It's a cheap thing to try tho...just a thought. Sometimes with carbs that won't dial in this works. Make sure you do a plug chop afterwards to make sure the burn is good if you decide to try this.
Dennis
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Baz,
The 40mm carb size has been bothering me since the beginning of this thread.
I did some research and found a carburetor sizing chart for Mikuni carbs in my VM tuning manual. There isn’t a tuning manual for TM carbs.
Aperture opening is based on total volume of air that the engine can pump at full RPM. I calculated 750 and 850 and put them on the chart below.
It shows that the 40mm carb is too big. In the notes from Mikuni and in the chart on the left you can see the effect of too large or too small of a carburetor. On the too large aperture the performance at high RPM increases while the low end performance decreases. There is no mention of other effects and this says that you could potentially get the 40mm carb to work, but the overlaps between the circuits may come into play.
The main consideration is velocity. Given the larger opening, air volume increases while velocity decreases. On a carburetor without an accelerator pump (an assumption on my part) this might explain the hesitation since there is a slight delay that occurs during throttle opening when vacuum drops until engine speed catches up. Normally you can’t feel this, but with lower velocity it might create the problem you are experiencing.
I personally would try smaller jets before I would give up, but there is a point of diminishing return when you cross over the recommended size of aperture. It’s a factor that could make it much more difficult to tune.
The 40mm carb size has been bothering me since the beginning of this thread.
I did some research and found a carburetor sizing chart for Mikuni carbs in my VM tuning manual. There isn’t a tuning manual for TM carbs.
Aperture opening is based on total volume of air that the engine can pump at full RPM. I calculated 750 and 850 and put them on the chart below.
It shows that the 40mm carb is too big. In the notes from Mikuni and in the chart on the left you can see the effect of too large or too small of a carburetor. On the too large aperture the performance at high RPM increases while the low end performance decreases. There is no mention of other effects and this says that you could potentially get the 40mm carb to work, but the overlaps between the circuits may come into play.
The main consideration is velocity. Given the larger opening, air volume increases while velocity decreases. On a carburetor without an accelerator pump (an assumption on my part) this might explain the hesitation since there is a slight delay that occurs during throttle opening when vacuum drops until engine speed catches up. Normally you can’t feel this, but with lower velocity it might create the problem you are experiencing.
I personally would try smaller jets before I would give up, but there is a point of diminishing return when you cross over the recommended size of aperture. It’s a factor that could make it much more difficult to tune.
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The diagrams below might also be helpful.
These show the fuel circuits and the overlaps for Mikuni VM carburetors. There are no charts for TM carbs but the circuits are the same.
An interesting note in the Mikuni manual is they suggest a plug chop at the RPM and load where the problem occurs. Might be hard to do this tho. They talk about doing plug chop for each circuit when changing carbs...I found that interesting.
Velocity is critical for fuel delivery and maintaining proper mix. With a lower velocity and more air, it is more difficult to get the proper mix and it may muddy up the range in terms of circuit crossover as well. Since fuel is being delivered by each of the circuits just changing one may not be enough.
These show the fuel circuits and the overlaps for Mikuni VM carburetors. There are no charts for TM carbs but the circuits are the same.
An interesting note in the Mikuni manual is they suggest a plug chop at the RPM and load where the problem occurs. Might be hard to do this tho. They talk about doing plug chop for each circuit when changing carbs...I found that interesting.
Velocity is critical for fuel delivery and maintaining proper mix. With a lower velocity and more air, it is more difficult to get the proper mix and it may muddy up the range in terms of circuit crossover as well. Since fuel is being delivered by each of the circuits just changing one may not be enough.
baz
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dennisgb said:Baz,
The 40mm carb size has been bothering me since the beginning of this thread.
I did some research and found a carburetor sizing chart for Mikuni carbs in my VM tuning manual. There isn’t a tuning manual for TM carbs.
Aperture opening is based on total volume of air that the engine can pump at full RPM. I calculated 750 and 850 and put them on the chart below.
It shows that the 40mm carb is too big. In the notes from Mikuni and in the chart on the left you can see the effect of too large or too small of a carburetor. On the too large aperture the performance at high RPM increases while the low end performance decreases. There is no mention of other effects and this says that you could potentially get the 40mm carb to work, but the overlaps between the circuits may come into play.
all help is appreciated ,it probably is just a 40mm carb is too big in the engines stock tune,but it interests me to find out for sure!,i have marked the throttle positions on my twistgrip and can confirm that the flatspot appears at 3/4 opening so more a needle problem?? i also went further down on the main jet from 145 to 120 a big jump i know but this made the bike run really bad pulling back and missfiring at 1/2 throttle ,the very best the bike has run so far is with a 165 main jet and the needle in the leanest position ie top groove i am going to do a plug chop after holding the bike in the 6500-7000rpm flatspot to see what the plugs are doing cheers baz
The main consideration is velocity. Given the larger opening, air volume increases while velocity decreases. On a carburetor without an accelerator pump (an assumption on my part) this might explain the hesitation since there is a slight delay that occurs during throttle opening when vacuum drops until engine speed catches up. Normally you can’t feel this, but with lower velocity it might create the problem you are experiencing.
I personally would try smaller jets before I would give up, but there is a point of diminishing return when you cross over the recommended size of aperture. It’s a factor that could make it much more difficult to tune.
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baz said:
Plug chop should be done under load. That's why I said it might be hard to do because you need to hit it at the spot where it fails.
It's interesting that the smaller jet showed what seems to be a lean condition...did you move the needle up with this jet? Change in engine response is what you were looking for so this is important.
How did the 145 jet run? Was it similar to the 165? If it acted similar to the 120 then something between 145-165 might make a difference. This is when you might need to drill your own size to get it right. I think the jumps in sizes you made might have been too much based on the information you provided. Once you are in the range the steps need to be smaller to find optimum combination.
A single 40 Mm Carb doesn't seem overly large for a 750. The total aperature area of the 40 mm carb is still about 12% less the stock twin 30 mm carbs setup, which works very well. I have a 40 mm Carb on my 490cc Maico, that bike pulls with brutal force from idle to 11,000 rpm. My project bike will get dual 40s for 1360 ccs. The Ducati 900 ss used dual 40s with a good result.
So a single 40 seems reasonable on a 750, maybe even a bit conservative?
Glen
So a single 40 seems reasonable on a 750, maybe even a bit conservative?
Glen
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'It is possible and maybe likely that this carb is just too big, but I really think you are close.'
If the air velocity in the carb is very low because of the big choke, you might need a smaller jet. Are you talking about a flat spot or a cough? If it is rich it will be sluggish but usually still run OK. If it is lean it will usually cough as you open the throttle larger. If it is rich at the large throttle openings you probably need a different shaped needle (fatter below the shoulder ) to suit such a large carburetor on a motor which might not need it. If you lower the needle you should be able to induce a cough at about three quarters throttle, otherwise the needle jet is too big. If you are talking about a hesitation when you are holding the throttle wide open and accelerating, I'd be looking for a fuel flow problem such as a blocked tank breather. If the hesitation comes as you wind the throttle open, it is probably due to the needle and needle jet. The brown plug colour you mentioned is largely irrelevant. The plug should have a black ring on the porcelain, right down inside where the porcelain meets the metal, if the plug is the correct heat range and the mixture and ignition timing is correct. I only use plug reading to check that the main jet size is right, usually for big circuits only. If you are using the bike on public roads, you will need to do a high speed run on a slightly uphill road and do a plug chop.
If the air velocity in the carb is very low because of the big choke, you might need a smaller jet. Are you talking about a flat spot or a cough? If it is rich it will be sluggish but usually still run OK. If it is lean it will usually cough as you open the throttle larger. If it is rich at the large throttle openings you probably need a different shaped needle (fatter below the shoulder ) to suit such a large carburetor on a motor which might not need it. If you lower the needle you should be able to induce a cough at about three quarters throttle, otherwise the needle jet is too big. If you are talking about a hesitation when you are holding the throttle wide open and accelerating, I'd be looking for a fuel flow problem such as a blocked tank breather. If the hesitation comes as you wind the throttle open, it is probably due to the needle and needle jet. The brown plug colour you mentioned is largely irrelevant. The plug should have a black ring on the porcelain, right down inside where the porcelain meets the metal, if the plug is the correct heat range and the mixture and ignition timing is correct. I only use plug reading to check that the main jet size is right, usually for big circuits only. If you are using the bike on public roads, you will need to do a high speed run on a slightly uphill road and do a plug chop.
baz
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the 145 ran pretty much the same as the 165 i did go down in .5 stages until i reached 145 the 120 i used was an 80 that i drilled out 1.2mm so maybe not ideal i did not move the needle when i had the 120 fitted maybe i should have!,in the mean time i have today and i tried running it with the 1-1 pilot air jet removed just for the hell of it,all this did was stop the bike form ticking over unless i adjusted the tickover screw right up and lots of popping and banging on the overrun so i replaced that and went for maybe a 30 mile ride as its the first chance i have had lately when the local roads are not underwater and when i was near home i opened up the throttle to get it into the flatspot to check the plug colour and i had difficulty finding it!.the flatspot is just a very slight dip at 3/4 throttle i now would have to induce it to make it happen by snapping the throttle could it be i just didnt have the engine hot enough?? the only other difference i had was when i went to start it today fuel was leaking from the overflow so i gave the the float chamber a few wacks with a screwdriver handle until it stopped,i am now very happy with this setup the bottom end and the mid range are just brilliant with a rock solid tickover i'm very pleased with it now regards bazdennisgb said:
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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.
I never drill main jets, only needle jets, where you can adjust the needle height to correct. The factory uses a flow meter to get main jets accurate, I don't even pass a wire through them.
I never drill main jets, only needle jets, where you can adjust the needle height to correct. The factory uses a flow meter to get main jets accurate, I don't even pass a wire through them.
baz
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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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worntorn said:
Each 30mm carb is operating independently feeding a single cylinder on twin carb set up. A parallel twin engine fires on opposite strokes of the engine so the intake is only on one cylinder at a time. Even tho the pistons move at the same time the valves open on opposite strokes. A single carb is feeding each cylinder on stroke demand...not both at the same time so it needs to be sized for one cylinder not two.
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