Texas Two Step Taco

[Texasstar]

Notes from the first dyno pull with two petcocks wide open.

After we connected the second petcock we now had 500 ml per minute flowing we also noticed there was no fuel in the float bowl. So that answered our question why we made great power at 6k from last Sunday’s pull and then fell flat on our face between 6-8k it was because the fuel flow couldn’t fill bowl the bowl fast enough. John was right we needed at least 400ml per minute and we only had 200ml per minute. Lesson learned and hopefully someone learns from this in the future. Watch your petcock it can blow your engine both literally and figuratively. Don’t run around half petcocked.

First thing I noticed on this run was the throttle was sluggish and not snappy like the last run. This run caught me off guard, because as soon as we got past 6k the vibration was gone and stayed gone. It felt the same at 8k as it did at 6k. The good news is now I know exactly what it feels like if I am running on one petcock. Bad vibrations. Bill was scratching his head saying we didn’t make any changes and we are only making 18 whp. As John said the torque numbers don’t lie either-we were only at 12 ft lbs. We were back to being FAT. Air fuel was 11:1 not 14:1 like the previous run.


Vibration gone, full float bowl, and fuel flow we could move forward tuning. BTW John I started looking at the Torque numbers way more than the HP numbers.

This pull had a 320 jet, 21 degrees timing, Br9es plug. C12 fuel with 20:1 BeNOL. We made only one change and that was to set the rev limiter at 8500 rpms. Bill Baxter says “that between the throttle and stinger on a two stroke is a vicious cycle.”

18 whp 12 ft lbs torque 11:1 air fuel 1100 degrees EGT. Still very rich. Throttle sluggish. Was not discouraged at all because of what happened when we had the air fuel at 14:1. Also we haven’t even started to tune the new expansion chamber.
Here is the video with the first run at 18 whp and the 3rd to the last run with 22 degrees timing and a 230 jet. We made 3 pulls with the 230 jet and they were all fat making right at 36 whp. We didn’t have any smaller jet and it looks like we are fat between 7-8k

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[John Murray]

Notes from the first dyno pull with two petcocks wide open.

After we connected the second petcock we now had 500 ml per minute flowing we also noticed there was no fuel in the float bowl. So that answered our question why we made great power at 6k from last Sunday’s pull and then fell flat on our face between 6-8k it was because the fuel flow couldn’t fill bowl the bowl fast enough. John was right we needed at least 400ml per minute and we only had 200ml per minute. Lesson learned and hopefully someone learns from this in the future. Watch your petcock it can blow your engine both literally and figuratively. Don’t run around half petcocked.

First thing I noticed on this run was the throttle was sluggish and not snappy like the last run. This run caught me off guard, because as soon as we got past 6k the vibration was gone and stayed gone. It felt the same at 8k as it did at 6k. The good news is now I know exactly what it feels like if I am running on one petcock. Bad vibrations. Bill was scratching his head saying we didn’t make any changes and we are only making 18 whp. As John said the torque numbers don’t lie either-we were only at 12 ft lbs. We were back to being FAT. Air fuel was 11:1 not 14:1 like the previous run.


Vibration gone, full float bowl, and fuel flow we could move forward tuning. BTW John I started looking at the Torque numbers way more than the HP numbers.

This pull had a 320 jet, 21 degrees timing, Br9es plug. C12 fuel with 20:1 BeNOL. We made only one change and that was to set the rev limiter at 8500 rpms. Bill Baxter says “that between the throttle and stinger on a two stroke is a vicious cycle.”

18 whp 12 ft lbs torque 11:1 air fuel 1100 degrees EGT. Still very rich. Throttle sluggish. Was not discouraged at all because of what happened when we had the air fuel at 14:1. Also we haven’t even started to tune the new expansion chamber.
Here is the video with the first run at 18 whp and the 3rd to the last run with 22 degrees timing and a 230 jet. We made 3 pulls with the 230 jet and they were all fat making right at 36 whp. We didn’t have any smaller jet and it looks like we are fat between 7-8k

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Encouraging results, with a bit of luck your next session will see you making 40hp+ at the rear wheel which is pretty respectable for such an old engine design. Should be pretty quick on the strip too.

The small main jet requirement indicates a very strong intake signal, which is to be expected I guess considering the engines high primary CR and shortish rod. There's very likely more power to be found with a larger carburetor (say 40 - 44mm) but I wouldn't even think about this until the current setup is tuned and you have a baseline. Nice work!

 

[Texasstar]

The M51 Bultaco probably has the worst head that Bultaco made. Single plug, poor flame propagation, and poor squish. We are making right at what the Bandido was published to make (and we know they lied a lot back then) https://www.klemmvintage.com/91oct.htm


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[teazer]

Progress is good. Fr the fuel flow, you can work out how much fuel you need for a given HP level. What size is the float valve? I like to use the largest available and then I make sure the sharp changes in direction in the carb body are eased - ie port the fuel inlet and fit a 3.5 float valve. Cross drilling the valve body often helps a little too.

HP numbers used to me measured at the crank either directly or by calculatiopn and were always much larger than rear wheel. Example was the GT750 quoted at 67hp net. In reality they made 48-50 at the rear wheel. I am sure that you are already making more HP than stock with more to come.

A head swap should be interesting and as HP rises, it will become more responsive to changes in timing and jetting. Watch for how the A:F changes with revs to see where it needs to be fine tuned. It's not easy to get the main jet right for all revs at WOT.

BTW, HP and Torque are basically teh same thing but the engine speed part of HP can lead you to incorrect conclusions especially after the torque peak where rising revs often cover the falling torque. You should see peaks in torque and HP getting closer as you zero in.

 

[Texasstar]

Paris Hilton sent me a note

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[teazer]

Beans and broccoli.....

 

[John Murray]

The M51 Bultaco probably has the worst head that Bultaco made. Single plug, poor flame propagation, and poor squish. We are making right at what the Bandido was published to make (and we know they lied a lot back then) https://www.klemmvintage.com/91oct.htm


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I hope the new head makes more power for you, but I have to say a well prepared trench head can make good power on these engines. On the dyno they did very well though they are under-cooled for any sort of WOT usage longer than a few seconds.

With my Pursang engine using the trench head I found it responded well to a projected nose plug; you may want to try a BP9ES in yours.

 

[Texasstar]

I hope the new head makes more power for you, but I have to say a well prepared trench head can make good power on these engines. On the dyno they did very well though they are under-cooled for any sort of WOT usage longer than a few seconds.

With my Pursang engine using the trench head I found it responded well to a projected nose plug; you may want to try a BP9ES in yours.

Thank you. How close are we to your jetting? We currently have a 230 main in the TM 38 and Bill was a little concerned to go lower. We also didn’t have any jets to go lower. We were able to lean out the last run with one needle change and we have one more to go. It really took care of the dip we had in the curve. Roller speed was way up on the last pull also. Vibration was consistent and felt way better! Still going to add stays.

I really think if we got the squish angle right on this M51 head it would make over 40. Do you think Bultaco went to two plugs to help cooling? Take a look at this M70 head we have.

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[Texasstar]

Plug gap is .035 here is what the plug looks like we are using a BR9es because the electronic ignition needs a resistor plug. Not going to pull the head yet. Going back to Bills tonight with a fresh plug and some jets. Going to also run some U4.4. Then we will pull the head. Any objections on going 32:1 on the Benol?


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[John Murray]

Thank you. How close are we to your jetting? We currently have a 230 main in the TM 38 and Bill was a little concerned to go lower. We also didn’t have any jets to go lower. We were able to lean out the last run with one needle change and we have one more to go. It really took care of the dip we had in the curve. Roller speed was way up on the last pull also. Vibration was consistent and felt way better! Still going to add stays.

I really think if we got the squish angle right on this M51 head it would make over 40. Do you think Bultaco went to two plugs to help cooling? Take a look at this M70 head we have.

I ran 320 in the TM38, but which is much bigger than what yours wants obviously but then I think your primary CR would be quite a bit higher (because of single transfer pair, shorter rod and smaller intake volume) and this would explain the very strong intake signal. When I stopped testing this engine I was playing with stinger diameter and there were indications that the existing stinger was undersized. Increasing the stinger size picked up some power and reduced the jet requirement - I'm not sure why. It seemed to me that being slightly too-big on the stinger was better than being slightly too-small as far as power goes, though this goes against classic theory.

It wasn't overly fussy about timing - it'd make good torque over a range of 3 or 4 degrees though of course the curve would be shifted up or down. It liked a fair bit of advance - around 20 - which isn't surprising considering the bore diameter. The trench and the modern chamber designs both wanted similar timing, but the better cooled Husky head would tolerate a more aggressive tune.

The trench head I used was a twin plug like the one in your photo, and I used a Dyna twin coil to fire both plugs. I really don't think the trench chamber is a bad thing, though of course all the forum experts will say nothing can match a modern semi-toroidal shape. The trench was used to help direct the flow in an orderly fashion across the top (allegedly), something that can help with the old engines and their very fast transfer flow velocity. You can actually get some useful squish areas on the sides of the trench with some careful machining, then afterwards use a die grinder to open up the trench to get the desired CR. Just remember that with an inertia dyno you don't get anywhere near enough run time to get the head up to normal riding temps, so be very very careful and listen for det with every bone in your body when you ride it for the first time, regardless of the head used. Also use a jet on the rich end of what works on the dyno help keep it cool in the real world.

I wouldn't be concerned about running under 230; hell, I run a 140 in the Sherpa. Just give it what it wants. I think given the very brief run time you'll see the torque drop off before you run the risk of a seizure. 32:1 will be fine for oil, it'll clean up as you start making more power anyway. Definitely add the head stays as soon as you can - vibration has an effect on fueling.

Make some dyno numbers starting with a 4 and I'll send you an NFI Racing tee shirt.

 

[Texasstar]

I ran 320 in the TM38, but which is much bigger than what yours wants obviously but then I think your primary CR would be quite a bit higher (because of single transfer pair, shorter rod and smaller intake volume) and this would explain the very strong intake signal. When I stopped testing this engine I was playing with stinger diameter and there were indications that the existing stinger was undersized. Increasing the stinger size picked up some power and reduced the jet requirement - I'm not sure why. It seemed to me that being slightly too-big on the stinger was better than being slightly too-small as far as power goes, though this goes against classic theory.

It wasn't overly fussy about timing - it'd make good torque over a range of 3 or 4 degrees though of course the curve would be shifted up or down. It liked a fair bit of advance - around 20 - which isn't surprising considering the bore diameter. The trench and the modern chamber designs both wanted similar timing, but the better cooled Husky head would tolerate a more aggressive tune.

The trench head I used was a twin plug like the one in your photo, and I used a Dyna twin coil to fire both plugs. I really don't think the trench chamber is a bad thing, though of course all the forum experts will say nothing can match a modern semi-toroidal shape. The trench was used to help direct the flow in an orderly fashion across the top (allegedly), something that can help with the old engines and their very fast transfer flow velocity. You can actually get some useful squish areas on the sides of the trench with some careful machining, then afterwards use a die grinder to open up the trench to get the desired CR. Just remember that with an inertia dyno you don't get anywhere near enough run time to get the head up to normal riding temps, so be very very careful and listen for det with every bone in your body when you ride it for the first time, regardless of the head used. Also use a jet on the rich end of what works on the dyno help keep it cool in the real world.

I wouldn't be concerned about running under 230; just give it what it wants. I think given the very brief run time you'll see the torque drop off before you get lean enough to seize. 32:1 will be fine for oil, it'll clean up as you start making more power anyway. Definitely add the head stays as soon as you can - vibration has an effect on fueling.

Make some dyno numbers starting with a 4 and I'll send you an NFI Racing tee shirt.

Thank you. Two things you have said that has resonated over and over in my mind is this...”the engine is ancillary to the pipe” and “I wish I did more time tuning the chamber.” We haven’t even started tuning the chamber yet. I do have some pieces cut to change the tuned length. Bill Baxter agrees with you. He has seen a jump in major Hp with 1/2” added to the header. That is why he runs a single cylinder on his dyno and tuned every chamber to be the same.

We will also test the stingers. We did one pull with 22 degrees timing and it was AMAZING how much that cooled the EGT.

BB has over 500 pulls on his current single that finally gave up the ghost.

We are on dyno pull 13. BTW our CHT has been around 220.

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[Texasstar]

If you are making your own gaskets like I am then here is a cost saving tip I learned from Bill Baxter. The metric leather hole punch set is 22.00 for a set while the metric gasket punch set is 150.00. Made my own gaskets this morning to get to .051-055” squish. Ralf said to set the squish at 1.4mm for his head.


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[Texasstar]

John when you said stronger pulse did you mean higher velocity? My friend Mark just made his transfers smaller to increase velocity on his land speed bike. Maybe he will chime in here about that. I was curious about how big I have seen the transfers and was wondering if they were becoming inefficient because they were so big????


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[Texasstar]

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[John Murray]

We are on dyno pull 13. BTW our CHT has been around 220.

This concerns me a little - when the engine is making power the head temp will be well into the 300s and possibly 400s after some time at WOT. Don't fall into the trap of thinking that the CHT will be 220 in real-world running; the very brief bursts on the dyno aren't enough to let the head warm up fully.

 

[John Murray]

John when you said stronger pulse did you mean higher velocity? My friend Mark just made his transfers smaller to increase velocity on his land speed bike. Maybe he will chime in here about that. I was curious about how big I have seen the transfers and was wondering if they were becoming inefficient because they were so big????


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Imagine two engines of the same size running at the same speed: one with a large crankcase volume and one with a small crankcase volume. Both will be pulling air through the carb at about the same rate per minute. But the one with larger volume will be pulling it at a steadier rate, the big volume acting like a buffer or cushion that smooths out the pulses. On the other hand the small volume engine will be jerking the air through in a series of strong pulses because it doesn't have that big cushion of air between the piston and the carb. Carburetors are very sensitive to this signal strength, so even though the total amount of air pulled through per minute will be the same for both engines the one with the stronger signal will want less jet to pass the same amount of fuel.

The flow velocity into the cylinder is dependent on the port window area, not the passage area. With modern engines the current practice is to make the transfer passages quite big - it provides a reservoir of pressurised charge right behind the port and helps keep the primary CR down, so this works well. The flow velocity with the big transfer ports is lower, more efficient and more easily managed. But with old engine designs with relatively small transfer you need crankcase pressure to get the air into the cylinder in the available time. The high velocity charge is harder to keep stable and wants to continue straight out the exhaust or go turbulent in the cylinder, hence aids like the trench chamber. The modern engines have lower pumping losses and make more use of the pipe to move the air, which is one reason why the best of them are making over 400hp per liter while the older designs might make 250hp per liter or so.

 

[John Murray]

View attachment 226588
View attachment 226589


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That piston has barely been warm; Paris Hilton is alive and well.

Just looking at that transfer port reminds me of something I like to do. The short turn radius has a big effect on flow, just like in a four stroke. It needs to be as big and gentle as possible. After shaping with the grinder I like to cut some wet-and-dry into long strips and gently round the short-side radius shoe-shine style. I don't really know if this provides much benefit but it can't hurt and makes me feel better

Apologies for thread-hogging.

 

[Texasstar]

Thank you that is a perfect picture!


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[Texasstar]

Ralf said we wanted to be 1.4mm or .055” for our squish and that would put us at 14:1 compression. Well we are at .051” for our squish which is a lot better than .017” last time. All I need to do is check the timing and torque the head.

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[Texasstar]

Making so much power we blew the clutch up...will post video soon. Huge slippage on the dyno I mean Bigly.


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