Texas Two Step Taco

Texasstar

Can't is a four letter dirty word
Watch my hand on the dyno last time. I know we can’t turn 16k but that is what the tell tale said on the tach. But we had to have been north of 10k. This engine wants to keep revving.


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Texasstar

Can't is a four letter dirty word
Original Bandido was 251 lbs.

The front weighs 105 lbs and the rear weighs 110 lbs.

We are at 215 lbs wet.


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Texasstar

Can't is a four letter dirty word
“I love the Raucous little Bastard.” -John Murray
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teazer

Well-Known Member
DTT BOTM WINNER
I am easily confused. Was that supposed to read as 10.0 or 1.0 ? Lambda range is around 0.8 - 1.2.
Or do you mean an Air:fuel ration of 10:1 which is rich and really safe. Almost past the Paris Hilton phase - approaching the presidential scale perhaps.

Room to trim that down step by step to around 12.5 - 13 potentially but a rich mixture might be what you need to avoid meltdown.
 

Texasstar

Can't is a four letter dirty word
Yes it was presidential at 10:1. Soooo we started with a 370 jet went down to 320. We made 30 wheel hp at 6000 rpms and then fell on our face. We’re making 15 hp at 8000 rpms. I have the rev limiter at 8000. I only have one petcock hooked up. May be starving for fuel?


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teazer

Well-Known Member
DTT BOTM WINNER
What happened to Air:fuel ratio as revs went up @WOT? It was rich down low. Did it get excessively rich higher up? People equate throttle position to RPM and those are quite different variables. At wide open throttle, fueling changes as revs rise which is what is known as the fuel slope. On the average older style two stroke the idea is to get slightly richer as revs rise.

On a 4 stroke the air (bleed) jet that feeds air into the needle jet to atomize fuel is used to alter the A:F where on a 2 stroke typically that air jet is missing or is set at 2.0mm and the primary needle jet nozzle height (primary choke height) controls the fuel slope.

And was the throttle wide open at that 6,000 peak? If "yes" to both, try going down on the main jet until it's a little cleaner at 6,000 and see what happens to A:F and power higher up. If it gets even richer, try a shorter needle jet. When A;F is optimized up to say 6,000, try shaving 1mm off at a time to see if you can trim the fuel slope.

If the A:F looks good the whole rev range, try changing the head. The stock head isn't designed for high revs and may be causing a problem with insufficient or excessive turbulence when it starts to come on the power band. It could also be a number of other factors including insufficient transfer time-area. Doug F should be able to advise what HP level his ports should be able to support.

Our calculations suggested that 30 was about right at 6,000 and that it should just be starting to hit its stride on the way to peaking at 8,000 or so.

Development, unfortunately, is a time consuming process of minimizing one adverse factor at a time and then going round that same loop a few times.

You could always try hydrazine. It's a little unstable and unsafe and other people may die, but you have to prepared to make that sacrifice (I hear people say that it's the presidential thing to do)

 

John Murray

Member
Yes it was presidential at 10:1. Soooo we started with a 370 jet went down to 320. We made 30 wheel hp at 6000 rpms and then fell on our face. We’re making 15 hp at 8000 rpms. I have the rev limiter at 8000. I only have one petcock hooked up. May be starving for fuel?


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You're trying to run this on one petcock?????? Measure the fuel flow through the needle and seat with both petcocks connected and if it still isn't adequate replace them with 1/4" ballvalves. Aim for 400ml/min. It should be making over 40hp at 8k, so at 215lb it'll be quick. If the fuel supply is insufficient it'll seize.

I think you're making this harder than it needs to be with all the distractions - forget about the a/f gauge, they are easily confused on a two stroke and can give misleading readings, at least until jetting is in the ballpark. Just listen to the engine and watch the torque gauge, it'll tell you what it wants. Once you get it making its best power make a note of the EGT and use that as your jetting guide in the future.
 

Texasstar

Can't is a four letter dirty word
You're trying to run this on one petcock?????? Measure the fuel flow through the needle and seat with both petcocks connected and if it still isn't adequate replace them with 1/4" ballvalves. Aim for 400ml/min. It should be making over 40hp at 8k, so at 215lb it'll be quick. If the fuel supply is insufficient it'll seize.

I think you're making this harder than it needs to be with all the distractions - forget about the a/f gauge, they are easily confused on a two stroke and can give misleading readings, at least until jetting is in the ballpark. Just listen to the engine and watch the torque gauge, it'll tell you what it wants. Once you get it making its best power make a note of the EGT and use that as your jetting guide in the future.
So glad you are back! Yea I feel totally stupid. Will correct the fuel flow issues before we move forward and I am very grateful we have been so rich!!!


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Texasstar

Can't is a four letter dirty word
We started the day on the dyno with 19 degrees advance, 370 jet in the TM Mikuni, 20:1 BeNOL C12. Our first run was an abysmal 18 whp and air fuel was at 10:1 Egt was 1050 degrees. The next two runs we kept going down in jet size and made 21 whp and EGTs went down. Then we realized that the O2 sensor had failed. We replaced the sensor and made a change to a 320 jet and 21 degrees advance which is 2.7mm BTDC. We made a pull and started logging at 4K. Air fuel was at 14:1 and we bam jumped 10 whp. Throttle was crisp vibrations minimum. As soon as we hit 6 k it fell on its face. You could feel it in the throttle vibrations increase big time and we were on the rev limiter in no time. Pulls were way shorter this time.

We had to cut the dyno short yesterday. The wife had found us our next family dog and we drove 2 hours to Paris, Texas. Zeke won this round on naming. His name is Desmo. Bulto was vetoed by the family. However, the family won on the breed...Desmo is a little Aussie. He is so smart.
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We will do the fuel flow test tonight with both petcocks plumbed and then make a pull.


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John Murray

Member
Something to keep in mind re the vibrations - the TM carbs are more prone to fuel foaming/frothing than the old VMs. I had problems with the air cooled 370 motor with a TM38; it'd pull strongly until the rpms reached a certain point where the vibration became excessive. Once it reached that point the power would drop off suddenly, so suddenly it was almost like a rev limiter. Reducing the vibration made the problem go away. I don't know if this is your problem - I'm hoping it's a simple lack of fuel - but if more fuel doesn't help it'd pay to check it out.
 

Texasstar

Can't is a four letter dirty word
Something to keep in mind re the vibrations - the TM carbs are more prone to fuel foaming/frothing than the old VMs. I had problems with the air cooled 370 motor with a TM38; it'd pull strongly until the rpms reached a certain point where the vibration became excessive. Once it reached that point the power would drop off suddenly, so suddenly it was almost like a rev limiter. Reducing the vibration made the problem go away. I don't know if this is your problem - I'm hoping it's a simple lack of fuel - but if more fuel doesn't help it'd pay to check it out.
That makes a lot of sense also. It is very difficult to hold on after 7500 rpms because it is vibrating so bad. I am going back through my mind on everything I felt when I was on the bike and I will post a video so you can hear what it sounds like it and compare it when it is on the rev limiter. Rev limiter is set at 8k for all runs.


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Texasstar

Can't is a four letter dirty word
Head stays are next on the frame modification list.


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Texasstar

Can't is a four letter dirty word
So Teazer sent us Keihin carbs for another project and I noticed he had weights epoxied to the bowl of the carbs. Was this because it was fuel foaming?


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John Murray

Member
Weight added to the carb and soft mounts can help - you often see lead wire or strips of lead sheet wrapped around the carb. But if it shakes so badly that it's uncomfortable to hold on to the bars you should probably do something about it. The Bul cranks are usually underbalanced from the factory but it doesn't normally matter because their rpm range is so low in stock form. I found that a balance factor of between 60 and 65% works well, and that a very light piston and pin is an enormous help. With your heavy piston you're probably going to need a couple of Mallory slugs to balance it. A head stay is essential.

PS. Desmo is beautiful!
 

John Murray

Member
I am going back through my mind on everything I felt when I was on the bike...


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Get a notebook and write down everything that happens every run - it'll be your most important tuning tool. I have pages and pages of notes from every engine I've built and tuned and it's surprising how often a solution will suddenly appear to you after reviewing the notes for the day.
 

Texasstar

Can't is a four letter dirty word
Weight added to the carb and soft mounts can help - you often see lead wire or strips of lead sheet wrapped around the carb. But if it shakes so badly that it's uncomfortable to hold on to the bars you should probably do something about it. The Bul cranks are usually underbalanced from the factory but it doesn't normally matter because their rpm range is so low in stock form. I found that a balance factor of between 60 and 65% works well, and that a very light piston and pin is an enormous help. With your heavy piston you're probably going to need a couple of Mallory slugs to balance it. A head stay is essential.

PS. Desmo is beautiful!
The snowmobile guys were saying the same think about their mikunis foaming. One guy hung a crescent wrench from his carbs to aid in dampening. But they were only having the problem on the dyno. I have an idea we maybe doing some crank work.

I have a confession. I have been running the Taco with only one petcock the whole time. I never saw a cross over for the tank so my assumption is making an ass out of me. I thought you just ran one side at a time.


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