MVT Digital Direct on Vertical Minarelli (MXS Race Cylinder)

[billwill]

There are a fair number of threads here on MVT's direct digital ignition but I've had a hard time finding any specific information or documentation here or other websites as it pertains to setup on a vertical race cylinder. 

If there is a thread here that I may have missed, please direct me to it and forgive me for missing it!

I have watched several videos like Brent's awesome and thorough series on this on YouTube but that was several years ago and for a stroker horizontal engine. In this case my bike is with the following setup:

- MXS Race Cylinder (not ported but transfer ports in the case have been matched)
- 30mm PWK carb
- a few different exhausts I've been testing with, all race quality (ST, Yasuni C16, SCR Corse handmade)

My questions are:

Is .35mm BTDC for the above build a good place to start? Or should I start with something less aggressive? My understanding is a lower number, for example .2mm or .15mm would be less aggressive.

Will I want to get a degree wheel and timing light (like Ryan and Brent used) to verify what the timing actually is? I'd rather not, but if I should, I should.

If above is "yes," what is considered a "safe" amount of advance at idle through ~13k rpm? What am I shooting for?

Thanks in advance for any advice or tips guys.

[90GTVert]

I've only used Minarelli horizontal and Piaggio versions of the MVT.

0.4mm BTDC is standard stroke, which is 39.2mm for a horizontal. 0.35mm for longer stroke. Some get more specific with stroke and conrod length, but MVT only gives those 2 figures.

The MVT is aggressive at lower RPM. That's why I swapped mine out. The timing that my engine was liking at peak required me to have 45 degrees or so of advance at lower revs... 42 is what I got as setup with MVT's specs.

You don't have to use a timing light and degree wheel or indicator. I'd venture to guess most don't bother. It's a tool to verify that it's where you want it.

[billwill]

May 9, 2024 22:12:38 GMT -5 90GTVert said:

0.4mm BTDC is standard stroke, which is 39.2mm for a horizontal. 0.35mm for longer stroke. Some get more specific with stroke and conrod length, but MVT only gives those 2 figures.

Thanks Brent. Ah ok so I had it backwards and .4mm BTDC is the one I'd want for my standard stroke 70cc kit?

May 9, 2024 22:12:38 GMT -5 90GTVert said:

The MVT is aggressive at lower RPM. That's why I swapped mine out. The timing that my engine was liking at peak required me to have 45 degrees or so of advance at lower revs... 42 is what I got as setup with MVT's specs.

I have heard that their recommended settings can be aggressive just as you state. What would I be looking out for if I don't have a timing light and degree wheel to know exact timing? Pinging or knock, rough running, high temps (especially at idle if you mention the advance is that way at idle) etc?

Starting with a lesser value for BTDC than .4mm such as .25mm or .3mm would mean less advance, correct?

May 9, 2024 22:12:38 GMT -5 90GTVert said:

You don't have to use a timing light and degree wheel or indicator. I'd venture to guess most don't bother. It's a tool to verify that it's where you want it.

Got it, thanks.

[90GTVert]

0.4mm is what they say for standard stroke, so probably.

I'd most likely go with their recommendation. Because the whole curve moves, it throws off the peak power timing if you try to back it off for lower RPM timing. I used one with great results on a couple of engines. The only thing that I've seen is that I could not get a flat slide of any sort in tune with it. Many others do though. Definitely listen for knock... especially part throttle. Watch CHT/EGT. You can look for peppering on the spark plug porcelain that can indicate detonation.

[billwill]

Got it, thanks man. Sounds like the MVT DD is good with certain builds and playing with fire with others. I guess I just need to slap it on and take my time and look out for all of those things. Appreciate you taking the time to hash this out with me. I know you've spent countless hours already on the MVT topic but your videos have been super helpful and I've watched them multiple times. Thanks.