Tomos hitting RPM limit, CDI box or something else?

I built a kitted 97 Tomos LX that used to rev to the moon, basically pinging the speedo all the way around and back almost to zero. After several years of ripping I started having some issues and long story short I have a new CDI box and the old original/stock box is gone. Now I can only get the bike up to about 40mph when it starts acting weird, kind of like four stroking but with a slightly different sound. I'm wondering if it's a rev limiter in the CDI box or possibly another issue.

It looks like my bike currently has the "Flash: made in India" box (https://www.treatland.tv/tomos-A35-OEM-cdi-box-p/tomos-a35-cdi-box-230627.htm) has anyone had any problems with this box on kitted bikes and if so do you have a reco on something I should replace it with, maybe the parmakit box or another "OEM" box from treats or the moped junkyard?

I've rebuilt the top end and replaced the gaskets, replaced the reeds, tried a different exhaust, jetted up and down to extremes, and tried a different carb body all with the same result so at this point I think it has to be electrical or maybe the transmission but I'm open to suggestions.

Thanks

Re: Tomos hitting RPM limit, CDI box or something else?

Brandon Weiss (Detective brandon to you) /

Did you adjust your timing? Any air leaks?

Re: Tomos hitting RPM limit, CDI box or something else?

It had an air leak before but that's been fixed and head temps are staying in a good range so I'm guessing no other leaks. I've never really messed with the timing, is that something that would change over time and need adjustment?

The bike runs perfect and hauls ass until it gets above 35 or so, then it starts freaking out.

Re: Tomos hitting RPM limit, CDI box or something else?

Brandon Weiss (Detective brandon to you) /

Are you using a resistor type spark plug, and have you tried new ones?

Re: Tomos hitting RPM limit, CDI box or something else?

I've just tried an NGK B7HS which is my normal go-to but looking at the description of resistor plugs that might be worth a shot. I'm not sure if I have a resistor boot though (it's an NGK), would that make any difference? Can you run both a resistor boot and a resistor plug?

Re: Tomos hitting RPM limit, CDI box or something else?

Brandon Weiss (Detective brandon to you) /

It would be extremely helpful to hear the noise if you could record it somehow.

Re: Tomos hitting RPM limit, CDI box or something else?

I was thinking that, just need to figure out how. I'll try to grab something in the next couple of days.

Re: Tomos hitting RPM limit, CDI box or something else?

> static frame Wrote:

> -------------------------------------------------------

Can you run both a resistor boot and a resistor plug?

Each resistor , the cap or the plug , is about 5k ohms . Both would be about 10k ohms .

If I understand correctly , some CDIs require a 5k ohm resistor . Which ones , I don't know .

I would think that anymore than the minimum required resistance would detract from the quality of the spark .

Re: Tomos hitting RPM limit, CDI box or something else?

Do you not have a mobile phone that can film? Or a tablet?

Re: Tomos hitting RPM limit, CDI box or something else?

I just don't have a phone mount on my bike and don't really want to try to hold a phone in my hand while riding 40mph on city streets, but I think an old phone case + duct tape may be a workable solution.

Re: Tomos hitting RPM limit, CDI box or something else?

Brandon Weiss (Detective brandon to you) /

Phone mounts are $10 on amazon, the Roam one is great, i have one on each of my bikes.

Re: Tomos hitting RPM limit, CDI box or something else?

Overpriced Parts /

> P D Wrote:

> -------------------------------------------------------

> > static frame Wrote:

>

> > -------------------------------------------------------

>

> Can you run both a resistor boot and a resistor plug?

>

> Each resistor , the cap or the plug , is about 5k ohms . Both would be

> about 10k ohms .

>

> If I understand correctly , some CDIs require a 5k ohm resistor . Which

> ones , I don't know .

>

> I would think that anymore than the minimum required resistance would

> detract from the quality of the spark .

Put ignition back the way it was stock

Re: Tomos hitting RPM limit, CDI box or something else?

DrGeorgeTompson (Alan) /

So does it not do it if you rev the bike on the stand?

Re: Tomos hitting RPM limit, CDI box or something else?

Benjamin LaPlante /

Sounds like you need a stock coil.

Re: Tomos hitting RPM limit, CDI box or something else?

Cool I'll pick one of those up, I've been meaning to get a mount for a while so I can use a GPS speedo to see how inaccurate my actual speedo is.

Re: Tomos hitting RPM limit, CDI box or something else?

Grabbed a recording, I wasn't able to capture it properly on the kickstand so there's some wind noise from riding, I ran a high pass filter to try to minimize the noise and uploaded both files in case one is easier to hear than the other. The best spots are probably about 6-7 minutes and 8-9 minutes, that's when I was able to maintain speed the longest.

Original

High Pass filtered

I have a trail tech tachometer (I don't really trust it though) and the bike seems to max out at around 7K RPMs at which the point the engine and the tach start freaking out. It feels like the engine is pulsing, like it wants to go higher but then pulls back, if you listen closely you can hear the engine noise kind of going in waves. I actually had a quick second when the bike broke through on the RPMs and got up near 45 but the tach was all over the place and it slowed back down to 40ish.

I tried a bunch of different throttle positions at the "top speed" and it seems to have no effect, when the throttle is wide open the engine sounds a bit deeper but overall acts exactly the same as when running in a lower throttle position at that speed/RPMs.

Hopefully this is helpful, if not I can try mounting my phone someplace else on the bike to get a clearer recording.

Thanks

Re: Tomos hitting RPM limit, CDI box or something else?

That makes sense, I checked the boot and it does have a 5K resistor so I assume I'm good running a standard spark plug then.

Re: Tomos hitting RPM limit, CDI box or something else?

My old stock coil is gone, If anyone know where I can get a legit stock coil in good condition (from what I understand there are a few different "OEM" ones and some of them suck) or has a reco for decent aftermarket/performance coil I'll give it a shot. I just don't want to spend a bunch of money ordering different coils in hopes that one will work. (edited)

Re: Tomos hitting RPM limit, CDI box or something else?

Brandon Weiss (Detective brandon to you) /

Is it the part that sounds like a plastic panel or loose part buzzing around?

It sounds like its running great and accelerating really well up until that point.

Re: Tomos hitting RPM limit, CDI box or something else?

> static frame

If you go to the top of the page and click View Flat , it'll make your life easier . ;)

Re: Tomos hitting RPM limit, CDI box or something else?

I'm not sure what's causing the buzzing sound, I don't hear it when I'm on the bike, might be something the phone was sitting against, but yeah you can hear the pulsing probably the clearest in that.

Re: Tomos hitting RPM limit, CDI box or something else?

I ordered a used stock iskra coil from moped junkyard, will report back if it makes any difference.

Re: Tomos hitting RPM limit, CDI box or something else?

♣Slew Foot♣ /

i bet it is your throttle slider creating turbulence. use a flat or slightly inclined one

Re: Tomos hitting RPM limit, CDI box or something else?

did you fix an air leak then not re-jet? are you just jetted way rich to compensate for the air leak?

the spark plug resistor thing has nothing to do with it.

the cdi boxes changed around '99 or 2000 from un-restricted, to one with a restrictor at about 8k rpm. you can see it wiht a timing light, the spark will retard dramatically (like 20 degrees) when the restrictor kicks in.

there are several versions of restricted one-wire tomos cdi's that were stock and aftermarket replacement over the years, but most bikes don't hit the rev limiter, 8000 is pretty high unless you are kitted

Re: Tomos hitting RPM limit, CDI box or something else?

I've jetted up and down the spectrum, from way rich to way lean just to see what'd happen. It does fourstroke when rich which it wasn't doing before the air leak was fixed.

Guess I should have busted out my timing light before ordering a new box, maybe I'll test it before I put the new box on just to confirm if it is restricting.

The bike is kitted – I'm running a 70cc 45mm alukit with single ring treats piston and high compression head, malossi 4-petal reed block with a 19mm PHBG, and a homoet exhaust.

Re: Tomos hitting RPM limit, CDI box or something else?

I was too impatient when I got home to mess with the timing light, I just threw the new box on and went for a ride... huge huge improvement, the bike runs like it used to and fully revs out.

For the record and future reference this was the box that had a speed limiter and this coil is the one that I replaced it with and so far seems to not have any issues.

Re: Tomos hitting RPM limit, CDI box or something else?

Resistor plugs and caps have no effect on spark. They were created to stop interference with AM radio signals and CDI systems are designed to work with them. I run both on my A55 with absolutely no troubles at all.

Re: Tomos hitting RPM limit, CDI box or something else?

> Lonnie Cavenee Wrote:

> -------------------------------------------------------

> Resistor plugs and caps have no effect on spark. They were created to

> stop interference with AM radio signals and CDI systems are designed to

> work with them. I run both on my A55 with absolutely no troubles at all.

If that's the case , why do I get a stronger spark when not using any resistance ?

No CDI involved , of course .

Re: Tomos hitting RPM limit, CDI box or something else?

> static frame Wrote:

> -------------------------------------------------------

> I was too impatient when I got home to mess with the timing light, I

> just threw the new box on and went for a ride... huge huge improvement,

> the bike runs like it used to and fully revs out.

>

> For the record and future reference this was the box that had a speed

> limiter and this coil is the one that I replaced it with and so far

> seems to not have any issues.

Your first link isn’t a coil.... it’s a TCI unit for an electronic ignition bike.... the seond link is a coil for a points bike....

Re: Tomos hitting RPM limit, CDI box or something else?

♣Slew Foot♣ /

Just for the record resistor plugs make the spark duration longer...

Borderline chaos in blowing up is the only way you reached top limit you could use the old three seashells thing you can adjust your mix you going to dress your timing or you add something...

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