New Member New SRX250 owner

I think you may be right Andych. The 1984 XT250 that had the higher HP ratings are what my "big" "30x" cams came from & I suppose thats likely the best donor bike to match the TCI to my 30x cams.
I did find both a 51y & a 3wp TCI in Japan listed on ebay that might clarify things.
51y part #TID11-08A 51Y-11
3wp part# TID11-14 2NV-10

A TCI listed in the UK & also my USA spec model has the 3wp part #
A TCI listed in Australia has the 51y part #

So it seems the USA & UK SRX250 use the same TCI as the later Japanese 3wp (which is rated at lower power specs than the earlier 1984 claims of over 30hp) while the Aussie bikes got a 51y ignition box.

 

I ordered a 1984 xt250 TCI, it was only $40 shipped & has 30x in the part #. There are 6 wires on all of the TCIs so I'm hoping that even though there is a slight difference in the wire connectors a rewire will be fairly straightforward.
I didn't realize Yamaha provides a graph of the ignition timing curve in the manual

 

Found the timing advance graph in the xt250/xt350 manual & there is a noticeable difference in the curve. 3 degrees more timing at the low end (9° on the SRX vs 12° on a xt250/350), the increase in spark advance happens sooner on an XT (2k rpm to 5k vs 2.5k to 5.5k on an SRX) and at the very top end timing stays at 34° till redline on a xt250/350 but on an SRX the spark hits 34 but is then slowly retarded to 31° as it nears redline. the SRX chart:

 

Going over the wiring diagrams & ignition specs there are several differences. The ignition coil resistance is different, the pickup coil resistance is different, & I am only able to make a correct match for 5 of the 6 wires going from an SRX harness to an xt250 cdi box. The ignition coil can be changed out with an xt250 unit easily, but the difference in pickup coil at the stator must be converted or rewound. That last wire I have not yet figured out though. It seems on an xt250 the cdi connects to a specific coil in the stator with a tan wire my SRX has no equivalent part shown on the diagram & instead of the Xts Tan wire going to a coil in the stator on an SRX that wire is red/white goes to the spark coil

I can upload the ignition spec section of both manuals if you're interested in a 30x TCI. The XT Tci box is smaller & cube shaped & on most the part number is worn off by the rubber shield but you can clearly read a 30x on many of them.

 

Yes it looks like the pickup is in the magneto/stator area & woild need swapped or rewound to match resistance. Same with the ignition coil (that ones an easy swap at least)
Here's the XT250 pickup specs

here's the same specs for an SRX

 

To me it appears the XT250/XT350/TT350 all get their power from a dedicated coil in the magneto, while the SRX gets its power on the red/white wire that feeds the spark coil primary winding. That same wire feeds out from the voltage regulato as a tan wire and goes through the engine stop switch where it then becomes red/white.
I wish yamaha gave us a voltage spec in addition to coil resistance so we could know if they both operate on the same voltage or if the XT/TT bikes are using an unrectified AC waveform directly off that coil in the magneto

 

Looking at universal cdi boxes there are 2 types...AC or DC.
Based on the XT/TT diagrams it is likely AC as I do not see any diode symbol or rectifier in the diagram & the CDI is powered straight from the stator which would be an AC waveform.
Sadly it looks like the $40 XT250 "30X" ignition box I ordered will not work on my SRX with its DC powered CDI But all hope is not lost ! There are many aftermarket CDI boxes available for dirt cheap, under $20 & some are even adjustable meaning we can set it to the 12° base timing 30x specs. I will see if I can find any aftermarket DC CDI boxes that show a timing curve or adjustability that would make matching the full 30x curve possible
Enjoy your week away, a change of scenery can give new perspectives

 

Andych the link you provided looks more promising than what I have found so far. Initially I was excited seeing the low cost Chinese CDIs but after much browsing it appears there are only a handful of designs most all of them of dubious quality, many reviews say they fail often & of poor quality.
I did find one manufacturer offering a timing graph for an adjustable CDI & it appears you can only adjust the rate of timing rise to one of 8 preset curves & the Rev Limit can be removed using dipswitches to change settings, but the base timing & max timing are standard it seems & max is only 20 degrees timing. So a cookie cutter cheap CDI may not be an option
The link you sent almost looks like a fully programmable computer like stand alone engine management. I will definitely look into it

 

XRACR did you happen to measure the base circle of your cam ? (to subtract from the lobe height to roughly find the lift height)
I have not yet gotten to install my 30x cams, still mired down with carb issues currently. But I am interested to compare as many cam measurements as we find, the info available on which models came with which cams is still a bit unresolved i think. The "big" cams i bought are supposedly from a 1984 xt250 & do not say 48y or 30x, but when comparing the base circle to the lobe height they check out to match 30x cams. Perhaps you can figure out which cam you have based on the lift height. Whenever i get around to my cam swap I plan to measure my original "US spec" cams, interested to see if ours match or if theres also multiple cams for the US market