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1988 Honda TRX250R with CDI and Carb Issues: Spark, Backfiring, and Jetting Help Needed

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Kaleb

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Discussion starter · #1 ·
Just bought a 88 trx250r some older gentleman had it, it was sitting for 2 years had it running for a day or so but was running terrible. Wouldn't kick start but had no problem roll starting. Pulled the plug, had no spark or little to none that you can visibly see, tracked it down to what I thought was the CDI box. Bought a cheap CDI off Amazon it now has all the spark In the world but will damn near rip your leg off when kicking it over then it back fires. That's problem 1



Problem 2
The guy had no clue what was done to the bike is what he told me, it has a keihin carb no clue what size, fmf gold series fatty, as far as I can tell maybe someone has been in the motor I guess there is no telling till I pull the cylinder off, anyways I pulled the carb and cleaned it put it back on and I think I'm having a jetting issue causing it to run super rough bogging down most of the time in about half throttle, I'm in Tennessee and wondering what size Jets would you guys try first, this is my first 250r and 2 stroke in general so any other tips and info are appreciated
 
#2 ·
Congrats on your 250R!

The Amazon CDI is the issue with your kickback. Unfortunately, only OEM CDIs are proven to work correctly and as they should, assuming they still work. That’s the issue. The CDIs are now 35 years old and beginning to fail. To my knowledge, there’s not a drop in replacement CDI box that’s known to be good and without issue (like the kickback you’re experiencing). Your best bet is to find a used OEM CDI that is known to be working. They typically run $150-200. However, it’s only time before it fails as well.

There are replacement ignition systems that you can purchase and use. For example, HPI, Zeel, or OEM CR250 ignitions (01 is the most preferred year, but, if I recall the correct year range, 98-07 can be used with the correct adaptor plate).

You mentioned it ran when pull started. You should try the OEM CDI again, but try gapping your plug to .018”. The aged OEM ignition is known to produce a weak spark; setting a tighter plug gap will help with this. Note that it’s still going to be weak and very difficult to see if trying to test for spark.

I’d personally get your CDI/ignition figured out before trying to mess with the jetting, as the ignition troubles could be the culprit there too.
 
#3 ·
Like bushwesl said get your ignition sorted out first than you can start tuning as long as you have a solid foundation to start from.... Post some pics of what you have so we can help you identify the parts. R's come stock with a Keihin 34PJ.. Most people switch to a Keihin PWK series carb, with a pic we can tell you if its a stock PJ or an aftermarket PWK.
 
#8 ·
Assuming it’s a stock setup, I’d recommend recreation (non-ethanol) fuel if you have it near you, which is typically around 90-91 octane. The ethanol in regular pump gas gums up and goes bad quickly. If you can’t get rec fuel then I’d go with premium.

If you’re not using fresh gas, that can also be an issue with the way it runs.

For the best success, I’d drain the tank and start with fresh mixed gas, clean the carb, use a new BR9ES or BR8ES plug gapped to .018”, and clean the air filter.
 
#10 ·
On my bike this week I put in a new BR9ES gapped at .018 to replace the B8ES which was at the stock .030-ish gap. The reason I didn't have an R plug in before is that's what the auto parts store had the day I had to buy a plug really quickly. Started first kick after sitting a week. It never (edit: not "never", but not in the past 10 years) did that before cold no matter how new the plug was. Sometimes I was kicking 15 times cold and going back and forth between ON and RES on the petcock while rocking it in gear. Warm was usually 1-2 kicks. Hopefully this keeps up.

So these things are clearly sensitive to plug setup when the ignition is old. 8 or 9 can end up being personal preference once you see how your bike runs in your locale. The question is... would be 8 harder for the ignition to fire?
 
#11 ·
I've been running B8ES plug's in my R for 35 years. The non resistor B8ES & B9ES were discontinued though about a year ago, I was able to find about 20 B8's but will have to switch once there all gone. Only standard plugs available now are the resistor type BR 8 or 9 series which is what Honda spec is.
 
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