The Hodaka Club Forum

I'm trying to race my 95 (Vintage MX) but I'm running into an issue I've yet to figure out and I need your help.

This is what is happening:
Bike is warmed and runs amazing off the line and for the next two laps, but then half way through the third lap it starts to backfire, then sputter and dies. No flicking of the throttle can keep it running for long. This has happened to me twice and both times were around 2-2.5 laps. The first time it happened I check the spark plug and it was wet and looked fouled. Since it was running so well at the low end I went to a smaller main jet. The second time it backfired/sputtered/died the plug looked OK (see below). Both races had brand new plugs. Once in the pits, after I looked at the plug, bike starts right up and runs amazing again...arg!

My questions:
1. What typically causes backfiring?
2. Any idea's on what is going on?
3. Steps you go though to diagnose and issue like this?
4. I'm open to any tips you want to share for setting up 95 to race.

Pertinent info to help with diagnosis:
Bike is stock except for new carb.
Race day was hot and sunny.
I race at high rpms.
I set timing the day before race with timing light.
Gas - Non-Ethanol 92
PreMix - Castor927 at 32 to 1
Plugs - NGK B8HS 5510, both races had new plugs.
Brand new 28 mm Mikuni running 35 pilot, needle 5EJ13 clip on center, 180 main on race that plug fouled and 160 main on that race where plug looked OK to me.
Inline fuel filter. Gas tank was treated for rust but I do have a small trace of rust still in it. I figure the filter will capture anything.
I rebuilt both top and bottom ends this winter.

Thanks for any pointers,
Jay

This is the plug after the second race.

Classic symptoms of a roached out condenser. Bike heats up - condenser dies. Bike cools down - condenser works again. While you're into the magneto, dress and check points gap.

your plug washer gasket looks over tightened shot. I would be using a colder plug just me ------------Clarence

Thanks Clarence - I'll pick up a few B9HS to try and watch my tightening.

Thanks Matt!! Will pickup a couple condensers from Terry.

Edit: He's out of stock.

Plug should be snug tight, no need to be any tighter than that. Best bet it's the condenser, but could be the coil or the plug cap also. But start with the condenser.

Be sure your tank is vented properly.

Thanks thrownchain - I would not have thought of the plug cap. I'll start with condenser then move down your list.

Thanks MTrat - I have the original cap with that tiny hole that I know is clear, but MAN that is a tiny hole. I'm thinking of looking for a cap with a bigger vent as I don't want to mess with the original.

Sure sounds like condenser - definitely ignition. Be careful with that 160 main jet - leaning out to chase an ignition problem almost never ends well. Could also be the infamous "blue point wire" issue. See the Resonator article from about a year ago. Finally, the tiny hole in the gas cap is perfectly capable of satisfactory tank venting. But you also need to check the vent hole in the rubber gasket too - I've bumped into units which didn't have hole at all.
Ed

Thanks Ed - I always wondered about that little vent hole. You have put my mind at ease. Once I get the new condenser I will definitely go back to the 180 and restart testing. Rather foul a plug then rebuild an engine.

I doubt if you have that HORRIBLE Spark Plug Cap #933002 but if you do get rid of it! I had the same problem . When I pulled the cap and put it back on the spark returned. Just the twisting on the cap and it would re- connect. Horrible! Joe

Also, the Service Bulletins #14 in the Library in the HPI website shows that Miserable Cap and other common Ignition problems. Joe.

Great point when chasing the elusive "intermittent spark" issues, Joe.

Joe thanks for reminding me of the Library. Great info there. After pursuing the articles my list is now longer; new condenser, check my plug cap, change points to .100" BTDC, change to NGK B10HV, modify front forks for better damping and more travel, weld my flywheel, check my gearing, etc.

Thanks to everyone for the help!! I replaced the condenser, actually relocated it following viewtopic.php?f=1&t=2186&hilit=condenser+relocate and bike is running amazing! You guys are awesome.

Great news, Dog! The CW is a really fun bike to race, isn't it?

Yeah Matt, really fun!