Misfire and backfire on decel - '88 Vmax

barber1303 · May 31, 2020

Before I go through the process of pulling the carbs, I thought I'd solicit the group for some advice. I bought this '88 Vmax from a co-worker who was "throwing in the towel" as he couldn't get it to run properly. The symptoms are a stumble coming off idle and a misfire (often triggering a backfire out the exhaust if revved high) on decel leading to a temporarily reduced idle speed until the misfiring cylinder(s) recover. Once recovered, it idles stably again until revved wherein the whole thing repeats. I'm assuming this is a fueling/mixture issue, but I checked ignition nonetheless and it is good. Compression is good all the way around. Fuel pump operates as it should. The carbs were recently rebuilt as evidenced by their external cleanliness. Co-worker sent them off for rebuild (I don't know where) in an effort to resolve this issue, but I cannot vouch for what's been done or whether it was done correctly. Anything else I should try/check before pulling the carbs to investigate further?

Linked is a video showing the carbs as the engine is revved. The #2 carburetor is pushing out a mist of fuel with each rev...none of the others are doing this...think it is related to the overall issue?

https://imgur.com/a/othCIoG

barber1303 · Jul 30, 2020

Follow-up to close out this hanging thread. I discovered that the misfire/backfire was caused by poor spark as a result of a faulty TCI box. Under low speed conditions, the TCI would not reliably spark the plugs, causing an erratic idle, poor take off, and misfire/backfire. Swapping out with a known good TCI box resolved all issues. Putting the suspect TCI box back in made all the issues return. Case closed.

mabdcmb@yahoo.com · Jul 30, 2020

Glad to hear you got it sorted. Thanks for following up with the remedy

Fire-medic · Jul 30, 2020

You might want to try re-soldering the circuit boards connectors from the box plug as some people have reported success in fixing intermittent issues with their CDI box by this work. If it was successful, you'd have a spare. Also, I am one of the people who would be interested to hear this worked for you. It's easy to disconnect the CDI box behind the steering head, and to re-connect the other soldered CDI box, to fire-up the engine.

barber1303 · Jul 31, 2020

Fire-medic, you provided this advice on another thread and I did make an attempt to reflow the solder. I have a decent soldering station setup and faithfully reflowed the solder at the circuit board connections, but it did not resolve the issue. I then tried to remove the solder from the connector terminals so that I could extract the circuit board from the housing. Unfortunately I ended up breaking a bit of the circuit board (top left) as I separated it from the housing and could progress no further. I'm guessing one of the components within the circuit board failed, but I don't think I would have been successful in determining which one, or replacing it. It was a fun exercise and worthwhile learning experience though...

Fire-medic · Aug 1, 2020

Don't they have kits to make circuit panel repairs? If you lost a printed circuit, knowing where it's supposed to-be should allow you to make a tiny jumper to connect things. Nothing to-lose.

I see the piece cracked-off.

desert_max · Aug 2, 2020

Agreed. There is nothing on that board that cannot be troubleshot with a handheld DMM and repaired with a decent solder station. That’s technology dating back to stone knives and bear skins. The most “sophisticated “devices in the circuit are the D1113 power transistors.