Serious backfire

Seemingly out of nowhere I'm getting a backfire .... prior to it even starting. where do I look first? Less than 2000 miles on rebuilt carb and valve adjustment . Thoughts?

 

Is it a one time thing when you start it? Does it happen each time you try to start her? Will the bike run and drive normally or are you having other issues with it?

Main reason I ask is that I have had bikes that the jets in the fuel bowl have backed out and they would backfire and run bad or not at all. I hope this is not the case.

 

"Why does my engine sometimes backfire when I first turn on the key (without attempting to start the engine)?:

- When you kill the engine, the intakes will still have some remaining (un-burned) air-fuel mixture remaining in them. Upon powering up the bike (turning the key on), the ignition system will apply 12 volts to the coils, thus charging them. Of course, that constant current is not good for the coils, so few seconds later (if the engine is not started) the TCI shuts down the coils by grounding them (to protect the coils from overheating) which cuts the current to the coils and thus triggers a spark to the plugs. If there is enough un-burned fuel in the intake manifolds or the combustion chambers, and if the valves are held open by the camshafts, a backfire thru the exhaust header and/or thru the intake manifolds/carbs/airbox can result."

From:The Information Overload Hour

 

This is the first time it has backfired upon cold start. Month or so ago it died on me (total electrical failure, during ride), upon restart it backfired. Once up and running warm it seems fine.

Thanks for info

 

Can confirm it's probably normal. I had some sidestand switch issues with my 650 Maxim (it was dirty and sometimes killed the ignition while driving!) The switch cut the ignition once while it was running and really quickly cut back in, BANG, huge backfire out the exhaust, I thought my back tire exploded or a gun went off behind me! Fixed the switch (thus the engine never cut out like that again) and it hasn't happened since

 

When you say Cold start, could you explain that? Had it been running that day? Was it really cold from sitting overnight or longer? I can understand that if the engine is warm and running and then you turn it off and then the ignition back on but not start it, the TCI would then discharge and Bang, I get that. But if it is a cold engine and had not been running for a day or so and you did the same, just turned the ignitions and Bang still? I will trust those that have this bike, if that is the case that is a good head scratchier to me.

 

By 'cold start' I mean sitting in garage for a week. And, no, not just turning on key. I mean choke engaged and turning over numerous times without actually starting.
Then letting it rest a minute or so, upon cranking it over again I get backfire. Thanks for asking.

 

You enrichment circuits, or pilot circuits are likely clogged. One or more may be allowing enough fuel to pass so that it loads up they system enough to backfire (or afterfire) when you proceed to the second round of trying to start it.

The first start after a long sit can be difficult, but that usually takes months of sitting.

 

Thanks K-moe. Are you saying pull carbs and clean .... not my favorite project. Other thoughts on solution?

 

Yes he is saying that, need to take them all the way into the church of clean, not just the steps. We all had to do it, some of us (like me about six times) before I got mine to run correctly.

 

Buy a fuel-injected motorcycle instead?
You bought an old machine, and she has needs. Give her what she wants, and you will be rewarded.

 

Fuel injected .... can't even work on those! Yea, I talk sweet to her and rub her tank as often as possible. Guess I gotta step up my game. Thanks K-moe

Did 'church of clean ' , two years back. Hoping not to repeat so soon.

 

Oh sure you can (well I can anyway).

The whole sermon (including wet-setting the floats)?
If so, then there might be something else going on.

 

Yep, the whole sermon including wet set of floats. Running perfect for two years, then just recently .........
And yes, I meant I cannot work on fuel injection. I'm sure there are masters out there.

 

Now she does not want to start at all. I ear a clicking sound from below the fuel tank. Sounds similar to a spark, but not really. How to trouble shoot?

 

click below the tank might be the starter cutoff relay you should have 2 relays that click sidestand relay and starter cut off relay.

Starter/Ignition cut-off relay (no color): behind self-canceller, under rear of tank.
Sidestand (kickstand) relay (blue): behind the left frame side cover, on the plastic regulator-rectifier mounting plate.

if you think your spark plug wires are arcing out try starting in the dark look under tank for arcing or cover your wires with tape to see if bike starts.

does bike turn over ? "Now she does not want to start at all" is a broad term .
turns over but does not start or press starter button and nothing happens are better terms.

also have you charged your battery recently? do a voltage drop test.

if starter is not spinning remove sidestand safety switch

 

Yep, she turns over just fine. Just grinds on battery. Just won't lite. New battery in spring and kept on battery tender. Voltage test completed on charger and electrical system during summer for unrelated issue.

Sidestand relay seems fine ..... shut me down (killed engine two months back). Is clicking sound normal? do not recall hearing it previously.

 

when key is turned on you should hear startersafety relay and sidestand relay click with bike in neutral 1 click.

do another voltage drop test on battery just to rule it out as a problem does not matter how new a battery is they can fail

turn petcock to prime for a few seconds to ensure carbs are full the gas can evaporate from the carbs.

as for trouble shooting ignition check for voltages on red/white wire at tci this wire splits to coils test for same voltage at coil connectors unpluged

the sidestand relay grounds out the tci remove it
if your starter turns then the starter safety relay is working it supplies voltage to solenoid.

have you ohmed out the ignition system lately to eliminate ignition coils plug caps wires and pickup coils as the problem?
pulled plugs and clean regap (or replace) check to see if wet after trying to start.

what do you have for a fuse block

 

fuse block upgraded - no rust, fuses good. Battery strong, volt drop okay. Will look at sidestand again.

 

Post values please. That will help with diagnosis.

 

to test side stand just pull the side stand relay it will over ride side stand function

 

Sorry it took so long to respond .... was on elk hunt. (don't ask)

Battery initial at 13.2 v and after crank @ 11.7 v.

 

k-moe - did battery values help w/ diagnosis?

 

with bike cranking and meter attached to battery what does the voltage drop to if under 9.8 volts bike wont start

 

as i recall it didnt drop below that value. I believe that is what you asked.

 

Well - either start replacing electrical components willy/nilly until I stumble onto solution or haul it to an old bike expert.

 

Before willy nilly go through EVERY connection (under tank, under side covers, inside headlight bucket, inside handle bar controls, etc) and clean, die-electric grease and reseat/ tighten.

Pencil eraser, small jewelers file, emery cloth for cleaning off any corrosion.

 

what was your total electrical failure?

Start with the basics and ohm out your entire ignition system. could be you are not firing on all plugs.
have you looked at the spark plugs?

did you clean out the enrichment circuit and pilot jets on your carbs as suggested.
could be the wells in the bowls are clogged preventing the enrichment circuit from working to all carbs

 

Okay, here is what I have continuing. (XJ750K) Continues to have electrical failure when cold but always (until now) restarts quickly. This time no restart. All lights and functional light indicators illuminate except neutral light. No starter turnover.
Packed it up and got her home, within a few (<5 minutes) all functional and will start. So, with battery only 1.5 years old and last checked it was charging within specs I'm suspecting the Regulator/Rectifier.
Will a bad rectifier act in a intermittent way or when they go bad will they just not function?
I'm trying to diagnose electrical issue without taking it to a shop. Any additional help appreciated.

 

if bike thinks it is in gear it will not turn over except if you squeeze the clutch and the clutch switch is working properly.

 

Bike was in neutral and clutch operated frequently. Is there a way for it to believe its not in neutral yet it is ?

 

bad neutral switch or the wiring going to switch may have issues, loose connection , corroded connectors.
if light does not come on could just be bulb.
clutch switch opperation should overide the neutral switch.
could also be an issue with the starter safety relay.

check neutral switch for ground connection with meter.
check clutch switch operation by disconnecting switch and putting sa jumper across it.
check the bulb I do not think it needs to work for operation but it is needed to indicate switch works and is easier to get to than switch and its harness

 

Thanks, I will check neutral switch. The neutral switch light once again illuminates after she rested for a few hours. Again, all lights functional except neutral light during failure.

 

Oooh! I saw this before!
On a Yamaha big bear, the neutral switch safety in that particular year was controlled thru the cdi unit, not seperate relay, something was effed up, same symptom, if you left the key on while waiting you could watch the light slowly get brighter over an hour or so, when lit, cranked again.
Changed CDI, mint.
Ironically, i tried the CDI on another big bear that used a seperate relay, not thru the box, it ran fine.
Yor bike i believe has seperate safety relay, this is likley the culprit. Id change that first

 

Have you tried to hold the clutch in and then bypass the starter button by jumping the starter solenoid? I would go back to the basics of fuel and spark. If you pull a plug, keep it in the spark plug boot and lay it next to the head. Turn key on, pull in the clutch and then short the starter solenoid. Hoping this will crank the engine and you will see spark. It the bike cranks and you see spark, Trouble shoot from the starter button to the solenoid.

 

I didn't read through the whole thread, but skimming this sounds kinda like what I went through on my Son's bike....all issues were traced back to the tci.

https://www.xjbikes.com/forums/threads/ugggg-xj550-shutting-off.115984/

https://www.xjbikes.com/forums/threads/boom.115586/#post-576834

 

Okay - back again. Last Fall ran like top, put away for 2 weeks, next attempt to start it would not. Plenty of battery and kept on trickle charge over winter. Try again in Spring and would once again not start. Turns overt just fine .... hoping Gremlins would fix over winter.
No spark as I firs try to diagnose. I'm working through Yamaha Haynes trouble shooting. Ignition coil would appear not to be an issue as both sides have no spark, seems unlikely both would fail at same time. Spark plugs all new. Fuse solid. Battery at start 12.7 v. Besides checking ALL connections, this seems to leave me with Pick-up coil and TCI / (voltage regulator). I would try to electrically test but I'm clueless and suck at electricity checks.
Any other ideas or encouragement?

 

first go to tci and disconnect the black wire with white stripe try to start bike.
if no start

unplug the connector from the tci that goes to pickup coils gray orange and black wires plus a black and white wire

ohm the black wire to orange then black wire to gray wire. also ohm black wire to ground should get no reading for black to ground

XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range


Ignition Coils:

Primary side (input from main wiring harness):
2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range


Spark plug caps:
1981-83 models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
1984 RL models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range

Spark plugs:
0 ohms per plug