Backfiring / Won't start

Hey there everybody, I'm new here, and pretty new to the realm of bikes, so please bear with me.

A few years ago I purchased an 82 Yamaha Maxim XJ650J. Odometer says about 10,000 km. I believe I am the third owner, and the bike had been dropped by its first owner resulting in some minor damage to the fins on the engine block, and the engine guard being scraped up.

THE PROBLEM:

It won't start at all, or if it does, it dies after about a minute. While attempting to start it will backfire loudly at random, and occasionally backfire when I release the starter button. The spark appears quite weak, white in colour, and doesn't seem to even jump the full length of the plug.

Resistance test on the coils show that the Primaries are at the correct level (5 ohms i believe?) and secondaries are both 23k ohm through the caps. One mechanic suggested replacing the Ignition Module. A freshly charged battery drops to about 12.3V while the starter motor is turning, and the motor seems to crank powerfully.

EVENTS LEADING UP TO THE PROBLEM:

At first the bike had starting troubles which seemed to stem from not getting enough gas. This was solved by putting the petcock position in PRI permanently. At the time I did not know what PRI was, just that "On" didn't seem to work. After making this change the bike worked well for a year, and would typically winter in a heated garage (snowed on a couple times from random Calgary snow falls during spring)

This summer I moved to Edmonton, and the bike had been running well. Then one random day it wouldn't start. I didn't have time to check why, though I did change the plugs because they were getting old. During the week an extremely strong wind gust tipped it over (only on kickstand, not centered) and it still wouldn't start.

After investigation it turned out it had suffered from fuel being dumped into the oil (either from tipping, or from being in PRI for a year and a half). Fortunately the tank was low. I did an oil change, changed the oil filter, and she starts up just fine, though runs slow on RPM while idling. The next morning she starts up at the normal RPM, so I figure nothing is wrong, and take her for a spin. I had switched the petcock to "On" during this time, to avoid the fuel dump issue from PRI.

I get about 10 km, and the engine starts to die as though it's out of gas (I switch to reserve to no avail). This is strange since the tank still had half a tank full. I get my car and fill the tank to full, try using PRI, and bike still won't start.

Next day it -does- start, but only goes around the block before dying again. I had to throttle up when idling at stops or else it wanted to die.

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Any advice and help would be much appreciated. My understanding is Ignition Module should only effect timing, not the spark strength, and that they rarely fail. What else could be the problem? Thanks!

 

I've been having similar problems that began when i removed the petcock for a thourough cleaning of the fuel system. I believe my problem is that the vacuum shut off isn't working. But then again I'm a complete nube and still troubleshooting this issue.

 

UPDATE:

So I tried running the bike again just to experiment some more. The bike actually started, with no choke, but only AFTER I stopped pressing the starter button. It ran quite well, mostly smooth but occasionally a little rough, in idle for about 5 minutes before it died. The bike wouldn't start again after that.

While trying to start it again, and monitoring the voltage levels of the battery, it had a massive backfire out the right exhaust, which continued to smoke for a long time...

I had put in a freshly charged battery not long ago, and it was already down to 12 volts even after running in idle. When activating the starter it would drop to 10.5 or 10.7 volts.

 

Classic symptom of low voltage. There is not enough to fire the plugs until the moment you let off the start button and the starter disengages.

 

So what would be the remedy? Like i said I put a freshly charged battery (have the motorcycle-grade charger/tender) in the last time I tried, and didn't have it cranking for that long or too often. It wont even give me any fire by this point. So what is causing the drain?

 

Have you checked the plugs after trying to turn it over to see if they are wet? If they are then you have a weak spark. If they are dry it could be that you are not getting enough gas. Have you cleaned the carbs at all? Backfiring is generally caused by a lean condition. Any tune-up like synching/dialing in the mixture of the carbs. Also make sure that the vacuum line is running from the fuel tap to one of the vacuum ports on the cylinders.

If you can get so far and the engine dies like it's not getting enough fuel it probably is not getting enough fuel. You are probably running it on the gas that is in the carbs and the carbs are not replenishing the fuel in the bowls and therefore run out and the bike dies.

Just a few thoughts and things I would look for.

 

The back firing has only occurred recently, typically while running the starter motor or immediately after running the starter. The short time she was running she didn't backfire, and when it quit it didn't have the "starving" sound, it just died.

 

Remove the Vacuum Line from the Manifold to the Petcock at the Manifold end.
Place the Vacuum Line to your lips and draw a vacuum on the line.
If you taste fuel; the Vacuum Operated Membrane in the Petcock is leaking and the Petcock will have to be rebuilt (Kit) or replaced (New or Used).

Inspect the entire Intake Air Boots and Manifolds for leaks, crack and ill fitting clamps. An air leak will have the Bike run lean and lean mixtures cause backfire.

Do you know when the last time the Carbs were removed and thoroughly cleaned?
You might very well be a candidate for a Carb Cleaning.
The Carbs cannot be tuned properly unless they are clean and providing the correct flows of fuel and air to the combustion chambers.

Are your Pilot Mixture Screws visible or do you have anti-tamper plugs covering the access to the Pilot Mixture Screws?

Once you get the Carbs Cleaned and the Throttle Plates Bench Synced; you'll need to be able to adjust the Pilot Mixture Screws in order to fine tune the engine.

Pull the Spark Plugs and line them up 1, 2, 3, & 4.
Take a picture of the Electrodes up close and in focus and let us examine the condition of the Plugs.
The color of the Plugs goes a long way toward stating the condition of the engine and its present tuned state.

 

fellow noob who bought an old bike: holy crud... meh bike makes these weird sounds like kerplunck... is there a magic button i can press to fix it?

xjpros: Remove the Vacuum Line from the Manifold to the Petcock at the Manifold end.
Place the Vacuum Line to your lips and draw a vacuum on the line.
If you taste fuel; the Vacuum Operated Membrane in the Petcock is leaking and the Petcock will have to be rebuilt (Kit) or replaced (New or Used).

Inspect the entire Intake Air Boots and Manifolds for leaks, crack and ill fitting clamps. An air leak will have the Bike run lean and lean mixtures cause backfire.

Do you know when the last time the Carbs were removed and thoroughly cleaned?
You might very well be a candidate for a Carb Cleaning.
The Carbs cannot be tuned properly unless they are clean and providing the correct flows of fuel and air to the combustion chambers.

Are your Pilot Mixture Screws visible or do you have anti-tamper plugs covering the access to the Pilot Mixture Screws?

Once you get the Carbs Cleaned and the Throttle Plates Bench Synced; you'll need to be able to adjust the Pilot Mixture Screws in order to fine tune the engine.

Pull the Spark Plugs and line them up 1, 2, 3, & 4.
Take a picture of the Electrodes up close and in focus and let us examine the condition of the Plugs.
The color of the Plugs goes a long way toward stating the condition of the engine and its present tuned state.

 

Ok some updates. Started a few more times, runs fine for a few minutes, dies. Occasional backfires while running the starter or just after releasing the starter button. I checked the petcock and fuel flows freely in PRI... unable to check on/reserve at the moment so just using PRI for now. There is fuel in every carb, but I won't be able to take carbs apart for a while. I also checked the alternator brushes. Not sure where one is supposed to measure from, but from the black plate they are mounted to their is an engraved line, and then more brush. One is about 5mm above the line, the other is about 10 or 12mm above the line.

Unfortunately where the bike is right now (where it fisrt died) makes it difficult to work on extensively... might have to just move it though if their is no possible "quick fix".

 

Sounds to me like you have to go through those Carbs and make sure they're all clean and available to be fine tuned.

You shouldn't have to run the Bike with the Petcock set to PRIME.
That indicates there is something going-on preventing the Petcock from operating normally.

Backfire can be caused by a number of things.
Lean Fuel Mixture leads the list.
Tight Valves is second.

My recommendation would be to have the Carbs Cleaned, Diaphragm Piston Bores Cleaned and de-oxidized, Bench Synced, Float Levels set and then reinstalled for further fine tuning.

Depending on the condition of the Carbs ... you're looking at a whole days work.

 

See this is where I get a bit confused. If its dirty carburetor causing backfire, shouldn't it also backfire while its running? It only backfires when I have the starter motor engaged, but not when the bike is running. Runs smooth when running, till it dies...

 

Alright time for another update. Started fine (though still after releasing the starter button...) and ran for about 8 minutes. Took her through idle to about 5000 rpm, held her at higher rpm for a few minutes as well and no troubles. Let her back down to idle and she dies.

Finally got it to a garage and checked the fuel lines and petcock. Fuel flows through with PRI, and sucking on the vacuum line is clean and causes fuel to flow when in ON and RES. Haven't bothered draining the tank and manually inspecting the petcock yet as it seems to work.

Yanked the carbs out of the bike (noticed that the air intake rubbers were attached 'lose' to the carbs... not sure if that is causing a problem) and that the rubbers on the cylinder sides are cracking on the outside, though appear un-cracked internally. All the butterflys visually appear evenly set. I drained the bowls and the fuel had a black sediment form at the bottom of the container. I removed the bowls and found they had a bit of red dust in the corners, which could be removed using a finger tip. I should point out I had drained about 5x as much fuel from the tank and there was no sediment.

The floats seemed to move freely, though thinking back it did require pressure to move the floats into 'open' position. I'll have to go back and see if gravity is enough to open them... of if their getting stuck. I attacked the fuel line to the carbs and blew through it with the floats open, and air came out though at a slower rate in some then others. The seal was tight with the floats closed. Seems i'll have to disassemble them further, and possibly look at a full cleaning.

Does any of this information point out some glaringly obvious problem I seem to be missing? Thanks for all the help!

 

Your floats should move freely - flop around, sounds like they are not.

I only had a bit of rust in the bowls but when I removed the needle seats the screens and fuel rail had a good bit. You may want to check it out.

The cleaning does not take that long and you already have them off. just take your time and be organized so everything goes back in the right spots.

Rick has posted a great tutorial on cleaning.

 

Well I found the problem. Took out the floats, float pin, and the well with the mesh screen. Each carb had some black sediment, bu the number 4 was just full of crud. Is there someway to clean the float wells and the connecting links that deliver the fuel across all four carbs -without- completely detaching the four carbs from each other? The rest of the carburator internals that I have seen so far (haven't completely disassembled) look factory clean, no varnish and no sediment except for the bit in the bowls.

Thanks!