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RSRBOB, Technician
Category: Motorcycle
Satisfied Customers: 1078
Experience:  Former Factory Service Rep, Dlr Line Tech, Service Manager, General Manager, Store Owner
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1997 Yamaha big bear. new cdi, new coil, new stator-pulse

Customer Question

1997 Yamaha big bear. new cdi, new coil, new stator-pulse coil, new flywheel, new sparkplug, new carb-adjusted, runs rich fouls out plugs eventually dies sometimes backfires
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Louie replied 1 year ago.
Hi again and welcome back to JustAnswer,
Thank you for the question and your patience. Should you still need assistance for information purposes only...
Initially, a review on potential suspects and proposed preliminary checks/tries (if have not been done):
• vacuum leak - ensure coupling of carb to intake manifold;
• over voltage - determine battery voltage during idling and when attempting to rev;
• have a compression and a leak down test.
Please post back results / observations of the above once tried.
Customer: replied 1 year ago.
everything there normal. how can over voltage have anything to do with it? the lighting coil and ignition coil are separate circuits.compression is 175psi, no leaks
Expert:  Louie replied 1 year ago.
Agree; but both are in the same magnetic field hence "proposed preliminary checks" for process of elimination.
Hopefully, another member can provide an alternate approach / second opinion to your concern. The question would be re-opened as I would rather opt out.
Still good luck with the Yamaha.
This is not an Info Request.
Expert:  RSRBOB replied 1 year ago.

Hi and thank you for your question.

I would like you to try some tests to see if we can affect the problem and narrow down the possibilities because of the test results.

The first thing I want you to try is remove the air filter and run it without the air filter to see what it does. On a normal Big Bear, it would start and idle but be very hesitant to rev up because it is so lean. If yours is just getting too much fuel, removing the A/F and running it would provide more air and help alleviate the problem.

If that doesn't work, start the bike and put your hand behind the muffler to feel how much pressure is coming out of the muffler. Then rev the engine and see if the pressure increases at about the same rate as the RPM's do. If not, there is a restriction in the exhaust. This gives the symptoms of running rich because the cylinder cannot evacuate all the burned gasses and they remain in the cylinder for successive power strokes.

Check those two things out and let me know how they went.

Also, if you are still having problems, I need to know if the entire ATV is box stock. No jet kit, no aftermarket air filter, no after market exhaust, etc. Also, are you starting with new spark plugs or trying to clean fouled spark plugs? Also, what brand spark plug are you running and what is the code on the plug?



Customer: replied 1 year ago.
The only aftermarket parts is a big bore kit. Air filter has been off for some time. Gone thru 3 sets of ngk plugs. Pressure from exhaust is strong. Compression is 175 psi. When I can get it running acts like it is firing on all strokes. Idles rough. Backfires when 1/4 throttle or more then dies. Has new cdi, new stator, new pulse coil, new ignition coil, new flywheel, new flywheel key, valve gaps to spec, valves working properly, cam lobes not worn within spec, No vacuum leaks, new carburetor, new intake boot, fresh gas. The only thing that hasn't been replaced is the wiring harness, rectifier, reverse switch,
Expert:  RSRBOB replied 1 year ago.

Thank you for the additional information.

I would like you to try something for me. Start the ATV, and with the cover off the air box, stick 2 or 3 fingers inside the throat of carb. We want to restrict the air flow. Try to rev the engine as you move your fingers to adjust the airflow. Try restricting more or less of it to see if there is a happy medium where the engine will rev better.

Let me know how this turns out.



Customer: replied 1 year ago.
Engine doesn't run any better. gets worse then dies. Like I said it won't rev past about 1/4 throttle. backfires and dies.instantly almost like it fired on the intake stroke or fired several degrees before compression stroke
Customer: replied 1 year ago.
Expert:  RSRBOB replied 1 year ago.

Thank you.

We have conflicting information here, at least on the surface. Correct me if I am wrong but at some point, you did say it was fouling the spark plug, or it was coming out black and sooty? Then you say it won't rev up, which sounds like a lean condition. That not withstanding, I need really detailed information here because of the extent of the attempts you have made to repair this and it has not helped.

1.) First, regarding the components that you have replaced, are they all brand new OEM Yamaha parts or are some used? If they were not all brand new, never used OEM Yamaha, please specify which components that were replaced were used parts.

2.)Next, about the carb. This is one of the biggies that I am concerned about from the last question. Is it an original OEM carb for your year and model of ATV or an OEM for another Yamaha atv?

3.)Is the jetting stock, and what do you have the fuel level set to? There are a couple of things in the carb that can cause it to run rich. The float level being too high, a leaking needle valve and seat, a leaking enrichener, jets that have fallen out (come loose), incorrect jetting of the carb slide not lifting.

4.)Also, do you have the lid on the air box and are there any holes drilled in the air box or lid?

At this point, again respecting the extent you have gone to, please help me help you by not telling me something is good or ok because you don't think its relevant or have not confirmed it since the problem cropped up. (Not that I think you would do this, but it is fairly common in this forum for customers to refuse to cooperate because they don't agree with the direction of the diagnosis and repair, so no offense intended) For me to be able to help you, I need answers to my questions and you are my eyes on the unit. I am trying to compile all of what I consider pertinent and potentially relevant information. The quality of my responses are solely dependent on the quality of the information I get from you. I may even ask you to recheck things you have said are ok just because the symptoms point to a particular component or assembly process.

5.) Have you done a compression test on the engine since the problem has developed? What is the compression now?

6.) Has it ever run correctly in its current, big bore, no air filter configuration? I would not expect any ATV to run properly without an air filter. It would just be too lean. It would start and idle, but not want to rev up. The one thing it should not be doing is fouling the spark plugs however.

7.) Next, have you ever put an automotive style inductive timing light on the spark plug lead to see if it is sparking when it won't start or develops the misfire? Where I am going with that is trying to eliminate variables here. If we are getting steady spark and the unit is acting up, not starting etc we know it is not a ignition issue, unless the timing is off. If the timing is off, like the flywheel isn't seated on the key way properly, it will not show up as a problem with a timing light because it is a crank triggered ignition. The best way to check that would be to slowly rotate the crankshaft to TDC compression and make sure when the T ***** *****nes up, the piston is indeed at TDC.

8.) When the engine does run, does it make any unusual noises that you don't think it should be making? If it is, where do they seem to be coming from?

I numbered my questions so I can keep your answers straight with my questions, please feel free to respond using the question # ***** then your answer. Thank you.

Let me know about this things and we can go from there.



Customer: replied 1 year ago.
1. All the components are brand new. Being the big bear is nearly 20 years old several of the parts are not available such as the stator, cdi, and are hard to find if you can get them. All parts are aftermarket brand new. New stator, pulse coil came with stator, new cdi, new ignition coil. 2. The carb is a new aftermarket carb for this specific big bear. After replacing the original carb with new one symptoms continued. Took apart new carb to see if carb parts were same as original carb and all parts were the same. 3.Stock jetting. Float is at .50in. No loose parts in carb. Has brand new enrichener plunger. Carb moving parts working correctly. 4.Air box is removed. No holes drilled. 5.Compression is 175psi. 6. Yes it ran like a champ when I installed the big bore kit. Of course it ran slightly lean without the aircleaner, but once i installed the air cleaner it ran great for over a year. 7. Had constant strong spark. Haven't used timing light. Used the see through spark tester tool. Replaced original flywheel with used flywheels as they are also not available new, twice. Flywheel key is normal no wear or offset. Removed head to check valve train. Everything looked normal. Cam lobes were within spec. Timing was correct before I took head off. 8. No unusual noises. One thing that it does do when it quits running is the crank rotation stops instantly, like it tried to fire before the piston was at tdc, like many degrees before tdc. Hope this helps.
Expert:  RSRBOB replied 1 year ago.

Thank you for the info.

There is still an unresolved conflict in symptoms here. The engine has to be lean without an air box. Fouling plugs is rich.

Who makes the aftermarket carb and where did you get it from?

When it backfires, is it backfiring through the carb or exhaust?

When it runs, how long does it take to foul the plug?

Customer: replied 1 year ago.
Without airbox it would run leaner, not necessarily lean. Fouls plugs within minutes just idling backfires thru intake. Not sure, was purchased before I got the wheeler. Just to remind you, it runs the same way with the original carburetor as it does with the aftermarket carburetor. I am going to just end this conversation as it has not progressed any from what I have already checked. Thanks for your help anyway.