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Discussion Starter #1
Hi, having some issues with my big bear,
I'll start from the beginning.
I bought it as a non runner, I've not ever heard it run but the previous owner bought it in a job lot, said it ran, stored it for a while and went to start it and couldn't get it to go.
So I bought it, had no spark, so i traced that to the stator/pick up so I had it rewound by West Country windings, renewed the battery, coil and plug, and cdi unit. So the ignition side it's basically new. It's sparking now if you use the pull cord but only sparks once on the electric start. I've checked the engine timing which was out due to the chain being stretched so I've replaced the chain and checked the valve cleareance. Which brings me to the issue. If I pull the bike on the cord it'll do maybe one full revolution and then snatch the pull start out of my hand. Almost as if the next spark is stopping the piston dead. It'll backfire out of the carb too sometimes. It just won't run.
I have had the carb apart and it seems ok?
Anyone have any suggestions? Any input is welcome.
Thanks, barney
 

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I don't work on 350's, but try pulling the plug and inserting a long straw to find the top of the piston to determine TDC and then look at the timing mark.
 

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1) Check the cam timing as ridgeway said.
2) Adjust the valves.
3) Do a compression test. If it is zero or near zero you may have a bent intake valve
 

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Discussion Starter #5
So I checked everything like suggested, couldn't see a problem so I went ahead and pulled the head off the engine to check the valves and camshaft, they appear fine, but I'm going to re lap the valves into the head just to be sure before refitting.
I'm stumped. Can the ignition timing be adjusted?
 

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The timing is adjusted in real time by the cdi unit & is not user-adjustable.

The piston stopping dead or reversing crank direction with the next spark and the backfiring out the carb seems to be one of two major issues:

First is potentially the position of the valves in relation to the piston aka cam timing. Problem areas here are the cam sprocket not aligned with the crank mark at TDC, valves not sealing (bent or stuck open or not adjusted correctly), chain stretched, cam sprocket locating pin or crank key sheared, cam lobes worn down (not common).

Second is when the spark occurs in the cycle. This is determined by the pickup, cdi box, coil, & plug. In some vehicles the pickup can shift to the side. In others there is a specific distance that the pickup needs to be adjusted away from the trigger on the crank. Others still can have the component on the crank containing the trigger (trigger locations vary by model) installed in more than one way i.e. backwards or at the wrong clock position. Usually these are solid state "Hall Effect" sensors and the distance from the trigger is critical for reliable signalling. I don't have your specific vehicle or the fsm for it so I don't know if any of these apply but you should check.


1) Can you explain the statement about using the electric starter & causing it to spark once? Do you only get 1 spark then nothing while cranking?
2) Did you replace the ignition pickup?
3) Is there any adjustment or movement on your style pickup?
4) Are you certain the component on your crank containing the trigger was installed correctly & is in place?
5) What did the compression test read?
6) Are you sure you have good grounds?
7) Are your crank and cam keys & dowels present and functional?
 

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Discussion Starter #7
Ok,
When I turn the engine over via the pull start, I get a nice healthy contiuous spark, when I use the electric start I only get one spark and that occurs as soon as I press the starter button, doesn't matter how
Long I keep it held down, I won't get another spark untill I
Let off and and press again. And it'll be just one spark per button push if that makes sense.
The ignition pick up coil was replaced the same time I had the stator rewound.
I don't believe there is any adjustment, seems to be fixed.
The flywheel on which the trigger is on is keyed to the crankshaft so It won't go on in any other way.
I haven't Managed to do a compression test as I didn't have the right adaptor. Not ideal I know.
I have good earths.
And all woodruff keys are present and in good condition.

I'm not sure what year yhe quad is. It looks old. My frame number is badley pitted so I can't make it out.

Barney
 

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First:
Obviously the starter use and 1 spark is not right. When you have a large current draw on the system with the starter you are not getting spark. I would measure the voltage at the battery while cranking and again at the coil while cranking. If there is a significant voltage difference between the two you may have a wiring problem or if the voltage at the battery is less than 10-11 volts or so while cranking your battery may not be up to snuff and may not be enough to reliably and continuously fire the coil. I would suspect the coil and/or battery first here.

By "renewing" your battery & coil do you mean replaced with new parts?


Second:
You can test empirically for a non-sealing intake valve (or incorrect cam timing) by turning off the gas, draining the carb bowl, pulling off the airbox to carb boot, holding the carb slide open, and cranking. See if you can feel pulses of air going backwards through the system. There should only be air pulled through the carb, not pushed back through. This is not a definitive test but should tell you if something is grossly wrong.
 

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Discussion Starter #9 (Edited)
Yeah new parts are, battery, plug, coil, cdi, stator/pick up assembly.
I've got the head off, have poured fuel down the valves and it does not leak past the valve seats. So all internals of the engine seem good.
When I had the engine together I did put my hand over the carb and it is sucking in air. I just get the occassional backfire through the carb, as if you had a distribter on a car timed wrong. That's the best way I can describe it.
I'm going to reassemble the engine tomorrow.

I've opened the whole wiring loom up to check for any shorts etc and the loom is good. I wonder if I have a faulty component. Maybe the starter is drawing far too many amps? The possitive cable does get fairly warm when cranking for an extended period of time.
Barney
 

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For one there's got to be some thing electrically wrong with the starter. Maybe you have a bad ground and as soon as you crank it with the starter the rest of your bike loses ground. And it continuously sounds like the timing is out weather its just off a tooth or what those motors are picky. So you replaced the timing chain. Did you replace the timing chain sliders. I know if there worn down it will affect timing. I'm assuming that has an auto timing chain tensioner on it. When u installed it was the chain slack on the side where the tensioner is. Have you rechecked timing after all assembled? Are you sure it was on top dead center I'm assuming compression stroke when u timed it? Another way to see if there is compression is to hold your thumb over the spark plug hole
 

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I would get a factory service manual available for download for about $10 US. It has a specific diagnostic algorithm for spark and will tell you exactly what to check in what order and the ohm ranges for each component.
 

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Discussion Starter #12
Orangeman,
I think the point you made about the starter making the bike loose it's ground is a good one, I'll check that. I haven't replaced the guide rails no, the chain wasn't slack it had a just stretched so the timing marks were slightly out, put a new chain on and it's all lining up as it should.
The tensioner is an auto tensioner. Seems ok.
One thing I have noticed is that the butterfly in the carb is always wide open, or as near as, whilst the flat slide is down?
 

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Discussion Starter #13
I will try get a hard copy of the manuel aurbrey.J
Sure it would be handy to have one any way. Going to be putting the head back on tonight. the engine was all in time, valves all ok, it's getting fuel. Has to be the when the spark is occurring. Could the flywheel be wrong?
 

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Could the flywheel be wrong?
It's conceivable since you bought it non-running that the component on the crank that contains the pickup is from the wrong model or is installed flipped over (yes it is almost certainly keyed but in some models you can reverse it/flip it over when assembling).

I would look carefully at the sketches in the fsm & see if you can find anything wrong.
 

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Discussion Starter #15
Hey guys,
So I rebuilt the carb and still couldn't get it to start. Still only one spark on the electric start.
So I decided to tow start it. It ran, but badley!
It won't idke by itself.
And I couldn't get it to restart either.
I think it would go again if was to tow it
Any ideas
Thanks barney !
 

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Discussion Starter #16
I think the running rough is the carb. Is there a better carb I could replace it with? It Has the mikuni flat slide carb on it at the moment.
Also it only ran while the choke was on.
 

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I know this is a old post but did the original poster ever resolve his problem with the one spark with starter being used and continue spark with pull rope ??
I have a 98 Yamaha big bear 350 4×4 with the exact same problem !!
 

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I know this is a old post but did the original poster ever resolve his problem with the one spark with starter being used and continue spark with pull rope ??
I have a 98 Yamaha big bear 350 4×4 with the exact same problem !!
I just bought a non-running 1987 Big Bear. I was looking on YouTube and saw a video of a guy with this issue. One spark and then nothing when cranking. He did a ton of checks (all on the video) and what seemed to work for him was heating the CDI up with a heat gun. It ran enough to indicate that this was his issue. I have read and seen videos of people having pretty good success heating up the CDI. I saw videos of a guy saying 300 degrees for 10 minutes and another post here on this forum that said an oven that has been preheated to 250 degrees and then put CDI in for 10 minutes. Let cool and then install and try.
I haven't had the opportunity to work on my new-to-me Big Bear yet so don't know what the issue is yet. It seems to be a very common issue.
 
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