I've got a 1987 big bear 350 4x4 I got as a non runner.
it has a good spark and getting fuel, compression seems fine (not checked yet as waiting for compression tester adapter to arrive).
it will fire and actually run for a second or two most of the times I've tried starting it now, but then stalls as if you've turned it off.

I got it from a mate who replaced the coil, stator and spark plug. He also got a Chinese Cdi unit as he was told it has no spark.
The old coil and stator are here in a box.
The Chinese Cdi is in the same box as I can see a spark all the time with the original Cdi unit.

So far since I've had it I've done a few little bits to try and get the basics in line.
Flywheel key checked, all looked good but replaced anyway.
Timing checked and looks spot on to me.
Valves adjusted and in the middle of the spec.
Carb bowl striped and jets cleaned, though the carb looks brand new inside. (I think it's the original carb, it's a mikuni carb).
I found the carb to engine boot was split and one o ring was split, so got another engine to carb boot (inlet manifold?) and that looks to be sealed perfectly now and obviously has no splits allowing air in.
Every wiring connector has been disconnected and cleaned then reassembled.
All earth points have been cleaned down and all wires have been checked for any dodgy connectors or crimps, all look good and get good continuity.
I'm using a 100ah spare battery from.my van so definitely got good power and started spins the engine over perfectly.

Every second or third time that I try to start it, it will fire and run for a second or two then die.
I'm slowly running out of ideas and would really appreciate any other opinions or ideas to try with it.

 

Fit a new plug.

I try bikes with gas from a Butane gas blowlamp, squirted onto the air filter, if the mixture is lean, the bike will fire up and run while you are spraying gas in, or may do when you stop spraying it in, for a few seconds. Same effect can be got with a spoonful of petrol on the filter, but you risk "wetting the plug" if it gets too much fuel at once, then it will never start until the plug dries out again.

 

Thanks for the response, much appreciated.

New plug is on the list of things to get today.
Am I right that it is a dr8es spark plug for this?

I've tried carb cleaner and easy start into the carb while cranking, but doesn't seem to make alot of difference.
Plug gets wet and black when cranking for more than 3-4 attempts though.
​​​​​​Maybe it's running rich?
it will only fire with the choke on full and no throttle.
Do you know what size jets are supposed to be in the carb?
Also, are rebuild kits available for the carbs? Would rather keep this one than get some cheap Chinese crap that probably won't work.
Can you also get new screws for the carb as some of mine are chewed up abit.
Or, does anyone know what thread they are?
​​​
The exhaust is also getting warm when I try to start it, after a few attempts you can feel it has warmed up abit which would tell me that there is fuel, spark and compression as it seems to be firing and burning some fuel.

I stripped the carb again last night and cleaned it out, all appears to be clean and no blockages, main jet needle doesn't block the main jet completely when no throttle is applied, not sure if that's correct or not?
Float and needle work perfectly.
Removed fuel tank and used a make shift tank to guarantee I was getting good fuel flow and used fresh fuel (v power, so no ethanol and rated as 98 octane) to eliminate fuel as an issue.

I've also put fresh fuel in and that's made little to no difference.

Hopefully the compression tester adapter will arrive today so I can check compression.

I'm slowly leaning towards it maybe having low compression as everything else seems to check out spot on so far.

 

You should be able to get a carb rebuild kit, Shindy make kits for most models and some kits have those screws included, they will be 3 or 4mm. Be very careful if you do a full rebuild as you end up removing bits that you normally wouldn't, and it is very easy to get a part upside down or in the wrong order.

 

Cheers for that, I've seen a couple online but need to find the carb model before buying anything as I need to get the right kit, just seems to be so many from China and I'd rather buy genuine if possible to avoid any problems.

Also, checked the compression last night, appears to be around 50psi if my gauge is correct, going to check the gauge on a good running engine tonight to verify it's correct then check the quad again.
If it is really 50psi, looks like the top end need a full rebuild and that is where my problem lies.
There does seem to be a good amount of pressure coming from the crank case when cranking the engine over too which I'm guessing points towards rings or head gasket?
Not ordering any parts until I've checked the compression again and stripped the engine to make sure I get what's needed.

 

350 single cylinder engines need to vent 350cc of air out of the crankcase as the piston goes down and suck in 350cc of air back in as it rises, only alternative is the one way breather valve used by Briggs & Stratton etc. Quad makers don't seem to use this, so lots of "crankcase pressure" on healthy engines. 50psi is way too low, try the thumb test, doubtful if it will blow your thumb off the plug hole at that compression. Get the workshop manual, there will probably be free downloads for a bike that old, and check everything, rings, bore diameter, valve stems, cam lobes and rockers, and give the valves some new seals and a regrind while you have the head off.

 

Yes, sorry for the late reply to this.
Checked compression again, 25psi dry, 50 psi with a cap of oil down the bore.
Rechecked everything and all in spec and fine.
Got a new plug (resistor type), gives marginally better spark.
Held my thb over the plug hole while cranking with WOT, didn't even have the go to push my thumb off so pretty sure it's a compression issue.

Got the head off, not checked measurements and tolerances yet though.
Head gasket is the 3 or 4 layer metal one, all layers still stuck together apart from the front where they're all separated and also around the cylinder to engine join.
No residue or air blowing out of head gasket when cranking though....
My thoughts are that maybe the cylinder was leaking into the engine and that's why I was getting excessive crank case pressure and very low cylinder compression.
Poured some petrol down bore and moved piston, didn't seem to loose any over the half hour that I monitored it. Put a small amount in the head (upside down) got a tiny seep around one of the valves but no drips or runs so thinking piston and valves were seating well enough.

Going to clean the head thoroughly and regrind valves then stick a head gasket on and see what happens.

If no luck with that, I'll have to try digging deeper, just reluctant to remove the cylinder and piston as I don't want to remove the engine from the quad if I can help it (limited space in my garage).


Any advice or tips are much appreciated.

Also, any specific head gaskets that are better than others?

​​

 

25psi is pathetic, and you have a big problem, something is way out. If there isn't discolouration on the old head gasket where gas has been leaking from bore, it wasn't that. If you are sure you had the valve clearances correct, you have a valve not seating or worn bore and/or rings. Fairly certain the cylinder should come off with engine in frame. Measure the bore and take the top ring off, put it in the bore and measure the gap. If it is OK, it has to go back on the piston the right way up. If the bore is OK de-glaze it by hone, or at least radial scratching with emery. A long time since I did a 350 Yam but I have a feeling the head has an oilway with a short tube between head and cylinder, that has to go in correctly, any make of gasket should be OK

 

Thanks for the info.
Yes, certain about valve clearances, checked them multiple times, always within spec.
Also checked the timing, again, multiple times, always in spec too.
Im glad you had the same thought as me in the compression. I've never had so little on any engine before!

I'll have a look abit further if it's that straight forward, I just assumed the whole engine had to come out to remove the cylinder. If it can be done in situ, I'll take it off sometime soon and check the rings and piston/bore, will give it a home before assembly too.
Valves will be removed and checked, then reground.

I'll carry in investigating before buying parts now, sounds like the problem could be deeper than I thought though. Fingers crossed I get to the bottom of it soon, I just want to ride the damn thing!

 

Does anyone know where I can find the specs for the bore/piston/rings please.
I was sure I'd bookmarked an online manual for the big bear 350 4x4 but for the life of me, I can't find the damn thing anywhere.

I have some measurements now I've stripped the cylinder off the engine but need to check them to see what's worn and what needs replacing.

All parts are available, but the difference between piston, rings and gaskets, or a complete cylinder kit is about £100, so want to make sure I know exactly what needs replacing.

I'm now off to recheck and note the measurements and will post up what I've got later.

​​​​​​
​​​​​

​​​