1994 Suzuki LT-4WD 250 Quad Won't take throttle?

[dlgg7]

• I've had this for probably 15 years. It ran great till 4-5 years ago, mainly riding the grandkids around our fields, till it wouldn't start. It sat for a couple of years till I bought a new battery, but couldn't get it to even hit, so rebuilt the carb, put on a new fuel pump, but no go, so it's sat. Several days ago, charged battery, cranked over and it started??? OK, idles good, but won't rev up, so emptied gas tank, flushed out and filled w/good 93 octane, pulled pump line from carb and have good full line of fuel cranking over. New spark plug, good spark, old one black BAD! Oil change, adjusted valves, no difference. Pulled carb and took apart and cleaned, didn't find any clogs or crusted up inside. Now I've had the carb back off probably 6 times checking, double checking anything I can think of. No gas blowing back in air box. It idles good, can increase throttle up some then starts popping and dropping rpms off and if I don't let off the throttle, it'll die. I've read for 3 days now MECHs and RANDY walking several thru tracing the same problem, tried MOST of the direction given. ???

[Mech]

Has it got good compression..  Have you checked the exhaust isn't blocked ?

[dlgg7]

Thanks Mech, gonna try to ck the compression soon as I FIND my gauge! Exhaust is good. I still need to ck my spark at higher rpm. battery is almost 13v.

 

[dlgg7]

Have to do all my scrambling once I get the grandkids we're raising off to school! Try some more thoughts today, Thanks again for the reply!

[dlgg7]

Spark is too hard to tell in daylight! Good spark at idle, at higher rpms, I can't see it even tho no studder in rpms till it starts bogging to die.

 

[dlgg7]

Well, yesterday pulled carb again after trying to ck spark, rebuild kit was same jets as the OEMs I took out. Set mixture screw to 2 turns by book, Got the stupid idea to pull main jet seat, yoke?, the Y shaped piece, those 2 security? screws were lock-tighted in, but I have the bits for them, BUT, broke the head off one. Cleaned passages thru and O-rings are shot, found 2 new ones and replaced. Finish getting it back together today. Good spark looks like, good fuel.

[dlgg7]

Just to add, book shows #9-Needle jet, which I couldn't get out of that 'Yoke?' that the Vacuum Piston slides on. Seems the 1 screw holding down the ears of the 'Yoke?' is holding it tight in the carb body. Shaking my head on trying to drill out that broken screw in the carb body, any suggestions or thoughts?

 

[dlgg7]

Compression good, 

[Gwbarm]

Patience is the key here, because those carb screws are tiny , get a center punch, i have one that pops when you push down on it to make the indentation to get it in the center of the broken screw, if its not in the center it will wander off and get into the threads, drill with the smallest drill bit you have, you will probably have to go a little larger with the hole later, but its best to start with a small hole, did i mention make sure its in the center of the broken screw. I dont particularly like ease outs but sometimes its the only way if its broken off flush. Soak the broken screw good with penetrating oil and since its in a carb i would also soak with carb cleaner it cleans out any varnish residue from the gas better. We want to try to get it out without damaging the threads but that does not always happen, if you get into the threads, dont panic there may be enough left to get the screw to hold , and if not you can tap for a size larger thread. 

I am working trrying to get one out of a throttle plate that the head stripped, it is being difficult, so i am taking my own advise and being patient, but it is so small, and they put them in with locktight and the screws are so soft , so i know what you are going through. 

[Mech]

Yup, what Gw said, small drill right in the center, and be patient. Sometimes warming the aluminium up lets the oil or whatever you are soaking the thing in soak into the threads and loosens any corrosion in there.

[dlgg7]

Reckon I'm kinda CHICKEN here! I have cobalt bits, just that small is hard to imagine going good! Already made a big goof while having the slide cover off and turned the slide upside down and where do you think the needle dropped straight down on the motor? Under the starter! Fishing w/fine wire was fun! It is back on the bike, starts TOO easy, but still the same. If I had another backup carb, I'd try it. Have to think,,,,,,

 

[dlgg7]

What I've been trying to pull to clean is needle jet holder, only thing that's not been out, can't get it to budge!

 

Gonna have to go thru some of your previous direction to ck the regulator and see if it's too much voltage shutting down the cdi at higher rpms.

[dlgg7]

Mech, you posted on another post on these-Your slide needle should be sprung loaded downwards. There's meant to be a small spring pushing it down, and the spring is held down by a plastic bit with an O ring on it. The O ring is all that holds the plastic bit from pushing up. If the O ring isn't on the plastic bit, or if it's old and hard, or if the plastic bit can be pushed up and out of place by you pushing the needle up slightly against the spring, but without actually pushing the plastic directly with the needle, they you should change the O ring.  The needles always wobble around sideways in the slide, that's ok.

By my book, the carb diagram for 1990-ON shows only the spring, spring seat, E-ring, then jet needle with ring underneath. Didn't have any O ring when I originally took it apart, am I thinking right?

 

Edited September 30, 2023 by dlgg7
added text

[Gwbarm]

They dont all have O rings mine just had the plastic piece and spring, nothing but the spring holding the plastic piece and needle in place,  and they are a little wobbly the main jet and plastic guide holds the needle in place.

[Mech]

I'd got my carb models muddled up there Digg.. Not all carbs have it as I was describing.

[Mech]

Every parts place I look up that bike in though shows a carb that doesn't have that particular carb for your bike..

Does that carb have a vacuum fitting for the fuel pump ? And for the fuel tap ? Have you checked the fuel tap and fuel pump are working and not letting either air or fuel get into their vacuum hoses ?

[Mech]

Just read back and I see the fuel pump is going right. I see the old plug was black, so one of either the pump or tap might be letting fuel into their vacuum hose.

Every time I've seen a suzuki LT regulator playing up the symptoms have been that it gets to a certain rev and then just sits there stuttering and missing like it's getting a rev limiter on a fuel injected car.. like the ignition switch is being turned on and off a hundred times a minute.. but the engine keeps running forever.. you can drive them around up till that rev, and then just keep driving at full revs with it missing like mad.. They don't die right off and stop running.

A black plug is either because of too richer mixture or burning oil, or there's an electrical miss. If it's got an ignition fault then it's likely to backfire in the exhaust because there's unburnt fuel in there from the cycles when there was no spark. If it's just getting too much fuel it's less likely to do that, it will probably blow black smoke and run rough but keep pulling more or less ok.

If it starts popping back through the carb when you open the throttle past a certain place, it's nearly always because of a lean mixture.

I'd probably be able to tell what system is playing up by riding it and testing things as I rode it. Sometimes a symptom is related to the temperature and is worse hot, sometimes they are related to the throttle and always occur at pretty much the same throttle position, sometimes they are related to load on the engine and the throttle seems like the thing except they don't play up at that throttle setting unless the engine is under a load, sometimes they play up as soon as the load comes on, and sometimes it takes a short time as the carb empties or some electrical part gets hot, and sometimes things recover as soon as you back off slightly, and sometimes they recover only after a time and after you've backed right off.. 

It's important to look at the symptoms, and the circumstances of an occurrence when we are trying to diagnose things. I always try, once I've formed an impression about what the problem is, to subject the bike to that particular condition while changing the other conditions.. So we can subject an engine to a heavy load by opening the throttle a lot, or by going up a steep hill with a lot less throttle..  We can ride it till it starts to falter and then back off the throttle very slightly and see if it recovers straight away, or whether it takes a few seconds, and we can back off a lot and see if it recovers quicker..  We can ride it until it starts to falter and then let the throttle off but not let the revs drop and see if that cures it, or we can let the revs drop while keeping the throttle held at the same position by starting up a hill. In the case of the regulators playing up, that last test is pretty conclusive. The regulators are really rev dependent once the battery is charged. They will start missing at a certain rev and then just keep on limiting to that rev no matter how much or how little throttle or load is on the engine.. they just rev limit pretty much..

Ride it and think about what happening, what's changing that causes the problem to start manifesting, what things make no difference.

It does sound like a fuel problem to me, and having those O rings broken could definitely be the problem. Perhaps you could drive it now with just one screw in it..  

As for getting the broken screw out, did it break off flush with the aluminium or with a bit proud ? Perhaps it can be gripped with a good pair of pliers. If not it needs drilling probably. I'd use a egg-beater drill if I was doing it and I'd turn it slowly and I'd change direction and direct the drill straight down the center. If you start with a very tiny drill, and it gets slightly off course, you can correct by using a slightly bigger drill aimed at an angle to drill to the bottom of that first small drilling..Then go back to the tiny drill again. If the screw is a four mill it should be easy to get out.

 

 

And just to clarify for me..  That carb is a diaphragm carb isn't it ?

[dlgg7]

Yep Mech, the carb has the vacuum diaphragm on top of the carb. But guess what, I pulled the front fender off yesterday figuring it wasn't any more of a PAIN!! than pulling the carb off the rubber boots again! Tracing the rubber hoses, watched a guy 'Schneid15' I believe on youtube on the vacuum hose routing and traced mine, the 2 AFTER the carb on the air filter side are just vents. 2 hose connections on the intake manifold side are vacuum, vacuum connections have flared ends on them, one on the right side of carb sitting on it is the one to the fuel pump, one under the throttle cable is for use to the fuel pitcock, which I put a new one in before when I couldn't get it going when it stopped running, With the front plastic off, I found this 2nd one not connected, I pulled the hose off and put a vacuum plug over the carb barb and it starts immediately w/no choke even and revs up good. Went for a ride w/battery strapped on the front and it runs just sometimes missing on acceleration and I have to feather the thumb throttle a little and it then will go on and accelerate. Now all I have to work out is the miss sometimes. I have to admit, I've tried cleaning the plugs, went thru 3-4 new ones trying to get this figured out and they where fouling out in no time! I'll have to see how it goes before I want to pull the carb just to try to drill-out that screw! Wish I could find another decent used one to go through! 

[Mech]

Good stuff..  So if it doesn't have the vacuum hose to the fuel tap either it's running on the prime position of the tap, or the tap has been changed to a manual tap. If it's still got the old vacuum controlled tap, there is no off position. They only have main and reserve, both controlled by vacuum, and a prime position to get them going after they have run out of fuel.

 

[dlgg7]

Lol, your right Mech, the fuel valve has been changed over to one that doesn't have the vacuum barb on it, Prime, Off, On. So that was part of the problem, that vacuum barb on the carb is now plugged. It revs up now, but now when I rode it, it starts missing around mid-range, I have to feather the throttle a little off and goose it some to get it to go on and accelerate more. I DID set the idle mixture by the book at 2 turns, It was about 2 3/4 turns prior. Not fun to get to! I'm waiting on an adjustment tool.

[Mech]

Yeah adjusting the idle mixture is the first thing to try now. That initial setting is just a ball park figure to get them running. If you can't get over the flat spot by adjusting the idle the next step would be to try moving the slide needle.

They are quite sensitive to the air-cleaner though, and I've cured some that had a slight hesitation by either cleaning the filter, or drying them if they are over oiled, and in other cases , by cleaning them and oiling them.

[Gwbarm]

So are you running it in the prime position or the run position, i have only seen the prime position on a vaccum petcock, usally has off, on, and res., but there are many different aftermarket ones available im not familiar with. 

[dlgg7]

Ok, Mech, the filter I haven't serviced yet, it is still oily by feel, so after that many years it probably needs cleaning and oiled. I've got a right-angle adjusting tool coming, only way I can see getting to that screw! And Gwbarm, my bad, it's the res on the fuel pitcock, your right.

[Gwbarm]

I have one of those angle adjusting tools, it works OK   i also got one of these if you have enough room, some do and some dont, but it is a little easier to hold on to and turn while adjusting.

 

[dlgg7]

Thanks Gw, I've got some of those too in diff sizes, can't get in to it. Hopefully the 2 different ones I ordered will be able to!

 

[Mech]

It's the hot exhaust that's the real problem.. The right angle or flexible drive is what you want.

This looks good..

https://www.amazon.com/dp/B0BHYW1PBF?tag=qcforum-20

[dlgg7]

Just an update, IT'S DONE! Pulled the carb back out 2 days ago, broke it all down AGAIN! Pulled the main jet seat? 'Yoke'? whatever it's called, finally got the seat to move out and all the side holes are good. Broke out my cobalt drill bits and pinged the center, kinda?, started small and worked up to one that just fit thru the threads of the other good one. Tapped back out to the 4mm .70 thread and replaced the junk 'security' torx screws w/4mm bolts. Back together, stupid rubber intake boot! Starts perfect, no choke, revs perfect, runs out great! THANKS Mech!

 

 

 

[Gwbarm]

Good work! Glad you got it fixed 

[Mech]

No trouble.. Glad to hear the drilling went well and it's all good now.