Ok my preffered screen name is 350maniac, but there were some problems I had logging in so I had to re-register. But that is beside the point.

I had a runaway Engine last week that I think was partially due to my idle screw coming all the way out (I forgot to tighten it) It was full throttle and not responding to killing the ignition or kill switch
The mishap cost me a belt and I am just now testing my Engine again. (I was lucky)
It ran great with the exception of being too rich. The real problem was that when I held the throttle down 2/3 or more for 5 seconds or more, there was still a little bit of power still on when I let off. I'm not sure where that would have taken me, because I hit the kill switch every time it did it. I would flick it back on when the rpms got down to idle.
I know this could mean I have an air leak, and I will do a leak down when I can piece the parts together this weekend. Any suggestions on what I could use to plug the intake manifold on a 350 would be helpful.
I still think it could be the carb though. I'm going to get a new air adjustment screw because I don't think I have the right one in it. I have a 38mm flatslide mikuni Any Ideas?

I

are you sure the throttle cable has the proper slack adjustment correct ?
slide in the carb returning to full off? or all the way down ?no burrs on the cable itself ?Make sure the needle is returning properly and not binding or misaligned with the jet in the bottom,this would keep the slide from returning to full off.
There should be around an 1/8" of slack in the cable at the carb or steering wheel,whichever one you want to check it at.
Also make sure the throttle cable connection in the carb is inline,meaning installed properly and not cockeyed inside thier.
Is this carb a brand new one ? or is it a used carb ? If it's new then the air mixture adjustment screw should be fine unless it is bent or nicked.
A lot of times a hanging throttle could be an airleak but not allways (depends).(do the test anyway for peace of mind).make sure you test or soap all the motor (( Internal Combustion Engine ? )) ! The right side seal will have to be checked and that means taking the recoil and flywheel off. crankcases,cylinder base,cylinder head,intake and reed gaskets and any plugs will have to be checked thouroughly.
If your running the throttle 2/3 with the motor (( Internal Combustion Engine ? )) hot and in neutral,there is a lot of fuel flowing and may take it a second or two to come all the way down.
Also are you sure you have the right primary or slow jet installed and it is not lean when the carb is brought down near idle?

I think your on the right track but other things to think about..

What plug are you running, have you had this thing apart before, I have seen a
Engine with a lot of carbon deposits do the same thing, when it was slightly lean
the carbon got so hot it was igniting the fresh air fuel mixture like a diesel
Engine.

A plugged or restricted exhaust can cause heat build up inside the Engine and
cause the same problem, you check yours for a mouse house?

when the mishap happened, it was no doubt dieseling. The idle screw being all the way out might have been a factor, but not the main cause. The Engine would not respond to shut off, and while I was trying to disconnect the fuel line, the belt failed. I was also running lean at the time. I'm not sure what caused the Engine to finally shut down.

I want to hook up an emergency fuel shut off, that I can reach while belted in. (I think the runaway Engine has made me a bit paranoid).

My throttle does'nt stick, but it is stiff. I have lubed it with spray silicone inside the steering yoke, but it only helped a little. There is sufficient slack and the cable has been run flawlessly. The cable does not stick at all. The carb is an older model I think, but I'm not that sure. When I first crank it, it runs high, but after a minute or two it will be at 2500 rpm. The Engine has never returned to idle very well. My carb vent hoses are in pathetic shape, if that matters. I'm going to expiremnt with the air screw settings to see if that makes a difference.

I'm using a standard NGK plug. I have only removed the head from the Engine to install a liquid cooled head. Just about every part of my 350 has been out except the Engine and gearbox.

I have a 500 main jet in now, I don't know about the other jets. The main is that big because my intake is that big. Its an automotive K&N with an outerwear and uni foam on the inside.

Pulling the choke when the Engine is running on usually helps, I would work on
covering the intake to kill the Engine over cutting the fuel, removing the air is
instant, the Engine will die waiting for the Engine to use up the fuel in the float and
fuel line after you turn off the gas might be too late for the Engine.

Compression still as good as before the mishap?

If in doubt change the throttle cable, you might check the routing to ensure it is
not being pinched or bound up by something else also conside stuff moving when
the Engine is under torque or while the buggy and or suspension is in motion.

I don't have a compression tester either. A point in the right direction would help me with that.

I tested again today. My throttle problems seem to be better, but now my egt is reading too high at idle after warm up, just over 600. I also reached just over 1200 degrees while running it today. Its not that hot here, because its been raining. The other day, my egt almost seemed too cold. It was just under 200 at idle and never got over 800. I have'nt changed the main jet either. I guess I'll do some plug chops. I've got to be too rich though, because I can't get full shift out before it starts to bust up. I don't get it?

Start doing plug shops and reading the plug forget the EGT until you get your jetting closer.

Check this one out its good stuff.

http://pilotodyssey.com/BillGivensjetting.htm


Here is a sears compression tester.

Are your running with the air box lid on and no modafactins. It sound like to me that you are having a mid range ( 1/4 to 3/4 throttle )lean problem. It would be better to pull choke on if it starts to run away adding fuel to cool down the pre ingnition. Air only will blow it up. As your running and egt rises notice throttle position and then apply choke and see if it helps if it does work on your mid range before moving up the the main jet. If not please check for an air leak. Also if your mid is lean your egt will run high and you will chase your main jetting for a long time. Notice what neddle setting your on, then try raising the neddle(lowering the clip) in order to richen it up in mid. Once youve done that you will feel it pull harder through the mid then stumble on top(put in big main jet to help find propper neddle setting) then work on the main. If you havent done anything with the air box and the motor (( Internal Combustion Engine ? )) has been bored. You might talk to some of the others about that and how it will afect and react to what your trying to chase down. Please take a close look and intake boot for tear or reed cage gaskets.

A 500 main jet ? seems awfully large.What carb size are you running ? 32mm 39mm ?
I have a 39mm pwk keihin and still only run a #162 main or so.Your airflow is going to depend on your carb size and not what you are running for an air filter.
On my carb i don't get an idle either and i adjust it at the wheel (when it is done this way you have to be carefull though because when you turn right it will pull on the cable and increase your rpm's.),i keep it a tad loose so i can readjust whenever i need,usually only for startup or warm up and after that i tighten it down for riding.I like an instant throttle response,so i don't leave any slack in my cable.
If your cable is a tad stiff i would check it for tight routing or rust or binding in the steering wheel and or carb slide.Try this unlock your adjuster and turn it all the way loose at the wheel then readjust it and try to see if it is still stiff.
If your carb is not returning for an idle check to see if the slide is sticking .You might also consider taking the wch off and cleaning up the carbon deposits on the piston top.You really need to know what jets are in this carb for sure,slow and main and recheck your midrange needle setting.
Since you are going to do a leakdown test then it wont be a problem to check the carb identification and jetting,making sure the floats are set correctly and it is clean.
you are probably way rich on the main causing the carbon and lean on the midrange and or primary jet causing the pre- ignition.

Ok,
I did my first plug chop. I put in a denso w27esr-u plug after I warmed it up. I took off down the street at WOT (Wide Open Throttle) for about 7 seconds. I killed the Engine and pulled the plug. I didn't notice a big difference, it still looked about the same with the exception of some rainbow coloring on the ground electrode. I put it back in and drove it aggresively to the shop. I took it out again and the colors were gone and the ground electrode looked a little dull. The ground electrode also had some dark brown on it where it meets the threads. Oh yeah, This was done with a 480 main instead of the 500. The top end feels much better. I can't determine what these plug readings mean?

I think I'm lean at idle and mid, because my idle egt is 600 degrees mid is about 800 to 900 and top end is a little over 1000. I should look into a larger pilot jet I think

I bought a compression tester, and it was 160 psi cold.

I took my carb apart and noticed that the bare cable coming from the slide was a little rusty, so that's why my throttle is stiff.

Leak down test is comming soon. Why does everyone mention using a bike pump for this test? Should I not use my air compressor? I don't think I have a bike pump

The

"I can't imagine running a 480 main. I run anywhere from a 145 to 152 in my pilot"

480 is a Mikuni jet size that's why the number is so high....

Some people don't like using EGT's and others live by them. I live by it, it will prevent an Engine seizure, plus it gives you something to go on. Plug tests are just a way to verify my readings. The digatron egt is alot better than most of the egt's I've seen. It gives you realtime data, not like an analog system that shows you 1380 after you've already seized.
The lean mid, rich top that you descibed seems to be what I have. I did'nt do it on purpose, but it sounds like a good setup.I have'nt figured out how to adjust my needle yet. I have been studying carb anatomy lately, so I'm sure I'll get it soon.
Your RandyLarry right?
The Mikuni numbers are different, but a 480 is big by their system too. I'm gonna up my pilot jet to see if my idle temp goes down. I've heard that it should be from 200 to 400 degrees.

I don't think carb tuning is gonna get me much faster than I am now. This thing takes off like a rocket.
I'll see you at coal creek (if your RandyLarry)

I have'nt figured out how to adjust my needle yet.

This might help
http://pilotodyssey.com/carbtuning.htm

Plug reading info, you might have already seen it...
http://pilotodyssey.com/PO/viewtopic.php?t=870

that's what I was looking for, something to use as a reference. Thanks hoser

Yep,

Well?

What did you find, or you had a chance to do some more testing?

Did more testing today. I rode for a while before plug testing. I was running pretty hard down one of the dirt roads at the farm, when I noticed that the Engine seemed to want to runaway again. I pulled the choke and killed it. I should have let it go a little bit longer just to verify that it was deiseling again. I get panicy when my Engine does something funny, because the last thing I want to do is major Engine work. I knew my choke could kill the runaway, so I limped it back to the pole barn and put a 490 main jet in to do some plug tests. I then decided to put the 500 main in too see if it would bog on the top end like it did the other day. It ran fine on the top end with the 500, not like the other day. This worried me a little bit, but it may be an effect of atmospheric conditions. I did not check the temp or humidity today or the other day, but it might be a little more humid today.
Anyway, I left the pole barn with my old plug in and drove about 3/4 of a mile where there is a hill that runs about 300 yards. I put in a new plug and ran at WOT (Wide Open Throttle) to the top, then spun out and headed back down at WOT (Wide Open Throttle). IT got a little hairy on the way back down, because the hill is terraced and I was probably approaching 70mph. IT was like jumping full tilt down hill about 7 times in a row. I run down it all the time, but never that fast.
My plug looked light brown on the bottom and still a little white with some gray at the top.
I can try to cut the metal off with my chop saw and post it. How do you cut the metal threads off without damaging the ceramic hoser?
I'm gonna try to do my leak down tommorow after work. Someone suggested spraying carb cleaner around the Engine seals and gaskets to see if it revs, therefore finding the leak. He said to do this before the leakdown because its easier. Is this good advise?

I forget exactly what size it is but one of the mikuni 500 series main jets is
actually leaner than a 400 series jet but I think it was a 510 there was a
lengthy article about this but I cant find it, when you change jets just make
sure you verify what it actually does.

Your discription of your plug sounds plenty safe to me but something is going
on to make your Engine diesel, if it is a intermittent air leak a plug or even a
EGT reading might not happen fast enough for you to save your Engine, a
seizure because of improper jetting when your checking your jetting as you
are wont happen as unexpected. you really gotta screw up. all that being said
what is the history of this Engine what mods has been done to this Engine.

You can spray around the Engine while its running but your not going to
check the seals that way simply because they are not accessible to spray
whilst the Engine is running, that is a generic statement that is used through
out the Engine trouble shooting industry and really does not apply to this
Engine, when I am looking for a vacuum leak I like the pressure test and
looking for oil spots, most Engine leaks will have some sort of trace evidence
where the leak is in this case oil will be found where it is leaking even if it is
just a slight amount like equivalent to one drop mixed with some dust stuck
to your Engine cases, if you see something like that you need to be suspect
and take a closer look, I would not want to wash off that evidence with carb
cleaner I would want to make the visual observation first, I wouldn't want to
wipe off something dirty on your Engine with a rag either without first looking
close you need to get into the detective mode (CSI) looking for little clues...

I cut my plugs apart by putting them into a lathe and turning down the area
where they crimp it together I have never done it any other way, I have
seen plugs where they used a hack saw to cut the side of the threads off,
if I were going to try a new process for the first time I would use another
plug first once you master the process then attack the plug your trying to
read no use destroying the evidence your trying to see during practice hehe

Let us know what you find you have a unique problem cant wait to see what
is actually causing it.

sounds like an air leak,what plug are you running in it ? i remember an ngk but which one br8es -br9es ?
I chased the jetting on my first motor (( Internal Combustion Engine ? )) similar to what you are going thru and it took a bit but eventually got worse and finally let go and four sided the piston and hurt the reeds (v-force 2).Though mine had a botched up case repair that i wasn't told about when i bought it,it eventually leaked and caused the siezure.
It wont take long or much either,better get the airleak test done before major damage happens.

I'm using a denso w27esr-u plug now.
I got the parts for the leak test. I hooked it up, and I found a major air leak where the exhaust flange meets the exhaust port. I don't think this is the leak I was looking for. That could'nt cause my Engine to deisel could it? It's an aftermarket flange, and I still have the stock one. That leak hinders the test. I'll put the stock one on after dinner to see if it will seal.
My mods are fairly light. I have a LQH, 102c clutch, DG Pipe, 38mm carb, digatron egt, high flow intake, (that I made myself) and I'm not sure about the reeds. I have'nt removed the intake flange to check the reeds , because it does'nt want to let to go and I don't want to damage it.
I should put 7lbs in right?

7lbs is ok

Not sure I follow you on the exhaust manifold you got some pics?

find any oil spots on the outside?

I'll post some picks tommorow. I'm talking about the coupling that is bolted on to the Engine that the exhaust pipe fits on. Maybe I'm not calling it by the right name. I thought it was called an exhaust flange or exhaust manifold.
I put the stock one back on, and noticed that there was a metal gasket still on it. I don't know why I over looked it when I installed the DG last month. I do some stupid crap sometimes.
I installed the stock one anyway, but I could'nt stop the plug from leaking once I installed it. I put a piece of pvc in it that was intended to plug the aftermarket manifold which is a larger diameter. I built it up with pieces of an old inner tube and tightened it down with a pipe clamp. It did not work. It slowly leaked air. I'll pick up a 1 3/4" plug from the plumbing supply store tommorow.
I had'nt been using the exhaust manifold gasket. That could'nt be the source of my problems, could it?

I'm pulling the Engine, I'll post my findings.