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Discussion Starter #1
this is pretty much a hypothetical question. was thinking today about spark plug heat range and fuel octane. is it possible that a too-cold plug combined with higher octane gas can make the engine look and run like its too rich? will a switch to a hotter plug and lower octane allow the plug to function properly and not foul up?
 

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Yeah, that's completely possible. Of course if the engine was run at race speed (something high octane and cold plugs are often intended for) then it might look fine.
JohnnyB
 

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hot plugs foul less. they burn off deposits better. octane, you won't be able to tell a difference. at least in the regard that you are asking.
is stock heat range pinging? if not stay with that. go colder if you have increased the cyl pressure with higher cr pistons, n20, or forced induction. Aslo run the most gap you can without the spark missing.
 

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Discussion Starter #4
the real issue is the fact that i need to jet down. it only takes a few miles before my plugs get gunked up and the motor starts to stumble. motor details, just in case: T120R, bored .020" over, twin amal concentrics, stock open header pipes. the current jets are #106, which i believe was the standard jet. helluva strong running motor, but just pushing too much gas through it. as i was thinking about jetting, i got thinking about all the other variables.

plugs used: NGK BR8ES, Champion RN3C, Champion RN4C. the 4's ran slightly better, but not enough to cook the deposits off.
 

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the 106 jets you mention........ those are needle jets

are they new?

how about describing exactly what it is doing when it starts stumbling (or fouling those cold plugs) and acting stupid

I run BP7ES and BP6ES in my hot street Triumph engines

I may run an 8 or a 9 in one but only if it's a WOT race engine

I've no idea why you are running resistor plugs
 

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Discussion Starter #7
you're the second person today to mention my resistor plugs! i've got a sparx ignition and that's what it recommends. it'll run on the normal ones, but supposedly will be damaged by them, maybe a little more research on my part is in order! the bike is a one or two kicker, idles very well. my problem lies in mid-range throttle. after a few miles it starts missing. if i'm able to stay high on the throttle (something i can't do on little city streets) it runs fine. after about 10 or so miles in the mid range, it tries to die. i've pulled the plugs, cleaned them and i'm back up and running. 10 more miles and the plugs are done. charging system is A-OK, batt is good. its not burning oil, the plugs are dry and sooty black. i'm going to check the jets and see if they've been drilled out. i should have a new set of carbs built up this weekend...so i'll know more when they go on. (i'm borrowing the carbs that are on now).
 

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I think the answer lies in Mikuni
Ken
 

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sounds for sure like you need new needle jets and certainly you do if it goes to bucking and jerking at part throttle over run

don't bother trying to measure them....... they will cause such a condition long before your eyes can even see the 106 hole starting to deform or egg shape

just buy a pair and screw them in

also note the float levels on those carbs are indeed adjustable, the plastic float needles are pure trash (use viton tipped only), and your main jets will have zero bearing on your problem

I know nothing of the Sparx ignition but I am quite familiar with their charging systems
 

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I don't think running higher octane does anything more to an engine that the octane level that and engine needs. In other words, running higher octane in an engine doesn't give the engine any more or less performance unless the engine has high compression such as a built motor. The higher the octane, the more the fuel can be compressed before ignition. Lower octane tends to ignite sooner when under compression. In stock engines, lower octane gas runs very well, while higher octane in the same engine really doesn't do any more for performance. Some folks report that their bikes run a little smother with higher octane, I'm not sure if this is just weather related or I'm feeling good about putting high octane in my gas tank kind of feeling. As a pilot, I could swear the engine sounds totally different when I'm flying over long water stretches that when flying over regular ground.

As to your plug fouling, that could be caused for a number of reasons like compression level, valve settings, ignition timing and or carb setup. I know that covers the entire gambit but all things should be checked / and adjusted to eliminate the obvious.

Before you can adjust a carb for mid-range performance, you have to make sure that your plugs look ok for full throttle operations. Try putting in new plugs and do a chop throttle test, run the bike at full throttle and kill the engine and check the color of the plug. If the plug is normal color - then your main jet is sized correctly. Now you can move on to adjusting for mid-range by changing the pilot needle clip up or down for mid throttle. Usually, this will adjust the throttle response in the mid range area. I've have not seen where a plug gets fouled at mid-range, but I'm sure it does happen. Your air screw is used to set idle. After all that, your pretty much completed adjustments to your carb.

If your ignition timing is a little too retarded or too advanced, it makes sense your not burning the full fuel load in the cylinder. If it was too advanced, you'd more than likely hear it ping from pre-ignition. That doesn't sound like what you have going on. If the ignition is too retarded, then you might see what your describing.

Finally - compression, This really tells you the health of your engine. I'd check it. I'm a real believer in covering the basics first. I don't know how many times I've been caught chasing a problem when I over looked an obvious thing. Not knowing any more than what you've said here, I'd check these items in the following order: compression - ignition timing - valve settings and finally carb settings. You may very well have carb problems, but checking the other items on my list does not take long and can be indicators to the real problem.

Keep us posted on what you find.
 

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I am going to agree with HackA on this one. Back when I worked in the Brit shop we had a bucket or two of Amal bodies and parts that were worn from vibration. It must be the side to side vibration because the slide bore would be worn and they would suck air past the slide and not idle. I seem to remember the needles getting worn where they went through the slide and you couldn't see th wear, but the mid-range would be all screwed up. With the needle flopping around, I think the wear the inside of the jet funny, too.

Ken
 

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good call ;)

another obscure possibility sometimes happens has to do with their being two different jet holders....... sometimes the wrong one or pair is inadvertently used

the likelihood of this happening increases when a carb builder isn't aware such wrong parts exist

quote:Originally posted by kenessex

I am going to agree with HackA on this one. Back when I worked in the Brit shop we had a bucket or two of Amal bodies and parts that were worn from vibration. It must be the side to side vibration because the slide bore would be worn and they would suck air past the slide and not idle. I seem to remember the needles getting worn where they went through the slide and you couldn't see th wear, but the mid-range would be all screwed up. With the needle flopping around, I think the wear the inside of the jet funny, too.

Ken
 

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also you do not mention slide cutaway

my bet is you are running 2 1/2 to 3 1/2

guess what? way to rich for straight pipes due to the resulting increased reversion at slower engine speeds

however, typically this results in an engine which warms up and "takes gas" uncharacteristically fast unless you are running something like a 2.5 for really restrictive and poor mufflers

for reference...... look up the carb specs for a TT Triumph

they came with straight pipes


what amazes me still to this day....... is how much mis-info abounds all over the net when it comes to tuning these quite simple carbs for custom bikes

my favorite BB.C reply to myself on this very subject, from one of their respected guru's, was something along these lines....

'Sir, that simply defies the laws of physics' although he was more bewildered by my suggestion that a similar bike would indeed require a smaller main jet than stock and he had no idea about the simple slide cutaway issues

would you like to buy a whole mess of amal tuning parts? they are cheap
 

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Discussion Starter #14
i've got #3 slides, stock for a bonnie. amals are definitely simple, but paying $25 each for slides to test and tune kinda sucks for a poor boy like me. what parts have you got? (i thought i heard an offer). the carbs i'm building have brand new needles, float needles, gaskets, springs, retainers, and...uh...a few other parts. brain is fried from a long week! a set of chokeless tops would freaking rule, much better than sealing that hole with goop or something. going to putz around tonight and see what's what. i want to get a couple long rides in this weekend.
 

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Discussion Starter #15
i've got #3 slides, stock for a bonnie. amals are definitely simple, but paying $25 each for slides to test and tune kinda sucks for a poor boy like me. what parts have you got? (i thought i heard an offer). the carbs i'm building have brand new needles, float needles, gaskets, springs, retainers, and...uh...a few other parts. brain is fried from a long week! a set of chokeless tops would freaking rule, much better than sealing that hole with goop or something. going to putz around tonight and see what's what. i want to get a couple long rides in this weekend.
 

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ill give you my advice from chasing a very similar problem on my t120r for a very long time. check your wiring. there is a connection under the motor, that when dirty, will cause your plug/s to foul. i only found it after pulling apart my harness and replacing it after weeks of frustration. it was soo dirty i couldnt see the connector, and the conection was filled with shit. a new harness fixed the problem, but cleaning those connectors probably would have too.

jc
 

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ill give you my advice from chasing a very similar problem on my t120r for a very long time. check your wiring. there is a connection under the motor, that when dirty, will cause your plug/s to foul. i only found it after pulling apart my harness and replacing it after weeks of frustration. it was soo dirty i couldnt see the connector, and the conection was filled with shit. a new harness fixed the problem, but cleaning those connectors probably would have too.

jc
 

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Discussion Starter #18
found it! its sooooo simple. stoopid simple, in fact. i was swapping on my new carbs and found that the jet bodies had started to back out. why i didn't pull the float bodies off before and check? i dunno, but problem solved. thanks for all the ideas though, good trouble shooting steps to keep in mind for future problems (i'm not naive enough to believe that i've solved everything).
 

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Discussion Starter #19
found it! its sooooo simple. stoopid simple, in fact. i was swapping on my new carbs and found that the jet bodies had started to back out. why i didn't pull the float bodies off before and check? i dunno, but problem solved. thanks for all the ideas though, good trouble shooting steps to keep in mind for future problems (i'm not naive enough to believe that i've solved everything).
 

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the number three slides will work with straight pipes but they are too rich

"stock for a bonneville" means stock for a bike with mufflers and no straight pipes

you'll know this to be true because your bike will have less than stellar fuel mileage and it will start up and take fuel with just a tickle or two..... no choke required

TT's with striaght pipes used 4's

put different megaphones on and even leaner slides are best suited
 
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