Velocity Stack/Fouling Plugs Tuning Question, a WWYD?

colonpee-:P

have a :P
K, have a CB500T that I just rebuilt, with open exhaust and velocity stacks, motor runs great, idles at 1200-1400, has a flat spot just off idle, then comes on like nobodys business! Carbs are all clean as a whistle new elec ign, etc. Now heres the WWYD situation:
Carbs were 145 jets, upped them to 155, then came back to 150. Plugs were fouling at 155 bad causing misfire... noticed with 150 jets after a few throttle blasts on the highway that the carbon is now just burning off and starting to become tan again...
Taking my stacks, open exhaust into consideration, what would you do?
Go back to stock 145 jets and keep the B8ES plugs, or stick with the 150s and run a slightly hotter B7ES plug? Considering the open stacks, I think that a 150 jet would prevent leaning out at WOT/high speed/RPM and the hotter plug would prevent the fouling, where if I go back to stock 145s that I would be too lean at WOT/high speed/RPM and lose some top end...
 

whataulooknat

Been Around the Block
Get some new B8ES plugs put the 150 jets in your carbs. warm the engine up with the old plugs then go out to a straight stretch of highway stop and put in the brand new plugs. Start the bike then rod the piss out of it through the gears and hold it WOT in top gear for a few seconds then hit the killswitch clutch it and close the throttle all at once. Coast to a stop, put your old plugs back in and ride it home then thoroughly inspect the new plugs that you just took out. don't let it idle with the new plugs in and don't cruise around on them just rod the bike through the gears WOT and then kill it. so you can get an accurate main jet reading. It sounds to me that your flat spot problem is in your idle jet/mixture screw. You want to be looking for a tan ring at the Base of the porcellin when you check for the main jet. The tip of the plug is not an accurate reading.
 

whataulooknat

Been Around the Block
My CB450 is almost the same engine as yours as far as the design and its bored out to 484cc's and I now run a 155 main jet. I used to foul out with a 145 main jet. but after changing the idle jet and properly syncing the carbs I don't have that problem anymore. The main jet wasn't the problem with the fouling in fact it was actually too small *just saying* doesn't have much in relation to your build's tuning though. Oh and BTW check for holes in your floats. a Sinking float can cause you so much headache if you don't notice it. It can give you all kinds of weird symptoms
 

colonpee-:P

have a :P
Floats are fine. I am reading the entire plug, not just the tip, the entire plug is black with the 150s in it, just starting to get tan now... I think a hotter plug with the 150s should do it, with the 155s in it, the plugs look like they were dug out of a coal mine... I have 38s in it for idle jets and the screws are 1.5 turns out... any changes to make there? I got B8ES in there now, would a B7ES keep things cleaner? BTW motor is completely stock, sans elec ign... stacks and open pipes... would a 155 jet give more top end over a 145 jet? Gonna play with it for a bit now, will update later...
 

Sonreir

Oregon
DTT SUPPORTER
Double check your compression and timing. If either of those things are out, reading a plug won't tell you anything useful.
 

colonpee-:P

have a :P
Compresssion/timing are fine... motor was re-ringed upgraded to elec ign, having a problem with fouling plugs... just got back from a ride, checked timing before i left, spot on, screws were 1.5 turns out, screwed them in to 1 turn, off idle response improved just a wee bit, went to 1/2 turn, didn't like it... went back to one turn and everything carbwise is fine. Havent checked plugs yet... got a nice steady idle at about 1400 on the tach. Flat spot slightly improved i think... wont know till I check the plugs tho... gonna try and bring that idle down to 1200 and see what happens...
 

colonpee-:P

have a :P
Carbs are spotless, took them apart again last night just to double check. Just idled down to 11-1200 and checked plugs, plugs were black but not as black as before, just a very very light dusting all over... cleaned them with steel wool and put them back in. Bike has a nice gentle idle at 1200 and revs nice. Just gotta take it out for a load test and see what the plugs read like tomorrow... Seems like my fouling tho is happening at cruising speed at part throttle, wide open clears it, idle is fine, but sometimes at part throttle the motor is spitting fire and crackling... once you WOT it, it clears up... could be fouling at idle being the culprit tho... need these plugs to stay clean so I can get back on the highway again!
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
This is interesting. On those carbs the "air screw" actually modulates the amount of fuel and air mix allowed into the pilot jet outlet by the butterfly. So OUT is rich and IN is lean. So changing from 1.5 turns to1 turn leaned it out and 1/2 turn was too little mix.

Let's assume that 1 turn is what your motor needs for idle. The next question is what does it need flat out wide open and that's the main jet. Must be tested at WOT long run high speed ie high revs and high load.

Then we get to the tricky part - high(ish) engine speed but not flat out - that's the needle & jet at work there.

Just off idle is the bypass drillings which sit/s just at the rear edge of the butterfly. It sounds as if your bike goes either full lean or full rich as the throttle cracks open and you need to know which. If it's too lean as the throttle is cracked open the bypass can be carefully drilled out with jet drills. If it's too rich just off idle then the primary air jet needs to be slightly larger. A dyno session with exhaust sniffer would tell a lot about that transition. It may be possible to drill out the slow air jet and tap it to take a smaller or larger Mikuni air jet.

Ask PJ if he an air corrector jet for those carbs. Stock is 0.90mm AFAIK.
 

whataulooknat

Been Around the Block
You said with the 150's in The plugs are starting to get tan again. Correct me if that's not what you meant to say in your original post. Thats why I said to check the 150's with Brand new plugs and don't run the plugs at all at anything other than WOT. The size of your Idle Jet and the adjustment of your air screw makes more difference than you think at cruising speed. Especially on these carbs that you can't adjust the needle. If you were able to adjust the needle I'd say to raise the clip. Try screwing the mixture screw In just a little bit. Remember IN leans it OUT richens it. Its backwards to most carbs. You Do Not wan't it to lean out too much at WOT you can destroy the engine. I burnt a hole through a piston once being too lean.
 

colonpee-:P

have a :P
Yes thats what I meant in the original post lol. went out tonight on her... marked improvement! The throttle position(1/8th roughly) at cruise speed/slow jet circuit where I had the problem is gone now! Bike revs smoothly and all the popping backfiring is gone... still a tiny flat spot now, but narrowed to just a tad off idle and gone at cruise.(Could just be me being extremely slow on the throttle trying to keep the noise down around town lol) just have to check the plugs for any new deposits. (slow jet is a 38 btw) Idles so nicely now and acceleration is smooth as silk with no popping banging on deceleration. Going to try and adjust the air/fuel screws to 3/4 turn and see if it gets even better, half wasnt enough and 1 seems spot on but still want to try 3/4... once I check the plugs and see they are still clean, will do a 3/4 throttle and WOT test sustained to see what happens to the plugs then. What the problem was I think was too high an idle and too rich on the air/fuel screws causing fouling which hampered acceleration/sustained high speed firing. Now for another question, If the 150 jets work and the plugs stay tan, assuming idle and slow jet circuits were causing the fouling, would stepping up to a 155 jet give more top end? IE not any more to the powerband as thats determined by compression/cam profile, but I just want to be safe at WOT and not lean the motor out at constant WOT at high speed....
 

whataulooknat

Been Around the Block
well honestly a tad lean engine produces more power than a tad rich. But your better off being a tad rich so you dont hurt the engine. Thats why Cars/Bikes/Trucks etc... seem to have more power on cool foggy nights. The air is denser with more oxygen so it leans the engine out just a little bit. if your already a tad lean you run the risk of the environment changing and leaning the engine out way too much. I burnt the hole in my piston on a cool foggy overcast day. Get 2 brand new sets of plugs and do a plug chop with the 150 and another with the 155 figure out which one has a better mixture. Remember to do a plug chop on Brand new spark plugs I can't stress that enough.
 

whataulooknat

Been Around the Block
Also when you do the plug chop remember tan ring at the base of the porcelain is the best. However if you plan on Racing and Hot rodding on a daily basis for extended periods of time you should go for a darker brown ring to protect your engine. As far as the spark plug heat range goes. Town riding/ short trips wants a hotter plug Highway riding/ long trips wants a colder plug. The stock B8es plugs are a pretty good all purpose plug.
 

colonpee-:P

have a :P
Yeah I want this bike optimal for extended highway use (road trip!) lol. So with an 8ES, say, if it still fouls, would it be better to go back to the 145 jets and stay with the 8's or run a hotter plug (7) with the 150s (assuming a nice tan plug is acheived) giving more fuel?
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
Er... That's a popular myth but it's incorrect. Max power is much richer than max economy.

Max power 12.6:1 air: fuel
Chemically perfect 14.7:1
Max economy 15.4:1

Those vary slightly form one motor to another but are basically right. More fuel makes more power up to th point that all the available oxygen is used. All engines have some unburned fuel exiting the exhaust pipe. So you will always use more fuel than theoretical calculations compute.

Go rich and do WOT plug chop. Repeat with smaller jet and inspect plugs. Best to do it on a dyno.
 

whataulooknat

Been Around the Block
Ok i guess i stand corrected on the slightly lean engine producing more power. But either way the tuning is still the same assuming you do not have access to a Dyno you'll have to do it by feel and by visual inspection of the plugs. Try for the light brown 1/4" or so thick ring at the base of the insulator slightly darker is ok as well. if that is correct at WOT with the 150 main jet great if not you'll have to go bigger or smaller depending on your reading I doubt you'll end up going bigger though since you said you were fouling badly with a 155. Hotter plugs are not good for long runs. Remember put a cold plug in a hot engine and a hot plug in a cold engine.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
You are correct that more fuel than the engine needs will drop power and it's easy to over jet. You are right that plug reading is done after a WOT run to get the plug up to operating heat and the place to read a plug is deep inside where insulator meets steel shell.

Trying to read plugs at any other setting gives a very crude indication of way too rich but not much else. If the plug is not up to temperature (Flat out) what is being read?
 

colonpee-:P

have a :P
k after last nights short rides, never did a WOT yet, just checked the plugs today... one plug was no dirtier than when I installed it after cleaning it last night, and the other one just had a light coating of black on it, readjusted the air/fuel screw on that side and will repeat tonight before I do the WOT test with new plugs. That way I know the idle circuit is good at least. But overall, performace has improved drastically!
 

whataulooknat

Been Around the Block
I never said there was a way to read a plug at anything other than WOT. I was just saying to do a plug chop to get the main jets dialed in. The scavenging effect doesn't work as efficiently and engine temperature/plug temp is different at lower rpms. You can't dial an engine in through the whole range just by looking at plugs you really need a dyno, but if your like me and don't have access to one you have to do everything from idle to 3/4 throttle by figuring out what works for real world driving situations by feel/trial/error. Its just really hard to do that if you don't have your main jets the right size to start with, the carbs synced to open at the same time, and the floats adjusted exactly the same. After these key things are done you can start working on the rest of the range. and you might have to go back and tweak some things afterwords but this will get you close. I think me and Teazer are on the same page when it comes to tuning/syncing carbs totally agree with everything they said. sometimes things just lose their meaning in writing.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
Hey, Whatchalookinat, I wasn't arguing with you - I was agreeing with what you had posted to reinforce it. :)
 

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