Spark plug tuning

biknut

biknut

Well-Known Member
I've been running a NGK BP5HS plug in my bike for a long time now. It used to run ok with this plug, but over time has started 4 stroking. New plug makes no difference. I've also tried a 6, but that makes no difference either. The jetting is perfect with a 68, but In an effort to stop the 4 stroking, I did try going leaner. All that did was lose power, and it still 4 stroked. The 4 stroking starts at 27 mph at full throttle. After that it will struggle to 28 mph, and that's about it. Once in a while when it felt like it, it would decide to stop with the 4 stroking for a few seconds, and I could tell that without the 4 stroking it would probably do 30 mph.

This wasn't bothering me too much because 28 mph is pretty fast, but I never liked the way it sounds when it 4 strokes.

So a couple days ago I'm looking at a chinese spark plug that came with a kit I recently bought. Its a LD Z4G. It looks just like a NGK B6HS. Normally I never use the chinese spark plugs, but I thought WTH I'll try it.

Surprisingly it runs great. No 4 stroking after it warms up, and it does 30 mph.

The only difference between this plug, and the one I was running, is this chinese plug isn't an extended tip, and the reach is slightly shorter.

What a surprise that those little chinesers know how to make a spaark plug for their own motors.
 
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Hey Biknut,
I'm not sure what 4 stroking is..... Is it when the exhaust note "sputters", making a exhaust note twice the frequency it should be making? Are you talking about detonation, that loud knocking noise. I've been happy about the performance from my NGK 8 plug. Since it's a cold plug, it has a somewhat recessed look as well. Maybe your engine likes a colder plug? Maybe as your engine gets more miles on it, the increasing carbon layer is taking less heat from the combustion, and it would also raise your compression a little. Well, anyway, I want to know what 4 stroking is cause I've heard the term quite a bit.
.duh.
-Fred
 
These little engines seem to like to 4 stroke. They especially like to do it at partial throttle, but what I'm talking about is at full throttle though.

You'll be accelerating toward top speed, and then right before you get there the motor starts sputtering and stops accelerating. You can feel that it would have gone faster it didn't start sputtering. Sometimes it will get past the rpm where it 4 strokes, and rev out all the way.

Usually a rich condition gets the blame, but mine was doing it no matter what jetting I tried. Then I changed to this different spark plug.
 
killercanuck said:
What were the gaps set at biknut? You could experiment with that to help dial her in. I like mine around .028"ish. You could try a run at .030, and one at .020 to get an idea if that's helping or not maybe.

crman, there's a good demo vid on the Tube about four stroking:
http://www.youtube.com/watch?v=0wzph3hL6EE
Click to expand...

I usually run 0.030, but I didn't even bother gapping the chinese plug, I just slapped it in there. It had a wide gap though, probably same as usual.
 
Crmachineman, 4stroking is where the mixture is a bit rich, & the motor fires every other stroke. If its only just 4stroking it'll fire properly (2 stroke) when under load, like uphill. Weedwackers & chainsaws r tuned this way 2 run powerfully when under load. If u wanna hear it, grab 1 & play with the "HI" screw on the carb while holding it WOT. Lean it a little so its screaming then slowly rich it up til it 4strokes. Where it just 4strokes is where u tune the HI adjustment 2. Now adjust the LO so it keeps running when u drop the throttle from WOT 2 idle & ur chainsaw is tuned. Cheers
 
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Also guys, a good way 2 know if ur using the correct heat range plug is 2 pull it & look at the threads on it. If the end 3 threads show blackening from heat ur using the right 1 & its maintaining the correct temp. Less than 3 threads & ur using too cold a plug & its transfering too much heat 2 the head 2 maintain running temp, more than 3 threads & its not transfering enough heat & running too hot. Cheers
 
Alright Biknut!
That video was just what the doctor ordered! That damn problem has been plaguing me too! Yeah, I've noticed a bit of funny running at part throttle conditions sometimes too, eh, not that much of a problem, but at full speed I get that problem too. It's very annoying. Maybe there is a better carburetor that has an air correction feature with an emulsion tube for the main jet. Dellorto carb.? It seems that as the pulsations start to get really strong at high RPM, the carb. keeps richening up the mixture, so some kind of corrective action needs to be taken. Simply leaning out the mixture will possibly compromise low speed performance. I'll try leaning out the mixture on my bike. I'm not as worried about lean conditions with the new head I'm running, so I think I can afford to do that. I'll let you know if it improves things on my bike.
Oh, and thanks for the tip on the spark plugs Ivan. I'll look at mine and see if I can make sense of it.

laff

-Fred
 
ivan H said:
Crmachineman, 4stroking is where the mixture is a bit rich, & the motor fires every other stroke. If its only just 4stroking it'll fire properly (2 stroke) when under load, like uphill. Weedwackers & chainsaws r tuned this way 2 run powerfully when under load. If u wanna hear it, grab 1 & play with the "HI" screw on the carb while holding it WOT. Lean it a little so its screaming then slowly rich it up til it 4strokes. Where it just 4strokes is where u tune the HI adjustment 2. Now adjust the LO so it keeps running when u drop the throttle from WOT 2 idle & ur chainsaw is tuned. Cheers
Click to expand...

Okay Ivan.
Now, I've seen those adjustments on my chainsaw, but we don't have that control on our motorized bicycle carbs. do we? I have a CNS "racing" carburetor. Is there a better carburetor in your opinion? Sure, I understand the idle speed screw, and the screw which affects the mixture for idle, but for full throttle operation, isn't it fixed by the position of the needle or the by the main jet orifice? There's no air correction screw for that part of operation is there?

.duh.
-Fred
 
Hi Crmachineman, I'm using a mikuni VM16 carb which is infinitely adjustable right thru the range & every jet u'd ever need is readily available, plus needles with different tapers & slides with differnent size cutaways. Alotta people use Dellorto SHA's but I think the PHBG's would be an even better choice if u wanted Dellorto. Cheers
 
Hi Crmachineman, ha,,, no, unfortunately these motors carb do not feature an externally adjustable hi speed circuit. If u do want the ultimate carb that does have this, check Rock Solid Engines webpage. They offer a Walbro carb (& lots more goodies) set up 4 Chinagirls. To use it the crankcase needs 2 b drilled & tapped so u can plumb in a fitting 2 send the carb's signal. Cheers
 
Ohhhh man! That sounds really great! I'm going to look into those carbs. I've heard nothing but good things about Mikuni carbs. I will look into that. I think this might be a very good step forward for my bike. Thank you Ivan!
dance1
-Fred
 
Yeah the mikuni's a great carb. 16mm is the smallest available. VM16-519 is the clamp on style mount carb. U'll need 2 turn up a manifold, I.D.16mm. The actual mount part of the manifold has O.D.25mm with the very end 3mm turned down to O.D.21mm for it to fit into the carb's internal O-ring. Or the whole thing can have O.D.21mm as the carb comes with a plastic spacer with I.D.21mm. I paid $88 about 6 months ago from mikunioz.com.au. Cheers
 
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