Lots of popping on decel and downhill rolls

C

cr2crf

Member
My 70 CT70 is a first kick starter. Very reliable. But on decels and down hill rolls it pops a lot. Found a few holes in the pipe. Fixed those but still the same. Any ideas?

Also, I want to install new rings. It's a totally stock motor. When can I just get a ring kit? Suggestions?

Thanks
 
cjpayne

cjpayne

Well-Known Member
Have you tried adjusting the valves, new points set @.016, new plug, air/fuel? Honda still sells the standard ring kit, but that's not all what's involved when changing them.
 
C

cr2crf

Member
Did the valves, points, plug. Air/fuel? explain. And I have done dozens of top ends on two strokes and four strokes so I am pretty familiar with the process. Just wanting another source for the ring kit, if there was one recommended by those who have been down this road before.
cjpayne said:
Have you tried adjusting the valves, new points set @.016, new plug, air/fuel? Honda still sells the standard ring kit, but that's not all what's involved when changing them.
Click to expand...
 
cjpayne

cjpayne

Well-Known Member
Just trying to eliminate the possible causes. I'm not an expert, but I assume the extreme popping is caused by alot of unburnt fuel getting ignited on deacceleration. Maybe caused by the timing being a little off or lean condition. My K1 will do a small pop every once in a while, but only on a long stint, say going down a long hill. Probably normal. There are a few rebuilders here that know where the best place is for ring kits and overbore ring/pistons. Hopefully they'll chime in.
 
Enginedoctor

Enginedoctor

Well-Known Member
dr atv has them, i'm sure chp and the like do too. tbparts maybe? i'd say OEM honda is the way to go. When you install the rings, the cylinder needs to be 'deglazed' , or given a kiss with a finish hone to allow the new ring set to seat. If the cylinder is scored, just buy an oversize piston and have it bored.

as for the popping, could be leaning out, or leaking air into the exhaust. did you put a new gasket on the head after you removed the pipe?
 
R

rjman36

Member
i'd say OEM honda is the way to go.


+1 on Honda rings. I learned the hard way. I just put a new set of rings on my Z50 from an aftermarket supplier that came with some old parts I bought. No orientation marks and no scraping bevel. Smoked quite a bit. Replaced with Honda rings and it was a huge difference.
 
C

cr2crf

Member
The bike is stock so stock rings will work, right? If this was a big bike is have no problem with it but small is new to me and I don't want to spend money twice.

rjman36 said:
i'd say OEM honda is the way to go.


+1 on Honda rings. I learned the hard way. I just put a new set of rings on my Z50 from an aftermarket supplier that came with some old parts I bought. No orientation marks and no scraping bevel. Smoked quite a bit. Replaced with Honda rings and it was a huge difference.
Click to expand...
 
6

69ST

Well-Known Member
With holes in the exhaust, you'll never be rid of the popping & farting on closed-throttle decel. Atmospheric oxygen will be drawn into the exhaust stream, transforming the exhaust into a part-time afterburner. A leaking/omitted headpipe "donut", alone, is enough to cause this.

You might be alright with a quick de-glazing and new rings, as enginedoctor outlined. When the cylinder & piston are clean & within spec, it's cheap "overkill"; it'll certainly buy you additional miles if you stick with the OE-style 1-piece oil ring. (IMO, the newer, aftermarket, 3-piece oil ring is a better design) First, find out if the cylinder walls are free of all scoring & rust first, then check the clearances. new rings won't do much, if any, good if the cylinder is worn to the point where the endgaps are too big, or the taper so great that they'll take forever to seat. Odds are high that it doesn't need new rings...if the cylinder & piston are both in good enough condition to make a quick re-ringing viable. In any case, it's not piston ring seal that's causing the rude exhaust noises..
 
hornetgod

hornetgod

Well-Known Member
In my earlier years when I did some heavy mods to a quad I owned, I got very familiar with jetting and the different circuits that are effected.

I learned,from several experts in this area that "popping is lean and bogging is rich". Have you tried to adjust the mixture screw? Back it out a half to full turn and see if the condition improves. Be certain to re-adjust your idle too. (Lean=too much air) (Rich=too much fuel)
 
OLD CT

OLD CT

Well-Known Member
Just to clarify,turning it out will make the A/F setting leaner. Turning it clockwise ''in'' makes it richer. Maybe your thinking about a two stroke H.G.?
 
C

cr2crf

Member
The holes are all sealed up so I know it isn't an air leak in the pipe. It also runs great when on a flat surface. As soon as I start up an incline the power drops off pretty good. I was thinking the compression (worn rings) was too low. Not sure how lean condition would cause that but on the downhill decel lean is what I thought. I will try the mixture screw to see if that helps.

racerx said:
With holes in the exhaust, you'll never be rid of the popping & farting on closed-throttle decel. Atmospheric oxygen will be drawn into the exhaust stream, transforming the exhaust into a part-time afterburner. A leaking/omitted headpipe "donut", alone, is enough to cause this.

You might be alright with a quick de-glazing and new rings, as enginedoctor outlined. When the cylinder & piston are clean & within spec, it's cheap "overkill"; it'll certainly buy you additional miles if you stick with the OE-style 1-piece oil ring. (IMO, the newer, aftermarket, 3-piece oil ring is a better design) First, find out if the cylinder walls are free of all scoring & rust first, then check the clearances. new rings won't do much, if any, good if the cylinder is worn to the point where the endgaps are too big, or the taper so great that they'll take forever to seat. Odds are high that it doesn't need new rings...if the cylinder & piston are both in good enough condition to make a quick re-ringing viable. In any case, it's not piston ring seal that's causing the rude exhaust noises..
Click to expand...
 
hornetgod

hornetgod

Well-Known Member
OLD CT said:
Just to clarify,turning it out will make the A/F setting leaner. Turning it clockwise ''in'' makes it richer. Maybe your thinking about a two stroke H.G.?
Click to expand...

Good catch "Old CT". I must have had my son's YZ125 jetting on my mind.
 
Last edited:
6

69ST

Well-Known Member
The "mixture screw" controls the amount of air entering the pilot (idle) fuel circuit, while the amount of fuel is determined by the size of the pilot jet. Turning the screw counterclockwise allows more air into the pilot circuit, thereby leaning-out the mixture. Those are the basics.

In actual operation, it's a dynamic system...affected by throttle position and vacuum. Once the throttle is opened past a certain point, ~1/8-1/4, the pilot circuit becomes inconsequential...the main circuit takes over. Below 1/8 throttle, the pilot circuit takes over. The smaller the throttle opening, the higher the vacuum inside the intake. At high rpm, the engine is displacing at lot more air volume per unit of time, unless restricted by the throttle. What's not been mentioned is the fact that closing the throttle at high rpm runs the vacuum level sky-high, increasing the air & fuel flow through the pilot circuit...until rpm drops down closer to normal idle level. A carburetor works on pressure differential...i.e. it needs vacuum to flow fuel. Abnormally high vacuum can draw excessive amounts of fuel through the pilot circuit, a phenomenon known as "pullover effect".

So, what causes the popping exhaust? The answer is the combination of just the right air:fuel ratio and an ignition source. Below about 3% concentration, there's not enough unburnt hydrocarbon to ignite. About about 15-20% the mixture is too rich to ignite. Under high-rpm/closed-throttle deceleration there's typically static high-vacuum on the intake side and dynamic/resonant vacuum on the exhaust. Unburnt fuel accumulates inside the exhaust. If enough atmospheric O2 enters the hot exhaust stream, the mixture goes "bang!" each time it reaches the sweet spot. Since the pilot circuit can't flow enough fuel to turn the muffler into a steady-state afterburner, the explosions are cyclical, though they seem almost random/erratic.

With all due respect, you may still have a pinhole leak somewhere in your exhaust. Just having a slightly loose nut at the port flange will usually be enough to cause this...even on a perfectly dialed-in engine/carb combo. Ask me how I know. That doesn't mean that excessive blowby, oil control problems or carb adjustments have no effect, they're just more likely to be secondary...if present. If the piston rings aren't sealing properly, then it's highly likely that glowing-hot carbon embers are the ignition source for the unwanted popping.
 
C

cr2crf

Member
Beautifully said.
racerx said:
The "mixture screw" controls the amount of air entering the pilot (idle) fuel circuit, while the amount of fuel is determined by the size of the pilot jet. Turning the screw counterclockwise allows more air into the pilot circuit, thereby leaning-out the mixture. Those are the basics.

In actual operation, it's a dynamic system...affected by throttle position and vacuum. Once the throttle is opened past a certain point, ~1/8-1/4, the pilot circuit becomes inconsequential...the main circuit takes over. Below 1/8 throttle, the pilot circuit takes over. The smaller the throttle opening, the higher the vacuum inside the intake. At high rpm, the engine is displacing at lot more air volume per unit of time, unless restricted by the throttle. What's not been mentioned is the fact that closing the throttle at high rpm runs the vacuum level sky-high, increasing the air & fuel flow through the pilot circuit...until rpm drops down closer to normal idle level. A carburetor works on pressure differential...i.e. it needs vacuum to flow fuel. Abnormally high vacuum can draw excessive amounts of fuel through the pilot circuit, a phenomenon known as "pullover effect".

So, what causes the popping exhaust? The answer is the combination of just the right air:fuel ratio and an ignition source. Below about 3% concentration, there's not enough unburnt hydrocarbon to ignite. About about 15-20% the mixture is too rich to ignite. Under high-rpm/closed-throttle deceleration there's typically static high-vacuum on the intake side and dynamic/resonant vacuum on the exhaust. Unburnt fuel accumulates inside the exhaust. If enough atmospheric O2 enters the hot exhaust stream, the mixture goes "bang!" each time it reaches the sweet spot. Since the pilot circuit can't flow enough fuel to turn the muffler into a steady-state afterburner, the explosions are cyclical, though they seem almost random/erratic.

With all due respect, you may still have a pinhole leak somewhere in your exhaust. Just having a slightly loose nut at the port flange will usually be enough to cause this...even on a perfectly dialed-in engine/carb combo. Ask me how I know. That doesn't mean that excessive blowby, oil control problems or carb adjustments have no effect, they're just more likely to be secondary...if present. If the piston rings aren't sealing properly, then it's highly likely that glowing-hot carbon embers are the ignition source for the unwanted popping.
Click to expand...
 
Top