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Engine chugs at 3000 rpm only after engine is warm

eenvader

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Trenton Ontario
Hi all, I have a '72. Ran fairly well until the other day. When the engine is cold, at start up idle, you need to keep your hand on the throttle or it will die, ...when engine is warmed up it idles a bit high...when the engine is cold, it runs awesome, no issues, but now after riding about 20 miles, its starts to chug around 3000k to 4000k, like loss of power, but accelerates fine....at any time you need to feather the throttle and not crack on it or the bike will stall....I'm most concerned about the chugging, its been getting worse over the past two weeks, does it on every ride now, gets me home but the ride isn't fun....any idea's on why its good cold ? or whats going on when its hot ? thanks !
 
Hi, thanks, I pulled the plugs this morning, all 4 are black carbon soot but especially the 2nd plug from the left....a plug website says "Overly rich fuel/air mixture, dirty air filter, too much driving at low speeds, or idling for a long time" .... and recommend "Switch to “hotter” plug. (The higher the plug number, the hotter the plug.)" The plugs are the D8ES, I always thought the lower number like a D7ES would be hotter....anyways what you think about the second plug ? also how do I lean out the mixture ? cheers ...mnot sure if this will foul the plugs but I ride around the 3 to 4 k RPM, maybe I should not be shifting up as I do and keep her in the higher revs ?

heres a cool description from NGK:

Causes of Carbon Fouling:
•Continuous low-speed driving and/or short trips
•Spark plug heat range too cold
•Air-fuel mixture too rich
•Reduced compression and oil usage due to worn piston rings/cylinder walls
•Over-retarded ignition timing
•Ignition system deterioration

Pre-delivery fouling:
Carbon fouling occurs when the spark plug firing end does not reach the self-cleaning temperature of approximately 450°C (842°F). Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached. When the heat range is too cold for the engine speed, the firing end temperature will stay below 450°C and carbon deposits will accumulate on the insulator nose. This is called carbon fouling. When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap. This usually results in a misfire and further fouling.

If the selected spark plug heat range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures. In some cases, the insulator nose can usually be cleaned by operating the engine at higher speeds in order to reach the self-cleaning temperature. If the spark plug has completely fouled, and the engine will not operate correctly, the spark plug may need to be replaced and the fouling cause identified.
 
...I have this going on also....."If the fuel air mixture is too rich, you will have unburned fuel in the
exhaust pipe, and you will hear low-pitched bangs and rumblings in the
muffler as you roll off the throttle"

gonna try this but counter clockwise..... "
The older style slide valve carburetors used on early Japanese
motorcycles have an idle AIR screw on the side of the carburetor. The
screw adjusts idle AIR *before* it mixes with the idle gasoline, so
turning that screw clockwise reduces air flow and richens the mixture.....The idle AIR screw will be on the side of each carburetor. About 1.24
turns out from closed is a good starting point"
 
but accelerates fine....at any time you need to feather the throttle and not crack on it or the bike will stall

This is a contradicting sentence. It does not accelerate fine if it stalls.

The 72 doesn't have a high idle cam, so you must do it yourself. This is normal and the reason the idle stop screw has a huge knob on it making it easy to turn the idle up when cold, and back down when warmed up, by hand.

It sounds to me like the coils and/or condensers are dying. Ignition is the first suspect in this circumstance (runs fine cold, runs bad hot).
 
Could just be old engine too, almost 50 years old now. Compression test to see if wasting all your time there. May not be a carb rich issue at all. Crap timing (slow) and especially dwell (point gap) being off will lower spark to let plug get dark too. Weak ignition looks just like rich carb as far as plug reading goes. Low compression too.

7 is the hotter plug, 8 for normal use and 9 is a colder plug in NGK. The numbers can go one way or the other depending on the plug brand. I'd stay with correct plug until you are forced to change, go hotter and you can incur engine damage.
 
Thanks guys for your comments..I cleaned the plugs and turned the air screw on all four carbs ccw about a 1/4 turn, seems to be running a lot better, the chugging is gone...should clarify the statement I made, it never dies during acceleration only when its cold after start and your at a stop light etc...you need to keep the revs up and slooowly release clutch or it will stall...more test driving today, i'll see if the plugs foul again
 
Headed in right direction, airscrew going CCW is leaning out idle and off idle. I personally never chase idle speed both hot and cold, I set engine idle when hot and then do what it takes to keep it idling while cold. Too much trouble doing all that over and over. Either keep engine idling cold by hand or buy a cruise control to artificially hold cold throttle on a bit until warm enough to not need it. I do the latter. That way the hot idle speed pretty much never gets messed with again.
 
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