Greyhound 196cc 6.5 HP backfires when throttle is closed....

#1
Carb is clean. After a warm up and a high speed run, quickly backing off of the throttle causes a couple of backfires ( too rich).
Also after the engine is hot and shut off for a few minutes it is hard to re-start. The choke may have to be put on.
This carb has a blocked off ( plastic square placed onto screw see photo). Is this the enrichment adjustment? None of my other clone engines have this screw.
I know the Phillips plastic screw is idle adjustment.

Greyhound carb.jpg
 

SAS289

Active Member
#2
Have you looked at the plug? I'm wondering if you are running lean now with the header pipe you added? Doesn't every clone including the 212 come out of the box too lean for the mini bike and go kart application? You should remove that 28 or 29 jet and replace it with at least a 33 jet minimum. If you are rich the first thing would be to make sure the float isn't stuck down.

And didn't you say that your pipe was getting very hot? I'm thinking too lean. Hopefully others will chime in on this.

I have 3 running 196's and I use the choke to start them every time, cold or hot.

That plastic cap you are pointing to is the air screw. I think it is part of the idle circuit and does play a part when the engine is running at higher speeds. You shouldn't have to pull off the cap and adjust it. I think I read somewhere that the starting point is 3 turns out after the screw is lightly bottomed by turning it in.
 
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SAS289

Active Member
#3
I need to correct myself. That is not an air screw. They call it a pilot screw but it should be called a fuel screw. If the screw was on the air filter side of the carb it would be an air screw.
 
#4
I ran the bike up my mountain for 5 miles,returned and pulled the spark plug. It does not look abnormal to me as the insulator is a light brown and the anode is black with soot.
I'm riding betewwen 7,000 and 8,000 feet ASL.
Greyhound spark plug after 5miles.jpg
 

SAS289

Active Member
#5
Interesting. Looks like you don't need to re jet. Not sure what is going on there.

Did it backfire with the stock muffler?
 
#7
The header pipe is scavenging the exhaust better (good), increase main jet one or two sizes (plug shows slightly lean fuel mix), your mixture is close to where you want to be. A light tan color on the plug insulator will indicate proper air/fuel mixture.
 

wjustice

Active Member
#8
I don't know if this the same as your problem but in 2 stroke micros if we heard a pop when they lifted for a corner, it was an indication of lean. Everyones head would look up and ask "was that a lean pop"?
 
#9
Thank you for the replies. It may be that I do need to go up one jet size.
I'm using alcohol free gas with and octane booster additive.I'll try running it without the additive after this tank is empty.
 

SAS289

Active Member
#10
I don't know if this the same as your problem but in 2 stroke micros if we heard a pop when they lifted for a corner, it was an indication of lean. Everyones head would look up and ask "was that a lean pop"?
Yes it's the same issue. When the throttle is quickly closed a lean condition is created during the RPM drop because there is less fuel being supplied from the main jet and nozzle. This is where the idle circuit comes into play. You would think richening up the idle circuit would help with the back firing. Isn't the main jet taken (mainly) out of play when the throttle is closed? I wouldn't change the main jet when my issue is happening when the throttle is closed (idle position).
 

SAS289

Active Member
#13
Is that the square black plastic covered jet?
Yes. You will want to back the screw out to get richer.

That adjustment can be done at idle after warm up. When you slowly back out the screw you may hear the idle go up. Stop there and lower your idle speed with the plastic screw if needed then ride to see if you can make the backfire happen again or not.

If you keep backing out the screw too far with no change (still backfiring) then you would have to go 1 step larger on the pilot jet.

I did this on one of my engines and it was a PITA because it was difficult keeping the tiny screwdriver in place and turning while the engine was running. Mine was too rich because my correction was from turning in on the screw.

Hopefully there is nothing restricting the flow in that tiny jet. It's something like .018 or .020 and can easily get clogged. It is located just below the plastic idle speed screw. You take the idle speed screw out then gently pry up and it comes out.
 
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#14
I warmed up the engine and gave the needle valve a complete turn ( little at a time CCW) the engine is now much easier to start, and the idle was adjustable down a little. If I try to set the idle to what I think is normal, the engine dies instantly.
The back fire has lessened but is still there.
I may very well have to change the pilot jet, but I'm at 7,000 feet ASL now and go as high as 10,000 feet on a normal basis out here.
 

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