Engine Knocking Problem?

[crash carden]

Hello, I have a 73 250 Harescrambler with about 300 miles on a rebuilt top end. It runs strong and has even smooth power through all the gears.However no matter what gear it is in when I let off the throttle the engine has a knocking sound.It runs fine with the throttle on "no knocking" with smooth power until I let off the throttle, then I get the knocking sound.I have the manual and have read up on the carb settings and the signs of running the engine lean and or rich. I am using Golden Spectrol,mixed one bottle to 5 gallons of high octane gas.I have the proper heat range spark plug.The needle is set on the second notch like the manual calls for. I have also set the idel acording to the manual.When it is first started it sounds like it is starved for fuel,but when it is warmed up good sometimes it idels perfect other times it idels fast and uneven,like it surges, gets fuel then does not get fuel. Fuel filter is new, float is also adjusted properly.My question,is this knocking sound normal? Is that the nature of the beast? Also why does it want to idel different at certain times? I have had this 250 for about two months,the guy I bought it from had the engine rebuilt so I can only hope that proper size"thickness"head gasket was installed.However he did break it in properly with a richer fuel mix etc. I just can't seem to get the knocking sound stoped.I have also repacked the pipe with new fiberglass, from Al B.. Also the bike runs so strong and smooth with great power that it does not seem to be running rich or lean. Also the idel being so inconsistant has me stumped.Any help would be great.Thanks in advance,Crash Carden. Go Team Penton!

 

[hrbay]

crash, if you are sure the sounds are motor related, not tranny, it sounds like it's detonating when you close the throttle(lean condition) and goes away when throttle is open?(normal)... advance might be too much or the piston to deck height may be a tad too much. Pull head, measure the piston ht. above head(deck) mounting surface and compare to spec., may need some thin base gaskets to adjust this ht. reset timing advance by the book. If it has been doing this for long check the piston top closely . You may get rid of the noise with 104+ octane boost or racing gas. GC

GC

[rob w]

Chuck, I think you need to retorque your cylinder head nuts & screws, that's all.

 

[crash carden]

Thanks for the feedback, hrbay,& rob w. When I got the bike I checked the torque on all the head nuts& screws,so that rules that one out.Whenever I buy a new bike I always strip it down to the frame and go over the bike,and replace what needs to be,and clean, grease, and oil everything, gear box lube etc. I am guessing it has to do with the head gasket thickness as mentioned by hrbay. My next question, would that also cause the inconsistant idel problem? Thanks again guys, any other help is welcome. Crash Carden

 

[Dennis Jones]

Crash,
I hope someone can help you because I have a 152 Sachs that does the exact same thing. It runs very strong on the gas and the plug does not read lean whitch I first suspected. Maybe to much compression?, but why the surging? Like yours this motor rebuilt top to bottom. Also I emailed you about the 250,let me know if you did not get it. Thanks

Dennis Jones

[DKWRACER]

Had the same sort of noise, and sooooo....went back to check the deck height and reset timing to 2.4mm BTDC......and also cont to the problem was a loose/wobbly woodruff key on the pinion gear that drives the clutch, backslapping!!!!

 

[tomale]

Crash, I has a few thoughts. I am wondering how the bike was broken in. could it be that they were not carful about the break in period? I am wondering if you are hearing piston slap? Also have you cleaned out the tank and what about the Petcocks, are they good and clean? This may cause the surging and lean situations.
At the race on Sat. I was talking to BigMac and he is having problems with his bing. He thinks that the carb may just be worn out. That is a possiblity too. Good Luck!

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[tlanders]

I had an inconsistant idle until I filed a small notch on the bottom of the carb slide skirt on the engine side. A soluion told to me by Doug Wilford.

Teddy

 

[Lew Mayer]

Teddy
I did that same mod to my slide years ago but for the life of me I can't remember where I got it. Did Doug say if that was listed in the original Keeping Tracks? It seems like I remember it was used to clean up off-idle mixture. Anybody else recall that slide mod?

Lewis A Mayer

[Paul Danik]

Crash,
When I am trying to sort out a problem such as yours I will sometimes slip in a larger main jet just to see what happens. I will also put the needle all the way down and then all the way up. This type of radical adjustment will sometimes tell you that the problem is somewhere else, or sometimes will point you into the right direction, lean vs rich, midrange vs topend, ect.  Hope this helps.
Paul