Pinging '74.5 250

[brian kirby]

OK, so I got this '74.5 250 that was built by Chi Jerry a couple of years ago. The bike has low hours on it since Jerry rebuilt. It runs really, REALLY, good, but after 2-3 laps when the engine gets fully hot it starts to ping, hard. It also does the uncontrolled revving when it gets hot if you miss a shift. When you start it up cold though, it does not display any typical air leak symptoms, it will not runaway rev when its cold, only after it gets hot.

I have done the "spray ether on it while its running" trick, and could not find any leaks in the usual places like gaskets or main seal that way. The jetting for the 36mm Bing is 165 main, 283 needle jet, 45 pilot, so jetting is stock or close, and the slide groove thing on the side of the carb is sealed, and the carb cap is good. The slide has some wear, but not enough for it to be leaking there, but I have an almost new slide I will try. I have not done a proper leakdown test, or checked the timing, but I seriously doubt the two owners of the bike between when Jerry built it and when I got it messed with the timing.

Anyone have any ideas? I am going to do a proper leakdown test soon, but I could be overlooking something simple...

Brian

[G Ellis]

Check the deck height I set them up at .051 to. 052. I think the height might be to low.

[brian kirby]

I didnt think of that Gary since Jerry built it I think the deck height is probably correct. It doesnt feel high in compression, I have ridden several that were harder to kick over, but I will check it. I will check that, do a leakdown test, and check the timing. It can only be one of those three things really.

Brian

[sixdazed]

Brian,Mine did that right before it blew a head gasket-just a thought.It had the same symptoms as yours for 2 rides and on the 3rd short ride it started chirping a bit so I started home then it totally blew the head gasket.I think it must have been leaking very slightly before.Luckily I was close to home when it happened.[8D]Cheers,Ric

p.s.after the first time it ran like that I sprayed around all gasket surfaces with no detectable difference.A pressure test was my next step until it became obvious when the gasket blew.

[tomale]

I was just wondering what are you running for fuel. My 76, which pretty much has the same motor. Pings when it gets warmed up if I use straight pump gas.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74' 1/2 440 maico
70' 400 maico (project)
72' cr125 Husky (project)
93' RMx 250 suzuki
Honda TL 125
2004 Suzuki DL1000

[Kip Kern]

Check the deck height, read the plug

[brian kirby]

Its not the fuel, I run the same fuel in everything I own from my '72 Berkshire to my '01 Honda CR500, which is 50/50 110 race fuel and non-ethanol pump. I dont need that race fuel in a lot of my bikes like the modern liquid cooled bikes but I am too lazy to mix different cans for different bikes.

Its got to be an air leak, timing, or deck height.

Brian

[brian kirby]

Deck height is right at .51-52, gaskets were goo and  the top end was torqued. Timing is at 2.5mm BTDC, I think I found my problem.

Brian

[brian kirby]

Dont have my manual handy, what should timing be, 2.2-1.8mm BTDC? I know 2.5mm is too much.

Brian

[brian kirby]

Hmmm...looked at my manual and timing should be 2.55mm I had it at 2.51mm so thats not it. Must be the head/base gasket, or someone else said maybe the carb doesnt have the 404 vaporizer in it...gonna put a new top end gasket set in it and check the vaporizer.

Brian

[derek martin]

Brian - worst case scenario; I've got a set of 250 cases out in the barn with the right side case cracked starting at the crank bearing fit and extending out to the right rear of the jug fit. These cracked cases came with my bike when I bought it - previous owner had replaced the cases solving the problem he had had of classic air leak symptoms but only when it was warmed up and ridden fairly hard.

[chicagojerry]

according to brian's facebook page, a blown head gasket was the culprit. i blew one at mckees farm too.  i mentioned that hopefully its not pulling a stud from the mag case threads. i have seen that too but its not a bad fix with a heli-coil. chi ejr

[brian kirby]

It was the head gasket as several people mentioned.



Brian

[brian kirby]

Update. After the head gasket, I did not ride it, but it sounds like it still has an air leak. Looks like I have to take the whole top end down, or to a pressure test.

On a side note, the top fin on the cylinder has been cut back.





Anyone ever seen this or know why it might have been done?

Brian

[sixdazed]

Brian,If I remember right I had a low spot on my head after the gasket blew.I just ran the head over my surface plate in a figure 8 pattern to get it flat again.I checked the cylinder with a straight edge and it was ok,so I ran it and never had any problems after.Ric


p.s.-is that fin cut on both sides,all the way around,or just the one?Maybe a damaged end of fin and was cut back to make it look a little better?