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
Check the deck height I set them up at .051 to. 052. I think the height might be to low.
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
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.
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
Check the deck height, read the plug
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
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
Dont have my manual handy, what should timing be, 2.2-1.8mm BTDC? I know 2.5mm is too much.
Brian
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
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.
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
It was the head gasket as several people mentioned.
(http://i1177.photobucket.com/albums/x348/kartwheel68/20131027_124838_zps8d66b757.jpg)
Brian
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.
(http://i1177.photobucket.com/albums/x348/kartwheel68/20131027_154324_zps1324afef.jpg)
(http://i1177.photobucket.com/albums/x348/kartwheel68/20131027_154305_zpsb1be6f9f.jpg)
Anyone ever seen this or know why it might have been done?
Brian
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?
Its on both sides, and if you look closely at the pics, its only on the sides, the fins still go full length on the front and back sides. I cant figure out why it was done, it doesnt seem to have increased access to anything...
Brian
Brain I have seen this before, I would guess that it was done when trying to take the head off.
i can solve the mystery of the cut fin. this bike used to belong to my buddy, kevin outland. it was raced quite successfully a couple of years back at some vintage races. it stayed here at my house during that time and i would help him maintain it. before jerry birky was working at penton racing products, the local options to repair broken fins were not that great. i had seen a few repairs and i was not impressed. that top fin was broken on both sides, with one side missing completely. i cut the broken fin with a abrasive cut-off wheel, albeit trying to save part of it and still look symmetrical. jerry B does such a good job of repairing them now that i would have gone that route then if i had been aware of it then. brian forgot that i told him most of this story in person at barber a couple of weeks back. chicago jerry
Yes, I had forgotten you told me that story. I know it doesnt make any difference in performance and I thought it looked like it had been done well.
I am going to go ride the bike today, I will know then if its still acting like it was. The only reason I think the head gasket didnt fix it is when I started it, it sounded VERY lean. We'll see. I ordered a couple extra head gaskets and a complete engine gasket/seal kit from Al so worst case I'll split the cases and replace everything tomorrow.
Brian
OK, latest update. I changed the head gasket, and only started it, I didnt ride it, but it sounded lean and no matter where I set the idle mixture screw it would not idle. I changed the pilot jet from a 40 to a 45 and that was a lot better, about 1 turn out on the idle mixture screw and it was perfect.
Once I got to the track, it was a lot better, but it was still lightly pinging when the throttle was in the 1/3 to 2/3 range so I raised the needle, putting the clip in the richest slot. It was in the leanest slot when I got the bike, and I had already moved the clip to the middle slot at Barber 2 weeks ago. I also saw this while I was changing the needle slot:
(http://i1177.photobucket.com/albums/x348/kartwheel68/20131102_180954_zps62869767.jpg)
It is obviously leaking, but could it leak enough air there to cause my symptoms? The bolt was 180 opposite at the top before, when I put it all back together, I rotated it to the bottom.
Once I did that, no pinging at all, and it ran perfect. I dont know if it was that clamp, the head gasket, the jetting, or a combination of all of them. I am going to order a richer needle jet and a new needle. It was 70 degrees today and I think if the temp was colder it would not be rich enough, and with the clip already in the richest slot I wouldnt have anywhere to go.
Brian
In that small gap I run a thin O-Ring that will half way compress as the carb is pushed to its correct spot.
Thats a good idea. I think the leak was because the notch at the top of the carb was almost closed, and when I flipped the clamp to the bottom, I guess it got a tighter seal because I cleaned the oil off and it didnt leak anymore. If that oil is leaking, then you know for sure air was leaking.
Brian
Brian, this is just a wild idea so take it for what it's worth. I have seen a lean condition because the carb is set up wrong. Most often it is a result of a float being set too low. What this means is that there is not enough fuel in the float bowl, this causes the pilot jet to run lean because the end of the jet is not covered in fuel, it is pulling fuel and air through the pilot jet circuit. This takes the air mixture screw out of the equation.
If the slide is set too high it will do the same thing, you will limit the control of the air mixture screw. So often times a larger pilot is installed to compensate. This sort of works but you do not have as much idle control as you should have. I had a problem with my 74 Maico I had an air leak, actually I had several that were so small it was hard to detect. I managed to deal with it by running a richer pilot jet and needle jet. When I finally solved my leak, I found that I was able to lean my carb a bit and I ended up with better idle control. The only way you will know for sure is to do a pressure/ vacuum test, then you can set up your carb right.
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
The float is right. I think the problem was three fold, it was a leaking head gasket, leaking at the carb/cylinder spigot, and on this bike a 40 pilot is too lean. It just will not idle or run with a 40. Put a 45 in it and it runs perfectly, and I can put the needle clip in the middle position with no pinging at all.
I've got GoPro footage from today uploading, you can hear its running crisp and clean.
Brian
Hey Brian,Did you remember to check for the updated vaporizer?
Yes, it has the updated vaporizer.
Brian
An update to this problem, I finally solved the problem, but I dont know why it solved it.
The bike would only ping when the right (sprocket side) crankcase filler cap was off. When I first got the bike the cap vibrated off, when the cap was on it did not ping. I can not figure out why this makes a difference because if there was an air leak in the cases from the transmission area to the crank area, the other cap is vented so it should not make a difference, but it does. With the other (non vented) cap on it does not ping and I can go back down to the size 40 pilot jet that was in it when I got it.
That cap being off shouldnt cause the pinging, but just to make sure I took the cap off, and it started pinging. Very very odd, but one way or the other its fixed.
Brian
It sure sounds like an air leak to me, regardless of whether the other cap is vented or not. Mike
Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R
I agree Mike, it was displaying classic air leak symptoms but why would it only do it when the cap was off? It makes no sense to me. There are three caps on the case, one in the trough behind the cylinder, one on the right side, and one on the clutch side, only the one on the clutch side is vented. Since the case is vented, if the case had a leak from the transmission area to the crank area, I cant figure out why it would make any difference that one of the non-vented caps was missing, but it darn sure did.
Brian
It almost sounds like the air leak is slightly more than the orifice on the cap provides, especially amid oil being thrown around from the gears. I would do a pressure test to be sure. A lot cheaper than a seized piston. Mike
Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R