| Author | Message | ||
     Alphadogcustoms |
Having rather unusual problems with a Blast. Customer tried to fix it first, claiming carburetor problems. He brought the bike to me with the carburetor and airbox off of the bike. So, I disassembled and cleaned the carburetor, put it all back together. Still didn't run right. Seemed really rich for some reason, and fuel was spraying out of the carburetor. Installing leaner jets had little effect. This was when I did a reset, and put aside the diagnostic bias of the customer telling me he had a carburetor problem. I did a cylinder leak-down test, and the intake valve was clearly leaking. Pulled the engine down and sent the head out for a valve job. Cylinder was in perfect condition, still showing original cross-hatching. I lightly honed the cylinder and put the engine back together. Still had the same problem with it huffing fuel out of the carburetor. Upon further investigation, it appeared that the cam timing was off. It seemed like the gear on the intake cam had slipped time. I got a used set of cams from eBay. I put the engine all back together, and put the carburetor jetting back to what it had previously. Engine ran normally now! Yay! A couple weeks later, the bike came back to the shop. It lost power and quit while being ridden. After a little testing, I decided that it had a hole in the piston. Pulled the top end off, and sure enough, piston is toast.  In the picture, it looks like the exhaust valve is kissing the piston, but there is no evidence on the valve that this is happening. So, this brings me to finding the answer to two questions: 1) Why did the piston get a hole in it? 2) Where do I get a standard piston and ring set? Why the hole? Reason#1: Fuel of too low octane. The owner filled it up with Sunoco 93. Reason #2: Timing too far advanced. I checked the timing, and the light on the module illuminated exactly at TDC. Reason #3: Thinner head gasket, increasing compression? I used a Sportster 1200 head gasket which appeared identical to the head gasket that came out. Reason #4: Something to do with the new cams. This is my prime suspect in tracking down the reason for this to happen. The new cams that I put in had all the same markings as the old ones. The one thing that gives me pause is that the exhaust cam seemed to be "off" a little from exactly lining up with the timing mark. It's possible that one tooth off with the exhaust cam could cause less "overlap" and cause detonation? Not sure about this one. It did seem like maybe the push rods were a bit long after the valve job, so I put 2 gaskets under the rocker box to make sure I had valve clearance within the range of the hydraulic lifters. I also installed new lifters. Now, as to getting a new piston. All I seem to be able to find so far is big bore pistons. I need a standard bore piston and ring set. The cylinder is in perfect condition. I'm searching twinmotorcycles.nl but it's a lot to wade through. The search function is proving to be kind of broad. | ||
| Gearheaderiko |
What year? 1st: the intake boot is infamous for leaking or splitting and it takes an off the bike inspection to be sure. If it’s not using an original OEM boot, you’re in for trouble. If it’s using “Dans Boot” you’re probably okay. (Lean) With the timing “jumping” a tooth that indicates the bike has been apart or another gear is shedding teeth. The oil pump drive gear is know to fail. If it has been apart that could explain the seemingly mismatch of parts. Blasts are very tolerant of bad jetting and bad timing, so you really have to mess things up. If the bike overheated, that could explain a sticking valve hitting the piston on failure. The pushrods are different lengths. To thin or too thick a head gasket won’t make much difference on a Blast. It’s not that critical, unless other parts are wrong. You can put an overbore (.015”) CP 10.5:1 piston in there without a problem. I’d stay away from the 515 big bore because some older one had serious failure issues. I don’t know exactly where to find stock pistons as I’ve never searched for them. Check the timing by the static procedure and a light. A hole in the piston indicates serious abuse or poor assembly. It doesn’t happen easily. Oil should be 20w-50 minimum. 2.25 qts MAX. Carb jetting should be 45/170-175. Idle mix screw about 2 3/4 turns out. If the customer threw in a “jet kit” it’s going to be problematic to figure out and get the jetting right. Non stock needles look more like sewing needles. The customer might have drilled holes in the carb. If so, good luck. In reality, it’s just a simple single cylinder engine-nothing special. Set it up and ride it. Pay attention for telltale signs, knock or ping, sluggishness, fat or lean sounding exhaust note, etc. The most high tech item is the tps on the carb. Unplug it and that problem will go away. It might run rich until it heats up, that’s it. What other mods have been done to the bike? Lastly, the ignition module is the wild card. I have seen them do a variety of puzzling things: low rev limit, high rev limit, sputtering, shutting down then firing back up (after a brief or long period of time), firing the cylinder after the bike is shut off or shut on, or anything else you might imagine. Twin carbs can be used on the Blast with minor modifications, if you need to try a different carb to help with diagnostics (in case the owner really messed up the Blast carb). Good luck. I hope that wasn’t to remedial. Keep us posted. | ||
| Gearheaderiko |
CP pistons are available in stock bore. Also try Buell Riders Online. I’ve been working on Blasts for 17years, but they might have a better idea on what’s currently available and where. | ||
| Alphadogcustoms |
Hmm. I can't seem to find a "Reply" button so that I can easily reply to each individual point made by Gearheaderiko. | ||
| Alphadogcustoms |
[[What year?]] 2001 [[1st: the intake boot is infamous for leaking or splitting and it takes an off the bike inspection to be sure. If it’s not using an original OEM boot, you’re in for trouble. If it’s using “Dans Boot” you’re probably okay. (Lean) ]] Ah. I didn't think of intake leak as a possibility. The intake boot appears to be in good condition. The rubber is soft and pliable. I used Permatex Gray Rigid to seal the boot to the head. [[With the timing “jumping” a tooth that indicates the bike has been apart or another gear is shedding teeth. The oil pump drive gear is know to fail. ]] The cam timing did not "jump a tooth." The one gear on the cam is pressed on. I think it must have slipped on the shaft. There is no key. As I turned the engine over manually, and observed the movement of the valve train, it appeared to be well into the compression stroke before the intake valve closed. I expect a little overlap of a few degrees, but nothing like I was seeing. [[If it has been apart that could explain the seemingly mismatch of parts. ]] [[The replacement cams came from eBay. I replaced them together. Seems unlikely that there is a fault with the cams. I DID get new cams from Harley once that were indexed wrong. Took me months to figure out what was wrong with the engine!]] [[Blasts are very tolerant of bad jetting and bad timing, so you really have to mess things up. If the bike overheated, that could explain a sticking valve hitting the piston on failure. ]] I don't think it was overheated. The owner said that it did not have as much top speed as before. [[The pushrods are different lengths.]] Yes, I know. I am an experienced Harley mechanic. I labeled the push rods when I pulled them out. [[To thin or too thick a head gasket won’t make much difference on a Blast. It’s not that critical, unless other parts are wrong. ]] Good to know. [[You can put an overbore (.015”) CP 10.5:1 piston in there without a problem. I’d stay away from the 515 big bore because some older one had serious failure issues. I don’t know exactly where to find stock pistons as I’ve never searched for them. ]] I don't want to bore the cylinder. The cylinder is in perfect condition. [[Check the timing by the static procedure and a light. ]] I only did static timing, with the light coming on right at TDC. [[A hole in the piston indicates serious abuse or poor assembly. It doesn’t happen easily. Oil should be 20w-50 minimum. 2.25 qts MAX. Carb jetting should be 45/170-175. Idle mix screw about 2 3/4 turns out. If the customer threw in a “jet kit” it’s going to be problematic to figure out and get the jetting right. Non stock needles look more like sewing needles. ]] Oil was correct. Jetting was 45/175 with a K&N filter in the stock airbox. The customer did not take the carb apart, (so he says,) and never did any jetting. I don't have any history from before he owned it. He rode it (hard) 800 miles before the initial problem surfaced. The only work I had done on it previously was things like flushing the brake fluid and changing the tires. The owner did change the ignition module in the nose cone. I asked him what procedure he used to set the timing. He didn't set the timing. He just stuck the unit in. The timing was a little retarded. [[In reality, it’s just a simple single cylinder engine-nothing special. Set it up and ride it. Pay attention for telltale signs, knock or ping, sluggishness, fat or lean sounding exhaust note, etc. The most high tech item is the tps on the carb. Unplug it and that problem will go away. It might run rich until it heats up, that’s it. ]] The TPS affects ignition timing, I believe. [[What other mods have been done to the bike? ]] None that we are aware of. I don't know if the K&N filter is stock. If not, it's possible it's running a bit lean with the 175 jet? [[Lastly, the ignition module is the wild card. I have seen them do a variety of puzzling things: low rev limit, high rev limit, sputtering, shutting down then firing back up (after a brief or long period of time), firing the cylinder after the bike is shut off or shut on, or anything else you might imagine. ]] Yes, that is a wild card. He had trouble with the ignition module, and replaced it with a used one that he got on eBay. [[Good luck. I hope that wasn’t to remedial. Keep us posted]] I appreciate the detailed response. You have no way of knowing what experience I have. I am a graybeard, and work mostly on vintage bikes of all flavors, so I have pretty broad experience. I am trying to figure out what could have possibly caused the piston to burn a hole when all I did was do a valve job. The cam change has to be suspect as does the ignition module. I did not use a timing light, because oil blows out of the timing port if you start the engine with no plug in the port. Somebody must make a window for that, but I have always just done static timing.}} | ||
| Gearheaderiko |
Thanks graybeard! I’d 20+ years into motorcycles when I got a Blast. It will change what you think you know or assume about them! No sealer on the boot, except in a dire emergency. Years of watching every other non stock boot configuration fail has taught that a dry stock boot is best. “Dans boot”is the only one that seems to last (though I’ve never seen or had a reason to use one). The TPS affects closed or open throttle advance curve. You’d never know the difference if it was unplugged unless you are really in tune with your motorcycle! Unplugging it is an easy way to check to see if it’s causing an issue (whether it’s the TPS or the module is reacting bad to the input). The auto enrichment of course won’t work but will open eventually from engine heat. Just for testing purposes. Is it just a K&N filter or is it still in the original air box? If it’s just the filter only, it’s now unsupported and the carb will bounce around possibly tearing the boot or making it bounce to lean and back. The same clear timing plug used for decades on Big Twin and Harley Sportsters also fit the Blast. It does suck when you bad parts that are new from the factory. Compression should be around 120-140psi. With a high comp piston 175-200psi Check to be sure the rev limit is at 6500rpms. If the module is bad, it can run right pass that and float a valve. I’ll give it a read through again and see if I think of anything else oddball. You could throw in a 175, it won’t hurt | ||
| Gearheaderiko |
Also a loose timing cup is possible, especially if the PO had removed it. | ||
| Gearheaderiko |
PS I only know one way to get a burn hole in a piston is by overheating the engine. |
