E: Engine Partial Single Seizure? Safe to ride?

CoreyL

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I had another small adventure with my Series B Meteor recently. Now that I have 500 miles on a new piston/barrel, I wanted to do a ride and change the oil and filter afterwards. I went out with two friends and we did about 90 uneventful miles at 35-60 mph. Coming home, I hopped on the highway for the last couple of miles (I was feeling confident). I wound the Meteor up to 65 to keep up with traffic (70 mph speed limit) and, after a few minutes, I felt a loss of power (like I was out of fuel, but I wasn't). I immediately grabbed the clutch and she died. I kept rolling along the shoulder thinking: Farkin'ell, I've seized it again.

Luckily, I was right at an off-ramp and I rolled down the hill and into a gas station. I put her on the center stand and pulled off my helmet, jacket and gloves. I tried pushing the kickstart lever with my hand and she turned over OK, to my great relief. So, maybe she was just starting to tighten up and I caught it in time. I let the bike cool for 30 minutes in the shade. I was only about three miles from home so I decided to see if she would get me there.

She started up first kick and I rode home and into my garage. I drained the oil while it was still warm, replaced the oil filter with a new one, refilled her with oil and then started her up just long enough to verify return from the sump. I shut her down.

Yesterday, I verified that the ignition timing was still 34 BTDC at full advance, I verified correct valve clearance, and verified that the breather hole in the gas cap was clear. I checked compression and got 160 psi which seems pretty high even thoughI have an 8:1 piston. I also took some photos with a cheap borescope that sends images to my iPhone. I could see some light scoring of the cylinder wall (and still some remnants of cross-hatching which puzzles me after 500 miles of riding). The piston looks like it has some deposits on top (what does that look like to you?). I've attached a couple of pics.

At this point, I think I need to richen up the mixture (raise the needle one notch and go back up to a bigger main jet). I'm wrestling with whether I need to pull the head and barrel again for a closer inspection or assume it's OK to ride based on the solid compression data. Thoughts?
 

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vibrac

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After I rebuilt it, my Trials Comet it seized on the A1 slightly up hill two up with trials sidecar. I caught it before full siezure much as in the above story. I let it cool then carried on the trial (well I had paid the entry fee:rolleyes: )
I used that engine for years with no more problems and with good performance including a solo track day (no not in Trials trim!) I stripped it this last winter for a full rebuild and I could see the marks of that long ago size but no blow by was evident.
 

Sakura

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Had this happen several times whilst testing bought in "rebuilt" bikes for a well known classic bike dealer. If it just tightened and then freed after slight cooling it will almost certainly be ok. Just take it easy for a bit longer and only build up revs slowly and for short periods. If it happens again add a little synthetic oil to the petrol and run in for 2000 miles. Try then without the added oil.
 

greg brillus

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piston to bore clearance not enough ...........Do not over torque the head stud nuts, 30/32 at most and re torque again after a couple of good runs and engine cooled totally first. The singles need plenty of clearance as the piston gets hot quickly, it is working harder than on a twin.
 

Oldhaven

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Hi Greg,

From Terry Prince's instructions for his big bore kit:

"clearance measured at 10mm from the bottom of the piston

should be: .0035. For racing .0045”-.005” must be used"


His recommended spec does not specify twin or single, but do you think the racing clearance recommendation should be used on singles, especially standard displacement Comets? I think you have done a few of these. This is what I have been told by a couple of others who know their business and what I was planning. I certainly don't want to risk partial seizure like Corey did.


 

davidd

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I spoke to one owner of a 600cc Comet who set the clearances at Terry's numbers. I don't think he will do that again. I suggested when he rebuilt the bike that he use 0.006". I don't think many singles will work with less clearance unless they only do a run to the pub.

With Nicasil aluminum bores I set it at 0.0035" for the single racer. They normally recommend 0.001".

David
 

chankly bore

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What carburettor, what jet size and what needle position? Also check for leaks at the inlet stub. I never run less than .005" bottom skirt clearance on a single and I think also that modern "petrol" burns hotter as well as quicker. When going up a long hill in top it is always better to drop down a cog and let the jigger spin faster as I found once to my detriment.
 

CoreyL

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piston to bore clearance not enough ...........Do not over torque the head stud nuts, 30/32 at most and re torque again after a couple of good runs and engine cooled totally first. The singles need plenty of clearance as the piston gets hot quickly, it is working harder than on a twin.

I'm running 0.004" clearance and torquing to 32 ft-lbs.
 

CoreyL

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What carburettor, what jet size and what needle position? Also check for leaks at the inlet stub. I never run less than .005" bottom skirt clearance on a single and I think also that modern "petrol" burns hotter as well as quicker. When going up a long hill in top it is always better to drop down a cog and let the jigger spin faster as I found once to my detriment.

It's running a 32 mm Mk1 Concentric (K&N filter), #3 cutaway, 0.106 needle jet with the needle clip in the middle groove. I've got a 260 main jet in there now. All the flanges have been made flat and flush (no air leaks). I plan to raise the needle (clip in lower notch) and go up in main jet to 280 or 300 before I take it out again. It's also got a SuperTrapp "muffler". I'm running a low expansion cast Omega piston.
 

davidd

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Corey,

I think that 0.004" is too tight on a Comet with iron liners. If that is the case (and some may disagree) then the clearance is still too tight. The Omega is a fine piston. The carb seems plenty rich. I think that all was well except that the piston got too large from the heat it generated.

I am inclined to say "pull it apart now, get a new set of rings, hone it for 0.006" clearance, clean up the piston a bit and ride it." If you are worried about bedding in the rings run some break in oil from Jegs or Summit. It should run like new and continue to run well (assuming the piston is just a little scuffed).

If you continue as is, another seizure may occur and damage the piston or bore a bit too much and that gets expensive. Whatever you choose, best of luck.

David
 
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