'82 Seca 750 Refresh Project: Now Scooby's Bike

Yea, you're right! Makes sense. Guess I'll just follow this chart for any nut or bolt that doesn't have specific torque value. Hopefully I don't bugger anything up so close to the end

 

Hey guys,

Been having some trouble with these bloody cams :\ I've turned the crank to TDC, put the Exhaust and Intake cams in correct spots, bolted on the cams and turned them so the dot aligns with the arrow.

Now comes the trouble. The sprocket bolt holes do not align cleanly with the cams when the cams are in the correct spot. The sprocket bolt holes are just maybe a few millimeters off... I can try to +/- the teeth so I can fit the sprocket bolts in and align the dots as close as possible . I'm able to get the sprocket bolts on but then the dots aren't perfectly aligned with cams. I heard if they're just barely off it's no big deal but doing this gives my chain some slack. So when I turn the crank over, the cams don't immediately move causing them to be out of position when the crank is TDC. With the slack, turning the crank over maybe 1/4th of the way causes it stop turning. I'm guessing this is from the chain slackening and tightening up again, perhaps jamming up so the crank stops turning clean. Obviously something not wanted.

I've read a few threads and I checked if the pistons were truly TDC with the straw in the spark plugs technique. All good there. Crank turns free when cams are off, also turns free when cams are bolt on, so no binding there. I'm thinking maybe it has to do with sprockets? I have a brand new chain but old sprockets. I just ordered new sprockets. If that's not it then I'm stumped. I'm at a loss on how to lift the sprockets onto the cams with little slack and have the bolt holes line up.

 

This might help.

https://xjbikes.com/forums/threads/1982-xj750-cam-timing-pictures.48419/

 

Thanks :^)

Yea I've been checking out multiple threads. The manual states gives some nice clear steps. I'm thinking it's just the sprockets atm. New chain is probably too taut for the old sprockets and thus can't align sprocket holes with the cam bolt holes. New sprockets should be coming soon and I'll try again

 

I fitted a new cam chain on my old sprockets. As the crank sprocket is smaller than the cam sprockets l would need another crankshaft if l wanted to change it. Theoretically it should wear more than the cam sprockets with half the number of teeth. I haven't seen worn cam sprockets even on high mileage engines. Your engine should be fine with the sprockets you have.

 

Ahhhh I think I figured it out. I'm a clown.

Re-reading the instructions in the manual. Only one bolt should be fitted onto each sprocket. Then automatic chain tensioner should be fitted on C mark and tensioned up then rotate the crank so the final bolt can be fitted on the sprockets and tightened. I was making the mistake of trying to rotate the crank to fit both bolts on the sprockets and then put in the tensioner. I'll try this later today and see how it goes.

 

Yes that is the way l fitted the bolts.

 

Hey guys,

Still having some issues with turning the crank. I've gotten both sprocket bolts on. Tensioned up the chain nicely. But there's still this issue of turning the crank. There's resistance when turning it 1/4th around. I've checked no valves are clashing pistons, no binding of pistons. It's all good until I put the sprocket bolts in and tighten up the chain. I have no idea what it could be. I'm thinking maybe the chain is getting caught on on of the chain guides? It's either I didn't set the back chain guide in nicely enough or the front chain guide isn't fully in the slot. The back chain guide should be pretty loose yes? I made sure it the inner bolt lightly touched the chain guide then I set the locknut. The auto chain tensioner should do the rest. As for the front chain guide. Does anyone have a picture of how far in the slot it should go? It sort of bounces a bit if you push down on it. Maybe it needs to sit firmly. Not sure. Though it might not be much help, I'll see if I can record a video.

 

The exhaust side cam chain guide should not bounce. There is a pocket the guide goes into. Not getting it in the pocket is an easy error to make. Try turning the engine more than a quarter turn clockwise with the tensioner in place to see if you get the same resistance. The back chain guide ( I assume you mean the tensioner side should not be loose when the tensioner is set correctly). The chain should be taught.

 

do you think the chain could be half way off the crank?

 

No I don't think so. I thought the chain could have been caught between the crankcase and the crank sprocket but the chain could only do that if it was off the camshaft sprockets.

 

The chain could be off of he crank sprocket. I've done that myself.

 

I've posted a short video of what's happening. It might be popping off the crank due to one of the chain guides, I'm guessing it might be the back side , tensioner chain guide. Can't really know for sure.

 

If you can get the guide out between the camshafts you might be able to shine a torch down the cam tunnel to see if the chain is on it's sprocket. Have you got your timing plate bolt tightened to the specified torque? How did you set the tensioner blade in the crankcase? The bolt should be tightened until it stops on it's seat on the tensioner blade, then it is backed off a quarter turn and the locknut is tightened.

Is the transmission in neutral while you are trying to turn the engine?

 

I think I found the issue. Its the intake cam. I've rotated the exhaust cam freely. it can hit 90 degrees no problem. Which is 180 for the crank right? The intake rotates only about 45 which is halfway to BDC which is where I'm experiencing resistance. The intake cam might be warped or the caps might have something wrong with them. Is it possible to switch intake with exhaust ONLY to test if they rotate? Obviously I dont want to run the engine with the wrong cams but I want to see if the intake cam or caps are truly warped or not. It could also be a warped head but I think it's more likely a warped intake cam. I might just buy a pair off ebay to see what happens. Will '82 Maxim 750 cams fit a '82 XJ750? Seems like Ebay mostly has Maxims

 

Yes for every 180 degrees of crank rotation cams move 90 degrees.

Are you sure the intake cam timing is correct? When the engine stops where you said it does shine a torch or put a dowel into the spark plug holes to make sure the piston is not near the head. Look at the cams on the shims on the inlet side to see if any valves are open. If you put the pistons mid way down the bores with the cam sprockets off and timing chain off and held taught turn the inlet camshaft through a few rotations you don't need to exchange the cams.

 

I am not an engineer but l think you would need V blocks and a dial gauge setup to check camshaft for runout. And of course a manual for the specifications.

 

Right, yea I made sure the pistons weren't clashing with any valves. Put a straw down the spark plug holes to measure how close they were to the valves. I took the chain off the sprockets and just installed one cam. I used intake cam on exhaust side with intake caps, keeping the chain off the sprockets. Still had an issue turning the intake cam, triple checked they didn't clash with any pistons. Then I pulled it off and installed the exhaust cam on intake side with intake caps. also keeping the chain off the sprockets. Rotated fine. Probably not the best idea for obvious reasons...and I hope it wasn't a huge mistake but nothing seemed to go wrong. I'm also not an engineer not have the tools to measure the camshafts but by that little experiment I'm gonna assume the intake cam is warped. Though I don't know how that happened.

 

Assume nothing.
Unless you beat on the camshaft, or dropped it several times, it's not going to bend.

If it was bent you'd notice as soon as you set it into the bearings.

Did you use oil or assembly lube on the camshaft bearings when you reinstalled the camshafts?

I'd like to see a video of what's going on with the cams and cam chain when you turn the crankshaft.

Also, the very last step in the whole process is to reinstall the camshaft chain tensioner. By installing it before everything else is in place it is possible for the chain to bind without you realizing, which is what I think is going on.

 

Hey guys,

I've done as you instructed, refit everything, Double, Triple checked. Intake cam still wouldn't turn. Unfortunately I didn't record a video as I just decided to order some cheap cams off ebay and check their specs matched my old cams, just to see if it was the cams or something else. They work great so I don't know what happened to the intake cam for it to bind like that. Got everything together and am checking valve clearances.

I'm not quite sure why the clearances I took mostly differ from the old clearances the previous owner took. I'm not sure when he took them or if he changed shims. Perhaps reinstalling new cams, new chain has something to do with it? Here are the clearances. Top was previous owner, bottom is mine. I'm not sure why they are sideways when I upload them here. I messaged hogfiddles about shim pool. Gonna pull these, swap some around and see what I need. Looks like I just swap the Intake shims with the Exhaust shims and it should be good

 

now would be a good time to measure the bearings

 

Hey guys,

Does anyone know the name of this little boot below the master cylinder? Looking to buy a new one as this one seems to be disintegrated. How do I replace it? Does it require a full disassembly of the master cylinder?

 

@scoobydew

this is a seriously important bit of advice. Get some plastiguage and make sure the replacement cams will work in the existing bearings. Usually they will, but if they don’t you will have a junk top end very soon. And you absolutely do need to recheck valve clearances with different cams and adjust as necessary.

 

The specs in the book for bearings would be "Camshaft to Cap clearance" right? I just measured it with some green plastigage. Intake, Exhaust right side had around a 0.020mm. Can't be for certain since the closest measurement they have on plastigage is 0.025mm and the mark was a tad larger. Intake, Exhaust on left side measured about 0.038mm. These are new cam measurements. Ran out of plastigage for old cams and no autozone sells plastigage green by me so gotta wait until Sunday.

What do ya'll think of those measurements?

 

If the specs are the same as the 900f engine then the book states

Camshaft journal to oil cap clearance
Standard 0.020 to 0.054mm

Service limit 0.160mm

k-moe will know if they are the same.

 

Yep, the specs in the XJ750 book look to be the same what you posted. I'll check the other cam on Sunday.

I have a question about valve clearances. Are there any feeler gauges that step up by .10, .11, .12... .20, .21, .22? My feeler gauge at the moment is .10, .13, .15 etc. So when taking clearances, the valve might have drag at .15 but too tight at .18. There's no way to measure if .16 is too tight or not.

Some shims I didn't swap out as they were .18 270mm. So I left them in but when reinstalling it seems to be .18 to .20 now but .23 is too tight. How can I know it sits at .20 and not .21 since I have no feeler gauge for that?

I attached a pic of the new specs which are circled. Replaced shims, swapped some and left some in place. Specs changed a bit :\

(I've just found some feeler gauges with .01 increments on partzilla, for some reason the Autozones by me don't sell .01 increments)

 

You are correct you are better with the proper feeler gauges. I had to buy a set too to make sure I was setting them correctly. When doing the clearances I aim for 0.15mm for the inlets and 0.20mm for the exhaust. The reason is the valves on our bikes pocket in that they get closer to the seats and the clearances therefore reduce from when the engine is new.

 

That is correct for all of the shafties.

 

KD Tools 2274

You stack the feelers to get the inbetween measurements.
Use the 0.11 and the 0.10 in order to measure for 0.21.

What really matters is that the measurement falls within the specified range. In-spec is in-spec, even if it's 0.01 mm in-spec. If necessary err on the loose side.

 

These valve clearances have been driving me crazy. Whenever I swap one shim that was out of spec, another one seems to go out of spec. I must've swapped shims three times now. I've had all my exhaust valves within spec:

Exhaust: .16 .17 .18 .20
Intake: .14 .13 .15 .09

So I swapped the 280 at .09 for a 275. Now the Intake valve is good at .13 but it seems like Exhaust .18 and .20 are both suddenly .21. I've double checked those valves and they were for certain .18 and .20 before the Intake shim swap and now they are both .21. Am I missing something here or is it normal to swap shims multiple times until finally they all line up correctly?

 

What you are seeing is the oil from under the shim squeezing out and changing the measurement. Being 0.01mm loose is not an issue at all. That will tighten up over time, so leave them in place as-is.

 

All right! and just like that the engine is done. Minus starter motor and what not. Thanks to you guys. Still a long way to go but it's coming together. Also posted a pic of valve specs here in case for some reason I lose that sheet, the specs will always be here.

 

Did you find out what was causing the play on the rotor at the ignition pickups end of the crankshaft?

 

The play was caused by the chain slackening and tightening back up because the intake cam wouldn't turn past a certain point.

Here's how she's looking so far. Left off a couple of things for cleaning off the rust. The fenders were very rusted up. Just finished the forks with some fresh fork oil.

One question. For the U-joint. because there's little space between the engine and the swing arm under the gaiter boot. How am I supposed to torque the U-Joint bolts? I can't get a torque wrench in there. Only a S-shaped wrench which I used to remove them.

 

You can get an offset short spanner that will fit a torque wrench.
Or you can just do it the same way 99% of others do and give them an adequate tightening.
One grunt and NO fart.

 

Hey guys,

Has anyone bought the MAC Performance aftermarket exhaust for 1982 XJ750 Seca? The fitment said center stand retention. The pipes are a bit long and they take up the passenger pegs, which is fine by me. I don't take passengers.

But I am upset that when I pull up the center stand it hits the exhaust mufflers. I'm pretty sure they're installed correctly but for all I know, I could have f'd it up. If anyone has any insight I'm all ears! I would love to keep the center stand. It's great for working on the bike.

 

Here is a picture of a 750 Seca I recently traded off that had the MAC 4 into 2 system. If you can zoom in on the muffler mount you can see it was to the factory muffler mount hole and behind the foot peg mount. I don't think you have yours all the way forward yet. Your muffler has the bracket for the center stand to rest against when retracted, the muffler just is not positioned quite right yet. I did not install this system so I cannot give more info than what shows in the pic.

 

Thanks for the help! I feel like a clown haha

 

Like this - it's precisely the way I do it, I had a colortune years ago, threw it out...

 

Hey guys,

Final stretch, hooked up all electrical today. Tomorrow gonna clean out the rust from the fuel tank and finally start it up for the first time if not this weekend then next.

Got some questions though. Does the aux and headlight ONLY turn on when the motor is running? Because everything else is good, brake lights, turn signals etc but the high beams and headlights aren't on with just the key in the bike. Also, does the kill switch also shut off the starter motor? I know some bikes still crank the starter with the kill switched on. Just so I don't turn over the engine by mistake cause I haven't put any oil in the crankcase yet.

 

If I recall, yes the bike has to be running and see power coming to the rectifier before the lights will turn on. Saves all the CCA for the starter to start the bike.

 

Today was first start. Engine turns over. Spark plugs fire but the engine doesn't start. Since I'm still cleaning the xj750 fuel tank, I used an auxiliary tank. Primed it and all. Engine still wouldn't start.

So I sprayed a bit of engine starting fluid and I got it to start for a bit then it died out. I hooked up the vacuum and fuel hoses. But I'm not sure if the cylinders are receiving any fuel from the carb floats. But they received the spray from the starter fluid.

I'm gonna check compression and gonna pull the carbs. Anyone have an idea why the carbs aren't pulling fuel to the engine?

 

Well, bad news.

The rebuild project is fudged. Compression on cylinders was around 120 cold, #4 was around 90 until the engine warmed up and it jumped to 120. Sprayed a little oil and the compression for all cylinders shot up to around 150. Already a bad sign. Started her up and there's excessive white smoke and popping most likely meaning the piston rings aren't sealing correctly.

I knew I should've re-bored the cylinders to the next size and gone with over-size pistons but I checked my specs, ring gap was good, cylinders were good, honed the cylinder and figured the stock size was good to go. I must've read piston to cylinder wall specs wrong.

Not only is this a lesson for me but I hope it's a lesson for anyone rebuilding an engine, don't just double check or triple check but be 100 percent sure your readings are correct or you'll kick yourself in the ass later.

I don't know when I'll tear down the engine again to re-bore the cylinder and change the pistons as this is quite a bummer and I'm left defeated. Especially due to the cost of rebuilding a bike. This was a nice quarantine project. But hopefully sometime in the near future I can get the engine running in perfect condition. Although it's no small job. All it most likely needs is a re-bore and a piston replacement.

 

Hold on here a while now!
There's probably nothing wrong with your engine - give it a little while before condemning it. Get it running on all four, starting and idling like it should, then warm it up with a few twists of the throttle to the red line, see the smoke stop. At any rate you need to give it some throttle to help the rings seal, if you can ride it, all the better.
Do not despair yet!

 

I was thinking the same, give it a chance don't pull it apart until you are sure. The gasses get between the piston grooves and the rings and push the rings onto the cylinder walls if am correct on the compression stroke. The smoke will be the oil burning off.

 

And also any oil used in the build of the top end goes into the exhaust and coats everything, slowly evaporating - or quickly depending on how hot and how quickly the exhaust gets.
Please give it a chance.
I re-read your build up - bad couldn't make out if you'd changed the damaged piston in the pic? You should have...

 

Of course, all four pistons were destroyed which is probably why the previous owner had trouble with his idle and popping sounds. Though somehow his compression wasn't absolutely terrible. It wasn't good but not as low as I'd expect for pistons in that condition.

I'll try it out again this weekend. From the short time I ran the bike since it flooded my backyard with white smoke, it held idle. But I heard some serious popping sounds which I'm not really too sure of the cause. I figured popping might be due to pistons from some blow by but reading up on it. It could be the carbs. I haven't touched the carbs since previous owner worked on them.

I'll take a quick video on how the bike runs.

 

Popping back into the airbox? This is weak fuel mix.
Popping in the exhaust is a missfire, could be anything, but usually lack of fuel if the plugs and leads are good.
Open the mixture screws half a turn each, before removing carbs. I'm not saying you shouldnt remove them, but I wouldn't without trying this.

 

Tried starting the bike today but it didn't want to start up. Yesterday I was able to start it up no problem. Though I was using a different petcock. This cheap 8 dollar Chinese made petcock I found on Amazon to use to plug up the hole while I de-rusted the tank.

Today I rebuilt the stock petcock and installed it to the tank instead of the Chinese one. When I started up the bike, it ran for about 5 seconds then shut off. Granted the tank was quite empty so I added a bit more fuel, turned the mixture screws out about half a turn each but it just didn't want to start. Despite rebuilding the petcock, I notice it leaks gas even though it's set to ON. I'm not sure if fuel is reaching the engine or not or if floats are flooded.

I imagine the carbs are getting fuel as when I pull the fuel line, it's full of gasoline. Though since the petcock leaks, maybe the vacuum isn't pulling the amount of gasoline needed and floods the floats?

Any ideas? Before I pull the carbs and make sure the floats aren't flooded with gas from start up attempts

 

Do you have an auxillary tank for supplying fuel when you would be using vaccum gauges? Use that temporarily to see if it runs and keeps running? Missed the part about it keeping running with the Chinese petcock. Sort your rebuilt petcock.