Jerrod's 82 Yamaha XJ-750 Maxim

Good to hear it is currently OK - the symptoms don't quite follow but sometimes things get lost in the communication.

Oh, and you have about 5 recent threads all on the same bike. Generally, it is best to keep your bike in one thread as the community more readily recognizes the history on the bike as well as getting a feel for the general knowledge of the owner. You can also add your bike to your signature to make it easier to for the community to know the model and year.

 

A rich mixture cools the combustion chamber.
Check your valves.

 

Merged the threads, and added the model and year to the thread title.

 

Even if you clamped a garden hose to fit, the fuel will still be restricted at the petcock nipple and the carb inlet nipple, either will flow all the fuel you need.
At what RPM is the dead spot occurring?

I haven't seen anywhere if you have the original airbox in place. You mention air filter, as in one air filter so I assume you have the air box in place, that is good.
The key words in your quote above "Just bought an 82 750 Maxim". Unless the PO had the bike running properly very few will get away with a few tweaks and get it running properly. My advice is to remove the carbs and thoroughly inspect them, then thoroughly rebuild them. Check the valves while you're waiting for the carb parts.
This must be approached methodically, valves - check, carbs - check. You know this, you're already riding and wrenching an old Honda.

 

Stock airbox, the filter was this big square thing, spent about an hour on with the air compressor and got it back to decent shape. Ordering a new one tonight. All stalling and backfireing has completely stopped. And it's running pretty good, unless I try to get into it. And at the upper end of the rev range where the power should come on is the dead spot. It did start a little lower down originally, but I took the lawnmower fuel line it came with off, and put on something that flows a little better. Dead spot is not as large, buy is still there.

 

It rides fine, cruising around town is no problem. You would think it was good until you get into it. It doesn't fall on its face, but there's no power band. I know it's not a 2 stroke, but that's the best way I can think to discribe it. It's like the top quarter of the throttle is unaccessible. It ran all the way up to 90, and had a little more to go. It just took it a little bit to get there.

 

Feels like it's not getting enough of something. Runs the same with the air filter in, as it does out. So it's not that I just didn't get it clean enough. And just so everyone is aware, I am not a Machanic, i rebuilt my CB's head learning as I went. Spark plugs in the XJ were black, I cleaned the air filter, and the plugs and now their on the darker side of golden brown after about 3 hours of putting around. Did not stall once, did not backfire once. So I consider that case closed. I do know that valves that are too tight can kill power, and cause overheating, but I was told that Lash is very unlikely to tighten over time. It will normally loosen. And it has no valve chatter to speak of. It actually sounds great.

 

The rev is very responsive and quick, it just has the dead spot. Maybe an issue in one of the secondary jets?

 

Proper maintenance. That's what it's not getting enough of. And that happened before you bought it.
Start from the beginning of the service manual and go through everything.
Also read this: The Information Overload Hour

You were told wrong. That's only true with valves that are actuated by rocker arms. The XJ has none. The valve clearances tighten as the valves slam into the valve seats over time, until a point where the seats are esentially set into place almost forever. Your machine is nowhere near acomplishing that.

 

Service Book is in the mail on its way, Has alot to do with why I haven't pipes the valve cover off yet. My very limited knowledge, is on a SOHC CB-750/4. Which indeed has Rockers. I'm in no rush here, seasons about wrapped up here in Ohio, and I have a running bike for whatever nice days may be left. And yes, this bike was neglected for the past 3 years at least. He was riding this thing around with no exhaust system at all. It had the downpipe's that lead from the head to the collector, everything was completely rotted past that, the collector itself had no top at all. It had completely rusted away.

 

Popped* the valve cover off.

 

The riding season in Ohio isn't wrapped up for me. At least not if this winter is a repeat of the last. It hardly ever got cold last year. You're a bit further north though.

The collective will help you get your XJ running like a brand-new machine.

 

Im about a half hour north of Columbus, and I don't plan on winterizing either of my Bike's. I'm gonna take em out once a week probably until January. And once the snow comes, I'll just start them up. Not interested in hitting a patch of black ice on 2 wheels lol, no thanks.

 

The great thing about our bikes is that the more miles they do the fewer shim changes they need, they still need checking though. I had a CB 750 K2 and I didn't bother trying to get a feeler gauge into the gap. I used to just slacken them of and reset them.

 

Bingo. Honestly, with screw-type adjusters on rocker arms, that's the best way. But you are correct; once the valves are in spec, and the engine is broken in, the "pounding into the head" lessens and all that is often needed is a simple check.

The absolute most important valve clearance check and adjustment is the FIRST ONE, which was supposed to have been done at 600 miles, then again at 3000 miles (that is considered "initial break in".) Then it relaxes to every 5000 miles. This was COMPLETELY NEGLECTED on a lot of these bikes because of the expense involved in having a dealer do it. And it's why you find 7000-mile XJs with 7 out of 8 valves TOO TIGHT.

Valve adjustment is the single most critical factor in the early service life of these motors, and if it has never been done then any bike with over 3K on the clock is not only going to need it, but the more miles beyond that it will begin to lose performance gradually until you end up with a bike that seemingly resists all efforts to get it running properly. 'Tain't gonna run right if the valves open too early and stay open too long.

You cannot simply ignore it, things will just get worse.

 

Winterize them anyway. Startups during the winter wthout actually getting the engine to work leads to condensation in the exhaust and crankcase, and acidified oil.

 

So disaster has struck kind of. My 1 and a half year old decided he was gonna give the adjuster knob on the back of the Yamaha's carbs a few good twists. And it's running really bad now. I had to shut it down, becuse I started hearing valves tapping. An it was running hot as hell. Didn't seem to want to rev up either.

 

1. You're supposed to hear the valves tap.
2. Have you checked the valve clearances yet?
3. It's not a disaster.

 

I'm not like freaking out or somethin lol, but they haven't made a sound up till then, when I started it they were good, and after about 5 minutes of running they started tapping. And the motor is very unhappy. And no, not yet. I was waiting on my service manual to come in the mail before I did anything else to it. But the boy kinda changed that lol.

 

It's reving slow, almost like a cylinder isent fireing, But all 4 are. And seems to be running hot. It's not happy at all.

 

Ok just got home and started it for a second. And the valve tap has gone away. So I'm officially taking a step back. Need to learn about this screw he was turning, pretty sure it adjusts the air fuel mixture. But I need to do some studying up before I go turning any screws I'm not familiar with

 

yeah that. and quit trying to run the motor we already know needs its valve clearances checked. The big knob on the back of the rack is the main idle adjustment for the whole rack, essentially it operates all 4 carbs' idle setting. If the bike weren't out of tune, the results of turning it that far would be more conclusive. Idling too low means insufficient oil pressure so the motor will be unhappy. Let me say this again: QUIT RUNNING IT until you have a better handle on things.

 

If you have not checked the valve clearances how can you be certain the noise is coming from the valves?

 

Talked to previous owner last night, and he says they were adjusted when he got the carbs rebuilt 2 years ago. But I'm not so trusting in him, so I checked them today. Counting from left to right, Cylinder 1- Intake: .004. Exhaust: .004- Cylinder 2- Intake: Between .003 and .004 Exhaust:.006- Cylinder 3- Intake: Between .002 and.003 Exhaust: Between .004 and .005- Cylinder 4- Intake: Between .004 and .005 Exhaust: .004

 

Are those measurements in inches or millimeters? (I actually think that I know, but again never assume).
If you're using an inch set of feeler gauges invest $12 or so in a metric set (KD Tools has a good set, available online or at NAPA). The feeler gauges in a SAE (inch) set just aren't as easy to use, nor are they as useful when determining the go/ no-go state of a shim on the shim table.

 

That's Inch/Thou. I have 3 different sets, the one I used just says OEM TOOLS, it has both MM and Thou reading on them. They go from .0015 up to .024. And yeah they ran me about 12 bucks. I'm not sure what the factory tolerance's are supposed to be, but if I had to guess based on what they were, I'd say 4 thousandths Intake side, and 6 Thou exhaust, maybe 5. But the lash was all over the place no 2 intake valves were the same. And both the Intake and Exhaust were both 4 Thou on the number 1 cylinder, which can't be in spec.

 

My Grandpa was a Tool and Die maker, and he thought me in Thou. So that's where I feel comfortable. Never really used MM.

 

The math is easier. I understand though. Dad was a machinist, and my lathe is from the mid 50's.

 

Bigfitz's AIRHEAD VALVE ADJUSTMENT with Pics - parts I & II

You will need to pull the shims one at a time to find out what's installed. Then you can use the chart to find the new shim that's needed. Keep in mind that you will very likely be able to swap shims around so you will only need a few replacements. @hogfiddles runs the shim pool, so don't go out and buy shims before contacting him.

 

Good advice, and yeah you've got some tight valves there. Big problem with using "imperial" vs. metric is that on the low side, a valve can be "in spec" using the imperial equivalent, when in fact IT'S NOT measured properly, in metric.

A set that is primarily metric is best for this, honest. I'd give you the specifics, but K-D sold out to/became Gearwrench so I don't have the part number anymore.

Those clearances are so "all over the place" that I strongly suspect your PO lied big time. Good catch.

 

Thank you, you guys are super helpful. It may be a few weeks before I'm working on it again, waiting in my service manual, and a few parts. Like a new set of intake runners becuse it appears someone attempted to do a polish job on the ones I have, which I've never heard of someone doing before. Which reminds me, what would the outcome of someone rounding off the edges on the inside of the rubber Carb boots? It looks like they tried to do a port and polish job on the intake runners. And I don't think it was a good idea...

 

I don't wanna turn the motor over 1 more revolution, until I get the new ones on it. Supposed to be here Wednesday.

 

If you mean the carburettor intake stubs be careful as the bolts can shear off when removing them.

 

Yeah, I've seen at least 4 different names for them when buying new ones. And I already have them removed, with rags in the intake port's until the new ones arrive. I'm thinking what has been done to them may have alot to do with how the bike is running. Parts like that, are the shape they are for a reason. Plus, they don't have seals, gasket sealer was used. Which to me is unexcusable, these parts are available and cheap. Using gasket sealer there was just lazy.

 

Yes should be no gasket sealer. Good they are removed with no broken bolts.

 

Figured I'd put up a few before and after's, not close to finished yet. But I think it's a pretty good start

 

I think he did too, but he knew I was fixing it up, and there was no real back and forth. But it's you guys who really pushed me to check. Thank all of you.

 

Had a fouled plug, so I replaced them today which helped it quite a bit. But this is what the brand new plugs looked like after about 5 minutes at idle. It appears my issue is in that number 2 carb. I have a friend that is a Machanic. I have to replace the starter, and then I'm hauling it to his place to dig deeper

 

I'm looking at these 30mm aftermarket carbs online, does anyone have any experience with these things? Or are aftermarket carbs more trouble then their worth?

 

All right guys, a bit of an update. I have discovered that my number 2 cylinder has low compression. Not sure if it's the valve seals, or the rings but I am about to start disassembling the top end. Going to replace all rings, and all valve seals, and will also be installing the correct shim's on reassembly. Is there anything I need to know outside of the normal everything must be reassembled in the order it was disassembled, and all parts need to be organized and labeled?

 

For example, my CB has to be reassembled dry, without any gasket sealer.. Just things like that I may not know. But could be a nightmare later on.

 

Umm....none of those plugs is indicating an acceptable fuel mixture.

 

STOP!!!!
STOP!!!!
STOP!!!

Low compression is often due to improper valve clearances, and/or stuck rings.
DO NOT take the top end apart yet.

If the bores are worn enough to equire new rings then you likely need oversize pistons, and those are only available as a custom made part anymore.

so...

STOP!!!

1. Valve clearances first.
2. Drop a tablespoon of a 50/50 mix of ATF and acetone down each sparkplug hole, and let it sit for a few days.
Run the engine for a several minutes.
Recheck compression.

 

Plus one for kmoe's advice there, don't rush taking this to bits. I would try riding it hard for at least an hour as well - you'd be surprised how this helps valves and rings bed in - when suffering from being idle as well as new. Then re measure clearances, get someone do a leakdown check as well as a compression check (assuming your valves are in spec). Slowl slowly catchee xj....

 

All my rings are fine, and all my valves are operating properly. Problem ended up being that both the Intake and Exhaust valve seals are worn out in the number 2 cylinder, it's going to a good friend of mines garage in Monday, and he's going to replace all of my seals. I could probably get the job done, but he does this kinda stuff for a living. So I figured I'm better off letting him take care of it.

 

A question I did have though, is there are these plastic sleeves that are on the cylinder studs. And they are all super brittle and coming apart. I'm sure their probably important, what are they?

 

Intake and exhaust seals worn out?
Firstly valve "seals" are there to stop oil weeping past the valve stem into the bore, they do nothing, zero zilch to affect compression.
Now, you may be meaning that your valves aren't sealing (ie closing tightly)? In which case go back up and re-read what k-moe told you, and I repeated -ride it untill it's good and hot. If you can't ride it then start it and warm it up as hot as you dare at least 4 times. Then recheck you valve gaps, and your compression. If you still have a problem then strip it, but there's mire than a good chance your issue will have gone away.

 

The engine studs are exposed to road dirt as you can see the studs if you look at the front of the cylinders. I think that's why the plastic sleeves are there. Someone said it was to stop them making a noise. Don't know if that is applicable or not.

 

Given that he mentions the plastic sleeves on the engine studs, he ignored us already and tore it down anyway.

I think seals was a typo, and he meant to write seats. In which case....I've never seen or heard of a valve seat being damaged at such low mileage, and certainly not all of the valve seats in one cylinder. It is possible though.

@Jerrod Robinson We never did hear back about the valves being adjusted. Was that done?