Is it just me or do Seca 650s throttle up and down with an immense lag time?

So I got a 650 Seca on Friday night for $300 including a new rear wheel and driveshaft because the one on the bike was locked up. After cleaning the carbs, I had a hell of a time getting them back on the bike, but through the miracles of elbow grease, white lithium grease, and a heat gun, they went back on this morning. Not wanting to mess up my newly cleaned carbs, I made a fuel container from a water bottle, and poured in some premium gas (non-ethanol gas isn't available near me) and primed the carbs. It started hard, but she eventually roared to life and settled in at about 1500rpm. I played with the idle a little, but it seemed to have no effect. I turn off the choke and it ran nice and smooth. I revved... Kicked it up to 3000 and let it fall, and it seemed to have a ton of lag. So I did it again, 3000, 4000, 4500, 5000, 8000... Each time it would take a while to respond to the throttle, but then kick up almost immediately, but would take a long time to get back to idle. Any time I revved over 6k, it would never fall back to idle without the kill switch, and would rest at 4000rpm until I turned off the bike.

I've found one thread about this and it seemed to be solved, but I'm at a loss, because my idle adjustment doesn't seem to do anything at the moment.

I have one other problem that is absolutely kicking my ass, and that is the locked up rear end. I jacked up the bike, and set it on jack stands because a previous owner put on a 4-1 header system that deleted the center stand. I removed the rear wheel and driveshaft, and unwrapped the other driveshaft I purchased with the bike. I compared them and they were identical except that the new one had a snap ring on the narrow end, and the old one didn't. I decided to reuse the old one, and it took a while to get it to line up with the teeth in the universal joint, but I eventually got it. I cleaned the brakes on the new wheel, and reinstalled it on the bike. After buttoning up everything else I had to take apart, I started the bike while it was still on stands, to test the new driveshaft. I put the bike in 1st gear, and to my horror, when I let the clutch out, the driveshaft or wheel would CLUNK on every rotation, and while I didn't see it myself, my son told me the whole bike looked like it was bucking with each clunk. We shut it down, and put the bikes away to work on next weekend. So now I'm here because I really don't have a clue what could cause the clunking. I added SAE 80-90W gear oil to the rear wheel, and the driveshaft has a sticker on it stating to use 80W oil there as well, but there seems to be no place to add gear oil to the shaft itself. I pulled the boot back on the universal joint, and it appeared dry inside, which gave me pause since I thought there was supposed to be grease everywhere in the drive mechanism. At this point I have no idea what's going on, but next weekend I plan to completely remove the drive side of the bike, from the transmission back to the wheel, and inspect everything. I'm hoping I don't find any broken splines, because that clunking was sounding horrible.

Any suggestions guys? My only other option is to load the bike on a carrier and take it to the shop, but I am remiss to do that because I bought an 82 so I wouldn't need to take it to the shop.

I think I've attached the video of first start today, which was actually the third start, which shows the runaway condition after a high rev.

 

That isn't normal.
You have a vacuum leak somewhere.

My preferred method is to use a can of carb cleaner. Slowly and methodically spray around the intake boots and carbs until you find the leak.

My thought is that the leak is at the throttle shaft seals, due to how it responds to throttle input. Though the leak could be at a vacuum port blockoff cap, or a cracked intake boot, or a number of other places.


The clunking is likely due to a failed u-joint. there is a rubber bood just aft of the engine that you can peel back for inspection. It is held in place by a thin coil spring wrapped around the engine side of the boot, so be careful when removing the boot.

Also:
Read This First
That link will get you a good start on understanding the XJ series of motorcycles.

 

From the sounds of it your carb synch is off , you will either need a 4 vacuum gauge manifold, or a two bottle manometer ( I prefer the vacuum gauges), I just finished resynching my carbs myself , if one carb or more is out it can cause a hanging idle or the idle mixture to have little effect on idle .

 

Thanks! I'm currently reading "In the Church of Clean" and plan on pulling everything apart again next weekend. My plan this weekend was just to get the bike running, and I did that, and now my focus is on fixing the rear end. I'm afraid I'm taking chunks off the driveshaft splines with that much noise, so my first order of business this week is to pull the rear end apart again.

 

1. Carbs....

- what exactly did you do when you 'cleaned' them
- sounds like they weren't' bench-synced, or running-synced either
-could be vacuum leaks but we can't' really get there til you have the carbs cleaned out right
-did you work with the idle speed screw at all
-did you adjust valve clearances yet

On gene in general, you haven't done a running sync, so that's the biggest thing here......and you're going to be adjusting the idle speed as you do your sun.

****you may still have lots to do here. If you need help, I you can call or text. Send me a private conversation and I'll give you my phone number. If you are completely beat,I also offer carb rebuild services.

2. Rear end

Sounds like your u-joint is set up

 

1. Carbs

I did a piss poor job of cleaning the carbs and I know it. My intention was only to get the bike running, then take the carbs down completely, re-jet, reassemble, and sync. The bike is running, and as I was running it on a water bottle fuel tank, I didn't have the vacuum tube plugged on cylinder #2. I capped that today and it ran a lot smoother, but I was just listening to the idle and didn't rev the motor at all.

2. Rear end

I came to this very same conclusion earlier today when I was daydreaming about the exploded parts diagram of my rear end. When I was assembling it the first time, it was difficult to get the drive shaft to seat, and offsetting it was the only way to get it to slide in, but it was crunchy. When I put it in gear, the whole bike flexed according to my kid, so today we jacked up the bike again, pulled the whole thing apart, sprayed the u-joint with parts blaster, greased everything and reassembled it. Still crunchy, but slid right in. I put the bike in gear and saw the flex my kid was talking about. The whole rear end is still knocking, so I shut it down and pulled the boot back. I can rock the u-joint backand forth, but can't rotate it in neutral. I believe the u-joint is seized, so this weekend we will be completely disassembling the whole rear end, and replacing the u-joint ($25 junk yard pull from a complete engine.) I'm debating whether or not to just buy the whole engine (it may be a xj750 motor rather than a 650 Seca) but I'm sure I can't actually afford an extra engine in my garage. Lol.

 

Sure you can...... We ALL have AT LEAST one extra engine in our garages here. Heck, most of us a have AT least one extra bike in our garage.... Lol

 

But I already have 3 bikes in my garage, and one sitting outside my back door. Lol.

I suppose, if the engine is cheap enough... But if it's a 750 Seca motor, and I have a 650 Seca, what parts are completely interchangable?

 

Swap the whole engine, then unswap the shifter cover and linkage.
Go to the information overload hour and scroll most of the way do and you'll find the engine swap section

 

Just curious why I'd swap the 650 Seca motor for the 750, when the 650 had better top speed, quarter mile times, and handling than the 750? I'm only asking after having looked up the times and I have no real clue about the 750 real life numbers, only a chart from a single website.

 

Because that chart isn't accurate, unless it was comparing the 650 Seca to the 750 Seca that had the touring package (and weighs quite a bit more than the naked Seca 750 as a result). The 650 is within 0.03/sec of the 750 In all the literature I have (nod going to the 750), and has a terminal speed that is about 10 MPH slower than the 750. As a practical matter there's no actual difference between the two, ulness you're getting paid for winning races.

Even if that chart were correct:
The 750 engine makes ~10 more HP than the 650, but weighs essentially the same as the 650 engine (IIRC they are within 5 pounds of each-other).
Now put that in your bike and what happens?
Same total weight, with 10 more HP.

 

The revving/ idle issue is likley because the pilot jets are plugged still and your idle is set so high it's running off the tube and needle not the idle circuit, this causes all the running issues you've explained. Buy full carb kits and do it right, than start over.

 

That's my plan. Waiting for pay day