The following was posted by me back in March:

I have a 1982 Maxim 650. The bike starts up easily and runs good, but I don't seem to have any top end power. Once the speedometer gets to about 70-75, it just wont' go any faster. This thing should easily top 100 mph. Not that I would want to, but I want it to run right too. I might be worth mentioning I weigh 225 lbs, but I would think it should handle that weight okay.

I've had the carbs professionally cleaned, acid cleaned the tank, new gas filters and gas, and new rubber intake boots. Compression is 125-135-134-135 across the cylinders, which I'm told is not bad for the age of the bike.

Any ideas on what the problem might be? I'm thinking either the valves need adjustment or maybe a bad diaphram in the carb, but wanted to get others insight. Thanks for any help.

I received lots of useful feedback, and since that time I have completely gone through all the carbs again. No problems with the diaphrams, the float levels are all the same and at the correct height and the carbs have been synched with a YCIS tool and a Morgan Carbtune and the mixture was color tuned. The spark plugs all have a light tan color, so the mixture is correct. New coils and HT wires, TCI is good and a strong spark is received at all plugs (which are also new). Yet I still have no top end power. At 60 MPH I can twist the throttle full open but the tach won't go above 6,000 and the engine really races (almost like it needs a sixth gear), but there is no additional power to go along with the racing noise. The engine seems to make a lot of top end noise and I also notice more vibration in the handle bars when this happens. At speeds less than 60 the bike just purrs. It starts right up, idles smooth and runs really well at lower speeds, but at highway speeds it's a pooch. Maybe I'm just spoiled with my 1450 CC Harley, but I'm thinking I should be getting more out of this, no?

Any more thoughts? Thanks in advance. Cary.

ok gonna try to add my 2 cents [just my opnion]. Never had any probs with my xj700 but it moves good enough my buddy with a 1200 shadow cant shake me and i got you beat on thw weight departmentby 50 lbs. Now we have a buddy that likes those gsxr's and got a hayabusa [?] that he let us ride for a day each [a great frind and we trust each other and are work buddies] . After rideing that my bike seemed slow. I would have the valves and timing checked as it sounds like something is breaking up at high rpm now does it rev out cleanly in the other gears?

Yes, it revs up cleanly in the lower gears. In fact, I can peg the tach in neutral with the bike on the centerstand. I'm leaning towards a valve adjustment, but I'm also going to check all the exhaust manifolds to make sure none of the seals there are bad. Thanks. Cary.

If the bike revs up when going down the road but speed does not increase, you might want to also check out the clutch system. You might have a set of bad fibers, springs or a glazing problem not permiting the clutch system to handle full power.

What Exhaust system do you have ?

It has the stock exhaust, which should be plenty to get the bike to 100+MPH (if so desired). Going to take the exhaust off this weekend to make sure it is not partially plugged or leaking at the manifold.

Need a little clarification on what you wrote in your post. You say something to the effect of "the engine races and makes a bunch of noise but will not go over 6000 RPM". It has to be one or the other. If it's racing then it's up over 6K already, or it's making a bunch of noise and will not go over 6k. The only other possibility with those 2 scenereos is that the tach is malfunctioning at precicely the same RPM that the engine is crapping out, not very likely.

There are a few "check this first" things to mention. If the exhaust or intake are restricted this will cause the engine to just run up against a wall at a certain RPM. Easy stuff are things like - pull the air filter element totally out and take it for a spin. After all the carb work I would just assume those are OK for the moment.

Checking the exhaust is a good idea, especially with a 20 year old stock system. I've seen old sets of pipes rust on the inside and fill the mufflers with giant flakes. If it's reasonable to remove the mufflers from the pipes and take it for a quick spin you may try that. If not then at least lightly bang them on the ground and see if you get a bunch of crap falling out.

Someone mentioned the clutch. If the engine is in fact racing (no matter what the tach says) but it won't pull the bike any faster then that's where the problem lies no dought.

Another possibility is that it is radically pinging (pre-ignition) at that load and that speed. That will be caused by a lean mixture, crappy gas, or the timing being too far advanced.

OK, before I have to publish this post in a doublebox set, get back and tell us if the engine is racing, OR will not run above that RPM.

Maybe "racing" was a bad choice of words. When the bike is sitting on the center stand, the RPM will hit 11K with a twist of the throttle. But when I'm going down the highway the top speed won't go above 75 MPH and the RPM won't exceed 6,000. Whilst this is going on, the engine starts to make a very loud, almost rattling noise, and a vibration is felt in the handlebars. It is a very distinct and audible noise different from the normal running sounds. It's almost as if the camshaft and/or valves are spinning so fast they can't keep up, which is why I said it was "racing". As soon as I back down the speed to about 65, the sound goes away and the engine runs smooth again. Maybe pre-ignition is the culprit. I

I have the exhaust off now. No cats or mice yet :). Will put it back together tommorow and take if for a spin without the air filter and see what happens.

Thanks again for the input. Cary.

I'd have to agree, it sounds like it's pinging to me also. Like I said above that will be caused by a lean mixture, crappy gas, or the timing being too far advanced, or a combination thereof. One easy thing you can do to check this is drain the gas out of the tank into someone’s car, fill the bike back up with brand new premium gas and add a full can (pint) of Turbo 108 octane booster. The fuel should be almost ping-proof at that point. Take it out for a ride and see if the problem occurs or is substantially better.

If I'm reading the above post right you say that this is happening about 6k RPM at 75 MPH (or so). You have all kinds of RPM left above that - what happens if you shift down?

Once you have determined that is where the problem is then we can work on figuring out what is actually causing the pre-ignition and how to cure it.

Well I'm happy to say I think I have the problem fixed. While I had the exhaust off Friday, I sprayed Deep Creep through the exhaust ports directly onto all the exhaust valves. Then I let it set over night. I put the exhaust back on last night and took it for a ride this morning and the problem is gone :).

Apparently, there was quite a bit of carbon buildup on the exhaust valves/seats that I couldn't get to by spraying deep creep through the vacuum ports or spark plug holes. It could also be there was some sticking valves, as the bike only has 14K miles, so it obviously has sat a considerable portion of it's life. This would have caused the valves not to seat completely, and thus I was losing power out the exhaust.

Now it throttles up smoothly and pegs the speedo easily and no more rattling noise from the top end. At 70 mph the tack reads 5,500, and if I pull in the clutch briefly it shoots right up to redline.

Thanks to all for your input. It gave me lots of ideas I might not have thought of myself. That was is great about this forum. :D

Cary.

Well, that's a new way to plug the exhaust! Always a new one....... ;)

Glad you have it fixed now.

Nice.

I may try that on my bike to see if I can gain any performance.

Mines seems like it lacks power of what I "think" it should have. I have never rode any other moptorcycle so I have no way to determine. COuld just be because its a small 650?

Ha...

But at about 75 my speedo sticks... I guess at that speed or higher doesnt really matter anyhow as Im already breaking the law :(

Cykle: during your next oil change, I would pour half a can of seafoam into your engine, and let it run for 10 minutes.

Also, next time you hit reserve on your tank, pour the remaining seafoam can into the gas tank, then take it for a run at 60mph on a highway for 10 minutes. Should help get rid of the rest of carb buildup.

Ed
XJOWNERS.COm