xj750 speedometer problem

Hi,
I'm new to this group. I recently bought a 1982 Seca 750 that had an instrument gauge with cracked glass but it worked. I bought a replacement from ebay and installed it saturday. The replacement gauge shows my speed at about half what it really is. The speedometer shows about 22 mph when I'm doing about 50. Even though the speed is off, the mileage on the odometer and trip meter is accurate. Anybody have an idea what is wrong? I suppose I could swap the speedometers but I hope someone has a better idea.

Thanks,
Steve

 

Perhaps the guts just need cleaned and lubed. Here's a thread by bigfitz that should help: http://xjbikes.com/Forums/viewtopic/t=15303.html

Not quite sure how the atari gauge cluster on yours comes apart, but the insides should be similar. Maybe someone who has an 82 with that cluster can help if need be.
Welcome to the best online forum there is! Plenty of knowledgeable people here to help and tons of information. To get you started, you need to check the rear brake shoes for delamination ( the lining separates from the shoe) http://xjbikes.com/Forums/viewtopic/t=1 ... art=0.html
Also check the date codes on your brake hoses (they only had a 4 year lifespan) http://xjbikes.com/Forums/viewtopic/p=3 ... tml#362577
Tires have date codes too. If more than 6 years old they should be replaced http://www.xjbikes.com/Forums/viewtopic ... =next.html
Safety first around here...we like our members to stay alive!

 

Welcome, as Moose said you would do very well to check (and ideally replace the shoes anyway) the brakes/tires/hoses before you ride the bike again. Could be nasty if a 31+ year old brake shoe fell apart at any speed... Plus (not wishing to overwhelm you, but these things honestly *are* required) you really should check and set your valve clearances as they tend to get ignored for years at a time on these bikes (due in part I believe to the perceived difficulty setting up "shim" based valvetrains, it really is not hard at all and a *lot* easier than replacing burned valves later. BigFitz52 has done a fantastic pictorial write-up on checking & setting the valves too, and Dave Fox runs a shim pool so you don't need to buy shims )

Also Bigfitz's guide for the speedo rehab should sort it, personally I'd swap in the original speedo head to keep the original mileage reading (but then I'm a bit anal like that haha )

Again welcome, you're in the single best place to get & keep your bike in good & importantly safe & reliable running order.

 

Thanks for the Speedo tips and the hearty welcome. This lookd like a great community to be a part of.

 

This.

Both that he's anal and just switch out the broken part.

New Speedometers and the tires occasionally need to be synched too, can't always just plug and play. Can't image that much of a difference tho would be accounted for an out of synch situation.

 

I second, that you just put the old guts in the new housing.

 

The speedo is probably reading low because it's gummed up to the max. Luckily the Atari comes apart allowing easy access to the instrument itself; and somebody has already thoughtfully posted the link to the instrument service article. There's no "synchronizing" or adjusting the speedometer, it should indeed be "plug-n-play." However, due to the purely mechanical design, they are very susceptible to becoming sluggish as the aging lube turns to sticky-goo.

I have to agree however, that there are a whole lot more important things that you need to be concerned about than the speedo.

 

It is very possible for the magnetic wheel that drags the speedometer needle along to have become weak. I would start with Fitz's suggestion of cleaning the speedometer, and then seeing if the accuracy improves. If it does not then the magnetic wheel becomes the next suspect. Ideally you would send the speedometer out to be re-magnetized and calibrated, but it can be done at home if you take care.

You will need:

1. a neodymium magnet that is not too large, nor too small, so it can make contact with the magnetic wheel at the back of the speedometer.

2. a short length of speedometer cable.

3. an electric drill with a known RPM

4. a clean workspace (absolutely no metal shavings present anywhere nearby)

You'll start by finding one pole of the magnetic wheel by holding the neodymium magnet above the wheel and allowing the wheel to come to a rest. Mark this with a sharpie, and the back of the magnet as well (when you see both black marks you know that the magnet is correctly oriented to the selected pole.

Use the magnet to rub the pole that you marked a dozen or so times (did you ever magnetize a nail in elementary science class? This is what you're doing now; remagnetizing the wheel).

After a dozen or so strokes of the magnet hook up the drill to the speedometer, using the short bit of speedo cable, and run it to the known RPM to check what the speed is given as. You will have to figure what speed should be shown based on the RPM that your drill turns. Japanese speedometers of the era turn 2240 RPM at 60 MPH.

Keep magnetizing the wheel in small steps until you get as close to accurate as you can, but without going over. If you go over you'll have to de-gauss the magnetic wheel and start over, and what you don't have is a deagussing coil (one can be improvised, but it won't work as well as the real deal).

 

bringing this back from the dead because I am having a similar issue 'k-moe'. anyone else done this before? not that i don't trust ya man just throwin this idea out for the other dogs to provide feedback

I've already dissected the cluster guts twice after rebuilding mine with a drop-in style similar to how Fitz did his. The magnet only causes the needle to rise only to a certain speed when I put an electric drill to the speedo cable. However, if I swap back to the original cluster and the same electric drill at same rpm the needle rises higher than my 'newer' speedometer indicates.

I must admit, some shavings may have been magnetized onto the magnet when I was doing my cluster rebuild several months ago so perhaps I need to clean it even better.

 

So yeah maybe I haven't been around enough to get a response from anyone, but that doesn't mean I don't ignore you guys. I could use some input from some gurus on this subject: I did a drop-in replacement like Fitz did with his speedometer unit. Only problem is the "newer" replacement speedometer doesn't read accurately.

The 'rise' of the speedo needle is not as relaxed as the old speedo; What I mean by that is the speedo will tell me that I am only going 35 or 40ish when I'm cruising normal 55-60mph. The odometer itself is accurate, I've double checked with a gps mounted to my bike. Also I've cleaned it properly and had taken it all apart twice to lube with Tri-Flow. Oh I also learned a lot about how the odometers mechanically work; the drop-in style speedometer I put in did NOT have the twist knob-kurl reset feature we have sticking out the side of our units. Originally it had the spring loaded reset feature on newer bikes - FZ1200 or early 90's style units - and I was dedicated enough to make it work like our units. Spent many hours just getting that concept understood to make it work.

Anyhow, back on subject: Pulled out the old speedo unit and inserted the cable into it. Unhooked the cable from the butt end at the wheel and inserted it into my drill. Gave it a whirl up to full speed with the drill and the old speedo will rise to 75mph give or take, but when I swap the cable into the new drop-in replacement and give it full speed with the drill, it only reads 55~60mph max.

What's the issue? Is it the circular spring's tension that sits on the needle shaft itself?

Here's proof to those who need a picture to believe me... its been modified beyond this point but its the closest resamblence to what it actually looks like, stored on my photobucket account currently.

 

The trouble is the lack of magnetism on the replacement speedometer. The spring should not be messed with, as it is wahat keeps the speedometer from going straight to 95MPH as soon as it spins. If the spring tention is the trouble it will typically cause the speedometer to read high, not low, as the spring can weaken over time.

It's pretty common for 30 year old speedometers to lose some of their magnetisim and cause the speedometer to read low. There are several places that will re-magnetize your speedometer if you don't want to go the DYI route.

This is one of the best shops for that sort of work (but not the only good one).

http://www.bobsspeedometer.com/1/120/index.asp

The advantage of having a shop do the work is accuracy, and most give a warranty on their work.