Real World RPM Check - '81 Seca 750

Tina the '81 Seca lives and is legal. Still have alot of things to sort, but the engine speed is the most pressing issues is the one I hope I can get some input on.

@ 4K on the tach in 5th, indicated speed is only 40mph.

This bike has 53K on it, new plugs look good after 30 miles (since purchase and resurrection). Stock exhaust, K&N filter in stock box, but the box has a bunch of holes in the lid. Unknown jet size. I have not even got it to 60mph, it sounds I'm flogging it to do 50mph.

I've been off a bike for a number of years, and previously I rode XL and XT thumpers so this is all new ground. There is a little lash in the shaft, and the clutch looks a bit worn. The clutch does seem to slip a bit on takeoff. I can't wait until I meet someone who has had one of these bikes so I can get a reality check.

 

could be the speedo is wrong, mine never goes over 55, i'll look at it someday
i just use a digital bicycle speedo from wal mart
if your used to thumpers it's a whole nother animal, think 125 mx bike
don't worry it'll do 5k all day long

 

I have the 81 Seca 750, Almost stock (Kerker 4-1 header and rejetted to match)
5k rpm=65MPH, 6K=78MPH, 7k=85MPH. Speedo pegs at 85mph but I have been informed in another thread that 8.5K rpm is, or should be, around 110mph. I've had mine running rock solid at 8.5k for several miles on more than one occaision.

 

Borrow a GPS, or get someone in a car to drive 55 while you ride.
My 900 reads 115 at an actual 104 MPH.

 

Thanks. I'm still tweaking with the carbs, I have over-revving at times with full throttle closure. I think there's still some blockages from sitting. I'm hoping as things get cleaned out I will have better response. There's also the clutch-slip angle, but I really don't want to think about it.

 

I did a few GPS tests on my speedo today after work. My Garmin Etrex fits perfectly between the speedo and tach.
100km/h on the dial is 98.4 km/h on the GPS, below that speed it is 2km/h or less out, I expected more error. You can see in the picture 0 km/h is right on
Eric

 

plansea, is that your 650? i realize its in Kilos and not Miles but your speedo and tach and indicators dont look like standard 650 instruments...

 

Hi, Ltdave
Someone more knowlegeable than me would have to answer that. This is the only 1982 650 I know. I see many 750's with the square instrument panel, but my bike is the only 1982 XJ650RJ I see around here. The instruments sure appear to be the originals, the red on the tach is very faded and the km's look about right for the age of the bike. You can ride pretty much all year here . Let's see what the experts have to say !

Eric

 

Plansea,
Does your GPS give MPH when you have it on your bike? I was wondering if this would be an alternative to a bad speedo.

 

That looks like the original to me. It looks just like the xj650 gauge cluster I'm using on my xj550r, except my speedometer is in mph.

 

I have a couple of those clusters too. They are original.

 

I have an 82 maxim 400 I get 4200 rpm @ about 45 mph. I have a 5 speed trans and from what I understand this is normal.

I was also told that if I wanted to eliminate some of the HIGH REV i could get a bigger front sproket ( 1 tooth bigger) or a rear sproket with 3 extra teeth and that this will lower the RPMs . but was also told I would lower trhe power in 1st 2nd and 3rd gears and gain more speed in 4th and 5th gears.

How true this is I have no idea but I am sure someone here can confirm or discredit this information.

 

I have a saying drilled into my head, "Four'll get you five five." Means (in 5th), 4K = 55MPH. That's on my '81 Seca. I use it to make sure I'm cruising in 5th without clutching and tapping up.

I second checking the speed w/ a GPS. I've got 5-10% error in my speedo.

Is your clutch cable set properly?

What oil are you running? Long ago I put some non-Yamalube in and my clutch slipped like mad. My clutch slips a bit on takeoff, but then hooks up solid. I'm going to refresh the clutch/springs this winter. Then again, I said that last year, too.

Where are you located, BTW?

 

Yes, you change the units from km/h to mph or even knots if you are a sailor. I will do my next GPS test run in MPH and RPM so I can relate to the other information posted on here.

The GPS is a not very expensive solution and is useful for direction and a trip log, too !

Eric

 

Plansea,I am not familar with GPS. How is this hooked up to the bike - power wise? What I do know of it, it would be useful. Any suggests on the type of GPS which would be a good match. I have '81 seca 750 and there's little room on the dash, but my speedo is off. Can you easily see the GPS for reading in day/night?