1981 Yamaha Seca 750

[Lazerhead]

Hi.

I am thinking about buying a '81 Yamaha Seca. The biggest problem with it is there is a bit of rust in the gas tank and the owner tells me the carbs need to be cleaned out. I know a bit about motorcycles, but not too much. We have agreed upon $600. I also want to do a compression check on the cylinders to make sure the seals are still ok. I had two questions for any gurus out there.

1) Is that a fair price assuming the only things wrong are the carbs being gummed up and gas tank?
2) What should the compression be on the cylinders?

Any help would be appriciated
Thanks,
Steve

[palanon]

$600.00 sounds about right. There will be ton's more stuff than the carbs and tank though. Lots of little things that will pop up and take time. Older bikes are fun but, you have to have the time and enjoy working on them.

I don't know the compression for that bike. Try getting an on-line manual. If you can't find one, a good rule of thumb is no more than about 5 lbs. variance between the cylinders. Any more and it will probably need a ring job and possibly cylinder honing.

good luck !!

[robertc729]

sounds like a good project bike, being that old it is bound to have more underlying issues than what you know upfront.

[omegajim]

Figure if the compression isn't over at least 100 PSI you're gonna have trouble.

The math is like so

1 atmosphere is 14.7 psi, roughly 15.

8:1 compression would then be 120 psi.

Can't remember the exact compression on this bike, but I would be surprised if it was much less than 9:1 or nearly 135 psi on the compression gauge.

As for the inside of the tank, a good bike parts store will have "tank slime" or "tank gook" or "tank liner" which is a 3 step process. The important thing to remember when lining the tank is that you can't let the gook settle anyplace you don't want it. For example, the fuel petcocks must be removed and a rag be placed in place of the petcock.

Also, find a service manual and the guys around here will help you through.

Expect to spend 3 - 400 dollars to get things going, but that's okay, you won't be spending this much every year to keep it going (figure at least one tire, spark plugs, battery, might even pay a shop to clean out the carbs.)

good luck.

[Lazerhead]

I am still talking with the guy he just told me yesterday that he doesn't have the title, which really worries me. But he says he is working on getting it. If he can get it I think I am gonna make sure the VIN and everything match up.

As for the work to be done, I am hoping to spend less than $200 on getting it running. The tires look pretty new and he says that the bike was working alright last year. I will probably just pay a shop to do the carbs and try to do the gas tank myself. But I still have to do a compression check. If there are any problems there, I think I will pass. I want a small project bike, not a major overhaul.

[palanon]

Unfortunately a bike shop is going to be around $200.00 for the carbs and that's if they only have to clean them ( $50. - 90. per hr. ). It's a fairly easy job to do yourself if your mechanically inclined and have the tools and chemicals.

To do a standard screw in dial compression test you really need the bike to be able to run long enough to warm it up and to keep switching from cylinder to cylinder while removing and replacing spark plugs. This means starting and stopping the motor at least 5 times and probably 20 minutes worth of run time.

If you can't find the exact compression spec look for a spread of close to 5 psi across all cylinders. You could probably get away with a little more but hope for the best.

good = 125 132 128 135 bad = 125 132 115 135.

good luck.

[omegajim]

You can't check compression just by cranking the engine for a few seconds?

I remember it like it was just last year. Wait, it was last year. Anyway, long story short - tested an outboard motor by cranking for a few seconds and got pretty consistent readings. Also found out there were some bad cylinders, right away.

[palanon]

You can skip warm-up if you expect a bad cylinder and your just double checking. A cold comp. test can be way off. A cylinder that reads real low may come up after a warm up especially after sitting for a long time. you need to give the engine time to scape away or burn off any deposits and give the rings time to heat up and re-seat in case they stuck for what ever reason. Depending on how long it takes to check a cylinder, you may want to run the engine for a few minutes between tests. If the plugs and tool can be changed easily this won't be necessary. If you get a real low reading on a cylinder add a table spoon or so of oil to the cylinder and test again. If the reading comes way up you probably have a bad ring. If it doesn't come up, probably a bad valve.

BTW - take the spark plug wires off while testing.

If your going to do a test on a bike you plan on laying money out on, you might as well do the test right. $600.00 is still $600.00.

[Lazerhead]

Ok, so he finlly got the title and everything checks out. It's been signed over and the VIN matches the bike. I went out to his house today and tried to do a compression check. I was able to get some numbers but the battery was pretty much completly dead, and even with jumping it the cylinders would only pump once before the juice ran down enough to have to wait a few more minutes. Here is what I got

Cylinder one - 110 # Three - 85 Two - 60 and Four 90

However I am wondering how accurate the numbers are seeing as how it was hard to get the pistons to pump even once and the bike has been sitting for so long. I am hoping that once the cylinders warm up the numbers will come up a lot. I was thinking of trying again but this time with some starter fluid sprayed in the other cylinders to get some power while doing the check. Any other suggestions would be welcome.

Thanks,
Steve

[notordinary]

Hello, I have this bike, It is a very fun bike. My bike has 33000 miles and has gave me very little trouble. just the regular stuff. the compression ratio is9.2:1. The carbs are very easy to clean 4 screws to remove the float bowl a can of carb cleaner, maybe have a small needle on hand to push through the jets, just to make sure there is no blockage in them. $600.00 is a steal for this bike. i ride my bike almost every day. if its not snowing. good luck.