Runs rough and dies when engine hot

Hi, after days of reading posts on this site and seeing so many helpful people with good insight I had to join up. What led me here is a problem with my '82 Seca 750. I have had it tuned up, new plugs, wires, oil change. I had the carbs cleaned, twice, at a local shop that specializes in older japanese bikes. I have removed and inspected the fuel line vacuum diaphragm in the petcock and it is in good condition. I also installed an inline fuel filter. I replaced my fuse panel, as that seems to be the first thing to do when you have a problem with these bikes. I have not replaced the battery. When the bike is cold it starts up right away and idles pretty good. After the engine heats up a bit, it begins running rough and has no power at all when the RPMs fall beneath the 3000 mark. It becomes increasingly difficult to get the bike up to 3000 RPMs once this starts. I cannot accelerate through an intersection after about 20 minutes of riding and have to let the bike sit for about 30 minutes just to make it back home, which is never a pleasant experience as the problem doesn't go away. I've had the bike into a yamaha shop and they couldn't figure out the problem without extensive testing at $89.00/hr, which is a little much for me right now, and I'd really rather work on it myself if at all possible. I have been tossing around the idea of rebuilding the carbs, but if that doesn't seem to be the problem I don't want to do it only to find out I lost out on the time and money invested in it. If I run the bike at all with the petcock on "Prime" I quickly flood the bike, and it doesn't change the rough condition at all. If there is any other information I could give to help with a diagnosis I'll be happy to post it. Thanks in advance for any ideas or advice you can give.

 

Hey Shad, and welcome!

Are you using an excessive amount of fuel when it is running? Have you checked the oil to see if may have fuel in it? You may also want to pull and read the plugs for a rich / lean condition as this may point to carb issues.

 

If it floods out on prime you have a float issue the floats should stop the flow of gas if they are not then that is your problem it is loading up.

 

in addition to all-too-common float problems, it could be leaky enrichment valves (check the "choke" cable, make sure it's not binding and the enrichment plungers seat fully back in when it's disengaged!!)

look at the plugs. if they're sooty black, your carbs need attention.

word of advice... don't trust shops to "clean" your carbs. there's enough experience between all the XJ nuts on this site to help you do them yourself and do them right :mrgreen:

 

I'll pull out the plugs tonight and see what condition they are in. They were replaced a few days ago, so hopefully they'll reveal something by now. If it is indeed the floats, would a rebuild be the way to go? I remember seeing a post that had links to rebuild kits, I'll see if I can dig that up. Thanks for the quick replies. I'll let you know how the plugs look tomorrow. Also, I didn't notice any gas in the oil, it looked pretty clean when I checked it earlier today. I did have gas coming out of the number 4 carb after idling for about 10 minutes with the petcock on Prime, so that is probably a good sign of flooding.

 

I checked the choke cable, it seems to be fine. The plungers are seating fully as well.

 

I'd say your right! Sounds like a float either not set correctly or stuck open. You may want to tear #4 down and see what you have. I know I had to use some fine sand paper in the passage the pin runs thru on the floats to make sure it moved freely during my rebuild. Of course, they had sat dry for 10 years. 8O

 

So I pulled the spark plugs today. They're just over a week old and already thick with black soot. As far as the fuel in the oil, that's there too. I tried adjusting the carbs to lean it out, but nothing even close to spec setting was working. Had to leave the bike at work after trying to adjust the carbs, so at least I got in some exercise walking home. I'll do a few searches for sooty plugs and fuel in the oil after I post this. Is there someone on the forums who would have Carburetor rebuild kits for an 82 Seca 750, or a website that is dependable for them? I guess I'm not exactly sure which model number the Carbs are on that bike either, and all of the searches I've done to try and figure that out have been fruitless. So if anyone happens to know that, or a good spot to look it up, that would be greatly appreciated as well. Thanks again for the advice on this, you guys are great.

 

Well, I definitely have some pretty good leads now to getting this bike running correctly. Thanks for the advice, and thanks Chacal for being around with all the right parts so I can get these carbs rebuilt and running correctly.

 

You might as well pull the carbs and take the float bowls off and see what you got going on there. I'm sure you have dried gas and all kinds of debris in the float bowls and your jets.

The best way to clean those bowls I have found is to boil in lemmon juice. (don't do it inside your house though) Its faster than carb dip. Make sure you check the thread on old school carb cleaning by RickCoMatic. He has it written so well and it is very easy to follow. I also recommend taking a unpainted golf tee and chucking it up in a drill with the head cut off. Spin the tee up and clean the seats up for the floats. It polishes the seat so the needle really seals well or you could buy the carb kits with new needles and seats. I bought my kits from www.carbkitcapitol.com But I think Charcal may have them cheaper. Good Luck