81 seca 750 quit running and will not start now

i have rebuilt the carbs replacing many parts and following the directions on here to the letter.
got it all back together rinsed out the tank and then put in new gas. petcock is rebuilt with new parts and tested working. new fuel lines new vacuum line. started up on the first push of the starter today. idle just right with choke on and around 1k with choke off. went for a ride up the street and back. lots of power and just a little hesitation between takeoff and full throttle. drove back to the house and it kind of sputtered and died.
good flow of gas from the gas tank on all 3 settings using a vacuum pump to hold the petcock open. unscrewed both end drain plugs and i am getting gas out both ends. strong battery and good spark. it just seems to be getting only enough vapor for it to almost start. you know that spot when you are trying to get a motor going and it is just above idle but not enough to run on its own. doesn't matter where i put the throttle or choke, no combination gets it above that barely running stage.
where do you experts think i should start?
thanks in advance for any of you experts willing to help.

 

Try turning the idle screw in a bit to make it idle a little higher. Sounds like the idle may be a little low under warmed up conditions.

You might have to wait a bit, you may have flooded it by messing with the Fuel Enrichment and Throttle a lot. I've done that, its frustrating.

 

thank you for your quick response.
i waited a few hours and it cooled down till the fins were completely cold.
could it be flooded after that long?
should i pull the plugs and make sure they are dry?

 

Did you install a fuel filter?

 

hey, thanks for your post.
I will assume you mean other than the factory filter? if you mean an inline one, the answer is no.

Could someone show me a a photo or video of how they got it to fit under the tank?
thanks in advance.

 

Inline fuel filters are etremely easy to install. Just make sure you have the right size before you cut your fuel line to put it "inline".

MN...are you thinking the filter may be clogged?

Narf...what happened when you tried to start it cold? I would check the spark plugs...make sure you are getting spark there , too.

 

How did you adjust the float levels?

 

thanks for your reply.

i measured them while rebuilding, then checked them wet while installed. they were soot on with the tube method.

remember it ran fine for a couple of weeks.

 

Just a thought; if you saturated the plugs it might explain it.

Crud clogging a jet or needle valve could also explain it.

Just for poops-n-giggles, throw in a new set of plugs.

 

damn. never in a million years would i have thought it was plugs. replaced them with properly gap'd ones and it started right up. runs like a champ and the hesitation is gone.

thank you so much for your help.

 

awesome !!!

 

You're more than welcome. I've been there; for some reason the XJs seem to be a lot harder on plugs and a lot more "plug-sensitive" than other motorcycles (mostly twins and singles) I've maintained over the years. Glad I could help.

 

I always recomend a small inline fuel filter. You never know whats or could be getting by the sock filter in the tank. Glad the plugs did it for you.

MN

 

what sock filter in the tank? the only factory filter I'm familiar with are the gauze screens INSIDE the carbs. The teeny tiny little bastards hidden behind the fuel needle seat that plug up if you so much as look at them cross eyed.

That's reason enough for an inline filter between petcock and carb inlet

What Fitz said (I should hot-key that phrase, I type it a lot) about plugs though my own experience seems to be yamahas are sensitive to proper gap more than other makes.

 

Check the petcock article. There's supposed to be a plastic-gauze filter "standpipe" arrangement sticking up out of the top of the stock petcock. Quite often it's gone missing.

 

no problem with the filter, has the regular plastic stand up pipe they all have.

and i did recheck the old gap, it was spot on. the old plugs just look black and fouled.

thanks again for your help. i still would have pulled off the carbs again before i ever tried replacing the plugs.

 

oh THAT... never did trust them things since I had one of them reserve straws come loose and MAIN/ON turned out to be drawing from the bottom of reserve. I just pretend that little thing sticking up in to the tank from the petcock doesn't exist. My life's been simpler since.

 

"Pretending it doesn't exist" won't help much if it becomes clogged; but as far as its ability to filter...

we'll just call it a "pre-filter."

 

Thank you again for you suggestion. i just haven't found an inline filter that will fit and allow vertical flow like exists already. if someone could suggest a place to purchase one or a part number or a picture of how they install it, that would be really helpful.

Since we are making this a discussion about those little plastic vertical filters, I wonder why Yamaha, Suzuki and Honda, use them in all their bikes, with great success, from the factory. And people ride for many tens of thousands of miles without running out and adding an inline filter when they purchase those very bikes.

I am also wondering about flow restriction with an extra filter, since these bike are just gravity feed. I wonder if there is any effect on open throttle long term like perhaps if you were racing. But i suppose the floats hold quite a bit of fuel so it probably wouldn't matter.

I do think it is a very interesting discussion. perhaps someone should open a new thread about "failing factory filters" or something like that.

thanks again all.

 

factory screens don't fail. they just can't do as a good a job on the rust particles.

people rides thousands of miles becuase the bikes are new with no rust.

after 30 years you have rust inside the tank. an in-line filter does a better job then the factory screens. the filters are usually glass filters and are 1/4 inch as are your fuel lines. and measure about 2 inches in length. they can be bought at any bike dealer or bike accessory shop. auto stores sell a plastic filter measuring the same. they fit very easily in the fuel line between the petcock and top of carbs. or if inclined, can fit between where the fuel line runs through the carbs and the tube that feeds the carbs.

they are basically a triangle shaped filter with a nipple on each end.

you might have to be creative but they will fit.