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.

 

don't have too much rust problem here, even in old tanks. But I definitely see your point.

Thank you for the description, but a picture is worth a thousand words. if someone could take a picture of theirs, since everyone has one installed , i could get a better idea of what i need to do. like if i need to take the tank off or buy new hose.

Thanks again for your help and suggestions.

 

First off: the rust in the tanks. Yamaha had a problem with their production of bikes in the early eighties: They were built quite a bit ahead of time, in large runs, and as sales lagged they SAT for months at a time, often on the docks. As a result they also had a widespread problem with rust in the fuel tanks on the brand new bikes. There were a series of tech bulletins advising the dealers on how to deal with the rusty tank issue, eventually requiring the process be performed on certain lines of bikes. The XJs were one of those.

In other words, these bikes CAME WITH rusty tanks in a lot of cases. The effects of ageing often just exacerbate a "pre-existing condition."

Neither the nylon gauze thing in the tank nor the beanie screens in the carbs can do anything with super-fine rust particles; I'm talking about paint-pigment fine particles. Hence the need for an inline fuel filter. It's pretty much a "given" on these bikes.

Inline filters come in different sizes and configurations. I have the straight-through, smaller diameter ones on both of my 550s; and the larger, 90-degree one on my 650 Seca and had no trouble tucking them in so they not only look "factory" you can't really even see any of them unless you're looking for it.

Here's the one on my '83 550 after I first installed it:



(I have sinced trimmed the "downstream" hose a tad to put the filter in a slightly more "downhill" orientation.)


The filters in this pic are the ones that chacal carries; they're very effective and are the ones I use. That being said, they are also available from a wide variety of sources, although some work better than others. You want the "bronze stone" type, not the paper element type.



As mentioned above, I use the smaller straight ones (on the right) on the 550s and the larger 90-degree one (on the left) on my 650 Seca.

I'll get you a pic of the one on the 650 in the next coupla days. (the above pic from my petcock how-to)

Highly recommended.

 

that is awesome. thank you so much. where do you source your filters? can you purchase them anywhere locally?

p.s. the 750 filter is a little different for those with 550/650 who aren't aware.

p.s.s. DAMN that is a clean bike!

 

Forum member chacal is the proprietor of XJ4Ever.com, our resident parts source. See the logo in the banner at the top of the page.

I got my filters, plus the OE fuel and vacuum line, from him (those are some of his other parts in the pic as well.) You can sometimes find that style of filters in an auto parts store as well. Just avoid the paper element type.

And thank you, that bike was a year-long recommissioning and updating project that resulted in:



One of the sweetest and without a doubt most comfortable bikes I've ever owned and I would not have been able to do it if not for chacal and XJ4Ever.

Honestly.

 

that is very nice. i have bought quite a few things from Chacal as well.
here is my project. not near as good as yours but its ok. :lol:

 

It's more than "ok." Very nice. +1 on the stock exhaust.

 

Look great, just be careful of the tire dressing as centripetal force will
work its way toward the tread.

 

thank you for the compliment. I spent a lot of time working on it. This is what it looked like when I got it.



I just did that for the photo. I wipe off the extra when I am done. Besides it all comes off when i peel out..... :lol:

 

You've done a lot of work.

Might I recommend a couple of additional appearance-care products that will help you improve it even further?

First off: The original black Yamaha wheels-- love original Armor-All, just keep it off the tires. Scrub 'em clean and use original AA on them, I use q-tips to get the nooks and crannies. I used my Dremel on the shiny parts of the rims.

For all that blasted faded-to-gray black plastic: Mother's "Back-to-Black." Apply, rub into surface, WAIT. Buff. Repeat if necessary. If the black plastic is really badly faded to where you can scrape the dead plastic off with a fingernail, careful use of gray Scotchbrite to remove the dead surface beforehand works wonders. The airbox, rear fender and plastic side trim pieces on my bike in the pic were as gray as could be; I brought them back to life using the method outlined above.

 

thank you so much. I will try those suggestions as soon as I can.

A question about your bike, those plastic covers over the part of the shocks that move, are they stock? or how do you get those on? or where do you find them?

also, how did you get the black on your wheels so black and the shiny part so shiny? is that what you were talking about above?

Thanks again.

 

You mean the fork boots? No they're not stock. Why, escapes me. They used to be standard on everything, back in the day. Every single bike in the 1968 Triumph and BSA catalogs has them.

In my case, they're a Kawasaki part (thanks, Wiz!) that happens to fit the 550 Seca perfectly, just by pure coincidence. That particular part number probably wouldn't fit the 750's larger forks. HOWEVER, somebody, I think it's Polock, has a set on his 750.

They're known as fork "boots" or "gaiters" and come sized by ID top, ID bottom and length. Relatively commonly available with some digging.

You do have to remove the forks to install them.

And yes, in regard to the wheels that's what I was talking about. As long as the black is still there and hasn't been scrubbed off the high spots, all you need to do is get them nice and clean (a soft nylon brush and car wash soap, you don't want to scrub the black off cleaning them) and then Armor-All them. Use a q-tip to get into the nooks and crannies. OK, four dozen q-tips (and about 3 beers.)

For the ribs on the wheels, look closely at the way they're made. The machining to cut the tops of the ribs back to silver is rather coarse, leaving a series of machining grooves perpendicular to the spokes themselves and a beotch to get clean once grey and stained.

Dremel to the rescue. I use my "extendo thing" because the handle is quite a bit smaller than the Dremel itself, and allows me to get the head of the tool nice and parallel to those machining marks. Then the spokes can be polished in the direction of the machining and you can get them nice and clean.

You have to be careful not to over-do it and clean off the black on the edges of the sides of the spokes; you just have to take your time and be precise.

I use a flat stitched-cotton (not the pressed felt) mini-buffing wheel on the Dremel with Solvol Autosol polish (German import, widely available) to get the initial cleanup, then finish up with Mother's Billet Metal polish.

The same technique is used on the rim area, but it's much easier, and if you slowly spin the wheel "into" the buffer it works quite well. Once again, be careful not to clean the black off the immediately surrounding area.

Once polished, they can be easily maintained by hand.