Main Fuel Jet / Pilot Jet Sizes

I picked up on Craigslist a 1982 750 Seca (43,000 miles) for 400.00 and am starting on a rebuild. Am I lucky I found this site! Hours of soaking up info has already saved me lots of time. I don't have much motorcycle experience, but would love to learn by returning this bike to it's proper condition/appearance.

First things first - clean the carbs. I pulled the carb bank and dismantled/cleaned the first carb (rack intact). Put everything back the way it was. I kept very careful notes and took many pictures along the way. I started on the second carb and noticed something that doesn't make sense. After I pulled the jets out of the cleaner and hit them with some compressed air, I noticed that the orifice of the pilot jet is a larger hole than the orifice of the main jet. I pulled the rest of the bowls and it's the same - the pilot jet is a larger orifice than the main jet. The threads for both jets appear to be the same and they may have been reversed at some time point in time. The screwdriver slots on most of the jets are all buggered up. Whoever maintained this bike was no professional by any means. I searched and keep finding reference to the pilot jet being a 40 and the main jets a larger number. But I don't know if jet size numbers are like wire sizes where the lower the number, the bigger the wire or vice versa.

I cannot trust anything on this bike as there is a whole list of things the previous owner rigged up or took off. For example, I could not locate the fuse box. When I found that it is supposed to be right under the seat, that happens to be the location where there are four large wires loosely spliced to four smaller wires. Obviously, the fuse box was removed and the wires spliced together with no fuses! (I have found the HOW TO for new fuse box) The igniter cover has one screw missing and the other three are each a different type screw. That drives me nuts! There is a birds nest of wires hanging out of the battery box. I was able to download the wiring diagram and should be able to sort this out.

Meanwhile, back to the jets. Shouldn't the main fuel jet have a larger orifice hole than the pilot jet? If so, the jets are currently installed reversed. (I'm identifying the main jet as the taller post with the washer.)

One more question - the top of the airbox, close to the front edge, there are two screw holes and about 8 inches away, there are another set of holes. It's obvious from the ridges near the holes there was something mounted there. I did notice some forum posts mentioning a baffle near the front of the air box. Maybe these are the holes that mounted the baffle? I included a picture that shows the 4 holes.

I am really looking forward to rebuilding the Seca. I can't say enough about this wonderful website. So much useful information and haven't yet run across an antagonistic thread - that's very refreshing. Sorry for the long post, but I'm trying to be as descriptive as possible.

 

Welcome to the site! Sounds like you got a bunc of work ahead. If you look at the jets the main jet should be a #120 and the pilot should be a #40 according to the yamaha micro fiche. The numbers are imprinted on the tops of the jets. They are really small so you may need to use some type of magnification. The main jet is the one that fits on the tall post. Hope that helps bud and good luck! Btw the holes on the air box are for chrome covers that go on there. If you look at my pics you will see them on my bike.

 

Wow, I think I may have found my problem here... reading your post.

While I was cleaning my carbs, trying to get this '80 650 Maxim back on the road, I found three carbs had #40 on the main and #110 on the pilot... one was the opposite... So I went with the odds due to my lack of knowledge... I made all the jets with #40 on the main and #110 on the Pilot...

I just bought this bike and it hasn't been running in 5+ years...

My problem is / was that if I gave the bike gas it would seem lean and bog out... If I feathered it up it would run up ok... I have a feeling changing all these jets back to the way they are supposed to be is going to fix my problem...

I made the changes this morning - put the tank back on, poured some gas in then went to get my keys so I could move the car out of the driveway...

When I got back to the bike, there was gas dripping down the right side of the bike... Dang it, what did I forget??? Oh, I didn't forget, I some how managed to poke a small hole in the tank on the underside, I guess rocking the tank back into position...

Gotta go get some JB Weld to put this fix to the test and then adjust idle mixture...

Got my fingers crossed...

Chris

 

Good luck with it, let us know how it turns out.

 

I find myself wanting to say "WOW" a lot on this site... I could barely hold on to the handle bars when I opened it up after putting the tank back on... Running a little lean right now, but I will take care of it Monday... I was beginning to think this 28 year old bike was never gonna run.

It is very important to know that the #110 goes on the main and the #40 goes on the pilot.

I rode it to work tonight... Ran like a champ! Idles a little high and still feels a tiny bit lean, but it's running better than it has since I have owned it.

While I am working tonight I pulled the tank off and am hitting up a second application of JB Weld to take care of the tiny puncture I put in the tank... Still dripping a tiny bit...

I feel like I owe you guys something, but for now I can just say thanks and share the experience...

I plan on learning a ton on this site... I have tons of questions already...

Thanks so far!!!

Chris
Chicopee MA

 

I take check or money orders or cold hard cash! I will send you my address in a bit! lol Just kidding! Good to hear you got it running better! Keep it up and RickCoMatic will be proud!

 

Hman.....it's still going to be running lean because the 750 engines should be using the #120 main fuel jets, not the #110 (the #110 size is for the 650 engines). I'll bet you that someone has swapped out a set of 650 Hitachi carbs for the original 750 Hitachi carbs, and that is why the main fuel jet sizes are wrong.

The 650 and 750 carbs are identical, interchange perfectly (bolt right up with no problems), but all of their internal jetting (with the exception of the #40 pilot fuel jets) is different from one another. That means that you really need to change the main FUEL jets (to the #120 size), the both the main and pilot AIR jets (up top, under the rubber diaphram), and the main jet Needle. Here is a comparison of the different jetting between 650 and 750 carbs:

650:
#40 pilot fuel jet
#110 main fuel jet
#195 or #205 pilot air jet (depending on year)
#50 main fuel jet
#Y-10 needle

750:
#40 pilot fuel jet
#120 main fuel jet
#225 pilot air jet
#80 main air jet
#Y-13 needle


Until you verify and/or remedy the jetting situation in those carbs, it will always run lean (if the jetting is the source of that problem), and running lean in an air-cooled engine is NOT a good thing..........you run the very real risk of burning up the engine via overheating it (i.e. burned valves, melted piston crowns, etc.).

 

Yes. I'm proud.
Every bike owner that comes in with a problem and leaves with a smile and a thank-you is another brick in the wall of Mission Accomplished.

 

It is a 650 Chacal...
1980 Maxim 650... But it wouldn't be running at all if not for you guys... Thanks... Hope I can return the favors as I learn more...

My next challenge is going to be fixing the starter... It just won't turn...

Took it apart at work tonight, cleaned a lot of crap out of it, but it looks like it is in need of some brushes...

I have experience with these electric motors...

Chris

Thanks again Rickomatic too... I still have to go through idle mixtures tomorrow night or Monday...

 

Whoops, sorry hman!....I read the first post in this thread and thought that was you!

 

Reading Chacal's listing of the jet/needle size comparison between the 650/750 explains everything about my carbs. I checked the needle, main jets, and air jet sizes and they are ALL consistent with a 650. Mine is supposed to be a 750 Seca. So now, I have to determine what I just purchased - either a 750 with 650 carbs or a 650 Seca. The side cover says 750 Seca.

I read there is a post somewhere that links to a chart that lists the engine number (mine reads 5G2-110700 stamped on the engine case) and will tell the model, engine size etc.

Off to find that link.

 

Yep, looks like an 82 XJ750J.

Here's that link: Vin ID

 

Thanks for the link Richard. I started searching for it, but got sidetracked when I found a link to the owner's manual I didn't have yet. Have been reading it.

So it turns out that I have a set of carbs from a 650 on my 750. Apparently, I will have to change out the jets and needle to get back to the stock sizes for the 750.

 

I just wanted to add my experience to this thread....
I've just stripped my carbs down this weekend and took a few photo's of my '81 650 carbs bits and pieces...here's what i found

#40 pilot fuel jet
#110 main fuel jet
#195 pilot air jet
#50 main air jet
#Y-10 needle

You can see the numbers stamped on the air jets below.
[justify][/justify]

And here they are in place
[justify][/justify]

And here are the fuel jets
[justify][/justify]

I took the pics for my own peace of mind (as i have never stripped a carb down before), it may help someone else out.
cheers

 

To answer you last question the holes are to mount chrome covers for the air box.