1981 Seca 750 carbs, really stumped

Bought a low mileage 1981 Seca 750 from the original owner. Trying to get this thing running right and I'm really stumped. I've had the carbs off this thing 5 or 6 times. I have completely cleaned the carbs out. I put in brand new pilot jets, main jets, needles & seats, adjusted the floats, cleaned out the air screws and reset, there are jets on top of the carbs under the chrome cap and under the half moon cover,, those were taken out and cleaned, there are two jets on the back side of the carbs, I took those out and cleaned them. All passageways seem to be open as I can spray carb cleaner thru them. The carbs were originally leaking at the fuel rail. I bought new O-rings and replaced them and cleaned that all out and replaced. There is no more leak. I cleaned out the fuel tank and have non oxygenated fuel with Seafoam in the tank. Lastly, I put in all 4 new spark plugs. Strong spark all 4 and good compression all 4.

The bike continues to run mainly on 3 cylinders. As you are sitting on the bike the furthest left cylinder isn't firing with any regularity. I put my hand over the carb as the bike runs and it has great suction. The run characteristics do not change if I put my hand over that single carb. If I do any of the other 3 carbs the bike will start to flood out. I've adjusted this single float at least 4 times with no change. The float moves freely and the needle is not sticking. Additionally, the float bowl always has fuel in it when I take the carb bank off. When the bike is running, if I manually squirt fuel into that carb, the bike will run on that cylinder until it burns the fuel out. Another squirt and she lights that cylinder up again.

I'm really frustrated with this. I've been in carbs many times including my 1982 Seca 550, many other motorcycles, and snowmobiles as well. I've always been able to figure things out but this has me really stumped.

Any advice on where to go with this can only help as I'm at a loss....

Thanks in advance.

 

no fire in #1 cylinder. have you removed the spark plug is it wet?
try triming back the sparkplug wire 3/8 inch.while you have the plug cap (boot) off ohm it out.
, tighten the screw inside if cap is loose.
have you tried a new plug?
what wet set level is carb set too? or are you dry setting the float height

 

Have you adjusted the valves? Checked compression? Take off the plug lead on cold cylinder trim some off and reinstall ...are you running stock airbox or pods...? No fan of pods here. Are your intake boots in good condition(airleak) ? These are all possible problems.

 

Sorry xj550 replied at same time....

 

Are you getting fuel to that carb? Could a chunk of o-ring come loose and clogged the passage? Try checking by opening the float bowl drain while the bike is running....maybe compare it with one of the other carbs.
Seems to me if you have spark and compression the only thing missing is fuel?

 

That's what I'm thinking. Fuel, air, spark, compression. It's pretty simple. The float bowl is full every time I remove the bank of carbs. The plug is dry. Again, if I squirt fuel manually thru the carb, that cylinder then lights up. I've been thru that one single carb at least 6X's and I can't imagine what is holding it up from delivering fuel. I've checked every passage and either cleaned or have replaced every jet in the carb. The needle & seat are brand new and I've adjusted the float up/down every direction you can imagine many times. I'm totally miffed.

To answer some of the above questions: 120+ psi in the cylinder, the O-ring came off the fuel rail intact, no evidence of debris in that passage, crisp spark. I will take a closer look at the intake boot but it looked good.

 

Based on the assumption that the pilot passages and enrichment passages really are clean, and that the float bowl on that carb is filling, then the other thing to check is the fuel level in that float bowl (really should check them all). If the float is set to shut of fuel too soon the pilot circuit will never be able to suck fuel.

Setting Fuel Levels.

The only other thing it could be is the intake valve on that cylinder not opening, or not opening enough, so check your valve clearnaces.

 

Dumb question...is the starter jet clear? If so try swapping the guts with a carb that's firing. Troubleshoot from there.

Gary H.

 

Does carb cleaner squirt from the starter jet?

Gary H.

 

One other thing ..have you swapped float bowels? If enrichment jet is pluggged this might follow to next cylinder.

 

The carbs are still dirty and plugged. Clean them, take the jets and emulsion tubes out . Can you see light through the jet hole? Are the little holes in the emulsion tubes clear? There has to be a clear path for the fuel to travel in order for it to work. Clean the carb slides and needles and make sure the diaphragms don't have any holes and they move freely.
If the fuel level in the bowl is correct and the carb passages are free and clean then it will work.

Don't forget to check the valve clearances , they are just as important as clean tuned carbs. They depend on each other.

 

Pull the idle fuel adjustment needles. If you haven't yet. Sounds like #1 pilot circuit is clogged still.

My rational is that it's not firing and the plug isn't wet. There's no fuel. If it was not firing and the plus WAS wet, then ignition issues are present.

 

I had the same problem with the #1 cylinder. Using a colortune plug I found that opening the enrichment circuit got it to fire, but of course made the other three very rich. I also found that opening the pilot screw to about 5 turns got it to fire. I finally found that my intake manifold gasket for that cylinder was torn and I was sucking so much air that it wouldn't fire. When I removed the intake manifold about 1/3rd of the gasket was missing. I made one out of some gasket material, installed it, and everything was fine. So you may have a large air leak on the engine side of the carb causing your problem.

 

I double checked the compression today just to be sure. 125# across the board. So the engine should be solid. I'm out of town for the next week so I won't get to it until I get back in town. Some very good advice here, stuff that I haven't checked.

One last question for now.... On both my Seca 550 & 750 it's always spoken about checking the cam shims. I don
T have a problem doing that, however, looking for the cam shims, most of them are no longer available from Yamaha as far as I can see. Is there an alternate source where I can get some of these shims?

 

Click the banner at top right of this page or contact member Hogfiddles as he runs a shim pool.

 

Many, but not all motorcycle repair shops (dealerships included) will allow you to exchange shims....for free...one issue you may have is you cannot have more than one shim out of the bike at a given time....DO NOT EVER ROTATE THE CAM WITHOUT ALL SHIMS IN THEIR BUCKETS!!!!!
So what you'll want to do is #1. verify all shim clearances. #2 pull each shim one at a time to read what size each bucket hold #3 determine which shims can be swapped from what you already have....ie, the intake in #1 and the exhaust in #3 are both out of spec but the #3 exhaust shim is the size you need to bring #1 intake into spec... #4 armed with the sizes you'll need to get everything into spec, go to repair shop and ask really nice if they'll float you the shims for the day...#5 get everything buttoned up #6 bring a case of cold Modelo with your used shims back to said repair shop.

Having valves in spec is one of the most important things you can do to make certain your bike runs correctly...make setting carbs infinity easier...if valves aren't in spec, you'll just be chasing you're tail...and soon you'll start writing posts about what $hitty bikes these are!

 

Yamaha uses 29mm diameter shims on the airheads. Yamaha still makes motorcycles that use 29mm shims but does not have all of the thicknesses in their parts catalog; they are pretty well leaving it up to the aftermarket to supply shims for shim-over-bucket applications.
Hogfiddles runs a shim pool, Chacal carries new shims in stock, and any decent motorcycle shop will have them as well.

 

Do other 80's bikes other than Yamaha's use the shims as well? Both my 1982 Seca 550 and 1981 Seca 750 have about 8500 original well cared for miles on them. Around what mileage typically should I be thinking shims?

 

1. Yes. Most of the 80's motorcycles use shim-over-bucket valve trains, but Suzuki is the oddball using 29.5mm diameter shims.

2. XJ valve clearnace checks are supposed to occur every 5,000 miles, but as the bike accumulates more miles the clearances will stabilize and you will not need to adjust the clearances as often (but clearance checks should still happen every 5,000 miles).

Airhead Valve Adjustment; Part 1

Airhead Valve Adjustment: Part 2

Part 2 is a supplement explaining an easier method for holding the valves during shim exchanges.

 

Some of the late 70's Kawasaki used the same shims as well as BMW "k" bikes and the Klr kawasaki

 

OK, here's the latest update. I couldn't for life of me figure out why the bike wasn't running on all 4. Well, something was plugged.... I pulled that single carb apart and this time I took 100% everything apart on the carb that would move. I then soaked that carb in carb cleaner for 3 days. I reassembled it this afternoon and put it all back together. The bike fired up and ran cleanly on all 4!! What a relief. I didn't want to move forward with any other improvements on this bike until I could get it hitting on all 4. Now I can move forward with valve adjustments, new tires, fork seals, etc, etc. it was really nice to hear it running on all four.