Seca help

Hey boys and girls
81 seca 750 here. Can’t seem to get this bike to run. Here’s where I’m at after a few posts over the winter that didn’t quite get me running.
valves checked and re shimmed 2x
Church of clean 2x
- split racks, new throttle shaft seals
Tons of carb cleaning 4-5x
bench sync 4-5x
Pilot screws set 2.5 turns out
Wet set a bunch
Checked for vacuum leaks a lot
Bike just doesn’t like to be synced while
running
Sometimes won’t even turn over. I’m waiting to do the starter clutch dance when I can get it running. I don’t want to split the motor if it’s just not going to run. So it’s a slow process. When the bikes cold I can get it to crank and start mostly. Plugs seem to get fouled quickly.
and then has trouble starting as well as the blender of rocks thing. Wait 1 hour try it again stuff.
if I can get it started with enrichment on it will warm up, and I’ll try a running sync with a 4x gauge. Usually with no success. The idle will run away or the constant adjustment of the running sync gets me nowhere. So carbs off, bench sync again repeat process etc. etc. etc.
have not done a compression test on the motor. Don’t have the tools. Any suggestions?
Thanks all

 

Hi @Joshua Howard

How about ignition/charging/battery? Have to looked at that at all?

If you are concerned about compression, many auto parts stores have a loaner tool program so you could probably get a compression tester for the cost of the trouble to pick it up and return it.

 

Battery is new. Cranks fine. Both coils and wires ohmed out proper. Trouble seems to be with fuel and air. It will run but not well. Often when it starts and the engine is cold I can give it throttle and it responds as it should. Once it get warms is another story. Once I attempt a running sync it gets all out of wack. Pretty sure after lots of reading and fiddling, I am doing the sync right. Open to better knowledge on the sync side.

 

Does it have pods or factory airbox?

 

Factory air box. Got it started with some cleaner plugs I had in the shop. Got the running sync dialed in. But when I blip the throttle it wants to big down and die. After leaning out the mix a little I blipped the throttle and it did die. Wont start now till it’s cold again. Rocks in the blender thing. I know I should pull the motor and fix that just want to hear it run first. When it first started up I could twist the throttle and it would operate correctly. Once warm the previous mentioned throttle bog sets in. I’m stumped.

 

Does it have the original Yamaha fuse box? How long have you had the bike?

 

Someone else put a blade type fuse box in. I got two parts bikes and have combined them. Had it about 8 months. Hasn’t hit the road yet.

 

I am wondering if you have stuck piston rings and low compression? You are doing all the right things.

 

Are you replacing them or cleaning? Have you tried with a new set since all the work?

They really shouldn't foul unless the carbs are dumping too much gas or are not jetted properly

One issue that happens occasionally is the air jets are installed per the manual, which is incorrect. They should look like this:

 

Thanks all for your help. Air jets are installed correctly. I’m working through the ignition coils right now. Can’t seem to get a consistent ohm reading. It’s jumping all over the place at each 2 wire connector. The plug wires at the caps are measuring right around 11k and 12 k. Wondering if the coils are bad.

 

I had the same problem on my bike recently. The red wire white stripe from the TCI to the ignition coil plug on the right coil had high resistance. I fitted a new crimp to that connector and it fixed it. Check the resistance from the TCI plug to the coil connector.

 

Mine seems to steady out at infinite resistance. the two plugs from the ignition control box(cdi?) both have 12 k ohms resistance. It seems that both coils ( used from 2nd parts bike) don’t have any or very erratic readings

 

It should be less than one ohm from the TCI box to each two plug connectors. That what mine are with the ignition off.

With ignition on are you getting 12 volts at the plugs that connect to the coils?

 

I’ll have to check that

 

One connection the red wire with white stripe at the TCI feeds both coils. If that connection is dirty it will affect both coils.

 

're carbs :
In early 80s government regulations for pollution made the bike manufacturers limit the pilot screw setting to very lean.
My Sec a would bog down when you accelerated. Broke seal on pilot screws back them out to two and a half turns, all good now.

 

I usually start at 2.5 turns out. Have gone up to 4 turns out and it still bogs down when warm. When it’s cold I can throttle it up and it revs well. After warm it bogs out and dies no matter where the puppy screw goes.

 

“Puppy”= pilot.

 

Check with a multimeter probe across the fuses too. I know you have a blade fusebox fitted. But you didn't fit it so better checking all connections.

 

How is that done Franz?

 

Fuse wiring connecters meant to the next connections.

 

I need to check my wiring diagram when l get home.

 

On second thoughts if you check you have 12 volts at the red wire white wires that connect to the ignition could the wiring should be ok. Trying to figure out why it's dying when you open the throttle.

Have you tried starting it with the fuel tap set to prime instead of the vacuum opening the fuel flow. Faulty petcock perhaps. When you start it let it run to see if it dies. Don't touch the throttle to see if it's fuel starvation. Then if it runs long enough for the engine to get some heat in it try revving it. Does the bike have a fuel filter fitted outside the tank. If so check if it has a direction arrow on it. If it's there it could also be partially blocked restricting the fuel flow.

 

All good thoughts. Vacuum is on prime
Doesn’t die unless I mess with the carb sync or something else. Will idle fine, just doesn’t purr when it’s idling. Yes to the filter( new), arrow, going with flow, into carb.

 

New air filter? Were these symptoms there before fuel filter was fitted? Any splits or blockages in the vacuum hose from the petcock to the stub on carburettor three?

 

New air filter all stubs are vacuum hosed to carb sync