1982 Yamaha Seca 750 Loping Idle, Needs Choke to Run

1982 Yamaha Seca 750 Loping Idle

OK, long story, get something to drink, then read this missive.

I put up my 1982 Seca 750 about 17 years ago in my garage. Last year after retiring I decided to crank it up again. The front brakes were frozen so I disassembled them and rebuild the master and brake cylinders. After laboriously bleeding them, they work fine now. Now I understand why you should flush the brake fluid ever few years. The brake fluid had changed to yellow crust.

When I got a new battery, I was able to change the oil, put gas back in it and it started right up. It had a loping idle that eventually stalled out but it ran. I got turned on to XJ4ever by my nephew so I could find parts for the bike. Removed the Carb rack and used carb cleaner spray to rinse out all of the passages. When carbs replaced the bike didn’t run better so I checked for vacuum leaks with carb spray and found a big one at the gasket between the intake runner and the head. The gasket had crumbled over the years so I bought new ones and replaced the rubber intake runners too since they were old and cracked. After reassembly the bike ran better, but not right. I retested the vacuum and found a smaller leak at the throttle shaft seal. Talked to Len and got replacement float bowl & throttle shaft seals, , idle mix screw O ring and fuel runner O rings and replaced them. I also polished the CV piston and cylinder until it passed the Plunk test. Inspected the diaphragm with a strong light, it has no holes and is still flexible and supple. Still rough idle.

I then tested coils for primary and secondary resistance. Coils are in spec but plug caps were out so I got new 10k and 5k Caps and put them on. Replaced plugs also with non-resistor plugs. I synchronized carbs and Colortuned them (~3 to3.5 turns out). Got the bike to run better. Took it out around the block over several weeks to test it out, and ended the season in November. But at the end it started to run badly again.

Pulled the carb rack again and found one of the diaphragms had slipped out from under the carb hat. Drained the fuel tank, took it off and sprayed WD-40 inside to store. When draining the tank, getting the last few drops with a turkey baster, the long tip fell off in the gas can. I thought it was Polypropylene and was impervious to gas, so I left in until I could drain the gas can later. I dry then wet set the carb float heights, but had much difficulty getting replicable wet numbers.

Then I rebuilt the forks and anti-dives since the seals were leaking. In February I reassembled everything and filled it with the gas from last year. It ran terrible. Couldn’t get it to idle very long and it would stall. When I refilled the gas can I realized the turkey baster had disappeared! Apparently, it was not PP but some other plastic that dissolved in gas. This is what the carbs had in them. When the gas dried in the carbs a coating of plastic was left.

I cleaned out the tank, got a new gas can and fuel line and disposed of the contaminated gas at the hazardous waste station. I disassembled the carbs completely, but again, and used carb cleaner spray and brushes to completely clean out everything. I found the fuel inlet screen on carb 4 was completely covered with gummy plastic. I sprayed out all of the passages several times and poked them out with the thin wires. I did this until all passages sprayed out carb cleaner as a forceful jet of fluid. I then replaced the floats, seats, screens and needle valves. (Floats looked like they were starting to crumble.) I then set the dry and wet float heights several times to get down to 3-3.5 mm (I’m getting pretty good at this now).

Still won’t idle right and won’t stay running without some choke on. I’ve tested spark from all plugs and it looks weak to me (however I’m used to HEI ignition sparks), but is blue/white and will jump a 6mm+ gap. Plugs have carboned constantly, even on some cylinders before I put it in storage. I’ve tested the pickup coils cold and they meet spec ( 634 and 645 ohms, spec 650 +/- 20%). I tested the spark on an old plug against the block. When I turn on the ignition, plugs 1-4 give the single spark as 2 seconds, but Plugs 2-3 give no spark. I probed the Coil leads from the TCI (black box Transistor Controlled Ignition) and 1-4 lead goes up to 10.8 V then drops to 0.5 V after 2 seconds. The TCI lead to Coil 2-3 goes up to 10.8V and stays there permanently. If I switch the coil leads so the signal from the TCI for coils 1-4 goes to coil 2-3 then 2-3 sparks. I reflowed one poor looking solder joint on the TCI, but didn’t help.

I took apart all connectors and fuses on the bike and cleaned them with DeOxIt contact cleaner. The TCI big connector was very corroded. Probed TCI lead to coil. Orange (1-4 coil) line compared to ground is >20M-ohms, the 2-3 grey lead shows 17.6 M-Ohms.

I wish there was a way of testing the TCI at all and coils at high frequency and voltage, as I suspect them. I also think I’ll have to pull the carb rack and see if I missed any plastic goop.

So, this is my tale of woe. I’ve gone through all of my ideas and drained Len of all his ideas, so does anybody have any other ideas.

 

Wow that was a lot of work... You probably did but the only thing I'm missing (or overlooked in your story) is performing a running sync on the carbs and adjusting the idle conforming.

 

Check the valve clearances too.

 

The set of plugs that spark is dependent upon the physical location of the reluctor. You should be able to bump the starter or turn the engine over manually and find a spot where the 2, 3 plugs will spark approximately two seconds after turning the key on.

The actual change should be when the key is turned on one set will go low for approximately two seconds and then when going high that set will produce a spark. The going high feature is built into the TCI to disable coil current to protect it from overheating. The TCI will become active again as soon as an input signal from the pick-up coil is detected.

Not sure exactly how this was done (was everything disconnected except the one wire to the coil?), but either way resistance greater than 10 megohm on the primary side would certainly be considered open and not an issue.

Likely not an issue, but XJ750 Seca 81-83 should be 5K per cap (4K to 6Kohms)

Running only with choke on is so typical of clogged jets / passages and I would be thinking about the turkey baster plastic still being in there. It sounds like it slowly dissolved, so it makes sense that it would take some extended soaking to remove.

Do all the exhaust pipes appear to heat evenly when first started, or at least all getting good and hot? Maybe try to keep it running with a bit of throttle and without the choke as that might help identify a particular cylinder that is having issues with the pilot circuit.

And as suggested what about the valves, do you know the history and have they been checked / adjusted? How many miles on the bike?

Has a compression check been done recently? That's always a good place to start with any motor that has been out of service for an extended period.

 

Yes, I’ve done that 2-3 times with the colortune and manometers. There are probably some things I was too bleary eyed to remember.

 

Checked valve clearances last year. They were in spec.

 

Just did compression test cold, got from 140 to 149psi across the cylinders ( did it cold because some cylinders were not firing then). But even cold this seems ok. Read orange box on this. Some of my replies mingled with quote text.

Again I appreciate the review and ideas.

 

Cold compression test is the right thing to do make sure the compression is right for starting.

 

Thanks, Franz

 

Okay if you read any of my posts on my Johnny Cash bike, I went through a mine field here are some items . Did you do a FULL tear down of carbs , all rubber seals removed . I use Berrymans carb dip 1 gallon container soak each carb bodies for several hours. Or boiling the carbs is another option . I could not get my carbs to run right finally figured it out problem number two slide would not move all the way up , and number four the diaphragm was not fully in the groove under hat . (You said you had that problem as well) did you do a bench synch on carbs ? I'm don't mean to insult you in anyway , it is the little things that get overlooked.

 

Maybe I'm crazy and I know this is going to sound weird... But what I do after rebuilding the carbs. I suck the vacuum port of each carb with my mouth... You can tell by feel if the same suction is needed to move the piston completely up. Just my scientific way to check the sealing of the diaphragm.

 

thats not what lifts the slides, the tiny hole in the bottom of the slide make it rise.

air sucked out of the little hole creates a vacuum it the top portion of the carb hat above the diaphram and the slide lifts.
it lifts and drops with the opening and closing of the intake valve.

 

Jetfixer, I did a full tear down of the carbs, taking all apart down to separate pieces ( seals, emulsion tube, inlet valve seat and screen, jets etc. I sprayed cleaner several times into all passages a blew them out. I didn’t boil them or soak them in Berryman. Did this Berryman soak take off the carb body finish and dull them? I also found the diaphragm seal test of blowing into the oval hole on the intake and seeing if the slide raised. Also, bench synced the carbs with a paper clip. As for insulting me, don’t worry, I’ll take all the ideas I can get. After 4 or 5 carb rack removals you get a much more mellow disposition (AFTER removing and replacing the rack, not during). Thanks for all your help.

 

I now do that test, but blowing into the oval hole.

 

The only true way to get into the small passages is to soak or boil . The Berrymans carb cleaner as long as you don't leave it for several days, I've never had any problems . Used this to do both of my Seca's carbs . Spray cleaner just will not get into the small passages, if your bike sat as long as you stated , it could be really gummed up . Although I doubt it is the problem, have you checked the petcock, are you running it on PRI or ON ? It is possible the diaphragm for the the vacuum is not giving enough flow.

 

I have an auxiliary tank I use so the tank & petcock are out of the equation. I was going to try the Berrymans but wasn’t sure if it would wreck the finish. Thanks for the info Jetfixer.

 

Hahaha, one single malt too many and it was 01:00 am but blowing the oval hole is it!

 

No doubt you're right. But blowing the oval hole as Mark said doesn't do the trick either? Because it does move the piston but maybe another reason?

 

I believe the oval connects to the chamber under the diaphragm. If the diaphragm seals the higher pressure under the diaphragm lifts the piston. If the diaphragm Is not sealed the added pressure leaks to both sides of the diaphragm so the added pressure doesn’t push up the piston.