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Author Topic: 1981 Yamaha Seca 750 starting issue  (Read 520 times)

Offline Troyman

  • Posts: 7
1981 Yamaha Seca 750 starting issue
« on: Apr 08, 2017, 20:07:18 »
Hey I'm wondering if anyone can help me get this bike running? I'm pretty handy and bought this bike thinking it would be an easy fix but I'm stumped. I bought it from a guy who couldn't get it running and said the previous owner/mechanic rode it to his place a year ago. So recently I put new plugs in and cleaned all the bad fuel out and pulled the carbs after I noticed gasoline spewing out the air intake side of one and replaced the float and boot gaskets. The carbs and when I pull the plugs tell me it's getting fuel and spark and will backfire once in a while when I choke it. With a good charge it turns over and wants to run. I've. Looked at and tested all the fuses with a multimeter but all it does is catch and turn over but won't start. The headlight doesn't work and I haven't had time to look at it yet if it helps. Everything in the carbs are clean and move freely and no diaphragm or other leaks. One thing I haven't done is borrow my buddies carb sync kit but I don't think the previous owner got that far and it shouldn't be that bad. Any advise would help thanks.

Offline hillsy

  • Posts: 4094
Re: 1981 Yamaha Seca 750 starting issue
« Reply #1 on: Apr 08, 2017, 22:14:47 »
Have you got the coils correctly wired? One coil will be 1 and 4 whilst the other will be 2 and 3.

Offline ApriliaBill

  • Posts: 827
  • Yeah...I'm old school.
Re: 1981 Yamaha Seca 750 starting issue
« Reply #2 on: Apr 09, 2017, 00:06:24 »
Yamaha, if I remember, the headlight won't glow till the bike starts. If you're getting spark, yes check the coils, also being that you went into the carbs, did you adjust them back to a base setting? How are the plugs showing you're getting gas? Didn't Yamaha have a prime setting? Is there enough gas in the tank? Maybe needs to be on reserve (I've seen this more times than you'd think), I heard somebody say once, that most ignition problems are usually fuel problems...
"Do not neglect to show hospitality to strangers, for by this some have entertained angels without knowing it."

Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #3 on: Apr 09, 2017, 01:49:53 »
Right on, thanks for the replies.
I didn't make any carb adjustments as they were very clean and the last guy apparently didn't touch them.
I meant that I made sure it got gasoline to the cylinders and even adding a little fuel directly in them and then tried quick start in the air intake.
This bike has a prime and the petcock works.
The headlight could be a "duh' issue for me (lol) but it doesn't flicker when the bike turns over and I haven't looked at it yet.
Yes it's got lots of new gasoline and has ran a lot of it through.
I'll check the coils in the morning, thanks guys.


Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #4 on: Apr 09, 2017, 17:57:44 »
Ok so coils are good. Manual says 2.5 ohms on primary coil test and both are at 2.4.
From coil to plug cap or "high tension leads" on secondary test as per illustration it says it's supposed to be 11,000ohms but both sides from plug cap to cap are about 21,000ohms and when I unscrew the caps and test from the wire tips through the coil it gives me about 10.8k ohm on both but the plug caps are registering about another 5k ohm each.
I understand the "plug caps" are supposed to have resistance but the illustration in the manual shows to do the secondary test with them on to get 11,00 ohm.
One video I watched showed a guy testing both via the low and high tension leads and I tried that but got nothing so I don't know if it's applicable to this bike.

Is this common to find and if not I wonder if the previous owner changed just the plug caps or maybe I'm still barking up the wrong tree?

Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #5 on: Apr 09, 2017, 18:06:35 »
Ah, manual says plug caps are 5.5k ohm which is what I have on them.

Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #6 on: Apr 09, 2017, 18:30:36 »
One thing I do see is the plug is bpr7es and the manual calls for bp7es. Do you think a little more resistance could be the culprit?

Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #7 on: Apr 10, 2017, 03:11:54 »
5k ohm extra plus a smaller plug gap might be the issue. I'm going to pick up the right plugs tomorrow.

Offline hillsy

  • Posts: 4094
Re: 1981 Yamaha Seca 750 starting issue
« Reply #8 on: Apr 10, 2017, 06:35:46 »
Are the coils wired up correctly? If they are swapped around they will fire 180 deg out and the bike won't start.

Offline Troyman

  • Posts: 7
Re: 1981 Yamaha Seca 750 starting issue
« Reply #9 on: Apr 10, 2017, 22:58:29 »
Yes coil one goes to 1 and 4 and two goes to 2 and 3.
Found out today there isn't a resistor less plug to match this bike but I can order resistor less plug boots. I'd just put some copper in where the resistor is but the two outside boots are not the screw out type and cannot get the resistor out.