Yamaha Seca 750, 1982 model

[Bonz]

There was a Yamaha tool you inserted into the YICS port that had a lever to expand rubber on rod to block the ports when syncing, don't know how much difference it made.

I believe it makes a difference. Sync'd w/o tool, thought it was good based on gauges. Looking back, they would be easy to line up sharing the same vacuum in the YICS port as has been said many times.

Used the tool, that showed they weren't good if having all 4 lined up is the goal. $68 front ebay, original Yamaha tool, shipped from Japan. Took a while but worth it.

 

[Bonz]

A little known fact on the Seca, of course you know it. The front brake MC is not on the handlebar. there is a cable from the lever to the MC.

How's the braking feel with that in between?

Missed this one a while back. Yes the cable between the brake lever and master cylinder takes away some feel for sure. Braking power is fine, however it's rubbery for lack of a better description.

Pulling the brake lever to the bar and securing it there with the bike on the side stand overnight and turning the bars left so the master cylinder under the headlight is the highest point has helped. That firmed things up a good bit beyond the initial bleeding of the calipers. But still not like the direct master cylinder connection on every other bike ever made. Top 10, if not top 3 worst designs ever on any bike system I've known.

Update on the bike:

Had an issue pretty much from the get-go, #2 was not firing at idle, it would come on intermittently as rpm/load increased. Carburetors are fine, ignition checked out electrical wise on the pickup/rotor, TCI and coils. I had fuel, spark and air. Compression was within 10 psi across all four, with #2 being the low one.

Only thing left that I had in the back of my mind was the valves had not been checked by my uncle when he used this engine to build the bike. #2 exhaust valve was .004", spec is .006"-.008". Very tight.

Compression check can't necessarily pick this up, however when there's enough fuel mixture getting pushed out on compression coupled with not creating enough compression at idle because the exhaust valve won't close during compression there's going to be no fire in that cylinder. At least that's how I visualize it in my mind.

Got the valves adjusted back into spec, #2 lit right up. Sync'd carbs with YICS tool, found a good deal on one from ebay.

For grins I checked sync without the tool, they were almost perfect which is to be expected since they share common vacuum via the YICS passageway. Inserted the tool, they weren't as pretty on the readings. Got those dialed in with the tool. Made all the difference with respect to butter smooth at idle and responds like a rheostat in any gear.

Ran the first real tank of gas through it in the last week on a mixture of rides. Approx 70% two lane 50-60 mph, 20% 75-80 mph on the interstate, 10% short runs into town. The bike averaged 47 MPG.

Going on a ride friday, should get a couple hundred miles. Interested want to see if mileage picks up once the engine gets a few more miles on it. It had sat for 20+ years and has 322 miles since it was restored 25 years ago.

Will take it on a ride tomorrow to get it good and warm. Change out the 15w40 HDEO I put in there when I started the process and the final drive fluid. 20w50 going in the engine, 75w90 in the FD.

 

[Bonz]

Going to have a little fun with this post... in another thread there was good discussion about 10w40 or 20w50 in a Nighthawk 750 and the difference in felt power.

One of my riding friends has a well cared for and good running 1998 Honda Nighthawk 750. That bike is running 10w40.

Went on 170 Mi ride in the Colorado mountains yesterday. The Nighthawk 750 and the Seca 750 were representing the carbureted UJM crowd.

The Yamaha Seca 750 had its first oil change after the work to get it dialed in, swapped out the 15w40 HDEO I put in for the tuning process for 20w50 as I had mentioned in the previous post.

I can happily report the 20w50 versus 10w40 power battle is a draw between the two bikes. One would expect the 16-year newer 16 valve head 1998 Nighthawk with 10w40 to trounce a 1982 8 valve head Seca 750 running 20w50. They were neck and neck, no discernible difference in performance. We swapped bikes back and forth, had a good time with it. Probably a good thing 20w50 was in the Seca 750 as a handicap so the bikes could stay even.

 

[Pete in PA]

Going to have a little fun with this post... in another thread there was good discussion about 10w40 or 20w50 in a Nighthawk 750 and the difference in felt power.

One of my riding friends has a well cared for and good running 1998 Honda Nighthawk 750. That bike is running 10w40.

Went on 170 Mi ride in the Colorado mountains yesterday. The Nighthawk 750 and the Seca 750 were representing the carbureted UJM crowd.

The Yamaha Seca 750 had its first oil change after the work to get it dialed in, swapped out the 15w40 HDEO I put in for the tuning process for 20w50 as I had mentioned in the previous post.

I can happily report the 20w50 versus 10w40 power battle is a draw between the two bikes. One would expect the 16-year newer 16 valve head 1998 Nighthawk with 10w40 to trounce a 1982 8 valve head Seca 750 running 20w50. They were neck and neck, no discernible difference in performance. We swapped bikes back and forth, had a good time with it. Probably a good thing 20w50 was in the Seca 750 as a handicap so the bikes could stay even.

Thanks. What was the ambient at the time?
When I did it, was 50F out. Yes, probably should have ran it longer, gave it a better chance.

Per the chart, 10w-40 is good till 95F so no issue.

Thought for sure the Seca was 4 valve. Yamaha even went for 5 valves for a moment around the same time.

 

[Bonz]

All correct with respect to your information. Ambient was 80° at 9500 ft, crazy hot at that elevation this time of year. Mid-90s coming through Colorado Springs on the 8 miles of Interstate in stop & go.

Plenty warm, 20w50 wasn't a handicap compared to 10w40 in an air-cooled 44-year-old Seca ujm versus a basically similar Nighthawk 750 ujm that first came out in 1991.

750 Seca and other displacements (1982, Japanese manufacturers) was the beginning of the end for two valve heads. Most started going to four valves in '83-'85 in the grand scheme, maybe some exceptions.

 

[Bonz]

My feeling is, like we dress for the slide not the ride, set the bike up for the hottest or most severe conditions and it will take care of itself. As I've mentioned before, it might be freezing in the garage come winter yet 20w50 cranks fine in any bike I've ever had for 20 years or more going for rides when it's a sunny day after a cold night.

Air density is 30+% percent less ( vs std. temp/sea level) at the higher elevation (9500') and 80° f during my ride. Meaning less cooling capacity. Low to mid 90° in Colorado, even at "only" 6,000', is nothing like a 90° to 100° day at close to sea level. Much more punishing and hard on oil.

 

[Marrk]

My feeling is, like we dress for the slide not the ride, set the bike up for the hottest or most severe conditions and it take care of itself. As I've mentioned before, it might be freezing in the garage come winter yet 20w50 cranks fine in any bike I've ever had for 20 years or more going for rides when it's a sunny day after a cold night.

Air density is at least 30 percent less than sea level at the higher elevations during my ride. Meaning less cooling capacity. Low to mid 90° in Colorado, even at "only" 6,000', is nothing like a 90° to 100° day at close to sea level. Much more punishing and hard on oil.

View attachment 349860

Thanks for the picture. Makes me wish I had a bike again. And a L5 lumbar disc.

 

[Bonz]

Bummer you aren't able to ride as a result of the non-existent disc it sounds like.

 

[Marrk]

Bummer you aren't able to ride as a result of the non-existent disc it sounds like.

Too many motorcycle crashes and car crashes. When you go off the road and say "Hello" to Mr. Armco and his friends, Gravel and Dirt, they leave you with a little something. It took a while, though, and it was really only the last one that finished me off. I had my fun.

Unless you were kidding. In which case, "HaHa" or whatever.

 

[Pete in PA]

My feeling is, like we dress for the slide not the ride, set the bike up for the hottest or most severe conditions and it will take care of itself. As I've mentioned before, it might be freezing in the garage come winter yet 20w50 cranks fine in any bike I've ever had for 20 years or more going for rides when it's a sunny day after a cold night.

Air density is 30+% percent less ( vs std. temp/sea level) at the higher elevation (9500') and 80° f during my ride. Meaning less cooling capacity. Low to mid 90° in Colorado, even at "only" 6,000', is nothing like a 90° to 100° day at close to sea level. Much more punishing and hard on oil.

View attachment 349860

Over here the most I have to deal with is about 1700 ft.

 

[car54]

My friend had a seca II. I put 20w-50 in and it ran great until he got stuck in traffic and left it idling so long it self destructed from heat. He wasn’t smart enough to turn it off when nothing moved for 45 minutes in florida heat.

It was a fun bike. Yours is gorgeous.

 

[Bonz]

My friend had a seca II. I put 20w-50 in and it ran great until he got stuck in traffic and left it idling so long it self destructed from heat. He wasn’t smart enough to turn it off when nothing moved for 45 minutes in florida heat.

It was a fun bike. Yours is gorgeous.

Thanks! It's a good runner now on top of the condition being pretty dang good.

No airflow for 45 minutes has a propensity to do un-fun things to air cooled engines. Sorry for his loss!

 

[Bonz]

Over here the most I have to deal with is about 1700 ft.

My folks are in Mechanicsburg, agree, not comparable elevation in that neck of the woods for sure.

Tuning gets me neurotic. My house is at 7,200', I go up to 9,500' and below 5,000' with respect to most day rides. When doing mountain passes, 10,000' to 12,000'+ is the norm. Or Pikes Peak or Mount Evans, over 14,000'. On a given day ride, could potentially have a 4,000'+ elevation swing within a 200 mile loop. I need to keep that in mind and take a grain of salt when it comes to how my bikes are running at any given point.

Lived in North Carolina for a bit a few years ago. Highest point east of the Mississippi is around 600 ft lower than our home here in Colorado.

 

[02SE]

How comfortable is that bike for someone your size? I recall the ergonomics of those old Secas being somewhat cramped for me, and I think you've got well over half a foot on me, IIRC.

 

[Bonz]

Yes, it's a tight fit. For the times I'll ride it which is shorter day trips, errands that kind of thing it's just something to deal with. I put a gel pad and an air pad on the seat for the ride last week, that gave me an extra bit of legroom.

I'll stand up on the pegs every so often to stretch, stretch a leg out to the side while seated that kind of thing. Then of course the stops for gas or hydrate a bit, get lunch that type of thing makes it not so much of an issue. But I'd be lying if I said my knees weren't aching and needing to be stretched while on the bike.

Swapping with my friend on the Nighthawk 750, the Nighthawk lower body position was more confining, however the upper body and a bar relationship on the Nighthawk was more roomy.

The plan is keep the Seca stock as far as adjusting anything for the riding position or otherwise. It's beauty is in being so complete and in the shape it is.

The ZRX 1200 has lowered pegs, seat built up to basically flat, bar risers and taller/wider bar. Riding position on that bike is much better, as I've set it up for my height over the years.