750 Seca almost there - need a little more help.

Just to avoid confusion I am going to start fresh with the facts here.

1981 Seca 750
-Valves adjusted to spec
-Carbs cleaned completely including pilot mixture screws with new o-rings and shaft seals
-Floats set
-Intakes have no cracks
-Vacuum caps and hose replaced with new
-bench-synched on a strip of card stock

Initially, it started with enrichment and idled up super-high, like 6000 RPM. Turned out the throttle cable jacket wasn't seated fully. Fixed that, it dropped to 5000 RPM.

After re-checking the synch and re-setting floats (thanks again, MN-Maxims) idle was lower, about 3000 RPM.

Just now I went and re-adjusted the enrichment cable. Apparently, it was too tight and the valves weren't closing all the way. After this I got it to start with the enrichment, then gradually closed the valves as it warmed up. Warm, it idled a hair above 1000 RPM, which seemed good to me.

Then I tried giving the throttle a twist. First time it stuck at 2000 RPM and took a while to come back down. Second time, it did the same thing. Third time it climbed to 4000 and stayed there. I tried pushing down on the enrichment valves, but that made no difference.

What do I check now?

 

It occurs to me that one thing I did not replace was slide springs. Can they be too weak and not return slides the way they should?

 

Did you replace the intake to head gaskets? Sounds like an air leak somewhere. Springs are probably fine though you may have a slide sticking.

 

Still sounds lean... Have you tried, just to see, if carb cleaner/starting fluid does anything? Are you sure the boots are all properly and firmly attached?

Guess you did the clunk test, right?

Where are your pilot/idle-mixture-adjustment screws out to? Tried tightening them?

Sometimes a float can be on the edge as far as working; one time I had a float that seemed fine when being wet set, but under actual use conditions would allow overflow. Consider attempting to measure it while the bike is running.

 

Did not replace gaskets. Matter of fact, haven't taken the intakes off at all.

The slides drop freely.

I think the screws are either 2 1/2 or 3 turns out. I will recheck. It's a PITA to keep pulling the tank...

 

Speaking of gaskets: Should they be dry? Or should I use a light coat of non-hardening silicone?

 

I would use sealer if I were doing it.

MN

 

Try dropping them down to 2 and see what it does? Also, you could leave the tank off and set it on a table nearby or something.

 

Throttle cable too tight or sticky? Check for correct play.

 

Oiled the cable and made sure the jacket was seated properly.

 

I am going to try that. I'm a little concerned about the bolts, but I'll take my chances.

 

I had the EXACT same problem with my Midnight.

Two things to think about:

Carb rack must be PERFECTLY aligned. I took two pieces of 90-degree angled iron and clamped the four carbs in it, then re-tightened the rack screws.

The vacuum boots on the intake manifolds may be dry rotted and leaking. This was ultimately the cause of my tuning trouble. I picked up a replacement set and voila! Problem solved.

 

Today I took another swing at it.

Took a propane torch and waved it around the carbs and intakes. No difference.

Reset the mixture screws to two turns out. No difference.

Tried playing with the synch screws - BINGO - RPM drop.

So, apparently I just badly screwed up the bench synch. Now, it still doesn't idle quite right, but I can pull the throttle a bit and have it come back down.

I tried the vacuum gauges I have. I don't have a YICS tool yet, but I wanted to see if they would give me a reading. The needle hops all over, as expected. I tried the straw from a spray can as a restrictor, but it's too big, needle still bounces a lot. I can squeeze the hose to make it relatively still. Need to find something else for a restrictor.