OK carb Guru's, Need help here. Choke issue.

Here is an interesting one for you guys.

1981 Seca 750. 28000Kms. Hitachi carbs. everything stock.
Runs like a scalded cat. Pulls like a drag car over 7000 rpm.

Running is not the issue, starting is.
Now the bike starts right up but never has a gain in RPM with the choke on.
Cold it will start with 1/2 to full choke, runs rich (can smell it in the exhaust) idles at about 900 but never hits 1050 RPM until it warms up.
sometimes it will get down to 500 RPM and then die, but starts right back up with a low idle.
If you rev it up it runs clean and then settles back to a low idle until warm.

Hot it starts right up with no choke and idles right at 1050 RPM

Why is the RPM not coming up with the choke on when cold?
I'm not complaining as the engine runs like a top, but sometimes you have to start it 3 or 4 times because it idles so low and quits until it gets warm.

Peter

 

That is a good question Peter. I haven't experienced that but I am thinking it may be your enrichment circuit (choke). I thinking the holes in the well located in your float bowls may be clear but the brass tube that goes into the well & pull the fuel up may be restricted. Might be a good thing to check. I am no expert just my opinion. I'm sure others will chime in on this for you. Keep us posted on what works for you.
Gregg

 

Maybe too much choke?... If it smells rich, give it less... Also, the enrichment circuits provide their own fuel _and_ air. There are passages below the diaphragms (in the front of the "trench") of each carb that provide air for the enrichment mixture. Make sure they're all clean. They're pretty big so they don't clog easily, but you never know...

Most likely you're not drawing up enough fuel via the brass tubes that greggvickrey mentioned. Shoot carb cleaner into every part of the starter circuit. It should cheerfully come out somewhere else, sometimes in more than one place. If carb cleaner does not come out or comes out slow, that circuit is clogged and needs attention. Cheap guitar strings work wonders for chasing these passages.

 

Well.....
I finally got time to pull the carbs (again). I went over everything with a fine tooth comb. All the jets were removed, cleaned and inspected. The idle jets in the bowls were cleaned and are as good as new (did the drill bit cleaning method).
Put a wire up through the little brass tubes to the enrichment circuit, and blew cleaner through every orifice there is.
I polished the slides this time and did the "clunk test"
Got everything back together on the bike, primed the carbs with gas, 1/2 choke and hit the starter.
She fires right up, idles at about 900 rpm.
Add more choke and she dies. let the choke off and she sits there like a puppy waiting to go for a walk idling away at 900 rpm.
Throttle response is instant and strong. and when she warms up she is dead on 1050 rpm.
I played with the choke and just can not get any rise in rpm no matter where it is set.
I guess that is just the way these carbs are.
I should count my blessings that a 27 year old bike will still give most modern sport bikes a good run. If a low idle and having to re start the bike once or twice when it is cold is all I have, well that aint so bad.

Peter

 

Something in the enrichment circuit is not right.
How does everything look in the 'choke tube?'
Does your cable pull smooth, and does the fork lift the enrichment shaft smooth and fully?

Sounds to me like instead of 'enriching' with more fuel, it is drawing air in through the enrichment tube due to either a bad air leak there, OR a minor air leak PLUS a semi-clogged enrichment circuit.

If you were getting any fuel through the enrichment at all, I would think it would at LEAST rise idle a bit. Not lower it. That's what makes me think vacuum leak with the 'choke' operating.

 

Good place to start.
Is the choke cable mounted correctly in the bracket at the carb end.
The end of the sheath should be in the bracket not behind it.
Just a thought.

 

I'm having the same exact problem.

Did you resolve your issue? And if so, how?