Fuel system tips, anyone got carb diagrams?

For all your mechanical queries, or for sharing your mechanical know-how. Also used for arguing about which oil to use...

Re: Fuel system tips, anyone got carb diagrams?

Postby Freddy » 26 Jun 2020, 00:35

Ryan550 wrote:but not sure how much difference exactly it will make,


A practical example of just how much difference a correctly operating stock airbox makes to a CV type carburetor. Bike is a 750, not your 550. Following some 'advice' from a related model forum, I once removed the rubber block that sits in the mouth of the air intake. In the parts listing its described as a 'noise dampener'. It takes up perhaps 30% of the intake opening.

The instant I started the bike I could tell it wasn't running correctly. Opening the throttle on the road to accelerate and the bike would just piss and fart going no where. Basically un-ridable, so bad had to bring the bike straight back home. This is from just removing a small rubber block in the intake opening, but still having an airbox, lid/snorkel, velocity stacks (intake rubbers), and stock air filter in place. Just one tiny change that obviously affected the pressure inside the airbox was enough to screw things up.
Freddy
 
Posts: 481
Joined: 06 Sep 2017, 11:06
Location: Sydney Australia

Re: Fuel system tips, anyone got carb diagrams?

Postby Ryan550 » 28 Jun 2020, 13:19

Freddy wrote:
Ryan550 wrote:but not sure how much difference exactly it will make,


A practical example of just how much difference a correctly operating stock airbox makes to a CV type carburetor. Bike is a 750, not your 550. Following some 'advice' from a related model forum, I once removed the rubber block that sits in the mouth of the air intake. In the parts listing its described as a 'noise dampener'. It takes up perhaps 30% of the intake opening.

The instant I started the bike I could tell it wasn't running correctly. Opening the throttle on the road to accelerate and the bike would just piss and fart going no where. Basically un-ridable, so bad had to bring the bike straight back home. This is from just removing a small rubber block in the intake opening, but still having an airbox, lid/snorkel, velocity stacks (intake rubbers), and stock air filter in place. Just one tiny change that obviously affected the pressure inside the airbox was enough to screw things up.


Thanks Freddy, after your example the running issues seem like they may be at least mostly down to the missing intake system, if it was a car I;d say it had a vacuum leak and simply wasn't pulling through all the fuel it should be.

I've actually got it starting pretty well now on the choke and idles OK as long as I keep the choke on. I'm going to pull the carbs out one (hopefully) last time, I ordered carb kits because that left end one still seems to have a leaking needle valve so it starts on 3 and eventually that one cylinder must unflood itself enough to light up. So once more off, beg, borrow or steal an ultrasonic cleaner to make sure all the junk is out, then put it all back together again with the airbox and ducting fitted, then see how it goes.

but now I'm pretty happy it will run decent and doesn't have any major engine issues, I need to get new indicators and refit all the bodywork so I can test it more than just free revving on the spot.
Ryan550
 
Posts: 13
Joined: 29 May 2020, 12:08

Re: Fuel system tips, anyone got carb diagrams?

Postby Freddy » 28 Jun 2020, 13:53

What have you set the pilot mix screws at? Haynes says they should be 1 3/4 to 2 1/4 turns out. I'd go for the larger 2 1/4 turns out.

The other specs I'd check seeing you probably don't know if changes were ever made in the past are....
Float heights are 15.0 to 17.0 mm
Pilot jet #35
Pilot air jet #160
Needle jet #6
Jet needle #N67J
Main Jet 1,4 cylinders #108
Main Jet 2,3 cylinders #110
Main Air jet #100
Starter jet #52

I would caution against using any non genuine parts in the carburetors other than gaskets, other than as a last resort when no other option is possible. All I've ever seen is seemingly a world of pain when people have used non genuine parts.

P.S. Even with excellent eyesight you'll probably need one of those illuminated magnifying glasses to read the sizes stamped on the jets. Genuine Keihin fuel jets are usually easy to identify as they have a 'star of David' type marking before the number.
Freddy
 
Posts: 481
Joined: 06 Sep 2017, 11:06
Location: Sydney Australia

Re: Fuel system tips, anyone got carb diagrams?

Postby Ryan550 » 03 Jul 2020, 02:42

Freddy wrote:What have you set the pilot mix screws at? Haynes says they should be 1 3/4 to 2 1/4 turns out. I'd go for the larger 2 1/4 turns out.

The other specs I'd check seeing you probably don't know if changes were ever made in the past are....
Float heights are 15.0 to 17.0 mm
Pilot jet #35
Pilot air jet #160
Needle jet #6
Jet needle #N67J
Main Jet 1,4 cylinders #108
Main Jet 2,3 cylinders #110
Main Air jet #100
Starter jet #52

I would caution against using any non genuine parts in the carburetors other than gaskets, other than as a last resort when no other option is possible. All I've ever seen is seemingly a world of pain when people have used non genuine parts.

P.S. Even with excellent eyesight you'll probably need one of those illuminated magnifying glasses to read the sizes stamped on the jets. Genuine Keihin fuel jets are usually easy to identify as they have a 'star of David' type marking before the number.


Thanks Freddy, I set the pilot jets to what I counted when I took them out, was more like 2.5 turns out from gently bottoming out.
I suspect the spring loaded plunger on the needle is maybe the problem, the left hand carb seems to be the problem, does not fill the cylinder with fuel, but takes a few minutes to clear out before it starts to run, as observed by the exhaust not getting hot. As I think I;ve said before, it is running well enough to put that bit more effort into going over the carbs fully.

Luckily enough, or unluckily, I've had a bit of practice reading jet sizes lately.

As I said I have had some experience with car type carbs, is the float level measured from the top of the float to the flat surface of the carb body? or is there a different reference point.
Ryan550
 
Posts: 13
Joined: 29 May 2020, 12:08

Re: Fuel system tips, anyone got carb diagrams?

Postby Freddy » 03 Jul 2020, 03:42

Ryan550 wrote:is the float level measured from the top of the float to the flat surface of the carb body? or is there a different reference point.


Correct, from the gasket face of the body, to the bottom of the float with the carburetor held just past the 90 degree position so the float is touching the valve but not compressing the internal spring in the valve. If held completely upside down the weight of the float will compress the spring in the valve giving an incorrect reading.

Set the floats at 15mm to give the highest fuel height. No doubt you are probably aware that fuel height affects air/fuel ratio and all these bikes were jetted on the lean side a simple thing like this that might make them a bit richer is worthwhile.
Freddy
 
Posts: 481
Joined: 06 Sep 2017, 11:06
Location: Sydney Australia

Previous

Return to Mechanical

Who is online

Users browsing this forum: No registered users and 2 guests

cron