Fueling question

[Scot]

1982 Seca 400.
The engine starts instantly and idles perfectly. Pulls nicely/smoothly up to mid throttle, but then then stumbles. Not missing, but it will not accellerate further. But if I back off the throttle it returns to nice, smooth running. (Manipulating the choke while it's stumbling at speed doesn't improve anything.) I can get it up to 55mph indicated on flat ground but nothing more due to the fueling situation. I kinda feel like it's not getting enough fuel, as opposed to too much.
Question: I'll pull the carbs back off, but where should I look first? What measurements should I check? It would be nice if it's float height or needle setting, right?
Thank you,
Scot
(Stock machine. New petcock rebuild kit. New plugs and air filter. Carbs cleaned. Jets are stock sizes, I believe.)

 

[CaptChrome]

Nice pic of a nice looking bike!

It does sound like a fueling issue. Start at the tank and work into the bottom of the carbs:

1. Check that the fuel cap is venting to atmosphere. Find the vent hole and make sure it is open so that the tank is not under vacuum when there is a large fuel draw.
2. Check that the petcock is flowing well - pull the hose off of the carbs and pull a vacuum on the petcock operating hose to visually check that there is ample flow of fuel.
3. The float needle valve seat has a mesh filter under it. When you get the carbs open pull the brass seat out with a pliers and inspect/clean/replace this mesh screen. Of course clean the crap out of the passage under the seat also. I think this is the most overlooked item when people clean their carbs and it doesn't take much rust scale on the mesh filter to cause issues with fuel flow.

4. Visually verify that your jets are clean. They probably are, but check anyway.
5. Put things together and reset your float bowl level using a piece of clear tubing (the DOHC carbs make it hard as you will need to pull the bowl drain plug and adapt the hole to fit a piece of tubing with a fitting or some other temporary means (Yamaha had a special fitting for this job). The manual specifies how you do this in more detail, but basically you fill the float bowls and use the tubing to visually measure the level in the float bow (it should be 3 +/- 1 mm below the flange at the bowl to body joint (see the small black dot on my float bowl at 3mm below the flange):

6. Put the carbs back on the bike and test things out.

 

[Scot]

Nice pic of a nice looking bike!

It does sound like a fueling issue. Start at the tank and work into the bottom of the carbs:

1. Check that the fuel cap is venting to atmosphere. Find the vent hole and make sure it is open so that the tank is not under vacuum when there is a large fuel draw.
2. Check that the petcock is flowing well - pull the hose off of the carbs and pull a vacuum on the petcock operating hose to visually check that there is ample flow of fuel.
3. The float needle valve seat has a mesh filter under it. When you get the carbs open pull the brass seat out with a pliers and inspect/clean/replace this mesh screen. Of course clean the crap out of the passage under the seat also. I think this is the most overlooked item when people clean their carbs and it doesn't take much rust scale on the mesh filter to cause issues with fuel flow.
View attachment 38869View attachment 38870

4. Visually verify that your jets are clean. They probably are, but check anyway.
5. Put things together and reset your float bowl level using a piece of clear tubing (the DOHC carbs make it hard as you will need to pull the bowl drain plug and adapt the hole to fit a piece of tubing with a fitting or some other temporary means (Yamaha had a special fitting for this job). The manual specifies how you do this in more detail, but basically you fill the float bowls and use the tubing to visually measure the level in the float bow (it should be 3 +/- 1 mm below the flange at the bowl to body joint (see the small black dot on my float bowl at 3mm below the flange):
View attachment 38871
6. Put the carbs back on the bike and test things out.

This is the best forum reply I've ever received. I don't know who you are, CaptChrome, but you're a digital hero in a dark world.
Thank you. I'll post up when it's running beyond the speed limit.

 

[CaptChrome]

Remember, the world's not so dark when you are on your bike on a sunny day ripping it at 65 mph. And that's what this forum is all about, right?

Thank you. I'll post up when it's running beyond the speed limit.

You are welcome. Please do post a follow-up, even if you run into troubles. Post more pics of the bike too, even if it is not running. You don't see nice, original Secas everyday!

 

[gpounce32768]

I was thinking a sticking/jamming slide- meaning they aren't opening far enough to let more air & fuel in- could maybe be a minor leak in the diaphragm gasket. I suppose needle height could be wrong too but that seems a stretch.

Suggest getting the bike up to the speed where the problem starts happening, then gradually open the enrichener. If power goes up then the slides are OK and there is a problem getting fuel thru the jets (possibly including a bonkers needle valve position). If it doesn't then maybe fuel flow issues as above (or the slides not opening)- +1 on the obscure places that can clog up in these carbs.

I envy your OEM tank- mine had so many holes rusted thru it looked like someone had blasted it with a shotgun. Replaced with a Maxim tank- it fits OK-ish but at least the original petcock fits properly, and in the right location.

 

[Scot]

I was thinking a sticking/jamming slide- meaning they aren't opening far enough to let more air & fuel in- could maybe be a minor leak in the diaphragm gasket. I suppose needle height could be wrong too but that seems a stretch.

Suggest getting the bike up to the speed where the problem starts happening, then gradually open the enrichener. If power goes up then the slides are OK and there is a problem getting fuel thru the jets (possibly including a bonkers needle valve position). If it doesn't then maybe fuel flow issues as above (or the slides not opening)- +1 on the obscure places that can clog up in these carbs.

I envy your OEM tank- mine had so many holes rusted thru it looked like someone had blasted it with a shotgun. Replaced with a Maxim tank- it fits OK-ish but at least the original petcock fits properly, and in the right location.

Another great reply. Thank you!

 

[kebabs]

Remember, the world's not so dark when you are on your bike on a sunny day ripping it at 65 mph. And that's what this forum is all about, right?


You are welcome. Please do post a follow-up, even if you run into troubles. Post more pics of the bike too, even if it is not running. You don't see nice, original Secas everyday!

65mph is that with the wind on your tail