DOHC hard to start, but runs great when it's warm?

It looks like I didn't clean my carbs out soon enough when I started to have issues and now my '82 XS400 Seca will only start if I remove the airbox and cover up one of the carb intakes. I can cover up either one and the bike will start.

I've cleaned out the carbs 3 times already. (I've heard "3rd time is the charm")
Air, Pilots, and Mains are clean. (Soaked in Alcohol and blown through with carb cleaner. Blown through again once installed.)
Pilot jet is set to 3-1/2 turns out. (Was 2-1/2 until 3rd cleaning)
Pilot jets are both 42.5 (though LH is the holey one and the RH is solid)
Main jets are 127.5 and 117.5 (LH,RH)
Chokes are clean.
Floats are level and to the book.
No mods to intake or exhaust... totally stock. I didn't even rejet anything.
LH Spark plug is usually black. RH is usually Ash-colored with black around the outside.
I've never changed the plugs or wires, though I have brushed the plugs; no change.

It has run progressively better each time that I've cleaned it, but will only start when I do the "Indian Trick" from The World's Fastest Indian. (At the 0:25 mark)
After she fires up and gets warm, she restarts easily... even hours later. Once she cools off overnight it's back to the carb trick again. I've been tempted to leave the intakes off, but since dust could get in I really don't want to do that.

Since I know that seeing and hearing are better than words:

Sounds like you are on the right track thinking that the carbs are dirty. Some thoughts:

- Fully disassemble the carbs (except the throttle butterflies) and simmer the bodies and float bowls in a mild acid. Vinegar, Lemon juice, or a water-based household cleaner that will remove rust or water spots (CLR, tub and tile cleaner, etc.)
- Find and clean the hole (close to carb body) on the brass tube that goes into the float bowls.
- Make sure the orifice in the float bowls where the brass tubes insert is clear.
- Use a fine wire and probe all the passages as best as you can, use compressed air to blow out the passages from both directions, then simmer again.
- Get matching pilot jets - the left jet appears to be delivering more fuel than the right.
- On reassembly, check the diaphragms for leakage by lifting the piston and sealing the opening at the top of the carb inlet. The piston shouldn't drop much and then hold until you uncover the opening.
- Running with the intake boots off will cause the engine to run lean and allow dirt and dust in, don't do it!
- Be nice to your engine, don't rev it above 4000 until the oil has warmed up (5 minutes of gentle riding).

Fresh plugs, wires, and boots may also help. Let us know how you make out!

What is the value the book says for the float height?If its anything like the sohc engines its wrong.My bike I have them set 27 measuring from the top of the gasket.

DOHC floats are set differently. The fuel level in the float bowl is checked on the bike.

the jet sizes are supposed to be different because the airbox opening is on on side... one side actually gets more air flow..... i will have to look in the book for the values...

Indeed, the DOHC is checked on the bike and it's a huge pain. I definitely take advantage of the +-3mm! I cheated and used the "does it leak when the bowl screws are loose?" and adjusted it until it just barely seeps over the course of a few minutes while running. I don't have the special probe that goes inside the bowls, so I had to make do.

The carb bodies are boiling now. I put 2 cups of vinegar in the pot and filled the rest with water. The whole house smells and I keep getting dirty looks from my wife for using "her pot." In any case, I don't have an air compressor, so I blew out the internals with carb cleaner again. I'll dry things out overnight and re-assemble tomorrow.

If I were to buy matching pilot jets, should I get the same size? 42.5?
If I were to buy a set of 45s would I need to back the pilot screw back to 2-1/2? or less?
I understand the reasoning behind the main jets being different (the Seca airbox), but would buying a matching set be detrimental? I'm not wanting to second guess the Japanese, but it makes my OCD a bit crazy knowing that things aren't matchy-matchy.
Lastly, should I use the Fuel Injector cleaner (the fuel additive kind) to help prevent this mess in the future?

Thanks!
-J

PS- Is my video helpful or am I a total jackass?

Find some clear tubing that fits into the drain of the float bowl and use that to check your float settings. The manual has the details on how to do it.

Since your bike is stock, you should be fine with the stock pilot jets. Just make sure they are the same design - preferably without holes. If you are keeping the stock airbox, keep the main jets staggered like the factory did. If you match them, the right cylinder will run rich, especially at full throttle above 7000 RPM. Read the write-up here to get a better understanding of how to test and jet your carbs.

Fuel injector cleaner won't help carbs much, and may damage the rubber in the carbs and intake manifolds.

I understood your written description, and the video confirmed it for me.

I boiled the carbs and reassembled everything. She fired right up but now she's smoking on the left hand cylinder.

While reassembling the carb I noticed I mis-stated the pilot jets:
LH - 45 holey
RH - 42.5 solid
Pilot screw is 3-1/2 turns out on each.

So, what could be making it smoke? Is the LH really getting too much fuel now at 3-1/2 turns?

I would try going back a half a turn to 3 turns on the left side to see if it makes a difference.

Also I forgot to ad check your plugs for color and read them.This will give you an idea if you need to adjust fuel air mix and how much to lean or enrichen.Also do a carb sync when you your finished.

I'm going to repeat myself - Match the pilot jets! Same size and design.

Once you have matching pilot jets installed, set the pilot screws to 3 turns to get the bike running. Then go for a brisk 15 minute ride to get the engine to full operating temp. Then adjust each pilot screw for the strongest idle and sync the carbs.

Scorpio1963 said:

Also do a carb sync when you your finished.

Click to expand...

+1

If you've never done this, there's good info in the tech area or search for "carb sync"

Backing off the LH Pilot screw to 3 turns out let the cylinder stop smoking on that side, so that worked well.

Dave, I totally heard you. I just need to wait for payday to get things lined up. MikesXS.net has matching jets, but I just see the holey ones. I'll go ahead and order those and match things up, then sync the carbs up (once I buy that as well).

I've gone about 40 miles so far since the carb-boiling: She's a bit anemic under 4000 RPM, but feels really strong beyond that. I imagine it's due to the pilot jets and whatnot. I'll report back when that's settled so someone else that stumbles upon this post has some answers.