No Throttle response help

[projectbuy]

Could I get an idea as to what could be the problem with a 1982 xs400 that has no throttle response when engaged into gear could be? It's dohc and currently missing intake boots to airbox. It runs great and cranks every time. Idles smooth and has throttle response when idling. The clutch is good. Could this be as simple as carbs needing to be synced? This is a bike I'm trying to buy and want to confirm the seller's claims that all I would need to do is a carb sync. Thanks

Edited to add more info

 

[tstidham]

Some additional details on the year, sohc vs dohc, pod filters or factory filter may help. Sounds like it's struggling under load, but I'd make sure your basics are covered like ignition timing, points gap (if equipped), valve lash, clean carbs, functional diaphragms, voltages, etc.

 

[projectbuy]

Some additional details on the year, sohc vs dohc, pod filters or factory filter may help. Sounds like it's struggling under load, but I'd make sure your basics are covered like ignition timing, points gap (if equipped), valve lash, clean carbs, functional diaphragms, voltages, etc.

Added some more details to the post. Other bit of info is that the bike is able to shift into second by very slowly giving it throttle. Can't check much else due to it being a bike I want to buy. Realize it's not ideal, sorry. Was just wondering how likely it was that a carb sync was the problem.

 

[tstidham]

If you have a compression gauge (or can rent one), I would check that prior to purchase. I think the carbs would have to be awfully out of sync to cause such an issue, and hopefully someone else will chime in that may know better than me. I have only ever bench synced my carbs by setting both butterflies to the same opening distance, then did my best to keep both idle jets close to each other when doing my running adjustments.

You can check the diaphragms when not running by using a finger to lift the slide into the carb body, then cover the oval air passage at the top of the carb opening and let go of the slide. It should move a little down, then hold position for as long as you keep the opening covered/sealed. Once you release the opening, the slide should smoothly return. If they don't hold, then the diaphragms are bad. I would also hold both slides up and look deeper into the carb bodies to see if the butterflies idle opening appears way different from each other. If the opening are WAY off, it could be a sync issue.

You can also (carefully) turn in each idle jet mixture screw (again, while not running) making sure to count how many turns (I like to count 1/2 and 1/4 turns myself) it takes to lightly seat the needle screws. Just use light fingertip clamping pressure on you screwdriver body when doing this so as not to actually "tighten" the screw into it's bottom most position, you don't want to snap off the tips. Then turn them back out with the same count, returning them to their starting positions. The point of this would be to see if the setting are far different from each other, again pointing to a sync issue.

If you suspect the in gear running problem is due to a lean condition, you could always try pulling the "choke" to the first position and see if it improves. Just know it's not really a choke in the traditional sense, where on a car, an additional butterfly restricts air to increase the fuel mixture. These just add additional fuel to the standard air intake amount (operationally the same outcome).

 

[CaptChrome]

Was just wondering how likely it was that a carb sync was the problem.

Not likely. It will be more involved than that most likely. If you are uncomfortable with all of the other checks and adjustments that @tstidham just outlined above, walk away from buying it.