Topic: PD Carb Air Leak

[550Resto]

Hello all, I've got a 78 550 and she runs well. I however have a high idle that hangs when coming to a stop at about 2 to 3k. If I apply forward twisting on the throttle she tends to somewhat settle down. When checking for air leaks, where should I focus? I was thinking carbs to manifold, would this be correct? This is the last item I've been having issues with. 4 into 1 delkevic exhaust. Stock slows and 100 mains, stock air box. Carbs were rebuilt by Harisuluv and bench synced. IMS is out a turn and a half. Any help appreciated.

[flybox1]

Do a vacuum sync, and then follow it up by a good idle plug chop.   Get your IMS dialed in.
See if the hanging idle persists.

[DaveBarbier]

If that doesn't the do it, check for leaks anywhere between the carbs and the head. There are o-rings between the manifolds and the head that are usually flattened out and hard. They should be replaced. I believe they're the same size o-rings that are under the tappet adjuster caps.

[eigenvector]

The air leaks I get on mine are usually between the airbox and the carb rack.  That oval gasket between the airbox and the filter likes to fall out for instance.

2 to 3k isn't bad - minor air leak at best more likely.  It's tough to sync the carbs with an air leak though.  I'd run down the leak first then sync.  Best option - spray starter fluid around the boots (sparingly - you don't want a flaming bike) and listen for rpm changes.  I'm guessing it's between the airbox and the rack.

[DaveBarbier]

The air leaks I get on mine are usually between the airbox and the carb rack.  That oval gasket between the airbox and the filter likes to fall out for instance.

2 to 3k isn't bad - minor air leak at best more likely.  It's tough to sync the carbs with an air leak though.  I'd run down the leak first then sync.  Best option - spray starter fluid around the boots (sparingly - you don't want a flaming bike) and listen for rpm changes.  I'm guessing it's between the airbox and the rack.

Flybox mentioned the sync because out of sync carbs can cause a hanging RPM situation. Might not be a leak at all.

And since air leaks before the carb also bring fuel with it generally leaks after the carbs are the issue. Because that leans out the mixture a lot. Either way, any air leaks aren't good so check everywhere.

[550Resto]

Would an idle chop be somewhat the same as a high gear chop? The carbs were vacuum synced and were spit on. I haven't done the clear tube test yet. I've replaced the o rings at the head so I know those are new. For the PDs fully turned in would lean the mixture while bringing them out would richen, correct? Also these only apply to idle, yes?

[Lostboy Steve]

No. An Idle chop is strictly an Idle chop.

[Lostboy Steve]

Wait, you can roll the throttle forward and it settles down? You sure your cable isn't sticking or your return spring weak?

[uksparky]

I noticed i have the same problem...smooth idle 1200.....twenty mile ride perfect, come to stop 3k rpm??...no cable hang up .....it would take a big vacuum leak for it to do that.....78 CB550k

[DaveBarbier]

I noticed i have the same problem...smooth idle 1200.....twenty mile ride perfect, come to stop 3k rpm??...no cable hang up .....it would take a big vacuum leak for it to do that.....78 CB550k

You need to set your idle when it's fully warmed up. It's normal to hold the throttle open on a cold motor. But your carbs have a fast idle cam which you can set to raise the idle when choke is applied. It's a screw with a spring around it between the center carbs. Should be set already but I had to adjust mine to about 2500 rpm at full choke. Works like a charm.

[550Resto]

My choke is adjusted to 3k, but should it go to 3k right away when the choke is applied from a cold start? I notice she hangs around 1000 until she's a little warmed up and with choke applied after warm up she goes up to the 3k mark.

As far as rolling on the throttle, it doesn't always settle it down. Movement for that is minimal and cable and return springs are not sticking. I notice the 3k hang after she's good and warmed up.

[Spanner 1]

When you put the carbs on did they tend to ' sink down' without the support of the airbox ? If so, then they are actually not all the way home in the carb to head rubber intakes. Unless the rubber is supple ( none of these old bikes have that !), it takes quite a force to seat the carbs over the bump in the intakes to where they are all the way home. I use a 3ft 2x4 levered off the frame downtubes on each side against the carb rack to get them to seat. If not seated all the way, then for sure you will have air leaks = hanging throttle, lean condition and popping on decel. Also, if seated correctly, the band clamps will not be visible from either side, only at the bottom where they tighten.
Simple test... sit on the bike with the motor running and grab the carb bank with both hands and try to pull it up and press it down, any difference in idle r.p.m.s. ?

[550Resto]

I think they might have had a little hang to them. When I put them in to the motor side I ended up taking the manifolds off the engine and doing it that way and then installing everything back on to the bike. I'll have to give this a shot and see what happens. Thanks for the input!