Topic: Diagnosing 550 with random high revs at idle

[zsno]

Hey guys,

So I've got my project almost all complete. It's a 78 550 with the PD carbs. Kerker 4-1 exhaust that the PO put on and ran with. I never rode the bike after I bought it three months ago, as I was busy putting on new seat, tank, tires, brakes, MC, and other things.  I finally got all the structural and cosmetic things done and went to give the engine a tune up. I took the carbs out and cleaned em, reattached them. Gave the bike an oil change (minus the filter as I need a new bolt to replace the stripped one), set timing, and set valve clearances. Had to replace a coil to get all 4 firing.

So the problem I'm experiencing is the bike will start with the choke, then once I take the choke off and it comes down to idle, it will randomly rev up to 4-5K RPMs. It will rev up, then calm back down a bit, then rev back down. I'm attempting to sync the carbs, but I just cant get them all to sync. As soon as I get close, one carb will go out of sync. When I give the bike throttle, it will sometimes stick, but its not the cables sticking.

I think this may be a lean running condition plus the carbs not being synced. What should I be checking/doing? I'm so close to getting this bike on the road and its killing me not being able to ride it.

[OldSchool_IsCool]

Did you check/adjust your tappets?  Maybe an air leak between the carbs and the intake.  Spray that area with carb cleaner or WD40.  If revs change as a result of the spray, then you have an air leak.

[Hush]

I'm not too up on the 550 but is it possible to put one or more of the carb slides in backwards?
I know this has happened to other bikes and will drive you nuts as everything else checks out apart from the random accelerations.
Also definitely check your carb boots are on tight and air filter also, I took my bike to a mechanic to sync my carbs only to be laughed at when he discovered that I did not have the carbs totally in the boot on one carb.

[Caseygroh1]

make sure all of the float levels are correct. The sticking could be from the adjuster on the carb sticking (the piece where the screws for balancing the carbs is) this attaches the slide linkage with the throttle bar linkage by means of a ball joint. peel the little rubber pieces back and spray some oil in there.

[Hush]

As an afterthought, check your automatic advancer unit, the thing with springs and flying weights behind the points system, if they become rusted or jammed they will give you some pretty variable speeds.
Mine were rusted almost solid and a clean up and light greasing of the pivot pins made a huge difference to my bike.
Probably wont affect the bike at idle but makes a massive difference when accelerating, they are a very forgotten area of the bike being tucked away as they are behind the points.

[Highwayhigh]

I used to have the same issue on my bike. One of the issues was my throttle cable is VERY slightly sticky, so even if you don't notice it under load, at revs sometimes it'll make the bike run up a tad. But the real solution for me was when I swapped my cam with a 650 cam. I decided to go through and adjust my tappets, adjust my timing, adjust point gaps, and while I was in there I did the Hondaman spring mod on the advancer. My timing, tappets, and gaps were pretty awful. Put the whole thing back together and she ran like a champ. It may be something else on your end, but it is a must to ensure all your other "tune-up" portions are pristine before trying to troubleshoot further. As stated many times, carbs are the final step in the process. Only go there when you've exhausted all other means.

As far as the carb sync, I haven't done one on my bike yet so I can't help you there. Though I've always heard it is a painstaking process to initially sync the carbs. Good luck with the bike man, hope you can get it straightened out and enjoy the road!

[zsno]

Well the bike seemed to not have this issue when I ran it around the block initially. Then I tore it down to put on new rubber, rebuild the forks, run new cables, and clean the carbs. I've also already done the normal tune up things. So today I'm gonna take the carbs back off and see if I can get em cleaned and find an issue. I'm gonna pay special attention to the float height and making sure the slow jets are clear.

[zsno]

As an update, I'm at the shop now and have the carbs taken apart. The first thing I noticed was that one of the float needle retainers was missing, so the flow of fuel to that carb is not being controlled properly. This may or may not be the root of my problem, won't know until I get some new float needles. Also went ahead and cleaned out the jets again while I was in there. Gonna have to wait on these needles to know for sure. An other things I should be looking at while I'm in here to eliminate this high rev issue?

[OldSchool_IsCool]

Since your carbs are out, check the slides to make sure none are in backwards.  Make sure all the holes in the emulsion tubes are clear.  Sometimes the crud can hide the holes closest to the threads.

[flybox1]

....what oldschool said!
plus, recheck float height, jet orings for good seal, a good bench sync, and then try vac sync when reinstalled.
adjust them slowly.  they will bounce around when doing this.  its like playing 'whack-a-mole'.  hit one, and another pops up. just get them close and be sure to have a box fan blowing on your engine.

[zsno]

How exactly do I tell if they're in backwards?

[TwoTired]

How exactly do I tell if they're in backwards?

The slide cutaway faces the inlet when properly installed.

Make certain the slides slide easily in the bore throughout their entire travel.

The 78 pipes were restrictive and the carbs were set lean to compensate (and keep EPA feeling like they were doing something worthwhile).

So, your diagnoses of running lean is likely correct.  If you use pods, too, then rejetting is a necessity.

Idle "hunting" is a symptom of poor carb sync, as is leaky carb boots.

[zsno]

Good news! I replaced the faulty float needle. That then allowed me to properly sync the carbs and get the bike running. I put about 25 miles on it tonight and it seems to run well. Perhaps a tad on the lean side, but that's tomorrow's battle.

[CBGhia]

As an afterthought, check your automatic advancer unit, the thing with springs and flying weights behind the points system, if they become rusted or jammed they will give you some pretty variable speeds.
Mine were rusted almost solid and a clean up and light greasing of the pivot pins made a huge difference to my bike.
Probably wont affect the bike at idle but makes a massive difference when accelerating, they are a very forgotten area of the bike being tucked away as they are behind the points.

I was having issues with my bike at about 7k,  it would act really lean. I had checked timing and valves.  I thought I might have gummed up a jet, then I read this.  Sprayed the hell out of it with a contact cleaner and PRESTO!!!  Bike screams all the way up to 7k now!  Thanks!