Help me troubleshoot revving problem?!

[Boswell]

Figure this has to have happened to someone else here, help me located the gremlin and shorten my frustration. I paid someone a lot of money, and this problem developed after they 'fixed' it.
73 cb350 four
So here's the deal, once it warms up and choke is fully open, after a few miles when i come to a stop the revs stay up around 3K, if I let the clutch out a little, the revs drop back down to 1200. It doesn't do this everytime, but when it does its annoying as shit and LOUD. The idle can not be set because it's all over the place. I'm constantly screwing around with the idle screw (honestly, no pun intended). This bike is ornery, it has good days and bad days, weather dependent, but this is common to everyday.

If it sits for a few days, it's a BITCH to get started too.

 

[Sonreir]

Refusing to return to idle is usually a symptom of running rich. If you're rich, the bike should tend to run better when it's cold as well.

Before we go too much further down that road, make sure compression, timing, spark, and valve clearances all check out OK. Then you can begin tuning the carbs.

 

[Boswell]

That's the idea, go through a thorough tune up procedure, just need direction of where to sniff out the problem. It backfires a little when warm and the exhaust smells a little like gasoline. I had assumed it was running rich or float bowls. This is frustrating as the $700 I spent on work went primarily to 'carb work'. I really can't afford down time, hope to find the solution quickly as it's my main mode of transport.

 

[Sonreir]

All of the following assumes you have clean carbs and the float levels correctly set. You may wish to do that first, but if your mechanic is worth the money you paid (I have my doubts), this should already be done.

First up, checking the static timing. This process is just a quick check to ensure the points are all opening when they should and are gapped correctly. The processes is detailed in about a dozen threads and should be covered in your manual as well.

Next check if you valve clearances. Tighten the chain tensioner before you begin. Check your manual for actual specs, but I think you want .002" for the intake and .003" for the exhaust. This involves rotating the engine until a given valve is fully closed and all tension is off of the rocker arm. Slip the feeler gauge into the gap between the rocker arm and the valve stem. Adjust the screw until the feeler gauge is just dragging. It shouldn't be tight. Now tighten the lock nut to keep things from moving around too much. Do this for each of the eight valves.

Now check compression. To begin with you will need a compression gauge. Get the bike running and let it warm up. Once everything is nice and toasty, check each cylinder's compression, in turn. You do this by removing the spark plug and inserting the compression tester. Now, with the throttle all the way open, the choke off, and the kill switch turned to off (you're not trying to start the bike), turn the engine over a few times. You can kick it or use the electric start, though the latter is preferable. You want around 165 PSI from each cylinder, though you can go down as low as 150, if needed. Anything lower than that and you've got some issues that need to be taken care of. Each cylinder should be pretty close to one another as well.

Assuming everything checks out OK, it's time to sync your carbs. This is done by letting the bike idle until a warm and then adjusting the air mixture screw for each cylinder until you achieve the maximum RPM you can get. A better way to do it is to use a carb sync tool, if you have one. After all the air mixture screws are adjusted to their optimum RPM, back the idle screw down until you're right around 1200 RPM.

If you're still having problems after this, let us know.

 

[eram]

In addition to the good advice given above, you may also want to check for air leaks in the system. I had an issue with a screw coming loose and the carbs squishing off the manifold just enough to give a small air leak that left my revs hanging very high.

 

[Boswell]

All very good information. That was basically my plan of attack right there. The mechanic did a lot of 'carb work' synching etc, I really don't want it to be the carbs. I will perform the above, including looking for air leaks (didn't think of that one myself, and highly likely, the boots look decent, not perfect), and get back with results. I just really don't want to take the damn carbs off!

 

[teazer]

High idle is usually caused by air leaks or lean mixture, but in this case I'd also look at the ignition advancer. If the springs broke or are loose the advancer stays advanced and doesn't return to the retarded position so revs hang up. Pull teh points cover off anc check that and while you have the cover off, set the points and timing.

And since it happened after the shop did that work check for the other obvious things like cables getting trapped or routed teh wrong way that holds the throttle open. Check the cables to see that they are not too tight and make sure there's enough slack in the "pull" cable.

 

[Rocan]

check that your carb diaphragms dont have a crack in them... its a common issue with the 70s hondas. also check that your throttle cable isnt binding; i had this issue and I scratched my head for a week before I found the issue.

 

[Boswell]

Still have to work on the timing, would like to set with a strobe, but oddly, the only PROBLEM I found in a tune up was the points gap was NIL, they were so tight they were barely in spec while fully open. This alone made the bike sound a thousand times better. That and the mechanic left the breaker panel screws so loose the panel could move around which, I'm assuming was causing most of my problems.

 

[djelliott]

You spent good money. I would make him fix it. Any mechanic worth a damn will warranty their work within a reasonable time frame. Especially if obvious mistakes were made.

 

[SrgtBear]

Im gonna piggy back that it might be an air leak. If you have an air leak between the carbs and cylinders it's gonna make the engine rev faster.

 

[Boswell]

Not revving at idle anymore, like i mentioned before, my timing is still a little out, but the damn thing is a whole new bike with a new set of plugs and points gap set. The oil was crystal clear, valve train looked spotless, now it's time to ride the piss out of it until i break something.

There's been a few things I found that the mechanic fouled up, small, forgetful things. Thing is, he's a 'famous' guy around here who's got a lot of supporters, so bad mouthing doesn't get you anywhere. I just have to find a garage spot where I can get set up to do all my own work, it's hard laying on the sidewalk in front of your house on the street in Kensington.