Topic: Cb175 spontaneously revving

[derelicte]

I stupidly picked up a 1971 CB175 a few months ago on craigslist even though I'm plenty busy with the cb750 also in pieces in the garage.  Was in great shape but not running, gave the carbs a good cleaning and then found I wasn't getting spark.  New plug wires, boots and condensor and still no spark, but it turned out a points adjustment got me up and running.  Got some new tires on there too. 

It starts up easy and will otherwise run fine but will spontaneously decide to rev up to 7 or 8k until I rush to switch off the ignition.  A few weeks ago, the first time I got it to start (it had sat for a year) it did this.  Then runs fine.  Then periodically out of nowhere will just rev the #$%* out of itself with no throttle input.  Again idle is otherwise smooth and sometimes it'll shoot up to 6 or 7k and just sit there (well it would if I weren't killing the ignition).  What are the obvious things to check?  Stock bike.  I really want to get back to the 750 and am close to forking it over to a shop so I can refocus on the other bike.  Throttle cable looks ok but again itll do this when it had been sitting there at idle without touching the throttle.  Any items I could check would be appreciated 

[trueblue]

My 200 was doing this and it turned out to be the throttle cable hooked up near the front mount of the fuel tank.  It would randomly rev without any apparent cause, I replaced the o-rings in the intake manifolds expecting it to be air leaks with no change before I found the cable hooked up.

[tbpmusic]

My 200 was doing this and it turned out to be the throttle cable hooked up near the front mount of the fuel tank.  It would randomly rev without any apparent cause, I replaced the o-rings in the intake manifolds expecting it to be air leaks with no change before I found the cable hooked up.

+10 on that - on both the CB200 and 175 the throttle cable is just barely long enough to reach, so routing is critical. Examine especially the "junction box" for evidence of a routing issue.
What you're seeing is textbook for routing issues - sometimes the bike will go faster when you turn the bars in a certain direction, stuff like that.......

[derelicte]

Sorry for the delayed reply, I thought I had posted a thank you but apparently not.  When I put the tank back on I routed the throttle cable to the outside of the tank mount instead of through the tunnel.  Fixed!  Great advice, thanks.  Now I have another problem.  Bike won't idle right, idle speed will drop too low and it'll stall unless I stay on the throttle a bit at stops.  I've done the idle tuning suggested in the manual (achieve max idle speed with air screw then adjust the stop screw accordingly), but the stop screw doesn't seem to affect the idle at all despite very large adjustments (pretty much had them all the way in and out trying to achieve an effect).  Intermittently one of the carbs is overflowing at startup while fiddling in the garage, but they were just soaked in simple green and sprayed out with carb spray / canned air & floats reset.  Ideas?  Aside from idle the bike rides out very well without any power loss or surging on the road, but as soon as I pull the clutch the idle drops too low and it'll die.

[trueblue]

I'm not sure on that one, i haven't had that issue with mine.  It sounds like the slides aren't rising when you work the idle screw.  Remove the air cleaner and look into the throat of the carb and work the screw, you should see the slides raise and lower, also check to make sure the slides are in the correct carbs.  The cutaway should be towards the airbox.

[grcamna2]

+1 I'd also be sure to fully clean out the pilot jets & all the passages that relate to the pilot circuit.It'll help a bit also when you synchronize both throttle cables so both carb. slides rise simultaneously.

[derelicte]

I got an ultrasonic cleaner for the 750 and took the carbs off the 175 to give it a try.  Just now pulled the lid off after the first cycle and the solution is fugly.  Thought they were pretty clean after the initial soaking w/ simple green then carb spray but maybe this'll do it.

[derelicte]

Apologize in advance..  Ordered an oem gasket kit and there are two pieces I don't know what to do with.  The shop manual shows what looks like an o-ring on the air screw, but it doesn't fit (see pic).  And the metal washer I don't have a guess for.  Pilot jets had some junk in them I missed so hopefully this will get her running right.  Carbs were also overflowing before and I've discovered a leak in one "buoy" of both sets of floats (one of them probably half full of gas) - going to try a repair with some JB weld after I get them drained. 

[derelicte]

The carbs were fully disassembled again and ultrasonically cleaned, and I went at the idle jets with a guitar string.  I've tried adjusting the air & stop screws with both .875 turns out and 1.5 turns out as starting points (both found online, don't see a specification in the shop manual).  Honda gasket kits installed - one extra o-ring per carb (the kit fits other models as well and I think it was extra -- at least as I got the bike, there was no corresponding o-ring when I first disassembled them. )

Bike starts easy, quick touch of the starter and some throttle.  Once warmed up around the neighborhood, at best I can get the bike to hang at 3 or 4k before it drops right down to below 1k or so and stalls.  Could this be weak spark?  As I said earlier it was getting no spark at all until I adjusted the points.  Everything else about the bike seems perfect, there are no flat spots, no hesitation, no smoke.. 

[trueblue]

Clean up the ignition system, it can't hurt.  I haven't had that issue myself before, once I fixed the cable routing mine idled fine.

[grcamna2]

derelicte,
     I think it'd be good to pull the points plate off and clean it all up,check your points...they may need replacing(use Honda OEM) ALSO service your spark advancer mechanism.  !

[derelicte]

 Running great!  Took out the manual this weekend and performed some of the engine maintenance procedures - tappet adjustment, static timing, checked points gap and advance mechanism...  Timing was advanced by my estimation one cm or so on the rotor.  Dialed that in and bought a timing light to make sure it was advancing properly.  Got everything back together and it's idling perfectly.  Could not be happier, first time trying any of this and its great to see it idling so steadily finally.  Much less engine noise at idle now and definitely felt like it was pulling stronger when I tooled around the neighborhood. 

While I had everything apart I put on an untested replacement condensor I found on ebay.  I noticed arcing at the points afterward so reverted to stock thinking the replacement was bad but the arcing was still there.  Read that can be from oil from the feeler gauges (I did use them on the tappets first) - had me concerned but the way it's running I think I'll just see what it's doing after another tank of gas. 

Thanks all    One out of the garage finally

[grcamna2]

Those little CB175's are great bikes;they do real good for what they are.Did you take the advancer unit off of the engine & take it apart & put new high temp silicone grease on the main shaft ? That'll help also.
They're a fun around town(or farther !)motorcycle. Glad to hear you're running well  !