Topic: CB350F runaway idle (various tuning problems)

[jpschroeder]

so the context here is at the bike was working pretty well. But at idle cylinder number four wasn’t firing consistently. I had just rebuilt the carbs and thought maybe the slow jet on number four wasn’t fully cleared out so I took the carbs back off and cleaned the slow jets again while I had everything off the bike, I decided to also check the ignition timing even though it seemed fine the bike is new to me so I thought I would check. It’s a Dyna S system - using their manual I saw the ignition was actually set a bit too advanced so I backed it off just a bit (static timing)

After putting everything back on the bike…it runs terrible. It is way harder to start (which I actually thought might be ok initially, the bike never needed choke before) and seems to “run away” at idle. If you blip the throttle (when it’s warmed up) it doesn’t return and will stay above the spark advance for a long while, sometimes it accelerates to that 3-4K range on its own.

The carbs were beautiful and clean. All new o-rings, and well seated ones at that. Could this be related to the ignition timing?

Heeeelp!

[RAFster122s]

Is timing advancer unit rotating freely, sounds like issues in that area

[jpschroeder]

Here is a video of it:

[denward17]

Throttle sticking?  Check cable routing and connections at the carbs.

[jpschroeder]

Sadly no, not the throttle sticking. I just gave it a bit of a test ride. It really doesn’t like to come down after throttling up and when it does it goes down so slow it sometimes dies.

[newday777]

Did you vacuum sync the carbs yet? If they are off like the 750 I just sync’d today you will get your runaway idle and slow return to normal idle.

[jpschroeder]

Did you vacuum sync the carbs yet? If they are off like the 750 I just sync’d today you will get your runaway idle and slow return to normal idle.

Oh really!? I haven’t done a vac sync yet, the tools to do it should arrive tomorrow or Sunday though. I would be thrilled if that was it. I assumed a bench sync was good enough to not have these kids of symptoms.


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[HondaMan]

Often those O-rings in the carb mounts to the head are leaking vacuum: they are hard as a rock now.
They are 27x2.4mm size.

[jpschroeder]

Often those O-rings in the carb mounts to the head are leaking vacuum: they are hard as a rock now.
They are 27x2.4mm size.

@HondaMan - are you talking about #9 on this diagram?




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[HondaMan]

Often those O-rings in the carb mounts to the head are leaking vacuum: they are hard as a rock now.
They are 27x2.4mm size.

@HondaMan - are you talking about #9 on this diagram?




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Yep!
I once rebuilt a [beautiful] show-bike 350F for a guy from Idaho that acted like you describe. It needed a full teardown for deep-cleaning of the engine, so he had me go an oversize at the same time (20k miles). It idled OK when it was real hot, but until then it either wouldn't idle or it would run up around 2800-3200 RPM, wandering around in speed. I fixed that first with the new O-rings, then rebuilt it. Afterward it purred like a kitten at the touch of the starter button, it was so cool!

[jpschroeder]

I assume since this print sits right against the head it must need to be heat resistant too? What material would you recommend? Also where do you find that size? It doesn’t appear to be that on any oring sites

[scottly]

It’s a Dyna S system - using their manual I saw the ignition was actually set a bit too advanced so I backed it off just a bit (static timing)

If you blip the throttle (when it’s warmed up) it doesn’t return and will stay above the spark advance for a long while, sometimes it accelerates to that 3-4K range on its own.

Could this be related to the ignition timing?

Heeeelp!

The advance springs are too weak, allowing full advance too soon.

[jpschroeder]

@scottly I suppose I could verify this theory using a timing strobe before I “spring” for a new advance unit.

Can anyone vouch for the aftermarket ones (David silver, 4 into 1, eBay etc)? I know everyone prefers oem but it seems crazy to drop $200 on an oem advance unit just for some springs.


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[HondaMan]

Just trim the existing springs back, to slow their advance rate. They have become softened over the years from heat-annealing. On nearly every SOHC4 I rebuild, it is standard practice to trim off at least 1/2 turn on both springs: on hi-mileage engines or the 500/550 it usually takes more like 1 full turn off at least one of the springs and 1/2 to 1 turn on the other (because they were too soft to begin with).

I'll look to see if I have any of those O-rings left: I bought a bunch of them years ago, made a special order of them. Not sure, at the moment?

Honda usually has them, too. You'd probably have to look up the number at CMSNL and order it from South Sound Honda, or something like that - although South Sound has a $25 handling fee for anything less than $200, while CMSNL is postage from Europe.

[newday777]

although South Sound has a $25 handling fee for anything less than $200, while CMSNL is postage from Europe.

Mark
I've been charged for shipping on $500.

[scottly]

@scottly I suppose I could verify this theory using a timing strobe before I “spring” for a new advance unit.

Can anyone vouch for the aftermarket ones (David silver, 4 into 1, eBay etc)? I know everyone prefers oem but it seems crazy to drop $200 on an oem advance unit just for some springs.


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I've tightened up weak springs by just squeezing down the end loops with a pair of pliers with good results. Try that first, making sure that the advance moves freely. The springs should have some tension at rest; free-play can cause your symptoms.

[HondaMan]

although South Sound has a $25 handling fee for anything less than $200, while CMSNL is postage from Europe.

Mark
I've been charged for shipping on $500.

Was this recently? I haven't ordered from them since May (2024). Then I ordered a bunch of bearing shells that came to almost $300, and they didn't charge shipping on it.

[newday777]

although South Sound has a $25 handling fee for anything less than $200, while CMSNL is postage from Europe.

Mark
I've been charged for shipping on $500.

Was this recently? I haven't ordered from them since May (2024). Then I ordered a bunch of bearing shells that came to almost $300, and they didn't charge shipping on it.

Yes, every order I've had, most are usually $300-$750.
I even spoke with Dexter about it.
It's a bit of a hard pill to swallow getting a delivery of a 5x5x5 box of carb parts, miscellaneous bolts and rubber bits totalling $400+.

[HondaMan]

although South Sound has a $25 handling fee for anything less than $200, while CMSNL is postage from Europe.

Mark
I've been charged for shipping on $500.

Was this recently? I haven't ordered from them since May (2024). Then I ordered a bunch of bearing shells that came to almost $300, and they didn't charge shipping on it.

Yes, every order I've had, most are usually $300-$750.
I even spoke with Dexter about it.
It's a bit of a hard pill to swallow getting a delivery of a 5x5x5 box of carb parts, miscellaneous bolts and rubber bits totalling $400+.

I went back and looked, and I have it 'switched': they charged me for shipping, but dropped off their $25 "service charge" that I've seen for orders less than $200 from time to time.
Sorry for the confusion! 

[jpschroeder]

Ok @HondaMan — before i start trimming these advancement springs, I checked with the timing light to see if I could detect something "off" — basically from 1200rpm its firing at roughly the F mark (just a bit advance from there) any RPM increase from there moves the firing position forward and simultaneously the bike starts coughing and wheezing like it has the black lung until it hits a much higher RPM, honestly its not happy till like 3000. It *really* doesnt like that middle range 1200-2500 or so — at say 1800 the timing is in between the F and advance marks still and it is running terribly there.

So the springs might be weak, but they are not way way out to my uneducated eye...I suppose it could still be related to the vacuum leak on those o-rings, and ill look for a replacement, but for some reason this feels more ignition related to me — probably because in the sequence of me jacking around with stuff thats where the issues really started. When i checked the bike the timing seemed quite a bit further advanced than I thought it should have been (at least 10deg) like a dummy i set it back to the "proper" timing mark (using the advance marks) and thats when my trouble started.

[scottly]

Is the advance at the full advance " marks at 3000 RPM and above?

[jpschroeder]

I checked it out some more this morning. The spark was fully advanced, when the engine was in the 2200-2400rpm, and not before. The advance started slowly moving forward around 1200rpm. So i think in this case the springs are fine.

On a whim I decided to move the timing forward more since that is how it was when I got the bike...huge difference. It runs so much better when its advanced further. is there any chance the plate is off? Is this just my bike? Is this a well known "thing" about the SOHC4 bikes?

Finally once it was advanced like this I did a carb sync, and she seems to be purring like a kitten now. I dont know guys, would love to understand this better.

[scottly]

On a whim I decided to move the timing forward more since that is how it was when I got the bike...huge difference. It runs so much better when its advanced further. is there any chance the plate is off?

Someone recently had an aftermarket advancer that had the index pin on the backside in the wrong position, which caused the timing marks to be in the wrong place, which appears to be the case with yours. There was also a case where the timing marks were incorrect for the model of advancer. The pin should line up with the T mark on the other side.
The 350/400 should have a 333 advancer, with timing marks spaced like the one on the left in the second pic.

[Herr Supersport]

The check is simple:
•   Bring cylinders 1 and 4 to the top dead center. Remove all spark plugs and use a dial gauge or positop.
•   The T-mark of the ignition timing adjuster must now align with the mark on the engine housing.
•   If it doesn’t, your ignition timing adjuster may not be correct.

Regards,
Daniel

[jpschroeder]

I will definitely try that daniel.

I rode the bike again some more just now and it definitely is running much better. It still doesn’t like going from idle to high rpm very much, it sort of burbles and coughs a bit when I advance the throttle off idle, and sometimes in need to keep just a hair of pressure on the throttle at stop lights to keep it from dipping too low, other times it seem bang-on at 1200.

Im wondering if this could have anything to do with the mixture. I’m at 1 full turn out, which, according to the manual would be a bit lean, but if anything it feels seems its a bit rich after looking at the plugs and what not. Elevation here is ~600ft above sea level so not very high.

Ok, other thought here, I came across this decade old thread, and specifically something @Deltarider said:

http://forums.sohc4.net/index.php?topic=142860.0#msg1622781

> I had noticed that when I checked the ignition after the bike had been at the dealers, ignition always had a few degrees extra advance. At first I was proud to discover this and correct the timing to exacly the marks, Honda had put there, only to discover my bike was not as lively as before. Also a friends 550 always returned with a few extra degrees advance. When I corrected this for him, proud to use my new dwellmeter and strobe and met him later on, he told me he had not been happy with the result and had the dealer set the timing like before. Later I found out more mechanics (some of them had owned a 500/550 or still owned one) had the experience of a better result with a few degrees extra advance.

He postulates there that it could be a bit early because the fuel quality in the US is generally poor. The only non-ethanol fuel I can get my hands on is premium 90 octaine fuel...what this means I don’t know. I’m out of my depth.