Topic: CB350F Carb Sync Issue - seeking advice

[Rayzerman]

Thanks to all the knowledge here and after much work, I have my CB350F running reasonably nicely but not where it needs to be.  Carbs cleaned and in excellent shape, ignition all dialed in, etc.  As background, carbs have all original brass AFAIK, float levels all dialed in via clear tube method at I believe 24mm.  Bench sync'd and adjusted the slides.  Carbs on the bike, and it starts and runs like it should anywhere above idle.... I did discover that after running it a couple of times, #1 carb is dripping from the overflow tube, all others good.  I suspect a sticky float, perhaps I didn't polish up the pivot quite good enough.... meanwhile, it runs OK at idle... mostly.

I am trying to dial in the air screws, and when I hook up my Vaccumate Allweather sync tool (best tool ever) to the ports on the carb boots, I notice the vacuums need to be sync'd but no matter what I do with the air screws, those readings don't change.  I attribute a lot of that to there being a large distance between where the screws are... near the rear of the carbs, but the vac ports are way forward on the carb boots....

I set the air screws at 1 turn out initially, and when I play with them I can notice changes happen but it is very difficult to determine an optimal setting... all I can go by is changes in engine rpm (using a digital tach).  I would have thought I could dial them in using the vac ports, but again, turning those screws has little to no effect on the vac readings.......

What's next?  Change out the pilot jets to aftermarkets (in the 4-in-1 kits I have)?  Recheck the originals, although I'm quite sure they were as clean as could be.... thoughts from the brain trust here is what I need.  TIA.

[BenelliSEI]

I’ll be interested to see what others say. Like you, I really can’t say I’ve ever had any different results than you! I rarely see or hear any change when I play with the air screw. For a long time now, I just set them at the number of turns suggested in the appropriate manual, and never go back.

I start by doing everything as you did, then a final carb sync with my Motion Pro sticks, and call it a day. I do find that on a “fresh” motor, it pays to go back and do it again after 3-400 miles. After that, I just ride it!

[Ozzybud]

To synchronize the carburetors you need to be adjusting the linkage on top of the carburetors. This changes the position of the slides and gets vacuum equal across all 4 cylinders  The air screw only adjust air/fuel mixture at idle.

[Mark1976]

To synchronize the carburetors you need to be adjusting the linkage on top of the carburetors. This changes the position of the slides and gets vacuum equal across all 4 cylinders  The air screw only adjust air/fuel mixture at idle.

The mixture screws are I think 1 turn out from lightly seated on the 350 (correct me if I'm wrong, it happens). Then sync the rack, that's it, they have a 1/2 turn of adjustment in or out. Turning (adjusting) them while its running is not going to yield anything but confusion. If they're lean, it'll be slow to return to a proper idle, if so slightly, if its a touch rich it'll be a bit soggy at idle (the idle will drop and then come back up ever so slightly, but trying to adjust the mixture scews while its running, even with vacuum gauges is just not going to work. Set it, sync it, ride it, see what you have.

[rotortiller]

Address the leaky float overflow situation, set the pilots to default and sync like Ozzybud says, is how I would deal with the sucker.

[Rayzerman]

To explain a bit further, I did the bench sync and the slides are set to 1/16" all anally and equal.  The slides initially do affect idle as I understood it, and if I'm not mistaken come more into play after idle, and the main jets at higher rpms.  This bike performs perfectly at anything off idle, no hesitations, smooth, etc..  I'm just wanting the idle fixed up.  The #1 float issue is just a steady little drip, nothing big, but yes that has to be fixed.  I'm doing all this while the bike is on the lift, in neutral, with a temporary fuel bottle, I can't go ride just yet without getting my main tank repainted and back on...  However, yes I would like to go ride it, but I'm fairly sure it will run perfectly at anything above 1500 rpm, but would agree a good road run would be beneficial..

The air screws on mine don't have the restrictors that limit you to a 1/4 turn, and I set them at 1 turn out to start.  I played with them several times and I came back to 1 turn as being as close to right, but not quite right.  I will play some more using everyone's suggestions, paying more attention to being 'soggy' or slow to return to idle (target 1250, but it likes it a bit higher.)  I can't get it to idle consistently at 1250.

My sync tool is easily as good as Motion Pro sticks IMHO, however... again, the vacuum readings don't change regardless of air screw setting.... Perhaps I should try tweaking the slides to get the vacuums even, with the air screws all set at 1 turn?  Might be worth a shot...... in the interests of science, lol.

Meanwhile, thanks again for the pearls of wisdom, it helps.  I will keep y'all posted.

Bike has 24k miles on it, compression is ~125 across the board, all valve clearances adjusted to .0025" uniformly.  4 into 1 Delkevic exhaust.

[carnivorous chicken]

To explain a bit further, I did the bench sync and the slides are set to 1/16" all anally and equal.  The slides initially do affect idle as I understood it, and if I'm not mistaken come more into play after idle, and the main jets at higher rpms.  This bike performs perfectly at anything off idle, no hesitations, smooth, etc..  I'm just wanting the idle fixed up.  The #1 float issue is just a steady little drip, nothing big, but yes that has to be fixed.  I'm doing all this while the bike is on the lift, in neutral, with a temporary fuel bottle, I can't go ride just yet without getting my main tank repainted and back on...  However, yes I would like to go ride it, but I'm fairly sure it will run perfectly at anything above 1500 rpm, but would agree a good road run would be beneficial..

The air screws on mine don't have the restrictors that limit you to a 1/4 turn, and I set them at 1 turn out to start.  I played with them several times and I came back to 1 turn as being as close to right, but not quite right.  I will play some more using everyone's suggestions, paying more attention to being 'soggy' or slow to return to idle (target 1250, but it likes it a bit higher.)  I can't get it to idle consistently at 1250.

My sync tool is easily as good as Motion Pro sticks IMHO, however... again, the vacuum readings don't change regardless of air screw setting.... Perhaps I should try tweaking the slides to get the vacuums even, with the air screws all set at 1 turn?  Might be worth a shot...... in the interests of science, lol.

Meanwhile, thanks again for the pearls of wisdom, it helps.  I will keep y'all posted.

Bike has 24k miles on it, compression is ~175 across the board, all valve clearances adjusted to .0025" uniformly.  4 into 1 Delkevic exhaust.

As others have said, you don't sync carbs with air screws, but the adjusters on the top of the carbs. You should consult a manual for this (and listen to people telling you not to adjust them with the air screws). Not sure if I'm misunderstanding Mark1976, but you do sync the carbs with the bike running. finally, if the bike's on a lift you might not be running it long enough for the bike to warm up, which is when you'll find the idle smooths out (if synced properly).

[Ozzybud]

As others have said . Fix the leaking carb before any other tuning is done. A little drip out the overflow indicates that bowl is full of fuel near the top and it will not idle right in that condition or run correctly.

[Rayzerman]

OK got it, will fix float issue first and get back to the sync...
FYI, bike starts well, runs well after a bit of warmup..... I fully warm up the bike, not a problem.... fans running for airflow to prevent overheating, can run it as long as I like....

[M 750K6]

Yes, the carb leak is 1st priority. After that, I would leave it alone until your tank is fixed. Ride it for 10-15 miles, before a carb synch.

You may be chasing the impossible dream, though. 50 year old motor. If you've not been through and replaced everything, highly unlikely you'll get a smooth idle at 1200rpm or less. Shaft in the points plate can be slightly bent, compression not consistent across all 4, cam wear, tight spots in cam chain, valve wear, HT lead resistances vary, imperfections / wear between carbs... There's probably dozens of things. None which affect how nicely the bike can ride.

I set my idle at 1400rpm where it's happy and the carb synch was quick and easy, made little to no difference. Many weeks tweaking the pilot screws and I ended up at one turn out on all four! Two plugs perfect, two slightly darker, but never fouling and a sweet running machine, which holds a steady throttle and pulls cleanly. It doesn't sound happy at 950rpm idle. I've accepted that, put the tools away and just enjoy riding.

[Deltarider]

A needed readjustment of the floats is extremely rare. Nonetheless it runs like an epidemic in this forum. The Clymer manual is responsible for this. Readjustment can be needed after a previous owner with itchy hands and nothing else to do, has fall victim to forsaid manual.
Here a few guidelines. With the cleartube method fuellevel should be 3-5 mm below the flange. This method can be a hassle and I have seen mixed results. Personally I don't recommend it.
My method to check fuellevels is foolproof. I have described it here* for the CB500/550. Provided you get the idea, you can apply the same method for the CB350F/400F. Tapped quantity for CB350F/400F carbs should be 46-48ml.
Adjusting airscrews does not noticebly affect vacuum. There are people who claim this, but you will not find a video by them to proof it, not for the SOHC CB Fours. Think of your engine as four 87cc lawnmower engines, four airpumps which happen to share one crankshaft.
After adressing the leaking, I'd check the ignition timing once more and the dwell in particular. In theory it is well possible that you have a fine running engine with perfect timing when revved, but when it drops to idle, timing is slightly off due to a too big difference in dwell. 
* Here's the method, be it for the CB500/550 with oldstyle carbs, but the principle can be applied for all models. Just make sure bike is on the center stand and check which fuelline feeds which carbs. Piece of cake.
http://forums.sohc4.net/index.php/topic,185754.msg2151647.html#msg2151647
Oh... and don't persue a 1mm accuracy; you will never ever notice a difference with 1mm +/-

[MauiK3]

+1, airscrews have minimal effect, I also just set them and do little else with perhaps a cursory adjustment just to see if I can tell a difference, I usually do not see much, if any, changes

[Rayzerman]

OK just fer fun, and I am not done yet, so this is just a first go for now.... have not fixed the #1 carb float level yet.....

Fired it up, starts right away with a bit of choke, warmed it up for a bit at half choke, then none.... got it up to temp.... set idle around 1250-1300.  Played with the slide adjusters, better when even, then played with all a little more vacuum, and all with a little less... I don't know what's better.  Got a bog off idle if I quickly crank the throttle and bike will die.  Gradual increase, no problem.   So, guessing a little lean, so I turned all the air screws in to 3/4 turn out.... seemed better.  That's all for today, just an experiment...... next carbs coming off to fix the float level, gimme a coupla days, got some other stuff going on.

[BenelliSEI]

If it’s an outside carb that’s drooling, need to take it off?

[Deltarider]

Standard procedure to follow when a carb overflows.
In general, carb peeing fuel out of its overflow tube, indicates its float needle doesn't close the valve. This is often seen when bike has been inactive for a long period of time. It can be caused by even a small particle of dirt between the float needle's tip and the valve or the float itself is hindered in its movement.
1. Detach all 4 overflow tubes from their brass pipes to identify which carb it is.
2. Tap with the stub end of a screwdiver its floatbowl and hope this will help to get rid of the dirt.
3. If not, close the petcock, unscrew the drain plug completely. Now with the drain plug removed, open and close the petcock a few times, to create a flush of fuel that can flush out the dirt between the float needle's tip (that is now down) and the valve.
4. When needed, combine step 2 and 3.
In most cases this does it. Ofcourse you will collect all the fuel - inspect it for dirt or excessive rust - and pour it in your tank again.
5. When the above steps don't help, it is necessary to remove the floatbowl for further inspection.

[Rayzerman]

The carbs are squeaky clean and freshly re-done by me.  I have to pull the carbs because some one prior to me stripped the screws, so I have a nut on them that I can't get off while the carbs are on the bike.. well sure maybe I could but ain't that hard to pull the carbs.  I did a drain twice (clean, no dirt) and test fill (twice) and verified fuel level is way high with clear tube... not thinking it's dirt holding it open but perhaps a pivot is hanging up... I might want to swap the float with one of the others and see whazzup once apart.

[Deltarider]

... but perhaps a pivot is hanging up... ...

It's good policy, once the floatbowl is removed, to inspect the float's swivel pin, which means cleaning and maybe polish it, but don't overdo the latter. Same for the float needle's conus and valve. After a long period of sitting, adding a socalled fuel system cleaner to the tank may do it for you. I've had good results with that and on some ocaasions didn't need to remove the floatbowl.

[BenelliSEI]

Also make sure the floats have not been “twisted” and hanging up on gasket or bowl sides……