Topic: Honda cb350 four coughs once warmed up on take offs?

[Mars704]

Hi everyone a bit stuck on this one could do with some help...please.

My Honda cb350 four starts up nice from cold and rides off smoothly with no problems, how ever once bike is warmed up the bike idles nice but it coughs and splutters and runs on at least 3 when taking off or coasting round corners, you give it a little throttle and it coughs until you get up to 3000 rpms. Once there bike runs smooth again.
I have done loads of work so far but still no luck solving this issue.

I have trimmed plug leads back put on new plug caps and new plugs, set and cleaned points and static timed motor, also set tapetts too, replaced inlet carburatuor o rings and checked for splits. Replaced fuel line and checked fuel tap and internal fuel filter all good, cleaned carburatuors out allso emulsion tubes replaced main jet seals, new float needle valves reset floats to 21mm , checked that all four needles were all the same height (yep) and bench set carbie balance. Replaced battery, air filter looks pretty good and has a good sealed lid on it.
Bike still has that little miss when taking off once warm and has a small bog down at about 90 -100 kmp.

Any help would be great :-). Thanks for reading

[dave500]

gday mars,is this like at around 2-3000 rpm?

[Mars704]

Hi Dave , You've done all these things too and still have the same miss when bikes warm?
yep that's the rev range it coughs and misses at, 2000-3000, very frustrating , one minute you think it's gotta be electrical then the next you think it's gas. So after doing everything I can think of in those two fields I'm now stuck.
As I've cleaned and replaced things along the way I must say the bike revs better than I first got it so I have seen some improvement but am just stuck with the coughing take offs once warm. 

Cheers Mark (mars704)

[Vinhead1957]

You also sync'd the carbs?    If not it may lead you to the problem

[Mars704]

Hi, I've only bench sync'd them at this stage, but worth a try, it's the only thing I haven't done yet.

Cheers

[Deltarider]

Advancer?

[Mars704]

Yep that's been checked off also , it's not jammed and doing all the right things, springs are free and working. Hmmm

[TwoTired]

Describe the spark plug deposits.

Elaborate on how the engine recovers toward power.  A clean, sudden resumption of power usually means mixture was too lean.  A dirty, burbly, throat clearing type resumption of power, means too rich.

I don't believe you can "see" a clean filter unless brand new.  And paper filters that have ever collected dust, can never be cleaned as new.  Filters that are holding dirt, create more of a pressure drop across them.  This makes carbs provide richer mixtures.
A quick test is to see how the engine responds with the filter temporarily removed.  Note any differences in  how the engine behaves.

Is the exhaust stock?  Or, does it hold less pressure than the the stock one?

Also, tell us the pilot screw setting.

[tomkimberly]

Have you adjusted the valves yet?

Tom

[Mars704]

Right spark plugs are sooty and rich looking but dry, it's a burble throat clearing sound when it's coughing,mixture screws are set at 1 1/4 turns out ,have tried 1 1/2 turns out still ran the same although plugs did run a lot cleaner ( might go back to 1 1/2 ),   the exhaust is a nice two into one on both sides.
The air filter I will order one in tomorrow, looks cleanish but like you say it still may be dirty, so I'll order one in.
I think I'm up to adjust mixer screws to 1 1/2 and replace air filter and test ride from there.

Hi Tom , yep tappets have been adjusted but weren't out by much. Made no difference.

Cheers Guys
Mark

[tomkimberly]

The air screws need to be set at 7/8ths, +/-1/8.

[Mars704]

HiTom

Yeah I read that they need to be set at 7/8 ths  but won't that make it run richer ?
I will adjust them back once I get the new air filter fitted, and try that.
Cheers Mark

[captain hopeless]

I'm fairly certain that my (350F) low-rpm burble is 'rich' one, sounding like you've described. If you switch off the petrol tap while riding, take note of whether the symptoms disappear as the float bowl drains. (eventually the bike will clap out from fuel starvation if you don't switch fuel back on.)

Might be something to try, anyway.

[Mars704]

Cool thanks captain am happy to try anything at this stage, worth a crack !

I used to own a cb350 four when I was younger and it was a little rocket for a 350, I guess I expect the same from this one, but they have changed the fuel octane since then, I have tried the standard 91 also have tried the higher octane 98, didn't notice any difference thou.

Cheers Mark

[captain hopeless]

float needle valves reset floats to 21mm

Conventional wisdom on this board calls for 24mm floats on the 350, which is what mine are set at. This is contrary to the service manual which says 21mm.

PS hello Kiwi!

[Mars704]

Yeah funny I was going to set them at 24mm as I have done a lot of reading up on the cb350 four and saw the 24mm measurement, was talking to the local Honda shop here in Hastings who has a few older sohc Hondas and he said to set them at what the book says,  I'm going to go with your suggestion and try out the 24mm.

[ADW]

HiTom

Yeah I read that they need to be set at 7/8 ths  but won't that make it run richer ?
I will adjust them back once I get the new air filter fitted, and try that.
Cheers Mark

Absolutely, 7/8ths is what you want. Just for the helluvit, even though my bike was running great at 7/8, I tried adjusting mine out to 1, 1-1/4, and 1-1/2 and it was markedly worse. Put them back to 7/8ths and your problem will likely go away.

[Mars704]

Cool thanks for all the suggestions guys, will do some of the adjustments one at a time and see what works best, I do have someone coming around one night to vacuum balance the carbs (perk job from a bike shop) so will hopefully get to the bottom of the issues.

Will let you know how I get on!  Fingers crossed I get it sorted.

If you think of any thing else that may help fire away please! 

[dave500]

plug caps?my pet hate is old plug caps.

[Mars704]

Yep done those thanks Dave, I think I've ticked off all the small easy jobs. 
But thanks for your suggestion they all help in the big picture.

Cheers Mark

[dave500]

check.