Topic: another mixture question

[petercb750]

Folks -
The picture: K2 750, standard exhaust, rebuilt carbs, new intake boots, standard airbox with foam element in good condition, size 40 pilot jets, size 110 mains, clip in middle setting on needle, checked for air leaks etc. Timed slightly advanced and points gapped etc. New plugs (8's). Running premuim unleaded (in Oz).
The problem: setting the mixture screws - wind them right in, bike still idles fine!! In fact, can not detect any difference!! With them out a half turn or so, bike has flat spot just off idle and fluffy until revs build (ie mains take over). Runs real good and strong once underway. Runs better with screws right in!
The question: why is it idling and running fine with the mixture screws right in? Should I instal larger pilot jets eg 45s? If it idles fine with screws right in, then runs fine through the rev range (maybe very slight flatspot at about 2k), should I even be worried about it and just ride it as is?
Battery is very weak, so am installing new one - will that have any effect on any of the symptons above?
Look forward to the responses.
Peter.

[TwoTired]

I noticed on my early Honda Cb550 Ks, that the idle screws are hollow tipped with a cross hole in them.  With this design you can't really close off the idle air bleed completely, effectively limiting their adjustment range.  If yours are the same type, it would explain why seating them fully doesn't flood the engine.  Take one of yours out and look a the tip design.

The Cb550F has solid air bleed screw tips.  Engine stops if you turn these in too far.

You might try a genuine Honda paper filter as an experiment.  If your performance issues go away and your idle screw settings return to stock positions, then perhaps the foam you are now using is less restrictive and a larger slow jet with give you the performance and adjustment range you need when using that foam filter.

Unrelated to your performance issue, but I don't see why you are using premium fuel.  Did you advance the timing far enough to really need that?

Cheers,

[petercb750]

Thanks TT, I wondered about the air filter - might give your suggestion a try.
Re the fuel - with 2 70's bikes and 2 70's cars I don't use anything else other than premium - here in Oz it has a higher octane rating than the standard, and certainly improves overall performance, especially noticeable in the cars. So it's not used because I advanced the timing, it's used because it's better.
It costs a bit more, but I think the benefits outway that. Most people I know with that period car/bike use premium (lead replacement fuel no longer available, and it was crap anyway), most also with a squirt of upper cylinder lubricant for good measure.
Cheers!

[dusterdude]

Thanks TT, I wondered about the air filter - might give your suggestion a try.
Re the fuel - with 2 70's bikes and 2 70's cars I don't use anything else other than premium - here in Oz it has a higher octane rating than the standard, and certainly improves overall performance, especially noticeable in the cars. So it's not used because I advanced the timing, it's used because it's better.
It costs a bit more, but I think the benefits outway that. Most people I know with that period car/bike use premium (lead replacement fuel no longer available, and it was crap anyway), most also with a squirt of upper cylinder lubricant for good measure.
Cheers!

oh #$%*,here it comes.

[petercb750]

 

[hcritz]

Grin...Hey Peter...that's from a previous discussion of Premium fuel not being need in the Hondas...
I drove the Vair the other day (with Premium) and noted the EGT and Cyl Head Temp. at several speeds, steady state and under load. Keeping the Turbo at low to no boost. Also keeping the RPMs up to around 3000.
Next tank will be regular...I know it's gonna #$%* and ping, but want to see what the temps run...
ought to be a fairly good indication of prem. VS Reg for engine temps in a an aircooled engine.
Ambient temps here are pretty stable now.

[eurban]

You say that you are using #40 idle jets but do you know for sure that they are clear?  Your symptoms sound a bit like impaired idle jets. . . . . .Premium fuel (At least in the US this means high octane) won't do anything for you unless your engine needs higher octane gas to function correctly.  In fact there is actually less energy content in higher octane gas.  High compression engines, turbocharged engines etc typcially require higher octane gas to prevent preignition and detonation but most engines do not.  Sometimes older engines will have large amounts of carbon build up in the combustion chambers effectively increasing the compression ratio and perhaps in other ways predisposing the engine to detonation etc.  These engines may show improvement from higher octane gas although removing the carbon build up is probably a better long term solution.  Far as I know none of the stock SOHCs require more than regular unleaded (87 octane in the US) and probably would not benefit from higher octane gasoline.  My 750 with a 10:1 CR 836cc kit requires higher octane gas to avoid preignition and detonation.  It really makes no sense to say that Premium fuel is better fuel.  If your vehicle needs it, then it is better.  If not, you are wasting your money and perhaps even getting less energy from it to boot!

[petercb750]

The idle jets are brand new, and carbs have been disassembled and cleaned throughout. I have also got a couple of in-line fuel filters between tank and carbs, so am pretty confident that no bits of crap are getting through. Despite serious tuning and attention to detail the problem persists. Bike is running quite well when revs are up and constant throttle, but at take off or when running and I back off the throttle and go to accelerate again it coughs and stutters very briefly then takes off, like it's momentarily starved of fuel or fouled a plug or something.
I haven't changed the air filter yet as suggested by TT, but local dealer suggests that as the problem is only off idle and low revs, there wouldn't be enough air going through to effect the problem anyway. They're now suggesting I go to #45 pilot jets which "should fix it".
What you think?

[TwoTired]

Bike is running quite well when revs are up and constant throttle, but at take off or when running and I back off the throttle and go to accelerate again it coughs and stutters very briefly then takes off, like it's momentarily starved of fuel or fouled a plug or something.

Symptoms are consistent with incorrect mixtures, but too rich or too lean?
What do your spark plug deposits look like?  Soot, tan, white?

Temporarily take out the air filter.   Do the symptoms  improve or worsen.  The filter has a choking effect, making mixtures richer.  Removing it should lean things down some.  If your symptoms worsen, then go with bigger slow jets (or stock paper filter).  If the symptoms improve, you are already too rich and bigger slows will likely make it worse.

Cheers,

[dusterdude]

You say that you are using #40 idle jets but do you know for sure that they are clear?  Your symptoms sound a bit like impaired idle jets. . . . . .Premium fuel (At least in the US this means high octane) won't do anything for you unless your engine needs higher octane gas to function correctly.  In fact there is actually less energy content in higher octane gas.  High compression engines, turbocharged engines etc typcially require higher octane gas to prevent preignition and detonation but most engines do not.  Sometimes older engines will have large amounts of carbon build up in the combustion chambers effectively increasing the compression ratio and perhaps in other ways predisposing the engine to detonation etc.  These engines may show improvement from higher octane gas although removing the carbon build up is probably a better long term solution.  Far as I know none of the stock SOHCs require more than regular unleaded (87 octane in the US) and probably would not benefit from higher octane gasoline.  My 750 with a 10:1 CR 836cc kit requires higher octane gas to avoid preignition and detonation.  It really makes no sense to say that Premium fuel is better fuel.  If your vehicle needs it, then it is better.  If not, you are wasting your money and perhaps even getting less energy from it to boot!

hey eric,its good to hear from you again.

[eurban]

Thanks Mark-  I've been offline decompressing for a while! Now if I can just get my 750 back together again from a couple of winter  time mods then I could really do some serious decompressing!