Topic: 1978 CB550 ... carb problems... please help

[jack_xx]

Well, this is my first post... and what a way to begin.

I'm having some challenges of late with my bike. I've converted the bike over from it's stock set-up, to a full-on racebike,.

My issue revolves around the bike as it sits running on the lift-stand. The choke is pulled out, running well, then pushing the choke in and adjusting the idle to maintain between 1500-2000 rpm ... the rpms drop suddenly, I adjust the idle to bring the rpms up ... blip the throttle a couple of times... adjust the idle again, and the bike almost dies. In some instances, the rpm suddenly rises on it's own beyond 3000 rpm - and then making a slight idle adjustment... again the bike almost dies.

I've checked the points and timing, everything appears clean, no arc... all looks correct. I've pulled the carbs off to check the jets, everything has been cleaned... everything intact. Something to mention is that I've taken the jets from stock 90s to 110s. I've had the 110s running on a friends 76 550 build... and this set works best. The 115s run too rich... and 100s have a rather sharp flat spot to them. The 110s work good.

Is there something that I'm missing here? I can't say that this is a mechanical (engine) issue. It's almost like the bike is sucking air at times... from another source. I've inspected the rubber boots for any cracking ... but nothing appears wrong with them at all.

Would appreciate any assistance, and could easily call to discuss further. Thanks in advance for any help you can provide.

jack_xx

[TwoTired]

The 78 Cb550K originally came with PD 46C carbs, 4 into 4 Exhaust (quiet), and a single paper air filter in the stock air filter box.

Is this what you are racing with?

edit: Fixed a typo, F>K, sorry...

[jack_xx]

This is the '78 CB550k model. Stock exhaust has been removed, and a 4 into 1 exhaust has been installed.

[Tom in Newcastle......Ontario]

The 78 Cb550F originally came with PD 46C carbs, 4 into 4 Exhaust (quiet), and a single paper air filter in the stock air filter box.

Is this what you are racing with?

And here I thought that the cb550f came with a 4 into 1..........and didn't even know there was a 78 cb550F

[Boner]

Not knowing Honda's at all but learning quickly I wont be of much help but you need to list the mods you made as this will give some our wizards the base to start from.
Engine mods ? cc & comp ratio ? cam ? head work ? changes to carbs other than jet size etc
Good luck

[jack_xx]

Not knowing Honda's at all but learning quickly I wont be of much help but you need to list the mods you made as this will give some our wizards the base to start from.
Engine mods ? cc & comp ratio ? cam ? head work ? changes to carbs other than jet size etc
Good luck

Here is the work that has been done on the engine. New piston rings, polished the piston heads, light porting, new gaskets/rings.

Carburetors have had the jetting moved from stock 90 to 110. Carbs cleaned. Stock air filter/pods removed ... replaced with after market pods (K&N).

Stock exhaust removed, aftermarket 4 into 1 exhaust now on the bike.

Any assistance would be greatly appreciated. Thanks fellas!

jb_xx

[TwoTired]

The 78 Cb550F originally came with PD 46C carbs, 4 into 4 Exhaust (quiet), and a single paper air filter in the stock air filter box.

Is this what you are racing with?

And here I thought that the cb550f came with a 4 into 1..........and didn't even know there was a 78 cb550F

There isn't.  I mis-typed   

[TwoTired]

Here is the work that has been done on the engine. New piston rings, polished the piston heads, light porting, new gaskets/rings.

Carburetors have had the jetting moved from stock 90 to 110. Carbs cleaned. Stock air filter/pods removed ... replaced with after market pods (K&N).

Stock exhaust removed, aftermarket 4 into 1 exhaust now on the bike.

OK, those changes have seriously change the carb throat vacuum, making it harder to draw fuel up from the bowls.  Likely ALL of the fuel metering devices need to be made larger.  This includes the throttle valve; which is the needle that is attached to the slide.  You will need to put the mounting clip in a lower position, thus raising the needles and allowing more fuel to pass into the carb throat.

No one is going to be able to tell you what the final carb adjustments will be for your unique design changes.  Now you will learn that the exhaust and air inlets are NOT just cosmetic changes.  And you will have to learn how the carbs work and how to get information about what part of the carb needs to be adjusted for the throttle position selected.  Reading spark plug deposits is one data source, even better is a fuel map from a Dyno test.
The chart below is a start.  Some of the metering devices are interactive, like Main jet and slide needles; meaning, changing one means changing the other, too.  In other words, raising the needles may require a smaller main jet than what you currently have.
You may wish to search the forum for "plug chops", and search your area for a test track where you can operate the engine under load in order to get relevant spark plug deposits information.

Best of luck!

[jack_xx]

Looks like the problem has been somewhat rectified. Small leak in the connecting tubes between carbs. All rubber tube have been replaced. Timing issue still there but running better now. Points and condensers being replaced, carbs to be sync'd... 

Thanks guys. Slow and steady wins the carb race... which helps lead to a  fast running bike, that actually wins the motorcycle race!

[jack_xx]

Gurgling noise, carbs are not the issue....   condensors and points replaced. Top end opened up ... and cam-chain tensioner (although it was not running correctly) was fixed, yet the gurgling ... and rpm's left hanging until idle adjusted. Swapped out set of different carbs/intakes... still does not run correctly. Timing checked again, bang on ... but bike is not doing well.

Crack in head possibly?

Rather frustrating to say the least.