Topic: Carb jet size opinions

[Tretnine]

I'm running a 1978 cb550k with 46a carburetors. The slow jets are 38s, the mains are currently 90s, needles are 2349f on position 3 with d7ea plugs. At that position idle seems stable but the engine feels flat/lean in the main circuit. It also hiccups when moving out of the idle circuit and into the main (right around the 1/8 mark, usually just  as throttle is opened.) The hiccup is pretty heavy, I wouldn't mention it if I didn't think it was significant.

Today I moved the needles to position 2. (2nd closest to long, tapered end.) This significantly helped the lean issues while at 1/4+ throttle, but the idle went to hell. It now 8 strokes when coming out of the idle circuit and hesitates much more. Once in the main circuit, though, it acts very nicely.

My plan is to move the needle back to position 3 and correct the main circuit issue with a larger main jet. My intention is to go to a 95 or 98, though I have no idea how far I need to go. Before I go ordering a million different sizes, are there any opinions? If possible to compare, how much of a 'jet size difference' is one clip position?

[TwoTired]

State air filter type and exhaust type

[Tretnine]

stock 4-4 and unifoam oiled filter.

[TwoTired]

I wouldn't go larger than 95 with stock exhaust and air box.

But, have you tried opening the Idle Mixture Screws more?  You aren't that far from stock with your mods.

Another good clue as to needed corrections would be the spark plug deposits.

Also, after the "hiccup" does it resume power cleanly or with a burble.

I assume you are using ethanol blended gas?

Other info points would be vacuum balance and the entire tune up schedule adherence.

Does the engine respond better with a bit of choke added?

[Tretnine]

I haven't tried playing with the idle screw mixture since moving the needle clip. I will try that today. Spark plugs look pretty clean, probably lean. The 'hiccup' with needle clip at position 3 is a hesitation. It feels almost like the motor stalls for a portion of a second then picks up again. With needle clip at position 2, the hesitation is much greater and gargles before it picks up again.

I'm sure the gas is blended, it's America, there is ethanol in everything.

Vacuum and tune up is all done, the motor was recently rebuilt, new timing and primary chains installed. This is the same set of carburetors that were on the machine when it was taken apart. They were not tuned correctly when they were pulled for the project. Now that I trust the motor is in proper order (I trust myself, at any rate) I'm working the fine tuning.

Vacuum balance, cam chain, points (Dyna system,) etc, etc, has all been taken car of.

Engine does not run well with the choke on, it will start initially with the choke applied, but after it is is warm it will not idle or drive with choke on.

Today my plan is to move the air screws and try a stock air filter to see how it changes operation. I will also call a local shop to see if they have any 95 mains lying around. Otherwise I have to order, which means more money and more waiting - so I'd like to try as close to what I need as possible the first time, or order everything I need.

The other thing I could check into, as I understand, is a thinner tapered needle - though this seems like a much more difficult affair.

[TwoTired]

Since you have them, use the #90 Mains.
Go back to the needle in # 3 position.
Verify float height at 14.5 mm
Open (turn out) the IMS to get rid of the lean stumble in 1/4 turn increments, testing at each new setting.  If it isn't gone by the time you have tested positions out to 5 turns, then find some 42 slow jets for the carbs, and repeat the search for the proper IMS position that will:
In top gear, at a crawl, at idle throttle position, Open the throttle to the 1/2 total travel position, the engine should pick up reliably and smoothly without any stumble. (Goal)
You don't want to have the IMS turned out any more than what is required to achieve the above.


With the UNI foam air filter (and bad gas), you may need to increase the Main jet size, if your top speed is reduced, or the engine runs too hot.  I'm not sure about this.  But, both the less restrictive ( less pressure drop) filter and the sabotaged gas make these originally lean burn setups to run even leaner.

Best of luck!

[Tretnine]

I had 42 pilots in there earlier and moved back to the stock (for the carburetor setup, not necessarily for the engine) 38s. I ordered a bunch of main sets, 92, 95, and 98 which I will try when they get in this weekend. The 42s were causing a loping idle, the 38s have produced a much more even idle and it stays in the idle circuit instead of acting really rough - then jumping up to about 3k.

Currently, I'm back at clip setting 3 and double checking everything in the carburetor. I will work the idle mixture screw and the stock filter until the new parts come - which will allow me a bit more potential for experimentation.

Suddenly, a couple of cylinders have started fouling plugs very badly - I'm double checking that the carb throats were installed correctly when the clip was moved last. Just to be sure, which side does the hole gone on?

I think you're probably right about using the #90 and working the idle mixture would possibly get the bike feeling smoother, but the main circuit is just too lean to run with #90s given the way it is running - be it the aftermarket filter, gas, altitude, humidity... or God's desire to challenge my patience. Whatever the case, I'll keep working on it.

Have you ever used different sized needles for tuning/performance?

[TwoTired]

Suddenly, a couple of cylinders have started fouling plugs very badly - I'm double checking that the carb throats were installed correctly when the clip was moved last. Just to be sure, which side does the hole gone on?

I don't understand this question.  Care to restate?

Have you ever used different sized needles for tuning/performance?

Not in PD carbs.
The earlier carbs had different needle tapers for the F and K models.

Cheers,

[Tretnine]

There are four small holes in the throttle valve (it's called in the clymer.) Two are used for mounting screws, one holds the needle and the clip rests there, but the third I'm not sure about. It's slightly smaller and sits a bit off to the side. How should that be positioned? I think I have the directionality of it sorted, but should be sure which way it's positioned before slapping it back on the machine.

[TwoTired]

There are four small holes in the throttle valve (it's called in the clymer.) Two are used for mounting screws, one holds the needle and the clip rests there, but the third I'm not sure about. It's slightly smaller and sits a bit off to the side. How should that be positioned? I think I have the directionality of it sorted, but should be sure which way it's positioned before slapping it back on the machine.

Ok, on my spare set the holes are off to the left.  More importantly, the slide cutaways on the bottom face the air inlet to the carb.

Cheers,

[Tretnine]

That's the way I was putting it together. I try to be fastidious about the way it's assembled, but once in a while I get distracted and wanted to be sure I put everything together in the exact correct way. After tweaking, it's running alright, but still needs some fine tuning. More will be done when the larger mains arrive thurs/fri/sat

[Tretnine]

I have upped the mains in this system. I went to a 95, but the plugs still looked white, too hot. I'm now at 98 (or 97.5) and they appear to be a nice shade of tan after a good round of flogging. The machine also seems to respond better while in the main circuit and does not smoke out of the tail pipes. (I believe the engine was overheating and causing oil to burn somewhere, and was doing the same in the valve train, resulting in smoke from the blow by.)

I still have idle issues - though they're getting better. The #3 carb was overflowing because of a bad float needle. This is fixed. Idle below 1500 RPMs is still rattly. Moving from the idle circuit to the main circuit still gives a hesitation and a 'burble' as Lloyd put it. A cursory sync has been done (again) but I need to figure out which pilot jets to use. The exhaust smells very gassy at idle - could I really need to go below a #38 pilot jet when I just upped the main jet by 10%?

Which way leans/richens the IMS (the screw located on the side of the carburetors CLOSER to the engine) I knew, now I'm second guessing myself.

The engine still resists moving into the idle circuit from the main circuit. (Tends to hang between 2500-4000 RPMs) is this indicative of either a rich or lean pilot circuit?

The bike just does not want to go in and out of the idle circuit, as it stumbles and hesitates going out and hangs going back in... thoughts?

Thanks.

[TwoTired]

What position is the slide needle in?

[Tretnine]

Needle position is back to stock #3.

[TwoTired]

With the stock bike, you are supposed to tweak the IMS for peak RPM at idle.  You'll need a sensitive tach capable of detecting 50 RPM changes.

The IMS, is normally set to 1 1/2 turns, stock.  I suppose I would start at 2 two turns out and then turn them inward (Leaner) until peak RPM is achieved.  Then check for throttle response from idle.

Cheers,

[Tretnine]

Have you got an example of a tach that would work for this?

[TwoTired]

Have you got an example of a tach that would work for this?

Nothing specific.  I expect digital tachs would work.
But, the manual just says to adjust each IMS for highest idle speed, and re-adjusting the idle knob to keep it at 1050 RPM.

Can't say I've done much adjusting on my 78 CB550 since I got it 15 years ago.

Cheers,

[Tretnine]

I found a tach like this at advance auto. It's worked alright and is giving me the information that I need - however - I was setting the float levels using a piece of vinyl tubing to attached to the float bowl drain and found that my #2 carb is overflowing quite badly. The gas is flooding all the way to the airbox (and probably into the engine)

I've blown compressed air though the float bowl overflow tube and it seems unblocked, but no gas is coming out of the overflow tube while the gas is on (whether the motor is running or not) I know gas is getting into the airbox, though, as I've seen gas there, and it has collected in the tube that attaches to the base of the airbox.

The carbs are clean and have brand new float needles. While it's possible that I have some debris impeding the float needle (and I am planning to pull the carbs to double check) I may have a bad seat.

OR, (and I don't know enough about this part of the carburetor) could the gas be coming from another location? This is the carburetor that receives gas from the tank. Is there something different on this carburetor that may crack, break, lose a seal, etc, or should I focus exclusively on the float / float seat?

[TwoTired]

There are two nipples on the carbs, large & small.  The larger one is the fuel inlet, and the smaller one is a vent line.
You aren't putting fuel into the vent line are you?

[Tretnine]

No. I even just double checked. The 46a carbs do not have a vent tube, the 46c carbs DO have a vent tube and a couple of other small differences.