Topic: Overheating=lean?

[Jeremy]

have a 77cb550k with pd46aapdi stamped on the carbs, these have a 38 slow jet and a 90 main, clip is in third groove. 4 into 1 mac  exhaust and uni air filter. Bike is difficult to start, need full choke, and seems to be very cold blooded, takes a while to warm up but pulls pretty good at wot and idles good after about 15 minuts of riding.. It is knocking under load pulling away from stops after a one hour ride, plugs are white after the ride. Have checked timing, dwell, fuel screws, air leaks, new plugs, dr7es. I'm pretty sure I'm running lean, do I adjust the clip, bigger main jet, larger slow jet? What have other people done with similar setups?
 

[Kixx007]

You could drop a clip (raising the needle) or go up a jet.  Honda's are always cold blooded.  But if after an hour of riding and your plugs are white, you better richen the thing up.

[TwoTired]

Your slow jets are #38.  They are pressed in at the factory.  (A lame anti polution attempt to keep people from tampering with the mixtures.)  Somewhere near the base of that tube is an orifice.  You should be able to use jet reamers to open the jets to a larger size.  And you can get jet gauges to monitor your results.

Have you tried turning out the idle mixture screws to enrich your idle mixture?  Factory settings will be meaningless with your 4 into 1 and Uni filter.

Or, you could try the 78 carb bodies that came with #42 slow jets pressed in.

One possible alternative, might be to reduce the size of the slow air jet.  As you look into the carb intake, you'll notice two tubes near the bottom.  The left tube is a slow air jet (deeper inside).  It is supposed to be a #150.
The other tube is the air jet for the emulsion tubes in the mains/needle jet circuit (#130).
I haven't done this, but in theory, air jet reduction should richen the mixture.

Interesting side point is that the Honda shop manual supplement for the 77 CB550 K3, pictures carbs with screw in jets at those air jet locations.  I'm guessing they took pictues of the prototype carbs before the jets sizes were determined after testing.

The above adjustments here should help the cold blooded warm up.

My all stock 78 is also cold blooded.  But, the rest of my 550s are not, as these don't have the lean burn idle tuning.

For 1/4 to 3/4 throttle settings enrich the mixture by raising the needles in the slide a clip position.

The main jet is for WOT.  Starting with clean plugs, do a full throttle acceleration, hit the kill switch, clutch in and pull and read the plugs.   Black insulator, too rich, white insulator, too lean, Tan, yeah baby.

Lastly, do you know that you have resistor plugs?  Do you still have the stock plug caps? These have resistors.  And it is better if you don't use resistor plugs with the stock plug caps. (probably not related to your current quest.)

[Jeremy]

Can I swap slow jets from a 78 into a 77, I have an old set here. Yes, I know I have resistor plugs. Couldn't get d7es from anyone, but better spark would mean leaner condition, that'll be another quest.
Also, should I raise the needles first or get a bigger main jet first.

[TwoTired]

I have been told that other people have yanked the pressed jets and and reinstalled them after cleaning.   So, in theory, you should be able to swap them , too.

The main jet feeds fuel to the needle jet circuit.  It has to be big enough to supply fuel under maximum demand from the engine.  The needle jet further restricts fuel flow.  Attached is a chart relating what part of the carb dominates the mixture for various throttle settings.

I expect you should determine if the main jet is big enough for the task.  Then adjust the needles, then the slows, and finally the Idle Mixture Screws.

http://www.sohc4.us/forums/index.php?action=dlattach;topic=1925.0;id=1489

[Jeremy]

can't read that chart, keeps asking for my password and then kicks me back to the genral area

[Jeremy]

and what would be a good starting point with jets, have a 90 in there, got to 100 or 105, 110,? 

[Caferacernoc]

All 500 and 550 fours have basically the same engine and fuel needs. The CB500 came with a 40 idle jet and a 105 main. The 550 came with a 100 main and I believe the 550F came with a 98 main. Then the 77-78 apparently went to a 90 main. With a more open flowing 4 into 1 exhaust and a more open air filter, if I were you I'd go straight to the 105 main. At least a 100. My '76 had a mac 4 into 1 pipe and pod filters and it liked 105 main, 40 idle and the clip in the richest setting.

[Caferacernoc]

Especially considering your plugs are white and it takes 15 minutes to warm up. You are way lean. Check for air leaks between the carbs and the engine. If you don't find any you need more gas, bigger jets!

[TwoTired]

can't read that chart, keeps asking for my password and then kicks me back to the genral area

Okay then lets try this.

[TwoTired]

All 500 and 550 fours have basically the same engine and fuel needs.

Well sort of.  The displacement change also effects the air velocity through the venturi and therefore the pressure drop that draws fuel through the jets.  Air filter restriction changes also changes the pressure at the jet outlets, fuel flow, etc.  And, the exhaust restriction changes the scavenge effect or how thoroughly the exhaust gasses have been removed.  If the fresh intake stroke charge is diluted with exhaust gasses, the mixture ratio must be leaned to account for the decreased available oxygen.

The CB500 came with a 40 idle jet and a 105 main.

The carbs I've dismantled (627B) for the 500 have all had 100 mains, which does match the Honda shop manual specification.

The 550 came with a 100 main and I believe the 550F came with a 98 main.

True.  Different exhaust pipe configuration!   Also, different slow jets,  needles and clip position.

Then the 77-78 apparently went to a 90 main. With a more open flowing 4 into 1 exhaust and a more open air filter,

Only the F models from 75 to 77 had the 4 into 1 exhaust. And, they used the same carbs and set up (069A).

The 77-78 CB550K models, had both 4 into 4 pipes and carbs of a different design.  I think these pipes are more restrictive than the earlier 4 into 4s.  And, the carbs had smaller air jets as well as smaller mains; #130 vs. #150.

if I were you I'd go straight to the 105 main. At least a 100. My '76 had a mac 4 into 1 pipe and pod filters and it liked 105 main, 40 idle and the clip in the richest setting.

I'm glad that worked for you. And, perhaps that's what Jeremy's bike needs, too.  I just don't think it is a slam dunk though.

The 77-78 K models were designed to be cleaner burning less polluting machines.  I'm not looking forward to the time when my stock pipes expire and I have to address all the carb jetting issues replacement pipes will bring.   I may just put an earlier carb bank on them.  We'll see.

Cheers,

[mgilvary]

Check for air leaks between the carbs and the engine. If you don't find any you need more gas, bigger jets!

What is the recommended procedure to check for air leaks?

[Jeremy]

Just put in some #100 jets and took it for a little spin, seems to go pretty good, picks up a little when I close the throttle, probably a little rich now. Seems to be be idling a little better too, wierd. Will do a plug chop on the weekend. But I thought by going from a #90 to as #100 that things would change a little more drastically.

[TwoTired]

How much did you operate at 3/4 to wide open throttle?  That's where the main jet has the most contribution to the mixture.

The pickup at throttle close is likely due to leaner mixtures at that throttle setting.

Cheers,

[Jeremy]

I did most of my testing at wot, it seemed like if i closed it a little bit it would pick up, Still lean? Really?

[TwoTired]

There is no one adjustment that will change rich or lean over all throttle settings. 
I posted a chart in this thread, that shows what part of the fuel metering in the carb applies to what throttle setting.

The main is choosen for WOT to 3/4 throttle.   The slide needle shape and clip postion determine 1/8 to 3/4 throttle positions.  The slow jet and idle mixture screw set 1/8 and below mixtures.

To some limited extent, each metering device has some effect at all throttle positions as none of them is isolated at any particular throttle setting.  But, they dominate the mixture when the throttle is placed in a position for them to be the major contributor.

Perhaps you've misinterpreted the terminology.  The main jet is the main source for fuel between 1/8 and WOT throttle.  However, the slide needles further restrict this flow, from 1/8 to 3/4 throttle as they are directly in the fuel path between main jet and carb venturi.  The slow circuit is a parrallel fuel path to the mains/needles.  They dominate when the slides are closed or nearly so.

[Jeremy]

which way to adjust the clip to make it richer?

[Bob Wessner]

Moving the clip down raises the needle, makes it richer because it allows more fuel to pass through the jet.

[kghost]

Bob,
 Your getting to be quite the expert. Have you thought of openning a Carb Shop?

 

[Bob Wessner]

No, too busy designing my easy-on, easy-off carb zippers, and oh yeah, glass float bowls.