Topic: Another carb identification question-Frankenbike?

[JohnMiller4827]

I bought a basket case  '76 CB550F and reassembled as necessary. It ran, but stumbled at low R's. I decided to rebuild the carbs before winter, so I bought Keyster KH-0151F kits listed for the CB550F1. The PO had a single K&N air filter pod mounted to the collector box, and the stock 4-1 exhaust. Upon disassembly, there are differences in the carb parts. The existing air mix screw has flow holes through the sides and end, where the replacement is solid. Slow jets are both #38, but the main is not marked with any #'s and larger than the replacement #98. Needle jets are both 1.8, and the only #'s on the exterior I can see is on the bank mounting plate which is a boxed in A,NL=.(Upon further inspection, I found out they're marked 087A.) I'll search for that # in previous posts. In the meanwhile, I'm going back to stock air filtration setup, so do I continue with the kits I have, and will the needle be all right? I'm using the same emulsion tubes that are in there, and there are no #'s on them. Any insight would be appreciated.

[TwoTired]

The only unique thing about the CB550F (engine-wise) was the exhaust and the 069A carbs that were tuned for the exhaust characteristics.  Unless your present exhaust closely mimics the stock muffler restriction (highly unlikely), it is futile to try to bring your carbs to those specs/adjustments.

The 087a carbs were tuned for the less restrictive 4 into 4 and most after market 4 into ones have even less restriction than the 4 into 4.  Unless there is something identifiably wrong with the original 087a internal metal parts, I would use those first in your carb tuning quest, including the #100 mains standard to the 087a carbs.

The 069A F model carbs do indeed have solid tipped air bleed screws.  K models had them hollow tipped and cross drilled.  The hollow tipped screws made the adjustment range limited, as you could not completely shut off the air bleed.
I'm not exactly sure why the 069A carbs needed a more dramatic adjuster, but I do know the taper angle of the screw tip changed.  My assumption (not measured) is that the 069A carb bodies had a seat angle to match the air screw design change.  Therefore, using solid tipped screws in the 087a bodies would be a bit of a mismatch.  And, I'm not sure why you would want to do that (other than to use up the parts purchased).

If your exhaust is of an open type, and your air filter is stock (or preferably the UNI NU-4055), and your intention is to make a street bike, rather than a track only bike, then I would start with the 087a carbs set to Honda specs.  Drive/drag for effect and check the spark plug deposits for mixture indications.  Then make adjustments as necessary.

Cheers,

[TwoTired]

As far as I know, the float needles and seats were the same for both the K an F models (not PD carbs).
How close to Honda dimensions the Keyster parts are is not known to me.  The risk here is that the mechanical float height may not give the correct fuel level, as Honda spec parts should.

Cheers,

[TwoTired]

Keyster kits are cheaper than Honda parts.  Why is this?
One way to make cheaper parts is skimp on quality control measures.  That is, measuring the parts supplied by the maker to ensure they meet tolerance specifications. Another way to make parts cheaper is to make the tolerance specifications larger.  This can make part variability greater on a part by part basis.  Can't often see this variability by eye.

There have been previous threads whereby people fixed poor carb operation by going back to original Honda parts.  The understanding was that the aftermarket parts were not made to the same dimensions as the original Honda parts.
Some people are happy with Keyster components, some are not.  This could be because there is variance in the Keyster part dimensions.  Could be any number of other reasons, too.

I have NOT partaken in the endeavor to monitor the Keyster component adherence to Honda specs.  I don't have any Keyster components to measure.
But, the word cheap is not the same as inexpensive.  Cheap can also mean poor quality in some regard.

The floats actuate the float needle and seat.  I have not scrutinized the geometry well enough to know that if the float needle seat dimensions, the float needle length, or spring pressure is different than what is found on the Honda components, that it will effect fuel levels in the bowl.  I merely note that it could be a possibility and alerted you to that point.  Since you have the parts in your possession, you have the opportunity to check that the bowls fill to correct levels after you have installed the replacement components.  If I were to use aftermarket components, I would not assume the new parts were gold standard, and would measure the difference to be sure they worked correctly.  Or, at least that the bowls were filling as expected, as high or low fuel levels in the float bowls can result in odd tuning behavior, especially if each of the four bowls is filling to a different level.

It may all work out straightforwardly with your parts.  I'm just recommending an insurance check, rather than blind faith.

Cheers,