Topic: Need carb advice (087 carbs on 550f after install - updated)

[mikethejeepguy]

I know setting up carbs when aftermarket parts are involved is a shot in the dark, but am hoping that someone can shed a little bit of light. I've read through FAQs and posts but still am not sure what to do with my set up. Sorry if the question is redundant.

The bike is a 76 CB550F. A while ago I had a problem with my #1 carb where the tube that hold the main jet has greatly weathered and pitted.

I opted to hold the jet in place with some heatshrink until I rebuild the carbs I bought off eBay. A patch that I think kinda worked. Being a 550F, I have the 069A carbs, and bought the 087s, the plan is to convert them to 069a. The bike thus far runs fairly well when warm, except that when I cruise in the 4000rpm range for some time and get on the throttle about 1/4 turn, the bike sputters for about a second and a half before it accelerates. The sputter is not really there if I climb to 4000rpm and get on it within a second or two, and she climbs to 8000rpm with no effort. Am planning to check my timing and my valves this weekend, but I suspect the carbs. Nevertheless I have the "new" carbs cleaned and ready to be reassembled. I suspect the bike is running a little rich as after a few rides the plugs get a little blackened. Planning to do plug chops soon too.

I have a UNI NU-4055 filter on order and will have it next week, I 4 into 1 exhaust (not sure if stock or not) the pipe on the bike installed by the PO is from a Harley Sportster, and offers a little bit of restriction but not as much as stock I'm sure. The mains in the carb are the stock #98 mains. Can be seen here...

Running stock ignition, but have a Dyna S on order (ordered before I heard of Hondaman's ignition unfortunately, I might still get it though) and am planning to run stock coils with the system. Everything else on the bike seems stock.

My question is about the carb innards. I recently realized that the local dealer ordered the Keyster kit for me that looks like is for the 550K (100 mains, 38 slow jets, and D26 needles). Should I use this kit? Or should I order the correct one with the proper 98 mains and needles? Do I need to compensate for the UNI and the pipe? I know the pipe isn't ideal but addressing this issue isn't that high on my priority list until I get the motor running smoothly. Is that bass ackwards?

Anyhow, any advice from you carb gurus would be greatly appreciated.

[TwoTired]

Just use the original 087  internal parts and settings with new rubber bits.  The 069a carbs were set up for the stock pipe which as been discarded.

[medic09]

BTW, that ride is very pretty.   

[mikethejeepguy]

Thanks TT... So just to clarify... you suggest to rebuild the carbs to 550K specs vs. 550F? So stay with the #100 mains and #D26 needle of the 550K? Needle clip positions the same as well? Is that because the UNI nu-4055 will potentially lean things out? And the less restrictive exhaust also leans things out? Am not doubting your advice in any way, just want to understand why for my own learning. 

Thanks medic09 ... there are times I can't stop staring at her. What's funny is how many guys, even once a long bearded Hells Angels dude in colors, come up to me and tell me that a CB550F was their first bike I never anticipated how iconic these bikes are in the bike community before I got one.

[TwoTired]

Thanks TT... So just to clarify... you suggest to rebuild the carbs to 550K specs vs. 550F? So stay with the #100 mains and #D26 needle of the 550K? Needle clip positions the same as well? Is that because the UNI nu-4055 will potentially lean things out? And the less restrictive exhaust also leans things out? Am not doubting your advice in any way, just want to understand why for my own learning. 

I don't recognize the #D26 needle nomenclature as being a Honda number.  The needles from Honda were 272304.  I don't know if the needle profile is the same between these two numbers.
Jet needle number              272304
Jet needle OAL                        5.57cm
Jet needle diameter at tip      1.25mm
Jet needle diameter at 1mm   1.63mm
Jet needle diameter at 2mm   2.06mm
Jet needle diameter at 3mm   2.49mm
Jet needle diameter at 4mm   2.52mm
Jet needle diameter at 5mm   2.55mm

Others have discussed tuning issues with some after-market needles in these Hondas, and have gone back to stock for corrective measures.  Let us know what works out for you , OK?

All the 550 motors used the same mechanical specs for the engine proper, regardless of year or model.  The only thing that changed was carbs and pipes.  The F model pipe apparently worked so well that the fuel draw at WOT needed to be restricted at #98.  Other 4into4 models with the same style carbs got #100.    You don't have that nice pipe anymore, so go up to #100 at least.  You may find out eventually that an even larger main is required, particularly if you aren't done with exhaust and induction changes.

FYI, I have a couple of F model 550s with the stock pipe and also use the Uni filter along with the 069a carb in stock configuration.  The set up works well for me.

In theory, the Uni filter lowers the carb throat vacuum, which pull less on the existing jet orifices and changes the fuel mix slightly. 
A good 4 into 1 race pipe system will help scavenge the cylinders at high, near red line or above, RPM.  This allows more oxygen to be available for the next cycle, and therefore the need for more fuel to pair off.  The trade off, is that below those lofty RPM figures, the exhaust pulses are reflected back to other cylinders, and foul their ideal scavenging.   Honda made their pipe a little more restrictive over the entire RPM band, in order to improve overall responsiveness in the RPM band most used on the street.  Since the cylinders don't have as much air replacement at screech RPM, they require less fuel delivery for those throttle positions/speeds.
Of course, operating at screech RPM reduces the overall life of the pistons/rings/cylinders.  But, racing enthusiasts frequently sacrifice longevity for immediate gains/thrills.

Cheers,

[mikethejeepguy]

A few months have passed since my post and my carbs have been rebuilt and installed. I thought I would sum up my experience thus far with the 087a set up (of the 550K carbs) in case someone is toying with using the 550K set up on a 550F.

My bike:
Dyna ignition with stock coils, Uni replacement filter in airbox, 4 into 1 exhaust mated to a Harley Sportster muffler (with baffle) which is likely less restrictive than stock. Tappet clearences triple checked, cam chain tightened, timing bang on, and carbs synced with Morgan and very close to even Steven.

The Update:
Due to availability I ended up going with the Keyster rebuild kits, BUT used mainly the o-rings and gaskets mainly (since that was cheaper than the $50 per carb for Honda OEM), I also used the jet needles from the kit. I opted to clean and polish the stock emulsifier tubes, needles, main jets (100), and slow jets (40). Am running the the needles in their stock slot position thus far. (stock F setup uses 98 mains and 38 slows).

My findings with this set up:
The bike fired right up and idled along smoothly from just a bench sync. The carbs were fairly close on the Morgan. The bike revs up nicely and rolls back down like it should. Riding, I found that there the engine begins to "garble" and hesitate slightly at 2300rpm (maybe 1/4 throttle) but rolling on the throttle (3/4) there is a brief hesitation, and then rpms climb nicely and fall off after rolling off the throttle. Also, the bike will not run for more than a few seconds on full choke when cold. It likes half choke the best (turning out screws did not help and am back to 1 1/2 out stock setup).

My assumptions:
I haven't done plug chops just yet. I figured I would ride and let things "settle in" for a week or so before I do. It seems to me that at 2300 the mixture becomes a little rich causing the bike to sputter and hesitate (the bike used to do a similar thing at 3900rpm with stock carb setup). I suppose I might have to lean things out by adjusting the clip position. (any suggestions here?) Also, maybe the 40 slow jets might be a little bit too rich vs. stock 38s.

So those are my findings so far... I will post more as I go along and tune. I have Frank Mrazek (a racer from the 70's with a few wins under his belt) dropping by my office today or tomorrow coincidently with his former pit mechanic, so am hoping to pick their brains.