Topic: 74 CB550 Running Rough After New Exhaust System Addition

[DBRider]

Where to begin…

Finally had my 74 CB550 running good. Within the last 50 miles I’ve rebuilt the carbs twice, adjusted points and cam chain, new plugs and wires, Magna coils, and valve adjustment. Clear tubed the carbs so it wasn’t even puking gas anymore. Went to great expense to restore the factory air box setup. All new intake rubber. After the second rebuild it was still hard to start but running solid and I was enjoying a little bit of riding. It needed an exhaust system, it was a rusty chrome 4-1 that had been chopped at the collector so there was no muffler. So, since it was running so good I splurged and picked up a new all black MAC 4-1 with canister muffler. Installed it and the bike ran really great and I took a 6 mile ride with no problems. And then I started back home and it died. Got it restarted and just limped home. The engine was hotter than normal, couldn’t touch it without gloves and it seemed like there was more noise from valves and bottom end. I read that maybe I had a leak at the exhaust manifold so I snuggled up the factory clamps a little more and went for a ride the next day. Low power, tries to die at idle, pops and sputters. All I changed was the exhaust system. I’m running stock jetting. I guess it could be lean, but shouldn’t it have been even leaner with no muffler and it was running good with no muffler. It’s been in the 90s before and after the change. I’ve spent almost two years working on the bike and really thought the exhaust would be the last big thing I'd have to tackle and then I could enjoy some summer riding. Now I’m back to square 2. Any things to check or suggestions are welcome. Thanks.

[desertrefugee]

I doubt your trouble has anything to do with the new exhaust. I suspect it's purely coincidental. Sounds like it might be time to go back to square one (not two).  Although my first instinct would be to suggest focusing on the carburetors, the sudden onset of trouble does suggest an electrical issue in the primary or (possibly) secondary ignition circuit. Recheck point gaps, all your connections, confirm fire in all four holes and maybe read the plugs.  If all that checks out then maybe start thinking about carbs.

[HondaMan]

Where to begin…

Finally had my 74 CB550 running good. Within the last 50 miles I’ve rebuilt the carbs twice, adjusted points and cam chain, new plugs and wires, Magna coils, and valve adjustment. Clear tubed the carbs so it wasn’t even puking gas anymore. Went to great expense to restore the factory air box setup. All new intake rubber. After the second rebuild it was still hard to start but running solid and I was enjoying a little bit of riding. It needed an exhaust system, it was a rusty chrome 4-1 that had been chopped at the collector so there was no muffler. So, since it was running so good I splurged and picked up a new all black MAC 4-1 with canister muffler. Installed it and the bike ran really great and I took a 6 mile ride with no problems. And then I started back home and it died. Got it restarted and just limped home. The engine was hotter than normal, couldn’t touch it without gloves and it seemed like there was more noise from valves and bottom end. I read that maybe I had a leak at the exhaust manifold so I snuggled up the factory clamps a little more and went for a ride the next day. Low power, tries to die at idle, pops and sputters. All I changed was the exhaust system. I’m running stock jetting. I guess it could be lean, but shouldn’t it have been even leaner with no muffler and it was running good with no muffler. It’s been in the 90s before and after the change. I’ve spent almost two years working on the bike and really thought the exhaust would be the last big thing I'd have to tackle and then I could enjoy some summer riding. Now I’m back to square 2. Any things to check or suggestions are welcome. Thanks.

Start by looking at the sparkplugs. If they are black, get new ones. It is VERY rare for the exhaust system to affect jetting, as these pipes are too short for any significant effects between types, unless one is just plugged up or the like.

When you say "rebuilt the carbs", did this include chaning mainjets? If so, put the old ones back in, as none of the current "rebuild kits" have the correct jets in them: they are all Keyster jets of sizes different from your original ones. Even if the Kyester jet number is the same (and it probably wasn't in the kit), the Keyster numbers run about 7% leaner than their size value. For example, their #100 runs like a #93 Keihin. Their slide needles need to be removed and thrown away: they are miscalibrated in every kit I have seen, causing rich low-end and lean top-end running. The holes in their emulsifier tubes are also too small, in every one I have seen. This makes for fuel that is not mixed with air when it enters the engine.

Also: if you removed the intake manifolds from the head, did you also change the O-rings in those castings? If not, they leak now. The O-ring is the same size as in the valve caps.

[TwoTired]

Whenever there is a sudden change in engine behavior, I find it best to first review the engine tuneup check list.  If the basics are off, then whatever you “fix” is not going to be a full correction.

During this, you look at the plug deposits.  Soot will interfere with ignition and if one or more of them shows a deposit pattern different from others, you have pointers  on where to look further.

Bad condensers and points do bear scrutiny, as the Diachi brand fails early in many cases.  New parts are not guaranteed to be good parts, even though they  should be.

Just be methodical and unemotional.  Bike doesn’t care about your feelings and desires.  It wants what it wants when it wants it.  It up to you or your mechanic to give it what wants so it has no choice but to serve you and your needs.

Cheers,

[RAFster122s]

Might not hurt to check your valve clearance also…

[DBRider]

Thanks, I’ll start over at the bottom. Your answers are what I was also thinking, but still encouraging, which is what I needed. To me, the sudden change meant something maybe catastrophic had happened. (Guess it still could have.)

Just for additional reference, the plugs were pretty black and sooty when I pulled them but I did have a little two stroke oil in the gas for extra lubrication after I had the rocker cover off last fall. I cleaned the plugs off which made no difference in performance. All the plugs were pretty much identical. I have new K&L carb kits. I understand that specs likely vary from OEM but the bike was running really good before the new exhaust - and even for a while with it.

I’ve never had the intake manifolds off but I have sprayed starting fluid around the area with no change in idle.

“Methodical and unemotional” has been the mantra up until the exhaust fiasco. Just working through problem after problem for almost two years. Admittedly , I was pretty content to yeet the d@mn thing into the backyard and walk away the other night.

[RAFster122s]

Keihin  brass in the carbs, the brass on Keyster and K&L kits is essentially junk…

JetsRUs sells brass that maybe aftermarket in some cases but it is properly sized to stock keihin specs….

[TwoTired]

Black soot on the center electrode insulator negates the effect of having and insulator and allows shorting the spark energy away from the spark plug gap.  Some methods of cleaning leave metal on the abrasive porcelain.  Replacing soot with metal deposits, leaves the spark plug no better for operation.  Sand bla works well and some report good recovery with torching off the black deposits.  But, I've actually made them worse with a wire brush.  Sand/bead blast fixed them right up, though.

But, if new plugs fixes your current run issues, expect it to recur, if the bike is running too rich, as those black carbon deposit will just happen again, if the engine can't self clean the plug due to just too much fuel to burn.
Cheers,

[DBRider]

How much effect does altitude have on these bikes? I am just over 4000 ft. Taking all the comments in, especially the sooty plugs, going from no muffler to a muffler with a baffle, and having some two stroke oil in the gas, I could well be too rich. I believe I have another set of plugs I can put in and report back.

[desertrefugee]

Living in Arizona and having run a number of carbureted motorcycles from which to draw experience and offer comments, I can tell you that from my home at about 1500 ft to about 5000 ft, I rarely notice any difference in any machine.  6,000 feet up, I begin to notice a distinct loss of power.  The over rich mixture definitely starts to catch up with it.  I've never pulled plugs at that point, but I'm fairly certain they would be dark. 

But behavior at increasing altitude really depends on what your baseline jetting is. 

[DBRider]

Update:

Points were pretty bad, especially 1-4. Installed new points and adjusted static timing accordingly. Bike started easier than usual. Still running like garbage tho. Idle hangs about 3200 rpm. Seems better at higher rpm but bogs and tries to die on acceleration. Blows a fair amount of smoke out of the exhaust when it does rev up also. Don’t dare put it on the street, likely have to walk back home. Plugs for 1-4 were white but the pipes were significantly cooler than 2-3, so I’m thinking washed out. Plugs for 2-3 are black and pipes are hot (much hotter than 1-4). I have spark on 1-4. 5 ohm Dyna coils. New wires and caps. New Ngk D7EA plugs.

Guess I should have kept the rusted out header with no muffler, least I had a sweet running machine. 🤷🏻‍♂️ I keep thinking carbs with the new exhaust but that doesn’t explain the differences between 1-4 and 2-3. That tells me ignition. But I have spark.

Haven’t had the opportunity to check the valves yet. But I just did last spring and have ridden less than 100 miles since.

[TwoTired]

Swap the condensers side to side to see if the  plugs show opposite color deposits.

You know fouled plugs will run like crap.  Insulators black? Spark shunted.

  Not convinced it’s your muffler.

Cheers,