Bike runs worse after carb sync.....

[DWade]

Hey guys
New guy here. I recently bought a 1981 CB750K. 100% stock except for a goofy sissy bar that needs to come off. This is my first 4 cylinder bike. It's in very good condition with 18k miles. On the ride home, I noticed it wouldn't spin past 5500rpm, so I threw some new plugs in it, adjusted the timing chains, and checked the valves (which were all spot on). Boy what a difference! The bike ran great!
A couple days and tank of gas later, I noticed fuel puking out the #4 overflow hose, so I rebuilt the carbs using the MacGregor DOHC carb rebuild guide. Bench synced them, and put the back on the bike. One of my boots between the air box and carb suffered a small tear, but it's not huge. The bike immediately fired right up on choke, but needed the idle screw turned in a turn or two to idle off choke. She was running very, very well, but I noticed some pops intermittently coming from the exhaust.
A few days later, I synced the carbs using the Motion Pro sync tool. I got them to flat line on all 4. Couldn't line up better.

Well.... now the bike runs much worse than it did before I synced them on the bike. I don't understand. The pops are much more noticeable, and it has some surging going on at part throttle cruising speed.

So what's going on here guys? Anybody ever experience anything like this before?

 

[DWade]

For the record, the popping seems to be coming from #1 or #2 exhaust, I can't tell. The tear on the boot is on #4.

 

[amc49]

Popping says fouled plugs from carbs flooding, weak ignition or compression down. Unless the popping only on decel, then it's an aircut problem.

You don't simply even up the levels all the same on a sync, you actually do what the race carb guys call 'crowds'. Crowds are light accels away from a set throttle point, you lightly rev the motor and make sure all 4 pull up exactly the same too for the first inch or so, not just set them the same at dead idle. You can fake the dead idle being even by having the idle mixture screws way off from each other and then the levels are the same at dead idle but motor goes to crap upon doing the crowd when the uneven idle mixtures f-ck with each other at the run-up. Valve sealing evenness messes with that off idle response too. Dead closed throttle and active increasing throttle are closely related but two different things.

These CV carbs are well known for driving mechs mad because they clog the pilot jet to run lean, then because the idle system is hooked to the primary main, the dead lean idle then makes a super rich point right as you pull away from idle. Something else the crowds will illustrate that you totally miss just syncing at dead idle only. Carb goes rich right after lean because the idle swings into being in essence an airjet right off idle, the same clog that makes it lean then makes the airjet action clogged to go rich as the flow switches to the primary.

Each mixture screw on its' own better be able to kill that cylinder when screwed shut or something wrong with that idle system.

Probably should do a compression test too just to have a solid confirmation the motor is sound, I say that because so many lose their butts assuming a 35 year old motor is sound even if it has only 18,000 miles. I've seen them totally messed up yet looking primo at less than 10K miles.

Dunno about the valve check done there but you use .005" as the number to hit, if you got valves at .003" or .002" they are probably being held open, they commonly burn at .002" yet the service manual calls for it as OK. WRONG, the engine commonly runs with LESS clearance running than you get with checks. How most of them get screwed up. That can make your popping too.

 

[DWade]

I'm aware you don't simply line up the lines. I have them all responding equally to throttle input, at least as equally as I could get them.

I pulled the plugs today and sure was surprised at what I found. Plug 1 is lean as could be, while the other 3 look slightly fat, but not bad.

I'm going to run a compression check in the next day or two.