|
|
Hi guys!
I'm a new owner of a MINT 1981 CB750C. I'm going to copy and paste 2 posts from another board so you can follow the issue and timeline but hopefully someone can help me out, this one has me pretty stumped so far.
Day 1 - Yesterday
I bought a bike a few weeks ago from a motorcycle mechanic. It hadn't been ridden much for about 2 years, he replaced it with a new GSXR, but it starts up and ran and rode awesome and has been meticulously maintained. Bike has fresh oil, the carbs have been re-synched, and other than old gas, was ready to ride.
Brought it home and went for a 100 km or so ride. It again, ran and rode amazing, and it seemed to be getting more power throughout the ride (I'd say this was 50% bike, 50% my own comfort level with a large CC machine). I managed to run the tank dry on the ride, so got all that nasty gas burned out of the tank. Filled it up with premium and parked it.
The past week and a half have been extremely busy with the GPS show and I didn't get a chance to ride it at all again until last night. Keep in mind the bike's been parked outside and its rained a ton in the past while. The bike was hard to start, and then when I got it running, had to hold the throttle open at around 2,500 rpm to get it to idle properly. When it was initially idling, there was a bit of blue and black smoke and it sounded like it had a slight miss, but i figured it was just cold.
I idled the bike like that for about 10 minutes while I got geared up and waited for a buddy to show up. When I put the bike into gear and tried to move it, it immediately died with the load. Fired it back up and it idled very poorly, had to hold the throttle just to keep it running. Tried revving the motor, and unlike the first time i rode it (that thing revs like a monster!), it revved extremely slowly if at all and wouldn't rev past 4,500 RPM. While I attempted to rev it up to hopefully blow out any crap in the motor, there was a significant amount of black (fuel) smoke and a strong smell of gasoline. The bike had a bad misfire while this was going on, throughout the speed range and literally if you snapped the throttle, the speed would increase a few hundred RPM instead of a few thousand.
Fast forward an hour and we started taking it apart. We figured a good place to start was to read the spark plugs and they had a significant amount of carbon deposits, number 3 and 4 were the worst, and 4 was completely black and kind of fuzzy. The center electrodes on number 1 and 2 were quite a bit shorter/more worn down than 3 and 4. All the plugs had a bad smell of gas (no surprise there) and the exposed ceramic under the center electrode was black on all the plugs.
Took a look at the condition of the wires, cleaned the plugs, and tested to make sure each cylinder had spark and each plug fired. They all fired about the same, though I personally thought the spark didn't look all that hot, not sure what a motorcycle spark is supposed to look like compared to a car though. We checked the coils and both seemed fine, no cracks, hot spots, and all the connections were clean and dry. I have a wiring diagram, checked the spark modules and ensured the grounds were solid. We didn't bother checking timing, because the bike ran butter smooth the last time it was ridden and I can't see timing falling off like that.
We went through the fuel system and the tank seems to be flowing properly into the carbs. I inspected the air filter and there was nothing out of the ordinary for dirt and containments. The fuel filter also looked nice and clean.
This was pretty much where we stopped. The PO offered assistance if I ever had an issue with the bike. I sent him basically what I've posted here via email but haven't heard anything back yet so I figured I'd float it out there to see if anyone had experienced a similar issue before on a carb'd bike.
It is essentially is too rich on at least one cylinder. Could be too much fuel, not enough air, or spark, but it really felt like a spark issue.
Day 2 - Today
Well, i think i can answer both of your questions in another long ass post:
I went and bought plugs today for an ignition test and a clean read of cylinder conditions. I pulled the airbox apart and sprayed seafoam into the carbs BAM! bike fired right up. It coughed and choked a bit right at first, there were tons of air bubbles in the gas lines, but it quickly smoothed out and revved great. After a few minutes of idling and playing with the choke and throttle the bike settled down to a nice smooth idle with no choke and no rev hopping. That sadly didn't last long and quickly you could hear 1 cylinder go out, and then another. Back to the same issues as before. Pulled the plugs and the same issue was apparant. Carbon on the plugs, and #3 and #4 had fuzzy deposits.
So, ignition doesn't seem to be an issue, time to tackle the fueling system. The airbox assembly was a real PITA, so I think al K&N cone filter on each carb should do the trick and they look badass! Got the carbs off the bike and started to take a look.
Everything on the outside of the carbs (and the whole bike in general) is completely spotless. Took #1 apart and everything looked brand new. The PO did say he cleaned the carbs and i certainly believe him!
The entire carb assembly on the other cylinders looked the same. After checking what is actually adjustable in the manual, we took some measurements on the floats. Unfortunately, the service description is sort of vague, so I'm not clear on whether the floats are within spec or way out. Also, there seems to be no way to adjust them. The manual states "adjust the floats carefully with the tang". WTF is a tang? I'm wondering if the floats are aftermarket from a carb rebuild sometime in this bike's life.
Gunk and deposits in the carbs seem to be ruled out. Tuning is starting to look more and more like the culprit or is at least playing a part. My buddy made a good point that the bike was tuned last on the old crappy fuel and now that there's a new tank of premium, it might've thrown the a/f ratio off. The bike has an aftermarket 4-1 exhaust, which Honda recommends calls for a re-jet, and if I do the cone filters that will also call for different jetting.
I'm going to talk to some people tomorrow, and bring the carbs into Universal to find out the float measurement point and figure out what new jets are worth.
|
Locked
