Been lurking around this site ever since I bought this bike (1981 cb750c). Great site with a ton of information! I bought the bike from someone who was a member here who recommended the site. First motorcycle I've ever owned but have been riding and working on dirt bikes for ages.
a little about the bike:
When I bought the bike one of the forks had a leaky fork oil seal and brake pads and disc on that side were covered in oil. I'd never rebuilt forks other than those on modern dirtbikes but figured since the forks I worked on before were a little more involved, having compression and rebound adjustments, these couldn't have been any more diffcult, and they weren't, did oil and dust seals on both forks with maxima racing med weight fork oil. Bushings were in good shape. Cleaned up rotor and pads.
The bike has 4 onto 4 exhaust, I really like the look. However, the guy I bought it from said the previous owner removed the baffles. So there are no baffles. The connection b/w the muffler and pipe was sloppy, so I fixed that also with reducers (its now a very tight seal). Bike is very loud, I would prefer if it had the baffles. I like straight piped engines but I believe there is something to be had for some back pressure. Right?
The problem:
I'm mechanically inclined but I have a weakness, carburetors. I don't know much about tuning carbs. Today I got the bike ready to go out for a rip, just got the mufflers fitted and forks back in etc. This is the first ride I've ever had on the bike. The engine fires up, i have to hold the choke up for a bit but not too long. It seems to idle ok, but when you rev the bike there is a hanging idle, it will take a while for the revs to fall. I wasn't too worried about it, so I went on my way. Throughout the ride there was a bit of popping on decel which I figured might be because of the baffles being removed. Also the bike seemed to lack a bit of power, and at times seemed to feel or accelerate better at times too.
After maybe a 10 minute rip (didn't get passed 4th gear, just cruising) I was coming to a stop, pulled the clutch in, revs fell and the bike stalled. I pulled off the road waited a bit and fired it up. went for another 5 minutes down the road in 3rd or 4th bike still popping, except now i can hear it coming from the 3rd or 4th cylinders/pipes. Pulled over into a parking lot to check things over. As I was leaving the bike just started popping like crazy when accelerating or decelerating, when I pulled over even at idle it was popping aggressively, way way too much fuel.
I didn't know what to do so I just turned the throttle adjustment screw (the one thats easy to reach with the black knob) out a few turns. Waited a bit and fired the engine up again, it was popping still but far less aggressively, so I headed back to the garage. Engine seemed to be running hot, but I've never owned an air cooled bike (nor a motorcycle) so I'm not sure how hot is too hot?
I will be doing compression test hopefully this evening and post the numbers.
My questions are:
1. Running no baffles in the exhaust cause excessive decel popping
3. What in the carb would stick to cause fuel to just dump in the cyclinder.
4. Can you buy just the baffles for these universal mufflers? is it worth it? are a good 4 into 1 system better
5.I like to have good throttle response, and have heard the 1981 generation cb750's on video that have excellent reponse, no hanging idle, revs fall immediately, how is this achieved.
6. What are your general thoughts on the problem?
Sorry for the confusing read and general questions, but I don't know much at all about tuning carbs. I've rebuilt 4 stroke dirt bike engines before but never had to touch the carbs. I have a feeling the carbs are whats causing me issues.
I will be buying new intake boots, as they might be suspect too. THey look fine but for piece of mind I ll order new ones.
A couple pictures ... Sorry for the long post. I want to get the motor running the way it should or better before I start adding changes to the way the bike looks. Any help is appreciated! Thanks
a little about the bike:
When I bought the bike one of the forks had a leaky fork oil seal and brake pads and disc on that side were covered in oil. I'd never rebuilt forks other than those on modern dirtbikes but figured since the forks I worked on before were a little more involved, having compression and rebound adjustments, these couldn't have been any more diffcult, and they weren't, did oil and dust seals on both forks with maxima racing med weight fork oil. Bushings were in good shape. Cleaned up rotor and pads.
The bike has 4 onto 4 exhaust, I really like the look. However, the guy I bought it from said the previous owner removed the baffles. So there are no baffles. The connection b/w the muffler and pipe was sloppy, so I fixed that also with reducers (its now a very tight seal). Bike is very loud, I would prefer if it had the baffles. I like straight piped engines but I believe there is something to be had for some back pressure. Right?
The problem:
I'm mechanically inclined but I have a weakness, carburetors. I don't know much about tuning carbs. Today I got the bike ready to go out for a rip, just got the mufflers fitted and forks back in etc. This is the first ride I've ever had on the bike. The engine fires up, i have to hold the choke up for a bit but not too long. It seems to idle ok, but when you rev the bike there is a hanging idle, it will take a while for the revs to fall. I wasn't too worried about it, so I went on my way. Throughout the ride there was a bit of popping on decel which I figured might be because of the baffles being removed. Also the bike seemed to lack a bit of power, and at times seemed to feel or accelerate better at times too.
After maybe a 10 minute rip (didn't get passed 4th gear, just cruising) I was coming to a stop, pulled the clutch in, revs fell and the bike stalled. I pulled off the road waited a bit and fired it up. went for another 5 minutes down the road in 3rd or 4th bike still popping, except now i can hear it coming from the 3rd or 4th cylinders/pipes. Pulled over into a parking lot to check things over. As I was leaving the bike just started popping like crazy when accelerating or decelerating, when I pulled over even at idle it was popping aggressively, way way too much fuel.
I didn't know what to do so I just turned the throttle adjustment screw (the one thats easy to reach with the black knob) out a few turns. Waited a bit and fired the engine up again, it was popping still but far less aggressively, so I headed back to the garage. Engine seemed to be running hot, but I've never owned an air cooled bike (nor a motorcycle) so I'm not sure how hot is too hot?
I will be doing compression test hopefully this evening and post the numbers.
My questions are:
1. Running no baffles in the exhaust cause excessive decel popping
3. What in the carb would stick to cause fuel to just dump in the cyclinder.
4. Can you buy just the baffles for these universal mufflers? is it worth it? are a good 4 into 1 system better
5.I like to have good throttle response, and have heard the 1981 generation cb750's on video that have excellent reponse, no hanging idle, revs fall immediately, how is this achieved.
6. What are your general thoughts on the problem?
Sorry for the confusing read and general questions, but I don't know much at all about tuning carbs. I've rebuilt 4 stroke dirt bike engines before but never had to touch the carbs. I have a feeling the carbs are whats causing me issues.
I will be buying new intake boots, as they might be suspect too. THey look fine but for piece of mind I ll order new ones.
A couple pictures ... Sorry for the long post. I want to get the motor running the way it should or better before I start adding changes to the way the bike looks. Any help is appreciated! Thanks
