wkong
CB750 Member
I’ve got a 71 (engine is from a 75 according to the case number) that’s fouling plugs. It doesn’t happen if I just let it idle in between checking adjustments or if I pop around town for a bit, can’t nail down when exactly it’s happening. It also won’t start or run at all with the choke on (yes it’s operating correctly and I’m using it right). I have to stay on the throttle for a few minutes or bump up the idle screw to keep it above 1500 then drop it back down as it warms up. Here’s what we’ve got:
7a carbs
110 mains (keihin)
40 pilots (keihin)
25mm float height (hondaman recommended dropping the float heights since I have ‘aftermarket’ float valves)
Air screw 7/8ths out
Electronic ignition first statically then dynamically charged at 2500 rpms
New coils, plug wires, and caps (switching wires between 1&4 and 2&3 has no effect)
Carbs synced which wasn’t easy when it has a tough time idling at 1100
Fuel flows out of the petcock and all four drain screws
11.5v at coils
4th set of plugs all gapped to .028
Needles at the middle clip (shouldn’t affect this, it’s fine off the line)
This bike was a drag bike, came with 40 pilots and 130 mains with pods and straight pipes. I’ve got a stock air box with k&n filter and 4-2 pipes with baffles. Idle sounds good out of 3 and 4 but not as smooth out of 1 and 2. Some sort of popping but it’s a low pop not a high one if that makes sense. Can’t tell which because of the 2-1 on that side. It’ll idle at 13-1400 all day, starts to have a tough time staying alive and the popping gets worse below that.
It had intake leaks after I soaked the carb boots in alcohol, got new boots and still had leaks so I’ve put on some heavy duty clamps that don’t show any leaks if I spray a moderate amount around the spigots, but if I douse it it revs up just a hair for a few seconds which I assume is the carb spray getting sucked into the air box.
It’s great on throttle, just a little bit of popping that only happened above 100 mph. It pops on decel.
I bought it and have essentially touched everything but the engine, compression test was within 10% across all four but I can’t remember the numbers anymore. Only time I get smoke out of the pipes is on start up if it sits for a few days but I’m in the PNW and it’s cold and humid so I’m not worried about that.
The only thing left I can come up with is the bungs in the carbs loosened up one of the last three times I cleaned the carbs, since this is the type with I think three of those.
I’ve been reading these forums for months and I was hoping to just post before and after pictures once it was finished and say thanks but I’ve gotta call in the experts. I’ve got a month left before I plan to drop it off a few hours away for the friend I’m rebuilding it for.
So! Any ideas on how to get the choke to work and get the idle even and lower? I assume that’ll fix the plug fouling.
And if anyone has a source on clamps that are something like 2mm smaller than the stock intake clamps I’ll try those instead of these torque clamps I’ve got.
7a carbs
110 mains (keihin)
40 pilots (keihin)
25mm float height (hondaman recommended dropping the float heights since I have ‘aftermarket’ float valves)
Air screw 7/8ths out
Electronic ignition first statically then dynamically charged at 2500 rpms
New coils, plug wires, and caps (switching wires between 1&4 and 2&3 has no effect)
Carbs synced which wasn’t easy when it has a tough time idling at 1100
Fuel flows out of the petcock and all four drain screws
11.5v at coils
4th set of plugs all gapped to .028
Needles at the middle clip (shouldn’t affect this, it’s fine off the line)
This bike was a drag bike, came with 40 pilots and 130 mains with pods and straight pipes. I’ve got a stock air box with k&n filter and 4-2 pipes with baffles. Idle sounds good out of 3 and 4 but not as smooth out of 1 and 2. Some sort of popping but it’s a low pop not a high one if that makes sense. Can’t tell which because of the 2-1 on that side. It’ll idle at 13-1400 all day, starts to have a tough time staying alive and the popping gets worse below that.
It had intake leaks after I soaked the carb boots in alcohol, got new boots and still had leaks so I’ve put on some heavy duty clamps that don’t show any leaks if I spray a moderate amount around the spigots, but if I douse it it revs up just a hair for a few seconds which I assume is the carb spray getting sucked into the air box.
It’s great on throttle, just a little bit of popping that only happened above 100 mph. It pops on decel.
I bought it and have essentially touched everything but the engine, compression test was within 10% across all four but I can’t remember the numbers anymore. Only time I get smoke out of the pipes is on start up if it sits for a few days but I’m in the PNW and it’s cold and humid so I’m not worried about that.
The only thing left I can come up with is the bungs in the carbs loosened up one of the last three times I cleaned the carbs, since this is the type with I think three of those.
I’ve been reading these forums for months and I was hoping to just post before and after pictures once it was finished and say thanks but I’ve gotta call in the experts. I’ve got a month left before I plan to drop it off a few hours away for the friend I’m rebuilding it for.
So! Any ideas on how to get the choke to work and get the idle even and lower? I assume that’ll fix the plug fouling.
And if anyone has a source on clamps that are something like 2mm smaller than the stock intake clamps I’ll try those instead of these torque clamps I’ve got.