Engine bogging down when revved

[Anonymous]

I have a 78 750, seems to be running fine EXCEPT.... when its idiling or in gear if I totally open the throttle real quick the engine sputters and dies out. If I accelerate normally or slowly it seems perfect but once I nail it it just bogs down. The carbs are rebulit, new kits and totally cleaned out, new plugs and I have been riding it around daily. I don't really get on it, not out there for the speed just for a little fun but I just want to correct whatever is wrong and have it running 100% Thanks again for the help...

[Anonymous]

i have a 78 750 also, i have the same problem also :roll: it runs a little rich so im guessing that has to be the cause.

[Anonymous]

I too have a 78 750 with that problem, and I had two ideas as to why. 1)Its gotten worse over time as the exhaust packing has deteriorated, thus maybe it's running too LEAN. 2) twisting the throttle pulls the slides WAY up allowing a LOT of air through, which actually DECREASES vaccuum produced by the engine temporarily, thus less FUEL but more AIR and becuase these old carbs don't have anything to compensate to keep the mixture a little more even, it stalls, bogs, stutters... just a thought.

[earlfor]

Youre exactly right, its a lean problem as you describe. I have the same condition with my 79 750. I have compensated for the problem by being very careful to balance my carb vacums precisely and to keep the vacum settings as low as possible to reduce the range of induced vacum between idle and full throttle. If you set the bike up to run a bit on the rich side, this problem is minimized, and power/acceleration is improved. Gas milage decreases a bit.
If you set a high vacum range, the problem is worse, power is decreased, but gas milage gets better. The "quirk" is very minimal on my 750. I look at it as just one of the characteristics inherent to the slide mikuni's on these bikes. Due to carb design, I do not think it is possible to eliminate this trait.

Earl

I too have a 78 750 with that problem, and I had two ideas as to why. 1)Its gotten worse over time as the exhaust packing has deteriorated, thus maybe it's running too LEAN. 2) twisting the throttle pulls the slides WAY up allowing a LOT of air through, which actually DECREASES vaccuum produced by the engine temporarily, thus less FUEL but more AIR and becuase these old carbs don't have anything to compensate to keep the mixture a little more even, it stalls, bogs, stutters... just a thought.

[Anonymous]

earl

earl my bike runs to rich and i stll have this problem, if i keep it wacked open it will die, gonna drop my needles tomorrow to see if that works better.

[earlfor]

Re: earl

Rick:
The transition from idle to opening the throttle is controlled by the pilot jet and airscrew. If opening the throttle causes the engine to die, I think its likely those settings are a tad lean. Dropping the needles will not help the bog because the condition occurs before the needles are active. If you observe the carb pistons as you "blip" the throttle, you will see that the "bog" due to a lean condition happens before the needles/pistons start to raise.


On your 2nd statement:

Dropping the needles because your bike is running too rich overall will work to cure that problem, BUT...dropping the needles means less fuel flowing and I would assume less power available.
It may be the better idea to leave the needles set as is and re sync the carbs to a higher vacum setting. This should result in more fuel flow, more air, more power and cleaner burning plugs.

Earl

earl my bike runs to rich and i stll have this problem, if i keep it wacked open it will die, gonna drop my needles tomorrow to see if that works better.