Bike says BUUUUUUUUUH.

[robinjo]

I've had a shop install seperate K&N filters and bigger main jets (116) and dyno it.
But if you start it when it's cold you got to keep the choke on for about 2 minutes or else when you give it some throttle it dies and goes BUUUUUUH .
Also, when you accelerate from a trafficlight and open the throttle and let go of the clutch it does the same thing only very minor, other than that it runs GREAT it goes from 65 to 110 mph in top gear in what seems like seconds and the throttle respons is great!!

Can I adjust the airscrews at the side of the carbs by screwing them in more? It still has the original pilot jets (15).

 

[Jay B]

Sound

Sound

My bike makes the same sound when its fuel starved. Sounds like the jets aren't big enough. Try the next size up.

 

[earlfor]

Re: Bike says BUUUUUUUUUH.

Robin
My70 GS 750 is exactly the same as you describe yours. As far as I know, this is normal for the models with the slide carbs. I also need to hold the choke on about two minutes to warm it up before it will accept throttle and too much throttle will result in the "BUHHHHH" you describe.
Its a result of the carb design and I dont believe adjusting the pilot or air screws will stop it. Getting them just right will help, but not get rid of it.

Earl

 

[robinjo]

Re: Bike says BUUUUUUUUUH.

Robin
My70 GS 750 is exactly the same as you describe yours. As far as I know, this is normal for the models with the slide carbs. I also need to hold the choke on about two minutes to warm it up before it will accept throttle and too much throttle will result in the "BUHHHHH" you describe.
Its a result of the carb design and I dont believe adjusting the pilot or air screws will stop it. Getting them just right will help, but not get rid of it.

Earl

Earl, it didn't do it when I had the stock airbox on, you would start it with the choke on and after about 20 seconds you could take off, I thought that if the engine ran without giving it any throttle the main jet is completely closed and it's running on the pilot screw? Or is this theory wrong?

P.S. If I do a really fast launch from a standstill it doesnt hesitate.

 

[earlfor]

Re: Bike says BUUUUUUUUUH.

Robin

I think you are right that if it is idling, the throttle is untouched and the slides/pistons in the carbs are at bottom, then it has to be running on the pilot and airscrew. On mine, if I rotate the throttle even the slightest amount, the carb slide/piston is lifted and it is no longer running solely on the pilot. There is an overlap of mixtures. On my carbs, at anything over 1000 rpm, the carb pistons are lifting and midrange is starting to assist. If I quickly open the throttle, my main needles will instantly go to full up.
I think what is happening is that the induction air is increased
faster that the jet can flow fuel and momentarily, you get a lot of air and no fuel, then the jet catches up and things take off. At low throttle settings there can be a momentary huge difference in mixture so we get the Buuuuuuu. At high power settings, there is already good fuel and airflow, so we dont get the hesitation.

The choke circuit is a separate circuit from the pilot and just richens the mixtures for starting, so I doubt it has anything to do with the Buuuu. Once the engine is warmed up and the choke is released, it is inactive.

Considering you have changed your induction, I can only suggest that you check the plugs for color to determine if the pilot and airscrew settings are about correct.

I found that adjustment of my pilot and airscrews minimizes the
hesitation, but I have been unable to rid myself of it. Its my opinion that due to the carb operating design, I probably will not be able to rid myself of this annoyance.

Earl


Earl, it didn't do it when I had the stock airbox on, you would start it with the choke on and after about 20 seconds you could take off, I thought that if the engine ran without giving it any throttle the main jet is completely closed and it's running on the pilot screw? Or is this theory wrong?

P.S. If I do a really fast lau
nch from a standstill it doesnt hesitate.[/quote]

 

[Anonymous]

In the Clymers manual, it discausses this hesitation under "Performance Mods" or something like that, and it suggests (on a stock bike) stepping the pilot jet up 1 and the main up 1 and something with the needles (I think, I don't have the manual here) if u have the same stock pilots in, I would step them up. (78 750)