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Rpms Drop Befor Reving??

  • Thread starter Thread starter deves3000gt
  • Start date Start date
D

deves3000gt

Guest
I have a 78' 750 all stock. I was just showing my wife the bike (i just acquired the bike from my father) i started it up (right up) i love these bikes) warmed it up and took her around the block. My qestion is i noticed that while it was ideling i was playing witht the throtle reving it up. It seems that when i opened the throtle fast it boged down. I think it would have staled if i didnt let up frist, but if i hit it a little then wiped it open it will rev right up with all it's proper power. with or with out it in gear. when i'm driving it i can open it up all the way and it revs right up. It idels at 1k, all cly. hot, the only thing that is off the bike is the air filter b/c it fell apart. I figure it might be a carb snyic. prob but i don't know. the gas in the bike is a bit on the old side. Any help will be greatly appreciated.
 
That is exactly why the Feds mandated the switch to CV (constant-velocity) carbs in 1980. When you open the throttle suddenly, there is no more vacuum to pull the gas through the jets. If you open at a 'more reasonable' rate, it maintains vacuum and will rev right on up there.

The CV carbs achieve this with the vacuum-controlled slide. Even though the throttle plates might be wide open, the slide is closed enough to maintain air flow over the jets. As engine speed increases enough to create enough vacuum, it pulls the slides up to expose more throat area, eventually opening up all the way.

For those that were into hot-rodding cars back in the '60s and '70s, this is like the difference between mechanical and vacuum secondaries on your 4-barrel carb.


.
 
Run the old gas through it and see if it still does it with new gas. If it does I'd check for leaks in the airbox and intake second. First I'd check the air filter like paco stated. Check the petcock vacuum line. Check compression and valve clearance. Clean carbs.
 
Steve said:
For those that were into hot-rodding cars back in the '60s and '70s, this is like the difference between mechanical and vacuum secondaries on your 4-barrel carb.
Click to expand...
That is a great comparison. I never thought about it like that. You don't even want to know how many quadrojunk carbs I messed with.
 
If you open up (blip) the throttle unreasonably quick on your VM carbs, you can momentarily gag them. This is normal. Correctly operated, there's no problem.
A fresh vacuum synch is always a good idea anyway.
 
chef1366 said:
That is a great comparison. I never thought about it like that. You don't even want to know how many quadrojunk carbs I messed with.
Click to expand...


If you want to complete the comparison, the CV carbs are like a vacuum-operated secondary,
but the secondary side feeds the primary.

It's a vicious circle, thank goodness it works most of the time. 8-[


.
 
I guess it depends if it is the normal "Lag?" experienced with VM carbs or if its a real Lean bog "problem" from too much air or not enough fuel.
 
hey guys thankyou for all the replies. It's good to konw that these carbs. are known for the temp. bog. I know the carbs. are running lean from the lack of an air filter but I don't remember the "lag" being so drastic. I guess I should put a filter on and see what happens hu? I'm just nervous about it, I the bike in peices (thanks dad) so i'm looking for something wrong with it. But, would the normal lag stall the bike? I didn't hold the throttle there long enough to see but the way it droped i'd be surpirsed if it didn't.
so this was so long.

P.S. This REALLY is a GREAT site\\:D/
 
If you're blipping the throttle as I think you are, too quick and not something you'd do to operate the bike normally, then nothing's wrong with the bike.
Removing the filter from the VM equipped models will not cause a bog of any kind.
 
I was going to ask the same question.When I hitthe throttle t about 4 or 5 grand the idle will drop to 900rpm's before coming back up to 11oo where they are suppose to be.I guess that answers my question.
 
Well, thankyou too all. Keith I guess your right. (not that you need me to tell you that) lol...the bike does opperate fine. So I guess it's noting to worry about. I'll let you know if it starts boging anywear else in the rpm range.

Once again thankyou to all who replied.
 
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