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Bike dies if I hammer on the throttle, sort of OK if I roll it on slowly

  • Thread starter Thread starter MrSelfdestruct
  • Start date Start date
M

MrSelfdestruct

Guest
My bike is a 78 750 (VM carbs). When I bought the bike it didn't have an air box so I just put on some cheapy pods. I live in Denver (52xx ft elevation), so the combination of pods + elevation has made getting this bike to run well challenging. Since I bought it 6 months or so ago I've replaced/adjusted:

pods
points
plugs
valve clearances
timing
intake o-rings
carb strip/dip/gaskets
oil/filter
petcock
fuel/vacuum lines
tires
carb sync
brakes
a bunch of other stuff i forgot to mention

I've got the pilot circuit pretty dialed in, and I think the needles as well. The problem is this: If I roll on the throttle slowly I can get to WOT but only after 5000 RPM or so. If I just hammer on the throttle from idle the bike will just bog down and die. The point at which this happens seems to be a little past 1/2 throttle...maybe about 2/3 throttle... just before the mains kick in. I can be as aggressive as I want with the throttle up until that point, but after that it will just die unless I'm past 5000 rpm.
Through searching I've found out that it is most likely from a mass loss in vacuum and thus a loss in fuel flow. What I haven't found, though, is a way to fix this. I've been through every combination of needle settings with every size main jet from 95 to 112 and everything in between (stock is 102) and nothing makes that throttle phenomenon change. Plug chops tell me that the stock 102 main is good once I get past 5000 rpm. What do I do next?
 
Mechanical slide carbs don't meter well when the throttle is opened suddenly like you are attempting. That said, it's hard to say if what you are experiencing is normal or otherwise. You might want to talk to Tkent02, he lives in CO and might be able to help you out with jetting suggestions.
 
Cheap pods cause more problems than they solve

Either find an airbox or get some K&Ns or ? (I can't remember the other ones, the K&N replicas)
 
as of right now jetting is completely stock so I'm hoping for a plug and play fitment. If anything needs adjusting it will probably be the pilot circuit. We'll see tomorrow. My guess is that when you switch to pods, the loss of intake length + loss of the air volume in the plenum screws with airflow too much for a 3-circuit VM carb to work right.
 
78 Vms are set kind of lean, so your jetting should be good in Denver
 
Raise the needles in the slides a couple of notches and see if it improves any.
 
re

re

It could be fuel starvation. Not from jetting, but from not enough fuel being delivered to the carbs. When you hammer it, you need a much larger volume of fuel. It is likely you have jet issues, but make sure you are getting enough fuel to the bowls, or you will be chasing your tail.
 
well what do you know. the stock airbox completely cleared up the problem, near as I can tell. I'll be doing some more riding this weekend to see for sure.
 
MrSelfdestruct said:
just ordered air box on ebay. we'll wee if that helps
Click to expand...


It absolutely will ! Your jetting headaches will dissappear when re-installing the OEM airbox ! The #1 most frustrating issue whan adding pods is trying to jet the midrange, few, if any are ever sucessfull unless they have a dyno in thier garage.
 
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