I have the 6.5 power valve in my 850DP and I thought maybe it was bad, we checked it out at the shop today and it is fine. My car is really running rich around town and was wondering if I should go to a smaller number size? 2.5 or 3.5. I have jets up front that are 78 for size. My motor makes only about 11" of vaccum with my Crane 296-2 cam. It seems like the power valve is opening too soon and making the gas go bye bye!
Hook up a vacuum gauge and see what the vacuum does while you drive around town. This will give a pretty good idea of what PV to get. Or, you could experiment and put in the 3.5.
My 850 runs rich too but I pull bout 16" of vacuum at idle with my 468's little gocart cam. When I find time I'm gonna do some jet-juggling.
brett's suggestion is good, drive around with a vac. gauge.
you should be well above 6.5 in around town driving.
smaller primary jets could help lots. they are cheap and easy to change.
the idle transition circuit is harder to fix, you could bypass it by shifting at a little higher rpm.
is your complaint it gets like 6-8 mpg?
Jetting a carburetor should be used as a "fine tuning" method, as it has only a slight effect on a motor's lean/rich mixture. To really straighten the motor out you need to make sure only a very small amount of your transistion slots are showing. Once you have verified/corrected this you can then lean the motor out through the idle circuit. The idle circuit should be used as a "gross tuning" method because it has the biggest impact on a motor's rich/lean mixture.
You do need to check to see if the 6.5" power valve is correct for your setup. I'm running a 6.5" in mine and my plugs are white. However it doesn't sound like you are running overly rich if your plugs are tan.
My friend who is a engine builder said that maybe because the motor only makes about 10" of vacuum that the 6.5 power valve might be opening around town under light acceleration. He also said after about 1500rpms the idle circuts are all done. I did try smaller jets in the primary and the motor fell on its face. Engine runs very strong, but it seems like even with easy driving its getting about 5-7mpg. When I really hit the gas the gauge almost moves infront of your eyes! :eek:
The idle circuit is never done. It always add fuel to the mixture, the transfer slots also always add fuel, then along comes the main jets and finally the power valve. If you are really worried about the power valve try blocking it off with a plug and see if it changes yours around town mixture. If it leans it right out yes the power valve is too rich but if after plugging the power valve the motor is still rich then it is in your idle circuit and transfer slots.
Shane you sure are learning :)
while i was experimenting with my 650, i discovered there was enough fuel from 1 idle circuit :eek: steady A/F ratio of 13:1 on both sides !
i can't imagine needing 4 idle circuits. So i would shut 2 off(the rears).
Sounds :crazy: but just for fun, see if you can tune it with just 1 idle circuit. if u can, look for a nice improvement in mpg.
Taping a vacuum gauge to the windshield will give needed info on that PV. on my 61 i put one in place of the clock. i do not miss the clock!
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i assume this 850 uses progressive linkage?
Matt, Not sure what you mean about progressive linkage, its got the normal looking mechanical secondary the farther you push down on the gas then the secondary opens. I will try closing the back two idle screws and see how it runs. I know its running fat because it just sounds wet, and my back bumper after a few runs gets black. Thanks for the info!
Make sure you don't have too much of the transfer slot showing. Check both ends if the carb isn't new people have been known to open the sec side to try to keep from drilling the throttle blades. You can try some lower number power valves just to make sure but remember the 850 DP is the worlds greatest design for a street car.
that's normal street linkage. the other type opens all 4 together :eek:
in traffic, what RPM does it usually see?
I think on my 750 dp there is two holes for the linkage, the first one allows the prim to open before the second (there is a slot in the sec throttle linkage with a rod connecting it to the prim) while the second hole moves the rod forward so it is in direct contact with the end of the slot pulling allowing the sec to open at the same time as the prim. Mucho gas to eng at one time. :eek:
another problem i've read about is fuel siphoning out of the acc. pump circuit. That would sure waste fuel.
Some carbs use a check ball(could be missing)to prevent this.
not sure on the 850. just a thought.
maybe someone with an 850 will chime in.
Never heard of fuel siphoning out of acc pump. Has anyone else heard of this??
Yes, there is a ball or a piston under the sqiurt. (piston in 850DP and ball in 750 vacuum in my case) to prevent that effect.
I have 13" of vacuum on idle and 18" highway. I my case I got stumbling until it go down under the pw limit (10.5 inHG) and the A/F meter going lean (but the A/F are rich on cruise and WOT). I hope the lean condition was an effect of my test with 76 primary jets to get 'normal' A/F figures during cruise.
Now I have installed 79 primary and will try to find some other adjustments to get it leaner during cruise. Will test when the rain stops dripping.....
Is this rich condition a normal effect of double pumpers?:confused:
i remember norval saying he richened the squirter to cover up the lean tip-in. maybe he will chime in.
sounds like it's worth a try.
i gather the 10.5 pv is too far away?(for use with the 76 jets)
DP are set up rich for racing and to lean out for around town driving i would reduce the size of the idle fuel restrictions(IFR),reduce the jet size while enlarging the power valve restrictions the same amount in area. the power valve has nothing to do with idle if the idle transfer slots are set correctly. most BBC need 8.5 power valves to prevent part throttle leanout.
Clem, I did try to go with smaller jets up front 75 size and the engine really stumbled then took off. I understand when the vacuum drops below the power valve rating then it richens the mixture by alot. Also are you talking about the idle screws IFR?
Power valve question for BB
