I summited a question to the Summit tech support about my low vacuum at idle. They told me something I didn't know. My 850 double pumper is new and new to me. This is what they said:

First ensure there is no vacuum leak between the carb and manifold



On the passenger side, rear of the carb there is a small threaded stud sticking through the base plate of the carburetor that will adjust the secondary throttle plates. (the carb may have to be removed) Turn the screw in until you can see a little light coming through the throttle plates. (more is not necessarily better)



When you now start the motor the idle should be much higher. Adjust the curb idle linkage screw until the idle is back to normal.



Connect the vacuum gauge to a full manifold vacuum port and adjust the air/idle mixture screws for the highest vacuum reading.



Place the car in gear with the brakes locked. Watch the vacuum reading. Whatever the vacuum reading now shows, divide the number in half, and add .5. This is the size power valve you should use as a starting point.



Run the motor through it's normal rpm range and read the spark plugs. You are looking for a dark tan color. Should the plugs appear dark, reduce the main jet size. If extremely light, or constantly look new, increase the main jet size.



NOTE: Check the threads of the spark plugs for traces of oil. This could indicate the intake manifold is not sealed properly and pulling a slight amount of oil from the lifter valley. This would create a vacuum leak, which will give you incorrect readings and will cause a lean condition.



Ok now I did what they said and I did see an increase in the vacuum signal but, now the engine surges some at idle and when I snap open the throttle once it popped fire and killed the engine. I have not changed the power valve yet. Is it acting this way because of the power valve or what?

TTT

Time to tackle the accelerator pump circuit. Start with a larger squirter. Try richening up the idle an 1/8 turn. Timing should be around 12-14 initial and 36-38 total. Power valve probably isn't the problem unless it is opening too late.

I don't think you understand what I'm talking about when I mention the power valve. I have around 5 to 7 inches of vacuum at idle. I'm sure the original power valve is rated higher than that. So I'm thinking its also opening up at idle. I can see fuel dripping in it once and a while.

I also didn't have a popping problem until I cracked open the secondary butterfly with the srew in the base plate to bring my vacuum up.

What's happening to your engine is a lean surge. I think Pat fully understands the operation of the power valve. If your power valve is malfunctioning, it would drip fuel into your carb at all times (like you stated), but it would cause a rich condition.

Is there any black smoke out of your exhaust?

It sounds to me like when you opened up the secondary throttle plates you introduced more air into the system causing a lean condition... especially at part throttle.

I recommend you tune up the idle and the transition circuit.

Of course you both could be right, where there is a blown PV allowing far too much fuel but still having a lean condition because of too much air? :confused:

Try replacing the powervalve with the correct one and then fixing the other problems.

I thought the PV in an 850DP was 6.5, so it's close, but 3.5 seems like it's what would work. That's very little vacuum unless you are running a really big cam or something. Unless I missed it, what happened to all the blow-by coming out of your breather(s)?

Mike

Well I put the secondary plates back and the popping stopped. I tried adjusting the mixture screws first but it didn't help. By playing with it I've got the vacuum reading up to around 7"@900 RPM. I still have some blow by comeing out of the breather.

bad Rat,

I just looked at the Holley site, and all of the 850's I saw had 6.5 Power Valves, so you are real close to having it actuate just like you thought. I would think that is a bad thing at idle.

Originally posted by Bad Rat 414:
Well I put the secondary plates back and the popping stopped. I tried adjusting the mixture screws first but it didn't help. By playing with it I've got the vacuum reading up to around 7"@900 RPM. I still have some blow by comeing out of the breather. I encounered this situation on my friends car also.

He has the standard old 830 Annular.Not an HP model.

He had turned the secondary screw to open the secondarys up a bit and set the idle mixture screws with a vacuum gauge.

The front was 7/8 turn out and the back was 1 1/8 out.The secondary throttle plates did not expose the transfer slots and the primary did not expose more than .040 of the slot.

Nothing REALLY wrong in my eyes.

I tried pump cams, squirters, and a 50cc pump.
Nothing helpped, till I talked to him a bit about his adjustments and when this popping/spit started happening.

I reset all 4 idle mixture screws at 1 turn out and backed out the secondary screw 1/2 turn.
*This means that I had to expose more of the primary idle transfer slot to regain the correct idle rpm.I know it now exposes more than .040 of the slot (supposed to be a no no) but the idle mixture screws still respond correctly.

No more poping/spit.
I'm not certain which adjustment (secondary screw or idle mixture screws) did it but he doesn't wan't me to mess with it.He tells me
"It's a bracket car, if it ain't broke, don't fix it".

** Example on PV's**
If your vacuum dipps from 7-6 then you need at least a 5.5 PV.