Should my motor be pulling more vacumn??

[jefflock]

I was using a vacumn gauge to adjust my carb and only had 6" of vacumn. I sprayed around the carb flange and the top of the intake runners with no changes on the motor. I'm running a 340 with an airgap, eddy heads and a comp cam 284/296 dur 507/510 lift. Would that be normal for this motor?

 

[ramcharger]

Whats the duration .050 lift?

 

[crackedback]

XE284H??? 240/246 at .050

What's your initial timing setting. That will make a huge difference in vacuum at idle. If it's not more than 16* it's not enough.

That cam should pull about 9-10" at idle would be my guess with timing correct. Idle around 900-1000

 

[jefflock]

XE284H??? 240/246 at .050

What's your initial timing setting. That will make a huge difference in vacuum at idle. If it's not more than 16* it's not enough.

That cam should pull about 9-10" at idle would be my guess with timing correct. Idle around 900-1000

XE284 is correct. I've been playing with intial timing trying different settings. Idle is 900 to1000 and currently initial is 20*. Still only about 5" vacumn. With this low of vacumn will it allow the 6.5 power valve to open also?

 

[ramcharger]

XE284 is correct. I've been playing with intial timing trying different settings. Idle is 900 to1000 and currently initial is 20*. Still only about 5" vacumn. With this low of vacumn will it allow the 6.5 power valve to open also?

It sure would and the problem will compound itself. I would plug it off and see if you can get it to idle. Is it pig rich and do the idle mixture screws do nothing? Compression ratio is also very important to your vacuum readings. This isn't a stock compression engine, is it? Comp ratio should be a measured 10:1 or better with that big of a cam.

 

[fishy68]

Sounds pretty close to me. I had a XE274 in my 360 and it only pulled 7" of vacuum in neutral.

 

[grimreaper]

Consider your elevation too. The higher you are above sea level the lower your vacuum.

 

[jefflock]

The compression ratio figures up to be 10.5 to 1. When you shut each side one at a time the motor trends to stall out. I believe that is normal and shows the power valve to be good. The plugs and the exhaust are showing it's to rich. I'll put a plug in place of the power valve tomorrow and see what difference it makes. Question? With the plug in place do you still adjust the mixture screws? I assume you do.

 

[jefflock]

We are at 1100'.

 

[fishy68]

Jeff if you plug the PV it'll be way lean on the top end. You can jet it up to make up for it but then it'll be rich at mid throttle. If you have 5" of vacuum put a 3.5" valve in it and it'll stay closed.

 

[moper]

If you measured and figured the compression at 10.5:1 then I'd say it's a little low. I don't like to idle that high either because you are probably partially off the idle circuit and that's why it keeps running. A lot depends on the quality of the hone finish and ring seating, and the quality of the Edelbrock valve job. If the heads were simply bolted on that may be a part of the problem. I agree that changing the primary power valve is important in any event. That vacuum level should be checked in geear too, if it's an automatic. Then choose the PV for that level.

 

[crackedback]

As Ramcharger suggested, put the plug in the PV only to get idle settings. Once you get the idle setting, select an appropriate PV and go from there. Use the plug as a tuning tool for idle, nothing more.

 

[ramcharger]

As Ramcharger suggested, put the plug in the PV only to get idle settings. Once you get the idle setting, select an appropriate PV and go from there. Use the plug as a tuning tool for idle, nothing more.

Exactly!

 

[SRT_DSTRHOLC]

really fish? I got 9-10 of vac in gear...and it got better with a new carb

 

[375inStroke]

With this low of vacumn will it allow the 6.5 power valve to open also?

I want an answer from someone that really knows, not what you've heard. Is the vacuum source to the power valve disconnected at idle, like the ported vacuum to the distributor?

 

[demon seed]

I want an answer from someone that really knows, not what you've heard. Is the vacuum source to the power valve disconnected at idle, like the ported vacuum to the distributor?

Excuse me but you want what? Maybe you'd like to start your own thread.

Terry

 

[moper]

The vacuum source for the power valve is the intake manifold. It's a little port that opens to the plenum on the base plate. So no, it gets full vacuum all the time. Now you have your answer. Anything else you'd care to ask or add?

 

[green1]

??????????????

 

[jefflock]

I swapped the power valve out for the plug tonight and readjusted carb. At idle (900rpm) the vacumn is still about 5". When I speed up the engine to 2000rpm it comes up to 10" and at 3000rpm it's 15" It pulls about 22" of vacumn maximum. I'm still wondering if that all I should get at idle.

 

[crackedback]

Cam retarded? On what Intake CL was it installed.

How are the rockers adjusted? They may be tight and causing problem.

 

[jefflock]

Cam was installed on a 106 centerline and the comp rollers were adjusted at 1/2 turn after slight drag was felt on the pushrod. (Hydraulic lifters)

 

[375inStroke]

The vacuum source for the power valve is the intake manifold. It's a little port that opens to the plenum on the base plate. So no, it gets full vacuum all the time. Now you have your answer. Anything else you'd care to ask or add?

Hot Rod magazine said they thought power valves operated like everyone else thinks they do. They talked to Holley, and the Holley guy told them about the power valves not getting any vacuum at idle. This is the reasoning for my question. I've only used Carters, so I have no experience with Holleys.

 

[moper]

Well, the passage you used to have to plug is in the channels that are connected to plenum space. I know it sounds nuts to say this, but Holley, if that was the source, is wrong. Manifold vacuum holds them closed. They see the same vacuum as the power brakes, PCV, etc. This is why the power valves used to blow out the diaphram when the engine backfired... The kit to fix that goes into the baseplate.