Low manifold vacuum?

[hunt2elk]

Wondering if any of you are running a combo similar to mine and we could compare vacuum readings. I don't have enough vacuum to utilize the vac advance in my distributor. I am trying to determine if I have a leak, or the cam is too big. Here is what I have: 400 block bored .030, 440 Source 451 stroker kit. 10:1 compression, Racer Brown SSH-44 cam (242/242 duration @ 50, 108 lobe separation) Stealth heads, Harland Sharpe 1.6 intake-1.5 exhaust rockers, Edelbrock Performer RPM intake. I am getting 7" of vac at 900 rpm idle. The car starts fine with no kickback at 16* initial, but needs more timing once it is started. At 28* it seems to run good, but the problem is that 7" won't pull in the can to get there. Did a leak down test. No hissing in the carb or exhaust, only the dipstick tube. I did the test when the engine was cold. My 1st gauge is at 95 lbs. 2nd gauge on each cylinder reads in firing order: 84,92,82,86,90,90,82,81. Had the plugs read and was told that 4,3,7,2 are showing lean, and #1 has excessive fuel deposits.

 

[superfragl]

I just started a thread on c-body forum (I have a 68 Fury) regarding low idle vacuum on my 440. My combo is different than yours, but I am facing similar issue - only 10" of vacuum. Freshly built engine, zero miles after rebuild and about 1 hour of run time. 9.5 static compression, Lunati cam with following specs

• Advertised Duration (Int/Exh): 268/276
• Duration @ .050 (Int/Exh): 226/234
• Gross Valve Lift (Int/Exh): .494/.513
• LSA/ICL: 110/106

Regular iron heads, Edelbrock intake and carb.
Compression test showed all cylinders within 146 and 150 - engine cold.

Cam manufacturer said I should have 14" at idle, but since the motor is not broken in yet, the vacuum can improve over the time.

I did a smoke test for intake leaks and found one in the carb choke unit. Will be fixing that.
Some folks suggested changing carb step-up springs. What carb are you running?

 

[747mopar]

Have you guys tried disconnecting the brake booster? A bad diaphragm could cause low vacuum. Other than a vacuum leak tuning can really bump up the vacuum but I'm sure you've already been there by now.

 

[Banzaiii67]

You want to do a leak down test on a warm engine with all the spark plugs pulled, coil disconnected and throttle held at wide open.

 

[moper]

The OP - What distributor do you have?

 

[Xp29h]

Hunt.. just finished a 400 block with 440 source stroker kit (512) used the Hydraulic Roller cam....

http://store.440source.com/Comp-Cams-Custom-Hydraulic-Roller-Retrofit-Cam-571/productinfo/145-1714/

Edelbrock intake TM6, CNC stealth heads, Holley carb, manual brakes, no booster..pulling 14 inches of vacuum. My initial is higher than yours at 21 and all in at 38.. Runs everything i need. I would follow some of the above suggestions and look at the power booster for sure.

Good luck

 

[superfragl]

I checked the power booster with vacuum pump, also ran the engine with vacuum port plugged. I might try bumping initial to 20-25*...

You want to do a leak down test on a warm engine with all the spark plugs pulled, coil disconnected and throttle held at wide open.

I think that is a compression test...

 

[hunt2elk]

Have you guys tried disconnecting the brake booster? A bad diaphragm could cause low vacuum. Other than a vacuum leak tuning can really bump up the vacuum but I'm sure you've already been there by now.

No vacuum accessories on this car.

 

[hunt2elk]

The OP - What distributor do you have?

Original style bought and set up by FBO.

 

[superfragl]

Then I would say - don`t bother. Adjust your vacuum advance can to start kicking in at 5" and be happy. You will have a lot more vacuum at higher
RPM - where you actually need the vacuum advance. If you blip the throttle and watch the vacuum gauge - you should see 20" or more

 

[moper]

Original style bought and set up by FBO.

Ah. That's unfortunate...lol. I try to avoid using FBO stuff. But - seems to me if it's adjustable there should be a way to set it so it works for you. Isn't there an adjustment screw to play with on it? Did they give you a vacuum range the can will work with?

 

[747mopar]

Ah. That's unfortunate...lol. I try to avoid using FBO stuff. But - seems to me if it's adjustable there should be a way to set it so it works for you. Isn't there an adjustment screw to play with on it? Did they give you a vacuum range the can will work with?

Unfortunate?? I've had nothing but good experience dealing with Don, very knowledgeable and hooked me up with a nice QF.

 

[747mopar]

The initial does seam a bit low, mine was a complete dog when set there. What's worked well for me (and gained vacuum) was setting initial at 20 and having the vacuum advance pull in 10 the second the motor fires for 30 at idle. Car starts and drives wonderful with this setting. Don actually helped me out with getting it dialed in and it's running better than it ever has.

 

[cdr]

as 747 said get the initial up & shorten the total.

mine likes 24 initial, 10deg of mech adv = 34 total,, then my vac adv is on ported vac, that ads 5 deg under light throttle, so total under light cruise is 39 ish

 

[lewtot184]

Unfortunate?? I've had nothing but good experience dealing with Don, very knowledgeable and hooked me up with a nice QF.

i agree! i've had nothing but good service with FBO stuff. i put one of their rev-limiter boxes and coil on my '65 and it's the best ignition i've ever run on a street car, and i've been doing this stuff since 1968. i don't use the FBO distributors, i build my own.

 

[hunt2elk]

Then I would say - don`t bother. Adjust your vacuum advance can to start kicking in at 5" and be happy. You will have a lot more vacuum at higher
RPM - where you actually need the vacuum advance. If you blip the throttle and watch the vacuum gauge - you should see 20" or more

The can adjustment won't go that low and there is none made for a Mopar distibutor that will. Don says they like to have at least 10" of idle vac to get the cans to work.

 

[hunt2elk]

Ah. That's unfortunate...lol. I try to avoid using FBO stuff.

I don't agree with that. I have his same setup on my Charger and it works great. Different cam though, that one pulls 12" vac at idle.

 

[hunt2elk]

Just talked to Don. He spoke to Jim @ Racer Brown and they both think I should be seeing 12-14" vac with this setup. He is going to pull the vacuum out of the dist and send it back to me. I will get the car started, warmed up and will do another leak down test. Got my fingers crossed that the motor doesn't have to come out because of a problem with the rings not sealing.

 

[lewtot184]

if you get 10" at idle with that cam i'd call it a day. i just tested a couple factory type vacuum units, an 8" and a 9". using a 3/32" allen wrench, turning the adjuster screw in the nipple clockwise until it stops, i got 2"-3" pull in start on the 9" and 4"-5" pull in start on the 8". i've set up aggressive centrifugal advance with vacuum advances before and am running them now. i suspect the centrifugal in your distributor either has too many degrees, or too heavy springs or may be sluggish due to lack of lube. i'd try to set up the distributor with 8-9 distributor degrees and aggressive light springs. let the initial be around 18 degrees on the crank and the light springs will start advancing quickly at idle to let you get about 25 degrees on the crank at idle. as long as the springs are heavy enough to pull the weights back when the engine is off you'll be ok.

 

[Meep-Meep]

A couple of things I see right out of the gate causing me to raise a big red flag. First, why are you concerned about hooking up vacuum advance to manifold vacuum? Second - and this is HUGE - you only have about 90 lbs of cranking pressure!?! Stop everything and check the following:

1. The cam is way retarded (intake closing way too late).
   
2. The cam is just too much for 10:1 CR
3. You don't actually have 10:1 CR as you thought.

The above assumes you don't have any major assembly malfunctions, etc.. Get some answers and report back.

To expand on my manifold vac advance comment, forget about that widely publicized article that suggests you need to run your vac advance off the manifold vac source. BS! Either lock the distributor to full advance (high overlap cam race engines like this) or run a quick curve and ported vac advance with as much initial as your starter can stand. For the street ALWAYS run the vac advance off the ported port. Always.