Hello,
I am a newbie to the wonderfull world of Hotrods. I have been reading many posts hear and i have been asking Chevelle heads questions too. Here is my problem. My car (1972 chevelle SS 350SB with edelbrock 4 barrel)seems to have lost 20-30 Power all around. The timing should be 8 degrees BTDC (accourding to haynes shop manual) but the car runns horribly there. It runs best about 20 degrees BTDC completely off the engines degree marker. I have replaced the plugs , wires, and rotor. The vacuum that comes from the distributor to the carb when plugged un plugged does not seem to change anything when checking the timing. Is this vaccuum pump bad? The car also rumbles in the exhaust at speeds of 55+ when i let off the gas. thanks you for any help!

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1972 chevelle SS 350

Hi Lavdog

#1 Perhaps your timing marks have moved especially if the dampner is a oldie but goodie. 10-14 initial advance should be fine at idle (vacume line disconected). Just hook up a long vacume line and see if your vac can will hold a vacume. Idle/air screws might need a bit of adjustment if your concerned about the backing off rumble BUT you just might have a small air leak also "UNLIT" propane gas around the carb area will tell. Good luck

Jim

thanks for the info jim. i checked the vaccuum line from the distributor to the carb and it did have a large crack. I thought i found my problem, but after replacing the hose it made no difference at all. This vacuum does not seem to be doing anything. Again the engine seems to have lost 25% of its power. There is a loud wistleling sound coming from the carborator but i think that is normal. thanks all. jl

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1972 chevelle SS 350

I just posted this reply in another post, with the car not running right at the spot where the book says it should, you should check the dwell on the ditributor. I was having somewhat the same problem... read the posts in this category about rough idle and timing being off. Just a suggestion, and it won't hurt to double check the dwell. Just remember that if you change the dwell you have to retime the car. On my 72 Elky 350SB, the dwell is supposed to be about 32 if i remember correctly and the timing is 8 degrees BTDC, the book you have should tell you what the dwell should be.

[This message has been edited by Midnight72Elky (edited 07-09-2002).]

If you are gettng no chaqnge in the timing when adding/removing the vacuum line to the distributor then you either have no vacuum going to the distributor or the vacuum advance unit is dead. If there is no vacuum at idle, then check it when the throttle is opened a little; it will then get a vacuum signal if you are hooked to "ported" vacuum at the carb. If the vacuum advance unit is dead, then you can get a stock replacement ($10) or a Crane adjustable unit ($25) and have fun dialing it in.

I will be checking the dwell tonight. I just realized i replaced the plugs with ac pre gapped at .35 I have a later model HEI distubutor on it, will changing the gap to .45 fix my power loss? you guys have been very helpfull! thanks

How recently was that Edelbrock carb installed? What, if anything, was changed at the same time as when you lost the 20-30 horsepower? I'm just wondering if your problem could be a vacuum leak at the carb or manifold. That would account for almost all the symptoms you mentioned.


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Bob Von Kaenel
70 SS 54k orig miles

the carb was put on 1 month before i bought the car, so about 4 months ago. last night i tightened all the bolts on the manifold and carb about 1/4 a turn. Do you think it could be the spark gap?

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1972 chevelle SS 350

Hi lavdog

ok you found a cracked vac hose to the dist can but did you check the can to see if it worked and held a vacume ?
Also on edelbrock carbs 600 & carter 625 the ported vacume is the passenger side above the idle screws. I set my plugs at .42 and they work well. Have you also checked the amount of advance at 2500-3500 your dist gives you ? Sometimes the advance weights need worked on. Good luck

Jim

Okay, i changed my gap to .45 and the car is better, i still think something could be screwed up with the vacuum advance. the hose is conected to the port on the passanger side. it has a vaccuum when i give the carb gas, not much when idleing. When i took a part thr distributor the vaccuum advance was very stiff. I used a screw driver to push on the vacuums moving part, it was very tight and i had to use alot of pressure to move it, could this vacuum be bad? Thanks!

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1972 chevelle SS 350

Hi Again

Well it's going to be a dab hard to push by hand thats why getting some vacume on will work better. See if you can find another vacume outlet and test it that way, either with a timing light or just the sound of the engine speed up/down.

Jim

Jim you where right on, the owner before me had the vacuum advance connected to the left port on my edelbrock carb, this port is reserved for cars with emission controls(good looking out chevelle chat people), i connectred it to the right port(which has a constant vacuum) and the car flies down the highway now when i push the gas down. Thanks everybody for your replies and help! I hope i can repay future posters with some usefull knowledge, Knowledge is H POWER keep sharing my friends! :)

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1972 chevelle SS 350

Hmmmm

Lavdog you are running full vacume now which aint all that bad (forgive me I-man, a weak moment)but really did not fix your problem. When you give it full throttle all vacume is gone including your advance. You should have mechanical advance starting about 1000 rpm + or - 100 rpm and at 3000 rpm or so 34-36 degrees of advance should show up "without the vac can hooked up. The vacume advance can hooked up to the ported side will varie the advance with light throttle cruising down the blvd. Advance cans will go anywhere from 8 to 24 degrees depending upon what you buy. Go ahead and mark your timing at 34-36 degrees and test w/out can hooked up then let it idle and see what your initial timing is, my bet is your around 24-26@ 3000 and idle is 10 deg. BUT if your happy with it leave things alone. Good luck

Jim