New cam, low vacuum, backfires - Team Camaro Tech
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post #1 of 28 (permalink) Old Dec 30th, 09, 05:21 PM Thread Starter
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sam g.
 
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New cam, low vacuum, backfires

just put a comp cams 280h on my 355 with sportsman 2 heads.the problem i have is theres low vacuum 5-6.it idles rough idles at 900 but sounds like its idleling fast higher rpm.it backfires.has a stumble, bog below 2k.also put a proform main body to my holley 650dp. it feels like it did before the cam.what gives?wont go any higher then 5,500 rpm
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post #2 of 28 (permalink) Old Dec 30th, 09, 05:38 PM
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Re: new cam,low vacuum,backfires

Make sure there are no intake leaks.
Full vacuum to the cannister, if needed.
More initial timing.
More air, as in open the secondaries alittle; use the setscrew on the pass side to open secondaries. Use a feeler gauge at first to help.

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post #3 of 28 (permalink) Old Dec 30th, 09, 06:11 PM Thread Starter
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Re: new cam,low vacuum,backfires

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Originally Posted by Everett#2390 View Post
Make sure there are no intake leaks.
Full vacuum to the cannister, if needed.
More initial timing.
More air, as in open the secondaries alittle; use the setscrew on the pass side to open secondaries. Use a feeler gauge at first to help.
how do i check if theres a leak at intake?i have msd pro billet dizz. no vac.canister.it seems like it wants me to open the primarys more for better idle,but i cant without gettn higher rpm
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post #4 of 28 (permalink) Old Dec 30th, 09, 06:23 PM Thread Starter
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Re: new cam,low vacuum,backfires

its aslo hard to start, when it starts it wont hold idle i have to rev it above 1,500 for like 1-2min then it will stay at 900.before the cam it would start and idle with no problem no need to rev motor or nothing.
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post #5 of 28 (permalink) Old Dec 30th, 09, 06:37 PM
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Re: new cam,low vacuum,backfires

need to change power valve to match new vacuum signal?
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post #6 of 28 (permalink) Old Dec 30th, 09, 06:58 PM
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Re: new cam,low vacuum,backfires

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.....how do i check if theres a leak at intake?
Spray WD-40 where the intake and cylinder heads meet while the engine is running. If there is a vacuum leak there, the WD-40 will be sucked into the combustion chamber causing the rpms to change.

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post #7 of 28 (permalink) Old Dec 30th, 09, 07:52 PM
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Re: new cam,low vacuum,backfires

Like I suggested, open the secondaries to allow more air in. This will allow the primaries to go more closed and bring in the idle circuit.

Once you get as good steady vacuum reading, as Derek suggests, divide the vacuum reading by 2 and install the power valve with this reading. If a higher numeral valve is installed, the low vacuum will open the valve and too much fuel will be dumped, and this equals low vacuum because the engine is flooded.

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post #8 of 28 (permalink) Old Dec 30th, 09, 08:20 PM Thread Starter
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Re: new cam,low vacuum,backfires

how do i check power valve?HOW CAN I FIND OUT IF ITS FLOODING THE MOTOR?
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post #9 of 28 (permalink) Old Dec 30th, 09, 09:41 PM
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Re: new cam,low vacuum,backfires

[QUOTE=Everett#2390;1351224]

If a higher numeral valve is installed, the low vacuum will open the valve and too much fuel will be dumped,

Ok..for enquiring minds,the open at idle vacuum power valve dumps fuel into what ckt?Now that you have answered that..how does head pressure in the main well affect the idle feed restrictor orifice?

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post #10 of 28 (permalink) Old Dec 30th, 09, 09:54 PM
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Re: new cam,low vacuum,backfires

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Originally Posted by sic7camaro View Post
how do i check power valve?HOW CAN I FIND OUT IF ITS FLOODING THE MOTOR?
Typically you would remove the power valve and vacuum test it with a hand held vac pump.
On car..if you bottom out all the curb idle mixture screws and she still runs..it indicates fuel leakage internally.Possible source =ruptured pv.leaking gskts/warped mating surfaces/porosity and the list goes on.
My power valves are externally referanced..makes this test a tad easier than the internal style.

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post #11 of 28 (permalink) Old Dec 31st, 09, 10:54 AM Thread Starter
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Re: new cam,low vacuum,backfires

should vacuum be so low with the cam?.whats normal for cammed sbc?will check pv.and all carb gasket.i sprayed carb cleaner and it seems to be a vacuum leak thru into the valley not into intake.could this effect pcv valve vacuum?and affect idle
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post #12 of 28 (permalink) Old Dec 31st, 09, 11:21 AM
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Re: new cam,low vacuum,backfires

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Originally Posted by sic7camaro View Post
.......I sprayed carb cleaner and it seems to be a vacuum leak thru into the valley not into intake.
Retighten all intake manifold bolts,then perform WD-40 test again on the intake manifold/head mating surface to see if the vacuum leak has been eliminated.

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post #13 of 28 (permalink) Old Dec 31st, 09, 11:22 AM
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Re: new cam,low vacuum,backfires

I have a 280h in the '69 and it pulls around 11-12. I changed the power valve to a 6.5 and it helped the idle a bunch. Also changed PCV to what ever # is called for in a big block.

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post #14 of 28 (permalink) Old Dec 31st, 09, 12:19 PM Thread Starter
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Re: new cam,low vacuum,backfires

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Originally Posted by rojo View Post
I have a 280h in the '69 and it pulls around 11-12. I changed the power valve to a 6.5 and it helped the idle a bunch. Also changed PCV to what ever # is called for in a big block.
any sound clips?just got a 6.5 high flow pv.or should i just put the one i got in the holley kit?
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post #15 of 28 (permalink) Old Dec 31st, 09, 03:53 PM Thread Starter
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Re: new cam,low vacuum,backfires

well i just put the heavy silver springs and it went from 5-6 to 9-10.so im thinking its a timing issue.will keep u guys posted.oh and i took it for a ride now it wants to go past 5k..if i have 67/and 73 jets will this be ok for my set up?
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