Thread: wont idle right with the lumpy cam

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i just finished putting my motor together. its got alot of compression and rebuilt heads.i have a lumpy cam and it doesnt idle right. it idles ok for a bit the it like looses it rythem or something like a vacum leak or something. i was told it was because i was running a quad and they don't run right with lumpy cam's
if anyone has any ideas let me know i checked plugs and wires...

What kind of engine, how much cam??? Have you checked for vacuum leaks???

350 chev
heres the cam spec
http://dab7.cranecams.com/SpecCard/D....asp?PN=110551
yes i've checked

I know for a fact that the Quad, does not like the lumpy lump camshafts, it was designed to save gas on a smooth idleing engine, so the primarys won't support the engine with enough gas.. because of all that nice extra lift, your motor now needs a larger gas supply to keep up with the demand.....

you could try reaming out the jets or replaceing them with bigger ones first, but you may need a 750 holley or something in the future

Pull the plugs. Determine color of electrodes and report back.

Abe

make sure that your firing order is correct and that you have the correct fuel pressure, try advancing the timing (if not advanced by vacume)

Jayman;

First of all, do not enlarge the jets.

The Quadra-bog carbs were designed to richen the mixture as the vacuum signal decreases. Similarly to that of the Holley carbs which use a power valve. This is done for the sake of fuel economy.

When you install a larger duration cam than stock, the vacuum signal goes south, tricking the carb into thinking it is under load, as in acceleration mode. This in turn opens the power valve or boost valve and richens the fuel mixture by 6-8 sizes. Now when this occurs during idle, just think of the fuel ratio in the engine.

Well it gets worse. The first thing the hot rodder does is turn the idle screw clock wise to try and compensate for the motor wanting to stall. As you do this, the butterflies open a slight bit more, which in turn signals to the carb to dump yet more fuel into the venturies. To make a long story short, you have a horrible running motor which is inefficient and doesn’t make near the power that it should.

The reason your motor idles alright for a bit is because you have cleared the chambers by revving it however as you come to an idle, the motor richens up and slowly floods itself. The other problem you have is that your vacuum advance becomes non-existent at idle with a big cam and therefore the engine idles at 10 degrees rather than 30 plus.

Is this all makes sense so far, please acknowledge and I will tell you step by step what needs to be done.

that sounds pretty right. i was thinking about trying my dad 700cfm carter to see if it ran better but i only have the stock intake for now until i find a good on an aluminum one..

To me a 284 duration cam is way too much. Power is from 2800 to 6500 rpm. Takes a good carb, intake, headers, ignition, and about 2500 stall converter to make it start working. By the way, is this a Pontiac motor or what???

Dave, read this. Maybe it will answer your question.

Originally posted by jayman54
350 chev

Ok Jayman;
For starters, you will have to consider doing two things.
1) you need to lean out the idle mixture. This can be done by either allowing fresh air into the manifold through an existing hole such as a pcv line or by drilling two holes (about .125 each) into the front two butterflies. For ease, you could remove the pcv and see if that is too much. If it is, try restricting the air flow a little at a time until it runs better.
2) you need to get rid of the vacuum advance, and recurve your distributor so that you only have at the most, 16 degrees of advance in it. This will mean that when you set your timing at idle and if you want a total of 38 degrees maximum on a 350 chevy, you will set it at 22 degrees. In this way you will create better vacuum at idle and better overall performance.

Keep us posted.

would changing to the carter carb work any better? its made for high performance and what about the intake should i just get a good dual plane aluminum intake....i'm runnning a HEI if that changes anything...

maybe i should just find someone who knows what the hell they're doing..Because i sure don't! i can barely drive the car let alone fix it. Someone help me!!

never mind the last post... that would be my brother being his normal loser self..he seems to think he's funny its pretty sad

i tried opening up the pcv valve it just tried to die on me. the idle speeds up when i open the one valve on the intake..where should i set the timming when i'm trying to set it up??