my setup is listed in my sig. this is my problem

1. when i drive the car there is a really bad stumble when i step on the gas, it feels like when it goes from the idle circut to the main circut, if I push it down further it will clear out, it does this when the car is in 1st, 2nd, or 3rd. I can lock it in low, and hit the gas and it still stumbles bad. If i go ahead and put it to the floor it gets up on the convertor, after stumbling, and blows the tires off.

2. when the car is idleing(sp?)i can hit the gas 5 or 6 times, revving it up to about 2500 and the thing starts smoking out the tail pipes. its not oil smoke, the car doesnt smoke driving or any other time. It smells like its running really rich . It makes your eyes water. and it does that if it sits in my garage running, yes the big door is open but it seems like its running rich.

It is a stock 650 DP. I am getting about 10-11 inches of vacuum and thats it.

This carb has been on two 350's with the same heads and about identical cam, and a mean poncho 455 and there has never been any problems like this.

I can't believe that its jetted to rich, hell everyone recomends a 750DP for it. I have checked for vacuum leaks several times and have found none. I think the power vavle thats in it now is a 956 or something, its been a while since i looked at it. Is this the right one? I saw on holleys site to pick the right one to drive the car around with a vacuum gauge hooked up and take the lowest vacuum reading and divide it in half, and thats the PV you need.

I havent gotten to tune on the car any, cams, squiter or anything, and i know every engine is different. but this problem i am lost on. Thanks.

 

It is meaningless to compare jet sizes between a 650 and a 750. Too many other variables affect that, like venturi size & design, etc.

Sounds to me like you've got too much pump shot which makes the 5 or 6 pumps of the throttle flood the motor; not enough advance, which causes the low idle vacuum; and once you get the vaccum up to where it should be, around 16" I would guess it should be, you'll need to ditch the stock 6.5 power valve in favor of a 8.5, so that it opens with light throttle, and doesn't require you to mat it to get the vacuum low enough for some enrichment to occur. You may need to up the jet size about 2 #s from where it is too.

A given carb will always run leaner on a smaller motor than a bigger motor. Sounds counter-inuitive, but that's what happens. The smaller motor is causing less air flow through the venturi, therefore less "signal" to the carb, under any given conditions; therefore the carb, if properly set up for the bigger motor, will always "think" the motor is at a lower RPM and doing less work than it actually is, because of the smaller air flow.

 

RB, thanks for the reply.

about the pump shot, do you think i should change cams, or put a smaller squirter in it. you also mentioned advance. intial is at 12 degrees and its on timed port, i thought about putting it on full port and see if it makes any difference. and with the low vacumm, i dont know how or if at all the vacuum advance is coming in. any thoughts on that?

also, i am confused on the power valve thing. with the low vacumm i have, why do I need a 8.5 to get me enrichment sooner if the car is running rich now? or is the stumble, and when I hit the throttle a few times and its rich two different problems? I am not saying you are wrong, i am just saying i dont understand.

thanks again.

 

On the pump shot, I'd try the cam first.

Your vacuum is lower than it seems that it should be. With similar cams I've gotten around 16", so I'd expect that's what you should see too. Something is causing it to be low, and most often, it's retarded timing. Try advancing the timing and see if it ever spark knocks. If not, keep advacning it til it does, then back it off a little. Don't rely on some mark, they're hardly ever right, it's a tuning thing, get it to where the motor runs the best and don't worry about the mark. I think that once you get that resolved, you'll need th ehigher vacuum power valve. Getting enough advance may even cure the stumble all by itself.

 

i bumped the timing up to 14 degrees and changed the vacuum advance to full port. when driving it now the stumble is gone. however i am still only getting 10-11inches of vacuum out of it and when i try to adjust the air/fuel mixture screws it doesnt really have any effect. i cant get anymore vacuum, and when i turn them in, like its supposed to, it dies.

Could the power valve be blown? would this cause it? It seems the more i run the car, the worse it idles, and it wont hardly run when its cold. and when i mean cold i mean like 80 degrees outside.

I thought I had taken care of that problem before when I found out the the timing was off because of to light of advance springs and the throttle blades were open to far to compinsate. it ran alot better after i fixed that problem.

For what its worth, car craft ran this cam in an engine and said that its borderline on being able to use power accessories.

Also now, after i moved the timing up, reset the idle screws and stuff, if i idle it at 750rpm like it used to be, when i put it in gear the rpms surge. if i put the idle at 1000 it doesnt do it.

its driving me nuts.

thanks again

 

chevk for vacuum leaks. 10-11 inches of vacuum is what I Pull, and my cam is 303/313 Duration! if your car is not a radical 400-450 horsepower monster, i would think there is somthing wrong with that vacuum signal! make sure your guage is correct as well. the first step is tracking down that vacuum problem. spray carb cleaner around the intake and baseplate of the carb and see if it changes the idle tones / level. also see if there are any bubbles around where you spray. also check your vacuum lines. if you have AC make sure your av lines are not cracked up, where they run to the switch.

 

i have sprayed carb cleaner around the intake and the the carb and it makes no difference. it bubbles some in the middle of the intake, between were the two bolts are, but i figure its sizzlin cause of the heat. i think the engine is in the 350-400hp range, after it gets tuned out. all the vacuum lines are new, hell everything is new.

 

ok if you see any bubbles at all, that is leaking compression! remember your engine is SUCKing not BLOWing and BUBBLES are a sign of BLOWING which means its air escaping the combustion chamber under the compression stroke and coming out of the intake port to haunt you! one of those can easilly drop vacuum readings, 2 of them can devastate it. I recommand swapping the intake manifold gaskets and useing the proper BOLT PATTERN to establish a good seal. Use red PERMATEX to make sure it doesnt happen again. Also, what are the specs of your camshaft?

 

Is this a crank post, or are you serious?

 

which post five7.

i dont agree with the escaping compression, if its escaping, it would be backfiring out of the carb. if there is 160lbs+ psi escaping from the cylinder, its going to go all the way out of the carb, and not stop at the intake gasket. just my opinion though.

 

i am going to replace the power valve and see if that makes a differnce. the thing is running stupid rich. i need to pull the plugs and look at them.

I will probably do this later this week, my car is taped up right now, i am primering it.

 

A compression leak causes rough idle, low vacuum signal, and poor driveability. the bigger the leak, the worse it gets. Bubbles around the intake manifold are a sign of compression leak.

A compression leak generates these symptoms for the same reason a radical camshaft does, except the reversion in the cylinder goes up through the carb and makes a radical idle, where YOUR problem is its existing through the intake instead of the carb. this is still a form of "reversion" but since the intake port is closer to the combustion chamber than the carbeurator you get MORE reversion. This is a crude way of looking at it, but it explains what your seeing. Compression leaks will only cause backfires through the carb if the mixture in the cylinder is ignited via spark before the intake valve fully closes, or sparks way too late in the cycle. such an advanced profile would lead to detonation at higher cylinder pressures as well. Remember your car does not make super-high pressure at an idle. Volumetric efficieny plays a huge role in cylinder pressure, and THATS compounded by cam timing. I asked what is your camshafts profile, it will explain alot about vacuum signals.

Im not saying im RIGHT but i AM saying its somthing to look at.

Now for power valves:
a power valve supplies fuel to the main circuit when the spring overcomes the vacuum signal supplied from beneath the carb. a BLOWN powervalve is simply a power valve with a diaphram that can no longer hold the spring closed because vacuum is leaking past it and down below the carb. when a power valve is open the extra fuel is pulled from the bowl into the main curcuit right before it is pulled into the idle curcuit, thus a blown power valve will enrich the idle curcuit and main curcuit. it will NOT however reduce vacuum signal, but it MAY cause a rough idle if the engine begins to miss due to fouled plugs. in fact, when a power valve blows, it is usually followed by a higher idle because of the extra gas, and this will only further increase the vacuum signal.

Your symptoms do not point to a blown power valve, yet this is only my opinion, and if changing it fixes your problem feel free to kick me and make fun of me.Im only trying to help, if you dont beleive this then im sorry.

 

king, i appreciate your reply very much. i have never heard of the compression reversion thing. I always like learning about new things. I had just never heard of this one.

What would cause the leak? do you think i need to readjust the valves?

I backed my car in the garage a few days ago, revved it up 4 times, and my eyes were watering, its running really rich at idle.
and since i moved the intial timing to 14 degrees and put the advance on full port, the bog is gone when driving, howver i am not quite sure why this fixed it.

my cam specs are as follows.
comp cams 268xtreme cam
224/230@.050 268/280 adv .477/.480 lift 110LSA

thanks again for your replys.

 

anyone else heard of the compression thing? or have any other ideas?

to da top

 

ok a few things cleared up. your cam, 224/230@.050 268/280 adv .477/.480 lift 110LSA, isnt exactly a normal every day cam. Now your vacuum reading makes sense... As does your timing problem. Any "radical" cam is going to want ALOT of intial advance. If you have your cam anything below 15-18* at idle your throttle response is going to suck right off idle. Being at 14 is good, but im going to recommend about 16 or more. Dont go over 18~21 initial though without some kind of starting-retard or you will encounter lots of starter load while cranking.

As for surges / idle problems, your engine is going to want to idle at about 1050 RPMS in neutral, and no less than 800 RPMS in gear, thats pushing it... a radical cam doesnt like to idle real low, and the tighter the converter, the worse it gets on the street.

Power accessories... yes 10" is about minimum vacuum for power brakes to even work decent. I pull about 10" and I have brake problems when driving around somtimes.

did you ever change your power valve? Im going to say you may want about a 5.5" Power valve in it, due to low idle vaccum, and a 650 Double pumper is perfect for it, I would not rev past 6200 RPMS however with that cam, and no where near 6000 if you still have stock valve springs.

Do you have four corner idle? Do you have electronically controlled timing? Do you have a computer in your car? or is it all carbed / no computer... Whats your stall? What kind of distributor? Whats your advance curve initial / total / vacuum advance etc... What size jets are in it now? and how many turns out are your idle screws?

I can recommend more to you if i know these things. I hope I can point you in the right direction...

For starters, remember with a non-CC holley you have COMPLETE control over the idle mixture. There is no excuse why it should be running rich at idle. Simply turn the idle adjustment screws IN(clockwise) until it stops running rich. Make sure they are all equally turned however, dont just turn one.

 

the surge problem didnt start untill i raised the timing and put it to full port vacuum. before that, there was no problems and i could idle it to 900 rpm idle and 700 or so in gear. i havent switched it back to see if it makes any difference. i havent started the car in a week because its taped up getting primed and sealed.

I have not changed the power valve

the convertor flash stalls at 2500, first gear feels great, pulls good, burns tires, but second leaves me wanting more. i know the deep first gear of the 700 helps, i have 3.23's out back.

I have the matched springs for the cam so winding it up i am not worried about, cam is supposed to peak at 5800

I do not have four corner idle, but my buddy has it on his holley 750DP, would it help to have it? are they interchangable?

Stock HEI ignition with lighter advance springs. i havent messed with my timing curve because I dont know how to. initial is at 14, and its pretty snappy, especially with the convertor.

All carbed no computer

Stock jets. i think they are 65's, i think

I think they are turned 1.5-2 turns out. and yes it runs rich at idle, i cant get anymore than 10 maybe 10.5 inches out of it. There are guys on here running the 274xtreme cam that pull that much and its 230/236@.050

I talked to mark shields and he says he is having problems with tuning his setup with this cam too

I car craft and hot rod and other mags, this cam, with a 750DP, air gap, and vortec heads puts out 400hp, my heads have been ported, polished and 2.02/1.60 vavles. so i hope to have 350hp, maybe maybe 365hp after i get it tuned out right.

Thanks again for your input.

 

First things first, Like i said, theres no reason to run rich at idle.
<b>I think they are turned 1.5-2 turns out.</b> Simply turn the screws in until it stops running rich. My 650 DP had the idle screws out about 1/4 of a turn to get it to not run rich at idle. 1/4 of a turn is NOTHING. It all depends on the carb and engine setup. Some DP Holleys have HUGE idle feed "holes" that lets tons of fuel through with just a small openening.

<b>the surge problem didnt start untill i raised the timing and put it to full port vacuum</b> Your engine will want fuel when you give it more timing. Advancing your curve will give you more power, but leaving the jets the same when you give it more advance can cause lean spots real quick. That cam will want FUEL under load and lots of timing.

<b>the convertor flash stalls at 2500, first gear feels great, pulls good, burns tires, but second leaves me wanting more. i know the deep first gear of the 700 helps, i have 3.23's out back.
</b> What is your total stall speed? Remember once again, your cam wants rpms and gearing. Your stall needs to match your gears too. Technically, with that cam, you want decent stall and pretty high gear. You should be able to get away with 3.23's and if stall is anywhere above 2800 then you should be ok. But remember, the more gear you give it, the more the cam will thank you, Up to a point. I would not go past 3.73's~

<b>four corner idle </b> is "just to cycle some secondary fuel" If you drive your car like a grandma and the secondaries never open your fuel would go stale... four corner idle just flows some of that fuel out into the engine to keep it circulating... you could close off the rear idle screws and just run front ones and still be ok. But you dont have it, so dont worry about it.

Things to play with:
Timing, idle screws, front / rear jets, timing, power valve(s), timing. With a cam like that, timing becomes paramount. It can make your car run great, or horrable.

<b>Stock HEI ignition with lighter advance springs. i havent messed with my timing curve because I dont know how to. initial is at 14, and its pretty snappy, especially with the convertor. </b>
If it was ME I would invest into an MSD setup, but thats just me, and it can get expensive ($150 for msd box, $35 for coil, $240~ for distributor) but I thought it was worth it, and it was...
But for YOU remember that word <b> Stock </b> your distributor is not giving you the kind of total advance you need for that cam, unless its been messed with already. 14 initial is ok. by 2400 Rpms technically you would want about 36* of timing in. but that also depends on your stall...

 

if my car is/was running rich at idle, with the added timing, why is it surging now? Everyone recomended a 2600 stall for the cam.

I will change the power valve and mess with the idle screws when i get the car back together.

 

<b>if my car is/was running rich at idle, with the added timing</b>

Running rich at idle does NOT mean running rich at part/full throttle. Your idle screws ONLY control idle mixture. Just because your plugs are black and your car is surging, doesnt mean your not running lean at WOT. Sounds like you need to lean out your idle mixture, advance your timing, and put bigger jets in your carb. Also, your power valve may be open at idle even though its not blown, simply because your cam pulls so little vacuum. You may need like a 5" or 4" power valve. For my engine with a blower I needed a 3.5" in the front because at 5" it would hang open just after idle... which is NOT conducive to performance OR gas mileage.

 

I appreciate your reply. Will the car act different when i get the "correct" power valve in it? Will it idle different? Thanks

 

well if your power valve is hanging open now, and you put a power valve in that DOESNT hang open, yes your idle will lean out a <i>little </i>. under part throttle you want the thing closed, and under half - full throttle you want the power valve open. its as easy as taking a ride with a vacuum tester and seeing where your vacuum goes when your cruising / accelerting slowly vs speeding up fast etc...

 

Tip: Use a power valve that is about 2 inches of mercury below the LOWEST manifold vacuum reading you get on cruise and idle (in gear for automatics). If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis. If the power valve opens a lot while you're driving down the road, your gas mileage will suffer.


I was reading a website about tuning carbs and I ran across this statement. Fuel drips from the boosters when you rev it up. I noticed it before, but didnt think to mention it. I got the car unwrapped today, so hopefully I will have time to mess with it tomorrow.

 

<b>If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis.</b>

thats a typical sign of a stock size power valve in a real cammed over engine. lumpy idles make for low vacuum, and low vacuum makes power valves open if they arnt the right size. Almost sounds like we pinpointed your problem... almost...

 

what do you mean almost?
oh, and i checked the plugs today and they are black
i let the car idle for about 30 minutes today, its fouled out atleast 3 plugs. so yeah, its rich at idle

 

good now get a smaller power valve (5-6") to start with and lean out your idle using the idle adjustment screws in the front of the carb. that should get your started.

I mean almost because while this is ONE of your problems, you still have the hesitation and radiation and celebration and... oops sorry just watched the robic williams movie again.