I have a 1967 camaro 427 .holley 780. points dist. Has a mother thumper cam. At idle it only makes 5 inches of vacuum,just off idle the vacuum jumps up. Is there a way to fix this so the power valve will know where it is at. Also with lower octane fuel this problem only gets worse. Keeping in mind the motor is killer off idle,starts great...all the good stuff. It is the idle that is just making me what to beat the car with a hammer.(anyone been there before?) Any thoughts. And if i put av-gas in it gets a whole lot better? WES:sad:

Holley says to find your power valve, take your vacuum and divide by half and that's the valve to run. Do they even make a 2 or 2.5 power valve? That engine should make more vacuum than that. What is the CR? 10:1 That cam should make at least 8hg of vacuum. At least it did in the Hot Rod magazine and that was in a small block. Do you think maybe the valves are too tight? Vacuum leak somewhere? What about timing and idle speed?

Charlie... I have rechecked the valves,and readjusted with no change. Timing is 34 total most power on dyno. Intial is 10. No vacuum adv.And off idle vacuum goes to 15ish.Dumb question as well..how do you stop mech. advance from spinning up to much.
Wes

What do you have your idle RPM set at?

We for sure have absolutely no info to work with.
Anyone who can say what the idle vacuum should be with a "mutha-thumper cam" at unknown rpm has a better crystal ball than I.

Standard stuff:
idle rpm, increase it.
10 degree initial is simply not enough.
Power valve has ZERO impact on anything happening at idle.

That cam is going to need more initial. Around 17*-18*. The way to limit mechanical varies for each dizzy. I run a Pertronix and they have these "L" shaped pieces that go under the advance springs on the inner post that limit how far the weight swings out. I believe MSD has bushings that they use. Each manufacturer is different. Set your initial to around 17*-18* and limit the total to your 34*. Then set your idle to the lowest RPM that will give you a stable idle. Is the car an auto or manual tranny? If auto, the idle is going to be the lowest stable in gear. Turn your steering wheel to full lock to make sure it doesn't stall if you're in a parking lot making a tight turn. The last thing you want is to be sporting the hot car and look like an idiot when the engine stalls making a turn(as me how I know).

Jim, what if the power valve is too high for the vacuum he pulls at idle. Won't that make the nozzles drip fuel at idle?

Lets start at begining...
427 30 over 9.8.1
mutha thpr cam
idle950
5 speed tremac
i was under the impression that if power valve was sized wrong it would
flutter?
(Some people are new to this and we all want to learn... but if your made to feel stupid it does not help)
if i bump up idle speed it runs on diesels.
and if i give more intial than i have to much totals and then it pings...hence my other question about locking it out.
I dont mean to sound mean but we all have to start somewhere in the learning curve ,and should be treated that way...make us keep coming back?
Thanks in advance WES:D

Wes, no one here is trying to make you feel stupid. We all started where you are and I know how it feels to not know something and are afraid to ask. What distributor do you have? I ask because not all manufacturers limit their mechanical advance in the same manner. For example, I run a Petronix distributor and they use "L" shaped pieces that sit under the advance spring and physically limit how far the weight can swing out. MSD distributors use bushings that go inside the weight where it slides over the stud to limit advance. The engine definately needs more initial timing. You can advance the initial to 17*-18* and then limit the mechanical to 7*-6*. That way you can run more initial to get it to idle better, but still limit the total so you don't get the pinging you heard. So lets start there. What type of distributor do you run?

It has a stock points dist.And yes i am going to put a conversion in it.But i do not want to change to muchat once.As i said off idle the car is AWSOME!! I reaslly hate the run on and the lame idle.
WES:D

It has a stock points dist.And yes i am going to put a conversion in it.But i do not want to change to muchat once.As i said off idle the car is AWSOME!! I reaslly hate the run on and the lame idle.
WES:DThe info stated above is basically stating that you should set your initial timing at 16*-18* total 34*-36* would give you more vacuum maybe only a couple of *. Don't forget to check your total if adjusting your initial and carb mixture/idel... My mild 427 with Comp Cam 280s 604 lift solid flat tappet, initial is 16* total 34* idle 750rpms with 10" vacuum.

Hi Guys ... How do i stop the amount of total advance??
WES

Hi Guys ... How do i stop the amount of total advance??
WESWhich dizzy do you have and don't forget to readjust the carb (check the amount of turns on the mixture screw take note should be around 1 1/2 turns out on each screw, then adjust ideal back down and fine tune the screws in or out 1/8-1/4 turn.

Hi Guys ... How do i stop the amount of total advance??
WES

That depends on the distributor, you said points, is that a factory distributor from way back when?

You can try a commercial recurve kit. At least the ones for HEI's come with different shaped wieghts and a diffent cam that limits the advance amount. Not sure what's available for a point distributor.

The "old school" way of limiting the advance was to weld then file the slots where the pins ride.

you might think about an aftermarket dizzy. some good electronics will make a huge difference starting and idling. MSD 8360 is a goodie, and comes with a selection of bushings to limit the advance.

Honestly, I am not sure if a stock distributor can limit its mechanical advance. I think you are going to have to invest in an aftermarket unit. It will be well worth the investment. The electronics alone will be more reliable and less maintanance oriented. .

Sorry in think you have a stock points dizzy. so it looks like you will have to change the springs and weights for total if it is more then 34*-36* at 3,500rpms.

Like Jim said the MSD 8360 is good or I like the Pertronix unit I have. I have used it now for around 8 years without any complaints. How concerned are you about original style looks? With it being a Yenko clone, are you worried about the "NEW" looks of a more contemporary dizzy?

Jim, what if the power valve is too high for the vacuum he pulls at idle. Won't that make the nozzles drip fuel at idle?

hehe, a real pet peave of mine. Flow thru the main circuit is a result of the pressure drop caused by the flow of air thru the boosters. At idle, there is simply not enough airflow to do this. Most engines / carbs are not drawing from the mains until over 2500 rpm!

There are a few past threads with some good reading on this. Or just try the test yourself. With an Ede or a Q-jet, pull the primary metering rods out. With a holley, take the primary main jets out completely.

Start the car. She will idle just fine.

The correct baseline power valve will be 1/2 to 2/3's of your vacuum reading at normal cruise

Wes - with the engine idling try turing the idle mixture screws all the way in. If the engine doesn't die you might want to replace the PV. Try staying away from terms like "mutha thpr cam" that doesn't tell anyone anything except maybe the cam isn't stock. If you don't know the cam spec's say so. Is it a hydraulic or solid lifter cam? Using your description of the cam maybe you will have to learn to live with rough idle. Read a few manufactures cam discriptions and they say things like "rough idle" on some performance cams. As was already suggested more initial timing should help but you'll have to re-curve the dist so you don't get more total timing when you advance the initial. If you are not running your vacuum advance on a manifold source give it a try, it will add advance at idle without effecting total advance. I know you said you don't have vacuum advance, I'm hoping you mean it's not hooked up.

Av-gas is formulated for altitude where the are is thinner. On the ground in your car where the air is thicker it will be leaner than pump gas. That kinda says your problem is rich idle and at 950rpm idle you could be off the idle curcuit. Check your idle mixture screws, I don't think I've tuned a holley that didn't like 3/4 - 1 turn but go ahead and start at 1 1/2 turns out from seated lightly. Warm up the engine before tuning then turn each screw in until the engine starts to stumble then back it out about 1/8th turn or until it smooths out. If you have a vacuum gauge look for most vacuum. Do both mixture screws. Next just to test advance your idle timing to 16 or 18 degs, this will raise your rpm, close the throttle blade (idle screw) until your idle comes back down to where you want it and gets you back on the idle curcuit. If this works you need to recurve your dist or run the vacuum advance on a manifold source (vacuum present at idle).

Do some research online and try to find a way of limiting the mechanical advance to 7*-6*. That way you can run 17*-18* initial. Like jim said, it may entail either welding the holes closed for the weights or doing as I and others suggest and go aftermarket.

I believe the reason he is having the idle so high is because he has such a low initial. 10* for an aftermarket "performance" cam is way too low. He has to set the idle to 950rpm to compensate for the low initial timing. As Dennis suggests, if you turn the idle MIXTURE screws in, not the idle speed screw, again, I am not trying to make you feel stupid, I am just trying to make things clear. If the idle doesn't drop considerably or even stall, then you are not idling on the idle circuit, but are on the main circuit. I would not be surprised if in fact you are on the main circuit due to the low initial timing and the high idle speed.

Here are the specs for the Mutha Thumpr:
235:I/249:E @.050"
522:I/509:E Lift
RPM Range: 2200-5900rpm
Comps Description:
Hydraulic Roller-(Mutha' Thumpr™) High performance street/strip, needs 9:1 CR, 2500+ stall, intake, gears and headers, rough idle

This is a very performance oriented cam. It's a hyraulic retro roller. This cam definately needs at least 17* initial and even with that it's probably going to idle at around 700-800rpm with manual tranny.

Yes that is another thing. This cam is also going to need vacuum advance at idle and also for a street engine, IT'S A MUST!!!! If you have simply plugged the vacuum advance off, unplug it and hook it to manifold vacuum. If you have deleted it, re-install it and hook it to manifold vacuum.

Hi Guys MUCH THANKS
Even though i work as a parts person,and have been for a long time and you learn always.so here is some more info..
cam comp mutha thumper 11-601-4 int 510 ex 495 102 c/l 107 lobe sep.
yes i woul like to keep orig looks.
yes the motor will die if you adjust idle screws...but no affect on vaccum.
if you cover carb with hand it will die.
when doing timing is vac adv hose on?...if so i find that the idle is up/down as vacuum changes.???
idle is good ... i didn't think it would idle GOOD. It is mostly the run on...nothing worse than going to a cruise and i park and it runs on and on.
i hope i filled in all blanks. I thank you for all the help.
WES:D

ahhh, now we're getting some info.

They actually call this cam a "mutha-Thumpr", a trademarked brand? Sheesh, sorry Charlie, I guess you had all the info you needed to characcterize this stick, sorry.

Wes, you got most of the blanks filled in for us now.
Typically run-on is due to too much airflow. Advancing the timing at idle will raise the rpm, allowing you to close the throttles a bit. This reduction in airflow should help the dieseling. Since you're idle mix screws are responsive, you do not have a lot of room to manuever here tho. Still, it's the next step. You can test with the distributor set the way it is, but once you settle on a new initial timing, you'll need to recurve the dizzy to get your total back where you want it.

Now see i am learning so much. I never realized that it was to much air i always thought it was fuel...so i need to put intial at 16 to start.Also to put some other info in the mix i checked voltage at coil on the resistor wire and it is12.9 volts and climbing ,so the wire is not working. So i am going to have to replace the igntion system.It makes sense now why the coil and cond where really warm to the touch. Could i put a coil that requires no external res. or are the points still going to be cooked by the high volts. Wes:D

Wes, on the air vs fuel thing, remember with a carbed system, you can't have one without the other, but air always comes first. It is moving air that draws the fuel. As long as the engine is turning, the fuel is flowing. (that's why efi cars never diesel!)

The other missing link is something to ignite the mix (after the key is off) That is something hot in the chamber, hot enough to lite the fire. Retarded timing contributes to higher temps, also too hot a plug heatrange is often a part of the problem.

On your reading on the distributor and coil, they don't mean a lot, depending on how they were taken.
coil plus voltage, engine not running... if the points happen to be open, you will read battery voltage. Not a problem with your resister wire, just no current flowing cause the circuit is open.

coil plus voltage, engine not running, points happen to be closed, you will read about 6-9 volts. The coil will get hot very quickly, as will the points, and the resister wire. These things were not made for continuous current, don't leave the key on without the engine running, stuff will get hurt.

If you read coil plus withh the engine running, you'll read somewhere in bbetween, and the number is really meaningless.

I took the 12volt plus reading while the car was running... is that to high?
And should we try for 16 degress intial timing?

Wes

Lordy Wes! You must be doing something right! These guys are really taking care of you. They're going to have your car laying rubber when you pop the clutch at idle. (BBC's can do that)
OK, after you get that vacuum advance hooked-up, be sure to report back. Your engine should work great. Remember, don't be afraid to increase initial advance. THEN apply vacuum advance.
Now if all that fails to stop run-on (dieseling) (and the problem should go away), you can always install an idle solenoid. My car came with one NEW. But it was a much more radical setup than yours. Good luck but you won't need it.

Do this Wes. Set the initial to 16*-18*. Then hook up your vacuum advance. When you read timing, the vacuum advance hose needs to be disconnected and the vacuum source needs to be blocked off. This is for getting your base timing. When you are getting your running timing, everything needs to be just as it is when the car is running down the road. Vacuum advance hooked up. Unhook vacuum advance and plug source. Take timing light and set initial timing to 16*-18*. After you set your initial, hook the vacuum advance back up. Set idle where you want it. With engine off, turn your idle mixture screws all the way in and then back off 1 1/2 turns to start. Start engine and hook vacuum gauge to a manifold source. Begin to turn your mixture screws out to get the highest vacuum reading you can, then turn the screw back in about a 1/16 of a turn. This will probably make your idle raise. Reset your idle to where you want it and do the process over again. Turn your mixture screws in and reset to 1 1/2 turns out. Begin to back out to achieve highest vacuum reading, then turn screws in 1/16 of a turn. Be sure to set both screws to the same point. You can't have one turned out 3 turns and the other turned out 5 turns. They must be set the same. If idle changes reset to where you want it. After this process, then you worry about setting the mechanical advance to limit it to a total of 34*. I was looking in the Jegs catalog and found a distributor curve kit that may work for your points distributor. Here is the link.
http://www.jegs.com/i/Moroso/710/72310/10002/-1

Also, here is a number for Moroso's tech line. They should be able to assist you in case this kit is not the one they recommend. I suggest you give them a call to be 100% sure this kit will work for what you need. It is a recurve kit, but it doesn't say anything about mechanical limit. In fact, none of the kits I have found say anything about mechanical limit.
For Technical Assistance, Call Moroso’s Tech Line at
(203) 458-0542, 8:30am – 5:00pm Eastern Time

Wes, have you been able to achieve any progress. I and I am sure others, are interested in how things are coming along. Please, keep us posted.

HI GUYS..
Sorry i dropped off for the rest of the weekend, 3 year old and winter work. That being said i bumped up the timing to 14 deg. and it seem to have helped. But i still notice no jump in vacuum and idle mixture does not seem to change vacuum either? Drove the car to the track(as a spectator) and it ran out as good as it usually does. Only thing i have noticed now is 1. It still pings. 2 when coming to a stoplight the idle is about 200rpm higher than set , but if you let the clutch out and load the car idle comes down.. push the clutch back and it idles where it should?
I am looking at various dist builds. I am leaning towards the fellow in Nevada ,that way i can have my total timing taken care of. Thanks again ,and if any thoughts let me know. P.S. Wait till i try to learn have to put pictures on this sight. HA HA
WES

When you set the initial to 14*, you now have 4 more degress on your total, unless you found a way to limit your total to 34*. Also, now that you have initial at 14*, you have to turn your idle down. With 14* and idle set to where it has been, you are probably idling on the main circuit now. That is why the idle mixture screws have no effect when you turn them in. That cam is going to need more than 14* initial. 17*-18* is going to be [B]THE STARTING POINT!!! Nothing below 17*. Once you have 17* initial, set your idle to around 650rpm. After that, turn your mixture screws all the way in and back them out 1 1/2 turns(with engine off) to start. After you have backed both out 1 1/2 turns, start engine and hook up your vacuum gauge. After that, slowly begin to turn one mixture screw out to achieve the highest vacuum reading. Then do the other one. Once both are set to highest vacuum reading, turn one in all the way and start over. The reason for this is to get the best possible setting.

Your problem is that the car still doesn't have enough initial timing. 14* is not enough for that cam. 17* is going to be the least you are going to be able to run. Once set to at least 17*, follow the steps I have listed. Setting the idle to around 650rpm, is important to get you off the main circuit so the mixture screws will have an effect.

I'll bet Wes is under the hood right now.:D
And of course he's following this great advice.:thumbsup:
Right Wes???:yes:

Fred and Gang,
You can't imagine , i print the thread take it to garage.That being said i have increased the intial to 17 and it is much much,almost perfect. Now i have to just get the total to stay at the 34 i need. Would welding the weights so they don't spin out so far work?
WES
P.S. Thanks Again...AND yes it really spins the tires now.:hurray:

I believe a stock dizzy has elongated holes where the weight sits. By welding those holes to a smaller oval, you can limit the amount of swing the weight can travel. I do not recommend this. I recommend buying an aftermarket dizzy. I know you want to stay with that "original look", but the adjustability and performance and reliability of an aftermarket unit far outweighs the "non original" look. Plus, with a little time, you can make the aftermarket dizzy look like an original unit. As from the wires coming out of my Pertronix unit, it looks much like a factory dizzy. The cap is small like a factory dizzy. It's not the large HEI with the cap in the coil. I run a Crane E-core type coil, but Pertronix offers round coild just like the original ones. They even offer a make or female style distributor cap.

Here is a link to the "male" style cap:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=PNX%2DD100700&N=700+4294925143+4294839065+4294881256+400428+4294 908216+4294840120+4294925084+115&autoview=sku

Here is a link to the female style cap:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=PNX%2DD100710&N=700+4294925143+4294839065+4294881256+400428+4294 908216+4294840120+4294925084+115&autoview=sku

Here is a link to the coil:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=PNX%2D45011&N=700+400428+115&autoview=sku

They offer black or chrome and all of their coils allow you to open up your plug gap for more complete combustion.

I run the second one I linked to. It has the male style cap. My wires have a female end that plugs onto the cap. I have had this unit in service for over 7 years and it has never let me down. It is a great unit and it's not too expensive.

I run a new MSD. It's a virtual copy of the original GM that your car came with except it's pointless. The weights are at the top too. Easy to reach. And you limit total mechanical advance to where you want it. It's all in the instruction. You can read them at Summit too.
And Wes, THANK YOU. We love success stories!

Sorry I didnt bother to read all of this but Id get the initial timing up around 18-20 with that camshaft, then adjust the carb. I fabricated a bushing that limits the movement of my weights that allows me to run 36 total and mine only returns to 26 or so.

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