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Discussion Starter · #1 ·
Hi Folks,

I have a 70 camaro RS with a 383 stroker. Unfortunately I have no clue on the engine spec, unknown cam, unknown heads and pistons or even compression.
What I know:
  • Aluminum heads with no marks
  • 4 bolt main, internally balanced
  • 650 cfm brawler carb double pumper with mech secondary
  • 6AL box with MSD distributor/coil
  • initial timing 20, all in 39 (without vacuum advance) I did try with and without vacuum advance at ported and manifold
- I tried much more aggressive initial timing the car likes it but am afraid of pinging and destroying the engine
  • Plugs look fine maybe too much timing as the color change is close to the base on the ground strap. I was using a cool plug earlier (R5671A-9) which would foul very quickly ( and went to a hotter plug ngk R5671A-7)
  • 2800 stall
  • TH350
  • 3.42:1 posi 10 bolt rear end
  • 255/60/15 tires
  • long tube headers
  • 6 in HG idle vacuum that fluctuates. The needle on vacuum gauge is not stable
  • have vacuum canister for brakes (barely works)
  • Rough idle (850 in park, 750 in gear) I had to drill small holes on throttle plates to help idle.
- Transfer slot is squared on both primary and secondary. air fuel mix screws out ~ 1 1/2.
- I have a AFR gauge, the idle ratio is around 14 and I see around 12-13 during regular cruse.
- cannot get the off throttle cruise to lean out, With vacuum advance I would expect 14 or higher ratio when I cruise around 40 mph and let off the gas but it sits around 12, I am guessing I need to jet down.
  • Takes 10 minutes to warm up even in warm TX weather. I adjusted the electric choke but still have to sit on gas for a while otherwise it will not idle.
  • Get about 8 mpg city/highway mix
  • No exhaust gases in the coolant
  • Could not find any vacuum leaks around carb, closing the choke by hand does not show any indication there is vacuum leak
  • I get around 20lb oil pressure when warm, over 60lb when cold, no smoke

There is no low end torque, cannot get the tires to spin without using brake, even then due to low vacuum brakes dont hold to well). Top end is not too bad.
Questions:
1- Could it be that valves are too tight?
2- The cam is too big and heads are too lazy, very low compression?
3- Could a big cam really cause such low idle vacuum (6 in) that is unstable?

Thanks for any suggestions.
 

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Question #1...Not likely, too tight means you hold a valve off the seat...it will just create a constant missfire. when too many are set too tight, engine then just won't start.

Question #2....Could easily be, but you'd have to investigate further...i.e you disassemble it partially to see what you are working with. You can degree the cam at the same time and that will tell you what it's spec's are.
It's just the time and the cost of gaskets and fluids, then you'll know what you are working with.

Question #3....Yes, it sure can.

Add to your list -
Cam installed retarded, or with way too much advance(tooth or more off on timing chain and gears, or if multi-keyway gears alignment symbols not matching)

Main jetting generally doesn't effect cruise, especially not at 40 mph, because fuel isn't flowing out of the booster yet....you are still running on the idle feed restriction, by way of the transfer slot.
May need to either reduce the idle feed restriction size, or increase the idle air bleed size, or maybe some of both.
The fact you have a decent idle AFR may be pointing to the fact you'll need to modify the carb to have a transfer slot restriction that is replaceable and tune-able.
Also need to check the rating on the power valve(s)

Have you run a cranking compression test yet?
 

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More info on the vacuum reading, is the fluctuation fast or slow, regular or irregular, and how large are the deviations.

To some extent a big cam will give a wandering needle so that has to be taken into consideration.

The fact that it will drop the idle down to 750 in gear doesn’t sound like a huge cam. But the effort to divine its duration and lift would be a useful exercise.

No name heads indicates they are imports but most of them are at least decent performers.

My knee jerk reaction is to suspect the cam isn’t timed correctly to the crank, another possibility is badly worn valve guides may be a contributor.

You might want to pull the distributor to look at the gear. A mismatch of materials eats gears pretty fast.

20 pounds of oil pressure under what conditions, hot idle that’s good, at 1500 RPM it’s not.

A common problem with modified engines and vacuum leaks is they occur at the valley side on the intake. The proper rich test is to bleed propane from an unlit torch into the inlet air stream, if the idle picks up it is an indicator that there is another air source beside the carburetor.

Bogie
 

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I am going to semi hijack this post, I am addressing same issue, but a slightly different puzzle.

350 60 over, dart 180/64 202 aluminum, stock intake, stock exhaust manifolds, comp cam 12-242-02(ex268h) and sniper qjet.

Car, 1980 Camaro, Th350, 342 gears, and car is complete dog and gas mileage 8 miles to gallon.

I also appear to have no vacuum leak, vacuum at idle is 13 with a bounce, will bounce between 11-15. Compression specs out at 9.9 to 1

I have looked online at tune/timing people have had good power with in this cam, 16 initial seems to be the norm, timing is set to 16, can is degrees in straight up.

I pulled plugs and they look slightly rich, afr ratio is pretty steady at 13.5. But plugs are different colors, meaning some are more black than others, and some are clean, makes me think vacuum single is not balancing fuel charge to all cylinders. I have 300 miles on engine, no oil fowling on plugs. Also when cold start using temp gun and checking exhaust temps, temps are not same or steady, seems like some are not getting fuel or spark is not burning what’s there, temps seems to equal out when warm.

Dist has been serviced and curve set, vac advance has been limited and total is in at 3000 rpm (36), all work was done on dist machine, no on car guess work.

If I dead stop stomp it falls on face, Bogie I am interested in your propane test, can you give a little more detail.

I cannot post photo of plugs via phone, could someone post them? I can send to your email.
 

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Discussion Starter · #5 ·
Thanks for the quick responses.
1- I tried using bigger idle air bleed. I have #65 and went to #78. This is while trying to fix the over rich condition. Car will not idle with #78 air bleeds. I may have gone overboard here. It might have worked with #70 or #72.
When I got the car it had 6.5 power valve, I swapped it to 2.5 I also tried 3.5. Before this it would be dog rich.
2- 20lbs at idle under 1k rpm. I think oil pressure is good. No blue smoke or anything. This engine maybe has 5-10k miles on it
3- I am not clear on one comment " The fact that it will drop the idle down to 750 in gear doesn’t sound like a huge cam "
Is this because a huge cam will not idle at 750rpm or is it something else. Also I do have a 2800 stall so in theory there should be little difference if any between Park and in gear. Car barely moves when I put it in gear due to the stall converter. I have to get on the gas for it to move. Also it barely idles. BTW if I increase idle speed to say 1k rpm range, after car warms up I get dieseling when I turn off the ignition.
4- I will try the propane trick. So I get a propane bottle and open it into one of the vacuum lines or just over the carb choke? I have not heard this before.
5- I did check compression (cold) when I first got the car. I cannot remember the exact numbers but I believe it was over 180 and pretty consistent. Essentially it was healthy.
6- So tight rockers would not cause this, I was hoping it was that and I was just going to adjust the rockers. BTW there is no lifter tick or any noise at all.

I am trying not to remove the intake and dig into the engine but I may not be able to get away without it.

Thanks
 

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There is also the possibility that the balancer has spun and then timing mark would not be accurate. This could happen with an older or even a new balancer that is out of phase. Best is to find TDC by putting your thumb over the #1 plug hole and see where the timing mark is. It should be pretty close to 0 on the timing marker. The other possible issue could be the at timing pointer is wrong for the application. This would only apply to balancers with rubber insulators.
 

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Discussion Starter · #7 ·
I did check the location of the TDC and confirmed the balancer is accurate. When I saw the engine liked so much timing I checked that first. I used a stopper tool and rotated the crank in both directions and marked the TDC. The mark on the balancer was dead on in between.
Thanks
 

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Bogie for the propane trick, if I pulled the PCV valve out of valve cover, and put the propane near or just inside the pcv valve hole, if there is an under intake vacuum leak would the propane get pulled into it?
 

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I have had two bad cams in engines I worked on.
Both ran OK but we're obviously down on power.
It would take degreeing 16 lobes to figure out the problem and I never did that.
Both cams were from Comp.
I always go with Isky now.
 

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If you were to measure and note the valve lift at the valve, you can get a ball park figure of the cam itself. It's a start at least in the right direction and wouldn't require a tear down.

You can also use masking tape with graduations on it. Wrap it around the damper, set up a DI on the valve,Note the degrees when the valve opens and closes on the damper, do the math or post it here and I'll do it for you. Crude, but it's a start anyway and doesn't require a tear down.
 

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Bogie for the propane trick, if I pulled the PCV valve out of valve cover, and put the propane near or just inside the pcv valve hole, if there is an under intake vacuum leak would the propane get pulled into it?
That would make a bomb, one spark and you’d be picking shards of valve covers out of your face.

The propane is a small bleed into the air horn of the carb, it richens the mixture without disturbing the carb’s metering as closing the choke does. If there is an vacuum leak the revs should pick up as the mixture corrects, if the RPMs drop then it’s going rich and likely isn’t sucking air from an unmetered source.

How old in miles are on the 383. This vehicle does not sound new to you?

It sounds like the cam is not timed properly, that can be age related wear on the timing set or an installation error.

Another common problem with flat tapped cams is lobe and lifter wear. You can check for this by idling the engine and looking for rockers that move less than others.

Unmaked cylinder heads speaks to them being Chinese imports, depending on how old they are the older ones just until a very few years ago were noted to chew up valve guides, this results in a vacuum leak and poor valve seating. Another common mistake on builds with aftermarket aluminum heads is they are thicker than GM iron or aluminum castings so they are taller, this typically places the rockers .1 inch higher than stock GM which requires .1 inch longer valve stem and pushrod.Inthe case of the pushrod if going to a bigger cam this is just the starting point in selecting the pushrod that matches the needed mid lift length. The spring pad moves up as well but this in your favor keeps the GM spring geometry pretty much the same.

The problem with vacuum tests on big cams is all the symptoms of problems with a mild cam are present when everything is in order. The long durations results in lots of overlap and the late closing intake all conspire to bring engine vacuum down so a normal would read late timing and the reversion causes the needle to wander which reads a bunch of problems if this had a “normally“ timed cam.

The bigger the cam the more touchy it is about it’s set up, there is no room for being a tooth off on the build installation, and very little tolerance for part wear.

Big cams move the torque and horsepower way up the RPM range, it takes gearing to recover the bottom end by both adding lever arm to multiple the force delivered from the crank and to speed the crank up to higher on the torque and power curve.

MSD distributors, modules and 6AL boxes are known for their electronic failures.

Not knowing the inside of the engine’s details is a huge information deficit.

383 should be a problem about keeping from frying the tires not a problem with getting them to smoke. So something significant is off, it could also be in the transmission.

Bogie
 

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Thanks for explaining Propane test Bogie. I want to clear up something, this post is addressing 2 different cars, 2 different owners with what seems to be same problem, low on power.

Eergin has 70 Camaro with 383, TH350, 3:42 gears with carb
Me (95 Truck) I have a 80 Camaro with 350 +.060, TH350, 3:42 gears with Holley Qjet TBI . My engine is fresh rebuild all new parts. Cam is Comp XE268H

I hijacked Eergin post, sorry guys

Now I have made some progress on my issue, Power is coming up, I have bumped my initial timing to 18 (from 12), I have readjusted the valves (I did not notice anything off) and I have made an adjustment to the kick down cable on transmission. I am thinking the timing has made the biggest improvement in power, but I also have noticed the kick down has also made normal driving improvement.

My next step is to add more timing and start looking at adjusting AFR
 

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The XE268H should be getting happy with where your getting to. Aluminum heads because of their thermal characteristics need to be pushed harder than cast iron and can be. Higher compression more advance and hotter coolant is a key to unlocking their performance.

It takes little effort to get over 400 horses and a like amount of torque out of a 350 with this cam and modern heads. If it’s not doing that something isn’t right in the assembly and/or tune.

Aftermarket TBI runs a lot better on an open plenum intake. If you’re running a dual plane intake that has a fully divided plenum then either cut the divider down about 3/4qtr inch or install about that thickness of an open spacer if there is enough vertical room.

Bogie
 

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Discussion Starter · #14 ·
I am out of town so I am not able to work on the car but here is my plan going back:
1- Try the propane test to verify if there is a leak under the intake
2- Find more about the cam by doing some measurements. I am not sure how to do this without at least taking the intake off. I will do some googling :)
3- Run the engine without the valve covers and observe the rockers. See if there is any abnormalities there. I will buy some old covers and cut them up as I do not want to make a mess.

BTW this engine has at most 10k miles on it. So it is still pretty fresh. I put maybe 1k miles in the last couple of years.

Thanks

Erhan
 

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Discussion Starter · #16 ·
I mentioned earlier, it does like a lot of timing, I tried 28 degrees initial and close to 50 all in it was running quite good but when I look at the spark plugs I see the ground electrode turning color at almost the base even now with 39 all in. I am afraid to push timing so much as I cannot hear or feel when it is pinging. Unless the break in was done wrong I do not believe the cams are worn as I said the engine has maybe 19k miles on it.
 

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I mentioned earlier, it does like a lot of timing, I tried 28 degrees initial and close to 50 all in it was running quite good but when I look at the spark plugs I see the ground electrode turning color at almost the base even now with 39 all in. I am afraid to push timing so much as I cannot hear or feel when it is pinging. Unless the break in was done wrong I do not believe the cams are worn as I said the engine has maybe 19k miles on it.

For one thing flat tapped cams are real touchy and they can go away very fast. Some start wiping lobes and lifters before the breakin is complete, there a several reasons for this but in general it doesn’t take too much research in this and other forums to see that this is pretty common..

Another is cam set up itself, most information has the builder set up the timing set with both the cam and crank gear in the 12 noon position thus having a great distance between the pip marks. This is very easy to get slightly off. The best way to set this up is with the pip marks facing each other where the cam gear mark is in the 6 o’clock position facing the pip mark of the crank gear in the 12 o’clock position, this is easy to see if you have alignment or not; in this position the engine is at cylinder number 6 firing which is one crankshaft rotation off for number one firing. So at this point you can rotate the crank clockwise as seen from the front one rotation to get to number 1 firing. Or chose to reposition the distributor plug wires starting with the number 1 plug wire installed to the number 6 cap terminal.

An indicator that falls out of your description could be that when the distributor was installed it didn’t mesh in the expected position with the oil pump drive to where the installation has the rotor not pointing at the terminal you are using for number one plug wire in the cap. You need to go back and carefully check this alignment.

It is clear that something is not set up right, this cam should not require this much base timing, it just isn’t that radical. It should run great with about 12 to maybe 14 at most of base timing.

Atmospherics will affect this if you live at 5000 feet as opposed to sea level the engine will like more advance for any given compression ratio because the air pressure is lower so the engine cannot get a full breath thus for any given compression ratio the actual cylinder pressure is lower which burns slower so the engine is looking for more time to get the burn at it’s best peak pressure in regards to crank rotation and piston position in the bore for force transfer; this happens when the cylinder peak pressure occurs about 15 degrees after TDC. So enough lead has to be provided to achieve this event. The other trick at high altitude short of supercharging is to up the compression ratio to force more absolute compression pressure which speeds the burn and provides more force from the burn.

The answer as always is start simple with the inspections then proceed to the more complex and expensive as you weed things out.



Bogie
 

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Eergin, I did find a fouled plug on mine last night, I changed them all, I also am thinking my cam is ok at this time. I am going to see how it runs tonight. I might add some more timing and start adjusting TBI idle, I am starting to learn these snipers units are not plug and play, they still need to be dialed in like a carb, with the added bonus of being able to date log and see the impact the change makes.

Eergin do you have a kick down cable on car? Adjusting that helped on this car, when standing on it she was not dropping a gear or gears, so engine would just load up not rev up, also trans would shift to fast thus lugging down the engine.

Bogie the divider between left and right side of plenum is open, GM drilled a big hole in it, about 1” to 1.25”, then GM put a big pipe plug in side of intake.
 

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Discussion Starter · #19 ·
Thanks for the quick response. I think the two car specs are getting mixed up here. For my case, 1970 camaro with a fairly recent 383 stroker, I have no clue what the cam is. I have no info on the cam spec. Based on the rough and lopy idle and 6 inch vacuum I think the cam is quite radical. Maybe Bogie you are commenting on the other vehicle which has the cam spec listed?

One question: If the cam in my engine is incorrectly installed and retarded by one or more teeth, could that be corrected by advancing the timing more? Like I said the car is very happy with 28* initial timing.

Also, I did remove the distributor before and made adjustments so I have a faster timing curve and added a bolt so mech advance is limited. I alsdo added a adjustable vac can so I can sort of control the vacuum advance.

Thanks

Ergin
 

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Discussion Starter · #20 ·
96 Truck, I do have a kick down cable and I adjusted it so I can get the trans to kick down when I smash the gas. My issue is not up top. My car is very sluggish down low and very rough idle. So kick down cable is not the issue.
 
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