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Old 05-18-2017, 09:35 PM
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65 psi low compression 388 chev stroker

I have a 388 chevy stroker that has extremely low cranking pressure of 65 psi. This was checked because of a low rpm misfire; as though 1 cylinder wasn't firing correctly. I did a complete tune-up- new cap/rotor, new msd wires, new plugs. The miss is still there it's slightly noticeably at idle, but, when driving- anything below 2200 rpm it runs mildly rough. It didn't do this before so something is off. The motor consists of .060 bore, block has been blueprinted, plate honed, line honed, and decked. Keith black 10:1 forged pistons, forged steel crank and rods. Pro-Comp cnc alum. heads with 2:05 intakes and 1:60 exhaust and 1.60 roller rockers. Cam I'm not sure of make, but has 540 lift. very choppy idle. MSD pro billet electronic dist., 830 cfm quick fuel carb. Idles with 9 inches of vacuum @ 900 rpm. Headman full length headers with 1 3/4 primaries and 3" collectors. 3" exhaust w/crossovers and flowmaster #40 mufflers. Trans is a 350 turbo w/ 2500 stall 10 converter. 12 bolt rear w/ 3.23 gears. Car is a 1965 Chevelle Malibu SS. When driving, if I am cruising at 2200 or less it is mildly rough,a definite miss, but when you give it more throttle it picks right up with no hesitation and anything over 1/3 throttle smokes the tires. Motor has approx. 2500 miles on it ,but is driven only occasionally and is 6 years old. Car is freshened before winter and also when summer begins. Started up at least twice a month during storage. Any help would be greatly appreciated!

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Old 05-18-2017, 10:34 PM
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compression tester is fragged.
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Old 05-18-2017, 10:40 PM
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Originally Posted by stanjr41 View Post
I have a 388 chevy stroker that has extremely low cranking pressure of 65 psi. This was checked because of a low rpm misfire; as though 1 cylinder wasn't firing correctly. I did a complete tune-up- new cap/rotor, new msd wires, new plugs. The miss is still there it's slightly noticeably at idle, but, when driving- anything below 2200 rpm it runs mildly rough. It didn't do this before so something is off. The motor consists of .060 bore, block has been blueprinted, plate honed, line honed, and decked. Keith black 10:1 forged pistons, forged steel crank and rods. Pro-Comp cnc alum. heads with 2:05 intakes and 1:60 exhaust and 1.60 roller rockers. Cam I'm not sure of make, but has 540 lift. very choppy idle. MSD pro billet electronic dist., 830 cfm quick fuel carb. Idles with 9 inches of vacuum @ 900 rpm. Headman full length headers with 1 3/4 primaries and 3" collectors. 3" exhaust w/crossovers and flowmaster #40 mufflers. Trans is a 350 turbo w/ 2500 stall 10 converter. 12 bolt rear w/ 3.23 gears. Car is a 1965 Chevelle Malibu SS. When driving, if I am cruising at 2200 or less it is mildly rough,a definite miss, but when you give it more throttle it picks right up with no hesitation and anything over 1/3 throttle smokes the tires. Motor has approx. 2500 miles on it ,but is driven only occasionally and is 6 years old. Car is freshened before winter and also when summer begins. Started up at least twice a month during storage. Any help would be greatly appreciated!

Tech made a good guess there LOL
What was the compression when you dry checked it and what was it when you wet checked it with a compression gauge cylinders 1 through 8 ?? Then do a leak down test ! with a good gauge Then we can start to narrow the problem down!

Making guesses will do no good till these tests are done!!

Jester
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Old 05-19-2017, 10:06 AM
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Was the throttle plated closed or WOT?
65psi sounds closed to me.

Those type tools don't tell ya much anyway. A leak down will tell you everything.
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Old 05-19-2017, 11:26 AM
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65 psi low compression

All tests were done with the coil disconnected and throttle held fixed in the full open position. I checked my gauge with my air compressor up to 150 psi and it matched my gauge,
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Old 05-19-2017, 11:41 AM
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Originally Posted by stanjr41 View Post
I have a 388 chevy stroker that has extremely low cranking pressure of 65 psi. This was checked because of a low rpm misfire; as though 1 cylinder wasn't firing correctly. I did a complete tune-up- new cap/rotor, new msd wires, new plugs. The miss is still there it's slightly noticeably at idle, but, when driving- anything below 2200 rpm it runs mildly rough. It didn't do this before so something is off. The motor consists of .060 bore, block has been blueprinted, plate honed, line honed, and decked. Keith black 10:1 forged pistons, forged steel crank and rods. Pro-Comp cnc alum. heads with 2:05 intakes and 1:60 exhaust and 1.60 roller rockers. Cam I'm not sure of make, but has 540 lift. very choppy idle. MSD pro billet electronic dist., 830 cfm quick fuel carb. Idles with 9 inches of vacuum @ 900 rpm. Headman full length headers with 1 3/4 primaries and 3" collectors. 3" exhaust w/crossovers and flowmaster #40 mufflers. Trans is a 350 turbo w/ 2500 stall 10 converter. 12 bolt rear w/ 3.23 gears. Car is a 1965 Chevelle Malibu SS. When driving, if I am cruising at 2200 or less it is mildly rough,a definite miss, but when you give it more throttle it picks right up with no hesitation and anything over 1/3 throttle smokes the tires. Motor has approx. 2500 miles on it ,but is driven only occasionally and is 6 years old. Car is freshened before winter and also when summer begins. Started up at least twice a month during storage. Any help would be greatly appreciated!

65 pounds in everything or just number 1. I think you need to explore the cam timing just to see what's there 9 inch vacuum on a 900 RPM seems pretty low on both counts. The relationship I mean at 9 inches of idle vacuum I'd expect a an idle at or over 1000. Knowing when the intake closes in crank degrees will be helpful with figuring why the cranking pressures are so low by letting you calculate the Dynamic Compression Ratio which is a compensation for stroke used to the point of intake closure which reduces the calculation of the Static Compression Ratio because the swept volume of the cylinder is reduced. With a long duration late closing cam this effect on actual pressures at cranking and under the torque peak can be quite pronounced. This is the reason why big cammed engines use lots of static compression in an attempt to counteract the effects of late closing intakes on idle and low RPM operation.


A constant miss fire on number 1 might indicate a cam lobe and its lifter is wiped. You can see that by pulling a rocker cover and watching the amount of motion in the rockers.


Bogie
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Old 05-19-2017, 11:47 AM
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Did you check all cylinders and did you do a dry and wet test? That would tell us at least which cylinders were the culprit (if any) and give us a good idea if it was rings or valves or head gaskets Etc! And like Bogie says watch your valve train while you crank the engine for valve movement.

And if you did all the cylinders every one was at 65 lbs.???

Pro comp heads are known for valve guides coming out ! I'm not suggesting thats your problem at all LOL Its just something a pro comp head owner should know ! I Received a lot of those heads that the guides were loose when delivered ! Of course they may have better quality control now

Jester

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Old 05-20-2017, 02:29 AM
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65 psi low compression

Pressure gauge was fragged. My gauge had two separate hoses- one with the correct spark plug fitting, and the other with the gauge. That hose also has the release valve. When doing the test the hose would leak where the two hoses coupled. Unfortunately, I was doing this test by myself using the ignition key. I had a friend come over and we discovered the leak but, couldn't fix it. But it was definitely leaking profusely. He had another tester we wanted to try. Unfortunately, his was for motorcycles and didn't have the right spark plug fitting/hose. So we use my plug hose and his gauge/hose. Unfortunately (again) his schraeder valve at the gauge wouldn't hold, but leaked very slowly. So once again the cylinders were checked and even with the loss of pressure, each hole would get to over 175 psi. So I drove the car and there is still a miss at very low rpm. Right now I am not sure of what direction to go; but tomorrow I am going to check vacuum and timing. These tests were done on a cold motor that had been warmed up but allowed to cool several hours so I could get to the plugs. My checkers tube length is such that only cylinders 1.2.7,8 could be checked without header removal. So for now I'm done compression checking. 9" of vac. was checked when engine was new, idle was 900-1000 rpm. My quick fuel carb has four corner idle adjustment which I will check. It has an enormous fuel filter(it's a Fram unit whos filter looks like an oil filter) Also, a few months ago I had to replace one of the floats needle and seat; it was allowing gas to just puddle constantly thru the front acc. pump nozzle. Hopefully, this new info. will help in tracking down the problem. I have a magnetic dial indicator that I can set on my headers and check the lift of all the valves. If I come up empty, does anyone think that recording the engine nose and posting a file of this would help? Also, if the vac. modulator was leaking could this cause a vac. leak enough to cause a problem like this Thank you all; once again
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Old 05-20-2017, 04:39 AM
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Originally Posted by stanjr41 View Post
Cam I'm not sure of make, but has 540 lift. very choppy idle.
So the compression is good, but you still have the low idle miss, stumble, rough running engine, or something.

Your cam has 'very choppy idle' and you have only 9" of vacuum at idle. I'm going to wager a guess that your issue is your cam. Without specific cam specs, it is difficult to tell for sure, but if your definition of very choppy idle is the same as mine, then I think you're going to be living with it, swapping the cam, or swapping the rear gear and/or tire size to get your cruising rpm into a higher range.

It would be great if you could determine if either leaning out the carb or richening it in this rpm range had any change on the miss/stumble. It is possible that you could tune this out - perhaps.

You don't have one of those darn air gap intakes do you? If yes, then low air speed and cold intake can cause the fuel vapor to drop out of the mix and cause all kinds of low engine speed tuning shenanigans.
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Old 05-22-2017, 07:05 AM
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So the compression is good, but you still have the low idle miss, stumble, rough running engine, or something.

Your cam has 'very choppy idle' and you have only 9" of vacuum at idle. I'm going to wager a guess that your issue is your cam. Without specific cam specs, it is difficult to tell for sure, but if your definition of very choppy idle is the same as mine, then I think you're going to be living with it, swapping the cam, or swapping the rear gear and/or tire size to get your cruising rpm into a higher range.

It would be great if you could determine if either leaning out the carb or richening it in this rpm range had any change on the miss/stumble. It is possible that you could tune this out - perhaps.

You don't have one of those darn air gap intakes do you? If yes, then low air speed and cold intake can cause the fuel vapor to drop out of the mix and cause all kinds of low engine speed tuning shenanigans.
You really don't like those Edelbrock Airgaps do you? Well I don't either, esp. on a street engine. Keep your money in your pockets and stick with the conventional non Airgap Performer RPM Vortec and non Vortec intakes.
Newer designs, aren't necessarily better. Don't believe the hype.

peace
Hog.
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Old 05-22-2017, 04:09 PM
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Thats a funny thing LOL many people like to have their car sound like it has A big Cam with a rough Idle and big lope! But big cams have a lower RPM miss by design (VERY LOW VACUUM) untill the RPM reaches a point that the cam and valve speed overcomes this and smooths out! My Old B/ G car had over 600 lift 300 duration Idled at 2800 RPM (to reach a point that it had a little vacuum) and smoothed out at around 6000 Rpm and my shift points were at 8,000 RPM, When I retired it and then Street cruised it, it sounded like a thrashing machine! But thats just something you have to put up with if you build a big cammed drag engine and use it on the street! They don't mix well ,LOL !! BUT THEY ARE FUNNNNNN to drive LOL!! Your miss may be something you have to live with if its cause is the cams design! If you try to smooth it out even with your mediocre cam and have it lose its natural miss characteristic it will suffer somewhere else in its power band.

If your miss is actually caused from some failure in a component it should be easy to trouble shoot !

Its also funny how 50 yrs ago a suggested idle RPM was included on a cam card or a custom ground cam card and I haven't seen a suggested idle in many yrs! Most any auto enthusiast knows you need some vacuum for an engine to even run and more vacuum to move out in gear or it will stall! Idle recommendations were given so you would know what RPM that point could be reached!

At the tracks when testing we used to have the vacuum gauge in the drivers compartment to see any vacuum discrepancy as speed picked up and at power shift points Many muscle cars had a vacuum gauge right in easy eye shot of the driver!
I also agree about the air gap intakes, here in the north even as warm as 50 degrees they actually frost in the bores and they can even freeze the throttle plates open at speed !! The air speed through the veturys creates a wind chill effect and mixed with humidity you get frost !!! You could add a heat source like a tube from the exhaust to the air cleaner !!! But now that I think about that "THE MANUFACTURERS DID THAT ALREADY" LOL

Jester

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Old 05-22-2017, 06:11 PM
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Hogg - you're right! I'm no fan of an Air Gap intake . I've been around only one and it reacted exactly the same as an inline 6 I built with headers (no intake heat) - absolutely impossible to rid it of a light throttle hesitation. Crank the RPM's (air speed) up and yup - all is well.

Jester - it makes me chuckle to hear someone want to build a motor for the noise it creates. You understand the consequences, but it seems most do not.

There's a solution - turbo .

All kidding aside, I think the OP needs a lower rear gear to resolve his issue and be prepared to cruise another 3-600 rpm's higher. My two cents FWIW.
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Old 05-22-2017, 07:27 PM
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65 psi low compression 388 chev stroker

You say you got the gauge problem fixed and you are sure you have 175 lbs. on each cylinder. Try pulling 1 spark plug wire at a time to see if there is any change in the way the engine runs. Then pull the valve cover one side at a time and start the engine and watch each rocker arm. Look carefully for each rocker arm to open the valve at the same height.
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Old 05-22-2017, 11:05 PM
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Quote:
Originally Posted by stanjr41 View Post
Pressure gauge was fragged. My gauge had two separate hoses- one with the correct spark plug fitting, and the other with the gauge. That hose also has the release valve. When doing the test the hose would leak where the two hoses coupled. Unfortunately, I was doing this test by myself using the ignition key. I had a friend come over and we discovered the leak but, couldn't fix it. But it was definitely leaking profusely. He had another tester we wanted to try. Unfortunately, his was for motorcycles and didn't have the right spark plug fitting/hose. So we use my plug hose and his gauge/hose. Unfortunately (again) his schraeder valve at the gauge wouldn't hold, but leaked very slowly. So once again the cylinders were checked and even with the loss of pressure, each hole would get to over 175 psi. So I drove the car and there is still a miss at very low rpm. Right now I am not sure of what direction to go; but tomorrow I am going to check vacuum and timing. These tests were done on a cold motor that had been warmed up but allowed to cool several hours so I could get to the plugs. My checkers tube length is such that only cylinders 1.2.7,8 could be checked without header removal. So for now I'm done compression checking. 9" of vac. was checked when engine was new, idle was 900-1000 rpm. My quick fuel carb has four corner idle adjustment which I will check. It has an enormous fuel filter(it's a Fram unit whos filter looks like an oil filter) Also, a few months ago I had to replace one of the floats needle and seat; it was allowing gas to just puddle constantly thru the front acc. pump nozzle. Hopefully, this new info. will help in tracking down the problem. I have a magnetic dial indicator that I can set on my headers and check the lift of all the valves. If I come up empty, does anyone think that recording the engine nose and posting a file of this would help? Also, if the vac. modulator was leaking could this cause a vac. leak enough to cause a problem like this Thank you all; once again
Sure post a video of the car running and the exhaust noise!!! We all love that This is a HOt Rod site

Jester
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Old 05-23-2017, 09:07 AM
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(When driving, if I am cruising at 2200 or less it is mildly rough,a definite miss, but when you give it more throttle it picks right up with no hesitation and anything over 1/3 throttle smokes the tires.)

This sounds like a carb idle circuit problem to me. What happens if you open the idle mix screws a full turn out? Is it better or worse?
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