Should be able to get down to 750 with a cam like that (stock was 650). Weve seen this topic covered before, but itsinformation worth repeating, keepingand even bookmarking.
You can hook a tee-fitting into an existing vacuum source or pull a line, such as one that leads to your transmission. Carb adjustments next. As stated, a stock engine should pull 17-21 In/Hg of vacuum at idle. If you're too tight, it will cause a pulse in the signal.
I just lowered the idle as far down as I could get it without stalling. I have about 13 deg vacuum at idle, and always seem to be 13.5 to 14 A/F. If I'm in fact is wrong, then please prove me wrong, by doing it the way I learned it. Mike Aguilar But, I also wonder if a larger cam is also playing a role in the lower vacuum readings. But you can go probably a full turn richer and not notice a change. The Fora platform includes forum software by XenForo, VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. The timing comment 66Satellite47 made was about installed cam timing, not ignition timing. The vacuum advance will be pulling off just before the mechanical advance takes over. It has horrible acceleration until 2nd gear @ David Fuller is OnAllCylinders' managing editor. EZ EFI 2.0. I sure I couldnt wat kan daar de oorzaak van zijn. Needle steady and increases at acceleration. So I can test the vehicle driving as my customer does, I get some neighborhood, city and highway driving during my 30 minute run. The aluminum Trick Flow 2.02 heads needed work and a couple springs that were weak. General Engine, How-To and Tech Tips, News, Tech You likely have a restriction somewhere within your exhaust system or a damage exhaust component. Cookie Policy - The key is understanding what your gauge is trying to tell you. lift .673/.648, @.020 286/286, @.050 250/254 lobe at 107 centerline at 105. Mine is quickly vibrating between 5 & 10 at idle. For a better experience, please enable JavaScript in your browser before proceeding. With the engine running, wiggle the vacuum connections and jiggle the lines while listening for the hiss or changes in the engine that vacuum leaks cause. Fix anything you find here. I don't use a vac gauge, I go for lean best idle. Conversely, if the gauges needle swings back and forth as the engine accelerates, your valve springs are probably too week for your engine. Mike's love of cars began in the early 1970's when his father started taking him to his Chevron service station. It barely A stock cam has will probably give higher readings. I have a 350 built by a speed shop that is supposed to have a Competition Cams 270 Magnum (224 at 050 lift) in it.
You ought to be pig rich in gear at idle and Everytime you mildly take off. The manifold on his car is all (relatively) constant vacuum. If your vacuum readings aren't steady when cold or hot I would suggest checking for vacuum leaks.starting with the carb's base, throttle plates, look for any brittle or missing rubber plugs on the carb, vacuum tree and brake booster line and swivel. Cookie Policy - He has also partnered with mainstream and trade publications on a wide range of editorial projects.
340 degrees advertised. Terms of Service - Finally after hours of trial & error it started with the carb unplugged I bought the vette with that Cam" That is the 1st thing I will be pulling out next winter. Actually, you can lean it to the point where the engine will not idle for long or at all and you won't damage anything. Often, a sticking valve will cause more sporadic drops during this pattern. I knew without a doubt my valve timing was right, I changed the plugs just because there was no where left to go and it worked. About Us - The ballast wire drops the voltage down, but the reason for that is to reduce the current that flows through the primary ignition circuit. Low quality gauges can leak internally and give unreliable readings. Need other technical information or engine specific advice?
Do Not Sell My Personal Information -, By logging into your account, you agree to our. Bearing this in mind do you still believe I must have a valve timing issue?Regards. Dad and BJ, more so BJ, taught me early on that a vacuum gauge can tell you almost everything you need to know about your engines health as long as you know how to interpret what youre seeing on the gauge, hearing and feeling. 4,000 rpm WOW, then nothing. Same idle, different initial timing. FFR4373RD MK3, nondonor, DIY options/mods/paint, Lexus indigo/Viper silver, Hydroboost PS&PB/discs/ABS, AC/heat, 3-link/Konis, FMS302E, Holley 600DP/Airgap, 4X4 exhaust, Halibrand deep lip chrome CobraIII, TeamIII spinners, rollbars: chrome full & single. Using a vacuum gauge to help you tune a carburetor can get you within 1/16th of a turn of perfect carburetor tuning. Engine vacuum will respond much more quickly than waiting for the engine to stumble. Thats the number of crankshaft degrees past TDC that the intake reaches max lift. Then I learned how to adjust that on my Mallory dist. JavaScript is disabled.
Required fields are marked *. Solid roller lunati 51019 with a 1.7 shaft mount rocker setup, put cam at about 256/264 @.050, .650" lift. Privacy Statement - The following errors occurred with your submission.
I have been watching a lot of videos on the same thing if you take a white marker and Mark your distributor and the bottom of the distributor losing the distributor and turn it just a little bit and see if that makes a difference and if it idles up high you might have to idle it down with the idle screw but if it does not do anything you marked it so you can put it back exactly where it was but I bet it works out you should be around 17 or 18 20 is a little too high for a 350 Chevy I have even saw them as low as 15 with aftermarket cam, [] Weve said it before, but an engine vacuum gauge is an extremely valuable tool. How fast the needle vibrates is telling of how many valve guides may be worn. Yours may be missing or old. You'd be surprised how many people don't do this. March 19, 2019 How about that bitchin' Howard's Cam sticker? It takes a 10 minutes to get the vacuum up to at least 15. I have a 1970 chevy 250 6 cylinder, at idle I get a quickly vibrating needle between 12 and 15 inches, whats wrong? Turn the idle adjustment screw(s) out all the way and then turn it/them in until the engine begins to stumble and back off a quarter turn. (You must log in or sign up to reply here. The first chance I get, maybe tomorrow, I am going to play around with the carb idle screw settings, and timing. When I drop it in to drive with the engine running at idle, she goes to 13/14 on the vacuum. A steady low reading between eight and 14 inches of mercury is usually indicative of incorrect valve timing. I rebuilt my motor (390 using original pistons w/ new rings, bearings, etc.) It needs to read up to 30 inches of mercury (In/Hg) of vacuum. Vacuum should steady. shouldnt have those kind of issues with a small cam like that. Before you rip the intake off, the oil could be coming throught the PCV system. Cookie Policy - Edelbrock Air Gap manifold Need help with 440-3 cutting out on heavy acceleration. BJ was probably the best mechanic Ive ever known. Put these three pieces of information together and youll know if youre looking at an easy fix or a complete engine rebuild. A vacuum gauge is an invaluable resource for tracking down engine issues. The engine seems really cold blooded, and takes its sweet time to warm up. I think you may be using idle speeds that are a touch too high and I am using a simplified model appropriate to a MT and not an auto. The engine has about 350 miles on it. Kim. Gremlins in the Thermoquad ? It is the receivers of life who do not live in the rational, honest way called for by the creator's behavioral law. Turn the idle mixture screw and watch the vacuum gauge. Do the idle mixture screws effect the engine? The cool thing about using a vacuum gauge to diagnose engine problems is that different engine problems show themselves by vacuum readings in different ranges. While its tough to diagnose remotely, in my experience, any rapid vibration will likely mean some sort of valvetrain issue: valve guides, springs, seat, etc. The car idles and sounds as if it has a decent cam in.
I understand that I can unsubscribe at any time. It is a Howard's 250021-11 cam that says Ford FE on the card profile.
Adjust the rockers, My dodge charger has a higher than normal reading on the vacuum gauge its obove 25. The question of the vacuum in the runner is not really the same as a multi-carb motorcycle with individual runners - those do have to be synced and can have different vacuum based on each carb opening.
it runs ok, but has a backfire under light throttle, please help. I just went and dug out my paperwork for the rebuild. Try timing it higher. To get started youll need to hook your vacuum gauge to an intake manifold vacuum source. It makes me wonder if the speed shop put a smaller cam in the the 270. Its high idleling and it has a code p0300 which is random misfire where should i start. But you didnt say how far the needle is travelling with each sweep.
Quick Tech: How to Read a Vacuum Gauge to Pinpoint Engine Problems OnAllCylinders Quote: I have not adjusted any timing. Check to make sure all vacuum hoses are connected and not leaking. and vacuum circuit, HELP! A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. I have the compcam XE274 camshaft in my 383 and am in the process of tuning the engine. No matter where you get your vacuum source, make sure its not ported. Engine RPM bouncing around with vacuum ranging between 10 and 20 are symptoms of weak valve guides. Unless the vacuum is really jumping around on the gauge, there is no reason to suspect a valve timing issue. Common causes of steady low reading are blowby due to worn piston rings or possibly late ignition or valve timing. The vacuum gauge pulses from about 6inHg to 14inHg at an idle of 750rpm in gear. Unfortunately, I do not know the timing of the installed cam - it was in the engine when I got the car. That's the problem right there. Check the simple (inexpensive) things first like your pcv and breather to ensure they're not pulling oil before you suspect a port leak in the lifter valley. Dont bang or drop it.
The swing on the gauge is usually anywhere from 10 to 22 inches of mercury, depending on the speed of the engine. Hallo John. HEI Distributor To confirm, youll need to perform a compression test. My mechanic tells me that during all our wasted hours hands on the vacumn I asked the Pertronix rep about the ohms question.
I run a Q/F 650 mechanical and my set screws are about 3/4 out. Its one of those tools for which you should pay more so you get the best quality you can afford. No, you're reading from that port or a manifold vacumm port on the carb will be the same. Can you kill the engine by adjusting them? Time it with the Vacuum Gauge. In loving memory of lost CorvetteForum Brothers and Sisters.
Read our Privacy Policy for more info. This site uses Akismet to reduce spam. Is your car making a strange sound or won't start? Check all vacuum lines and fittings for cracks or breaks.
The runner just makes for a bit more needle shake. Just shoot for the highest at idle. Copyright 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Yes, at the base of the carb or right in the plenum is better, but total vacuum should be about the same. hallo mijn vacuum meter meet een snelle trilling boven de 20 inch Thanks for that great information Tedster! Problem: we 1st started with a brand new Holley 650 Double Pumper It depends on a few things. I like to spray carb cleaner around the base of the carburetor and intake manifold as well as all the vacuum lines, check valves and connections. Advertising - 408 ci Gordon Levy motor with his T5 trans and independent 3.55 rear. a few years back and put an "RV" cam in, so nothing radical. Thanks Tedster! A forum community dedicated to Ford Mustang owners and enthusiasts. Adjust your carb and distributor. I no longer run it on the MSD dizzy and can't say I miss in terms of drivability or MPG. So here is what I have: for sharing your talents to people around the globe. Well, I have been fiddling with the truck today, and I can't coax a reading of more than 16/17 out of it at idle in park (750 rpm). I said that I did a compression test and that all is at the right. When you are at the lean best idle, a 1/8th turn leaner makes a very noticeable difference in the engine. resivor can.
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Comp Cams Mother thumper cam for something that WORKS.. It seems when I had my old cam in the truck, I remember getting vacuum of closer to 18/19. You obviously have a problem. Could you repeat the test with the linkage to the secondary carbs detached? never started. Steady vacuum is good. advance,didnt work for me,lots, Found my problem,long story short,took a long drive That mix.
Please tell me if thats the proper I haven't checked it so i dont know what its at. 20 degrees at idle? Test vacuum connections to make sure theyre tight. a SBC, but only if all the parts were perfectly matched. I have a vacuum gauge and A/F meter in my dash so I see these readings all the time. Do you have a way to tune the fuel table? It is set at 10 deg BTDC right now. Can you verify your efforts with another vac gauge?
While my cam is no race cam, I do wonder whether the slightly higher lift/duration in an "RV" cam can lower the vacuum reading a bit. http://www.onallcylinders.com/2015/0gine-problems/ [], Excellent information, more complete that what came with my vacuum gage. JavaScript is disabled. I plan on adding a Chevrolet Vacumn Canister and a vacumn pressure pump 17" isn't too bad, but there are some things to check. you probally know it already ,if you have power brakes .
). You are a wealth of knowledge. Okay, I guess! Thanks A restriction in either the intake or exhaust is indicated if the gauge shows vacuum decreasing slowly instead of quickly. If the needle vibrates rapidly between 14 and 20 inches of mercury, but then steadies as rpm increases, you may be dealing with worn valve guides. An extremely low reading could also indicate an air leak at the intake manifold or throttle body. Can't say. Terms of Service - I have a baby by comparison 246/256 @ .050 on a 110 centerline with .580 lift I have a 1100 idle with 25 degrees locked in timing and 8-10 inches of vacuum solid flat tappet cam and 1150 carb on a nitrous fed 408W. was 3-7 inches. I did notice however that I have a film of oil in the intake plenum and runners.
I need an opinion from all you pros out there! Next, go for a test drive. Stamped rocker arms vs. adjustable, hydraulic cam. If the needle drops quickly and returns to a normal reading, a burnt or sticky valve is indicated. It is a relative reading. 327 small journal rebuilt 2000 miles ago. Since its hopefully a fine instrument, treat it properly. Pop the hood and look over the engine compartment with the proverbial fine-tooth comb. Instead they live as they please and finally are forced to depart this life., The Fora platform includes forum software by XenForo, My detailed instructions for adding ABS to a Fox width rear axle FFR, https://www.youtube.com/channel/UC7cBpeIW286xO3U4l4RtVMQ, http://www.youtube.com/user/joeembery#p/u, http://s207.photobucket.com/user/JoeE13/library/FFR Cobra?sort=6&page=1, http://outintheshop.com/faq/Vac Adv Spec.pdf, VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. A couple of years later I realized that I got 20 deg of advance at full vacuum. Contact Us - All graphics from forums.neon.org unless otherwise noted.
hello, sorry for my writing I am French, I have a pontiac firebird 2.8l manual transmission of 1988 and on my vacuum gauge I read 16 hg with the occasion very slight increase to 16.5 hg and after having all check all the ignition it turns out that all works well and with the good advance besides if I try to advance the ignition i have a misfire can someone have an idea? Solved. Start the engine and listen for leaks in several locations around the engine bay.
Very interesting to a Chevrolet fanatic that probably could assemble What does a rapid fluctuation at 16-17 in of one inch with a erratic barely visible flick down to 14 inches mean in a four cylinder ecotec 2.2 l engine? In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing. To directly answer the PV question, yes you need to change it with those vac readings. A good source for many is the vacuum line going to the transmission. To What vacuum reading is okay with a lumpy cam? Gonna see if I can't get the vacuum up some. To me that 'no less than 1 turn out' is simply a guideline. Watch the timing with your timing light as you increase engine speed and you'll see it in action. :cheers: The following errors occurred with your submission, CorvetteForum - Chevrolet Corvette Forum Discussion, V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette. How much vacuum you get is mostly dependent on your cam. [] the engine passes the leakdown test, start looking for vacuum leaks. tell you if the bearings were not the right size. I tried connecting to manifold once and it was terrible. This is pretty much the accepted method of tuning a carburetor; however, it will only get you in the ballpark. I hate to ponder pulling our fresh install to change out the MK4 7869, gen 1 Coyote, TR-3650, 3-link, power steering/brakes, ABS. With that much duration and low vacuum, it is just a massive vacuum leak. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register. Obviously this isnt true for huge lopy cams that can barely idle. Post it here! with 18-20 " vac at 800-900 idle thats not a 224 deg int dur @ .05 in only 350 cubes,no way ,thats stock type idle vacuum reading you would get with a 350 250-300hp cam & not a 270 deg/224 deg @.05 int dur cam in a 350,end of story. If you notice the needle slowly fluctuating between four or five inches, chances are you have an ignition-related issue. VP 5.7GSI wont start - good fuel pressure, good ecm, wiring - Page 53, Vac Visual: Quick Guide to Vacuum Gauge Readings - OnAllCylinders, Vac Visual: Quick Guide to Vacuum Gauge Readings | MEP Precision Racing, Can Low Compression Cause a Lean Engine Condition Like This? you'll need to run vac. Mild performance cam profiles should give vacuum readings of between 15-20 In/Hg, although their performance profiles may decrease average vacuum by approximately three In/Hg and cause mild lopiness with attendant vacuum fluctuations as overlap increases. Your email address will not be published. Thinking of adding power with a new combination? adjustment carburetor With help from Dutch translator software, it sounds like youre asking about a rapid vibration above 20 inches of vacuum. Below are some of the most common readings and diagnoses youll find: This a normal readingbetween 17 and 22 inches of mercury. Flooring it should cause vacuum to drop quickly, maybe to zero.
Something like a plugged cat or muffler is usually the culprit here. You can lean the idle fuel mixture to the point of engine stumble without causing any damage. I can unsubscribe anytime to withdraw my consent. On a 383 I'm surprised you have that much vacuum Really - I thought where I am currently at is way low. idles at 950-1050 rpm. When people talk about how much vacuum they have at idle, does the rpm you idle at matter ? Once on the throttle 11 to 12 A/F, I am generally too rich. Most engines will run 17-21 In/Hg while idling at normal operating temperature. Releasing the accelerator quickly after a few seconds should cause vacuum to quickly bounce back up to above normal (usually around 25 in/Hg) then quickly return to normal (17-21 /Hg) as engine RPM slowly returns to idle.
A larger cam in a small cubic inch motor explains the reason for a lower vacuum signal. As I did some serious homework out if kilter.
Begin your engine check-up by making a thorough preliminary inspection. Now perform the wiggle test. Steve,you are correct the idle vac will come down some when he reduces the idle speed but a 650 rpm idle is for auto trans in gear/not when in paryk where ot would idle higher & many manual trans setups idle in the 750-800+ rpm range. That old "151" cam that you have a copy of there, is well and widely known to give low vacuum; has been since about 1964. Learn how your comment data is processed. JavaScript is disabled. Increasing the idle speed to 1000 gives another 1" and decreasing it to 800 drops another 1". What cam are you running?
During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. Do Not Sell My Personal Information -, By logging into your account, you agree to our. He's done pretty much everything in the automotive aftermarket from gas station island attendant, parts counter, mechanic, and new and used sales. Run with a breather to atmosphere on one side and hose to the air cleaner on the other. My machine shop guy really likes Howard's Cams, and that is what he wanted to put in. Idle is 9" vacuum at 1100rpm. screw dialed in by itself overnight !
Nothing left to check?
Is 1200 rpm to high to get a good vacuum reading ? Which brings another question, how much have you needed to turn the idle speed adjustment up? When I was a kid, my father and BJ taught me how to adjust a new or rebuilt carburetor by watching and listening to the engine. it shouldnt move around that much check for leak or valve to tight. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Alcohol Injected All Alum 632 cid Race Engine, BLOCKS-BLOCKS-BLOCKS in Stock Ready to Ship, Everything You Need to START a Chevy Performance Connect & Cruise System (LS and LT Engines), What To Look For When Your Hot Rod or Restoration Doesnt Start, An LS3 Swapped 1992 Buick Roadmaster Estate Wagon Project: Part Five. Has mild rv cam. You do get more advance at idle, but your total advance with the engine at say 2500+ RPM is unchanged as the vacuum advance is no longer in play. I believe the original owner of the truck had the engine rebuilt at one point, but, I don't know if he switched the original cam out or not. There is no oil/coolant mixing that I can see. Its a 383 with a Holley 770 Street Avenger. Check your spark plug gap as this condition often indicates too narrow of a gap. For comparison, 496 stroker w 230/236 duration at .050, .541 lift hydraulic roller, at 14 initial and 900 rpm, 12 inHg. I have sprayed starter fluid around the manifold and carb base and didn't notice any jump in rpms. Please list your cam, vacuum, and at what idle RPM it is. Just a good torque cam. Manage Preferences - Advertising - 1.
Anyway, I will play around with it and report back if I can get it closer to 18-20. I always read about how a big cam will give you lower vacuum numbers at idle.
I was curious as to whether the oil in the intake could be due to an intake gasket leak in the lifter valley or could this be blowby around the pistons because the rings haven't seated yet? What will happen is that the tip-in of throttle will be less responsive because now you'll need to re-tune the accelerator pump timing / shooter size. A forum community dedicated to Factory Five owners and enthusiasts. You can have any bigger cam ground to diffrent lobe center to get more vaccum at idle. It is not the giver of life that destroys human life. However my vacuum is 21 inches when idling at 1200 rpm. Or the valve train being I agree to receive emails from the site. Terms of Service. I know on my setup if it is too fat at idle and i lean it out, the map sensor will start showing more engine vacuum where the engine actually wants to idle with less fuel. Hot Plugs Extra-Long for aluminum heads
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For a better experience, please enable JavaScript in your browser before proceeding. What can this be? The needle on my vac gauge moves about an inch or two until you are accelerating then it is smooth, on decal and light acceleration I see about 12".
If your valve covers don't have baffels in them like the stock one did, it is common for oil to get spashed into the PCV valve. With a big cam you can uncover the transfer slots and start drawing fuel from the main circuit rather than idle circuit. My Throttle body light comes on sometimes. Remember though, a solid cam seems milder at idle than a hydrolic of the same duration @ .050 due to the lash. Conversely, a choked or restricted air cleaner would result in a higher than normal reading, depending on how hard the engine works to pull in air.
