low vacuum at idle
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low vacuum at idle

  1. #1
    Aquacraft Eddie's Avatar
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    Unhappy low vacuum at idle

    well I thought I figured out my problem with my timing and low vacuum, I pulled the motor and took the intake off (eddy air gap) and noticed the intake gaskets were soaked with oil at the bottom, it had been sucking oil in threw the bottom of the intake , the outside had no vacuum leak at all just inside where the valley is. The machine shop milled the intake to fit , apparently the engine builder either milled one head more than the other? or who knows what but I got the intake back on and verified my timing and put the motor back in , just started it yesterday and I have the same damn problem!!!!, very low vacuum at idle (5-7 inches on vacuum gauge at 1400 rpms and 20 degrees of advance at idle) the MACHINE shop did a leak down test when I had the motor out which came back good, they also pressurized the intake and checked it for leaks and nothing, so now where do I go??? maybe the piston rings are screwed? maybe wrong clearances ? this is the only thing I haven't checked but at this point I am ready to just buy another motor , this thing has had strange issues since I got it , and it was a crate motor!?,,,,anyways I'm wondering if the low vacuum could be a carb problem? or cam lobes going to shit? , it does not back fire so I don't think its the cam but I'm not an expert and you guys know more than me so any help is appreciated, or anybody have a decent 454 for sale at a good price I might be interested , I trust you guys way more than ebay or the stuff I got around here .......

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    Senior Member LiquidDan's Avatar
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    Have you checked the Cam Timing?
    Originally Posted by gn7 "Stick to making the worlds best engine stand."

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    Aquacraft Eddie's Avatar
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    Quote Originally Posted by LiquidDan View Post
    Have you checked the Cam Timing?
    well I took the timing cover off , got motor tdc, and made sure the gears were on right, but I do not have the full cam specs , so it has not been degreed, I do know the lift is .553/ .578 and duration is somewhere in the 234/244 area , the builder never sent me the specs, but gave me this ball park....and now he doesn't answer the phone,,,go figure....he said it was a custom ground cam, made it sound huge, then he gave me those numbers which are not that big at all so I should have more vacuum at idle,, I need to get a degree wheel or make one,,,,it has low compression also , lowest was like 100 and highest was like 120 if I remember right, ill have to look for my paperwork, I really thought I found the problem (leaky intake) so I went no further. I should have just rebuilt the motor when I had it half way torn down , but I thought the problem was solved and I was trying to save money,,,,,im thinking I could just order a decent cam and lifters and put them in, then ill have the cam numbers for sure and see what happens, but at that point I should just tear down I guess , I don't want this to keep happening , so I feel bad because theres another summer gone, that's why I was thinking about just buying a motor that is known to be good, if theres such a thing and drop it in , but that's probably me just being frustrated and getting in a hurry , i talked to the machine shop here and they have at least a 6 month wait so for sure I'm pretty screwed ...
    Last edited by Eddie; 05-07-2016 at 07:34 AM.

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    Muffler City Old as Dirt's Avatar
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    Quote Originally Posted by Eddie View Post
    well I thought I figured out my problem with my timing and low vacuum, I pulled the motor and took the intake off (eddy air gap) and noticed the intake gaskets were soaked with oil at the bottom, it had been sucking oil in threw the bottom of the intake , the outside had no vacuum leak at all just inside where the valley is. The machine shop milled the intake to fit , apparently the engine builder either milled one head more than the other? or who knows what but I got the intake back on and verified my timing and put the motor back in , just started it yesterday and I have the same damn problem!!!!, very low vacuum at idle (5-7 inches on vacuum gauge at 1400 rpms and 20 degrees of advance at idle) the MACHINE shop did a leak down test when I had the motor out which came back good, they also pressurized the intake and checked it for leaks and nothing, so now where do I go??? maybe the piston rings are screwed? maybe wrong clearances ? this is the only thing I haven't checked but at this point I am ready to just buy another motor , this thing has had strange issues since I got it , and it was a crate motor!?,,,,anyways I'm wondering if the low vacuum could be a carb problem? or cam lobes going to shit? , it does not back fire so I don't think its the cam but I'm not an expert and you guys know more than me so any help is appreciated, or anybody have a decent 454 for sale at a good price I might be interested , I trust you guys way more than ebay or the stuff I got around here .......
    If the leak down came out ok.. the rings are in good enough shape.. having the same damn problem using the same damn gauge?
    Why does a 8.5:1 454 with a mild cam have to idle at 1400rpm? because it won't idle @ 800?
    What is the total timing set at? 36 degrees @ 3000rpm? Then when it idles down to 1400rpm, 20?
    Compression test OK? or are the valves set too tight?
    Sure you have that bumpstick installed correctly? cheap timing set not indexed properly?
    Manifold end seals done right?
    Gasket under the distributor?
    If you think the lobes are going flat.. use a dial indicator and check to see if it lifts the valves the same as the others when you turn it over..
    Hyd. or solid cam?
    The Cam in my 496 has 271/282 @ .050 duration and close to .700" lift.. it will idle down below a grand and still has 9" vacuum.. you should be 11-13 minimum @ under 1000rpm.. easy..

    Edit:
    didn't see the previous post.. same time.. Comp test looks low.. you checking the manifold vacuum off the fitting where the brakes or trans would usually hook up? or are you checking off the carb ported vacuum source? The latter will read low..

    A primary factor in manifold vacuum is throttle plate position. When the throttle plate is closed, manifold vacuum should be higher than when the throttle plate is open. With an open throttle it is easy for the outside air to get into the engine and there will be less difference in pressure between the outside air and the air found in the intake manifold. As the throttle plate closes, a restriction in airflow is created and manifold vacuum increases.
    Last edited by Old as Dirt; 05-07-2016 at 09:15 AM.

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    Senior Member californiadreamin's Avatar
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    Is it running poorly in addition to low vacuum? Low vacuum is a relative term as different builds will produce different results

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    Senior Member lbhsbz's Avatar
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    Vacuum is a result of restriction upon volume that the engine is trying to pump. Under no load, if you're at 1400RPMs, the throttles are open a bit, reducing any restriction. I'll bet it runs like crap because excessive idle throttle angle is being used to compensate for something else. Get the throttles closed and out of the transition circuit, get idle mixture and timing set such that it'll idle under 1000.
    GN7: "If you were to have ever had two brain cells you could rub together and make heat, you be dangerous"

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    Aquacraft Eddie's Avatar
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    Thanks guys sorry to bring this up again,,,the intake currently has the vacuum port plugged, going to put a fitting in and see, I was checking vacuum from carb,pcv port not sure if that's ok or not? the intake end seals are good, used the right stuff and no leaks at all. The carb plates being open so far is probably one of the reasons I have low vacuum , I was going to adjust the secondaries so I can turn the idle speed down but I don't think I should have to do that with this cam, its got to be the timing some how , either cam is wrong or bad timing set? or maybe dizzy? its a Mallory hei dizzy , and I didn't like the compr test but it supposedly has molly rings and it does not have that much run time at all, and it seems to run ok when revved but idles like shit? plus I have not had it in the water so once a load is put on it , it may run like shit at all rpms or may not ? who knows just hate to drive 4 hours to put it in water and get disappointed, I would like to figure this out before I get to the water, thanks for your cam specs and idle , vacuum old as dirt , that makes me think there is something wrong with this for sure. It might be a stupid question but how can the cam be in wrong? unless the timing set is bad?

  10. #8
    Senior Member LiquidDan's Avatar
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    Is it at 1400 idle because you have a vacuum leak?
    Cam can be in wrong if it's a tooth off...need to check cam timing @ intake valve opening....degrees and lift
    Last edited by LiquidDan; 05-07-2016 at 10:01 AM.
    Originally Posted by gn7 "Stick to making the worlds best engine stand."

  11. #9
    Village Idiot fc-Pilot's Avatar
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    Eddie, bare with us as we ask questions. Some my be way below your level and you may feel we are talking down to you, but we don't know your level of experience and we are only trying to help. I built a SBC decades ago as a young teen and used a new 9 key-way timing set. That meant that there were 18 marks on the crank gear. Well, not understanding how it worked I got it three teeth off. It did the same thing you are talking about. Had to open the throttle way up to run and when idling (if you can call it that) the rpm's were high and the vacuum low. Had I know what degreing a cam in was about back then I would have saved myself an engine. I am not saying that you may have screwed it up, but timing sets and cams are not perfect and can sometimes bee a few degrees off. If both are a few degrees off in the wrong direction that could easily cause this issue. If the carb has an issue like having no fuel flowing through the idle or transition slot then you would have to have the throttle way open that that could cause some serious issues like what we are talking about.

    You can degree your cam with a timing tape around your balancer if needed. In a pinch I used masking tape with carefully marked degree lines. This would be my first thing to check. It the cam is retarded then it will cause low vacuum and low cranking compression. You don't need the cam specs, as you just need to find out the lobe center-lines and adjust accordingly. If I were in your shoes I would want the can advanced 3 to 4 degrees. Have you ever timed a cam? I am not trying to be a jerk, just trying to help get you good results and on the water.

    If that looked good then I would put the dial indicator on the valve spring retainers and see how much lift you are getting. If you lost a cam (I don't remember if you said it is a flat tappet or not) then that could cause these issues too.

    Paul

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    Muffler City Old as Dirt's Avatar
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    Checking @ the PVC port on the baseplate is OK.. personally, I would pull the carb and flip it over.. that way you can see the little transfer slots, so you can turn the idle screw down until the little slots against the blades are in the shape of a square to a very slight rectangle.. this will get you back into the idle circuit instead of pulling fuel from the transfer slots(they are meant to help transition from idle to off idle creating a "no bog" situation with assistance of the carbs "Squirters") also reset the carb air/fuel screws on the side of the metering block 1.5 turns out as a starting point..

    without being in the idle circuit, you'll never get it to idle properly and will end up with a big flat spot getting in the gas..

    After you have done that and verified the carb base gasket is sealing good.. we can work on getting the timing right in the distributor.. you'll probably have to find a way to limit the mechanical advance so it will allow "more" initial without exceeding the 36 degrees total.. relax..

    A cam with 236/242 duration(your ballpark) and a fair amount of lift isn't going to idle smooth.. probably somewhat choppy.. but with the cam timing right, it should idle under 900rpm..
    BTW, my timing is set for 36 degrees @ 2800rpm, with 28 degrees initial.. I had to make a bushing to limit the mechanical advance.. bigger cams like higher initial timing.. and idles @ 1000rpm
    Last edited by Old as Dirt; 05-07-2016 at 11:01 AM.

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    Default Solid - hyd???

    I experienced something like this when someone put hyd lifters on a solid cam, no lash.... Waaaaay too much overlap to idle under 1500, but sounded like Godzilla above 2000..... Try loosening the rockers ONE turn across the board and see if the vacuum comes up any... Even a solid cam with solid lifters might act like yours if the valves are all too tight....

    Ray
    LOUD BOATS SAVE LIVES

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    Aquacraft Eddie's Avatar
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    I thank you all for your help!, I really want to figure this out , and I know you guys are helping and I don't feel like I'm being talked down to , and honestly if you guys need to talk down to me and it helps my situation I'm all for it,,, the last motor I rebuilt was in 1998, 351c , have not messed with a bbc since like 1990, I have done no motor work until recently ,,,except I do work on vintage motorcycles , rebuilt bike motors but no car motors since 98. I cant seem to turn the idle down at all , because it will die, and I cant get less than 20 degrees of advance at idle , if I try it will die. I can open up the secondaries on the carb and try to lower the idle and see what happens? the cam sprockets have multiple key ways and I made sure they where right unless the timing set is miss stamped?
    Last edited by Eddie; 05-07-2016 at 01:05 PM.

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    Aquacraft Eddie's Avatar
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    these pics may not be great but I think you can see the marks, I took pics before I put the timing cover on,not sure why they came out sideways, but theres a dot on the cam sprocket which lines up with the crank sprocket which is also on 0, you cant really see the dot on the cam gear in the pic but its there..actually you can see the dot in the second pic, at the top, #1 tdc..
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    Last edited by Eddie; 05-07-2016 at 01:18 PM.

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    Muffler City Old as Dirt's Avatar
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    Quote Originally Posted by Eddie View Post
    I thank you all for your help!, I really want to figure this out , and I know you guys are helping and I don't feel like I'm being talked down to , and honestly if you guys need to talk down to me and it helps my situation I'm all for it,,, the last motor I rebuilt was in 1998, 351c , have not messed with a bbc since like 1990, I have done no motor work until recently ,,,except I do work on vintage motorcycles , rebuilt bike motors but no car motors since 98. I cant seem to turn the idle down at all , because it will die, and I cant get less than 20 degrees of advance at idle , if I try it will die. I can open up the secondaries on the carb and try to lower the idle and see what happens? the cam sprockets have multiple key ways and I made sure they where right unless the timing set is miss stamped?
    You could give that a shot.. the secondary transfer slots are sitting higher in the bores and it may help to open the secondary blades some to be able to reduce the primary exposure so it can get back to the idle circuit.. what do you have to lose?
    You have a fuel pressure gauge? 5-6psi max
    Floats set at the bottom of the sight plugs?

    Timing:
    Make sure the total timing is 36 degrees or a little less @ 2800-3000 rpm, vacuum disconnected, will work with initial after that..
    Last edited by Old as Dirt; 05-07-2016 at 01:27 PM.

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