Trying to figure out missfiring

Discussion in 'Street/strip 400/430/455' started by josehf34, May 17, 2018.

  1. josehf34

    josehf34 Well-Known Member

    My '70 455 for some unknown reason is missfiring on the cylinder #3 and I can't still figure out what's going on

    Already changed 2 spark plugs, checked compression and is 120 PSI Dry / 135 PSI WET (10W30) pretty much like all the other 7 cylinders.

    I feel like I could have a burnt exhaust valve on that cylinder , I tried putting a sheet on paper on that exhaust tube and it sucks the paper, but as far as I know if the valve were burnt I shouldn't have compression on that cylinder, or at least not 120 PSI and taking in mind that all the other 7 cylinders are between 115 and 130 PSI so is pretty much like compression on that cylinder is "fine"

    Also something really weird that I've noticed is that if I don't torque the rocker shaft mounts but instead just barely tight the nuts the engine idle is awesome and vacuum @550 RPM is too

    I think another chance could be sticky valves due to car has been sitting like 30 years but again, should the cylinder have compression if it has sticky valves?

    Any idea what should I be looking for?
    Last edited: May 17, 2018
  2. buick64203

    buick64203 Right wing conservative Staff Member

    Odd issue for sure. How does it run with the rocker shaft bolts torqued down? Is this a bone stock 455? Rebuilt? Milled heads? Hydraulic or solid lifters?

    Just a thought, but maybe the pushrods are a bit too long?
  3. Stevem

    Stevem Well-Known Member

    Was a valve job done to the heads and was the block and or heads milled?
    Also if you hook a timing light up to the cylinder and if flashes then you at least know that plug wire is ok!
    DasRottweiler likes this.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    My thoughts as well. I’d get an adjustable push rod, adjust it for 0 lash, and 1/2 turn more, then measure it and compare it to the others. 120 sounds low for a 70 motor.
  5. Stevem

    Stevem Well-Known Member

    Wouldn't be the first time that a C clip popped out of a stock lifter and made proper adjustment impossible!
    HotRodRivi likes this.
  6. josehf34

    josehf34 Well-Known Member

    I don't have idea if engine has ever been rebuild, I think not because the car has been stored since 1988 but maybe there's a chance. I think the issue doesn't has to be with long push rods, maybe more with oil pressure because If I finger tight the rocker shaft the engine inmediately starts running rought

    Here's a video about what I'm talking, is a bit hardly to notice but when the the shaft is adjusted the engine idle quality immediately deteriorates

    Also I'm pretty sure that these rocker shaft and arms are from a 400/430 instead 455, There's no way I can find some 455 rockers where I live (I'll have to order it from U.S) but is there any modification needed to be performed in order to successfully uses this kind of rockers on a 455?
    Last edited: May 18, 2018
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Oil pressure has nothing to do with the issue. If you tighten the rocker shaft down and the engine starts to run rough, it is because the valves are being held open a bit and not sealing. You will burn a valve if you don't correct the issue. More than likely, the push rods are too long. Get yourself a push rod checker and see what the correct length should be, then compare it to what you have now. Stock push rod length for the 400-430 and 455 was 9.350"

    You can use 400 or 430 rockers on a 455, you just need the correct push rods. The ball end of the push rods is different on the pre 70 rocker arms. The push rods are also solid for a pre 1970 engine. The 400 and 430 oiled the rockers through a passage way in the block and heads, the 455 oiled the rockers through the lifters, and hollow push rods. You may just have incompatible parts there, but it can be corrected. There's no way to know what mistakes were made on this engine unless you can ask the previous owner?

    More info on oiling differences between the 67-69 400, 430, and the 70-76 455. You can interchange rocker assemblies with the right parts.

    Last edited: May 18, 2018
  8. josehf34

    josehf34 Well-Known Member

    previous owner never used the car, he's a collector that buys and stores cars so this modification was done previous 1988 so there's no chance to know what else was done on the engine. I think is very probably that the 455 push rods are still there, is there a way to identify 430 and 455 push rods with only taking them out ?

    I'm not sure if the issue is about valves hanging open because if I tight the shaft only with my fingers the issue appears and I'm not 100% sure but I beleive that isn't enough to have a valve hanging open, also, I think the push rods should be very hard to turn by fingers with the piston at TDC if they were hanging open, or am I wrong? Is the first time that I'm chasing an issue of this kind

    I'm going to check it today and post feedback
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    455 push rods are hollow. I believe the original 400-430 push rods are solid. Read the attachment I posted, The 67-69 push rods have 3/8" ball ends, 70 and up have 5/16" ball ends.

    With the engine running, you could easily hold a valve open a bit if the push rods are too long, by tightening the rocker shaft bolt down. I'm am sure the bolts are well lubed and it doesn't take much to turn them. I can't think of anything else that could account for the symptoms you describe. Measure the ball ends and the overall length of your push rods.
  10. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Looks to me like the #3 intake valve is barely moving like the lobe is flat. I'd replace the lifters anyway. I bet the lash cups are stuck. Either way with the intake off you can see what is going on.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Good pick up Joe. It is harder to see with the plug wire in the way, but it does look like it is moving less. I wonder how tightened down the pedestal bolts are.
  12. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I've been thinking about this more. Pull the rockers and put a straight edge across the valve tips. I bet you'll find that exhaust valve is tall. I think the seats have sunk therefore increasing the stem height which would have the same effect as long pushrods.
  13. BadBrad

    BadBrad Got 4-speed?

    You've likely already ruled this out (and I can't tell from your initial note when you have a miss), but plug wires can be rubbish with cylinder pressure from a load, and perfectly fine at idle even when the ohmeter says it's trash. Check the wire for resistance; I've seen exactly what I've described.
  14. No Lift

    No Lift Platinum Level Contributor

    When you tighten a rocker arm as you're doing while the engine is running after the rockers are loosened the cup inside the lifter has raised itself to a higher level than would be allowed normally by the pushrods. As you tighten it down the lifter doesn't go back to where it belongs instantly because the oil pressure is holding it up. Therefore the valve is held off the seat for a period until the lifter cup can collapse slightly to allow the valve to seat fully giving you the rough idle for a bit. What you're doing is a non-test. Now if you loosened them and the idle got better then you may have something. Along the same line a slightly long pushrod should not affect the idle quality because the lifters will collapse to a point where they reach a balance with the oil pressure vs. spring pressure and they will stay in adjustment and everything will work normally. Not good for performance but not a bad idle either. The odds of one being abnormally long is small with your mash-up of '69 rockers with 70 pushrods and lifters. It may have too much preload for max performance but it wouldn't cause an idle problem and if it did what are the odds that it would only affect one cylinder. Even a missing c-clip in the lifter shouldn't make a difference because once the lifter cup height and the valve spring pressure has balanced out the cup will be where it belongs. The lifter cup should never ride up against the retainer anyway unless it is a special "no preload" type like the Comp Pro Magnums. Obviously you cannot do an adjustment of that lifter to set preload but it should work ok unless the cup has come out of the lifter body but that is whole 'nother thing.

    I agree with Joe on checking the stem height and Brad on the plug wires which speaks for itself. The worn exhaust seat is quite possible. In my experience it usually causes an off idle miss at lower rpm, say 1500 to 2000. You can feel it in the engine as you rev it lightly. Probably because it doesn't seal completely when the rpm goes up. At low or cranking speeds it has time to land on the seat and seal it. I think a worn intake lobe would tend to lower the compression test pressure. However I've had worn lobes on an engine and it really didn't make a difference in idle. Off idle power maybe. Remember unless the lobe is very bad the engine may make slightly more torque down low just like having a smaller cam.

    If you want to try to see if preload is the problem because of the mash-up you can loosen ALL the rocker bolts maybe one turn exactly from tight. A 5/16-18 bolt would loosen up .055" with one turn. Run it for a bit and see if idle improves possibly indicating too much preload. If the lifters tick then they are too loose so maybe try 1/2 turn loose which would be about .026" looser. At that point though preload probably isn't your problem because from the factory GM probably has it set up for about .060". Remember you cannot tighten them and expect a good idle until the lifter have collapsed slightly and returned to balance.

    I have used adjustable 5/16" pushrods with the '69 rockers and had no problems. Not the best thing to do but it worked. I may be mistaken but I believe that setup with stock length non-adjustables may actually have less preload then normal because of the 5/16" ball end on the 70 pushrods and the 3/8" cup on the rocker arm.
  15. Stevem

    Stevem Well-Known Member

    Get some white out and some brakekleen.
    Wipe All the oil Off the push Rods somewhere where you place a white mark and you or someone else can see it with the engine running.
    Bring the motor up to 1500 rpm and note how each push rod is spinning.
    Those that spin at a slower rate are a sign that the Cam lobe and lifter are going south and or shot already!
    The push rod should fully spin 1 time for every time that valve opens and closes.
    You will never get the needed proper lifter adjustment on a valve like that period!!!
  16. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    You have to pulll your rocker shafts , or at least bump it to top dead center for the cylinder in question and check to see if the valve is depressed. Big rocker cups with small rod balls arent a good mix either. You dont want the valve depressed on top dead center.
    Your compression also is way down. 150 for a worn out motor , You should be 160 to 180 psi. But if your pushrods are slightly too long that would explain why the low nums all the way around. I would also pull that pushrod and use a light to see if the clip is still in the lifter.
    150 psi is just my interpitation of a worn out motor. You can still get lots of miles out of a motor with 150psi.
    Last edited: May 19, 2018

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