350 rebuild gremlins still lingering

Discussion in 'Small Block Tech' started by 70skylarkcusto, Oct 8, 2018.

  1. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    As the title suggests I’m having issues with a rebuilt 350 for over a year now. I’ve posted in the past on issues related to the rebuild but here is the FULL rundown. If anyone has any suggestions please let me know where to go next.

    Had my 350 rebuilt over 2 years ago now. Since the beginning the motor has never run right, I’ve had low and bouncing vacuum readings at idle, and a very rough idle. runs OK under wide open throttle but still sounds like something isn’t 100%.
    350 bored .030
    TA-284 cam
    I’ve checked ignition timing, tried multiple HEI distributors, checked compression and done leak down tests with 5-7% loss so everything seems to check out. I’ve had the carburetor and ignition professionally tuned. I’ve checked for vacuum leaks, checked firing order, ohmed out my plug wires, checked ground and anything else I could think of. Yet the motor STILL won’t idle smooth no matter what I do. I understand I have a slightly bigger cam in it, but this almost seems like an intermittent miss.

    I’ve beem pulling my hair out over this for the last 2 years and am finally out of ideas. If anyone has any ideas at all or any suggestions I would appreciate the input
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Was the cam degreed? What vacuum is it pulling at idle? I would do a compression check.
  3. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    I actually do not know the answer to that question only because I physically didn’t rebuild the engine. It was built by a very reputable engine builder, but I can only assume it was assembled properly.
    I’ve done a compression check, all cylinders were within 5 psi and ranged from 146 to 151 psi in each cylinder

    Vacuum at idle at the highest reading I was able to achieve was 13-14 inches with the needle bouncing
  4. Mark Demko

    Mark Demko Well-Known Member

    Maybe the pushrods are too long, holding a valve or valves slightly openo_O
    Any noises from the valvetrain?
  5. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    Funny you mention that, I actually did have an issue with that, the valves that were installed had valves that stems which were different lengths than the original, however the pushrods are now the correct length and there doesn’t seem to be any valve train noise.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  7. 70skylarkcusto

    70skylarkcusto God, Country, Cars

  8. buick64203

    buick64203 Just plum crazy Staff Member

    I hate to ask dumb questions but....

    Is it possible that what you consider a rough idle is just the idle a TA 284 cam has in a 350? I had a 212 in a 455 and it had a noticeable lope to it. A TA 284 is a larger cam than a 212 and its in a 350. I'd think it would sound a little lumpy. Regardless, its not going to idle smooth like a stocker
    MrSony likes this.
  9. Mark Demko

    Mark Demko Well-Known Member

    Could you post a video with the car idling?
    The way an engine idles tells a lot :D
  10. Stevem

    Stevem Well-Known Member

    You did not state what your running for a Carb, but if it's the stock Q jet then the motor will only run ok once it's in the Cam's power band!
    The stock Q -jet was never set up to run at low rpm ( under 3000) properly with the reduced amount of vacuum that the overlap that size Cam makes in that cid motor.
    Screw making a second compression test and get your hands on a leak down tester or a adapter to pump up each cylinder with air at TDC and listen for leaks out the Exh, back into the Intake Manifold or into the Crank case with a valve cover off.
    Entropy11 likes this.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yes, if 1 cylinder does not drop RPM compared to the others, it is weak.

    I second Jason’s comments about the 284 idle, it will be lumpy.

    Also, the Q jet needs to be modified to run right with that cam. Stock calibration won’t work.
  12. telriv

    telriv Founders Club Member

    Yeah, things like idle air bleeds need to be modified for that, AND MANY OTHER cams. With the lumpy cam you have changing jets/metering rods isn't going to cut it. Ignition timing is another thing. It needs A LOT of initial advance to get ANY TYPE of low end. MHO is you need to install an A/F ratio meter with a bong in ea. exhaust pipe to tell where you're really at.
    Just my 2c.

    Tom T.
    Entropy11 and 300sbb_overkill like this.
  13. buick64203

    buick64203 Just plum crazy Staff Member

    A wideband is the way to go for advanced tuning. And its a bung Tom, although after all this he may need a bong. LOL. Sorry, couldn't resist. Made me laugh

    You put the right carb on a car and get it tuned properly, you would be surprised how much better the car will idle. That wideband transformed my Aquamist car
    MrSony likes this.
  14. partsrparts

    partsrparts Silver Level contributor

    What is your idle RPM? I'm running 10 to 1 comp. with a custom ground cam from JW at TSP and it won't idle smooth under 800 rpm in gear, 1000 in park.
    I live at 5300' elev. and can not pull more than 12.5 vac at 1000 rpm in park. I also have a AF meter and at idle it runs 12.5-13.2 it will not idle any leaner. Running a Holley 750dp with 4 corner idle 1/2 turn out on primary screws and 1 turn out on secondaries. Timing is 16 initial 36 total at 2600. I've been messing with it for a year and am now very happy with the way it runs!
    I'm not trying to hijack this thread just wanting to give the OP different ideas to look at.
    One other thing is check your motor mounts, when I first installed the motor it rock an rolled bad until I found a bad motor mount. Installed poly mounts and smoothed things out quite a bit
  15. buick64203

    buick64203 Just plum crazy Staff Member

    It will idle leaner if you put in different idle feed restrictors.

    No Holley is plug and play. They need to be calibrated to the invididual car. My new 950 has been apart three times already and it's coming apart again
  16. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    I have seen the 284 in other 350 engines and it might have a slight lope but my engine shakes the whole darn car. This doesn’t seem like lope from a big cam, it seems more sinister than that. I’ll try to take a video and post it today.

    I am running a stock q jet, i really hope that could be my problem, that’s the only thing I haven’t changed or checked. My compression and leak down tests yielded nothing out of the ordinary. I did have the carb and ignition timing tuned by a local speed shop who used an a/f meter to set everything. I was upset when I got the car back and it still idled hard. My next plan was to bring the car back to them and have them try a different carb on there to see what that might do for me.
  17. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    My idle is set around 750 in gear and 950 in park. The lower the idle the more rough the engine gets (surprise) but it should never run as rough as it is. The higher the rpm the better it runs but it isn’t until about 1500 rpm that it finally starts to smooth out. My ignition timing I believe is almost identical to yours. I have it written down somewhere I’ll have to pull out the paper how it was finally set
  18. Matt Knutson

    Matt Knutson Well-Known Member

    Wiped cam lobe and or cam not degreed properly
    alec296 likes this.
  19. 70skylarkcusto

    70skylarkcusto God, Country, Cars

    I know for a fact the cam lobe isn’t wiped. At one point I thought that might have been the problem so I had it gone through again by the engine builder and he reported the cam break in was fine and the cam and lifters looked as they should. As for the cam not being properly that is a possibility as I did not the to rebuild myself. But again it was done at a very well known shop, not to say they don’t make mistakes
  20. alec296

    alec296 i need another buick

    If the valve stems where at different heights, sound like a bad valve job. Valve stems should all be even . you should do a leak down test. You can narrow it down by doing the cylinder test mentioned by pulling a spark plug wire one at a time to see what cylinder has least drop of idle or no effects on idle.

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