Mercruiser/Comp Cams- timing question
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Mercruiser/Comp Cams- timing question

  1. #1
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    Arrow Mercruiser/Comp Cams- timing question

    Curious...I used a 330hp rotating assembly, replaced the heads with a set of ported big ovals, installed a comp cams 11-236-4 cam, lifter and spring set. Used stock Mercruiser balancer, timing cover, and chain. Ignition is a gm hei unit with a lightly springed mech adv. No vac. adv.

    When base timing is set at 16* and total timing is @ 36* the thing pops through the carb upon acceleration and runs real sluggish.

    If the base is set up to 20* and total timing is 40* the thing idles gr8 and responds gr8.

    I have read of others that have experienced this same issue with Comp Cams.
    Is there possibly a quirk with the timing mark on the dampener being retarded? Or is the 330hp Mercruiser timing possibly intentionally retarded for the Mercruiser dizzy?
    I did not degree the cam. Is there a possibility of the cam being ground specifically for the Comp-Cams timing chain? I also have read that Comp grinds their cams with advance built into them?

    WTF is the deal here? Your thoughts?

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    Default

    Sounds like the balancer could have slipped or perhaps the balancer and the timing marker are not matched. I would suggest you verify you show 0 degrees on the balancer with the engine set at TDC.

    Doug

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    Senior Member jetboatperformance's Avatar
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    Default popping

    RE the "P" is it a constant rythemic popping or an occasional ? or does it cough and then continue , if its contant its important ot know ! Tom

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    I doubt that this is a comp cam problem. They work very well. Did you break in this cam properly? Were the springs set up on the heads properly..spring heights and pressures? Something else is going on here. There are a lot of possibilities , sounds like its out of time but you have that covered. Have you checked for a crossed plug wire? Don't be insulted, it happens...Checking the balancer for accuracy is a good idea also. Good Luck Jim

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    Senior Member GT Jets's Avatar
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    Default

    Probably a spun harmonic balancer, easy enough to check...FWIW, very common on the GM balancer Merc used, I have two sitting in my garage right now, one is almost 20* off, the other about 5* off (both 330 HP 454 Mercs).

    Plug in a vacuum gauge, the proof is in the results of the vacuum reading...

    GT
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  8. #6
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    Wink Timing

    Well guys..
    The balancer has not slipped, wires are all correct. I will certainly check for tdc. on zero. The balancer and timing cover and tab are factory Merc. which was re-used on this same matching rotating assembly. Springs are set to the correct installed height and pressures as indicated per Comp Cams.

    The pop through the carb occurs upon opening the secondaries of the carb when timing is set @ 16* and 36*. There is no miss, just slow resonse upon acceleration.
    Bump the timing up a 4 degrees and the engine is very responsive and runs and idles nice.

    I was reading that this cam should be installed @ 4* advance. The Mercruiser sprocket used did not have any advance keyway slots in it. Just the single factory key slot so that is where it is set. This should be irrevelent to the timing issue at hand from my past experience however.

    It is driving me nuts to get to the bottom of the issue being dealt. It is raining out right now so I will have to check the balancer and tdc when the weather clears.

    Thanks guys for the input. Any thoughts that may come to mind are much apprieciated.

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    Default Carb?

    Is the carb vac secondaries or a double pumper?....If its mechanical secondaries, (d/p) type it could be a lazy secondary squirter. The advanced timing "crutches" the engine so the carb doesn't bark under the lesser fuel supply at the critical transition into the secondaries. Is it a D/Pumper?....Ray

    PS Cam timing and ignition timing are not related as far as what the "mark" is showing.....
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    Default Carb

    750 mechanical. Carb has beed gone through twice. and adjustments to the accelerator pump are I could find goofy.

    Also acts the same with another 800 demon that I had laying around. I thought the same thing with the carb.

    I am glad that you clarified the ign. timing thing. I have advanced cams in the past and have never had an issue with the ign. timing.

    I will check the balancer a second time when the weather passes and post my findings.

    Thanks guys!

  11. #9
    Boat Nut sleekcrafter's Avatar
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    Try upping your squirter size on the secondary, sounds like a lean situation.
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    Default Ya but....

    Quote Originally Posted by sleekcrafter View Post
    Try upping your squirter size on the secondary, sounds like a lean situation.
    Ya but he has tried 2 different carbs.......Same results....Kinda rules out the carb. I would take a close look at the rocker action. Make sure they're all working the same as the motor is cranked over.....A lazy exhaust valve can cause popping in the carb under load conditions.....Ray
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    Default Balancer update...

    Checked the balancer and location of tdc w/timing tab location. Balancer has not slipped and is located dead.
    1.72 Roller rockers are all in sync and working as they should.
    Pulled the plugs and they are all black in color as they should be from an idle.
    Not sure where to turn from here. I guess I will just have to run it and listen for detonation.
    Goofy situation.

  14. #12
    cfm
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    What is probaby happening is that the extra ignition timing is creating more intake vacuum, which means the engine is running more effecient. Less intake and exhaust reversion, which is letting more air go thru the carb in the correct direction, thus the carb is adding more fuel. Remember, air pulls fuel out of the carb.

    Set initial for best idle. Set total, including rate of advance for best power. Set up your distributor to make this happen.

    I've run into same thing with low compression decent cammed BBc's as you are.

    Always do ignition timing before carb tuning, as carb tuning can depend greatly on timing because of how the engine will run.

    Edited due to spelling mistakes. Also, like to add combustion charge at lower rpms is pretty 'polluted' with low compression and decent cam, thus need for more timing. Ignition needs to happen earlier because combustion takes longer to happen.
    Last edited by cfm; 04-26-2009 at 05:15 PM.

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    Arrow Cam/compression

    I must agree. The dynamic compression is moderately acceptable given the use of the stock 8:5:1 compression ratio. The combination of this set of heads and the use of a steel shim gasket did bump it a small fraction but it is still on the low side.
    It just baffles me. This is a first for me.
    I have wondered the same as you on the compression thing here, but did not really realize that it would have this big of an effect. Really it makes the most sense as everything else looks good.
    I guess next winter we will have to install a set of pistons to bump it into the 10:1 ratio pending how it acts when we get the thing wet.
    Last edited by progressiveauto; 05-01-2009 at 10:35 PM.

  16. #14
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    Default Fyi..

    Spoke with Dewey today at Comp Cams. Timing is in the normal range even though it seems to be a bit advanced from the norm and is expected to run extremely well where it is.
    I love anticipating the worst. It keeps the mind busy.

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