Engine has small tremble - Team Camaro Tech
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post #1 of 7 (permalink) Old Jan 5th, 07, 06:40 AM Thread Starter
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Steven
 
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Engine has small tremble

This is a small blck 350, recent rebuild with vortec heads, electric fuel pump, SMI Stage 2 carb, stock point dist with crane conversion kit ... new water pump, alternator, headers, GM performance Intake, Taylor wires, NGK Plugs , etc ...

The builder said he degreed the cam 12 degrees

This is the cam ... Comp Cam XR276HR

http://www.compcams.com/Technical/Se...umber=12-423-8

The engine seems to have a small tremble when idling. Is this solely due to cam specs or does it indicate some other kind of problem? Revving is fine... WOT runs appears to be fine ... Mid throttle acceleration also seems to stutter a little bit. I am looking for some tuning advice. I use a Sears dial-back timing light, but I sort of question it's accuracy. Setting the dial to 30, then adjusting the timing to read 0, I am assuming I have set 18 degree of static timing. Am I interpretting this right? Applying a little throttle, I run about 45* of total timing. Please correct me if I am blowing smoke here.

I want to know if I should look at fuel or ignition issues or, whatever you guys may suggest .... Thanks in advance ... Steven
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post #2 of 7 (permalink) Old Jan 5th, 07, 08:45 AM
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Re: Engine has small tremble

IMO, after reviewing the cam card, I believe he should not have degreed the cam. Plus, 12 degrees? This is a great amount.

Ibelieve the cam should have been installed straight up because it already has 4* advance ground into it. Lobe Separation Angle (LSA) is 110* ans Intake Lobe Center Line (ICL) is 106*. So any more advance, and its going to be a step or two above stock with the duration & LSA.

Plug in a vacuum gauge and see what the intake vacuum reading really is.

Your subtraction of timing, you dial in 30* and adjust for 0*, you have timed in for 30*. Now if you have 48* at cruise rpm, 48-30= 18*. But at what rpm?
Initial timing is taken with no centrifugal advance and no vacuum advance. Unplug the vacuum line, and idle down the engine to below 1,000 rpm. Measure timing.
Now, rev up engine to say 2500-3000 rpm, and measure timing. This will be centrifugal timing.
Now plug in vacuum hose and measure the change. this will be vacuum advance.

Hypothetical:
Initial 10*
Centrif. 14*
Vacuum 12* Total timing at 3000 rpm is 36* BTDC.

Once timing is straight, you can view the vacuum gauge and adjust mixture screws for max vacuum.

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post #3 of 7 (permalink) Old Jan 6th, 07, 08:31 AM
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Re: Engine has small tremble

I think you must be mistaken on the cam degree statement. No one moves a cam by 12 degrees.

Best to check static (idling) timing with vacuum advance disconnected and get the idle as low as possible (500 or 600 rpm), so that the centrifugal advance is not kicking in at all. Then, as you rev engine, you should see timing mark move. Dial-back timing lights can be confusing, so be careful. For initial timing set I would use no dial-back. Just make sure you can get timing at idle set to 8 to 12 degrees.

I agree with most of what Everett said above, except the centrifugal advance allows much more then 14 degrees. It is actually around 22 to 24 degrees, which starts kicking in as you rev engine and should all be in by about 3,000 rpm. This means you would have a total of about 34 to 36 degrees of timing (with vacuum advance DISCONNECTED) at about 3,000 rpm.

For maximum performance and driveability, you'll definitely want to experiment with initial timing and with the advance curve (both mechanical and vacuum). Here's a nice web page that talks about timing curves: http://www.corvettefever.com/techart...curving_power/



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post #4 of 7 (permalink) Old Jan 6th, 07, 01:50 PM
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Re: Engine has small tremble

The example I gave was hypothetical. As said by Chevy-SS, dial backs can be confusing, except for the high dollar units, Snap-On & MAC.

It may be better to measure the circumference of the harmonic balance, divide by 360 and mark it every 5* and put the dial back at zero. Also, a good check point/calibration for the tool. If you divided and measured with some accuracy, the mark method will be a better tool.

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post #5 of 7 (permalink) Old Jan 6th, 07, 03:05 PM
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Re: Engine has small tremble

Everett, not trying to be too picky, but I just wanted the guy to be sure he obtained 34 to 36 degrees of advance WITHOUT the vacuum advance.

Yes, I agree with you about marking the damper. I use a "Fluidampr", which is already marked, but I used to measure and mark dampers all the time, back before dial-back timing lights. Way back before those lights, heheh. Marking the damper is pretty much foolproof.

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post #6 of 7 (permalink) Old Jan 6th, 07, 03:32 PM
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Re: Engine has small tremble

Hypothetical:
Initial 10*
Centrif. 14*
Vacuum 12* Total timing at 3000 rpm is 36* BTDC.

Sounds a little low Everett. I know you state it as hypothetical, but maybe more like
Initial : 14
Centri : 20
Vacuum : 12
Total Timing would be 46 at cruise with vacuum and a Mechanical+Initial of 34

You didn't list your distributor make/model, I would dial in 14 at idle with vacuum plugged, set the springs and/or stopper on your distributor for a 20 degree at 2500-3000 rpms (whatever your 75mph cruise speed is), then hook up full manifold vacuum and see what timing you get at idle (800 rpm), you are looking for 26 at idle, if its higher, use a 3/32 allen to turn the adjuster inside the vacuum canister a full turn counter clockwise for each digit over, or full turn clockwise for each digit you are under. That is if your vacuum canister is adjustable.

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post #7 of 7 (permalink) Old Jan 6th, 07, 05:58 PM Thread Starter
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Re: Engine has small tremble

Hey guys, I appreciate the responses. Yeah, maybe I am mistaken on what I thought the guy said he degreed the cam ... I thought 12 was sort of out there also... I'll have to get give him a call. My invoice lists degreeing the cam, but it doesn't list how much ...

Anyway, I got caught in honey-do land today and didn't get a chance "play" with the car. I will definitely find some under the hood and around the road time Sunday. I'll post some numbers then ...

Thanks,
Steven
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