Ignition Timing jumping around

[johno270583]

1999, 1900SR, 5.0L carbed

Have had the 'problem' for a while, but never thought too much of it.

When setting base timing, I am able to set it perfectly on the mark.

However when I unplug the base timing module, and motor is started and running at idle, the timing light shows that it is jumping around slightly.

eg.
I think base timing is 8 degrees btdc, so timing is set at this mark.

Then when I restart motor and recheck timing, it shows most of the time at about 7 degrees BTDC, then will fluctuate occasionally up to about 9-10 BTDC.

Also when the motor is revved up to about 2000RPM, the timing DECREASES to about 6-7 BTDC. Is this normal?

I am wondering if ICM Module is a problem? SHould the timing jump around? I know its not a huge fluctuation, and the motor runs pretty well, but could definitely run better.


As a bit of background, I have had a bit of trouble with it running in the year ive owned it. Most recently has been fine, except for the occasional backfire.

I have replaced Coil, leads, spark plugs, ignition sensor, rotor and sensor wheel, dist. cap.

I have also just replaced stock Intake manifold with an Edelbrock Performer, and stock carb with Holley 4160 Marine.

I have just realised I havent adjusted idle mixture screws, but doubt this would have anything to do with this issue, would it?


Appreciate the help and opinions!


Andrew

[seapuppy]

andrew....
when your looking at the timing marks....at idle...with the module unplugged...it's ok to see it move about a few degrees...but the backfire is a real question in my mind...at what point does it backfire....if it's just as you push the throttle down and it comes off idle...then it might be trying to force too much gas into the carb before the throttle plate can open quickly enough....but if it's caused by a hot intake valve or something like that..then the engine needs looking at..might be a valve lash issue....not being there makes diagnosis tough.......

SP

[johno270583]

SP
With the stock carb, it went through a phase of popping and backfiring when warmed up and had been running for 30 mins. Replaced leads, cracked coil, regapped plugs (very narrow gap out of box) and ran well most of the time. Would still occasionally pop and backfire when hitting the throttle for skiing, which I put down to stock carb running too lean and probably needing a rebuild. Also I felt lacked power.

Instead I upgraded!

I havent had the boat out since putting on the new intake and carb last weekend. Started with muffs on to set timing and idle. Gave the throttle a few hits to check the response, and it did backfire twice out of the probably 20 times I hit it. As mentioned, I have not adjusted idle air/fuel mixture (Holley reccommends lightly seating screws, then out 1.5 turns, then adjust with vacuum gauge). Would this cause it to backfire (too rich)?

If I get the chance this weekend, I will get it in the water, do some run tests, and then maybe try resetting idle fuel mix, to see if makes any difference.

Any other suggestions as to what may cause this would be good too!

Thanks

[johno270583]

Also I should mention I have been a little worried about vacuum leaks in the intake since installing. I have read spraying carb cleaner around the sealed edges is best way to find a leak. Is it, or is there any better way? I think I will knock together a vacuum gauge also to check this out.

[seapuppy]

well..first off...get an inch/lbs torque wrench and torq the carb down to about 122 in/lbs....then use a torque wrench in ft/lbs and start in the middle and torque the intake manifold down...I believe (if my ram is still working) about 35 ft/lbs...THEN!!!!...get soapy water in a squirt bottle and spray around the intake and carb base...at this point the engine should be warmed up and idling about 600-650rpm.....
..if the engine suddenly smooths out....there is your leak........you can also use a hose or one of those stethescopes for mechanics to find a leak....but the easiest is to use spray soap...ie dish soap in a squirt bottle works best.........I hesitate telling you to use carb cleaner or anyother spray due to the explosive nature or being in an enclosed space...so ..soapy water works best here.....


also "As mentioned, I have not adjusted idle air/fuel mixture (Holley reccommends lightly seating screws, then out 1.5 turns, then adjust with vacuum gauge). Would this cause it to backfire (too rich)?"

too rich and too lean will cause popping thru the carb.....when you adjust the idle mix screws...make sure the engine is off...then run them into seat (lightly) and back out 1.5 turns...start engine....let settle....then back out 1/2 turn at a time...bump the throttle a few times to get it to settle..then check with vac guage...you should be getting at idle between 10-15 in./ .....depending on where your getting the vac. reading...it will either tell you your running right or you need to adjust valves...or weak valve springs....a vac gauge can tell you alot of info if you know how to read them...



SP

[ss3964spd]

The timing decrease with RPM is wonky, and is the first place I would look to diagnose the popping/back-firing. I do not know what the timing advance curve is but it simply must advance with RPM. I don't know what the proper procedure is to check the timing for your boat's ignition system but it seems you do. I also don't know if timing advance is managed by the ICM or by old style weights and springs under the rotor. If there are no weights and springs then it must be the ICM.

If you are confidant that you are measuring the timing correctly, both base and total, and you are still seeing the timing decrease as RPM is increased, you're gonna need to find out why that is happening.

Dan

[seapuppy]

although the ECM controls spark....if it's not advancing to the curve that is mapped...then you need to have it hooked up to a computer where they can remap it...


SP

[johno270583]

well..first off...get an inch/lbs torque wrench and torq the carb down to about 122 in/lbs....then use a torque wrench in ft/lbs and start in the middle and torque the intake manifold down...I believe (if my ram is still working) about 35 ft/lbs...THEN!!!!...get soapy water in a squirt bottle and spray around the intake and carb base...at this point the engine should be warmed up and idling about 600-650rpm.....
..if the engine suddenly smooths out....there is your leak........you can also use a hose or one of those stethescopes for mechanics to find a leak....but the easiest is to use spray soap...ie dish soap in a squirt bottle works best.........I hesitate telling you to use carb cleaner or anyother spray due to the explosive nature or being in an enclosed space...so ..soapy water works best here.....


also "As mentioned, I have not adjusted idle air/fuel mixture (Holley reccommends lightly seating screws, then out 1.5 turns, then adjust with vacuum gauge). Would this cause it to backfire (too rich)?"

too rich and too lean will cause popping thru the carb.....when you adjust the idle mix screws...make sure the engine is off...then run them into seat (lightly) and back out 1.5 turns...start engine....let settle....then back out 1/2 turn at a time...bump the throttle a few times to get it to settle..then check with vac guage...you should be getting at idle between 10-15 in./ .....depending on where your getting the vac. reading...it will either tell you your running right or you need to adjust valves...or weak valve springs....a vac gauge can tell you alot of info if you know how to read them...



SP

SP

According to Edelbrock instructions that came with manifold, should only be torqued to 12 ft/lbs, which seems very loose, so we went to 18 ft/lbs, which is what the Mercruiser manual reccommmends for Vortec heads.

Didnt check the carb bolts, but will do this again, just tightened with ratchet originally.

Will go over these to double check again

[seapuppy]

I think (if my math is about right..)..12ft/lbs is roughly about 122 in/lbs.....(so my ram is close)...

SP

[johno270583]

Just deleted my post that I ha fully written out on my phone! Annoying!

Update

Took boat out for a test run, but ended up just wakeboarding!

No adjustments made prior to dropping boat in.

Started right up. When powering away from ramp, coughed and farted (ie. Popped and backfired) when trying to gas too much. Once warmed up was fine and no backfires at all.

After warmed up, tightened down carb bolts, which needed it, and adjusted idle mix screws. They were out 2 turns before adjusting, now 1.5 turns out. Haven't made vacuum gauge yet to check and to fine tune.

After initial warm up, boat ran very well, no backfires or popping. Ran 50mph easily, and was hitting rev limiter, about 5200rpm, which isn't good but just need to keep an eye on it. Previous stock carb ran 46-47mph @ 4800rpm, and struggled. Alot more pick up with new carb and manifold, as expected.

Only thing I did notice, apart from backfiring when cold, is that motor bogs down when going from standing start to full throttle, for ski/wakeboard take off. Guessing because all barrels open, and too much fuel? Is ok when I go to about 3/4 throttle, seems to take off well then.

Is there anything I can do about the bogging down, without rejetting?

Any reason that it would be backfiring when cold? I wonder if adjusting mix screws will make a difference next time out?

Apart from the above, I am very happy with performance upgrade now, though obviously still a few bugs to iron out. I will recheck timing and idle again (did check this after idle mix adjust, but not after retorquing carb). Also want to check the timing advance and what is happening there, ie. Is timing advancing as it is supposed to. Also hope the adjusting of idle mix will fix backfiring when cold.

Anything else or other suggestions?

Andrew