Mercruiser Carb Rebuild - Idle Mixer/speed questions

tobyguy

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Nov 27, 2007
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Have a 4.3L Mercruiser with a 2 barrel carb (807764) on a 2001 Bayliner 1850. Bought a spare carb locally from someone who had the same carb (got it from someone who didn't winterize their boat properly and cracked the engine - so they were parting it out). Paid $50 for it, so figured it would be worth it to just swap carbs in and out as I rebuild them (to not be down if I can't get one going for whatever reason). Carb looks in pretty good condition (fairly clean).

I've been looking at the manual at the carb rebuild instructions (and youtube videos). Seems fairly straight forward. I believe normally these carbs are suppose to have a tamper resistant idle mixer screw shield to stop people from adjusting it (the one I picked up does not, guess someone removed or rebuilt it).

I just ordered an Ultrasonic Cleaner to make sure it's cleaned thoroughly. So has been a bit of an investment so far.

Anyhow, the steps I'm uncertain about (my question) is if I have to set the Base Ignition timing, ground purple/white wire to engine, use a "shop" tachometer, timing light, etc. etc.

I've noticed on some threads people are not mentioning performing the above, just setting the Mixer/speed screws (and I assume they just use their tachometer on their dashboard).

Didn't want to have to purchase any items if not really required (or set the Base Ignition timing if not required)

Appreciate the pointers and thanks in advance
 
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Fun Times

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Hi there, the cover over the mixture screw didn't start being used till a bit later than 2001... Think it was more around 04ish for non TKS carbs... TKS was 05+.

On some engine models, getting access to the air mixture screw to turn it can be challenging so be prepared for that before hand just in case.

The below info is some info taken out of a Mercruiser service notice when the TB 5 system first came out in late 1994.
Timing Engine or Adjusting Carburetor
Because the Thunderbolt V Ignition Module has Idle Speed Spark Control, it has to be locked in ?Base Timing Mode? whenever you:
Set or Check The Timing
Adjust Engine Idle RPM
Adjust Idle Mixture
BEFORE turning ignition key on or trying to start the engine, connect a jumper wire from the Timing Lead (PUR/WHT wire, see wiring diagram) to a good ground. This locks the Module into ?Base Timing Mode?. Now any of those three adjustments can be made.
IMPORTANT: The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the ?Base Timing Mode?.
Adjustments are made the same way they were done on Thunderbolt IV engines. The Idle Speed Spark Control in the Module is only active within a certain RPM range. Because of this, engine idle RPM has to be set exactly to the specified RPM shown and in NEUTRAL GEAR. The boat can be either in the water or on a trailer.
IMPORTANT: Use an accurate shop tachometer when setting engine RPM because it has to be at an exact RPM in forward gear.
Timing at Idle RPM: 10? BTDC
Idle RPM in Neutral Gear: 650
IMPORTANT: Be sure to disconnect the jumper wire between ground and timing lead or ignition will remain in ?Base Timing Mode?.
 

tobyguy

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Nov 27, 2007
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Thanks for the responses. Guess I better purchase a tachometer then.

A couple of follow up questions.

1. Can I use a laser digital tachometer? If a laser tachometer is suitable, where could I place the tape? (see picture - wheel in dead center of engine?)

2. 2nd question - it says "connect a jumper wire from the Timing Lead (PUR/WHT wire) to a good ground".
Here's a picture of my engine. I see a Purple/white cable on the top left of the picture (looks like there's a plug I can remove and simply wire it there). Does that look like the right wire? Also, where can I attach the other side to? Where's the best place to ground it?

thanks in advance!
 

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Bondo

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Guess I better purchase a tachometer then.

1. Can I use a laser digital tachometer? If a laser tachometer is suitable, where could I place the tape? (see picture - wheel in dead center of engine?)

Ayuh,.... Welcome Aboard,... The Best investment might be a digital tach/ timin' light,....

One tool that can give ya the Info of several tools,....

I've got this one, bought it at my local Napa,...
INN-5568.jpg
 
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Fun Times

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Yes, the purple/white wire you see with the black rubber plug is the engine service wire you'd ground out to any clean ground connection whether it be a bolt, negative side of a battery post (< black or yellow cable), etc. 'that you can find/reach. Use a near similar type of wire size as the engine service wire and attach the other end to ground.

If you already own a laser tachometer then give it a test before adjusting to see if it works for your working procedure. Like Bondo, I have the same timing light that has a built in tachometer gauge that make it more convenient as it's right there in your one hand vs using two hands for the same job.

Also check the front of your engine for a timing tab that has the numbers on it already like these for example, http://www.bing.com/images/search?q=GM+engine+timing+tab&FORM=HDRSC2

Some engine models have a tab mounted to the engine that may or may not have markings on it. Other engines may have markings on the harmonic balancer.

In most cases, marking the line/s with something like a white grease pencil helps you see the timing marks while using a timing light.

Hope this helps, good luck.:)
 
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