Bizarre Idle Situation

1977 Mercruiser 228 (350)
Symptom 1: Runs great at higher RPMS, cuts off when bringing back to neutral at about 1100 RPM. I have got it to idle great at about 700-750 RPM twice, by adjusting distributor, timing 8 degrees above TDC. However, I go back after a while and try it does the same thing and nothing has changed from when I had it idling perfect.
Symptom 2: Blower switch kills engine.

What I have done:
1) Factory reman Q-jet
2) Removed intake manifold, new gaskets, etc.
3) Vaccuum pressure good
4)Timed right on
5) Sprayed carb cleaner to check for leaks
6) Adjusted idle mixture screws
7) Tested neutral safety and shift interuptor switches, both good
8) New gimble bearing, bellows, etc. (shouldn't affect my problem, etc.)
9) New fuel line
10) electronic ignition

I am stumped, could it be linked to the obvious blower problem or could mechanical advance on the distributor be froze. I took it off and the big spring was a little corroded. I have finally given up on the vaccuum leak theory.

Thanks.

Re: Bizarre Idle Situation

I would look at 4 things.

#1 by above TDC you mean Before top dead center right?
#2 the blower must be shorted and it must be on the motor harness in error instead of the boat harness.
#3 some vacuum leaks will not show up externally. use a gauge.
#4 unhook interuptor switch from coil and try it.

Re: Bizarre Idle Situation

Thanks for the info. Yes, before TDC. Also, I have used a vacuum gauge and that's how I determined good, steady pressure. I will definately try the interuptor switch.

Re: Bizarre Idle Situation

Hate to ask a silly question BUT does the carb have the normal idle screws? Saw a California rig with seals over the adjustment. ( they went into the file 13)

Re: Bizarre Idle Situation

Beachcraft 1 said:

Thanks for the info. Yes, before TDC. Also, I have used a vacuum gauge and that's how I determined good, steady pressure. I will definately try the interuptor switch.

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What did the vacuum guage indicate? A good vacuum should be around 20hgm at idle. Did the needle bounce around, vibrate, etc? Q-jets are very sensitive to small bits of dirt getting in the jets, tubes and passages. Too many things to go wrong on an old carb. I got so fed up with my old quadrajet and replaced it with a Edlebrock 1409. All my problems went away and it's sooooo easy to adjust when necessay.
A model 228 is a 305 cu in. You can verify that by looking on the port side rear block just below the head.

Re: Bizarre Idle Situation

The gauge was steady, 19-20 at idle, that is why I no longer suspect vacuum problems. Also, I put the 350 in the 228 replacing the old 305. I HATE this Q-jet, very finicky. To answer Lyle29464's question, the idle adjustment screws are accessible and open, although they are the water valve, straight screw driver style which I hate too.

Re: Bizarre Idle Situation

Are you running a wedge place under the carb or is it mounted directly to the intake? can you reposition your vac gauge to the vac pulloff port on the front of the engine? this will give you a reading of ported vac and if its running lower than 15 you have a leak somewhere. when you have it running, snap the throttle, watch the gauge as the engine RPMs come back down, if it drops close to or below 11 on the way back down, you have a leak, even the most tiny one will cause the q-jet to freak out and i couldnt detect mine by spraying around the base.

If national put yours together with matching jets and rods on the primary side, then you should be able to achieve 800 rpm if you set the idle mix screws 4 turns out and the idle speed screw 2 turns in (assuming all base gaskets are 100% sealed).

if you opened the carb at all (as i did) replace the airhorn gasket, especially if you have even the slightest tear anywhere (even a small one where its cut to go around the primary rods).

Re: Bizarre Idle Situation

Does the blower work?

Chris

Re: Bizarre Idle Situation

Has the coil been replaced recently? I replaced my coil with an MSD blaster 2. While I was at it, I cleaned all the contacts. My distributor has a Pertronix module. After doing that, my engine (2.5L) would only run at hight rpm. Turns out, the blaster coil was such a low resistance that I had to add a resistor inline because the pertronix module was cutting out after I cleaned the contacts because it has to see at least 3ohms of resistance or it will cut out! Hope this helps. Also, really check into that blower thing. YOu may have a short that is cutting the ignition out.... Hope you get it fixed...

Re: Bizarre Idle Situation

Scott Farley said:

Has the coil been replaced recently? I replaced my coil with an MSD blaster 2. While I was at it, I cleaned all the contacts. My distributor has a Pertronix module. After doing that, my engine (2.5L) would only run at hight rpm. Turns out, the blaster coil was such a low resistance that I had to add a resistor inline because the pertronix module was cutting out after I cleaned the contacts because it has to see at least 3ohms of resistance or it will cut out! Hope this helps. Also, really check into that blower thing. YOu may have a short that is cutting the ignition out.... Hope you get it fixed...

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I have a high performance coil and pertronix ignition and took out the external ceramic resistor becuase I thought it needed a full 12 volts. What contacts are you talking about? Coil, etc.? Why was the pertonix module not cutting out at higher rpms? Where exactly did you add the resistor, between positive coil and positive ignition (pertronix) or positive igntion and positive coil?

Re: Bizarre Idle Situation

chrismurr1 said:

Are you running a wedge place under the carb or is it mounted directly to the intake? can you reposition your vac gauge to the vac pulloff port on the front of the engine? this will give you a reading of ported vac and if its running lower than 15 you have a leak somewhere. when you have it running, snap the throttle, watch the gauge as the engine RPMs come back down, if it drops close to or below 11 on the way back down, you have a leak, even the most tiny one will cause the q-jet to freak out and i couldnt detect mine by spraying around the base.

If national put yours together with matching jets and rods on the primary side, then you should be able to achieve 800 rpm if you set the idle mix screws 4 turns out and the idle speed screw 2 turns in (assuming all base gaskets are 100% sealed).

if you opened the carb at all (as i did) replace the airhorn gasket, especially if you have even the slightest tear anywhere (even a small one where its cut to go around the primary rods).

Click to expand...

I have been running the gauge off the choke pull-off as I have no other vacuum ports on intake manifold. I have a spread bore intake, so no adapter. Is that what you mean by wedge? Should I be using a small amount of Ultra Black, etc. on the intake gasket?

Re: Bizarre Idle Situation

because i am running straight inboards, there is a plate under my carb that lifts the back of the carb up 12 degrees to get it back to level. same spreadbore but a different setup. if your setup mounts the carb directly to the intake then that is one less thing to leak i wouldnt put anything on the gasket at all, just make sure your surfaces are as clean as possible and if you running a 1/4" gasket, it should seal any minor imperfections when it compresses.

Im going agree with others, first things first, correct the blower switch issue.