Can't adjust idle runnin out of options HELP PLEASE!

I have a 1983 Mercruiser 305 the serial number is [FONT=&quot]6218492. So far I have replaced the sensor in the distributor and the Ignition Module Thunderbolt IV. I have rebuilt the 2 barrel Rochester Carb and done a complete tune up on it. Here is what is going on now which I found after the carb rebuild because I was follow the procedure in the manual for after a carb rebuild as far as idle adjust and fuel mixture adjust. When I put the engine under load which for me is in gear at idle with throttle cable disconnected. I turn the idle speed screw to try to get my RPMs to between 650 and 700 what is happening is as I turn the idle speed screw I will get to a point when I get close to 650 where the engine speed will jump to about 850. What I have noticed is coincidentally the timing also changes which is why I replaced the Thunderbolt Ignition Module. So when it jumps up I then try and turn it down to 700 and when I get to about 700 it will drop below 600 and the timing will change too. If I leave the RPMs above 700 when I take it out of gear the RPMs not under load are about 1100 - 1200 RPMs If I l leave the RPMs low 550 - 600 under load I'm afraid it will stall going in and out of gear. I'm really lost hear. I'm not sure where to go the only thing I haven't changed is the distributor itself. If I change that and I still have the problem I'm done. Did I miss something? I have one other question there are 3 black wires coming out of the Thunderbolt IV module one is short and goes to one of the bolts that hold it on the manifold. Where do the other 2 go. When I bought the boat neither were connected so when I put the used one on I didn't connect the long two. It would be great it this was the problem but I doubt it. Any help would be greatly appreciated. [/FONT]

The TB-IV has 5 wires. from left to right grey, White w/ green stripe, purple, White w/ red stripe, black

The other 2 connections turned out to be 1 black wire with a large terminal on one end and a small terminal on the other that was coiled up and wrapped in with the other wires.

that might be the ground wire for the dist body.

Thanks Ed. Now I just have to figure out why the idle is all messed up.

I would check the dist shaft for side to side movement.

Brian, I'm assuming from your description, that you're question regards the idle speed screw, not low speed mixture adjusting screws.

In addition to Chris's suggestion, here's a long shot that may be worth looking at.
Look at the end of the idle speed screw. It must be nice and flat... (I.E., face perpendicular to the screw shank)
These do not always strike the stop plate squarely. If there is a hump or non-flat surface, it may actually raise the idle stop causing an increase in RPM as you turn it one way or the other.

If need be, you could chuck it up in a drill motor or lathe, and turn a fresh flat or pointed tip on it.


As you noticed, a change of only a few degrees in BASE/Initial advance will cuase a change in RPM if nothing else is touched.
Chris would know more so than I.... but I'm thinking that the ignition module should be holding a constant BASE or Initial advance at below lets's say 900 RPM. IOW, there should be no change in BASE advance from 0 rpm up to approx 900 rpm.

Chris..... any thoughts on that?




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When base timing jumps around as described above, I go right to the distributor and check the housing for wear. If this is the original distributor, it's 31 years old.

Docksidemarineservices said:

When base timing jumps around as described above, I go right to the distributor and check the housing for wear. If this is the original distributor, it's 31 years old.

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Chris.... this would be an EST system. I'm thinking that other than the distributor shaft rotation...... there'd be no moving advancing components!
Unlike a mechanical advance kettering or conventional electronic triggering system, these use a Photo-Eye wheel and sensor.
Shaft wobble would not cause a change to the dwell angle, or any significant change to the spark lead....... yes/no?

However, you no doubt have more experience with Merc's TB system than I do.


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To check the housing for wear do I have to remove it? That's no a problem I've removed and installed a few, I'm not sure I have ever checked one for wear.

briang0 said:

To check the housing for wear do I have to remove it? That's no a problem I've removed and installed a few, I'm not sure I have ever checked one for wear.

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I can't answer that one for Chris.... but I believe that he's talking about the actual vertical shaft within the distributor housing. The upper bushing may be worn.

If so, remove the distributor cap, and see if you can feel excessive side-to-side play.

If this was a mechanically advancing system..... in either VR, Hall Effect or contact points......, play here will cause a change to the dwell angle.


Since the photo-eye "rotor wheel" breaks/makes the infrared signal at a larger diameter, shaft wobble may not affect the dwell angle quite as much.
As for it affecting the BASE/Initial timing.... I doubt that as well.... but Chris may point out that it can or may.




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The fact that he can not adjust idle and it fluctuates badly when adjusting the main idle screw kind of suggest his carb base may be shot.
Maybe the throttle shaft is very loose causing a vacuum leak or maybe he is running to thin a base gasket???

One other thought is the timing chain is shot.

Put your timing light on and bring throttle of to ~ 2000 rpms and watch the timing mark........if it is jumping all over the place the timing chain is stretched out.

One other thought is the timing chain is shot.

Put your timing light on and bring throttle of to ~ 2000 rpms and watch the timing mark........if it is jumping all over the place the timing chain is stretched out.

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Would you please explain!


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When a timing chain wears out and there is slop in it, typically you can see this when having a timing light connected and watching the timing mark move when throttling up and hold it steady at 2000 and if the timing mark moves all over the place the timing chain is stretched out. Very common problem especially if the motor has had a history of over heating.

If this is the case (stretched timing chain) then maintaining a idle can be very difficult.........Idle will fluctuate because timing is not staying steady.

I will check the timing mark.... I'm using a Mercury base gasket for the intake to carb gasket. I have a advance timing light so I can bring the rpms up and still see the timing mark.

The base gasket issue is, There are ones that are very thin and cause a vacuum leak. There are others available that are ~ 1/8" thick. these work very well and do not allow vacuum leaks.....if yours was paper thin then either buy a thicker one or make a thicker one........

Also check the throttle shaft for excessive slop side to side......if the throttle shaft/bore is severely worn it will also cause vacuum leaks.

One last thing is the throttle cable, make sure the cable is adjusted to have positive pressure on the throttle shaft so it returns to base idle every time. This is adjusted with the barrel (trunion) on the throttle cable..........also make sure there is a return spring o for the throttle linkage if one is supposed to be there.......

I checked the timing at 2000 RPMs and it was solid. If I remember correctly it was around 20 degrees but i'm not positive. I checked the throttle assembly on the carburetor to make sure that there was no play and there wasn't. I made sure that I had positive pressure on throttles assemble with the throttle cable. I did all this and still had the same issue. What i did verify was that I was getting about 15 degrees of timing advance when the idle shifts by itself. I checked the distributor and there is some side to side play in the shaft. I can rotate the rotor I'm not sure if this is normal or not. I decided to pick up a distributor to see if this fixes the problem. I'm wonder if the gear on the end of the distributor could be warn.

I think the new distributor will fix your problem.

I finally replaced the distributor yesterday and I'm happy to say that everything is working as it is suppose to. Thanks to all who contributed to this thread. At least I will have a few trips out before I have to pull the boat out for the season.