View Full Version : Fast Idle after warm-up

Dan K
07-26-2004, 08:57 AM
Can anyone offer some insight into this problem for me?
1981 S&S 351 Powerslot electronic ignition.
I had my carb rebuilt (4160) as preventative maintenance since it has been awhile. After reinstalling the idle has been giving me problems.
It will idle low when cold and will stall when put into gear unless I give a little throtle imediately. Then after it warms up the idle is around 1100 rpm or so which is pretty high since the boat moves pretty fast at this rpm. When I try to lower it down by adjusting the idle screw it will die out. I changed my timing got 10 BTC it was at 14 but my valve cover indicates 10. I also tried to adjust the idle air mixture screws but this didn't produce any better results when I lowered the idle.
I have to say that the boat ran better before the rebuild but sucked a larger amount of fuel than it usually did. The fuel consumption is better now, but the idle isn't what I believe it should be.

lakes Rick
07-26-2004, 10:48 AM
I would check my choke for proper adjustment and look for a vacuum leak...

Dan K
07-26-2004, 12:46 PM
Thanks Rick,
The only possible vacuum leak I could find is on the port that is near the electric choke coil. There is a suction on this port, but I don't know what it is for. Do you have any ideas. I wish I had a picture of it handy it would make it easier to explain.

lakes Rick
07-26-2004, 01:48 PM
Hopefully Jimn will sign on here..

two things...

1. Is your choke a hot air or electric??? If its hot air I would say your are missing the tube from the intake to the choke...

2. If your choke is electric you need to plug the unused vacuum port at the choke.... If its what I think a brass cap or possibly a rubber vacuum plug would take care of it....

3. Spray some carb cleaner at the port.. IF your idle changes thats the problem.... From what you have said, sounds like a vacuum leak....

07-26-2004, 02:18 PM
Check the throttle plate to see how far open it is at idle. It should be almost closed. Remove the throttle cable when you check the idle. I have seen these stick and stretch, giving more or less throttle than it should. Talk to the place that rebuilt it and they should be able to help you out here.

Dan K
07-26-2004, 02:35 PM
I have an electric choke, I have covered the port with my finger to see if anything changes and could not notice a difference. I will trying spraying some carb cleaner to see if there is a difference.

Jim, The entire rebuild experience was a nightmare. I took it to a performance parts place that sent it to their rebuilder and was promised delivery in 5 days. 9 days later when I needed the carb for a two week vacation they told me they still needed some parts. But the performance place did not know which parts the rebuilder needed. In 1/2 hour I located two different locations that had complete marine rebuild kits for this particular model, I notified them of this and they said that the rebuilder found the parts he need and would have it completed at the end of the day. I got this carb in the nick of time since the boat is kept at our cottage and we were on our way. This is why I usually do these things myself.
So I don't know how much help these guys are going to be, I believe they did a proper job, the only things that I may suspect are not optimal would be the powervalve and jetting, but these do not come into play with my idle problem as far as I know.
Do you have anyother thoughts ? The idle is high even when the cable is disconnected, since I always disconnect it to adjust the carb.

07-26-2004, 10:37 PM
They need to get it back and adjust it right. 1100 is way too high and is hard on things when it slams into gear. It should be closer to the right setup than it was when you got it back and they should guarantee their work and the work by their carb rebuilder.

Dan K
07-27-2004, 11:02 AM
I agree with you Jim. They should, but I do not have a lot of recourse with this and they are 2 1/2 Hours away. My best bet is to adjust it properly myself.
Do you have a suggestion on what order to tackle things ?
looking for and fixing vacuum leak
Setting timing
Lowering idle
adjusting Idle air mixture
any other items you may suggest.

07-27-2004, 02:13 PM
Why don't you have much recourse? They had it done for them, so they need to cover it not working properly. I know it's a haul to get there, but if they sent it to someone to have it done, it's as if they did the rebuild.

Dan K
07-28-2004, 11:34 AM
I certainly can take the carb back to them and ask them to get it adjusted properly, but then I will be faced with the downtime during my peak usage. I am just trying to get this to a livable situation until September when I won't mind the downtime as much, or ideally get it adjusted properly myself with no downtime and no need to get the performance shop involved.

Ron Grover
07-28-2004, 12:22 PM
I would look at adjusting the electric choke mechanism. I had a 1981 but it was a few years ago. If I remember the choke mechanism you should be able to loosen the adjustment and turn it slightly to allow the choke to release once the engine is warm. Sounds like from your problem the choke may not be releasing. Then adjust your idle screw on the other side to set the proper warm engine idle speed.

For future reference, I have a 1993, 205 that has a carburator. It was acting up so I called Holley and they have a rebuild service. Sent carburator off to them and within 2 weeks it was back to me looking brand new. Put it on and the only adjustment I made was to increase idle speed. It was set to about 600 rpm and that was too slow, engine would sometimes die during idle. Sped it up to 800 and it purrs like a kitten. Total cost for this Holley rebuild was $312 that includes shipping. I did mine in the winter, Holley said they have an express service that is faster, (more money too). I'm not complaining about cost at all considering how well mine runs now and I know it was probably done by people that REALLY know that carburator.

07-28-2004, 04:14 PM
Maybe your mechanical advance in your distributor is sticking or your advance-weight springs are broken.

Michael Gamble
08-01-2004, 07:17 AM
Dan, after it warms up, can you back the idle down and it still run? I tend to think along the lines that your choke is giving you fits. I'd check the butterfly to see if it's opening like it should, and verify that your choke is working properly. I'd be willing to bet that's the problem.

Dan K
08-02-2004, 10:35 AM
I had a chance to work on the boat last Friday and have solved the idle problem after several trips to the local parts store (automotive). I isolated it to a vacuum leak between the carb and the 1" riser plate I have the pvc vavle tied into since I have a edelbrock performance manifold. When I replaced the carb after being rebuilt I used the universal gaskets that I was provided. It turns out that there are several bolt holes on this gasket and two of the unused holes happened to overlap slightly with a couple of the channels cut into the riser for the PVC Vacuum. I found these by spraying carb cleaner arounfd the carb and noticed a surge when sprayed in a couple of spots. I went to this small town outo parts tore and ended up in the back room looking at every gasket they had, none fit like they should so I cut my own from a piece of 1/16 thick gasket paper using the riser as my template. I was able to them put it all back together and get this baby to idle down around 700 rpm without dieing out when put into gear. What an improvement. Had a great day skiing after that.

09-13-2004, 11:52 AM
Great to hear that your problem is solved, Dan. I have an 88' w/ 351, and am noticing a similar problem. The carb has never been adjusted or worked on, except when I tweaked my idle after installing an electronic ignition.

The idle screw is the larger screw, with a spring backing, near the throttle linkage correct? What is the smaller flat-head screw, on the same side of the carb, and where should it be set at? Could be possibly be time for a rebuild on this carb?


Dan K
09-13-2004, 12:19 PM
I would check around the Carb for a vacuum leak as I did, by spraying carb cleaner at different points to see if it changes the idle speed momentarily. This quickly lead me to determine were my leak was located. As far as rebuilds go this site was the most informative for me.
Carb Rebuild (http://www.fordmuscle.com/archives/1999/12/holley/index.shtml)

Keep us posted on what you find.
As far as the other screw that you are talking about, I can't place it, can you post a picture of it or describe if it is vertical or horizontal and the approx location front or rear?

09-14-2004, 08:26 AM
Thanks for the response, Dan. After closure inspection, the forementioned screw is on a horizontal axis (as opposed to the vertical axis of the idle screw), two inches or so from the front of the carb, and about in the middle of the housing. It appeared to be backed out substantially, so I brought it in 2 turns or so to make sure it wasn't going to fall out. Again, not sure what it is, or what I just adjusted. It's a small screw, flat head, approximately 1/8" in diameter. Thanks for your help.

Dan K
09-14-2004, 09:29 PM
The screw you are talking about is probably the idle air adjustment screw, there is one on each side and they are supposed to be backed out about 2 turns. If you turned it in you made your idle mixture too lean and this will cause stalling. Read the previous link, this will explain proper adjustment. My first idle problem dealt with the gasket missing around these screws, causing a small vacuum leak.

09-15-2004, 09:26 AM
Great information, Dan! I'm concerened about a vacume leak at this point that the previous owner compensated, by bringing out the air adjustment screws. Or possibly (hopefully) over the years, the air adjustment screws may have just worked their way out, because when I checked them the other day, at least 6 full threads were exposed, and then I brought it in 2 turns. leaving 4 threads exposed.

I'll go ahead and take them in all the way tight, then back them out two turns as you and your article suggests and see what happens. Sound good you think? The boat has been stalling some during idle and gear engagement. Hopefully that was simply due to the fact that it had been running too rich. :confused:

09-15-2004, 05:23 PM
Two turns out is a STARTING point. you need to (gently) bottom the screws and then back them out. I usually start off by backing them out 1.5 turns.

Then with the engine FULLY warmed up, at idle in neutral, you want to screw one side in and out to find the "sweet spot" that the engine likes the best. You're looking for the setting that yields the highest idle speed, and the smoothest running. Then move to the other side of the carb and do the same thing. Hopefully, if done correctly, your idle will be nice and smooth at this point, and if it was as out of whack as it sounds, the idle speed should be much higher, due to the engine getting a much better air/fuel ratio.

SO, now you need to screw the THROTTLE STOP screw out, to lower the idle back down the the proper setting. Doing this make the idle circuit in the carb more sensetive, so now you can go back to the idle a/f ratio adjusting screws and do the same thing over again. first one side to achieve the best possible idle, then the other side. Some times it's good to go back to the first side just to "make sure".

After that, make a final adjustment to your throttle stop screw to get the desired idle speed, and that should be it. It should now have a solid, stable idle, and not stall in gear.

If it does still have issues, there is another problem elsewhere.


09-16-2004, 09:47 AM
You are red hot, Tom! Awesome explanation and detailed instructions, I completely understand. The flusher is coming out tonight, and the driveway will get watered. Thanks a million.


09-16-2004, 04:30 PM
No prob. Let us know how it worked out!


09-20-2004, 06:42 PM
It worked out perfectly, Tom. Thanks again for your guidance. The screw near the throttle linkage was almost all the way seated, and the other side was 1.5 out. After adjustment, the boat is idleing 75% better no question.


01-05-2016, 03:05 PM
Not sure if I missed it but can someone tell me how to adjust an electric choke. I have been told to turn the choke to the right, with the ignition off, until it opens. Then back off until it just fully closes. Then go one more tick to the left. Does this sound right?

01-05-2016, 03:36 PM
Not sure if I missed it but can someone tell me how to adjust an electric choke. I have been told to turn the choke to the right, with the ignition off, until it opens. Then back off until it just fully closes. Then go one more tick to the left. Does this sound right?

With the engine fully cooled down, loosen the 3 screws and turn the choke cap so the choke closes with a light amount of pressure. You may need to fine tune this a little. If you can't find a setting that works well, you may need to replace the choke cap. The choke should open fully within 90 seconds of turning the key on.

01-06-2016, 07:12 AM
With the engine fully cooled down, loosen the 3 screws and turn the choke cap so the choke closes with a light amount of pressure. You may need to fine tune this a little. If you can't find a setting that works well, you may need to replace the choke cap. The choke should open fully within 90 seconds of turning the key on.

Thanks a lot. You are the Guru for Mastertcraft boats.

01-06-2016, 11:56 AM
Thanks a lot. You are the Guru for Mastertcraft boats.

You're welcome, but you are way off on that Guru thing. There are still plenty of things about these boats that I have no experience with.