Mercury inline primary pick up 4-6, which is better, 4 or 6?

[tavacska]

My motor is 1977 mercury 1150, serving me well, except it has to be set around 1000 rpm not engaged on idle to not bog down. If setting at 800 rpm idle non-engaged, it has 10% to bog down in some certain situation, such has rough water area. When engaged, it's always 650-700 rpm.

I've read recently that the primary pickup degree has effect on idle performance. Just curious for 4-6 degree, which one could be better for idle performance. My understanding is 6 degree but could be wrong. My current primary pickup is 5 degree, and my idling is 5.5 and just trying to open the throttle.

 

[racerone]

Open low speed mixture screws 1/8th turn at a time and test run.----I suggest you leave timing alone.

 

[Texasmark]

600-700 is the right range idling and in the water boat moving in F gear per the manual....slow enough to shift without a "klunk" yet fast enough to keep the engine running. I agree with Racer on what he said.

On later engines of that size idle speed is set by engine timing so changing the jets would help your hole shot yet may not affect your idle rpm. If so, on later engines just adjust the spark advance for the correct number.

On your engine that is probably powered by a belt driven magneto I don't know how the idle speed is controlled but it has carbs with a low speed adjustment screw.....I know that much, having had an '88 115 hp "tower" once.....one fine running machine....I loved it.

 

[racerone]

Need to understand that idle ---" primary pick up " ----is the point where throttle plates start to OPEN.-----So in fact that has nothing to do with how the engine idles.-----So a " no money " spent procedure is to open low speed mixture screws 1/8th turn at a time till you get smooth acceleration !

 

[Chris1956]

Normally in those engines, idle pickup timing cannot be set to a 1* tolerance. It can be set to 4-6* BTDC, and will show on the timing light, that it is always in that range.

Racerone is correct that hesitation is likely caused by a too lean idle mixture.

Just so you know, the top carb will tend to run a bit leaner than the mid carb, and the bottom carb a bit richer than the mid carb, when idle screws are set the same. Thus the idle mixture must be set on the water, motor warmed up, in gear at idle. The trick is to open the mixture screws, one at a time, top carb first, then mid-carb then bottom carb. Open them 1/8 turn at a time and test after each adjustment. Make sure to use freshly mixed 50::1 fuel mix, as old mixture can lead to bad adjustments.

Eventually you will find the correct compromise between good idle and smooth acceleration. You may need to adjust the idle speed at the end to get it back to the correct RPM. You want about 1000RPM in neutral, so it will runat the correct RPM in gear to avoid stalls.

 

[tavacska]

Thanks everyone for your feedback, always.
Whenever I am not able to get on water, I'd like to find some trouble with my boat.

So my boat works good last season. It has 1000rpm idle non-engaged, 650 engaged, 5000rpm at WOT with 17 pitch on a 17 ft boston whaler. The powerhead is new, I mean brand new for 2 years. Ignition system is distributer + trigger, all brand new. Has a enricher installed so the cold start is fantastic.

My only concern is this 1000 rpm idle. I really wish it is 800 rpm non-engaged and 650 engaged. 1000 rpm introduced no trouble. But 800 rpm really keeps me always worried.
When I was with 800 rpm, it bogged down after I rapidly turned full throttle to idle. I believe it is the water coming along with the full speed boat causing more load on the prop. It also occasionally stalled when I tried to launch the boat.

Last week, I re-wired the plug wire of 40 years old and re-checked the timing. My pointer is 2 degree off TDP. I use the 0.464 method precisely and repeatedly this time. I don't know why it can be off, because last time it was also set by the same way of 0.464. So the max timing was not 20 deg, it is 18 deg now. The primary pick up was not 5 deg, it's 3 deg now. I have redone the primary pick up to 5.8 degree and max advance to 19 degree. I am also switching the fuel from 87 10% ethanol to non-ethanol 87.

I hope these adjustment can help to set the idle to 800 rpm non-engaged.

As back to the primary pickup timing, some guy mentions when you have more advanced timing in idling, you will have more power under load in idling, thus more stable idle performance. For example, when I use idle screw to adjust the idle rpm, 0 deg - 6 deg will increase the rpm from 500 to 1000 rpm, then the throttle is picked up at 6 deg, and more power will come from the throttle as I keep advancing the timing from 6. If I set the primary pick up at 4 deg, 0 - 4 deg will increase the rpm from 500-800 rpm; the 4-6 deg will increase rpm from 800 to 1200 rpm because more fuel kicks in. I want more room for the idling to increase the power purely from advance of the timing not from the open of throttle. So I think 6 deg is better in my case.

But the other guy also mentions some race guys use 21 deg as the primary pick up. It will be advanced to max timing before opening the throttle. That is just an extreme case as others point out, which will cause detonation and damage the engine.

So far, I have set up the timing to 6 deg at pick up and 19 deg at max, and switched to the non-ethanol fuel, with a brand new set of spark wire. Hope I can tune it up at 800 rpm. BTW, I am also setting the mix screw with the method repeatedly mentioned in this forum.



Edit:
Some other thought here, maybe it's better to ask here:
1. I use the static method to set the timing. But I did check it with a timing light. The static method is pretty accurate, at cranking, idling. But some mentions at full advanced and WOT, the max advanced is 2 degree less. If it is set at 19 deg at cranking, it will be 17 deg full speed. This statement is not trust-worthy, but still worth to ask.
2. My motor is 115 and I set it to 19 deg max. Most time I cruise around at 3500 rpm and 25 mph. So I guess non-ethanol 87 octane is good enough? No need to the premium?

 

[racerone]

Simple feed back here with few words.----If motor is idling too fast you adjust the idle stop.------Idle pickup is where throttle plates opens.----If motor bogs on acceleration the richen the low speed mixture.

 

[merc850]

Use the idle stop screw to lower the idle rpm (turn it out to lower). You might have to adjust the cable to bring the distributor to the screw

 

[Chris1956]

Those motors require the timing belt to be installed when the pointer on the distributor pully points to the dot on the timing decal. Often they are 1/2 tooth off. You can change yours by 1 tooth, and the idle may improve. This adjustment is outlined in the OEM service manual, I did not make it up.

Not sure you can get 850 RPM in neutral and 650 in gear with a 17" pitch prop. The available power at idle may not allow that.

Advancing the primary pickup timing, changes the synch between carburation and timing. Might cause lean running.