TOPIC: Rebuilt 115 - poor idle still.

OK so my '84 ADI fired 115 is up & back in action again. Prior to its rebuild it was very poor at idle, you had to have it set quite fast or it would die the moment you put her in gear. Not good on the gearbox so I started her in gear a lot.

When the motor was stripped the 5 little restrictors in the inlet ports were all missing & no 3 piston was badly melted. Good, there's my idle issues methinks. So a 015" rebore on all 6, new hi domes, squared off ex ports (NOT raised), s/steel restrictors refitted (locktited & peened over) - 0.034" holes, modified reed blocks & she runs strong again. But - you guessed it - she still won't idle slowly, 900 - 1100 at best (in gear) normally the latter sometimes higher. Also if you have had it idling for a while it will die as soon as you hit the throttle & is a bastard to start again - it will do this 2-3 times before she is off again then it will be OK until it runs slow/idles for a while. Bizzarely if you pump the throttle (only part way) as you try throttle up when she is in the dying mode it will often help? When OK she doesn't bog just takes off big time. Basically in this respect she is EXACTLY the same as before the rebuild so I have missed something. This motor has always done this since I got it in 2008 no matter what I do with it.

Carbs are original (with bigger main jets due to fitting hi domes) & cleaned & serviced, float levels set as per the OE manual, backdrag system disabled by blocking the pipe. Idle stabiliser removed, different switchboxes, same stator (looks fairly new) & trigger. Modified square style late fuel pump fitted (new). Link/sync as per manual. Idle jets probably near 2 turns, maybe more. Ex relief well above the water. 22p Laser2 prop.

Can late ADI 90 carbs be used (with 115 jetting) or are they too different? I have a complete untouched 90 powerhead here.

It is feeding from a belly tank through a Quicksilver spin-on water separating filter. Now I know some of you say this is a no no but I can't see it would cause these issues & in every other respect it is good?? When I first ran it after the rebuild I ran it on a 6 gal tank with 25:1 on a separate fuel line & it was no different then, I still couldn't get it to idle any different.

I have posted this on Johns & some have suggested more timing whilst Joe Poole suggested a problem with the bleed system - I assume (he hasn't replied to my last post) he means the thin pipe that takes the oil from the bottom bearing to the top bearing. the btm check valve on this in new but not the top one.

Any help gratefully received gentlemen.

Nice detailed write-up!!
You have obviously been doing your Mercury Homework!
I see a couple things I'd like answers to though.
1) When you say 'modified reed blocks' Are you just referencing opening the stops, or did you also check & correct worn out labrynths? If the labrynths have more than 1.255" I.D. there will be too much crankcase pressure sneaking between the labrynths & the crankshaft - consequently throwing off the beast's ability to idle.
2) When you say that you cleaned the carburetors, did that include removal of the idle jet's vertical brass tube & clean the tiny holes about 1/2 way up the tube + make sure the small brass washer is still in place?
3) Have you checked for undesired movement in the throttle linkage?
I commonly find that there is too much movement due to wear between items 18, 24 & 22 on the 'throttle lever & link rods' parts break-down page. Or that the torsion spring (item 20, is not set strong enough or the shimming or center cap-screw of the throttle housing top & bottom (items 20 & 22) is dragging too much to allow the return of the linkage (item 22) back to an idle with firmness.
Between the wear on those components and the drag of the trigger wires as they exit the top end cap, I have seen as much as 10-12 degrees of 'drift' in the idle RPMs.
Just when you think you have your idle timing and idle stop set right the RPMs can change.
Also take a look at the nylon barrel on the primary throttle pick-up link to see if one side is worn off. As this barrel rotates, your primary throttle pick-up setting can change.

I can not answer whether or not swapping to the 90 adi carbs would be okay.

I see no reason why the fuel wouldn't feed well from a belly tank & through a water separator. This set up works best with the prime bulb placed between the tank & the separator.

'Nuff fer now.
Doc F

Hi Thom

Thank you for your prompt reply.

1) I modified the 'throat' of the reed blocks (removed nut, bolt & web), they are not the originals but replacements as the originals were worn as you say. I cannot say if they are worn beyond 1.255" a I wasn't aware of that measurement when I built it.
2) Short answer - no!
3) I did wonder about that, will go through it bit by bit. I have had trouble getting max advance at times as the trigger wire drag does overcome the spring tension at times.

Thanks again, Geoff

Geoff,
I just remembered something else that could be causing you idle issues.
The SeLoc Manual shows the Primary Pick-Up should be set at 4-6 BTDC on the adi 115, but Mercury shows that it should be set at 5-7 ATDC.
Another thought,...Since you do have an adi 90 there, be sure and not use the 'Throttle Cam (item 18 on the "throttle Lever & Link Rods page in the parts Break-Down) off the 90 on your 115.
It is different and consequently opens the butterflies at a different point at different amounts.

Just a note for everyone:
Merc states that Primary Pick-Up should be set at:
1-3 BTDC on the 90 hp adi
5-7 ATDC on the 115 hp adi
4-6 ATDC on the 140 hp adi

Doc

Geoff, another thing to consider is you have increased the volume of the reed block when you removed the web. I did the same. That is enough to hinder fuel draw and atomization at idle. If your reed block labyrinths are worn as Thom has mentioned you could get erratic operation at idle. Another thought might be the correct pick up points are ATDC as Thom has mentioned. This may have been designed with the idle stabilizer in play. The idle stabilizer adds advance if it detects a drop in idle speed. It may well be that if you are running without an idle stabilizer (as I am) you might have to add some timing to compensate.
Food for thought, what do you think Thom?

Dave,
I never really considered the need for Primary Pick-Up timing to be altered with disconnection of the Idle Stabilizer. Interesting thought.
I must confess that I don't really understand how the IS works when it is wired in, but I don't think that either the timing or carburetor linkage moves based on the IS - does it???
Inquiring minds want to know.
Thom

Morning Thom, the idle stabilizer electronically delays the timing via the switch box. There are no mechanical changes involved.
It does this through the bias inputs on the switch boxes. Think of it as a "timing reservoir". The stabilizer holds back X degrees of advance. If you time the engine with the stabilizer connected, say at 21 degrees, you are mechanically timed at 21 Plus X and electrically timed at 21. Thats why when the stabilizer dies you burn pistons, you just added that timing X back in.
At idle the stabilizer looks for consistent idle speed, if it senses a drop off it adds timing in an attempt to recover.

Thank you gentlemen, I will give it a thorough going over when I get a chance, won't be for a week or two though. Some very useful pointers here.

Well the week or two turned into a month but I finally got on it this morning.

First thing I checked was the throttle advance/retard body (in two bits with a spring in between items 19 & 22 (on Midas) that the trigger arm attaches to) for movement compared to the throttle cam (18 - opens the carbs) & there was a LOT of play. So there was no way you could accurately set the timing/sync at idle as it could move a LOT. It really needs a new shaft (22) but I trimmed the ends off slightly in the lathe & put centre pop marks in the recesses each end into which it fits. Now there is no play at all between the two ends of the shaft which has to help. I also altered the trigger wire clamp that sits on top of the water jacket above no 1 cyl so it allows the trigger wire harness to move without restriction as it is only being moved, in effect, by the spring (20) in the linkage. I will order a new shaft at some point so I have it in reserve.

I have run out of time now so I will re-time her on Monday & check a few more bits out then. I am off to Falmouth in Cornwall next week for 4 days fast boating down there so I will report back if things have improved at all.

What do you chaps feel about timing her both idle & max spark with her running & the throttle cam on the top carb disconnected so the carbs don't open? I feel it could be more accurate but welcome feedback?

Hmmm, no one has an opinion on timing it running then?

One word gentlemen. Timing.

Checked it this morning & it was set about 10-15 deg ATDC (this is with 90% of the slack taken out of the advance mechanism, see above) so I adjusted it to 5 deg but found if I used a screw driver it would advance with the weight of the screwdriver pushing against it so I adjusted it with my fingers with it pulled back against its stop to 5 deg. The adjuster screw is waaay different to where it was before. The reason for the slack is due to the worn shaft the assembly moves on (item 24 on MIDAS) - I have sorted the rotational movement (as mentioned in an earlier post) but there is still lateral movement on various points of the shaft. I have a new shaft on order (pt no 87937).

Anyway I fired her up - she started easily & idled well (if fast), adjusted the idle down to about 1000 in N & she was sweeter than she ever has been. Put her into gear & she sounded fine - although it was only for about 8-10 seconds before the drum emptied. So I head off to a rally tomorrow MUCH more confident that I will be able to set the idle as it should be - finally.

So a word to the wise all you ADI chaps, sort any play in that advance assy before trying to set it up.

A big thank you for all your help gentlemen.

I've been following this as I'm hoping to build an ADI motor. Thanks for the great tips and sharing how you got through it. Please post back on how she runs once on the water.
~Jersey~