Merc 150hp late 70's inline 6 - Carb question

[bga4]

Hey, so finally got my boat out on the water, motor seemed to be running perfectly while on the trailor, starts no worries, idles nicely, rev's up with no problems etc. However when under load I couldnt get it past 12kph at full throttle.

So I thought first thing to check would be the cable adjustment (as I had to take them off while mounting the motor etc) so ive adjusted the cable and seems to be correct - full throttle on the controller appears to move the throttle arm to max, so im assuming thats correct for starters, as I said it revs right out (I dont have a tach, but judging by how loud it is I wouldnt want to go to much higher, at least not out of the water)

What I did notice though while adjusting the cable was that while moving the controller to full throttle I noted the long black plastic bit thats parallel with the block (1 of the two verticle bits that join all three carbs), on the top carb that links to the middle carb was attempting to turn and it appeared to be twisting instead of actually smoothly turning as one... so the question is, while at full throttle or even during advancing the throttle should those two black plastics bits that link all three carbs been turning in sync and turn evenly as one? im assuming they open the carbs up, but what im not sure about is whether or not all three carbs open the same amount throughout the rev range or do the bottom two carbs open up at different times depending on the rev's etc. I wouldve thought they would open evenly so each cylinder gets an even spread of fuel/power etc.

my theory is that the middle and bottom carb isnt opening sufficiently to give enough power to go beyond 12kph. What confused me though, is that the motor doesnt skip a beat, so if the bottom two carbs where not in sync (or opening sufficiently) then surely it would be obvious, ie. not running smoothly, rough idle or missing etc.

sorry for long winded dialogue, just not sure whats the bits an pieces are called, let alone how theyre meant to work. Im about to order a manual but I know from experience that people on here can often explain things easier than a manual can

Thanks in advance
Blaire

 

[Laddies]

Re: Merc 150hp late 70's inline 6 - Carb question

NEVER free rev a engine or you won't have to worry about how it runs it will be blown up. You need to do a complete Synchronization on the engine and it probably would not hurt to clean the carbs while your at it, a engine that runs good without a load or back pressure on the exhaust will not necessarily run in the water and the idle must be adjusted in the water in gear to make the engine run right.

 

[bga4]

Re: Merc 150hp late 70's inline 6 - Carb question

Laddies: thanks for prompt reply. haha I get your point about free revving an engine, I wasnt thrashing it so with any luck no harm done.

I did wonder about syncronization, ive read a few forums about link an sync etc, seemed easy enough.

So with that in mind, im assuming then that if all three carbs are sync'd then that means they do operate together?? ie. all three should be open the same amount at the same time?

thanks

 

[quicktach]

Re: Merc 150hp late 70's inline 6 - Carb question

If you're going to start working on the motor, get yourself a manual for it. Study the motor with the manual in front of you and get very familiar with both. Set the motor to spec. Take your time. Carb disassembly and cleaning in order too.......

 

[Chris1956]

Re: Merc 150hp late 70's inline 6 - Carb question

I assume you are discussing an inline 6 motor. I will further assume you are talking about the black plastic connectors that connect the middle and bottom carbs to the top carb. The three carbs need to operate as one. If the mid and bottom carbs do not open all the way or don't open at the same time as the top carb, you need to make an adjustment. The top of the mid and bottom carbs have a small bolt. this allows you to preload those plastic carb connectors to have them open at the same time.

 

[Chris1956]

Re: Merc 150hp late 70's inline 6 - Carb question

Also, make sure you have good compression and good spark on all cylinders. Inspect the spark plugs for consistant color. Advise on any that at black or real clean.

 

[bga4]

Re: Merc 150hp late 70's inline 6 - Carb question

Also, make sure you have good compression and good spark on all cylinders. Inspect the spark plugs for consistant color. Advise on any that at black or real clean.

Hey Chris, thanks for taking the time to reply, an yea your assumption is correct, sure is an inline 6, as per thread header haha

also im pleased you answered my question about the fact that the three carbs are meant to open at the sametime -as I wasnt sure and didnt want to go chasing ghosts. But yea thats definately not happening now, I can manually turn the bottom plastic bit that links the 2nd - 3rd carb which appears to open the third carb slightly, so it would seem that the link on the top of the 2nd carb is where the problem is, as I cant move that one at all....

Good point about checking the compression and colour of plugs etc, will get onto that today and will post back with my findings. I know the plugs are quite new and I had them out not long ago and nothing seemed out of the ordinary, im quite experienced with 2 stroke motorbikes so I know what a fouled plug looks like, discolouration etc and there was definately nothing untoward about these ones, which is encouraging I guess.

Im contemplating just pulling the 2nd carb out, as probably the easiest to get too anyway and breaking it down giving it a good clean up, if need be replace gaskits etc, that way I know for certain.

When you say bolt thats used for preloading the carb, im assuming you mean the one thats actually fixing the plastic bit to the carb?

If I cant figure it out will post a pic to make sure im not tampering with the wrong one.

Anyway watch this space.

Cheers,
Blaire

 

[Chris1956]

Re: Merc 150hp late 70's inline 6 - Carb question

Blaine, Those IL6s were known for leaking water into the bottom cylinders via bad crank seals, or any cylinder via bad exhaust gaskets/baffle/ inner exhaust cover. if you see brand-new looking spark plugs on a cylinder or two, suspect water infiltration.

If distributor model, watch the spark plug wires for arcing to block.

 

[bga4]

Re: Merc 150hp late 70's inline 6 - Carb question

Hey chris, quick update so i pulled the middle carb off an sure enough its seized up good up proper, the throttle shaft was completely stuck, huge mission to get out, looks like sea water or something was left inside the carb an simply corroded the shaft virtually welding it in place.

Eventually got it out an cleaned it up, need a new shaft now but everything else looks gd.

Was able to have a good look at the other two carbs and they look good, thankfully.

Couldnt find my compression tester so thats my next job. No doubt its buried somewhere.

Will take a gd look at plugs as you mentioned an will see what theyre like an will post back.

On plus side, this motor has cost me next to nothing, so either way it goes its been a great way to learn haha

Cheers

 

[bga4]

Re: Merc 150hp late 70's inline 6 - Carb question

Ps. it is a distributor model aswell, once its back together ill fire it up at night seeif it puts on a light show.