'74 Merc 650 OK slow, won't go fast

Cohoho · Jan 10, 2003

Hello, all you helpful souls, I have just bought my first boat - a little 16 foot Bayliner with a Mercury 650. It had been sitting for a couple of years and would not start. With some new gas (sour fuel was most of the problem) and a little tinkering, I got it running, and it runs pretty well. I tried to replace the water pump as a preventive thing, but the bottom half only comes off a half inch or so, then it appears that the drive shaft or shift shaft (or both) prevent it from opening any more. So I punted on the water pump replacement. Also I tried to take the prop off, but it seems to be stuck. So I took it out today and it starts and runs good. But the shift/throttle lever only puts it in gear, and barely gets it above an idle. I can throttle up with the warm-up lever, but not fast enough to get on a plane. I am hoping that it is something simple like an adjustment in the throttle linkage cables. My other suspicions/fears are that it might be a gummed up carb, a spun prop, or any number of things that haven't occurred to me.

I will have some time this weekend to tinker with it a bit. I would appreciate any suggestions for what the problem might be, and any tests I can do to confirm or eliminate these possibilities. Thank you in advance for your assistance.

mellowyellow · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

1) get a manual if you don't have 1 2) get that lower unit off and change impeller 3) pull and clean carbs and get a rbld. kit 4) rebuild fuel pumps, new diaphrams etc. 5) check to see if prop is spun. mark it and see if the mark moves. any shop will have a puller. 6) do a compression check on cylinders, to start pull the plug wires 1 at a time and see if RPMs drop (they should) if no, that cyl. ain't firing a few hrs. of your time and a few bucks in parts and you'll be all set (Ihope) good luck! M.Y.

Cohoho · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Thanks, Mellow Yellow Here's how I'm doing on your suggestions: 1) I have a Clymer shop manual - it's the first thing I got when I bought this boat - but it is only of marginal help. It is somehow too specific and too vague all at once. 2) It was actually a Mercury shop that tried to get the lower unit off to get at the impeller. They warned me that forcing it might get the thing open or might pull the shift shaft out of the gear box, requiring a rebuild. The motor has a decent indicator stream, so I thought I'd just keep an eye on it. But you really think it's critical to get that impeller replaced? 3)and 4) How big a deal are these rebuilds? I'm not super experienced with this sort of thing. My credo has always been to figure out everything that might be required to repair things, then I start by fixing/eliminating the cheapest/fastest/easiest options. I was still hoping that this thing was as easy as some adjustment. I'll do the rebuilds, but I'm not excited about doing them if they're not necessary. 5) I'll mark the prop and check it. It was the same Mercury shop that couldn't get the prop off. Remember that some of these parts have been mated for almost thirty years. But It seems that if it spins on the shaft it ought to come off of the shaft with some persuasion. 6) We did a compression check and all three cylinders were 150-160. I will try removing wires while it's running to make sure they're all firing. Thanks for all the suggestions. I saw some adjustment screws where the throttle cables connect to the engine. I am still thinking I should monkey with these a bit and see if I can't get it running a bit faster. Any guidance on this front? Thanks, Sea Lemon

alcan · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Hi Sealennon Make sure you return the warm-up leaver to the down position. Then shift to foreward, then push further forwarward for more throtle. With the engine turned off, check to see if the butterflies in the carb are open at full throtle. If not, disconnect control cables at motor and try by hand. I am a firm believer in the credo, if it ain't broke don't fix it also, but if you can't remove your prop and lower unit, your motor is broke and this problem needs to be repaired.

rickdb1boat · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Sea Lemon, # 3 and #4 are a must do on a Motor that has sat for a long period of time. It's not that hard to do if you have the manual, which you do. Just go slow and use a clean area to work on them. As for # 2, this is also a must do because the impeller, being rubber, has surely rotted from non-use over the years. It may put out a good stream now, but it will quickly fall apart and then you will not only lose the cooling, but you will have rubber parts floating around in the water passages, which can clog up easily(Not Good). As for the prop, it should be splined on the shaft, so I am a bit confused when you say that it "Spins" on the shaft. Unless of course, it has a shear pin design.(Mine Didn't) I would also be looking at the Throttle Cable as part of the slow running problem, but I am sure the Carbs are causing at most of your problems. Have someone push the shift/throttle lever(Not warm up lever) all the way forward from neutral and observe to see if the Carb plates are opening all the way. Also, check to see if the timer base is advancing when doing the same procedure. Let us know what you find out.

Cohoho · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Thanks, everybody: Boy do I feel stupid (but relieved). Alcan hit it on the nose. User error.

I had not returned the warmup lever back to the down position. So the main throttle/shift would only get so many revs. So I was using the warmup to throttle once I had it in forward. I put it in the water today and everything appeared to work correctly while tied up at the dock. So I took it out for a spin and kept the warmup lever down, just used the throttle/shift. The controls are a little strange - I move the thing 90% of its range before it gets above an idle. But it does throttle up and it popped out of the hole and got on a plane just like a regular boat!!!

Hooray. It works, and it took me no money and almost no time.

Now what to do? I'm torn between "if it ain't broke don't fix it" and Rick's point that rebuilding the carb and fuel pump and replacing the impeller are "must do's". I don't want to ruin my motor, but I also don't want to spend a month of weekends working on a 30 year old motor that might run fine (and currently does) without any more work.

rickdb1boat · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Sea Lemon, If you got her running good, then I would just be concerned about the water pump only, at this time. If you don't replace the impeller, just be sure to keep an eye on the cooling system output to be sure it's still pumping OK. It's up to you to decide weather to run it like it is or go forward with the repairs. Who knows, maybe you are the lucky one! Good Luck which ever way you go.

alcan · Jan 11, 2003

Re: '74 Merc 650 OK slow, won't go fast

Hi Sealemmon Yes I agree with rick. The water pump should be serviced. This isn't a very expencive to do. Most will say it is cheep insurance. Better $15. now for an empeller than a new motor later. Also not being able to remove your prop and inspect for fishing line is a big problem that needs attention. I am not real sure as to the exact rating, but I'll hazard a guess. These two issues are at the top of the outboard killer list. Fishing line is massively abundant at all boat ramps. It wraps around the prop shaft and either screws under the prop shaft seal, causing a leak or forms a melted mass which rubs and burns up the seal, causes leak. The prop shaft should be inspected on a regluar basis,after every trip in some cases. Your water pump empeller should be serviced at least annually. Don't be to concerned about this motor being 30 years old, a good running motor is worth taking care of. Most outboards don't wear out, they get killed. I'm gald to hear yours' is runs well. Now it't up to you to keep it that way. Have fun.

Cohoho · Jan 12, 2003

Re: '74 Merc 650 OK slow, won't go fast

Thanks, everybody! I got my boat out for a real test today. Yesterday I launched in a no-wake zone on the canal, and could only just get on a plane before I had to throttle down. today I took it out on Puget Sound and It ran perfect! I am really thrilled, and I think I love this motor. I agree that it is a good motor and worth protecting/fixing/maintaining. I will try to get at the impeller, but I want to go over what the shop said after they failed to get the bottom of the motor off. Keep in mind that these are experienced outboard (and specifically Mercury) mechanics and have all the right tools, and have seen a lot more situations than I have. They said that the lower unit started to come off, but stopped after a half-inch or so. They said that either the drive shaft or shift shaft was stuck where it splined in to something. They said that I could force the thing, maybe driving some opposing hardwood wedges in between the upper and lower, but I might cause more harm than good. They said the stuck shaft (whichever one it happens to be) might come undone at the wrong end. Apparently if the shift shaft is stuck and I pop it out of the gearbox I am screwed. What do you guys think? I know the repair wants to happen, but how do I go about it so I don't buy myself a lower unit rebuild in the process? Any pointers, techniques, opinions??? Thanks

Troy_from _Oz · Jan 12, 2003

Re: '74 Merc 650 OK slow, won't go fast

Sea Lemon, I too am a complete novice to boating mechanics and recently (and successfully) changed the impeller on my 71 merc 650. Mine did the same thing - came about 1/2in apart then stopped. I started by using two prise bars (screwdrivers) and a fair amount of force to no great avail (other than minor damage to the LU - not reccommended). Out came the wedges and bingo - the lower unit dropped free - after that it was plain sailing. (remember to support the LU before it drops!). had the whole lot finished within a bout an hour an a half - and im a scientist - so not that mechanically adept. I guess the moral of the story is that if i can do it - anybody can. I cant vouch for your seized shaft though - but if you decide to change the impeller - consider giving it a go for yourself. If it is going to break, I'd rather break it myself than pay a mechanic to do it! Best of luck with whatever you decide. Cheers - T

Troy_from _Oz · Jan 12, 2003

Re: '74 Merc 650 OK slow, won't go fast

Sea Lemon, Just another thought, with the seized prop - my manual says that if the prop wont come off you need to apply heat to the prop shaft. It does caution to stand back as the rubber hub is likely to explode! This I think Ill pay the mechanic to do

-which is contrary to my previous post with respect to paying to break stuff, but if its going to explode then thats another story.... T

alcan · Jan 12, 2003

Re: '74 Merc 650 OK slow, won't go fast

Hi Sealemmon These are good questions. This is the reason your mechanics stopped work. They know this will require time. And since time is money, it wouldn't be long before they had more labor in this unit than it's worth. Now I am also willing to bet they didn't try very hard to get your L/U off. The shift shaft dose spline together at the L/U separation. And they do get stuck. There are several ways to go about this situation including the wedge idea. Unfortunitly there is no guarntees. This is a make or break deal. How ever if left as is, your motor will only last as long as the empeller. And since things seem to be stuck I'd say it's been to long since the last service allready. If you do attempt this job, take your time, and be carefull. As far as pulling the shift shaft apart at the wrong place and being screwed goes. If no other damage occured, I'd call that a win. Sorry to say it, but you're already screwed,it just hasn't hit you yet. I feel from what you note, that you have better than even odds of total victory on this one. You do have a manual right? If not this will be your first step. Keep us up to date.

Cohoho · Jan 14, 2003

Re: '74 Merc 650 OK slow, won't go fast

Ok everybody, You talked me into it. I'm ordering the h20 impeller kit. I am still worried about causing damage in taking the thing apart, but I don't want the impeller taking it apart for me - from the inside out!

I have a couple weeks before I have both the parts and the time for this undertaking. Does anybody have any specific advice for how to take this stuck LU off, with the least possibility of further damage? I read somebody else's post about a similar problem - he said it turned out he just needed to yell louder while wrenching the lower off. Anybody got something else - things to be careful of, etc? Thanks. I really appreciate having all your collective knowledge to draw off of. We don't ALL need to make the same mistakes if we can learn from each others'...

mellowyellow · Jan 14, 2003

Re: '74 Merc 650 OK slow, won't go fast

since impeller is 100% responsible for cooling your motor, it's a great idea! MANY people post about difficulty removing lower units, but most are successful....eventually. do a search above and it will give you good advice. if impeller starts falling apart, she'll be throwing pieces of rubber into the entire cooling system. pay careful attention to oil in L/U to make sure there's no water in there. look for metal filings on the magnetic bottom screw (grinding gears) just love my I6 '70 1150

M.Y.

'74 Merc 650 OK slow, won't go fast

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