'79 50hp merc blueband, wont go over 2500rpm when in gear?

[keiron49]

hi all
excellent forum. I have spent over an hour so far searching for my problem. so here goes for you clever folks.
I took my boat out today for 1st time 12ft shakespeare with blueband 50hp thunderbolt O/B. it starts first time everytime hot or cold and idles fairly well. when i first got it out in the water it sounded like it was not firing on all cylinders (4 of) and had a popping noise every so often. after about ten minutes the popping and uneven running noise stopped and it sounded fine. going from 1500rpm cruise to full throttle only went up to 2000 rpm's and was very slow. after another ten minutes it would increase on full throttle to 2600rpms and no more. yet still starts first time every time when switched off. out of gear it will rev to 5000rpms all day long just not when in gear. anyway got it home and flushed out with muffs andstarted first time so i hope it is not something expensive and a simple fix. when idling it sounds uneven and the engine does 'wobble' a bit. Any wiews, i have not done a comp. test as i dont have the gear to do it, so am trying here before it gets expensive i.e mechanics.
many thanks this forum is very helpful.
keiron

 

[DutchMerc]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

This could verry well be an ignition problem. I have the same thing on my '74 40HP. I found out after i put a scope on each plug.. Exchanged the coils between cilinders, problem stays on same cilinder. I think i have a defective switchbox. I am still looking for a (not to expensive) solution..
Hope your problem is less expensive...

Good luck.

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

many thanks for that , i am going to the repair shop today to see if theycan shed any light on it , but i know the answer already ,"bring it down and we will strip it "erc... sounds expensive, but may have to bite the bulllet.
will update you soon

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

guy said today that it could be my stator faulty as it controls the second carb and allows full power, hope hes right, but its gonna cost ?280 for the bit. boo hoo

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

right, i have compression tested the engine and get 50psi on all cylinders, is that good or bad? also i have found this on ebay and would like to know if anyone can confirm if it will fit my engine? here is the link http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=290245473807
many thanks n advance to you all that read this and can offer any assistance.

 

[DutchMerc]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

I'm not sure about your 50 PSI. How did you check this? You should check at WOT and with a full batery, to give your starter-motor full rev. My 402 '74Thunderbold still has 150 PSI ... 50 seems a little low to me. Did your motor overheat before the problems appeared? Maybe you have a leaking gasket on your cilinderhead?

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

hi , i used a compression tester, and pulled all plugs out and then in turn tested each cylinder by cranking engine over 5 or 6 times until the tester showed a reading. how can i test it running at WOT ( by the way what is WOT? ) i presume its full throttle??? do i have to have it in the water?

thanks for your input.
keiron

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

sorry forgot the answer to overheating, as far as i know i dont think so but i have only had the boat a few weeks and only been out on the water once, however i did have to change the impeller as it did not have water coming from the telltail and when i got it out it was melted good. when i took it out it seemed to run fine as in starting / idling after about 20 minutes it was all good but for the power, i was out for at least an hour and it stopped/started all that time first time.

i am waffling now so i'm gonna shut up.
thanks

 

[DutchMerc]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

Wide Open Throttle is needed so you don't loose speed in the first part of the cycle (no vacuum). You just pull all remaining cables from the plugs, so the motor does not start. Make sure your tester is not leaking and turn the startermotor on for a few seconds. This should give you a fair reading... good luck.

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

will have another go tomoro at WOT and see if its different. i'm hoping it will be higher. will let you know what happens, cheers.

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

well, i tried again today with throttle on full, and same readings 50 psi. am i doing it right?? all plugs out, fuel line off, tester in hole, crank engine a few turns, take reading.?? repeat on each cylinder.
cheers

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

finally took her down to the shop and after twenty minutes diagnostic and elimination it was firing on two cylinders and the switchbox was goosed. new switchbox and ?200 later all four cylinders firing at 120psi ( my meter must be faulty ) now got to get her in the water tomoro to see if all is good.

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

took her out into the solent today and she started first time and stalled once in gear , did this twice, then ran well up the river for 15 minutes. once in the open water , let her open up and she went superbly up onto plane and away, absoolutely brilliant. had her full on for 45 mins and then chugged back down the river, no problems back onto the trailer. hopefully this is now the start of my summer. thanks.

have fun out there.

 

[keiron49]

Re: '79 50hp merc blueband, wont go over 2500rpm when in gear?

the next trip out and it was back to missing and the power was gone, back down to the engine shop and the switchbox was fried again. he took off the flywheel and the stator was not the right one and had all the wires adapted apparently it was a 14 amp instead of 9 amp or vice versa, anyway he replaced the stator and switchbox, then off on to the water. all is good now doing upto 35mph and was out for 3hrs on saturday and then two hours on monday, everything is as it should be, but it cost me a fortune, i might take up golf again haha.