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Mercury 40 hp wont achieve full rev

C

chwinc

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I need help please my 1987 40

I need help please my 1987 40 hp 2 cyl 2 stroke thunderbolt just wont achieve full rev the throttle does open fully so no linkage problem. the boat feels as theres too much weight at the back when I open her out is this called "bogging down" as I have heard mentioned?
your help would be greatly appreciated thanks
 
"Terry, is this something that

"Terry, is this something that started happening recently? did it work good before? or is this a new set-up for you??"
 
"they didn't make a 87 2cy

"they didn't make a 87 2cyl 40, the last 2cyl was 1983, they did make a 87 35hp 2cyl..What pitch prop, how big is the boat"
 
"Its older than I thought, its

"Its older than I thought, its a 14 and half foot cjr, not sure of pitch.
it worked ok untill day before monday!!but thinking about it it has been gradually slowing down all summer and yesterday was when it really developed, it starts and idles ok !
I havent changed nothing!
its as if its not getting enough fuel but that would make it cut out at high rev surel? any ideas ? thanks"
 
"its running on 1 cyl then, co

"its running on 1 cyl then, compression and spark test time. post serial number"
 
"thinking it might be around 1

"thinking it might be around 1978, 402 model, blue band with sparkles. If you have a timing light you can do a quick check for spark, put light on each wire and check for flash. Really need compression and spark test first before we start guessing. High speed stator could be gone, but lets do the free stuff before spending money"
 
"Terry/JB - that serial would

"Terry/JB - that serial would make it a 79, twin cylinder, the first model that they simply called the "40" horse. At least it does have the Thunderbolt IV ignition (nice - and mostly available).

New/recent problem - at least we can rule out the prop being too big.

I do have all the specs/test tolerances/proceedures for this model if you find no/weak spark on prelimary test..."
 
yes it is a 402 blue band.
th

yes it is a 402 blue band.
thanks gentlemen I will go to work tomorrow and get a strobe light.
She's definately running on two cylinders on idle .
someone suggested a leaking fuel connection causing the pump to suck air????
 
"possible but to test that all

"possible but to test that all you need to do is manualy pump the fuel bulb while running, problem goes away and you know its fuel related. The compression of the engine cycles the fuel pump, thats why its important to have those numbers, low comp means no fuel."
 
aha got it thanks a lot will t

aha got it thanks a lot will try compression test tomorrow.
thanks JB and Graham will let you know what I get
 
"I had a similar problem on my

"I had a similar problem on my 40hp 2-cyl 1 carb. 1995 mod. Mariner. I have been trying to solve this problem all summer. Runs as a kitten on low rev and idle, starts fine. Thought it was the idle screw adjustment, and almost got is to throttle up properly, but still not the reaction I had expected. Last week it really bogged down after 15 min running. Plugs were fine etc. Went better if I choked it shortly, but still no good. Decided enough was enough. Handed it over to the repairman, took him a few hours to fine tune it in the shop and figure out nothing else was wrong. Now it runs perfect. He mentioned it could be a cracked block and water in cylinders, but it was not. Most likely just a samll piece of dirt in the carburettor, or too little air through the tank valve. Do a professional adjustment and buy a new tank and hose before you use it again. Good luck. It is probably less dramatic than you think."
 
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