low RPM at full throttle

cpt. sundancer

cpt. sundancer

New Member
Oct 23, 2009
273
red wing,mn.
Boat Info
1981 sundancer 260/425 h.p.
Engines
merc.355 ci, B&M blower
I have a '81 260 sundancer with 5.7 ,stock motor,points ign. system, motor will rev up to 4300 rpm out of gear,but when i shift to forward and try to get on plane,it only revs to about 2200 rpm. motor sounds good,just stops at 2200 rpm, have a new carb.(last year and ran great)changed fuel/water filter,plugs....any ideas :huh:
 
This happend to me and it was caused by one spark plug not working. Also check compression in all cylenders
 
I had the prop refinished,it had a couple dings but nothing major,put in new plugs and also checked each wire to make sure spark was reaching it,but the bottom does have the damn zebra mussle attached,but they where not there at the beginning of season and still ran like crap.how can it run great in nuetral up to 4300 rpm and run good up to 2100 in forward and seem to stop :huh:,it's hard to believe one spark plug could keep rpms that low if it was bad and revs great out of gear. my next step besides a compression check is a pertronix electronic ign.system and a diff.set of plugs:smt100
 
My boat will not run full RPM or even get up on plane with barnacles on it. They make a huge difference. The first time I had a fouled bottom on my 27 ft I thought I had major engine troubles untill I pulled it out of the water and saw how many barnacles it had. Cleaned them off and was just fine again.
 
Are you saying the engine won't rev past 4300 in neutral? Or are you feathering the throttle up and stopping the rpms at 4300, it will do more in neutral, your just stopping at 4300?

Can you confirm the secondary's are opening up on the carb?
 
Something real simple, check the oil for any discoloration, if it is grey or milky in color you may have another issue.
 
Oil looks great,only have about 6 hours on oil change. I rev to about 4300 in nuetral and stop,might go higher,just dont know why it wont rev that high in drive.bad cable ?,If it was barnicles or zebra mussles on hull wouldnt that effect speed and not engine RPM ? :huh:
 
cpt. sundancer said:
...,If it was barnicles or zebra mussles on hull wouldnt that effect speed and not engine RPM ? :huh:
Click to expand...

Not necessarily. My bottom was fouled with river muck and I lost both 6 mph top speed and 500 WOT rpm. I can't rev my engines past 3,500 in neutral before a SmartCraft overrev alarm shuts it down.
 
I had the same problem with a 26 Chapparral 5.7 V8. As a powered up I couldn't get the RPM over 2,200 while on the water. We found that my spark plug wires had areas that were worn and permitted electrical flow towards the engine block thus gounding and reducing the electrical requirements to the plugs. The best way to check for this is to wait till dark startup the engine and Safety check the sides of the engine were the wires are located to see if electrical flow is seen, small sparks or lighting of the area. This was what was robbing us of power. Note! If this is the case they should be changed quickly since this also represents a fire hazard with fuel fumes. Hope you find the problem.

To good weather and calm seas.
 
Thanks for all the input,Iam going to do a tune up....new plugs[even though i just replaced them],I will try a different brand,new wires,cap and rotor,a pertronix electronic ign.,to get rid of the points,new coil and check cylinder pressure and timing. and also clean the hull.,thanks again :thumbsup:
 
Clean the hull before you spend any money on anything else. You'll be pleasantly surprised with the results I think.
Cheers.
 
I second the coil issue but it should happen once the coil gets hot. I would also check fuel pressure. Spinning an unloaded engine up to 4300rpms is also hard on the main and rod bearings. A friend of mine loves to rev his 454 to warm it up.....lots of engine issues, Mike.
 
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