downpitched prop with no increase in rpm?

[wolf396]

on my '80 18' glastron carlson with a 228 mercruiser sterndrive (305 chevy 4 barrel) i used to be able to turn 4600-4800 with a stainless 21 pitch. the 21 got damaged and I used a 23 for awhile and the rpm dropped to around 4200-4300. about a month ago I bought a 21 pitch prop same brand, design and dia. as the 23 and there was no increase in rpm. I have already checked to make sure im getting wot. by the way, it has a pertronix 1 in the distributor. any ideas where to start looking? thanks in advance<br />-wolf

 

[QC]

Re: downpitched prop with no increase in rpm?

Have you swapped between the 21 and 23 on the same day? i.e. eliminated conditions of the boat today vs. conditions when it pulled the 23.

 

[TilliamWe]

Re: downpitched prop with no increase in rpm?

As quietcat says you have to make all sure all variables are the same. After that, is it possible that the 21 you bought is really a 23?

 

[wolf396]

Re: downpitched prop with no increase in rpm?

quietcat- thats a good idea, I'll give it a try, as well as a 19 i have. i thought i would add that it pulls real well all the way to 4200. i read elesewhere on this forum about exhaust baffles becoming lodged in the y pipe. possibility? thanks.<br />-wolf

 

[QC]

Re: downpitched prop with no increase in rpm?

Yeah, this way you can make a comparison with no regard for bottom condition, weight in the boat etc. Without this type of info it is impossible to determine if this is a prop issue or an engine issue or a boat issue. Does not mean that those are not the issues, but you have to eliminate a missmarked propeller from your list. <br /><br />My guess is that the 23 will not pull as it once did and either there is more weight in the boat or the bottom is dirty (unlikely on a trailer boat if you keep it stored out of the water) or you have something limiting performance (big list of possibilities). Good luck!!

 

[wolf396]

Re: downpitched prop with no increase in rpm?

well, i put on the 19 to see if it would rev a little more, but it rained all weekend here. i thought i might add that i have checked the timing three times and it comes up different every time (i havent adjusted yet). my readings have been everywhere from 40 advance to 6 retarded at idle. this is troubling me some.

 

[rbezdon]

Re: downpitched prop with no increase in rpm?

Something wrong with your readings. The engine cant idle at 40 degrees advance. The fact that the engine is running says the reading has to be off. Also, that much advence would also effect RPM. Is rpm constant?? If it is, I'd get e new timing light.

 

[wolf396]

Re: downpitched prop with no increase in rpm?

tried a different light (tested both lights against my car engine and worked fine) and it made no difference. i also was pretty sure that there was no way the engine would run at 40 deg. the engine runs fairly consistant. i'm thinking either bad balancer or possibly distributor problem, but i thought if the balancer slipped it would vibrate like crazy?

 

[rbezdon]

Re: downpitched prop with no increase in rpm?

One other thing that caused wandering timing (at least in my days playing with old cars) was a stretched timing chain. Chain drives the cam, cam drives the distributor. The extra length due to wear/age and wander changes timing erratically. If your certain the timing is wandering, it has to be the distributor or whats driving it (except for the balancer but I think you would see it if you just pull/turn it a bit by hand.

 

[Richard Petersen]

Re: downpitched prop with no increase in rpm?

Check that wandering timing on a couple of other cylinders. All wander ? Change the distrib with a loaner. Then find the bad parts.

 

[wolf396]

Re: downpitched prop with no increase in rpm?

the timing doesn't wander as im checking it, it just comes up different after every time i run it. borrowed a dist from a 72 chevy. will try it this weekend. thanks to all for the suggestions. ill keep ya posted.

 

[ron7000]

Re: downpitched prop with no increase in rpm?

sounds like you had the lead on a wire other than #1 cylinder for the timing.

 

[wolf396]

Re: downpitched prop with no increase in rpm?

well i put the 19 on this weekend and i did gain some rpm but not enough. when i bought this boat it would turn 5000 all day with this 19. now im getting aruond 4600. i found tdc and figured out my balancer is slipping, so that explains the bogus timing readings. im fairly certain my carb needs an overhaul, since it idles pretty crappy right now. hopefully after i solve these issues it will be all good.

 

[rbezdon]

Re: downpitched prop with no increase in rpm?

Wolf, while you have the boat out to rebuild the carb, run a compression test as well. It is a good idea and may give some info. Also, pull the plugs and do a good inspection before cleaning. Kepp track of which one comes from which cylinder.

 

[wolf396]

Re: downpitched prop with no increase in rpm?

still not fixed. so far, I've replaced the plugs, wires, coil, fuel filter, harmonic balancer, set timing, rebuilt & adjusted the carb. no change. i'm leaning towards fuel pump or fuel line. possibly the pertronix module is bad?

 

[rbezdon]

Re: downpitched prop with no increase in rpm?

Did you do the plug inspectiuon by cylinder and compession test so we can compare the compression levels in each cylinder with the plug condition??