RPM's & Prop

Joker2 · #1 09-25-2011, 06:37 AM

I am only getting 4-4200 RPM's with my prop 15- 1/2 x 15 Alum. get 32 mph on gps. One person, no gear, 20 gal. gas, trim adjusted. The boat is light, recently rebuilt the whole transom and rear deck, totolly dry now.Checked the linkage at motor, it is maxed out. Engine height is OK the fin on lower unit is even with keel. Engine just repaired, runs great. Could a prop change make up 1500 RPM's ? I don't know what else to try.

spareparts · #2 09-25-2011, 08:27 AM

If you are sure your tach is accurate, go back to the basics, check spark on all cylinders, check compression. Remove the front air box and make sure the bottom two throttle plates are opening, they are attached to the upper plates with a coupling that can slip. IF that checks out, take a spray bottle, put some gas in it. Remove the cowl and front air box of the engine. Have some one drive the boat at a moderate speed(3500) and spray a shot of gas into each carb. If a cylinder doesn't make any difference, you know that's the one not running, its either running rich or you have an ignition problem., if the engine drops rpm when you shot gas in the cylinder, thats a cylinder running good. IF it picks up rpm when you shoot gas in it,its running lean and isn't getting any gas. BTW, don't make a full power run with the air box removed, OMC products don't run lean at wot with the air box removed. I've got a 115 eagle series motor with the plastic carbs that's been kicking my but all summer, I finally scored a new set of carbs off of a V6 to try out on it, I've been thru these carbs four times and can't find anything wrong, but when I put the new carbs on it, it instantly ran batter. IF the loaner carbs fixes the engine, I'm going to order new bowls and main carbs bodies for the 115 and put them together with a new kit. They can be real frustrating.

Joker2 · #3 09-25-2011, 02:10 PM

Even if my tach is not accurate shouldn't I be going 40-45 MPH at full RPM with my setup. These are the numbers I have seen with similiar setups on this forum. I am running on all cylinders I just changed a bad ignition coil, so I have been through these checks, all butterflies open, compression is 120 on 1,3,5 and 100 on 2,4,6. I guess I can try the gas in the bottle thing. Thanks

THEFERMANATOR · #4 09-25-2011, 04:05 PM

Quote:

Originally Posted by Joker2

Even if my tach is not accurate shouldn't I be going 40-45 MPH at full RPM with my setup. These are the numbers I have seen with similiar setups on this forum. I am running on all cylinders I just changed a bad ignition coil, so I have been through these checks, all butterflies open, compression is 120 on 1,3,5 and 100 on 2,4,6. I guess I can try the gas in the bottle thing. Thanks

Thos numbers are not very good to see. You want to see all of them within at least 15% of each other,b ut I prefer to see them within 5% of each other myself. I would expect to see 115-125 on all 6 for a healthy engine. Try and see if you can find a shop with a dyno and see just what it's making, I think this will show you your problem and upset you at the same time. Your speed sounds correct for your RPM's with that prop, so all that leaves is the engine or lower unit.

tsubaki · #5 09-25-2011, 05:12 PM

Just for the meanness of it, raise the engine up about an inch and a half and see how it does.
It's easily done without a hoist.
What you got to lose?

spareparts · #6 09-25-2011, 06:03 PM

have you tried a decarb of the engine? It might get those compression numbers a bit closer

reelapeelin · #7 10-21-2011, 01:14 PM

Quote:

Originally Posted by Joker2

I am only getting 4-4200 RPM's with my prop 15- 1/2 x 15 Alum. get 32 mph on gps. One person, no gear, 20 gal. gas, trim adjusted. The boat is light, recently rebuilt the whole transom and rear deck, totolly dry now.Checked the linkage at motor, it is maxed out. Engine height is OK the fin on lower unit is even with keel. Engine just repaired, runs great. Could a prop change make up 1500 RPM's ? I don't know what else to try.

Good example of why it's helpful to post yer boat/motor in your profile info...can't remember what motor you have...but obviously you've got some RPMs to make up, but my thought is a 17-19 P prop of same diameter..and look into the suggestions others have made...

THEFERMANATOR · #8 10-21-2011, 09:45 PM

Quote:

Originally Posted by reelapeelin

Good example of why it's helpful to post yer boat/motor in your profile info...can't remember what motor you have...but obviously you've got some RPMs to make up, but my thought is a 17-19 P prop of same diameter..and look into the suggestions others have made...

He's got a mid 90's OMC 150 2 smoker, pretty simliar set-up to tsubaki's boat I believe. With all the problems he's had I stil lthink something is up with this engine. It should have around 120-125 pounds of cranking compression out of that engine I believe. I know some only had 90, but those were engines with idle reliefs(the late 80's-early 90's 200/225's), but some others have a140(my old 85 140 has 155 out of it, but it is built up a bit

).

reelapeelin · #9 10-22-2011, 03:59 AM

Quote:

Originally Posted by THEFERMANATOR

He's got a mid 90's OMC 150 2 smoker, pretty simliar set-up to tsubaki's boat I believe. With all the problems he's had I stil lthink something is up with this engine. It should have around 120-125 pounds of cranking compression out of that engine I believe. I know some only had 90, but those were engines with idle reliefs(the late 80's-early 90's 200/225's), but some others have a140(my old 85 140 has 155 out of it, but it is built up a bit

).

Yeah, that much variation in compression is a problem...he needs to find another mechanic who's not trying to "defend" the motor....

aussie · #10 10-22-2011, 09:31 PM

yes he probley dont want to do the engine again free of charge and is trying to cover his workmanship in my eyes 100psi is too low for a 2strokes all u need is a small scuff mark in the bore and it kills the performance of a 2 stroke pull the heads of and check the bores