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

THEFERMANATOR · #7 09-25-2011, 06:48 PM

Quote:

Originally Posted by spareparts

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

this is supposed to be an overhauled engine that he just bought, so I wouldn't think a decarb is needed for it. I still think your mechanic isn't getting the job done for you from all of teh trouble you've had out of this engine from him.

Genie Aye · #8 09-26-2011, 12:43 AM

If I had 100 comp on any cylinder and it is more than 10% lower than my highest---bad cylinder/s.

With only 100lbs comp--I would wonder what type of freshen up your mechanic did--almost sounds like some ports may hooked the rings from burr's.

I can tell you with a injector out on my engine--I lost about 10 mph on the top end--that is a engine rated at 218 at the prop.

Good luck--but the news you are giving is NOT good.

Bummed for ya.

Joker2 · #9 09-26-2011, 03:38 PM

You guys are scaring me, but I am not surprised with this guy. If it means anything, I did the test on a cold engine, only taking one plug out at a time, I did the test when I was troubleshooting my problem. At that point I had a bad ignition coil only firing on 5 cyl. I know this wasn't the proper way to do a Comp. test, but do you think it would make that much difference. I don't know. For what it is worth ( nothing ) he told me the engine was good when he got it, that he just changed the crank. Maybe now that it is running nice I'll do the test again. Thanks for all your input.

aussie · #10 10-21-2011, 04:23 AM

Quote:

Originally Posted by Joker2

You guys are scaring me, but I am not surprised with this guy. If it means anything, I did the test on a cold engine, only taking one plug out at a time, I did the test when I was troubleshooting my problem. At that point I had a bad ignition coil only firing on 5 cyl. I know this wasn't the proper way to do a Comp. test, but do you think it would make that much difference. I don't know. For what it is worth ( nothing ) he told me the engine was good when he got it, that he just changed the crank. Maybe now that it is running nice I'll do the test again. Thanks for all your input.

recheck your comp with all plugs out 100psi is too low in my eyes they guys are 100% rite to much variation on comp i only like 5 psi or 10 psi max difference could be broken top ring good luck