Repitching Rude 8 1/2" X 9" aluminum prop

[jauguston]

I am pushing a 12' Crestliner jon with a 8hp 1985 Evinrude and we are loading the engine a little heavy at WOT. Is there a shop in NW Washington that can take a little pitch out of it. I have searched for a lower pitch prop and can't seem to find anything between it and the 8"X7" that will be too small.

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

I'm no expert but it would seem if an 8 X 7 will be too small that your not loading it that heavy.Do you have rpm figures? Rpm range on the 8hp is 5,000 to 6,000.If you don't have a tach you can get a tiny tach easy to install
about $40.

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

Spike,

The service manual says 4500-5500 and I haven't stuck my Tiny Tach on it yet. It just has that too much prop sound. I want to be in the upper part of the RPM range. Any info on a shop in this area than can depitch the stock prop.

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

Spike,

The service manual says 4500-5500 and I haven't stuck my Tiny Tach on it yet. It just has that too much prop sound. I want to be in the upper part of the RPM range. Any info on a shop in this area than can depitch the stock prop.

Jim

My after market manual says 4,500 to 5,500 on the 7.5 and 5,000 to 6,000
on the 8hp.I think if you check your actual rpm you'll be able to tell where your at better. If you want to be at the upper end it would seem to me your allready pretty close if you feel a 2 in. change in pitch is too much.
Does the boat plane now?If it doesn't then I don't see a 7 inch pitch being a threat to over rev.

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

Spike,

I am using the 4.5-7.5 OMC manual and was told here the 8 was the same as the 7.5 just with a different way of rating the power. I see the 8hp four stroke is rated for 5000-6000. I wonder if that is what your book is refering to.

Getting back to my engine I have found I was looking in the wrong place for rpm improvement. I sat down with the manual and the engine and went over all the carb and mag timing linkage adjustments. Not one of them was as it was supposed to be!

I went to the lake last evening with my Tiny Tach hooked up and saw 5500 rpm with me alone in the boat. Sweet!! ---But-- after about 10 minutes of running at various power settings I could not get more than 5200. I don't think it is overheating as there is lots of water flow but to be sure I am going to take my infared heat gun and check it, the manual says where on the engine to check the temp. Any ideas??

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

My manual covers 84-89 2 stroke Your reading 7.5 specs.In fact 4.5 and 7.5 are the same rpm spec.8hp listed seperate at 5,000 to 6,000.
I did see a note that while L77J4 is reccomended, QL78V is reccomended for extended high speed operation on 6 & 8 hp prior to 89.I don't know if this might have any bearing on your fading rpm.You may want to check the thermostat.
I wonder if it might need a decarb?

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

That is interestng about the higher RPM for the 8. This group told me the engine was the same as the 7.5 but power rated differently. I don't doubt what your manual says but it sure seems strange. What is the part number for your service manual. I have not found any physical differences between what is in the 7.5 manual and what is on the 8.

I tried to get L77J4 sparkplugs for it and was told they are no longer made. The replacement was said to be L77JC4, the C denoting some kind of copper coating. I am using this engine on a fishing boat so I should probably stay away from the high speed sparkplug. I am using the Evinrude synthetic two stroke oil the local shop recommended and has in bulk at a good price, bring your own container (-: I am mixing at 50-1 even though there is a sticker by the fuel hose connection that says 100-1.

I am going to try and check the temperature with my infared heat gun but I took a look at where the manual says to check it (on the core plug with the numbers on it) and it is so well hidden behind the coils I am not sure I can get the gun beam in there.

I have given it two doses of Sea Foam through the carb and got lots of black gunk out of it. I also have some in the fuel. I intend to do that several more times, there is still carbon showing on the piston heads.

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

I have an after market Intertec manual.I skimmed through to see if I coiuld find any obvious difference between the 7.5 and 8.The only thing I noticed was The 8 with a powerpack marked CDL has a device to limit rpm at 6,200
and the 8s reeds have 3 retaining screws instead of 2.
Also checked NAPA parts catalog it appears the 8 uses a different power pack than the 7.5.As none is listed for the 8.
Not unusual for similar motors to develope different hp ratings by using subtle changes in induction etc. some as simple as a throttle stop.
Also not unusual for the higher hp to be rated at higher rpm. ie::
the 9.9 and 15 9.9 5,000 to 6,000 15 5,500 to 7,000 of course the 15 has
more changes than a throttle stop to achieve its output.
Stick with the 50:1 mostly to combat corrosion during storage and little use times.Sounds like you got it pretty well dialed in.You could raise the vertical height to see if you can squeeze a little more out of it just be sure it pumps water.

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

Spike,

I really appreciate all the research you have done for me. Overall I am pretty happy with how this is turning out, I was a bit disappointed at first but as I get familiar with this engine it is starting to run pretty well. I am still puzzled with the drop in WOT rpm after it runs for a few minutes. As you say it may be it is still suffering from carbon build up and I will continue with the Sea Foam treatments.

Reassure me here --- is it OK to prop for and run at 6000 rpm WOT???

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

I'm kind of conservative and would prefer to run below the max.But if it continues to run at 5,200 you aren't likely to make 6,000 with a prop change.
The 7"pitch would give you better load performance and if she runs too close to the max you can always back off a little.I don't think the 7" will push much faster empty.

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

I think I have found the reason for the rpm drop. I said I didn't think it was overheating because there was lots of water coming out of the bleed hose. Now I know that doesn't mean much. I ran it in a barrel today and got my infared heat gun out and ----OH S**T ----- that sucker is HOT!! After 4-5 min at 2500 I saw 200 degrees on the core plug and 260+ on the head. I pulled the thermostat and there was a mess of "stuff" around it. Salt related for sure. I cleaned things up as much as I could and put things back together - still getting hot. Lots of water flow out of the thermostat hole with out a thermostat or cover in place so the pump is putting out lots of water. I left the thermostat out and the temps are nice and cool. Ordered a new thermostat. The saga continues (-:

Jim

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

I probably should post this on the Evinrude list but the Mods haven't moved it yet so here is another related question. I replaced the thermostat and it still seemed to be losing rpm after a few minutes like it was getting hot. I pulled the head and the water passages were pretty cruddy with more salt related crap. I cleaned all the crud out and took my Dremel with a carbide rotary file and opened up what looked like a narrow spot in the casting on both sides by a bolt hole. In the test barrel at 3000rpm the infared heat gun shows 140-145 degres on the cover over the thermostat and that is what the thermostat is (143).

One more thing has me concerned. I read on another thread someone recommending some kind of sealant should be used to seal the water pump housing to prevent sucking air on a similar vintage engine. Is there anything to that suggestion and if so what if anything should I do.

Jim

 

[steelespike]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

It would appear that you are running right on with good water flow.While sealant might seal the pump if you were careless excess might come loose and get stuck somewhere.Leave well enough alone.It has worked for 20+ years.

 

[jauguston]

Re: Repitching Rude 8 1/2" X 9" aluminum prop

Thanks Spike. I will leave it alone. Hopefully I will get a chance in the next day or two to go to the lake and test run again. I am happy with the way it runs generally but hope it doesn't have the rpm drop after it warms up it has been doing. WOT at first will be 5600 and after 5-10 min of running at varied speeds WOT would drop to 5400.

Jim