85hp charger speed/prop/rpm concerns

[Guy707hc]

Good afternoon, I have recently upgraded from mid 70's 70hp chrysler to a mid 80's 85hp charger. It runs great. Idles pretty strong. (1040rpm) in neutral. 980-1020 in gear. Recently installed a tachometer so I went testing on the glass this morning. I was able to reach 35.4mph @ 7040rpm.... ? It is over revenge badly. So I zeroed the max rpm and did it again. Almost identical peeked at 7020rpm for about 1 second and held 6900 for 10-20 seconds. Pulled it back and cruised at 5200rpm @25mph for a min or two. Went to holeshot from 15mph and it took off and slowly fell on its face... only would barely idle... (fuel pump gasket tore probably from 7000rpms)...
So I'm on a 13x19 prop now. Will a 21 be enough or 22? It's on a restored 16' arrowglass tri-haul. It was just me and my gear in the boat during the test runs. Would have one more person with me at most on a normal outage. I should shoot for 5500rpm any suggestions would help alot. Thanks

 

[Nordin]

Generally going up or down one inch in pitch will lower/rise the RPM about 200rpm.
Going from 19 in in pitch and 7000RPM to a 21 in pitch prop will end up at about 6500-6600RPM

 

[jerryjerry05]

All the numbers you posted tells me that you might have too much motor for the size boat you have???
Loading the boat with another person and the equipment needed might slow it down a little?
Finding a 23p prop might be tough??

 

[Guy707hc]

I agree.. the hull is stamped at 85hp max by the coast guard. I even built a plywood deck on the bow out of treated 4x4's and 3/4" treaded plywood to add weight and help it get on plane better. (That was years ago when I had the 70hp) they are about the same weight though.. practically the same motor.. there is a 23p in good condition on ebay I will purchase today. Hope it is enough. I haven't seen anything else that high pitch unless you spend big money on a ss race prop or something and I'm not interested in that..

 

[topgun3690]

Recently installed a tachometer

Are you sure your new tach is compatible with your motor? Is the pole setting on the back set correctly? Maybe get a tiny tach to verify the new tach is showing the right RPMs.....before shelling out money on new props.

 

[Guy707hc]

I'm kind of confident that the tach is close to correct... it is a cheap ebay digital tach that you install on the #1 plug wire and tell it how many counts is a revaluation. It is set on 2p = 1rev. Since it's a distributor motor, I tried it on 1p=1rev. But it was idle at 2000 and cruise at 10000@25 mph. I felt that was wrong. I thought since the distributor fires every pass of rotor button. But I could be wrong here. It is a two stroke... so 2 strokes per fire right.

 

[topgun3690]

I think I set mine at 3P = 3 spark per rev. for a 3 cylinder motor.....might try that and see what happens. Am not near my boat right now to verify that setting but pretty sure that one is correct.
You have a 2 stroke, but it's not 2 strokes per fire.......it is 3 sparks per rev (for a 3 cylinder engine).

 

[Guy707hc]

OK that's sounds right. It's very confusing to me. But since every cylinder fires on the same stroke right... as the crank rotates, cylinder 1 fires and then #2, then three. All in the same Rev? Sorry to make this complicated. I am just reading the plug pulse not the stator.. I think it is 12p per Rev. I couldn't find a tach to read 12couns for a deal. Thank you again. I don't know if my tach has 3p p1.. I will confirm and report today.

 

[jimmbo]

I doubt that Engine would hold together for long at 7000 rpm, so I suspect the Tach is way off. Idle speed in gear should be around 600 - 650 rpm. If you have 1000+ in gear Idle, the gearcase will be taking a Beating when shifting

 

[topgun3690]

This is the one I got off ebay......they are all pretty similar.

 

[Guy707hc]

OK yes, my tach Is very similar. I was able to set to 3p1r. It is showing 640-660rmp at an idle on muffs. I didn't keep it WOT for long while I was out testing but I agree 7000rpm will tear up things pretty quickly. I did purchase the 23p prop. I will retest with this 19p that is currently installed... then for fun or if needed I will test the 23p prop. My luck has it that I will need closer to a 21p? We will see. Thank you for straiten me out on all the confusion. It really is a great motor and runner. It has no lack of power. This is my second 3cylinder chrysler and they both have been strong. Thanks again. I will update as soon as I get the boat out again. Next weekend unfortunately.

 

[Guy707hc]

Would just like to clarify why I believed the uncalibrated tachometer...when I would get towards the top of the rpms @ 3/4 throttle and up.. the boat would gain speed and rpm until the point that the rpm would jump up a bit and actually loose a mph or 2... like the prop was slipping or spinning out. I just assumed that it was the excessive rpm causing prop wash or cavitation.. also on a side note. The bottom#3 cylinder had the carb set to 1/4 turn from seat to even run... (it actually idled better when turned all the way in lightly shut) while the top two carbs matched each other @ about 7/8s turn out from seat. So i rebuilt them. Drilled out the Welch plugs, cleared passes, seats and needles. With no change! I checked plugs often and have been wet every time. But since I replaced the tired fuel pump they all three are within a 1/4 turn from each other. They all are a little less than one full turn. (This is under muffs) I will adjust in gear on more time since it has the fuel it needs now. So thank you guys!! You have helped me answer multiple questions today and in the past!!

 

[topgun3690]

This is from the lync n sync stickie at the top of the forum.....

10. I can NOT stress this enough! NEVER set the low speed needles less that 3/4 turns out no matter how poor the idle or acceleration. To do so will run the engine too lean and detonation and melted pistons at or near full throttle operation will result.

Most folks on here use one (1) turn out from lightly seated as the minimum setting.....just to be sure it is not run too lean. But 7/8s is good according to the info above from the late Frank A.

As for the prop possibly slipping, you could make a mark across the hub and prop at one point, then check it the next time after running and see if the marks are still lined up or not. If not, then it is slipping......

 

[jimmbo]

OK yes, my tach Is very similar. I was able to set to 3p1r. It is showing 640-660rmp at an idle on muffs.

Now 650 on the Muffs is gonna be about 500, even less in gear, when in the Lake.
However if it was reading 1040 in Neutral, was that on the Muffs or in the Water? If these revised Rpm Numbers, due to resetting of the Tach, didn't require any adjustment to the Idle Stop, then I suspect you might be Over Propped. Your Engine WOT Range is 4500-5500 Rpm

 

[Guy707hc]

The 1040 was on the water not muffs in neutral. But after that run I replaced the (going)bad fuel pump and had to reset the idle ans all three fuel mixture screws... but I do understand what your saying about the difference from changing the count on the tach... It will be close to being right or I may even have to downsize prop... But now that I have a good fuel pump and all three carbs are very close to echother on their idle mixture screw, I'm hoping to have more stable power all threw the rpm ranges. Unfortunately I will start over from scratch. I will also set all three carbs to 1 full turn out from shut to be safe, even if I have to bump the idle up a touch to have a clean idle. ... I will say after installing a new pump and setting the carb again. It idled for 1.5 hours and just started to barely smoke a bit. (I checked Temps often and use AMS oil for mix) it's healthier than its ever been! Thanks all. Will keep post results with next weekends outing.

 

[Guy707hc]

This is from the lync n sync stickie at the top of the forum.....

10. I can NOT stress this enough! NEVER set the low speed needles less that 3/4 turns out no matter how poor the idle or acceleration. To do so will run the engine too lean and detonation and melted pistons at or near full throttle operation will result.

As for the prop possibly slipping, you could make a mark across the hub and prop at one point, then check it the next time after running and see if the marks are still lined up or not. If not, then it is slipping......

Oh I didn't even consider that prop slip between the hub and the prop... i figured air cavitation is what it felt like. I will mark them and add that to the list as well.

 

[Guy707hc]

Guys sorry for the long delay... I was able to get on the water yesterday between rain events. The tach set up is 3p1r now and idle in water is 640-680rmp in neutral. And 620-640rpm in gear. Idle at 3 mph with 13×23p prop. This prop is slower idle speed than the 13.5x19p prop @4mph. The boat does not like the 23p prop at all. Whole shot was barely acceptable.. took too long to plane out. Every time I got up to 30mph or so., it would Rev out and loose a little speed(3 or 4mph). I could let out of some throttle and the rpm would would come down and grab again...and be fine for cruising. I marked the hub with a couple marks across the hub with paint marker and the hub is solid with no slip. When it revs and slips, I trimmed down and it instantly grabbed and started to gain speed and rpm.. top speed was 32mph. The tach said 5420rpm (but it reved out a couple times so this is false) but at speed was less. I bet rpm was closer to 4000rpm before it revs out and looses traction.
I will install 13.5x19p prop(marked with timing marks) and do another run when but it doesn't slip nearly as much. And it achieved 35mph with motor running pour. I got rained out before I could swap them and compare on the same day.
So I wonder if it is worth a whale tail to be able to trim up a little and get the boat out of the water a little(to gain some rpm) I'm literally plowing water trimmed down so far with 13x23p.
And is 1/2" smaller prop the reason for cavitation? 13"vs 13.5" that big difference?
Any thoughts and suggestions is greatly appreciated. Thanks all

 

[Guy707hc]

Guys after putting the props side by side... the original 19p is much bigger OD of the paddles and longer on the hub/paddle? The OD of the barrel is the same? Just longer and wider. The inner portion that mates with lower unit measure the same as well. Maybe why the holeshot was poor and wanted to spin out at high rpms. Not enough grip on the water?

 

[jimmbo]

Looking at the Pics, I see little, if any Cup, on those Blades. Cupping help the Prop's ability to keep a Grip on the Water if there the Water is not Solid, as in Undisturbed.
Where is the Anti-Ventilation Plate, with regard to the Hull Bottom height wise, when it is Parallel to the Keel?

 

[Guy707hc]

Unfortunately it is 3/4" above the bottom of boat when they both are parallel with each other. I held a strait portion of my net handle up against the bottom of the hull and trimmed the motor to match and had 3/4" above the bottom of the hull...? Is this way to high? The hull is a 1965 arrowglass trihual. Not rotted or saturated. I replaced the floor and pulled the glass to confirm 7 or 8 years ago now. But stored indoors and dry since.