No top end....why?

[Turn4fun]

I have a '67 chrysler 35 horse and I swapped the powerhead for an early 70's 55 horse. I got it running and it starts easy, idles well and runs like it should up to about 3/4 throttle. Once it gets to 3/4 throttle it's like hitting a brick wall, it dogs out bad and speed drops to 6 MPH. If I throttle back speed picks back up and it runs fine. The best way I can describe it is it's like the timing stops advancing with the throttle.

When I got the 55 powerhead it was missing the tower shaft so I used the one from my 35 horse. The best I could figure is that 55 tower shaft was different so I found a guy who was parting a '75 55 horse and I bought his tower shaft. It was a little bit different but not much. I installed it and then re timed and re synced the ignition. Both points are set to open at exactly 28? BTDC on each of their respective cylinders at WOT. When the tower shaft is in the full advance position the throttle plate is perfectly horizontal. When the remote control arm is in neutral the throttle plate is just beginning to open from the completely closed position.

Everything should be set exactly as it is supposed to be, so why do I not have any top end? When it's in the "sweet spot" around 3/4 throttle the engine sounds and feels right and speed is 11.5 - 12 MPH on my GPS (which is exactly what my 35 horse ran at full throttle). As I move past 3/4 throttle it sounds very restricted and "flat" and speed drops to around 6 MPH. Anyone have any ideas what I might be missing here?

 

[john from md]

Re: No top end....why?

Take a look under the flywheel and make sure you don't have a chafing wire that is grounding out. The other possiblility is that you have a restricted fuel line or vent line.

John

 

[Turn4fun]

Re: No top end....why?

Wiring is all fine and not rubbing on anything. The engine is not missing when it bogs down and does not appear to be starving for fuel. Have you ever opened the throttle a little bit by hand right at the carb without moving the spark advance linkage? You know how it kinda lugs and feels flat and the RPM's don't increase, that's how this acts past 3/4 throttle.

 

[john from md]

Re: No top end....why?

Increase the advance to 30 degrees and see what happens.

John

 

[Turn4fun]

Re: No top end....why?

My suspicion was correct, the timing does stop advancing, and then retards as it approaches WOT. Is this right, seems like the spark advance should be constant from idle to WOT. See attached video clip.

 

[pnwboat]

Re: No top end....why?

Looks like the tower shaft is not the correct one. The geometry of the arm that moves the mag plate is not right, or the link between the arm and mag plate is not the right length? It looks like the arm on top of the tower shaft may not be clocked correctly. Needs to be moved clockwise on the shaft itself. Not sure how it's held in place on the shaft. Most of them have a roll pin that goes through the arm and a hole drilled in the shaft. If it does have a pin, remove it so that you can re-position the arm and experiment with re-positioning the arm. Only problem with that that I can see is the idle adjustment screw or tab will limit how far you can re-position the arm before it hits the block.

Looks like you should be able to solve this problem. May take some thinking, but it's basically a geometry problem. Getting the mag plate to move the correct distance. Try experimenting with the length of the link between the arm and mag plate, and re-positioning the arm on the shaft itself.

 

[Turn4fun]

Re: No top end....why?

My tower shaft is a one piece casting so the top arm cannot be repositioned. The one from my 35 that I was initially using is the same way. Changing the length of the link rod only effects how far the plate rotates but does not correct the issue of it dropping back.

 

[Turn4fun]

Re: No top end....why?

OK, so I retimed the engine to 30? and it seems to work much better but still isn't quite right. Engine speed now steadily increases with throttle throughout the range but faulters slightly just before WOT then smooths out. The engine sounds]/I] like it's operating properly at WOT now, yet boat speed is just 12 MPH which is exactly how fast the 35 horse powerhead pushed it. I found a vintage Chrysler tach and installed it the other day. It is definitely the correct tach and was N.O.S. still in it's original box with instructions so I know it's hooked up properly too. At idle in gear I'm getting 700-800 RPM but at WOT I'm only getting 3000 RPM. Does it sound like I need to get a prop with less pitch? I can't imagine that a pitch change alone is gonna bring me up 2500 RPM. I'm also baffled why a 20 HP increase hasn't increased boat speed at all.

 

[jerbear56]

Re: No top end....why?

May be a dumb question, but do you have the motor trimmed correctly?

 

[john from md]

Re: No top end....why?

Start pulling plug wires one by one. The one that doesn't affect engine operation is the bad cylinder. The problem can be fuel or spark so you will have to do some troubleshooting.

John

 

[Turn4fun]

Re: No top end....why?

I've done that and it's definitely firing on both cylinders. Fuel system does not seem to be at fault either. Like I've said, it really seems to be a timing issue even though the timing seems to be set right. When I switched from 28? timing to 30? timing it helped but the mag plate still reaches a point where it stops moving and then retards. It's not nearly as severe as it was, and now happens somewhere between 7/8 and full throttle. I guess I'll keep tweaking it to see if I can eliminate the drop back altogether.

Now this low RPM thing has me baffled because it sure sounds like it's turning a lot more than 3000 RPM. I have a good idea that I'm getting a false reading. Last night I read through the instructions that came with my tach again just to make sure I hadn't missed anything. Everything is hooked up like it says, however I did miss one important step; POLARIZING. Basically it says to hook it up just like I have it, BUT, then you are supposed to run the engine to about 3000 RPM and reverse the wires to see if the reading changes and go with the configuration that yeilds the highest reading. I can't check this until the weekend but I've got a hunch that it's really turning in the upper 4000's if not low 5000's. If this is the case then I'm in the range where a pitch change may actually be what needs to happen.

 

[john from md]

Re: No top end....why?

Buy a cheap inductive tachometer off ebay or at sears. You need one when you are adjusting your carbs anyway.

John