Lack of Power-Tach Over Run-Lack of Power!

[Peaceful Pirate]

We have an older M/C 140hp (mid ?70?s) that is now in a deck boat and has always run just fine; even to the point of being able to jerk our 19 yr/old our of the water on skis. Then this past week it developed some rather odd problems that have us a bit stumped on how to best approach a solution.

Normally when giving it throttle, the engine comes up to WOT of about 3500 RPM in a smooth and steady run-up. However, this past week, when giving it power, the tachometer would run way past the normal 3500, and max out on the tach to 6000; but by engine tone and sound, you can tell it is only running in the 2000-3000 RPM range. At the same time, when you reach about half throttle, the engine begins to cough and lug like it is being fuel starved. Power out put appears to be less than half of normal WOT.

When the throttle is backed off just a bit, the engine settles down and runs along at about a third to maybe half speed/power, but the tach is showing it running at around 5000-5500 rpm; with the occasional engine cough and/or lug.

We had a Pentronix ignition conversion in the distributor, so we pulled it, put the old point system back in, but doing so made no change in the problems.

The power loss and cough and lug brought the accelerator in the carb to mind, but that would not account for the tach RPM run-up. The tach RPM run-up brought up maybe coil (a ballast system) or maybe distributor problems, and even a possible alternator problem.

So rather continue on chasing our tails and just replacing parts for the sake of replacing them, it is time to sit back and reconsider our approach. We would appreciate any thoughts and/or recommendations anyone might care to offer.

 

[Bondo]

Re: Lack of Power-Tach Over Run-Lack of Power!

Ayuh,... I think ya got 2 separate problems...

For the Tach,... With the motor off,...
Wiggle the switch on the back, 'n see if it comes into it like it should...

For the Runnin' problem,...
Check, 'n change the fuel Filters....

 

[Peaceful Pirate]

Re: Lack of Power-Tach Over Run-Lack of Power!

Bond-o, Thanks for the reply.

This is part of our frustration, and we apologize for not also adding additional information. Following the initial problems, we thought of a possible fuel issue, so we drained the fuel tank and pressure air-dried it out for a full day. At the same time, we pulled the Racor filter and the inline micro filter, then blew all the fuel line out, and replaced the filters with new ones.

As for the tach, the really strange thing is that at idle, say dockside, the tack shows a normal idle RPM (about 750-800 in gear). We put an analyzer on the coil and the RPM matches the tach. We know that when the tach is showing 5500-6000 RPM, the engine is actually running in the 3000?s, because as you know, while a M/C 140hp is a solid engine, if you tried to run one in the 6000 RPM range for very long, especially under load, it would not be long before it would have an internal hemorrhage!

Up until it reached that point where it begins to bog down and cough, it appears to be running normal. It starts right up, idles smoothly and will cruse our of the cove in a no-wake run showing around 1500 RPM. Then run up the throttle and the tach immediately begins to spin around toward 6000, power starts coming up, then hits the tach pin just past 6000 and the engine starts coughing. It just never gets anywhere near the normal WOT.

Question ? Could a bad coil cause this mess?

 

[Peaceful Pirate]

Re: Lack of Power-Tach Over Run-Lack of Power!

Also I should add that all this takes place on initial start-up and is not something that comes on after the power plant has run for a while and heats up.

Another thought - Alternator?

 

[Peaceful Pirate]

Re: Lack of Power-Tach Over Run-Lack of Power!

Ok, the original problem was and still is:

At idle and power up to 2000-2500 RPM, everything appears to be working well and runs well. Bump the starter and the engine fires right you. Continuing to throttle-up, engine starts to bog and sputter like it is fuel starved.

However, at this point, the sound of the engine tells me that it is not running wild. It simply is not that loud. It sounds more like it is in the 3000-RPM range. Also, power flat-lines and does not go up.

The boat had been running just fine for the last 3-4 months. Had taken kids water skiing and made our usual lake runs. Then we just start it up to take a run and the problems manifest themselves.

So far, I have:

Checked that coil with a multi-meter since I do not have an extra one on hand. Checks out OK.

Check and verify timing and insure proper advance ? No change

Changed out the Pentronix E/I and reinstalled points/condenser ? No change

Changed out the distributor, cap and rotor and verified timing? No change

Drained the fuel tank, air dry our the tank and fuel system; all lines, fuel pump and change out the Racor water/fuel filter and a micro-filter ? No change

Refueled with ethanol free, adding marine Sta-Bil ? No change

Changed out the prop to insure prop bushing is not slipping ? No change

Pulled engine and changed out coupling bushing to insure it is not slipping ? No change

I changed out the tach with another one. Actually, I had three extras and installed the one that matched the engine analyzer. All three were within +/-150 RPM. Then took it out and low and behold, the RPM continues to run all the way up!! Same for the tach on the analyzer. However, I again note that by just sound alone, the engine is not running that hard and no way is it close to hitting 6000 RPM. By changing out the tach, I able be able to verify that the tach is receiving the reading from the engine. However, since changing the tach did not solve the RPM issue, then I am looking at another problem. Coil? Resister?

So here is where I am and what is on the table:

Have ordered coil just to make sure there is not some internal short/open in the coil that the multi-meter test is not picking up.

Have ordered a rebuild kit for the carb. With the way it is acting, and leaving out the tach/RPM issue, with the bog and sputter and power loss on run-up, I keep coming back to the accelerator in the carb as a possible problem. Want to replace needle/seat, check the float function and clean the jets and ports.

Thought I would also check the valve adjustments just to make sure nothing has changed there. I have not touched the valve train yet, but did pull the cover and all eight were functioning and nothing was obviously out of the normal.

After changing out the prop and coupling, I am fairly confident that it is not the drive line slipping. Also, by eliminating the above, I am now taking this from the prospective that the tach issue is a separate issue from the power issue.

Well, there it is! So any words of wisdom or recommendations? I do appreciate anything you care to offer. I know the answer is out there ? I just have not hit on it yet, but all things considered, I am slowly working through just about everything that make this puppy run!

 

[Peaceful Pirate]

Re: Lack of Power-Tach Over Run-Lack of Power!

OK, the boat is back in the water and running better that ever and after fighting the Texas sun for several day trying to resolve this issue, we finally hit on the problem. Just thought I would close this thread out with what we found.

After all of the above, we were down to:

Checked the alternator ? no help.

Checked the coil ? no help.

Rebuilt the carb ? no help.

Changed out the carb ? no help.

BOUGHT an inductive tach ? no help other than to establish that when the boat tach was showing 5000-6000 range, the engine was actually running in the 2000-3000 RPM range; just as I thought.

Finally changed the ballast resister ? BINGO!! She runs like a top. Voltage to the coil in run: 7.8v

THANK YOU ARABIAN180 FOR THE POST RECOMMENDING THE AUTOZONE #8214

Reinstalled the old resister ? Back to the same old s*%t! Voltage at the coil: 3.1v.

Changed out the resister with the new one and off we go again. Voltage back to 7.8v.

The best we can tell from all of this is that with a bad resister, the coil was not receiving enough voltage so that when the engine load reached the WOT range, the coil simply could not fire either fast enough or strong enough. Since the boat?s tach is driven off the coil, this also somehow threw the tach into a fit and it was reading an RPM that simply was not there.

I am still not totally sure why all of this works together and will do some more study to better understand it all. But for now it is time to enjoy the summer!