First time out, major shudder / vibration...

[cp4000]

First time I took the 1976 70HP out on the water. Starts great, idles a little rough but not too bad...when I went to open it up, it shuttered pretty bad, (Vibration) to the point where I didn't want to continue. Had it up to 12mph at about 2500 rpm never getting up to plane. (Older boat, didn't want to rip the transom off.) Ideas? I was thinking bent prop shaft? Bad prop? Timing? Not running on all cylinders? When I turn the prop, it doesnt wobble...The prop has one blade that is cupped / bent a tad? I have put a timing light on each cylinder, they all spark...Did run engine with one plug out at a time and seemed to get highest rpm when all three were attached? Would greatly appreciate the help...was a pretty disappointing day

 

[cp4000]

Re: First time out, major shudder / vibration...

Oh yea, check the compression a month ago and and three were good...

 

[Bifflefan]

Re: First time out, major shudder / vibration...

Firing order correct?

 

[cp4000]

Re: First time out, major shudder / vibration...

I'll check in the manual today...but the plug wires will only reach one plug so I'm not sure how that could have changed? The only thing that sticks out in my mind was....when I pulled a wire on each cyl to check rpms to look for a non-firing cyl. All read @850rpms with one cyl unplugged except one when I was measuring off of cyl 3. (Think it was cyl 2) It read 1500rpms off of cyl 3, so I stopped the motor, switched it to cyl 1, then it read 850 like all the others?? Fluke?? I did buy a spark tester today and will check again. For a shudder / vibration this bad, is this the area you would focus?

 

[cp4000]

Re: First time out, major shudder / vibration...

Checked spark again on all three cylinders with a spark tester, 7/16" gap, strong spark on each. All three plugs had the same oily look to them. Straightened out the one blade on the prop that was cupped / bent...plan to time the motor tomorrow and test this week. Can the carbs cause the major shudder? I really haven't done anything to them yet?

 

[tashasdaddy]

Re: First time out, major shudder / vibration...

carbs surely can, best way to ruin that motor. also a bent prop shaft.

 

[Haffiman]

Re: First time out, major shudder / vibration...

'All read @850rpms with one cyl unplugged except one when I was measuring off of cyl 3. (Think it was cyl 2) It read 1500rpms off of cyl 3, so I stopped the motor, '

This indicates that you have a double firing cylinder. (two cylinders fires at the same time)
Hook up to a spark tester and double check.

 

[Sixmark]

Re: First time out, major shudder / vibration...

First off, check the compression again now, the compression a month ago can be drastically different now, diagnose today's problem today, do not rely on things from a month ago.

Check your spark again....

By the way, when you did this compression check a month ago, did you ground the plug wires?

I had this same problem with my 75hp it idled a little rough, and I decided to run up the rpm a bit and see what she would do, she bucked and shook like mad, did some quick tests at the dock, replaced the power pack and everything was fine.

Do not take this as a cheat sheet to pinpoint the power pack, but rather an example of how important the proper testing, done in the proper manner can quickly pinpoint the problem.

 

[cp4000]

Re: First time out, major shudder / vibration...

Sixmark,
That sounds just like mine...I will check the manual and test the power pack tonight. I'm pretty sure the prop shaft isnt bent, I see no wobble whatsoever, but one blade of the prop was bent so I guess its possible. I will check compression again...The double firing cylinder idea has me kind of stumped as to how I would go about checking that with a spark tester? Will report back when I have some further answers. Will be ordering somne carb kits too...

 

[cp4000]

Re: First time out, major shudder / vibration...

Oh yea,
No I did not ground the plugs when checking compression. I basically removed all three plugs, taped up the boots and checked each cylinder with a tester. Is this not right?

 

[Haffiman]

Re: First time out, major shudder / vibration...

Sixmark,
That sounds just like mine...I will check the manual and test the power pack tonight. I'm pretty sure the prop shaft isnt bent, I see no wobble whatsoever, but one blade of the prop was bent so I guess its possible. I will check compression again...The double firing cylinder idea has me kind of stumped as to how I would go about checking that with a spark tester? Will report back when I have some further answers. Will be ordering somne carb kits too...

Hook all 3 cylinders up to the tester.
(hopefully you have one with at least 3 terminals)
Look at the spark sequence. It should go 1-2-3-1-2-3---
If 2 cylinders sparks at the same time = double firing.
You may even use a timing light on the fly wheel.
Mark all cylinders at TDC, hook up on one plug cable at the time. Move the timing light around and see if you may read another cylinder in addition to the one at TDC. If you do = double firing.

 

[Sixmark]

Re: First time out, major shudder / vibration...

You can get a good spark tester at most any auto parts store.

For future reference, the spark plug wires should be grounded when performing a compression test to avoid damaging the ignition components.

As for a bent shaft, you won't always see it, so don't rule it out just yet.

 

[cp4000]

Re: First time out, major shudder / vibration...

I'm gonna start by installing 3 carb kits, tashasdaddy has me kind of worried, with good reason of course. I removed all three carbs today and will order the kits soon and then go from there. All reed valves look good. I noticed you do not get the slow speed valve or seat in the kits? Not a big deal?

 

[cp4000]

Re: First time out, major shudder / vibration...

I took apart the carbs and they were absolutely spotless. I am still going to put kits in. One major question though... There are technically no valves for the slow and main jet? All three basically had what I thought were valve seats behind the screws that were tight, no adjusting needed and no adjusting valve behind the seat. Hmm...ok? I am starting to think the prop may have caused the shaking like mad under throttle. I am going to buy a prop, along with the carbs and test again...So much for a cheap project haha.

 

[mrcj001]

Re: First time out, major shudder / vibration...

I had a `75 135 crossflow that was shaking like that......It was low on compression on one cylinder and would not fire correctly.

Re-check the compression on the engine. Something could have happend since the last time you tested it.

These outboards can screw up and you wont know or realize that it has.

 

[cp4000]

Re: First time out, major shudder / vibration...

I rebuilt all three carbs today and adjusted the floats, replaced all fuel lines in the upper half and put in new spark plugs. It idled just as before, pretty good...a timing adjustment would probably make it near perfect. I did re-check compression, I have 120, 120, 115. Hooked all three cyl up to spark tester but they fire too fast to see if there is a firing order. Would be water testing in the AM but now its not peeing...From when I had it in the water until now, I cranked it over for about a minute out of water. I can't believe I took the impeller out, but will be trying to find one tomorrow I guess...

 

[cp4000]

Re: First time out, major shudder / vibration...

I kind of ruled out double firing. I did mark all cyl TDC on the flywheel and verified with a timing light, all matching up under the light. Want to test on the water but it stopped peeing and getting very warm, fast! I bought an impeller (replaced a month ago), pulled the lower unit again, impeller was fine. I put the hose on the water tube that goes up into the motor, got water out of the tell tale. I poured water down the impeller housing and came out the inlets. Nothing appears to be blocked. The housing is a bit scored so by my deduction, that has to be the problem, am I right? Just kind of wierd how one day its works fine, the next it doesn't?

 

[cp4000]

Re: First time out, major shudder / vibration...

Almost ready to take it out for a second time to water test. Today, used the Joe Reeves method of timing, was a few degrees off. Idles better and sounds strong. Bought a different prop for testing purposes. (13x19). Just waiting for a new water pump kit with housing. Hope that major shudder is flushed out. Running out of time, its getting cold

 

[cp4000]

Re: First time out, major shudder / vibration...

Can't believe how well the OEM water pump rebuild kit worked. The tell tale is the strongest Ive seen. Will be taking on the water for the second time to see if that shudder / vibration is still there on Saturday.

 

[cp4000]

Re: First time out, major shudder / vibration...

Back on the water this morning, 35.4 MPH without trim and correct prop. Shudder / vibration is gone. Not sure if it was some of the work I did to it or the diff prop but it is gone! It's got alot of power and sounds good! Thank you guys very much!

The only issue it had and maybe you guys know off the top of your heads...When you throttle up slowly, she will bog down and eventually die at about 1500-2000 rpm. If you bring it back to nuetral and throttle it fast, she gets up and goes. Any ideas?

Thanks again! I am a happy camper, I mean boater