problem solving engine trouble

Hey everyone.   I have a 76 nautique with a ford 351.   The engine was completely rebuilt a couple years ago.   After the rebuild, the engine ran great other than one minor problem.   When I driving at a slow speed like when I was trying to pull kids skiing the engine would start sputtering and want to shut off. I would have to pump the throttle a few times to keep it running.    It ran perfect at higher speeds and I didn't have any trouble accelerating or idling.    I thought it was a carburetor problem and tried everything I could think of to adjust it.   The carb was also rebuilt/cleaned during the engine rebuild.   

Also, the carb was a holly Automobile carb so it wasn't even supposed to be on the boat. I thought that may have been the problem anyway.   I wasn't sure if they were tuned differently since boats and cars are under different loads.

Since that was a safety problem that needed to be fixed anyway I bought a new carb for the boat this year. It is a Quick Fuel M-600.    (Here is the thread I started about that: http://www.correctcraftfan.com/forum/forum_posts.asp?TID=36281&KW=&title=where-to-buy-a-holley-carb)

The QF carb install was simple.   The boat fired right up and ran great in the drive way.   I took it to the lake to check everything out and do some final adjustments but I was still having the same problems except this time it was trying to sputter off at slow and cruising speeds. It seemed to be idling pretty high too but I think most of that is because I needed to re-adjust the throttle cable after installing the new carb.

Does anyone have any thoughts on what could be causing the engine to want to choke off when cruising?   does it sound like a carb tuning issue or could there be something else going on?   Thanks for your help!

thanks, JQ.    We cleaned the fuel tank out when we rebuilt the engine so I'm going to mark that one off the list.    I also just put a new fuel filter/water separator on a couple months ago.    I'm going to do some checking tomorrow to see if the fuel pump is working correctly and go from there.    I'll let everyone know the progress.   

How would you go about checking the fuel tank vent?

I'm not able to replicate the problem I'm having when I'm running the engine in the driveway so i'll have to get on the water to do some of these tests.

I did just check to make sure the fuel pump was working by unplugging the coil wire, pointing the fuel line from the pump to the carb into a glass jar, and turning the engine over. The pump is shooting blasts of clean fuel directly into the carb so I'm guessing the pump is working correctly. I also don't see any debris in the fuel.   I'll try to get the boat on the water as soon as possible to check the fuel tank vent.

Any other ideas?

Also, my throttle cable is sticky. (When I bump the rpm's up from neutral and then return the throttle to neutral the rpm's don't come back down)   Is there a way to loosen up the cable or is it time to replace?    Thanks.    

good question.   Before I decided to change the carb it was acting sticky like I said in my last post.   After the carb install it was improperly adjusted.   When I bumped the throttle forward, the engine would rev up before it went in gear causing the boat to jump.    I adjusted the cable exactly like someone mentioned in this thread:   http://www.correctcraftfan.com/forum/forum_posts.asp?TID=26601&title=shift-cable-adjustment-pcm-123-and-morse

It is working fine now other than when I bump the throttle forward and then bring it back to neutral, the RPM's don't return to idle.    

Also, The quick released on the cable should be adjusted so it provides some light pressure to push against the idle stop every time.

Lift Films wrote: Gottaski, that may be my problem.   I'll check when I get back to the boat. Back to the engine troubleshooting, I've checked all the fuel delivery options and it seems like the carb is getting fuel like it is supposed to. I called quickfuel'd tech line and the guy I spoke with said I needed to go up a jet size.   He said the m-600 comes with 67 jets and I need to go up to a 69.   I also ran the boat this morning and doubled checked the float levels. They are at the recommended height.    I am only getting the hesitation problem around 2000-2500 rpm.   If I let the boat cruise at that speed it will stumble.   If I don't touch the throttle it will stumble for a few seconds and then resume running normal.   After 5-10 seconds it will stumble again and repeat. Any thoughts on any of that?   I appreciate yalls help as I learn a lot of this stuff.

67 jets stock? That is strange. My M-600 came with 65 jets in the Primary and 72 jets in the Secondary. It was my understanding from QF that 65/72 was the base or stock setting for sea-level. Well, as jets are relatively inexpensive, that may be a place to start.

The intermittent stumble is a head-scratcher...

2 spacers in series? That doesn't sound kosher. You could always try removing one spacer. How thick/tall is the single spacer? The 2 combined?

Be sure to use the marine carb/spacer gaskets. Thicker and stronger.

http://www.skidim.com/GASKET-CARBURETOR-SPACER/productinfo/RM0054B/

JQ

Might be good to have a timing light on to watch the advance under rpm change. It's possible the centrifugal advance is rusty and not moving outward and then snapping out as rpm increases. Lack of timing advance could explain some of your intermittent stumbling.

To check for a vacuum leak spray starter fluid around the base of the carb. If it is leaking it will increase rpm as it sucks in the starter fluid.
Soapy water wont find a suction leak, a pressure leak yes a suction no.

Disconnect the cable from the carb and see if you can get it to idle. Sometimes missadjustment of the cable will cause it to hold open making a high idle.

Vacuum leak could also be at intake manifold gasket to head. Unlikely, but may be worth ruling out.

I wasn't able to find a leak with the starter fluid test.

Here is my vacuum reading. Also notice the hard start:
vacuum reading

Do you know if my engine is right or left hand rotation?
'76 351 PCm
My firing order right now is set up as 1 8 4 5 6 2 7 3

Timing is supposed to be set at 10* BTC. I can't even see the numbers unless I rotate the distributor which brings the RPM up above 1000.

thoughts about any of this?

Ok timing is adjusted properly so I can mark that off the list.   I'm still getting a high idle (approx. 1000 rpm).

Can a bad PCV valve cause a high idle?   Should it be air tight? It's loose enough I can wiggle it around and pull it right out with almost no friction.     

I've tried unhooking the throttle cable and loosening the idle screw on the carb.   Neither will bring the idle below 1000 rpm.

Plug the PVC entirely, for a test. Then you will know if it is the cause.

Both