View Full Version : new carb, possible tuning issue?

08-01-2013, 05:39 PM
Just installed a new 4160 carb on my PCM 351. Got it running on the trailer the other day, seemed to run great once the fuel bowls filled up. Adjusted the idle so it was idling at 800 rpms, seemed to rev fine. Took it to the lake this morning, and the damn thing very nearly wouldn't start, on the trailer in the water on the ramp. It did start eventually, and ran fine on the lake, no problems at all. But I didn't turn it off lol. Got it back to the trailer at the end of the run, and before pulling it out of the water shut it off, and tried to restart. Was just as difficult to start as at the beginning!

Now it's home in the garage, and just for laughs I went out there and tried to fire it up. It fired right up immediately just like it's supposed to. I shut it off immediately b/c I didn't have water connected. Disbelieving, I tried it again. Fired right up the second time too.

The only possible explanation I've come up with is that the times it fired up perfectly, the boat/trailer was level. The times it was difficult to start, the nose of the boat was higher than the stern by the amount you'd expect on the ramp. Possibly a float adjustment issue? I'm open to ideas though. It runs really really well other than the starting issue. Pulls hard all the way to 45 mph. Runs smooth.

08-01-2013, 05:46 PM
It sounds like you may be right on the angle causing an issue. One thing I have learned over the years is that by the time a problem manifests itself, there are often multiple causes combined. Address all the basics: plugs, wires, points?, dist cap & rotor, timing, coil, corroded connections at coil, ballast resistor......blah, blah, blah. If one of these are out of spec you might not notice, but if two or three gang up....you are hosed.

08-01-2013, 06:24 PM
It's a new coil, new ballast resistor, and the wiring connections to it are in great shape. Cap, wires and plugs new this season. The boat has a Crane Cams XR700 EI conversion kit in it. Have not checked timing, don't think it has points anymore because of the EI. Definitely would be possible for me to be wrong on that though!

Any tips on checking the float level? And setting it as necessary?

08-01-2013, 06:34 PM
You are correct, the EI replaces the points. I forgot about all the work you have been doing. Two things to look at are timing and that your mechanical advance in your distributor is moving freely and returning to it's base setting. If your timing is too far advanced, or if your mechanical advance springs are not returning the timing to base timing, you will have an intermittent hard starting problem. If you don't have a timing light, you can pick up an adequate light at Harbor Freight.

08-01-2013, 10:52 PM
You might be on to something there. It backfired once before it started at the lake, and I noticed that more than once, the engine rotated the opposite direction briefly after letting off the starter. I'll go get a timing light and check. Of course with my luck whatever caused it is currently in remission! Hopefully the timing light will reveal something.

Edit: well I'm getting a light later today. In the meantime I popped the cap off, and I can confirm that the springs return the rotor back to its base position if I grab the rotor and rotate it. Seems like there is about 30 degrees or so of advance (I'm guesstimating) that is mechanically allowed by the mechanism, and the springs are capable of snapping it back to base position every time.

08-04-2013, 10:35 PM
Just went through the exact same problem! Put all new firing components on it (coil, plugs, distributor, EI ignition ) put a brand new holley marine carb with exactly the same issue. You are right it is a float problem. The primary float is likely to be flooding it because it is set too high. I removed the front bowl and tweaked it to close sooner not allowing as much fuel in the bowl. Also if it back fired could have ruined the power valve . I also had to replace after a back fire will it was flooding. Hope this helps . I just got back from a 3 day week end with no issues , man I happy!! That is the first time this season. Hang in there you will get it.

08-04-2013, 10:42 PM
I would check to see if you can see fuel going into the carb under the spark arrester after you have had it running . My boat u could see fuel pouring in . Check that be four you adjust float.

08-04-2013, 10:48 PM
Since it is a new Holley I didn't go the float route, but as his distributor appears to be functioning properly, it is a possibility.

Any updates strad?

08-06-2013, 12:13 AM
Yeah. I had it out again today. Between last time and this I noticed that the coil lead was not seated all the way. Same with one of the spark plug leads. I corrected that. It has started up fine since. Dunno how they became unseated since they hadn't been touched. I do have a light but I did not check timing today.

08-06-2013, 12:20 AM
That make some sense. Intermittent coil connection will cause a backfire from having no spark while cranking and flooding, then firing. Make sure there is a little slack in the coil wire and that the coil bracket is tight to the intake and has a good grip on the coil if you haven't already.