540 BBC running poorly
 

1991 Cigarette 35 Cafe Racer. Twin 540s built by White in TN. 34 hours since built.

I pulled the motors in August in order to replace both transom assemblies.

Everything is reassembled now. Motors not running well. The motors both backfire through the carburetor if base timing isn't well above 25 degrees, which is way too high.

I pulled all the plugs today. Most look ok, some are white and look almost unused but many are dark, some sooty, and some wet. It seems like they should all look similar, but they don't.

I will attach pics of the plugs shortly.

One more.

How much dust got in your carbs :) or dist cap ?

Good question. Could have been a lot. They were on my garage floor for 2 months.

what ignition system?

How old is the fuel in the tanks?
Is this last years gas or was new gas put in and the boat run this summer?

Both engines have the same problem so it has to be something common to both. Fuel...

MSD distributors and marine 6m-2L boxes. Mallory coils.

Tanks have about 20-30 gallons about 3 months old, 20-30 gallons fresh in the last week, and maybe 20 gallons old fuel from when I bought the boat in June.

have you removed the dist caps yet? had a set that ran bad after sitting a few months and when I opened the distributors they were rusted up bad and the springs had fallen apart. put in new distributors and crane boxes with the dist locked and the crane box controlling the timing and they have been flawless since.

Was anything removed from the engines or were they pulled 100% complete??
Why mess with the timing?? They should have been timed when they were removed.

First thing I would do is pull fuel filters and dump them into glass containers and check for water contamination.

I removed the caps and rotors, they appear to have been new from when the engines were built, which means only 35 hours of runtime but a few years old. Nothing obviously wrong with them on visual inspection. No visible rust on the distributors advance mechanisms. I sent the msd boxes in to be checked and they said all was well with them.

Prior to pulling the motors, i had an issue where the starboard motor would only deliver about 4300 rpm while the Port was still pulling to 4800 and more. Also running hot. So, timing seemed like a possible reason. Unfortunately I did not check the timing prior to messing with it.

I'll check the filters this week and report back.

Prior to pulling the engines I removed caps, rotors, carbs, plugs/wires, and headers.

I messed with the timing because of a discrepancy in the way the 2 engines were running, both in power and water temp.

I just pulled fuel filters and dumped them into a glass pitcher. Few bits of grit but no obvious water or other visible contamination. I'll let the pitcher sit to see if anything settles out but not looking like massive water contamination.

many things, some setups really need a lot of base timing, many run dist locked to help idle, I personally have 0 faith in MSD(may soon die) parts, boxes fail when they feel like it, magnectic pickups in Dist fail as well(chinese made) Next need a wideband to tune carbs correctly, without it you are pissing in the wind. As far as dist are concerned the DUI's are one of the best out there if you have room for the large cap HEI.

what was the reason for this

To minimize the chance for breakage when pulling the motors out.

are these standard firing order cams ?

I don't know. Any easy way to check? Certainly a slightly incorrect firing order would explain the way the motors run.

I would call the engine builder......if its a 4-7 swap cylinder 7 will be 3rd to fire

I'm gonna go with Speedy on this one.............probably has an LS firing order.

Spoke with the builder briefly - they said it's a standard firing order.

Put new plugs in it and try again.

I'm replacing all the plugs (NGK BKR5E) and both sets of plug wires first with Moroso Ultra 40, the Merc wire sets that I have now are probably from 2014 when the motors were built (or older). They ohm out OK but I have not checked for arcing at night, etc.

I would start by double checking everything you removed and put back on, especially firing order.
What is timing supposed to be set at?? Base and full advance?? I'm assuming full advance should be around 34*

Interesting you mention the main Merc plug. I had some intermittent connectivity in that plug on both motors. I did the best I could with cleaning and expanding the pins to make good connections again but I suppose it's possible there's still something iffy.

I use battery cleaner, then blow out. Awesome for trailer plugs too...just sayin. Do decent of snowmobile trailing and those plugs get nasty.:)Then my carburetor files for the female part. Male parts usually okay,but if not some emery cloth or fine grit sand paper small strips and use like polishing a crank. Slightly split male prongs. Slightly ! I've pilled some boats harness plugs apart and some people over did it. Doh !

Oh, pet peeve of mine: Do not put Dielectric grease on contact part of electrical plugs. Why do people do this ? Ugggh. It does not pass electricity. uggghh. It helps seals out the elements.:)

Sounds to me like a firing order issue. Double check your plug wire routing. Std firing order is 1-8-4-3-6-5-7-2 . Clockwise rotation.

a local buddy of mine did this. He replaced his caps and rotors on his new to him 572's. Engines idled like crap, down on power, etc. He called me a few times to pick my brain. We talked about carb adjustments, fuel system stuff, etc. Finally one day he calls me and tells me he found the issue. He had two plugs wires swapped on both motors!

When i built my 598 CI for my Daytona - on first fire up it wouldn't idle and ran like crap. Turns out I wired the plugs for normal firing order.
Checked the cam card and it didn't say anything about 47 swap. BUT there was a 47 on it so I did the swap.
Ran great.