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Bf30A - Questions after Carb Rebuild

S

Samav

Contributing Member
Motor is a Bf30A LHSA pre-1997

Hey all, I just finished rebuilding my carbs for my motor. It was running very rough before, but after I rebuilt and cleaned the carbs, it started up quickly and sounded beautiful. I am still seeing some concerning signs though, so I am hoping someone can help me understand if any of them are causes for concern.

First of all, I have recently been having problems with telltale strength. Before I rebuilt the carbs, it was a mere dribble, which was much different from what it used to be, the thing used to blast water from its telltale. After I rebuilt the carbs, the telltale blasted strong again like usual. But, three days later, I started the motor, and the telltale was pretty weak again -- not as weak as it was before the rebuild, but noticeably weaker than when I first started it up. I used a carb synchronizer to measure the vacuum in each intake runner, and found that, even though they were in sync, they all had a very weak vacuum. I just replaced the intake manifold gasket and every gasket in the carbs. Why could they all have a weak vacuum, could this weak vacuum be related to the weak telltale, and is it even a problem at all (this motor is very old and has a lot of hours)? I noticed that there was a water passage next to the intake manifold, so I thought maybe a vacuum leak could cause the telltale to weaken. Am I just being over cautious?

Another thing is that the motor was running extremely rough before the carb rebuild. Afterwards, it ran very smoothly. I adjusted the mixture screws and changed the idle speed and got the motor running perfectly. I started it three days later, and it was running slightly rough again. What could cause this quick deterioration? I noticed that the air coming out of the exhaust felt sputtery. Is it supposed to feel that way? The motor seemed to sound like it was missing every now and again, but I'm not sure (I'm no expert here).

I haven't done a valve clearance adjustment yet, could that make a big difference?

So, are these things causes of concerns, and if so, what are the possible causes/solutions?

Totally beside the point, but the fuel pump hasn't been replaced since the motor was bought (20 odd years ago). Is that something I should consider replacing?
 
racerone said:
Carburetors have nothing to do with the telltale.------Replace the waterpump impeller !
Click to expand...

I know the carbs have nothing to do with the telltale; I was wondering if a vacuum leak at the water passages between the intake manifold and the intake runners could have anything to do with telltale strength. Sorry if I didn't make that clear. I know it seems far fetched but this damn telltale has been bothering me for a while now.

Also, I have replaced the impeller recently, all gaskets, the housing, and the impeller itself, with no noticeable improvements in the telltale strength.
 
Blow compressed air back through the tell tale nozzle to unblock the barb fitting that the hose goes on to . Also make sure your thermostat is not stuck open as that will also reduce water pressure. Vacuum is always low, as long as they are all the same it's good.
 
I have a 1998 BF40 and definitely recommend the following if it has never been done before. I consider this maintenance especially with your vintage of motor:

1.) To ensure your carbs are synced properly, you should use a gauge like the following: https://smile.amazon.com/gp/product/B004MSJ7E6/ref=oh_aui_search_asin_title?ie=UTF8&psc=1

Here is a video on how to do it: https://youtu.be/bsHCMJnRY9A


2.) Water issue: Step 1: Replace the impeller & HOUSING (its not that expensive or hard to do and you have already done this step), Step 2: Like RACERONE said, when the lower unit is OFF blow compressed air back through the telltale. Step 3, remove the water inlet cover, clean it out, replace the thermostat and gasket, then reinstall. Then you will have complete peace of mind and years of trouble free pleasure.

3.) Valve Clearance: If it has never been done, I definitely recommend this, you will need two 10 mm box end wrenches and a feeler gauge. Here is a YouTube video from Dangar Marine, on a BF40: https://youtu.be/aVBWNAhs3XU

Outside of that the only other things to check would be:

1. Timing, this will result in a new belt
2. Spark, test and replace as necessary
3. Checking for a vacuum leak at the manifold gasket. You can use a propone torch unlit with just the gas on while the engine is running moving it around the gasket to see if you hear any changes in how the engine is running. If you do, replace the gasket.
 
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In addition to the very excellent advice above, I will say that you may want to remove the thermostat and housing and check in there for corrosion. Salt water can really take a toll over the years and, yes, it's entirely possible that the cooling passage in the manifold has been eroded.

More likely than a breach in there though, causing the erratic performance, is simply the possibility that the tstat isn't opening and then "modulating" as it should. This will definitely effect the way the engine runs and could be a variable condition of heating/cooling making the problem intermittent.
But it doesn't hurt to get in there and see what things look like in that head.

If you encounter a lot of white, powdery corrosion, resist the urge to dig and scrape it out. As you remove that stuff you are removing aluminum oxide and that comes directly from the casting. So be gentle and use vinegar or Salt Away to clean as best you can.
Renew the tstat if you haven't recently or, at least, stove top test it for reliable operation.
Good luck.
 
My short answer is I don't know. But.....

Do you have a source for the replacement 25hp carbs? Because a quick check on boats.net shows they too are unavailable.

The carbs will probably fit but I know the main jets are smaller and the intermediate and nozzles will likely need to be changed too in order to avoid having your engine run too lean.

Hopefully someone else on this site can give you a more definitve answer but you will probably need to start your own thread to reach a maximum number of members.

Good luck.
 
While the valves of the 30 are the same as the 25 (same part numbers) the camshafts likely have a slightly different lift and/or duration (different part numbers).

It's a common way that Honda and other manufacturers increase power output on some models using the same block and head.
That is why you will want to rejet the carbs to avoid lean burn conditions which could cause you problems. Not to mention the loss of horsepower.

Good luck.
 
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