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2007 bf30d vaccuum readings

C

Chubbsdad

Member
Hi all new to this forum
I have a bf30 which had a complete overhaul early last year . My unit was overhauled because of low rpm approx 4200 despite changing rings and carb overhaul , my wot compression then was approx 180 psi. A new block was fitted and i managed to run for the season at app 5200 rpm .
I have only managed a couple of outings this season ,however I am back to square one with the running rpm at around 4500 rpm I have reset pilot screws to 1-7/8 turns and done a few syncronisations but it didnt seem to make much difference to the output rpm.
I understand that my carbs might require cleaned again ,but can anybody tell me what vaccum psi I should be pulling . My understanding of Of 4 stroke vaccuum is that it should be around 18 -21 at idle rpm .the higher the better ,that tells me my rings are good . I havnt yet done a compression test
Can anybody give me some vaccuum figures to compare
Cheers
 
Your understanding is correct. 18-21 in.hg. is optimum for a 4 stroke gasoline engine of ANY ilk. The vacuum should rise slightly with the engine turning 2 to 3k rpm. Although... the closer to 21in.hg the reading is at idle. the rise will be almost negligible.

The engine is basically an "air pump" that you spray gas into and light it. The mechanical "seals" for good vacuum are:

The pistons and rings fitment to accurately honed cylinder walls.
The throttle plate(s) to throttle bore(s) "fit"
The valve stem to guide "fit"
The valve face to valve seat "seal"

And, of course, intake manifold gaskets and casting.

Exhaust restrictions will lower vacuum readings. An outboard that is mounted too low on the transom will have increased exhaust "resistance" depending on the depth of water the exhaust gas is forced to pass through.

But, your lowered rpm sounds like a case of not enough fuel being burned.

Do you ROUTINELY drain the carburetors if the outboard is sitting for more than a week or two? Not doing that seems to be the number one cause for problems with Honda carbs.

With today's fuel blends containing more and more alcohol...which is "hygroscopic"...meaning: attracts WATER. Fuel left in the lines and the float chambers will typically have some "drawn in" water vapor that then condenses. Water...even vapor in the air....will hold small mineral particles which then have an opportunity to attach themselves to the insides of the carb passages and cause restrictions. Fresh fuel and frequent carb draining is your best way to avoid this problem.

And what about throttle adjustment? Is the engine getting full throttle? A PRECISE carb syncing WILL provide for a bit greater power but does more to enhance idle quality than anything else.

There are many things that can have you down on power....from spark plugs to rough bearings in the transmission. The trick is to figure out what it is.

Good luck.
 
Hi I seem to have difficulty replying to the thread
Thanks very much for the info. Seems like I need to clean my Carbs as I have not been draining them .
reading the threads on the forum I can see that most of the low power problems are due to fuel contamination .
My trottle etc has all been checked and seems to be ok.
my interest in vaccuum is that on doing the sync I only get around 15 inches vac at idle
I was looking for some real time figures to confirm mine.Hi anybody out there doing a sync on your carbs what sort of readings do you get
thanks
 
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Well, your low vac readings could simply be a result of where you are tapping in. I was discussing main manifold vacuum but the balance ports on your carbs would be AHEAD or upstream of the throttle plates. This is considered "ported vacuum" and is almost always lower than vacuum accessed in the main manifold plenum.

They (engineers) used to actually take advantage of this reduced vacuum source on automobiles to provide for "switching" of various controls like distributor timing advance and such.

If you are really interested in obtaining true "absolute manifold vacuum" readings to assess the health of your engine...you may need to drill and tap a "diagnostic" port of your own. This is not something always provided for on outboards.

Good luck with your carbs.
 
Hi jgmo thanks for the info .I thought that the tappings were direct into the manifold of each carb thus giving me the absolute vac readings on that carb.
 
That is correct...for THAT carb. That's the problem... I think. Each "balance port" is in the manifold runner for an individual carb. If it were a single cylinder engine, then that might be the most efficient tap point. But on a multi cylinder engine a point that is COMMON to all cylinders MIGHT give a better reading and will be less subject to "pulses" that may be taking place in each individual runner.

Outboards being in such compact "packages" don't allow for much room to find a "common" point to tap. I was just speculating...something I shouldn't always do I guess...as to why your readings might be so low.

Let me ask you this: Have you thoroughly gone over all the "air vent" tubing and connectors for cracks and leaks? Also, have you vacuum checked the decel "dashpot" along with it's check valve for leaks? These are all places vacuum could be "robbed".
 
I have gone over the main joints,rubbers ,with Wd 40 when running but it made no difference. I have not checked the decal dashpot , so that might be a possibility . When the unit was being repaired last year we had difficulty reaching the compression specs . I eventually took the unit out and as I said I managed to get about 5200 rpm at Wot With engine not reaching full rpm ,we thought that the boat might be over propped ,a smaller prop to bring the rpm and power up. However this year due to one thing or another I have not been able to take the boat out until recently. On my first outing it got to about 4500 rpm so I reset the mixture screws and gave it a sync according to the books it starts and idles ok but does not deliver full throttle.
When I was trying to tune some extra rpm I noted that my vaccuum was around 15 inches last year prior to the ovehaul my readings were around the 10- 12 at best . With the overhaul its come up due to the compression increase but never having taken readings I dont have a base . So I was wondering if I have the same old problem but its a vacuum leak that has been missed. Its a long story
 
When you performed the OH, what was done to the head? You mentioned a new block but I didn't see where you listed any head work.

I'm afraid I'm just making a long story longer but going back to valves sealing and guide fitment...that can lose you a lot of "efficiency" if not set up. Also...timing is another factor neither of us has mentioned as yet. Although, if it starts and idles well...the timing shouldn't be too far off.

But if you are running an old, stretched belt or if the "cogs" are significantly worn (belt OR sprockets) the "error" might get exaggerated at higher speed and load demand.

I'm afraid I've exhausted most of my ideas for you. Maybe someone like hondadude will jump in and provide a DING DING DING moment that would cover something we may have missed. He's GOOD for that!
 
Its good to talk and throw thoughts around , your comments are most welcome ,its dark outside here in scotland and its the end of my vacation so my problem is now how to test the unit apart from stripping it down .
Going back to the overhaul the head was leak tested and corrected but no new parts , the timing was checked and seemed to be spot on but again my low vac readings had me puzzled. I thought that the timing was out but I checked it outmyself while I was at the shop . Read somwhere about the timing cog moving on the key ( seems its a 2 part fitting) centre boss moved its interference fit . In general my unit is in good condition belts cogs all ok . So its got me puzzled .
Im a retired chief engineer so I know a bit about engines in general,however when it comes to this engine as I said above its got me puzzled ? Happy boating and thanks for the input
 
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