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Selecting a prop for BF75A

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pagraham

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"I derigged a 1970 Whaler hull

"I derigged a 1970 Whaler hull and re-rigged everything on a 1978 whaler hull. I have a BF75A (1996) with a Attwood Stilleto # 335132, 13 3/8"x19" prop. Previously the motor turned 5400 rpm's now it turns 4700 rpm @wot. I borrowed a 13x17 prop off a 135 yamaha and got 5100 rpm. What should I do? Thanks for any help"
 
"I'm not that familiar wit

"I'm not that familiar with the two types of Whalers you are talking about in terms of weight and hull configuration. You want to get your WOT RPM to 6000 +/- 200 to 300 RPM. Assuming your engine is running correctly, AND it is mounted properly, then drop your pitch to 15", which should get you very close to the 6000 range."
 
"Someone suggested that I make

"Someone suggested that I make sure my engine is firing on all 4 cylinders, that if I am loosing spark from 1 plug that it wouldn't run that bad but would affect rpm's on the tach. How can I check this? Thanks"
 
"At idle, use an insulated pli

"At idle, use an insulated pliers and pull one spark plug wire off the spark plug one at a time. If the motor rpms change, the cylinder is firing. If there is no change when you remove the spark plug wire, then that cylinder is not firing and there is an issue with that cylinder. Unfortunately, this test is only good at idle.

If you have a timing light, you could have someone watch it as you are running and compare how the light looks on the different cylinders.

One question I would have is...if you did the rigging yourself are you sure you are getting full throttle? Also, did the motor run ok before you took it off the other boat?

If it sat for any length of time, it could develop some carburator issues.

Are the two boats different lengths and weights? How much different?"
 
"The boat is the same length a

"The boat is the same length and weighs approx. 100 lbs. more than the older hull.I was told by the whaler pros on whalercentral.com that the older hull is a slicker running hull. I rigged it myself. I have a Seloc Honda manual and I have gone thru the carbs a couple of times to clean. Then syncronized with vac gauges I checked and at full throttle the carb cam is in the full open position, wouldn't that indicate I am getting full throttle?"
 
"It does sound like you are ge

"It does sound like you are getting full throttle from your carbs.

Is the cavitation plate approximately even with the bottom of the boat? Motor height can make a big difference in rpms and runability.

It sounds like the 17 pitch was probably closer to the right prop for your first boat. WD is right, the rpms should be right around 6000.

If your newer hull has more friction, then a 15 should to get you closer to 6000.

Again this is assuming that the motor is at the proper height and all cylinders are functioning."
 
"I believe there is a proble w

"I believe there is a proble with cylinder #4, at idle when I unplug the spark plug boot there is no change in how the engine runs, I am getting fire to the cylinder, shouldI go thru the carbs?"
 
"Yes, that is what is sounds.

"Yes, that is what is sounds. You should check the compression is you can to see if there are problems in the engine itself.

If you do not have one, try one more test - although it won't prove the cylinder is good, it will verify that the bottom carb is having issues.

There is a black plastic lever that connects all the carbs together at their chokes. You should be able to pop that off with a screw driver at each carb. Run the engine again and manually operate the choke at the bottom carb only. The engine will probably pick up in speed. If so...Carb job time... If it does not change, open the drain to the carburator and see it you are getting any fuel at all into the carburator. If there is no fuel, it has a stuck float valve and still needs a carb job.

If you do not have a manual, I strongly recommend getting one to do the carbs. Try to keep them in order so any vacuum balancing that is needed would be minimal. \

Happy checking..."
 
"I identified #4 cylinder as t

"I identified #4 cylinder as the problem, cleaned that carb, re-installed. Started motor now # 2 cylinder has the same problem, went thru that carb and made no difference, still no change in rpm when I pull the #2 plug boot and I do have fire. Carb is full of gas, does no change when I choke that carb only. Do I need to go back thru the carb?"
 
"Yep! That happens sometimes.

"Yep! That happens sometimes.

There is some passage blocked somewhere. Be sure to pull out the main jet and the emulsion tubes that are going up through the carb. Be sure to clear out the very little passage up through the very thin tube going up through the carb.

If your idle mixture screws do not have caps on them...remove them and make sure that passage is clear also.

While you have them off, I would do them all..
There may be some particles loose in your fuel lines.

Flush your fuel lines. There may be some particles loose in your fuel lines."
 
"I've gone thru the #2 car

"I've gone thru the #2 carb 2 times now, inspected the O ring on the idle jet, looks good. Still no change when I pull the plug boot on # 2 cylinder. A Honda mechanic told me that some times the idle jet gets small cracks in them and they don't clean them they just replace them.I have the 4 original jets and found that they all have cracks in them. Inspected the #2 jet and it looks good, soaked it 2 days this time. This is very frustrating, to think the #4 was not working, sone as I get it going the #2 quits working. I have ordered another jet and another fuel pump. Saw that the fuel pump was leaking the other day, But I would think that you would loose fuel to both carbs at the same time, anyway it does not seam to be leaking now. I could try moving carbs around to see if there is any diference, any ideas? Thanks"
 
"Have you removed the idle mix

"Have you removed the idle mixture screws and cleaned those passages? I will try to attach a pdf showing the slow and transition stages and where the fuel flows. The air comes in from the hole in the upper part of the front of the carburator and blends with the fuel coming up the idle jet. This might help you cleaning these paths. Spraying brake clean through them will show you they are clear. The fuel for idle comes out some very small holes in the top rear of the barrel of the carburator See number 5.

293626.jpg


You can change the top three carbs around any way you want. The carb balance will probably be out, but you have to get them to fire before you worry about that."
 
"Mike, thanks for all your adv

"Mike, thanks for all your advice, that's an exellent illustration, better than what is shown in the Seloc manual I have. Yes I have cleaned the idle screw passages, counted the turns from seat before i removed them so I could put it back like it was. I pulled the carbs again, went thru and cleaned #3, 5 & 7 in the illustration. Put it back together but swap #1 & #2 carbs to see if the problem was the carb or the cylinder. Turned out the problem followed the carb to the #1 cylinder, no change in rpm when pulling the plug boot, now on #1 cylinder. Started backing out the idle mixture scew on the problem carb and noticed an increase in RPM's, once I felt like that cylinder was operating, pulled the plug boot and noticed a drop in rpm. So now I feel like all 4 cylinders are firing. That idle screw is now backed out 2 1/4 turns from seat, next one down is backed out 1 3/4 turns from seat. Is this normal? Shoud I replace the Idle screws? O rings? Sync the carbs? What do you think, thanks again"
 
"The initial setting for your

"The initial setting for your engine is supposed to be 1 7/8 turns. So you are not too far off. The main thing is if you turn it out more, will the cylinder start faultering? If so, note where that is and turn the screw back in until the cylinder improves then faulters again. Then find the midpoint of those two spots. Hit the throttle quickly...if it sneezes then adjust the screw out a little. Turning them out just a little after you hit the sweet spot will probably fix any sneezing. You may have to adjust the other carbs as well.

It would be excellant if you can sync the carbs, especially since you are moving them around. If they are out of sync, you will see a definite improvement in how it runs as you zero the carbs in.

You should again go back to the idle mixture screws to assure they do not need further adjustments.

It is all an iterative process.

If the idle screws are not damaged, worn or corroded, there should not be a need to change them. If a carb is giving me problems, I always change the o rings, especially on the idle jet to remove all doubts. Since the o rings come as a set, change them all.

While doing these adjustments, you should keep the rpm around 950 plus/minus 50. I tend to set the idle closer to 980-990. The idle will increase as you keep making adjustments. Be sure to readjust idle as you go.

When everything is ready to go, be sure to use a good fuel treatment (all the time) such as PRI-G, Sea Foam, etc. There are several out there.

If you do not already have one, adding a water separating fuel filter in the fuel line between the tank and the engine will minimize any water problems that may arise in the future."
 
"I syncronized the carbs, adju

"I syncronized the carbs, adjusted the cable linkage, set idle speed and set the idle adjusment screws at 1 3/4 turns out on each carb. Runs pretty good. Still no increase in rpm's, I guess I will go for the 15" prop. Also died when I nail it from idle speed. What could that be?"
 
"1. Be sure that you are gett

"1. Be sure that you are getting full throttle at the carbs. That can affect the top rpm.

2. Back out the idle mixture screws a 1/4 turn each. See if that helps the acceleration.

Check the Acceration/diaphragm device. It is the saucer shaped device (about 2 inches in diameter) with a small rod coming down out of it that is just to the left of the #3 carburator.

Look in your manual for how to check. It actually shoots a small amount of fuel into the carbs when you are trying to accelerate. May sure it is being activated when you accelerate.

If you push the rod up gently while the motor is running, the engine should speed up some then come back to idle. There is also a dashpot checkvalve in the hose that should be checked too. It may be in backwards. Check your manual for proper direction as well as how to check.

I have emailed you a Service Bulletin that can help you with this. It has great pictures.

If you do not get it, let me know."
 
"You never did answer one of t

"You never did answer one of the 1000 questions that I asked about how high is your engine mounted. If the cavitation plate (horizontal plate above the prop) within one inch plus/minus with the bottom of the boat? Trim the motor until the plate is parallel to the bottom of the boat and put a straight edge on the boat bottom or the cavitation plate, whichever is lower ...to check.

If the motor is mounted too low, it will affect both the take off as well as the top rpm."
 
"The cavitation plate is 3&#34

"The cavitation plate is 3" above the transom, the motor is on a 4" fixed bracket mounted all the way up. I have not observed how the plate is @ WOT but I do no slip or blow out at all when accelerating. It is deffinatly not to low. The throttle cam is full open minus maybe an 1/8" or less from hitting the end of the cam slot at full throttle. Aren't you suppose to make almost touch. I have a set of have carb sync gauges I used"
 
"Disregard the plus or minus 1

"Disregard the plus or minus 1 inch since you have the motor on a bracket. This changes the basic rules. The 4 inch difference may be ok.

The main thing on wide open throttle is that the carb plates are horizontal at full throttle.

Check out the Service Bulletin about the acceleration device and try adjusting the idle mixture screws out a little.

Did you set them initially at 1 7/8 or 1 3/4, where they are now?"
 
"Ok, I don't remember now

"Ok, I don't remember now 1 7/8 or 1 3/4, I'll go back and check again. I read the service bulletin, thank you, you are improving my outboard manual.Sounds like that may be my acceleration problem. If the throttle cam is full open how could the carb plates not be horzontal?"
 
"Paul, I don't think your

"Paul, I don't think your mounting is correct. The rule is 1" of elevation of the cavitation plate above the bottom most point of the keel for every 1 ft. of offset off the transom. With a 4" bracket offset, your cavitation plate should be parallel with the keel, or maybe a 1/4" above. Three inches above is too much. And although you are not feeling any slip, you are not using the engine's power efficiently."
 
I had it mounted 2 holes lower

I had it mounted 2 holes lower and the boat felt sluggish and the rpm's were about the same. Still 47 to 4800 rpm @ WOT
 
"Well, okay - what can I say?

"Well, okay - what can I say? If it works, it works. Just contrary to conventional wisdom and my own experience.

My current boat is a 25 ft. C-Hawk with a 9.6 foot beam. The engine is mounted on a 3.5 ft extension off the transom. Cavitation plate is slight over 3" above the keel. The 225 pushes that 5500 lb (plus) boat at 39.5 mph at 5950 rpm with trim tabs full up."
 
"The number of turns on the id

"The number of turns on the idle mixture screws are initial settings. It they are set too lean, your acceleration will suck."
 
"Leaner would that be backing

"Leaner would that be backing it out from seat? I hope to get back to it tomorrow, real nasty weather here today. I ordered the dash pot check valve today and picked up a 15" pitch prop, hope for another water test tomorrow evening"
 
"Turning it in makes it leaner

"Turning it in makes it leaner. If you turn it out a little more, it should give it more fuel which would help with acceleration. Turn it out too far, the the engine will not idle correctly.

I am talking about 1/8th to 1/4 turn more out to check the throttle reponse.

Also, make sure the engine is up to operating temperature to make this adjustment.

Good luck tomorrow."
 
"Mike, I put on the new 15&#34

"Mike, I put on the new 15" x 13.5" prop, started and warmed up the motor , adjusted all idle screws to 2 turns from seat. Checked the diaphram module and it operated properly, engine spit when I operated the push rod it is also ajusted right. Then backed out the idle screws another 1/8 and it smoothed out and did not spit any more, a good sign. Launched the boat, exellent hole shot, no slip. Motor tops out at a solid 5300 rpm @ WOT. Pretty rough out, might have squeezed 5400 under better conditions. I sure expected a bigger increase in rpm's. Observed the cavitation plate at plane it was just above the water, also have a Doel fin on it it was just clear of the water. I felt like it ran really good, nail it from idle and it does not hesatate at all, what do you think? Thanks"
 
"Progress is good!!!

Review

"Progress is good!!!

Reviewing all the posts...

You appear to be about where you were before rpm wise. You increased the prop diameter 1 5/8 inches but reduced pitch by 5 1/2 inches.

You got all cylinders to fire and improved acceleration.

One thing you never did say is how big your boat is... length, weight.

The rpm's ideally should be closer to 6000 rpm at full throttle and fully trimmed out on smooth water.

As far as motor height...it does seem high but see if you experience any cavitating, slipping or overheating on longer runs. That would be a sure sign of the engine being too high.

It would seem that the rpms would go down if the motor is lowered...however, I do not know if it would be counteracted by the ability to trim the motor higher. I think that is something that you just have to experiment with if you want to pursue it."
 
"Mike, I increased dia, by 1/8

"Mike, I increased dia, by 1/8", and dropped pitch by 4",the boat is a 17' Montauk and should weigh around 900 lbs bare hull, I think 1600 lbs. fully rigged. Should I just accept it a 5300 rpm, or should keep trying to get higher rpm,s? How will this affect the motor? I think I will drop the motor 1 hole to see how it affects trim. I can't remember when the motor ran this good, acceleration wise. Thanks"
 
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