Yamaha V4 carburettors circuit at WOT

[YamV4130]

I have a Yamaha V4 130 BETO 93 mod (6L1 306706) that just don't want to make full revs at WOT. Can turn 5000 rpm "regardless" of weight and trim. I have rebuild the carbs 2 times, but reding through the internett, the only solution is to rebuild one more time.

What/witch circuit is active at WOT for this engine? IF I had debris in the idle circuit, would that affect the WOT performance? Last time i rebuild I used ultrasonic cleaner and checked the main jet and nozzle..spotless!

+ I found the correct service manual for this motor, and it states non resistor plugs, B9HS-10 (I have resistive spark plug cap @5Kohm), but there is a sticker in the cowling saying BR9HS-10?? Is it correct to use resistive plugs AND caps?

Thanks!

[ausnoelm]

Idle circuit has nothing to do with high speed, you need to clean everything, not just the jet, but before all that, did the motor ever reach better RPM, have you changed the prop recently, have you tested spark on all cylinders at high RPM?

[Edgan Egals]

Spark Plug B9HS-10 / BR9HS - 10
SPARK PLUG.jpg
Clean the Carb and follow the turn out of Pilot Screw
Pilot Screw.jpg

[YamV4130]

Idle circuit has nothing to do with high speed, you need to clean everything, not just the jet, but before all that, did the motor ever reach better RPM, have you changed the prop recently, have you tested spark on all cylinders at high RPM?

Thank you. I rebuilt the cars yesterday and I did use safety wire on all jets and nozzles/circuits. No debris.
The motor did reach 5800rpm with yamaha 13x19 prop. Needed more bow lift so I changes to Turning Point express 13 1/4x19 and the rpm went down to 5000rpm with exact same speed of 40mph. The boat/motor combination defies logic. The boat weighs 1200 lbs, 16,5 ft and I have no extra weight.
The boat has a 2ft "pad" back on transom but does not have the motor power to "get up". I have tried all mounting holes and was advised to raise the motor even more. I got a jack plate and the antivent plate is now 65mm above pad. Lowest I tried was 10mm bellow pad, but the holeshot and handling was terrible. Lost 2mph raising the motor though!

I know this boat/motor combination is supposed to feel lighter and have more top speed, but I just cant figure it out.

The motor shoots the boat on plane and revs up to max speed at 40mph and just stops, like hitting a wall. Its like im having 90hp at WOT, but the engine runs smoothly. I have tried to play with ignition timing to no sucsess.
I also tried running the motor to near WOT while pulling on plug at a time, but this gave a terrible hole shot so all cylinders are firing <4500rpm.

One thing that is strange is nr4 cyl does not alter the idle speed and the color of the piston does not look like the rest (picture).

[YamV4130]

Spark Plug B9HS-10 / BR9HS - 10
SPARK PLUG.jpg
Clean the Carb and follow the turn out of Pilot Screw
Pilot Screw.jpg

My 93 mod service manual states the same, but I have resistive plug caps. Would that be overkill to use resistive plugs and caps?

[ausnoelm]

A couple of things, are you saying the RPM was OK, then you changed the prop and now it's not? if that's the case, it kind of indicates you are over propped? the one cylinder being different might be an important clue, if it's running lean, it could spell disaster in the long term, it could still be a spark issue on that cylinder too, pulling plug leads off is not testing, it's fooling around.

[YamV4130]

A couple of things, are you saying the RPM was OK, then you changed the prop and now it's not? if that's the case, it kind of indicates you are over propped? the one cylinder being different might be an important clue, if it's running lean, it could spell disaster in the long term, it could still be a spark issue on that cylinder too, pulling plug leads off is not testing, it's fooling around.

RPM was ok, but not speed and handling. Still felt the combination was heavy and underperforming.
Yes I know it could end up in disaster, but I cant find any reason for it to run lean. I got spark, fuel/mixture, and compression (130psi). I think a practical test is telling more than a DVA and Ohm reading from the ignition system?

[boscoe99]

I am going contrarian here and suggest that the idle circuit will affect the operation of the carburetor at high power settings. Fuel is flowing through both circuits at the same time IMO. Not saying that the particular problem is within the idle and mid-power circuits however.

At a high power setting have someone mist some gasoline into the air intake system. Does the motor pick up and run better or not? If so, I would presume there is something amiss within the carburetors.

[boscoe99]

Spark Plug B9HS-10 / BR9HS - 10
SPARK PLUG.jpg
Clean the Carb and follow the turn out of Pilot Screw
Pilot Screw.jpg

What am I missing? I don't see the idle mixture setting for the 130 model listed on the sheet that has been posted.

[99yam40]

how many fuel pumps does that motor have?
is one run off of the #4 crankcase?
looks like #4 maybe getting some extra fuel due to a leak in fuel pump maybe causing it to run rich
piston looks washed

[YamV4130]

how many fuel pumps does that motor have?
is one run off of the #4 crankcase?
looks like #4 maybe getting some extra fuel due to a leak in fuel pump maybe causing it to run rich
piston looks washed

It has one pump getting its pressure pulses from #4 crankcase! Bought it last year and the previous owner said they just changed the diaphragm, but that could be BS.
IF it was a leaking fuel pump I would think the motor would run rough and very badly? My motor runs "smooth" on higher revs, but again, it shakes and has a "2 stroke" running <2500rpm.

Thanks guys!

[99yam40]

a leaking pump can show up more at lower RPM, at higher RPM a motor burns more fuel so it would be less noticeable .
never hurts to look to make sure.
a previous owner of my C40TLRX drilled out the main jets.
it took me a long time to figure that out, proper numbers on the jets, and I cleaned the carbs many times trying to figure out why the WOT had a miss from time to time.
never rule out something until you prove it.
I had to order a new jet to measure the hole because I could not find specs for that

[YamV4130]

It could be many things, I agree! I will pull the pump today for piece of mind.

Just block the output port (with a gauge?), remove it from the crankcase and prime while looking for fuel leak?


Thanks.

[99yam40]

leave it hooked up to carbs no need to pull the hoses off, prime and look for fuel coming out of pulse hole..
since they said they changed the diaphragm , maybe the piston top got that look from back before they changed it.
you never know until you make sure

[YamV4130]

I just got finish with testing the fuel pump. No leaking from pulse hole but it needs new soft parts! Or a new pump. It was leaking out from the main seal when the 2 screws where missing.

But the throttle cam roller has 2mm more travel at wot, could that mean anything? I can see that the throttle plates inside the carbs move from 86-90° when manually push the roller the extra 2mm of travel.