Rodbolt, motor still cutting out at WOT

[99yam40]

This is really getting strange. Yamaha 1999 C40TLRX still has the high speed cutout or miss, like it is dropping a cylinder at high RPMs . but when I take the cowling and silencer off of the front of the carbs it runs fine WOT, no miss or cut out. Reinstall the silencer and it cuts out a lot at WOT.

I have been through the carbs 5 or 6 times in the last month cleaning and blowing out, looking for something causing the problems, even put in kits. Only thing I have found so far is a small piece of rubber tubing stuck in a needle seat that came from me pulling and reinstalling the carbs so many times, and that was apparent as that cylinder would not fire at all.

Fuel pump pressure is 5psi at WOT and there are no air bubbles coming through to carbs. All of the Ohm readings check out on the ignition coils and other coils also. Compression is good on all 3 cylinders 118-121 psi.
I have checked and #1 & #3 carbs have #115 main jets in them and I am almost positive #2 has #118 jet but will have to look closer next time I have it off. I was mainly looking to see if someone had swapped the jets to the wrong carbs, but when I saw the two #115 in the top and bottom I assumed the middle one had the correct one in it also.

I found all 3 of the Idle jet rubber plugs loose and in the bottom of the bowls last time I opened the carbs. Have you seen this happen before? I have read where someone else found this also after having the carb kits installed by a yamaha shop but no one ever gave a explanation for it or said how the plugs being out would affect the running. My thoughts are it would cause it to run rich at high rpms but am not sure.

Am I correct in the positioning of the jets?I have not seen anything in the manual about where they go, just that the specks call for 2 #115 and 1 #118 for the 40 hp.
Maybe the jets have been opened up by someone(I am the 3rd owner)but I do not think so. It had just a intermittent problem ever since I bought it almost a year ago but nothing as bad as now.

how do you find out what size the orifices are suppose to be?
I am sure I could find a real small set of drill bits that I could use to gauge the holes with and then measure with a set of calipers or mic.

Does this thing seem to be running rich if removing the restrictions (silencer)from the front of carbs makes it run better?and have you heard of this before?

I have put in 2 new sets if sparkplugs but no difference. Do you think a larger plug gap or hotter plugs would help? The 50 hp uses different plugs, but I do not like installing the wrong plugs in a motor.

Any and all thoughts will help

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

how do you delete these things?

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

bump^

 

[rodbolt]

Re: Rodbolt, motor still cutting out at WOT

dunno
the middle carb has the 118 jet though.
do the plugs indicate a rich condition?
although its really really rare to have a high speed over rich condition but its kinda what your describing.

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

Plugs look a little wet, never completely dry, but the color is tan (normal) but I do not run it at WOT much at all. Normally just 1/2 to 3/4 throttle except for the hole shot. That is when I first started noticing the cutting out and started opening it up WOT to see what was going on and kept hearing and feeling the cutout.
Can you direct me to something that could give me the hole sizes in the jets so I check to make sure they are still the correct size and give me anything else to look at that could be my problem?
If the rubber plugs come out of the idle jet holes will that cause a rich high rpm problem? And what could cause them to come out of the holes?

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

Also could reusing the o-rings for the needle seats cause a leak through the threads that could cause this? They were flattened but not cracked or torn.

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

I found a place on line that said the #115 jets have a hole that is 1.15MM in size and the #118 is 1.18 MM. Does that sound correct for Yamaha jets?

If the rubber plugs come out of the idle jet holes, will that cause a rich high rpm problem? And what could cause them to come out of the holes?

What else could cause a high speed over rich condition?

Looking for more ideas to look at.

 

[rodbolt]

Re: Rodbolt, motor still cutting out at WOT

never seen new plugs pop out of the pilot fuel jet hole.
yes if the orings for the needle seats leak it will flood but will be most noticable at idle.
index the flywheel, use a test wheel and check timing on all 3 cylinders at WOT, use a peak reading meter and check CDI output voltage at WOT on all 3.
if the above passes continue with fuel.

 

[hevysrf]

Re: Rodbolt, motor still cutting out at WOT

Just bought new oem carbs for a 1990 pro 50, runs super rich, sheen on water, bad bog out of hole, opened up carbs, darn plugs 7/8ths out of jet hole. Reassembled, porblem not fixed. MartyD

 

[99yam40]

Re: Rodbolt, motor still cutting out at WOT

never seen new plugs pop out of the pilot fuel jet hole.
yes if the orings for the needle seats leak it will flood but will be most noticable at idle.
index the flywheel, use a test wheel and check timing on all 3 cylinders at WOT, use a peak reading meter and check CDI output voltage at WOT on all 3.
if the above passes continue with fuel.

I marked all 3 pistons TDC and 25 BTDC but have not been able to put it in the water yet for WOT timing check with light. I will this coming week.

I tried to read the CDI output with my Fluke 87 set on peak and 1 Milli Second. but did not get anything but 8 - 10 volts even thought the motor was running.
I made sure the small solid copper wires I worked into the back of the connectors were making connection to the 3 connectors by checking with a ohm meter with the connector opened up.
The manual says to check from the B/O, B/W, B/Y to B. I take it the B is a good ground not a certain black wire.

Is that a correct assumption?

I guess that I will need a DVA to take these Peak voltage reading instead of my Fluck87 set on peak.

So could someone tell me if the attached Diagram of a DVA is something I could use, if I made one, to take these readings. Or is this not worth the effort and I should just buy a DVA online or somewhere to read it properly?

 

[rodbolt]

Re: Rodbolt, motor still cutting out at WOT

beats me about the DVA but with an intermittant you may not see it with a digital meter.
for intermittants I shift to my CD 77 meter or the sierra meter that can be had for about 100 dollars at most NAPA stores. may even be availible here on the Iboats store. look in the sierra catalog under mercury tools or shop tools.