Low idle, reduced power

[stwaxy01]

2001 Yamaha c40 TLRZ Saltwater use

I recently replaced the water pump, the fuel lines (lines from the tank to the engine and from fuel pump to carbs), and cleaned & rebuilt all 3 carbs. Also replaced the diaphram/gaskets in the extra assembly that is part of the middle carb.

Initially, it ran great (5500 RPMs) which I think is the most I've ever gotten it to go. What I'm experiencing now:

1. At low idle the engine cuts off. I have to put the choke on, rev it up and then throw it into gear to get going. When I slow down to dock, it also cuts off.

2. Slight vibration in engine at very low speed

3. Engine was running fine at full power and then dropped from 5500 RPMs to around 4000 RPMs

Not sure if these are related or if they may be tied to the work i've already done. I didn't fine tune/adjust carbs when i put them back on other than count the screw turns when took each apart and put the screws back in with the same number of turns. What should I check? Should I replace the fuel pump? Adjust the carbs?

[99yam40]

That motor does not have a choke, it has a prime start enrichment system .
is the red lever in the normal position?

did you follow the link and sync procedure after installing the carbs?

check the fuel pump to see if it has a leaking diaphragm

[stwaxy01]

Thanks for the quick reply. Good to know about the choke. By choke, I meant I push in the round rubber cover on the side of the throttle arm which allows me to rev the engine without engaging the propeller.

I know where the red lever is. I was fairly certain I put it back in the position it was in when i took it a part but I can certainly take another look. Will it be obvious which position it should be in (e.g. on/off, up/down, etc.)?

I followed the steps in the SELOC Service Manual but don't recall link and sync steps. Any thoughts on what I need to do to link and sync?

I didn't see any obvious leaks from the fuel pump but was planning to replace anyway since I've replaced most of the other fuel assemblies.

[cpostis]

Just a shot into the dark, is your fuel tank vented (cap or otherwise)? Try loosening the fuel cap and see if you still have the problem. May just have to replace the fuel cap with a vented one?

[99yam40]

full counterclockwise should be the normal/automatic position

one notch clockwise should be open
one more is closed

Need to get all 3 carb throttle plates to be closed exactly the same.
it also deals with timing, the pick up roller, RPM and other stuff
need a Yamaha manual if the one you have does not have the procedure

when installing the carbs there is some slack in mounting bolts, so need to do the link and sync every time you put them back on.

[stwaxy01]

Going through the Service Manual. There are a few terms I'm hoping you can help me decipher in the specification tables. For example, under a heading called Ignition Timing Degrees there are three Sub items:

1. Idle - it says 6-8 ATDC. I know it stands for "At Top Dead Center" but not sure what that means? What is Top Dead Center and what does 6-8 mean?

2. Carb Pickup - Also says 6-8 ATDC. Is this referring to Pilot Screws? Or, an Idle Adjustment Screw?

3. Full Advance - 24-28 - Anyone know what this is telling me? 24-28 turns? From fully seated or from when you start the screw thread?

Thanks.

[99yam40]

TDC is top dead center of the piston in the cylinder
means piston is at the very top and will move downward if you turn crankshaft in either direction
BTDC is Before TDC ( Advanced timing)
ATDC is After TDC (retarded timing)

all deals with degrees of crank shaft

You use a timing light to check it on the flywheel

[stwaxy01]

Thanks 99yam40. What does the 6-8 mean? is there a gauge or something I am supposed to hook up to the piston/cylinder?

[stwaxy01]

You guys suggested that my problem may be that I didn't link and sync (which I didn't). In the service manual, it outlines steps for several adjustments.

1. Throttle Linkage Checking/Adjustment (involves manipulating the Magneto Control, full-retard stopper, CDI, etc.)
2. Throttle Cable
3. Idle Speed (checking idle and *****ing RPMs, adjusting throttle screw, adjusting pilot screws, etc.)
4. Oil Pump Link
5. Shift cable adjustment
6. Adjusting Starter Lock Out

If i rebuilt carbs but didn't mess with anything else, do I need to go through all these adjustments? Only trying to prevent getting in over my head.

[99yam40]

6 - 8 degrees

sounds like you have never worked on any type of motor before.

I do not think this is the place for learning basic motor stuff.

there should be books / classes that will help people try to understand basic motor design and theory before trying to work on them

[stwaxy01]

I appreciate the spirit of your comment. It’s a fair assessment but in general I think these forums are fine for all levels/questions. You know how it goes, answer what you want, don’t answer what you don’t want. I’ve done pretty good so far from folk’s help. Regardless, I appreciate the site's help up to this point and for any other elementary to advanced questions I plan to ask.

[99yam40]

then look up and understand timing of spark

[stwaxy01]

I completed the link and sync. That seemed to get the motor running without cutting off at low idle. However, it still appears to be bogging down. I could get around 4200 RPMs max and then dropped to about 3900. It held there steady the rest of the ride. I noticed when I got it back in the slip, the bottom carb was leaking fuel (steady stream) when engine was running. The fuel was coming out at the top, on the left of the two port holes in the corner.

Could this related to why it is bogging down? Any thoughts on why that carb might be leaking if it was just rebuilt? The rebuild included cleaning all holes, jets, etc with compressed air, spraying with carb cleaner, and replacing all rings, float, needles, etc. (all parts that come with the carb repair kit on boats.net plus I also bought replacement o rings for the needle valve assembly that did not come with kit). Thanks.

[cpostis]

As far as the leak, I would think you must have a piece of debris lodged in your needle seat. I would pull and be sure it's clear. Hook up a piece of fuel line to the carb inlet ,blow through it to see if it's clear, turn the carb over (inverted) and see if when you blow through it again the flow is stopped. If not, your needle seat is leaking.

Out of curiosity, did you check your fuel tank cap/vent? If not vented properly, you can get a vacuum in your tank and fuel line and starve your carbs at high rpms. Maybe this is the reason you drop rpm's?

Good luck and let us know.

[stwaxy01]

Thanks Cpostis. I'll take the bottom carb out tomorrow and make sure nothing is in there. The tank is vented. It's a permanent tank w a vent on top (not from the cap).