Runs like crap at low-mid RPMs, Timing/Thermostat Questions 2003 T60TLRB

[zenoahphobic]

I think a lean mixture burns slower because, although the same amount of oxygen molecules are present, they need to travel further to "mate" with lesser HC molecules in the same space.

Where this leads to an economy limit, I think is due to the corresponding slower rate of gas expansion able to be harnessed as the piston moves down at the explosion or power stroke.

In a carbon free clean/new engine, if not the spark plug, a very hot exhaust valve can be a source of ignition. It can be red hot and may just ignite the first few mixture particles getting to it before they cool the valve; the inlet valve still being open. There does not necessarily need to be any overlap where both valves being open briefly at the same time.

I guess newer engines thus pose a design challenge, because of wanting hotter running for efficiency, good swirl aerodynamics for power and efficiency, increased intake speed for more power, and the getting rid of the increasing no longer wanted heat after each burning cycle out of the combustion chamber. Higher temperature resistant valves are necessary that inherently therefore will hold more heat energy.

[95GeeTee]

Thanks for everyone's input, where would you start to troubleshoot this problem? Steps? Air/Fuel mixture? Thanks.

[TownsendsFJR1300]

Thanks for everyone's input, where would you start to troubleshoot this problem? Steps? Air/Fuel mixture? Thanks.

Was part #16: https://www.boats.net/catalog/yamaha...lrb/carburetor actually removed and checked that it was clear? Same goes for all jets / orifices as stated earlier.

Just a spritz of carb cleaner, compressed air doesn't cut it. If you can't see light thru it, it don't count.

Should ALL the jets pass, again, as noted, treat them to an Ultra Sonic cleaning to clean the carb body. Once done, double check with carb cleaner or brake cleaner that all the body orifices are clear...

Your issue is in the carb(s).

[95GeeTee]

yes,i made dang sure every hole on part 16s and anywhere else on carbs were clean. Also the local boat repair shop supposedly rebuilt all 4 carbs as well. I have rebuilt carbs many times my self, even tried the ultra sonic cleaner on all carbs. Anywhere else to check? Thanks!

[zenoahphobic]

If you have synced the Carbies with a vacumn gauge then it is unlikely an air leak from places such as connection to inlet manifold and its seal to the head.
The only thing is maybe the butterflies have play in their fit and bearings that appears when they go through their arc.

You say you have had this problem a long time, maybe has been there since it's original assembly. So maybe never correct, midrange problems occur from the transition from idle fuel to main jet fuel. In slide carburettors this usually can be adjusted, however butterfly carbies you have less adjustability.

I am only guessing of course, some are critical about advice given on little information.

Whats left?

I would:
Check your floats and their levels.
Critically look at the butterflies opening by having someone move the throttle slowly their full movement without engine running.
Check that all the jets are the correct ones. Consider replacing them for new ones or changing them to different sizes. You need advice which ones but I would increase the main jet a little.
Determin that there is not excessive shake in midrange that could somehow contribute to fuel leaking or it not getting through (sloshing in the bowl).
Determin your fuel pump is working at these revs. There is a possibility that there is a leak, although you could have strong pulses at higher revs that supplies adequate fuel at full throttle it may be poorer at those lower revs.
You also still can have an inlet valve not sealing, that then seals under power.
You could also have the ignition retarding at those revs, difficult to check but you need to know what the ignition timing is doing.

Bit of a brain storming exercise, I'm afraid only you know what's happening.

[TownsendsFJR1300]

Tks for the update. To add to the above, any carb I can't find the issue with, I'll hook my Mity brake vacuum to.

What I do is install the float/needle, flip the carb upside down and put the gauge on the FUEL INLET. Pump the gauge up, there shouldn't be ANY leakage.

I'm not sure what type floats you have (and very likely NOT your issue), but shake, listening for fuel inside and also holding (with say a small wrench) under water looking for any bubbles.

As noted above, if still no progress, inspect the size of the jets and see if their appropriate for your altitude.


**How do the spark plugs look? If very lean, especially after running WOT or at higher RPM's, obviously richer main jets should help.

[rodbolt17]

why not simply clean the carbs,correctly set up the linkage and go play?
an intake pop is either an intake valve leaking or a carb issue.
running lean or an intake valve leak.
it does not get any easier.
9th grade auto shop.

[99yam40]

leak down test should show if there is a valve leaking

[zenoahphobic]

We have pretty much gone full circle here.

Anyone mention the enrichment valve? Could let unwanted air in, don't know how far they can be dismantled.

Use a bit of Townsend improvisation with air pressure and water.