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

[95GeeTee]

I have been fighting this problem for years. Motor runs ruff and pops in the intake at low to mid RPM. Runs fine full throttle. Anyways i have had a long journey trying to fix this problem with no luck but i think i am getting close.

What i have done:

- Rebuilt the 4 carbs atleast 5 times already
- Had head completely rebuilt, milled surface clean, new head gasket, etc.
- reran new fuel lines
- changed plugs multiple times
- Checked valve lash many times
- changed intake gasket o-rings
- sync carbs
- multiple compression checks, leak down test. all good.

Same results after all the above. Did zero good.

All of this and never checked the thermostat. motor was peeing but theromstat may have been holding back some. I do have the overheat alarm which works but motor must have been cooling because it never goes off.

Before pulling thermostat, checked timing and it was dead on the T at idle. Pulled thermostat, it was pretty dirty inside so flushed it and put cover back on without thermostat.

Ran motor and then checked timing and now it was off the T on the next notch to the left. Wierd. Motor did run alot better, couldnt make it pop in the intake like before.

My Questions:

1. Why the difference in timing? Does something control the timing automatically (retard/advance) while motor is running?
2. Thermo Switch, is it only for overheat alarms or does it tell something to adjust timing depending on temp???
3. Can thermostat and/or thermo switch cause weird problems like i am having?
4. is there a way for me to adjust timing on this motor while it is running?

Thanks!

[walleye1]

How did you sink carbs? You need a vac mate to do it rite.

[95GeeTee]

How did you sink carbs? You need a vac mate to do it rite.

Vacumm Synchronizer just like this one https://www.youtube.com/watch?v=XqOfSMq1DlY

[pstephens46]

Was this one of the motors with the shtty undersized jets? Maybe Boscoe knows?

[99yam40]

what are the timing specs for that motor?
does what you see match the specs?

I have not seen many motors with Idle timing at TDC before, but I have not messed with any T60s

a 2 stroke will lean sneeze when too lean,( burn the fuel air mixture in the crankcase before entering the cylinder)
but a 4 stroke has valves to hold the burning of fuel in the cylinder unless it fires while the intake valve is still open.

maybe a hot carbon spot could light off the fuel charge as it comes in
running cooler without the stat may keep that from happening.
just my thoughts

[boscoe99]

My Questions:

1. Why the difference in timing? Does something control the timing automatically (retard/advance) while motor is running? The CDI assembly automatically advances or retards the ignition timing. 0* TDC (+/- 1*) at idle to 25* BTDC at full throttle. (+/- 3*).
2. Thermo Switch, is it only for overheat alarms or does it tell something to adjust timing depending on temp??? It is a switch. Normally open. Closes due to excessive temperature. Only when closed it might affect timing.
3. Can thermostat and/or thermo switch cause weird problems like i am having? Don't think so.
4. is there a way for me to adjust timing on this motor while it is running? No.

Thanks!

Backfire is indicative of a lean air/fuel mix. The carbureted models are known to run lean. Very lean.

What were the idle mixture screws set to? The base setting is 2 and 3/4 turns from being lightly seated. Plus or minus 1/2 turn. I would not be afraid to open them up further to see what happens.

Rebuilding carburetors is a step after they have been thoroughly cleaned. If not completely dismantled, cleaned in a heated ultrasonic cleaner, using a good cleaning solution, all orifices and passageway rodded out with soft copper wire, the orifices and passageways verified to be free and clear, it may be a waste of time to rebuild them.

Ditto on the need to properly link and sync them.

If and when the motor is backfiring what happens if some gasoline is misted into the air silencer intake?

[boscoe99]

I just posted and answered your questions. My response was flagged as spam. WTF over.

Hopefully it will show up in due course.

[rodbolt17]

TDC is where the idle timing should be.
25* BTDC is WOT.
go fix the fuel system and quit mucking about.
that intake pop is a sign of a lean condition in the intake.

[boscoe99]

My post finally got approved and showed up.

To the top. If the OP is still around.

[99yam40]

what sets off the air fuel mixture in the intake?
is there an over lap of the intake and exhaust valves being open at the same time that can spark off the mixture in the intake?

If you are misting fuel into the intake when it pops it will catch the misting bottle spray head on fire and cause you to throw it away from your body.If it breaks open it will set fire to what you threw it at.

[boscoe99]

The theory as I understand it is that with a very lean mixture the air/fuel burns slower than it should. The combustion gasses may still be burning in the combustion chamber when the intake valve opens. Fire in the combustion chamber lights up the air/fuel mix in the intake system. A flame front then goes back out to the carburetor. A back fire.

[99yam40]

it would have to be burning very slowly to last thru the power stroke and exhaust stoke and still be burning as the intake stroke startsand the intake valve opens.

but then back in the 70s we would dump some cologne into a empty 5 gal water bottle, roll it a round a little to spread the liquid.
Set it down and light off the mix with a match
there is usually enough O2 in there with the fuel to burn like a blow torch and burn your finger if not careful.

do it a second time and not as much O2 so the flame slowly burns down into the bottle in a slow wave.
with the clear bottle you got to watch the flame front slowly ripple down to the bottom.
something to entertain the group as we sat around blowing smoke

[zenoahphobic]

I don't believe on a four stroke, a burn can last through an exhaust stroke.
However, if you have some carbon or indeed a spark plug still glowing, there might be a chance, if there is excessive fuel in the inlet, that that may ignite shortly after the opening of the inlet valve.
Thus instead of thinking lean burn (pop), this might indicate too rich a mixture at times. That can result from leaking into the intake (spitting at times).

[boscoe99]

All I know is what I read in text books. This snippet from the FAA Aviation Power Plant Handbook.

[boscoe99]

In a brand new motor, with no carbon build up whatsoever, what might cause the air/fuel in the intake system to catch fire, resulting in a back fire?

Let's say it does not happen if the idle mixture screws are turned out but does happen when the idle mixture screws are turned inward resulting in a leaner than normal mixture. What is causing the combustion to occur in the intake manifold?

In my old Mercury it ran funny at the normal mixture setting. As I turned the idle mixture screw in, it sounded better. At some point it would start to back fire. A lean sneeze if you will. What was the cause of that?