My MVVS 1.6 IFS, equipped with the tuned pipe is hard to start and runs unreliably. The first start of the day is fairly routine but on subsequent flights it gets harder or impossible to start. When starting hot it will "start" but dies like it is running out of gas. When it does continue to run it sputters and won't accelerate as though it is too lean but is in fact too rich. If I slowly open the throttle , after about a minute of running, it clears out and will run OK. If I readjust the mixture lean to make it run, it will go lean in flight after a time.
The engine seems to restart and run OK when the air temperature is in the 70s (F). In the 80s it's a little balky. In the high 90s it may be impossible to start.

I think the problem is that the carburetor heat soaks between flights and is vapor locking or boiling and cools enough to work after a few minutes of running. It is exacerbated by air temperature plus the extra heat generated by using a tuned pipe. What are your thoughts on that?

I've been able to get it running by closing the spark plug gap to about .015" from .025" when the OAT is near 100 degrees. (EXCEL ignition).
Why would that be?
It looks like changing the gap, changes the mixture requirement.
Is that possible?

Would 50:1 oil rather than 40:1, cause the heat problem? I'm using Stihl Ultra synthetic at 50:1.

The engine is out in the open. Prop is 18X8. Premium gas.
Once cleared out, the engine usually runs fine in flight, it is just the restarts and getting it to run prior to flight is a problem.

Thanks
Jerry

Jerry,
In your other post you mentioned a carb that should not be on the 26cc MVVS. I would check the pulse line tightness and connection. Also the inner strain gauze of the carb.
It seems your engine gets too hot, and you indeed have some vapour lock issues. Out in the open, cooling is not the best you can do, especially whith plane static. Engine temperatures will soar. Gas engines need good cooling air, directed at all the cooling fins.
Do You run gas, or some other like alkylate fuel? White gas and Alkylate (like Coleman, Aspen) is known for vapour lock problems. The fuel just evaporates too quickly.

Pe,

Thanks for your reply. It seems I read the wrong number on my carb. It is a wt 481D.

I'm using the highest octane gasoline sold at the locale filling station, 91octane I believe.
The strainer gauze looked clean but I replaced it yesterday and will test the results today.
I will go back and recheck the integrity of the pulse line and carb seal.

I am perplexed that when hot, it will run only with the spark plug gap set less than recommended. Does closing up the gap have a virtual effect of changing the ignition timing?

The timing is set around 30 degrees BTDC. If the timing is retarded from there, would that make it run cooler or hotter?

I enjoy reading all of your posts. They have expanded the horizons of my knowledge.

Thank you again

Jerry

30° BTDC is too much for the little 26cc engine, which runs best at 26 - 28°. Too early timing can destroy the connecting rod bearings and crankpin, as well as run hotter. Adding a head shim helps a lot here.
Adding two shims will cause a slight power loss of 100-200 rpm, but makes the engine run extremely sweet.
A plug gap of 0.020 should be about right with the RCexcl ignition. Normally, smaller gaps give erratic running.

Now I'm getting somewhere!

I changed the ignition switch and battery and got the three best flights to date on this airplane and engine combination. It is still suffering boiling off of the carb between flights but with choking, the engine will start and run, even at the 98 degrees we had today.

The ignition switch seems unable to support the current draw of the ignition, especially after the battery is drawn down a little. Maybe the Vlach ignition wasn't bad after all.

I shall retime it and regap the plug.

Thanks again for your help

Jerry

The current draw is quite low on these ignitions, provided you keep the voltage low. Four NiMH cells are more than enough! All extra voltage causes extra current draw and is transferred into heat.
I had the ignition working down to 3.2V and my normal 0.5 - 0.6 mm plug gap.

All this time I was thinking the problem was heat and ignition because of high air temperature and previous ignition trouble. Every time I changed something in the ignition (batteries, switches, plug gap) it ran better. It was running perfectly when all of the symptoms suddenly returned. With the cool weather I could now tell it was going lean (then quitting).
On the fifth trip into the carb I noticed what appears to be an anti-syphon check valve between the regulator chamber and the needle valve supply passage. The flapper in the check valve is gone so I'm guessing it parts are plugging the high speed discharge nozzle. I have installed a different carb and have yet to test fly it, but here's hoping this will be the fix.

Jerry

You have a lot of issues, and narrowed them down. One major issue seems to be the carb going lean indeed, due to a malfunctioning high speed cirquit? That explains the early advance running better with small spark plug gap (which retards combustion onset).
Beg-steal-borrow a good carb, or bring the carb to your nearest garden appliance shop for a checkup. (if they have good craftsmen there!). Be sure to tell them your findings and history.
You also can order a new carb, and save you a lot of hassle, budget permitting. Your problem seems to need a major carb overhaul that most are not willing to do (included me, I just do not have all the required tools).

Pe,

You have mentioned adding two shims to the head. Is this two shims in addition to the existing shim or is it a total of two shims?

Thanks

Jerry

Add two shims for a total of three. Adding one shim already is an improvement that will not reduce power. The third shim (total) will reduce rpm slightly.

Pe,

Just thought you'd like to know, and I must share this with someone!

I think I can claim victory over this engine. The problem was it was not pumping enough gas, but not all of the time. A leaky pressure tap and an acetate pump diaphragm in the first carburetor wouldn't consistently supply gas. Then replacing the tygon pump tube with softer neoprene made it worse.

The next two carburetors had black pump diaphragms which apparently don't like California gasoline. The check valve flappers curled up, causing the same problem.

I Installed a teflon pump diaphragm in the last carb and replace a damaged regulator lever (which caused the same symptoms), and replace the pump tube with tygon.

Now it starts and runs as sweetly as it should!

When you add in the trouble caused by ignition failure, connecting rod failure, a fouled plug caused by connecting rod failure (which caused the same symptoms), to the carb problems it was an interesting trouble shoot.

Bottom line is: Leaky nipple and soft pump pressure line plus obsolete pump material caused this trouble.

Jerry

and do not forget ethanol

Thanks for the update. Carb is the first to check. I have develloped a good feel to check for nice flabby membranes that ere needed in any engine.
I am glad the engine now runs as required.
Modern carb materials will resist the oxyginated gas (ethanol added) very well.