Sudden GRP .28 sticking at high revs! please help
 

Hey guys, need some help here. I have a very well maintained and nice condition GRP Tuned .28. It was
broke in properly and not rushed. It's still not fully broke in but almost. Its under a gallon of fuel. Has a lot
of compression and everything is factory on it. The only things I have changed here and there are carbs
and plugs. Lately it has been running worse than ever!! Its so frustrating because its a great engine
and runs well when it wants to apparently. It idles fine, and revs fine, but if I rev it past half throttle
the engine sticks at what ever rpm I let off the trigger at!!! This is on the box mind you without wheels/tires.
The clutch is fine, I tested it in another car/engine a couple days ago. So its not the clutch, not the exhaust.
Its using factory head and shims. I noticed the GRP plug Im using that came with the engine doesn't sit flush
with the combustion chamber. It sits a bit higher up .5mm or so. Come to think of it, it must be the plug. FUNNY
as it is the manufacturer plug for goodness sake! When I first wsa breaking the engine in I was using OS plugs
and it ran very well and tuned pretty well. It never stuck at high revs like this. Was also using the Novaroos carb then too.
he tank and lines are fine and the throttle servo and linkage are set up well and fully returning to idle. Its so crazy to see a nitro engine stick at revs
after the throttle has been fully closed!! That's pretty scary to think that can happen. I really don't see what the mechanical issue would be if there was one.
The engine internals are fine, all is smooth and operational internally. There are no mechanical defects externally either. Could it hang at
high revs due to a defective plug or it not seating flush? I haven't checked the button and piston since this. I will tho.
The only possible thing I can think of (that hasn't caused this issue or any issues before) is Im using a Novarossi carb without the
carb o'ring so it seats fully all the way down in the crank case. Could there be an air leak there at the base of carb without the oring?

Sounds like an air leak to me. Can you seal the carb with silicone?

Sound like you have blockage stuck inside the carb needle. This will cause engine to rev high and not stop until you close the carb. The solution is clean out the carb set back to factory settings and install a fuel filter. Check to make sure you throttle linkage isn't binding and opening and closing carb. You may have to readjust the throttle linkage. Check rx battery for good power.

It happens after the carb has closed. He has an air leak. The motor's leaning out.

As stated, it sounds like an air leak. If that o-ring is not installed air will defiantly will get through.

Don’t most tanks come with a clunk aka stone filter already installed? Why would you want to add another filter? Seems to me just another spot for issues when you start adding things.

I believe it uses a turbo plug? If so and the current plug is not sitting flush then there is your air leak.

If I was you I would do what I ALWAYSdo, tear down the engine clean it and use a wd40 and qtip soaked of it to put it back together. If there’s an issue you will see it. I even tear down my carbs cause I can see where the needles are by sight and pictures and it makes the world or difference. My opinion is you just have a dirty engine and you need to go threw it and make sure no orings are split.

I did tear the engine down fully and cleaned it entirely!! I always do. Thats why Im baffled. Some good response here guys thanks. Running without the carb throat oring in the past hasn't caused this issue. Although it may have been there before just to a much less degree.
That could be why I could never get a truley dialed tune on it. I always thought about not having that 1 little oring on carb throat can lead to issues or not?

So its either the oring on carb throat not being there or the factory plug not seating flush in the combustion chamber, is what we have so far. These were my thoughts as well. Ok time to go back to an OS plug (which was what I was going to try anyway) and the stock carb I will try.
I want to use the Novarossi carb of course with the throat oring but its doesnt seat all the way in the block with the oring. That's the only reason why Im not using it.

NO!

Tearing down an engine completely. every time you run is the worst thing you can do, You can lose parts, damage threads or worse??. That o-ring for the carb throat is important. That is causing an air leak and over time will get worse causing problems later. A loose glow plug won't cause the problems you're having. It will just start easier and vibrate looser and loser until it pops out and engine will stop. Swapping carbs from one brand isn't such a good idea either?

Ill be testing the engine very soon again. I will have a base o ring on it even tho I hate how it doesn't fully seat with that. I always thought that would be prone to an air leak. I think its more the plug not fully seating in the head.
Ill try an OS plug and report back.

Has the throat of the carburetor been deformed AT ALL by the clamp that holds the carb in-place? I don't think I've ever seen one that hasn't been deformed at least a little over time. Without rubber O-rings at the top and bottom of the throat, air leaks past the gaps caused by that deformation will get worse and worse each time you disassemble the engine for servicing.

I know! its damned if ya do damned if you dont. Unless you oil the rings each time, which I dont. I use residual oil around the carb or engine and rub it on but dont bust out the oil can each carb install. I dont think most do.
I need to stop swapping carbs! lol Im trying to find out why the grp carb isnt tuning as well as the Nova carb. There may have been a leak somewhere at the time as well, may not have even been the grp carb giving running issues.
The one thing I hate about nitro is the fact that you need to check 8 different things to find the cause of 1 issue. I think I have an air leak at the head currently, not the carb. Ill resolve this with a better fitting plug hopefully. But the carb issue, I really want to love the grp carb on the grp engine but Im just not at all
confident in it so far. I ran the carb twice and didnt like how it ran both times. Couldn't dial in a tune, extremely sensitive to small needle changes. The last time I ran the grp carb was before this thread started and this thread issue.
When I ran the grp carb there was no throttle sticking. Everything was good then except the carb was so sensitive to tune, extremely. Not any fun at all. I heard grp tune well and easy. I haven't seen that yet.
The engine isnt fully broke in tho either. under half gallon on it.

It's NOT the plug!!!!!!

If the engine is too sensitive and can't seem to hold a tune, try decreasing the compression ratio with an extra copper gasket under the cylinder head.

do you have his engine in your hand to say that? - i cant believe your still allowed to comment dribble on this site.

Thanks man, so last night I went out and tested a different engine, not the GRP. I have to finish installing that in another truck.
But last night I tested my Novarossi .28 in the meantime because I was having a separate issue with that. The GRP .28 was sticking at higher revs once it got to like 20k rpm.
The Novarossi .28 was pinging hard coming down to idle off high speed passes on a rich tune. This tells me (I think) if its pinging hard on a rich tune, then it's gotta be either too much compression,
too hot of plug, or both at the same time? My question still remains- How do you know if you have only a too much compression symptom, or only a too hot of a plug symptom, or both at the same time? This is the tricky part.
What's the best way to distinguish between either too much compression or only the plug being too hot or too cold? I know head shims dictate compression and plug number is the heat range. Does higher nitro content increase compression (I think it does), and too much compression can blow plugs. But my issue I usually see is Ring aping pinging. It was happening every time very consistently. I added the shim back in it that I took out. Its back
to the factory shimming now at .9mm shim stack!! I was shocked. That's the biggest shim stack I've seen. I didn't think taking a shim out would cause that drastic of a running change
but it did. It was ring aping pinging. Adding the shim back in (used the same plug the whole time) calmed it back down almost 100% until the tank got down to almost empty and started
1 or 2 pings every so often which I didn't like. Id like to get all pings gone. With this Novarossi .28 I think Ill just leave the shim stack even after its broke in fully, and maybe go to a #6 plug
from the current #5 Im using. But it was running and idling like a champ and had a lot of power with the factory shims and a #5 plug. I guess I ended up answering part of my own question of distinguishing some of the symptoms. I only added a
head shim and the engine calmed down noticeably at idle. Id say its good to go. Next up is finishing the GRP. Getting that running well again.

The shim stack does not have to be equal with the real head clearance but 0.9 looks like a lot.

Ill post a pic of it. It seems like a lot to me too. I was shocked wen I saw it. The only thing I can think of is that the head is designed very low
in the sleeve or it has a very long stroke?

What is driving me nuts is why does it still ping here and there after a high speed pass with .9mm shims?
1 other thing I noticed is that there is a metal ringing sound sometimes at idle. I heard that is the rod?
It was running rich.

There's no hard-and-fast rule about whether the problem is best fixed with a different temperature plug or a different compression ratio. You basically just have to play with it until you get it to run well in your climate, your running conditions, and your fuel.

Pinging hard while decelerating sounds like the LSN is too lean, but if you can't enrich the LSN without the engine slowly loading-up on fuel and choking-out when accelerating again after idling for a minute or so, then the pinging during deceleration might indeed be the plug is too hot.

The general rule I use for selecting a compression ratio is to look at the range of "acceptable" tunes that I can achieve. If I have to tune the engine so rich that it loads-up on fuel just to keep it from overheating, then the compression ratio is too high and I should add a shim. If I have to tune the engine so lean that it makes bad "lean engine" noises just to make sure it warms-up properly when running, then the compression ratio is too low and I should remove a shim.

So what do you think about this then? I got the GRP.28 running great again tonight. Its got the 2 needle nova carb and oring installed. 9mm venturi, reds quattro all aluminum clutch, os rp6 plug, and a .7mm shim stack on 30% nitro. I got it running great accept I cant get the lsn and idle to idle high and fast like you normally can if you lean the lsn a lot to idle high, then drop the idle like most guys suggest tuning the idle and lsn. I tried doin it this way and it would never just start idling at high revs consistently. I leaned the lsn 1 full turn and it did rise up but sounded like pinging almost, not a fast and high idle. It wasnt acting right on the low end tune. It still idled rich and still four stroked a bit when too lean on lsn. The power was there when I took off but the idle sounded rich and like it would die soon. Not a confident consistent idle at all. I managed to get it idling a little fast after it warmed up after 2 tanks of fuel but its still not idling strong, but there is a ton of compression. I cant even turn it over by hand on the flywheel in the truck after shutting it down hot. What does this sound like? Too low compression ratio? The engine came with less shims from the factory, cant remember how much less exactly. I want to post a video. Maybe tomorrow if I have time.

I found the GRP manual that came with the engine. It has all the info in there. Very nice, informative manual. It is excellent. It only shows 1 shim in the exploded view. The legend for the view lists it as a .2mm. This seems super tight,
only .2mm deck height. It lists another shim but I dnt know if its optional. The view only shows 1 shim. This part needs to be more clear. The other shim listed is a .1mm but may just be optional. I think I will just put a shim stack of ~ .4 or .5mm and run a os rp6

Then measure the real gap by using a piece of 1mm solder and squeeze it between the piston and chamber when turning the engine one turn around. The squeezed part will give the real head clearance. That should be around 0.6mm

If Im going to measure the real clearance its not going to be pressing the piston against solder. I would do the caliper method measuring the depth from top of the sleeve to piston, then chamber to head and subtract them.

Hello,

I am new to this forum. Reading it for some years but registered today.
May I ask how old this GRP engine is? I mean, I haven't seen an GRP engine for at least 10 years.

Do you usually make your needle adjustments in full-turn increments, like you mentioned here? Because that's WAY too much. The typical recommendation for coarse needle adjustments is 1/4 turn at a time, and fine needle adjustments is 1/12 turn (1 "hour" on a clock") at a time. If you make adjustments larger than that, you could be overshooting the ideal tune over and over.

After you lean the LSN because the engine is idling too rich, you need to lift the truck off the ground and run it full-throttle for a few seconds to blast-out all the accumulated extra fuel from the crankcase. In my experience, a very rich idle can continue to manifest for up to several minutes after the tune has been corrected, simply because the excess fuel in the crankcase hasn't been expelled yet -- especially if the engine was previously running so rich the crankcase couldn't get above ~150°F to vaporize the excess fuel. But even if the crankcase is hot enough to boil-off the fuel, there will still be a bunch of oil left behind that can cause bogging-down until it's blasted-out by running full-throttle.

A lot of my tuning issues have come down to an air filter that wasnt that dirty, just a lot of onroad use. So i didnt change it in a long time. I noticed a huge difference in how much more amazing it ran when i had no filter on because i forgot to install it. It ran so good without a filter. So that led me to putting a new filter on and the engine ran great. My tuning issues and running issues have come down to the filter. The tune and running was so much more consistent with a new filter on it. It wasnt oiled either. I never knew the extent at which the filter can affect tune and running so much. I always wondered and now I know. I always changed filters for maintenance and health of the engine but it has a big affect on tune.