HPI RS4 - Sputtering....

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We need to get the touring forum going so I thought I would post it here instead of in the general nitro.

Where I race most of our TT's and Features are 10 minutes long. In my last race my car was starting to sputter when I would go in and out of a corner then hit the throttle. It wasn't doing this before but I was adjusting the needle a little bit.

I was getting close but I wasn't out of gas. The engine was extreamly hot. I plan on getting a better heat sink with in another week. I have the left half of the windshield cut out. A friend suggested that I do the same for the back windows too. I have done that but I havent been able to race it yet to see if it helps becasue of the crappy weather around here.

I was wondering if anyone had any ideas on the sputtering. Is it to lean or rich? Which way should I adjust?


My first impression is that it's too rich. But that depends I guess. If it's running really hot and sputtering, that may be lean. I get sometimes confused because excessive rich and excessive lean can both cause stalling. I haven't figured out how to tell the difference yet. Some help, huh?
I cut the front and rear windows on my cars to get flow. I usually use a 2" hole saw on my drill. Punches a quick hole. And then I just sand the edges so I don't cut myself.
If it is running very hot it is too lean, not enough lubrication. Turn the needle counter clockwise in tiny increments until it runs cooler and does not sputter. But if you go to far it will be too rich and still sputter. Overly rich wont hurt it but overly lean will KILL it. That one needle carb is gonna give you problems for ever, like I said before mine ran like junk until I replaced my carb. But richen it up a little and let us know how you make out. BTW are you using a temp guage? If so what are your actual temps?
NCNitro - Thanks for the info. Hopefully the rain will stay away today and I can try it out. I still haven't dismissed getting that new carb you mentioned but for the life of me I can't find just it. I must be completely over looking it. I don't know of many online vendors. The only place I have checked is TowerHobbies.com. Temp gauge? No I am not. Is it worth investing in. Do you have a certain one that you would recommend? Again TIA.

Alot of people will tell you to buy the Raytek mini, which is very good but also very expensive, $80 - $100, I use the radio shack temp gun, $39, which uses a Raytek p/c board in it, and I have compared it side by side to the Raytek mini and they both read exactly the same. This is one of the best tuning tools you can get. I will save you from running too hot and killing your engine. I will see if I can find a link for you where to get a carb.
A 10-minute race is a long time.
For a long race, anything that will require a pit stop you will want to run rich. As the race progresses the engine will lean out to the point of flaming out near the end of the race. 10 cars at the halfway point and 4 cars at the finish line I m sure you’ve seen this. Its better to run a bit rich than to not see the flag.

You may want to play with your gearing a bit. Going down 1 tooth on your spur will go a long way to keeping your engine cool. Speed doesn’t win a race, a good line does.
NCNitro - When you get a second could you post the Cat No for the Radio Shake temp gun. Thanks.

FastEddy - Thanks for the idea. I will keep that in mind. I agree a good line will beat a fast car. Some of the guys I race with seem to go with the idea balls to wall. Most of the time they are the ones flipping their car or spinning out in the corners.

One of the guys in my race group was able to help me out. He told me that it was a little to lean.

I have the needle at 2 1/4 turns right now. The engine runs much cooler than it did before. Now I can through a full tank of gas with no problems.

One more question. With the needle at 2 1/4 turns the top end seems nice but getting off the line is weak. What way would I adjust the needle for a better start off the line?

Of course a better top end could be more benifical towards and end of the race.

I am also looking around for a tune pipe and header combo. Any thoughts? A friend of mine just bought a Dynamite pipe and the one thing I liked about it was that it used springs to hold the two pieces together instead of the rubber piece.


Well, the drawback on that stock engine is that there's only one needle. To get better launch, a 2 needle carb would work wonders. That engine loads up when it sits, and you have to give it a chance to clear before it'll get up on pipe. That will kill you off the line. I don't think leaning the HSN will do much. And it will take you back to where you were when you started. I say find a nice 2 needle carb and swap it out. That way you can lean the LSN for good clean launch.
That is what I thought. In a different post NCNitro suggested a OS 10E Carb. I have been looking high and low for this carb and I haven't found it. I checked TowerHobbies.com and they don't carry it. Do you know of an online retailer that carries it? I need to make a trip to my LHS tomorrow. I will ask one of the guys there to see if they know.

I see that hobbypeople.net sells the carb. It lists for $54 but it's on backorder right now. Tower Hobbies also has them for the same price and they note limited stock right now. That should be a start. Also check on Ebay. Someone may be auctioning the carb cheap. Or try to find a blown engine with the carb included. You may be able to pick up the whole engine cheap and scavenge the carb.
Candyman, the OS carb that Tower has is the new OS 10 er, it does not have the threaded screw holes in the sides for the two set screws that the hpi engine uses. It is designed for use with a pinch nut (man that sounds painful, lol) anyway I had the exact same problems when I was using my single needle carb, now that I am using the OS 10 e it was a night and day difference. With the single needle carb you have to compromise one end or the other. My RS4 3 idles great, launches great and runs great at top end, my only regret was I did not do it sooner.
Ya I noticed that too. I had it at a nice idle at about 2 1/4 turns out but now it is kinda weird. When the car warms up for about 5 minutes or so it wants to die when we are lining up for a race. I have no idea why it is doing this. I also noticed that my body (the side where the muffler is) seemed to be more grimey than normal. What is the cause of this. Is the car to rich and spitting burnt nitro fuel out. Should I lean it a bit? I am going to order some extenisons to I can try and get it away from my body a little bit.
It defiantly sounds as if you are running rich. How’s the power? Is it puffing smoke? This is a great reason to get a temp gun. If you could see that it was running @ 180f you would have your answer right away if on the other hand you were @ 300f you would know exactly what to do as well.

This is only half the answer if you are racing.
If this is the case let me know and I will give you some tips for race day tuning.
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Well, since you richened your mix, it will tend to send a little more fuel and oil out the exhaust. So a little more grime can be expected.

If you still have the single needle carb, it will tend to choke off the idle if it sits too long. Usually when they design them, they err on the side of too rich. Because of that, when it sits for a few seconds, the crankcase fills and the engine wants to die. Not much you can do. Hopefully they'll get you off the line quick before it dies.
Getting off the line sucks but the top end is there. About puffing smoke. Do you mean when I jump on the throttle? A little. When I am WOT the blue smoke is pretty heavy. As soon as I get my payment from a deal on ebay I plan on getting a temp gun.

Yes I race every Saturday and with a friend sometimes during the week for fun. When I purchased this car from my uncle he told me that he never broke in the engine properly. Because of that I was able to get a good price on the car. I think I ran into problems when I adjusted the carb. I never recorded the orginal setting so when he was running the car during the break in period he might have had it set too far one way. Now when I set it according to HPI specs it has problems.

I have been thinking about getting a new engine. What would you recommend. I am looking at purchasing an OS .12 CV-X. Tower carries em for $120. Is this a good price? Does the .12 CV-X come with a 2 needle carb? I was looking at getting an OS 10E carb for my current engine but if I get that plus a rebuild kit I would have most of the money for a new engine. Can the damage be reversed if I wanted to rebuild the engine. HPI doesn't really carry a rebuild kit but I know the part numbers of the items I need. Would I also need to buy a new case? All total the rebuild will cost around $68. Plus the 10E carb which is another $60ish.

I guess right now I am a little confused on what to do. When I tore into the engine I noticed a discoloration on the piston head, a little burn spot on the side of the piston and the piston wall. What are these signs of?