bugsiegel

My Feedback: (2)

what would happen to an engine if I changed the carb on an engine resulting in an increased inside diameter (throat) of .040?
assuming the outside diameters of the carbs were the same and therefor were interchangeable on a single engine..
anyone tried this????

GUNSHIPGUNNER

I switch carbs around on my engines and have been doing so for years.

I have a Perry carb from a Webra 90 two stroke. It is huge with an inside diameter of .375". It just so happened the outside diameter is .510" which is the exact size to fit one of my K&B .61 ringed engines. The K&Bs carb venturi was tiny at .270".

It worked perfectly on the K&B and gave it another 1200 rpms on top while still maintaing a good ide and transition.

Next I tried it on my OS Max .32 F-ABC. The stock 3H carb on the 32 has a venturi size of .240"

So Iwent from a .240" venturi all the way up to a .375" one.

The carb's outside diameter was a bit too large so I chucked the carb body in my drill press and turned it down using sandpaper and a flat file. EASY!!

This time I gained about 1800 rpms on top but it would not idle and the transition was very poor. So I installed an old Varsane pump I had lying around. Now it works perfectly.

I also raised the exhaust port a bit to increae the exhaust duration to 158 degrees. This really helps make a tuned exhaust more effective.

Now with the big carb, the increased exhaust port duration and a carefully tuned expansion chamber it pulls stronger than my OS .46 SF (and the 46 has a tuned pipe and is almost new)

Here is a link that you will find helpful:

http://www.perrypumps.com/

They sell the Perry carbs and the Perry pumps. Check out the spec page and you will see that they offer two different sixe carbs for every engine. A smaller stock sized one and one that is quite a bit larger but is designed to be used in conjunction with a pump.

Usually you can increase the carb size about 20% and still have a good running and idling engine. When you go much bigger the carb begins to lose it's ability to draw fuel hence the need for a pump.

I also have an older plain bearing OS Max .15 SP. I did the exhaust port work on it and installed a Mac's tuned exhaust system. I also removed a bit of material from the trailing edge of the crankshaft inlet port (under the carb mount) and installed a much bigger carb off of an RC car engine I have. The carb is nearly twice as big inside and strangely enough it idles perfectly and has excellent transition without a pump.

The little 15 will now turn an APC 8X5 prop at over 21,000 rpms.
(for how long I do not know but it already has had 2 gallons of 30% nitro through it with the modifications and it runs stronger than ever)

So by all means go ahead and try a larger carb and get a pump (under $35.00 and it will work on any engine and last for years)

And don't let anyone scare you away from making modifications like these. Most of the time the fellows that attempt to discourage someone from making engine mods have never tried it themselves and don't really know what they are talking about.

You can contact me at my home e-mail if you need any help.

Curt!!

Hobbsy

My Feedback: (102)

Guns, I have a Webra .91 that the carb had been modified by someone before I got it and was real touchy. The carb off of an Irvine Q.72 is exactly the same neck and venturi diameter as the stock Webra carb. The .91 is a perfect running engine with the Jetstream carb on it. It has very good idle and a clean stumble free midrange. There are way too many engines that run rich in the midrange. Irvines don't.

bugsiegel

My Feedback: (2)

probably all the other work inside of an engine is not for me but I will ask anyway to get an idea.... how do you increase exhaust ports and and tune an expansion chamber ( briefly I don't plane to attempt this )

what I'm actually asking is this... I have a rossi 45-200 which I think I am going to change the carb on. I have it running OK but am looking for a little cleaner mid range on this particular application because the engine is enclosed partially in a cowl.
Perry makes a carb which is close to the original ,#4600 which has a venturi of .340 . I measure the stock venturi at about .333.
they also have a carb to fit this engine with a venturi of .375. This carb is already big for a 45 I think because I have the rossi 60 and the two carbs are identical.
Will I gain or will I disrupt the midrange some more with a larger venturi. I don't want to get into a fuel pump for this plane.

Hobbsy

My Feedback: (102)

Bugs, if you look at top of the column the larger carb is listed in, it will say if it is a pump carb. The pump carb has finer fuel passages in it and needs pressure to push the fuel through. Some engines will run on the pump carb without the pump but most will not idle with it.

machinist man

I just made some bushings for a guy that put a OS 7Dcarb on a Super tigre 3250. He needed a 15mmX17mm bushing. Must work. Let me know if you guy need somthing boared out. Ken

bugsiegel

My Feedback: (2)

Ken,
I will definitely let you know and thanks for volunteering your help.
there are some good people in this hobby
Paul

GUNSHIPGUNNER

PIPES AND PORTS:

It is a rather fiddly process if you take the time to check the stock port timing with a degree wheel. Since I have modified so many RC engines over the years I generally have an idea of what the stock timing is and how much to increase it without actually measuring port timing.

My method is to pop out the cylinder liner and remove anywhere from one to three millimeters (.0393 to .1181 inches)
from the top of the exhaust port. I use a moto-tool with a carbide bit.

The tricky part is cleaning up any burrs or rough areas left on the inner surface of the liner. If you don't you will ruin your piston the first time you insert it.

To prevent this I use a tiny jewelers file and hand file a small radius around the entire upper edge of the port.

The designer of the famous Veco and K&B .61 engine, Clarence Lee, uses Cascade dishwasher detergent in hot water to clean his engines after working on them. Now this guy makes a full time living off of modifying engines (especially K&B) so he knows his beans.

Well I thought, "How ridiculous" DISHWASHER detergent. Until I tried it. Now it's all I ever use followed by spraying the inside of the engine with a mixture of Marvel Mystery oil air tool oil and ATF (automatic trans oil) mixed fifty - fifty.
This prevents rust and also is about the best after run oil you can find.

Here is the link to how to tune a tuned exhaust system.

http://www.macspro.com/tuning.asp

This leads to Mac's site. They make one of the best systems for RC applications.

CARBS:

Go with the #4600 carb. I can almost guarantee it will clean up the mid range, provide you with an even better idle and produce more power on top!

BTW: I run 25% or 30% Nitro in every engine I have and have been doing so for over 20 years.( ABC engines like this hotter fuel when they are newer) The one thing to check for is detonation. And you can usually add a head shim or two to correct this. (All my engines except my Big Moki's which HATE any fuel with more than 10% Nitro.) Most of the helicopter guys run high Nitro fuel because besides increasing power some it helps the engine idle better and run in the mid-range better.

If the RC Universe guys want, I can post a short but complete article I wrote on finding T.D.C (top dead center) the easy and accurate way without any tools (just 2 plastic protractors glued together and bolted on where the prop goes) and how to measure all of you engines port timing. Also which ports I usually change and what effect it has on what part of the power band.


CURT

Nathan

My Feedback: (12)

You got my vote... I'd really like to read that.

Macgyver

Hi,I like to open my carbs too and match and port engines.I love it when I can see A improved engine.Go to my web site and see the Rossi 105 carb I did work on.Take care-Mac http://community.webtv.net/macgyver4u/DuctedFanEngines

GUNSHIPGUNNER

All I can say is what a neat webpage and thank God there are others who DARE to modify their engines.

I'm sick and tired of doing a web search for "modified RC engines" only to find some lame-ass site where the guy tells you:

"Attempting to modify you RC engine is not advised as it takes extreme precision and you won't gain anything anyway".

or

"Don't modify your engine. This will void your warranty."

or

"The current batch or production engines are already making as much power as possible. After all they have muli-million dollar companies designing them with engineers that are much smarter than YOU"

or

"The guys in your club will laugh and point at you and your privates will fall off "

I hope you get the idea so thanks again for the great webpage.

Macgyver

You know when you get A stock small chevy and then put a cam,headers,intake and timing on it and then you see the difference on the performance and you get A big smile.There is more acceptance to cars than there is model engine hop up's,if you know what I mean.The first engine I did I scewed up but I found out some stuff and I didnot have much money in it.You go buy A plain bearing motor like A OS FP 40,used and you take off the rear plate and close and open the timing on the crank and match where the crank pin where the rod goes and bolt patern where the back plate goes and that is your starting point.Then you get a better motor like a OS FX 40,46,and do the same on that motor and then you cut the timing port on the crank on the FP to match the timing on the FX and you open up the carb and then you test and I forgot to tell you to tach the engine before and after porting.I just did these two things and got over a 1000 rpm increase.The FP timing is so user freindy and you also open up the muffler hole and air bleed hole in the carb will need to be opened up a little bit.I did a super tigre 45 and without A muffler it did 17000 Rpm's with A MA 10/6 and I sold it on ebay so I can do some more.I did the same thing on the Rossi105,the rossi block is A work of art but the timing is mild so I matched it to the K&b100 and the intake was way to small so you know what I did.It went form 20,000rpm's with A Ramtac fan to 23,000 rpm's.I will talk some more soon,take care-Mac.

ChuckAuger

My Feedback: (12)

I've done a bunch, mostly on Rossis, and I find a Dremel cut-off wheel the only thing that will cut thru the chrome in the top of the exhaust port..I couldn't scratch it with a carbide cutter. But as soon as I am thru the chrome, I use a carbide cutter to remove the brass.

And you guys with OS engines, try this at your own risk. I have done a couple of OS engines,, but with the peeling problems now days, I would think hard before altering the sleeve on one. One thing to look for is if the sleeve is plated both inside and out, use extreme caution. If it is only plated on the inside and in the ports, then it will most likely be OK.

Macgyver

Hi,that Super Tigre I ported and sold is still on ebay if you would like to see it go to the search engine and punch in item # 1659482917,I don't touch my ex.ports I match the ports to the block,clean everything up retime,open up the carb and bingo!,I have cut about 30+ motors and I cant get enough! You know allmost all those engines from Italy have small throated carbs ,Rossi's,OPS,Super Tigre,Some I put ASP 61-91 carbs on them,The Iside Di. is 4.30",That K&B100 DF carb is I think is 4.73",it tels you at there web site and they make two size's.I put that big K&B carb on A K&B 7.5,fi,re,on A quickie and it tach in the 20,000's before it made runway paste!,little bit's everywhere.The carb arm came loose and it got radio hit so i got mad and picked up a formula plane to make myself fell better witch it did.When your running these engine's at high rpm's like me you have to lock down everything or else it all comes loose.Anyway I love doing this I just don't go out when the older guy's are flying so they can fly in peace.I'll talk some more later,take care-Mac

bugsiegel

My Feedback: (2)

someone post or email some detailed photos of what you guys are doing to the sleeve.

Macgyver

Hi,there is A March 2001 Model Ailplane News that shows Col. Robert Thacker porting K&B.82DF engine and if someone out there could scan the pic's in the book and post them here would be great.I have the book but no scaner but give me A little bit I will see what I can do,In the meantime check out this other web site,http://community.webtv.net/bolt34/Jets ,take care-Mac

GUNSHIPGUNNER

Great posts Mac.

I got so sick and tired of most of the other club members looking at me funny when I showed up at the club field with all my extensively modified engines in various planes like the Shrike or some European Formula one pylon racer.

Also noise was becoming an issue. Not with any neighbors of the flying field (there weren't any) but from some of the more "scale like flying" members. ("scale like flying" is an excuse used when a fellow can't fly very well)

As it turned out we made a move to a home in the country and it had 7 acres some of which I had logged and cleared so I could build my own private flying field. This solved any noise problems and I dropped out of the club!!

That was in 1987. In 1998 we moved to our present home which has 9 acres adjoining 200 acres all of it fields. Needless to say I have my own airstrip again.

No rules, no whiners!!

Take care Mac,

CK

Macgyver

That is realy nice,yes I like flying at the fying feild when only A few is out there.I fly at the Madera club,which is part of the Madera airport.We fly off the cropduster landing strip.Even Mr.Nelson has brought he's formula teem to test there.There is alway's some type of large scale racing planes.I love my serenity at the feild too but when you see a unlimited or a F4 Phantom going 200mph+ it is A site to behold.Gunshipgunner,when can I come over for A flyby.Realy nice!-Take care-Mac