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K&B 3.5 engines

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Old 04-22-2009, 03:04 PM
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Crash-RCU
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Default K&B 3.5 engines

I was given 2 k&b 3.5 front intake rear exhaust engines that run very eratic at the very best. Engines have Perry carbs on them, one engine will run good enough that it can be flown but the other one won't no matter what. I can make it run worst but can not make it run right. Dose anyone know of a 2 needle carb that will replace the Perry carbs. these engines are on a twin. Engines have very good compression and smooth bearings.
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Old 04-22-2009, 03:15 PM
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Default RE: K&B 3.5 engines

Are they new? have they been broken in?

I am using the carb off of a K&B Sportster .20/.28. It's a perfect fit and works great.

You can get them here:

https://shop.mecoa.com/shopdisplayproducts.asp?id=156

David
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Old 04-22-2009, 03:19 PM
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Default RE: K&B 3.5 engines

Have you tried cleaning those carbs and putting in new o-rings? The reason I ask is, those engines haven't been made for quite some time and we generally love Perry carbs. The two together suggest that something has gone off with the engines/carbs you are holding. Have the o-rings dried out or has oil dried in the smaller passages? Have you pulled the low speed disk out to have a look? Remember, turning the low speed disk +/- 5 deg is a big adjustment on a Perry.

Bill
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Old 04-22-2009, 03:49 PM
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Default RE: K&B 3.5 engines

rainedave that is the engine that I have. They are nit new but look to be in good shape. Mecoa only show remote needle valve carb instock. Wjvail, the 2 orings on mixture disc look ok but the o-ring that seals the carb to the engine looks trashed. I am going to replace all three o-rings. How many turns on the high speed needle do I start with.
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Old 04-22-2009, 04:03 PM
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Default RE: K&B 3.5 engines

2 1/2 to 3 turns to start, and lean in from there.

While you have the idle disk out, take a dollar bill and run it through the very tiny slot in the section between the two o-rings. It's hard to see, but if the slot is clogged the carb won't perform at its best.

I am also a fan of Perry carbs. they are one of the easiest to adjust when clean.

David
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Old 01-12-2013, 06:43 PM
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Default RE: K&B 3.5 engines

Sorry to resurrect such an old thread, but if anyone's still around, I'd like to discuss this engine a bit because I just got two of these in an old collection I bought. I had no idea such engines existed. The rear transverse muffler is really unique, and probably won't fit most airplanes.

Mine are in great shape with superb compression so I put one on the test stand today. They have the Perry carb. With a 9x4 Zinger and 10% nitro I found the needle extremely sensitive. I briefly got over 14,000 rpm, but then found I could not get over 13,400 for some reason. But after that it was really steady and idled beautifully. Extremely smooth and a very linear carb. A real pleasure. But I wonder if it would work in flight because the mixture was so critical.

I went to a Bolly 8.5x4 and got a smooth and steady 15,000. Great! Needle was still sensitive, but once properly adjusted the engine was steady as a rock.

So how is this engine in the air? RD, I'm wondering why you went to the Sportster carb? What prop do you like for it?

Anyway, I'd love to hear more about this odd duck. It seems really well-made and it's so unique. The whole thing weighs barely over 9 ozs, so it really might be worth building a plane for it. Thanks in advance for any more light you can shed on this.

Jim
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Old 01-13-2013, 04:48 PM
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Default RE: K&B 3.5 engines

Not sure anyone saw the post above as it remained buried far back in the list.

Anyone have any more info about this engine?

Jim
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Old 01-13-2013, 07:41 PM
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Default RE: K&B 3.5 engines

I have one on an outboard hydro.  First I will admit I haven't run it yet because I moved and can't find a place to run it. (5miles from Lake Erie?!!)  The guy that sold it to me said he had a lot of problems with the carb and couldn't run it at all.  He bought another carb.  I just looked at the package that it probably came in, and it says HPI racing 4.6mm 12E to15FE $69 whew.  I think it only runs good because the original carb was just too big.  K&B was good/bad for that, they wanted performance.  I thought I would mention that while I was taching a bunch of motors last winter, I noticed that when I leaned them out to get a peak reading, they would sag and not run as fast until shut down and cooled off.  That is on ABC motors.  I think the cyl expands and loses compression.  I had one in a .21 inboard hydro that went pretty good many years ago.  I think it had an open control line carb and an exhaust throttle though.  I noticed a Bluebird .21 is a fairly close copy to the K&B if I remember correctly which I don't always.  They are kind of neat motors as they have the removeable front end that can be rotated 90 degrees to run in reverse. The ones I have anyway.
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Old 01-13-2013, 07:53 PM
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Default RE: K&B 3.5 engines

Yeah, they mostly seem to be used on outboards. It would look pretty odd on an airplane with the transverse muffler and pipes sticking out either side, but it might look kind of cool on the right plane. I really like the carb a lot, I'm just concerned about the sensitive needle.

Maybe it sagged on the 9x4 prop. It happened real fast.

These engines were also used with pipes on ducted fans, I think. Must like to rev up; I assume a small prop is the way to go, like an 8x5.

Thanks,

Jim
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Old 01-13-2013, 08:22 PM
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Default RE: K&B 3.5 engines


Quote:
ORIGINAL: buzzard bait

Yeah, they mostly seem to be used on outboards. It would look pretty odd on an airplane with the transverse muffler and pipes sticking out either side, but it might look kind of cool on the right plane. I really like the carb a lot, I'm just concerned about the sensitive needle.

Maybe it sagged on the 9x4 prop. It happened real fast.

These engines were also used with pipes on ducted fans, I think. Must like to rev up; I assume a small prop is the way to go, like an 8x5.

Thanks,

Jim

I still have about 4 of these and did build a Bridi Sportster 20 in the early 80's. I did use the stock mufflers and it did require some rework of the front end but nothing too extreme. It was very reliable and I flew it for years. I still have the engine but have converted it to control line use. They really do come alive on smaller props and pressure feed fuel systems. They were also used for competition free flight and those events are not noted for using anemic engines. Oh and if you can ever find a davis head and use it you will probably have one of the best engines that you have ever owned. Powerful and really cheap on fuel. But your car will smell like the energency room forever but you get used to it quickly.

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Old 01-14-2013, 06:33 AM
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Default RE: K&B 3.5 engines

Dennis, thanks very much for the info. Amazing that you mentioned the Davis head because in fact one of the two engines I got has a Davis head on it. I have flown diesels off and on in the past so I have the fuel and the familiarity with the technique. I got pretty tired of the stink, but I'll have to at least run this up on the test stand.

It sounds like you've run the Davis conversion on this engine; do you remember what props you used on it? I assume the diesel would handle 9 or 10 inch props of moderate pitch.

I'm puzzled by the negative comments on the carb above. The Perry carb on my engine does not have a very big venturi...it is slightly smaller than the venturi on my K&B Sportster 20. On the test stand my carb worked beautifully, except that the needle was quite sensitive. It would go from perfect to too lean or too rich in one click. Makes me wonder about the difference between the beginning and the end of a tank in flight. That's my only concern with using the engine on a plane.

Jim
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Old 01-14-2013, 07:09 AM
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Default RE: K&B 3.5 engines

These engines have a pipe timed sleeve. They will really come alive with a tuned pipe or a "minipipe"
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Old 01-14-2013, 07:17 AM
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Default RE: K&B 3.5 engines


Quote:
ORIGINAL: controlliner

These engines have a pipe timed sleeve. They will really come alive with a tuned pipe or a ''minipipe''
Could this be the reason as to the over-sensitive needle? I know someone that has one of these engines that's new/unrun. I think it has a K&B carb on it though, looking similar to the Sportster carb. It's not a Perry.
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Old 01-14-2013, 07:18 AM
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Default RE: K&B 3.5 engines

I'm not surprised, since it was used as a ducted fan engine. A header and mini pipe would be terrific.

This post and the above "crossed in the mail". Yes, I wonder if this engine/carb is just not that great without a pipe.

Jim
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Old 01-14-2013, 08:15 AM
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Default RE: K&B 3.5 engines

Hi!
I had one 25 years ago when we flew Quarter midget pylon (Yeah! we used 3,5cc engines instead of 2,5cc) here in Sweden.
We used no nitro fuel and 20% castor oil. The K&B 3,5 ran good and could hold a steady needle setting on that fuel so I it might be that your engine is set up for less nitro. Too much nitro will rendor a very sencitive needle setting! We used 7x5 props and tuned pipes and Magic mufflers in those days.RPM was around 21000-23000rpm.

Soo...for a scale plane or sport type model, use 5% nitro and a 9x4 prop, APC, RAM or Graupner "Sonic" are the best props.

If the engines are old, have been laying for months or years, it's mandatory to replace the bearings.That's very important!
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Old 01-14-2013, 08:25 AM
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Default RE: K&B 3.5 engines

Usually things are more sensitive with a pipe. If the needle has a coil spring to tension the needle, maybe a piece of tubing may help a bit. A piped liner is usually close to 180 degrees open on the exh port. You can easily tell this by looking at the prop when it comes up on compression. It will be at pretty close to the same place as it started with the other blade, a normal liner will be about 30 degrees before with the other blade. It will run poorly without a pipe if it is a piped liner. I think an 8-5 or 7-6 would be ok. It isn't much different than a .15. I had a perry carb on a K&B .40 that never worked as good as the K&B one. I don't remember how my Perry is on my Conquest .15 other than the cheesy spring on the needle. Edit, funny, this got crossed in the mail, and the first time , I replied because I thought no one else would.
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Old 01-14-2013, 10:33 AM
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Default RE: K&B 3.5 engines

I have two small engines with Perry carbs - OPS .15 and Conquest .15. The OPS .15 (piped) has lightning quick throttle response and was very easy to adjust. I have not run the Conquest enough to dial it in and have only adjusted top end on the bench so far.

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Old 01-14-2013, 12:12 PM
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Default RE: K&B 3.5 engines

back in the '80s these were right at home on the hammer 20, mounted sideways with an unmuffled pipe. best power was with an opened up control line venturi (or nothing at all...), case pressure to the tank and an exhaust throttle....balls to the wall!
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Old 01-14-2013, 12:22 PM
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Default RE: K&B 3.5 engines

I've got a few of these. I noticed that a couple of them have a very large carb, & the others have a "normal" size. Also, as Dennis stated. These engines really come alive with a smaller prop. As was also stated, I prop them like a strong .15. My guess is that a combination of poor tank placement (without pressure), a large prop, the big carb, & lo nitro could make for a challenge. and yes, there are pipes available for that engine. Un-piped, with the big carb & cc pressure, I get near 20,500 RPMs on 10% while spinning a 7x6 prop. .......... Geirge K.
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Old 01-14-2013, 12:51 PM
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Default RE: K&B 3.5 engines

Mecoa still lists a full range of parts: http://www.mecoa.com/parts/30/30-89/30-89.htm?id=943

Go to the price list, which also has stock status...looks like everything is there: https://shop.mecoa.com/shopdisplayproducts.asp?id=189

The "Tuned Muffler" is the stock canister type, but there is an exhaust adapter and a header tube. With those I assume you could fit a pipe or a mousse can.

Very interesting to hear from all these people about an engine I had never heard of until I wound up with a couple of them.

When I tried a 9x4 the engine was only about as strong as an OS 25 FP, but when I went down to a Bolly 8.5x4 I got much more from it. As people suggest here, even smaller is probably better. Jaka, I bet those quarter midgets really moved! I tend to prefer slower more scale-like types so it doesn't really suit my style, but since I have the engines it might be fun to try a "change of pace".

Jim
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Old 01-14-2013, 01:51 PM
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Default RE: K&B 3.5 engines

I have a brand new one that I hope to run soon. Tim, mine just has a CL venturi. on it ...which is huge! It also has a pressure tap on a rear cover bolt, which I'm sure it will need with that big of venturi. If someone has some tuned pipe dimensions I'd love to hear them. I was thinking about a 7x6 or 7.5x6 prop at largest seems like what it might like. The piston/liner fit on mine is really tight, as is the fit on a 6.5 K&B with the same setup I have...although it looks to have been run a few times anyway. My 6.5 has no venturi or Carb on it....I wish I knew what it ran stock?
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Old 01-14-2013, 02:30 PM
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Default RE: K&B 3.5 engines


Quote:
ORIGINAL: Max_Power

I have a brand new one that I hope to run soon. Tim, mine just has a CL venturi. on it ...which is huge! It also has a pressure tap on a rear cover bolt, which I'm sure it will need with that big of venturi. If someone has some tuned pipe dimensions I'd love to hear them. I was thinking about a 7x6 or 7.5x6 prop at largest seems like what it might like. The piston/liner fit on mine is really tight, as is the fit on a 6.5 K&B with the same setup I have...although it looks to have been run a few times anyway. My 6.5 has no venturi or Carb on it....I wish I knew what it ran stock?
For some reason I thought yours had an RC carb like the sportster... I guess old age is setting in early. [&:]


My new engine is as tight as that K&B 3.5 is if not worse. Better have a blow torch handy.. LoL.
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Old 01-14-2013, 03:51 PM
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Default RE: K&B 3.5 engines


Quote:
ORIGINAL: mtntopgeo

I've got a few of these. I noticed that a couple of them have a very large carb, & the others have a ''normal'' size. Also, as Dennis stated. These engines really come alive with a smaller prop. As was also stated, I prop them like a strong .15. My guess is that a combination of poor tank placement (without pressure), a large prop, the big carb, & lo nitro could make for a challenge. and yes, there are pipes available for that engine. Un-piped, with the big carb & cc pressure, I get near 20,500 RPMs on 10% while spinning a 7x6 prop. .......... Geirge K.
The large carb was for a pressure system the smaller one was for sport flyers and did handle larger props better also the idle was better. The large was simply to satisfy the requirement of having a carb. Idle was not really necessary more then 30 seconds but the ability to shut the motor off was of key inportance
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Old 01-14-2013, 04:49 PM
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Default RE: K&B 3.5 engines

A Mac's pipe would be ok, I used one of the .20 size on my old boat. My outboard has a curly dune buggy pipe to keep it out of the water. On planes with a rear exh, the pipes are usually in the way unless a header with a quick up bend can be found or made. Back in the day, K&B were the ticket for going fairly fast cheap. A Rossi was a lot of coin, and a Supertiger .15 or.19 wasn't schneurle or rear exh. Little Foxes were a joke untill the BB one came out, and there wasn't a lot of other stuff around.
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Old 10-12-2013, 07:36 AM
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My old .21 with pipe, irvine carb, 10% nitro and taipan 7x6 prop 22000rpm.
Iam buildin 1:12 ta152H aircombat plane for that engine, but proplem is that rules Prohibit pipes.
http://www.steinmodel.cz/index.php?id=st_ta-152h
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