Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

K & B 7.5 DF on prop ?

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Old 09-16-2005, 07:34 PM
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rmorhan
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Default K & B 7.5 DF on prop ?

got a k7b 7.5 pumped df in a package deal with a d dust. the previous owner had problems getting it to run properly on the K & b carb so swapped it with an os 8b. what prop/ fuel would i run on this combo & is it gonna be real finicky? if so i might save this motor for a future df project & put on a jett 50 i,ve got kicking around ( the dust is drilled for the k&b so i,d like to try it if i,m not gonna be pulling my hair out getting it to run properly). can this unit be run without the pump or will it be damaged with no fuel going through it ? anyone running this motor on a prop have any issues?? any advice appreciated....thanks rob m
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Old 09-16-2005, 09:17 PM
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r-c-guy
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Default RE: K & B 7.5 DF on prop ?

Haven't run this df engine on a prop, but have run the os equivelant. It ran strong and fast using only pipe pressure.....

randy
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Old 09-16-2005, 11:20 PM
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Default RE: K & B 7.5 DF on prop ?

***This is just my experience***

I got an old K&B- 7.5CC (45) that came on a quickee (Scat-Cat ) it was FIRE and had a Perry pump and carb. I believe and it could turn a Rev-Up (Wood) 19K with the full tuned pipe- I ran it on 5% DF fuel and 15% Powermaster on separate occasions... I could not get it to run very consistently and it was VERY loud so I ended up selling it- Keep in mind this engine was 20 years old, I have heard of MANY folks who have had great results with this engine on a prop.

Just break it in on the bench and get the settings dialed in- I ran mine on the pump and bypassed the pump, no noticeable difference on the ground- I still want to get a new one of these and run it--

Go for it and keep us posted!
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Old 09-17-2005, 12:17 AM
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Default RE: K & B 7.5 DF on prop ?


ORIGINAL: rmorhan

can this unit be run without the pump or will it be damaged with no fuel going through it ? anyone running this motor on a prop have any issues?? any advice appreciated....thanks rob m

I don't know about the pump... but I don't think it wil run long enough WITHOUT fuel to do any damage...
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Old 09-17-2005, 01:18 PM
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Default RE: K & B 7.5 DF on prop ?

just wondering if the pump would be damaged if i ran the engine with the pump by passed....thanks rob m
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Old 09-17-2005, 01:40 PM
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Default RE: K & B 7.5 DF on prop ?

here,s some pics of engine & pump...thanks rob m
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Old 09-19-2005, 10:43 AM
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Default RE: K & B 7.5 DF on prop ?

kewl...... that looks like one of the modified engines that were sold for a while - If I recall properly Tom Cook sold them configured like that. Has an OS carb on it - BIG carb!! Was a huge improvement over the K&B that was available with the Perry.

I would not try to bypass the pump. The engine will not run well with that carb without the pump. It simply can not draw enough fuel.

The pipe is set REAL short. Looks like about pipe setup - around 10 " or so ??? In that case, its going to need a very small diameter prop or the engine will not run properly (unless you make the pipe assembly longer). You may want to try one of the APC carbon QM40 props to start with. Let that sucker turn up.

Bob
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Old 09-19-2005, 06:04 PM
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Default RE: K & B 7.5 DF on prop ?

thanks again bob ! any idea on the pump setting i should use ? i ran it today on a 8.5 x 7n apc & i think the pump needs to be set, seems real rich. any idea on the reccomended setting? the distance between the glow plug & where the pipe reaches max dia is 8.25 " & to the end is 14". i,ve got a few pipes kicking around what would be the optimum length to keep the prop tip speed down ? appreciate the input....take care...rob m
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Old 09-20-2005, 08:58 AM
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Default RE: K & B 7.5 DF on prop ?

What you have set up there right now needs to run at above 20K rpm. (QM40 prop will get there)

What was the 7n turning ?

Do not adjust the pump. Use the needle valve if its rich. Just like any other engine.

The pump adjustment is more for pressure regulation, and will only affect midrange and transition. At top end, the needle is what is the adjustment.

Keep in mind, this was a ducted fan engine. Was designed to run hard and fast at full throttle 90% of the time, and an idle speed of 3000 rpm was acceptable.

Right now you are running with a 'way-to-short' pipe to evaluate the engine with that prop. Id set the pipe closer to 11" and go from there of you have another header available. Again, the engine is high-timed, and this is still a short pipe. Start with a small prop, and then work up. When you find a prop where the engine starts becoming hard to set the needle, you have found the critical rpm (Tuned length). Note the RPM as you go
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Old 09-22-2005, 03:13 AM
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Default RE: K & B 7.5 DF on prop ?

i appreciate the advice bob. no tach readings available ( was close to dusk & my lantern battery was weak) put the jett fire 50 on the stand as well, what a beast, this thing screams ! me thinks the new dust needs this mill on it while i dial in the K & B (like the jett will ever come back off !) i,ve got a new dust kit kicking around as well , guess it,s time to throw it together for the fan motor.. take care....rob m
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Old 06-17-2006, 10:14 AM
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Default RE: K & B 7.5 DF on prop ?

I had this same engine mounted on a 20 size stick. It would turn a 9-10 prop at around 23000 rpm. The plane was fast to say the least. I also mounted pizza cutter tires to reduce drag. I sold the engine to a friend and mounted a OS 40 LA on my little ugly stick. It is still fast but not as fast as it use to be. I did not use the pump. My pump was bad . It ran fine on exhust pressure . My pipe was 13 1/4 in long
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