Perry Backplate pump question
 

Hi all. I'm preparing to run and tune a Webra with a backplate pump and pipe. My question is this: I assume that I should not use muffler (pipe) pressure to the tank, correct? Also, any other pointers on this setup will be appreciated, e.g., initial settings for the Dynamix NV, etc.

Thanks,
David

RE: Perry Backplate pump question
 

No muffler pressure to tank

RE: Perry Backplate pump question
 

Never ran a Dynamix w/ a pump...but you'll probably need to set the needles leaner than usual to keep it from flooding while you're cranking it. With a good working pump, you'll see fuel squirting into the venturi with each flip, once it gets primed.
If you use an elec. starter, be careful...

RE: Perry Backplate pump question
 

Thanks y'all. I'll start with the NV open about 1 3/4 turns, I guess.

I'm assuming that this drawing from the Webra manual is stating that the hole in the spraybar should be off center with the venturi fully open. Look at the red arrow I added to the illustration. The manual depicts the hole over to the side away from the NV. This is how I adjusted my carb. I guess I'll find out if it's correct.

David

RE: Perry Backplate pump question
 

Very nice test stand. With the spray bar fuel discharge hole in the center of the venturi it would seem that you might have better mixture/atomization of fuel and air?

Bill

RE: Perry Backplate pump question
 

Thanks Bill. I agree that logic suggests the spraybar hole be centered. But, there must be a deliberate reason why that illustration was draw the way it was. The draftsman/illustrator certainly must have made a conscious decision to locate the hole off center in the drawing. I can't imagine it was simply a haphazard act (having done some technical illustrations myself).

Test running the engine will be the only way to know for certain, unless someone else here who has experience with Dynamix carbs chimes in.

David

RE: Perry Backplate pump question
 

You will find the proper location through trial and error. The location of the hole itself is not important, since the entire spray bar tube slides along with the air valve slide...it's the relationship between the two that's important.
The "cats eye" tapers to nothing, or almost nothing as the slides slide together, out towards idle..and that leans out the mixture.

Just run that sucker and you'll figure it out ;):D

RE: Perry Backplate pump question
 

I thought Perry pumps were only designed for use with Perry pumper carbs. I tried one with a stock carb on an O.S. 40 and ran way too rich through mid range. After looking at their website It seems as though they make regular carbs and special ones for running with the pump. I ended up buying a Perry carb for the pump and it worked beautifully. You may want to do more research if you have running problems when you start it.

Max

RE: Perry Backplate pump question
 

Thanks Max. I also have a standard backplate. If it is too troublesome to adjust, I might just remove the pump. But, I have read that Dynamix carbs work well with pumps, too.

David

RE: Perry Backplate pump question
 

I've got a 120AX and the perry regulating pump works great. I had midrange trouble at first, but was shown that I improperly had things setup and that it was due to the pump set too rich. Best thing to do is to tune the engine without the pump, then adjust the pump to the engine.

I asked about the oscillating pump, and perry said it's meant for four strokes and giant scale two strokes, but should work the same so no need for a Perry carb.

RE: Perry Backplate pump question
 

As I recall, when they were introduced in the mid 70's they were sold as a set...the backplate pump and big bore "pumper" carb. They were first available for the K&B .61 and then the O.S. .60 FSR and ST G .60 and Webra Speed .61...then they were available as an option, already installed on the K&B .61 and .40 series 75.

Some (non Perry ) carbs work OK with the pump and some just don't have the adjustment range necessary to get 'em to work properly.

RE: Perry Backplate pump question
 

Dave, that spraybar you have to be careful you have it pointed down into the bore if anything. If you set it pointing upward at all it will blow back into the fuel line. The V groove location is covered well enough by the thumbscrew on the throttle plate so that can be adjusted out no problem.

The rotation angle of the mixture tube I figure has a great effect on how well the low and high speed mixtures pan out on it, because this angle has everything to do with how the spray pattern develops.

Looking at the Webra sketch, and my own line of thinking, the top of the V groove should start to form a hood over the groove looking straight down at it to help form a vacuum in the groove as the air passes.

It might be worth the time to rig up a simple flow bench using a piece of clear tube to the carb and then that to a shop vac, and flow water through the carb and try different angles to see what pulls the most water up a loop of tubing.

RE: Perry Backplate pump question
 

rdave,

I seem to remember reading, in results from a search on RCU, that the Perry pumps come with the stock adjustment set for too much fuel flow for a 'regular' carb. The spraybar adjustment is similar to the 'problem' that some people experience with Super Tigre carbs.

Terry in LP

RE: Perry Backplate pump question
 

The reason the discharge slit is off center is because most of the time except at full throttle the airflow is off to the side.

RE: Perry Backplate pump question
 

Thanks everyone. I'll just put the stock, non-pump backplate on and run it that way. The first things I need to do are get the Dynamix set up properly and then start trimming down the header. I certainly don't need an extra variable in the mix to mess things up.

Hobbsy, I think I see your point, except that the spraybar moves along with the slider as one assembly; they are attached to each other.

David

RE: Perry Backplate pump question
 

Suggestion....

I have used the dmix carb on a few webras - good carb. Works differently, so you want to set the low speed mixture with some care (do not get it rich - the carb works like an accelerator pump)

Leave the pump on, but run the engine without using the pump. Use the muffler/pipe pressure.

Get the engine dialed in close - both high and low speed.

Then hook up the pump.

When running, make small adjustments to the engine. It is probably very close. You will have to lean both needles a little.

But if anything more is required than half a turn either way, adjust the PUMP to deliver the right fuel flow.

Most folks do not adjust the pump to match the engine, and it sometimes delivers too much or too little fuel. Those are the people who tend to give up on the pump.

One one Webra/pump setup (external pump, but still perry) I cut a screw slot into the pump adjust ment hex. Pump was inside of the fuselage, and I was able to adjust it using the screwdriver. Was set up on the bench first, and I could adjust while running. Final tweeks were expected after install, but I never touched it.

Bob

RE: Perry Backplate pump question
 

Thanks Bob. Can you elaborate a bit on the above warning? Are there any dangers involved in having the low speed too rich? I don't want to damage the engine or myself.

David

RE: Perry Backplate pump question
 

no, no damage. Its just going to load up a bunch in transition or when you hit the throttle hard. The way to set the low speed up is to keep turning it in, checking transition, and when you get to where it cuts off, back off just a little on the low speed mixture and thats the spot.

The dynamix carb is a bit different in design. A bit ahead of its time in some ways.

Some folks got frustrated with it because they expected it to just run out of the box and expected it to run like a tight-neck venturi carb. It was designed as a performance carb. It needs serious pressure to perform and for the engine to run well. It does not draw fuel on its own. Pipe pressure or a pump is required. Usually the webra or other aftermarket "mufflers" did not produce enough backpressure or fuel delivery pressure to allow it to work well.

One of my best running webra speed engines was given to me by someone (1980?) after the guy litterally ripped it off of his airplane. Two weeks later it was in a Curare. The guy was dumfounded :)

RE: Perry Backplate pump question
 

Thanks again, Bob. One question... Since turning the low speed adjustment in richens the Dynamix and out leans it (opposite of a low speed needle) are you saying I should richen it until it quits during transition, and then lean it just a little? Or, do you mean to lean it it until it dies during transition, and then richen it a bit?

Thanks,
David

RE: Perry Backplate pump question
 

no, lean it out.

If I recall, the low mixture adjustment actually moves the spraybar position.

My old CMB .60 had the same arrangement.

Oh...!

One more thought..... after looking at your photos....

The only thing that keeps the slide in the carb body is the throttle lever. Make sure your throttle linkage does not permit the wire to leave the slot. If it is possible for the wire to leave the slot (especially under vibration with the throttle pushrod shaking around a bit) the wire WILL leave the slot, and the slide will fall out of the carb body (and of course depart the airplane at the worst possible time into thick jungle/grass). In a best case, the slide does not fall out, but you loose throttle control.

One way some of us avoided that ...... drill and tap the slide end toward the high speed needle side. I had a machinest friend do it, and it was a #1 screw. Small screw, washer,locktite installed after the slide is in the carb body. Creates a stop flush on that side of the carb at full travel position. Prevents it from falling out.

it was a good fix. Webra adopted it. The current version dynamix comes like this from the factory.

(reference ..... http://www.webra-austria.at/motor.ph...BURETTOR&lang= )

RE: Perry Backplate pump question
 

Yes, Bob, I have already discovered that problem with the older slides! I also saw that current photo on the Webra site and the stop screw fix.

Since I have no way of drilling and tapping the slider I thought up another fix. I was going to try and find a longer threaded rod and have one end extend far enough to actually hit the crankcase where the carb mounts. The length of the threaded rod could be adjusted to make the wire stop at the desired spot. Maybe my photo will make my idea clear.

David

RE: Perry Backplate pump question
 

I've purchased the Webra throttle arms before. There are two different lengths offered, or have been, just so your aware.

The thread size for it is an M2.5 and threaded rod for it is at [link=http://www.smallparts.com/s?ie=UTF8&refinementHistory=subjectbin%2Cmaterial_ browse%2Csize_name%2Cmeasurement_system%2Cthread_s tyle%2Cbrandtextbin%2Cenc-merchantbin%2Cavailability_class%2Citem_length_str ing%2Cavailability%2Creview-rating%2Cdate%2Cis_prime&pf_rd_r=06365SNDY05MV77K2 GYN&pf_rd_p=417978501&searchNodeID=16410711&pf_rd_ i=16410711&searchRank=salesrank&field_size_name=M2 .5&pf_rd_s=center-2&searchSize=25&pf_rd_m=AIUBT5HP6PMAF&pf_rd_t=101& searchBinNameList=subjectbin%2Cmaterial_browse%2Cs ize_name%2Cmeasurement_system%2Cthread_style%2Cbra ndtextbin%2Cenc-merchantbin%2Cavailability_class%2Citem_length_str ing%2Cavailability%2Creview-rating%2Cdate%2Cis_prime]SmallParts.com[/link] and can also be used for replacing the throttle plate adjustment.

RE: Perry Backplate pump question
 

I managed without the stop screw for a while.... its just one of those things that works if you keep an eye on it