Loss of power at 4,000 and up.

84 R100RT Loss of power

Good evening all,

I am experiencing a flat spot in the power band at about 3800-4000rpm in neutral and 4000 and above under load. In neutral, once past the small hiccup, rpms rapidly increase. Under load, in first gear, acceleratiion is more aggressive than stock. At 4000 it will continue to accelerate, just drastically slower to about 5500 where it doesnt have enough "grunt" to accelerate further. If I upshift and the RPM drops back to the 3000 range, will continue to accelerate as if nothing was wrong untill the 4000 rpm mark. Any gear above 1st will not accelerate past 4500.

The bike starts and idles fine, albiet with a bit of a lope to it. I currently have the idle speed set at about 1,200rpm which is a tad high for these. Its set that way to balance between happy bike and stable carbs.

Carbs are sync'd, Ignition system is new, Engine is, for all intent and purposes, rebuilt.
What I believe I'm running into is a lack of knowledge for accurate diagnosis on my part as well as a good bit of contradicting information on the web.

The rundown:

1984 R100RT
Siebenrock 1070cc Big Bore Kit w/ Asymetrical Cam
Alpha Ignition System v3 (Hall sensor and ignition module: 20deg to 29deg)
450w Generator and Diode Board (EME)
Lectron Slide Carbs
NGK 5k Wires
Bosch Super RO751 Plugs
Bosch 6v Coils
Valeo Starter
New Battery
Mega Ton mufflers (not that it has a huge impact beyond the noise)

Im trying to make sure I have all the electricals correct beofre I go diving into tuning the carbs further. What should total timing be for this application?

I realize this is probably considered "hot rodded". Just looking for a little more information/input, primarily due to the amount of variables and my limited knowledge.

Any input would be appreciated.


Side note: As far as concerns with the Lectron Carbs and why they are on the bike... The bing on the right cylinder had a fracture causing a consistent rich condition previously. This lead to the bike not running on the one cylinder. I have used Lectrons on other bikes in the past and "tuning" them is the easiest of any carb I've used prior hence the current installation of them.

Off the top of my head...plugged air filter, torn carb diaphragms. Slide carbs...maybe no diaphragms! What are the carbs?

20774 said:

Off the top of my head...plugged air filter, torn carb diaphragms. Slide carbs...maybe no diaphragms! What are the carbs?

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They are Lectron Carbs. I've edited the post to reflect.
Filter is a new OEM, Ive also tried it with no filter or air box. No change.

Can't say as I've heard of Lectron...not been mentioned on the forum before. Can you find an original set of carbs? Putting those back on could help to see if the problem goes away. Given the choice of carb, I'd be trying to figure out what's going on with fuel flow - jets, slide, etc.

Tell us more about the spark plugs. For some reason I can't find any reference to Bosch RO751 plugs. Are they resistor plugs? You don't need the extra resistance with the 5K ohm caps. Additional resistance can make it harder for the spark to jump the gap...basically weakens the spark.

I'm sure you've read about changes to the "breathing" of the engine. BMW designed the air flow in and out of the motor for some optimum effects. Changes to the muffler could create some back pressure or standing waves that could be affecting performance.

Given the internal changes to the engine, I suspect you may be having issues getting the fuel flow right and one or more regions of performance is starved for fuel. As you said...somewhat hot rodded. Generally, changes to the Airhead engine/intake/exhaust can be thought of as a zero sum game...you might be able to improve one region of running, but it might have a detrimental change at another.

Looks like you'll need to do the substitution game until you find something that effects the issues you're seeing. Good luck!

+1 to what Kurt said.

The bike in question is far enough removed from stock that the OP is going to have to get methodical about tracking down the source(s) of the problem. It certainly sounds like a bike whose carburetor is badly off from midrange on up. I’d start with baselining the bike by checking valve settings followed by verifying timing at idle and getting an exact read on how much advance is being supplied—a degree wheel mounted to the alternator rotor would pin that down exactly. Then beg or borrow a functioning and correctly jetted set of OEM carburetors and see how the bike runs in that configuration. If it’s running well then, the aftermarket carbs would be suspect. It might be that they can be rejetted and adjusted to work correctly, but that is likely to be a long process requiring repeated “plug chop” runs. So goes the process of hot-rodding and second-guessing the engineers who designed the bike.

Good luck,
DeVern

20774 said:

Can't say as I've heard of Lectron...not been mentioned on the forum before. Can you find an original set of carbs?
For some reason I can't find any reference to Bosch RO751 plugs. Are they resistor plugs? You don't need the extra resistance with the 5K ohm caps. Additional resistance can make it harder for the spark to jump the gap...basically weakens the spark.

Click to expand...

So the Bosch Plugs are the OE plug from BMW FROM 1984, my father has the same bike and bought 12 sets from a shop as it was closing. They are resistor plugs. That would be fantastic if this is whats going on, I'll pick up a set here in a few minutes.

As far as the carbs, I took his bings off his bike, he was none too happy, put them on mine and I still experienced the same flat spot at 4000 and above. Due to my inexperience with bings, and my experience with Lectrons, that why I went that route.
The company started in racing 2 strokes (sleds, dirt bikes, 3-4 wheeler, etc.) and branched into 4 strokes about 6 years ago IIRC. I like the simplicity of them.
The Lectrons run a metering rod system. Essentially, its a rod with a flat taper, the more air that passes past the rod, the more fuel it pulls. To richen, you tighten the rod in, exposing more of the widest part to air flow, and the opposite to lean it out.
There is also a secondary adjustment for WOT but this happens regardless of how far the throttle is open. Benefit to all this is infinite tuning and no need to worry about altitude changes either, it has a balancing system based on atmospheric pressure for that.

saddletramp said:

So the Bosch Plugs are the OE plug from BMW FROM 1984, my father has the same bike and bought 12 sets from a shop as it was closing. They are resistor plugs. That would be fantastic if this is whats going on, I'll pick up a set here in a few minutes.

Click to expand...

Kicks and grins, I put the OE wires back on, no change. They didnt have the new plugs in-stock so back to waiting.

I'm sure there are any number of other plugs that you can use. NGK offers good plugs. Snowbum has a chart with some cross references. I think my /7 takes the W7DC, maybe the W6DC. I would think NGK BP6ES (or whatever the latest numbering is) would work in a pinch.

https://bmwmotorcycletech.info/sparkplugs.htm

NGK BP6ES sparkplugs would be perfect, IF you can find them. NGK has discontinued the BP6ES plugs and many vendors are now shipping BPR6ES as a substitution, but the R indicates a resistor plug and is not correct for your bike. There are still some BP6ES plugs in the channel and you might luck out with a visit to a FLAPS or three.

The Bosch W5DC, W6DC, and W7DC plugs are now only available from BMW dealers under a BMW part number, at just under $7 each IIRC. Your only other correct plug I know of at this point is Nippon Denso 3043 W20EP-U U-Groove Plug and they are reasonably priced but not usually carried at the auto parts store. They can probably order them in, tho.

Best,
DeVern

This is a Frankenbike. There is very little BMW specific wisdom to guide us through this one.

But, back to basics. This is a four stroke cycle internal combustion engine. Suck squeeze bang blow. Air, fuel, compression, ignition - all at the proper time of course.

Swapping carbs might eliminate part of the air and fuel equation. But the rest is not so clear. And I have no idea what the new hot-rod pistons and cylinders mean, nor the modified exhaust.

Fuel flow rate from petcocks tested?

I realize that you don't have Bings but instead are using whatever those carbs are. If you did have Bings, what is happening is a classical condition when the diaphragms become torn and you are losing vacuum. So, what about those carbs would cause that condition??? Again, I know nothing about those air/fuel mixture devices that you are using BUT I AM VERY FAMILIAR with the condition happening as it is in your bike.....Perhaps you could do a little research and figure out the vacuum loss or fuel starving that is happening under load at a mid-high RPM...……..God bless....Dennis

DennisDarrow said:

I realize that you don't have Bings but instead are using whatever those carbs are. If you did have Bings, what is happening is a classical condition when the diaphragms become torn and you are losing vacuum. So, what about those carbs would cause that condition??? Again, I know nothing about those air/fuel mixture devices that you are using BUT I AM VERY FAMILIAR with the condition happening as it is in your bike.....Perhaps you could do a little research and figure out the vacuum loss or fuel starving that is happening under load at a mid-high RPM...……..God bless....Dennis

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Exactly! +1

Being slide carbs, I very much doubt that the Lectron carbs have diaphragms. In slides, the throttle cable is directly connected to the slide whereas in CV carbs, the cable controls the opening closing of a butterfly...which brings into play vacuum above the diaphragm.

see https://www.lectronfuelsystems.com/ ... no diaphragm.

I'm actually a bit surprised that several "old-timers" haven't heard of these...

GTRider said:

NGK BP6ES sparkplugs would be perfect, IF you can find them.

Click to expand...

I managed to track down a set of BP6ES plugs, getting them installed here in a bit.

As far as petcock flow rates, the fuel lines and float bowl are transparent with these so I am able to track the fuel flow, no issues so far there. Knock on wood...

Lectrons do not have diaphrams so nothing there. However, the metering rod covers low RPM to Midrange and the "Power Jet" Covers Mid to high RPM. I have been incrementally increasing fuel based on the butt dyno, no major changes yet.

So, out of curiosity, what should the ignition timing advance be at max? Being that I have that level of adjustibility that stock configuration does not, I'd like to set it to match for now to narrow down possible issues.

Looks like you're on the cutting edge using those carbs. Under 4-stroke kits, BMW is not an option. Maybe that's why no one is familiar with them. They seem to be for mixed usage machines - xcross, ATV, singles, etc.

As for advance...do the people you bought the big-bore kit or the Alpha ignition have any suggestions for you? Since this isn't dual plugs, I would think that the stock advance scheme for an R100RT would be a good starting point. I have no understanding what an asymmetrical cam does, but maybe that means it gulps more air at some point...that's certainly going to require some adjustments to mixture. What's the compression ratio you're achieving with this setup? If it's greater than say 9.5:1, there might be issues with detonation/pinging, which might lead one to consider a dual plug setup.

Pauls1150 said:

see https://www.lectronfuelsystems.com/ ... no diaphragm.

I'm actually a bit surprised that several "old-timers" haven't heard of these...

Click to expand...

.

Learn to read ….Sorry they don't teach that any more. Anyway, not to be RUDE, as the "YOUNG TIMER" is......Having gone through slides and vacuum carbs those are not suggested. Instead what is advised is to find out loss of vacuum and starvation of fuel that would imitate what happens with a vacuum type carb to cause the condition that is being experienced at 4K RPM...……Ever even put your palm over the intake of a lawn mower back when you were in the 4th grade and felt the vacuum??????…..LEARN TO READ and keep your rude comments under your hat...…….

20774 said:

Looks like you're on the cutting edge using those carbs. Under 4-stroke kits, BMW is not an option. Maybe that's why no one is familiar with them. They seem to be for mixed usage machines - xcross, ATV, singles, etc.

As for advance...do the people you bought the big-bore kit or the Alpha ignition have any suggestions for you? Since this isn't dual plugs, I would think that the stock advance scheme for an R100RT would be a good starting point. I have no understanding what an asymmetrical cam does, but maybe that means it gulps more air at some point...that's certainly going to require some adjustments to mixture. What's the compression ratio you're achieving with this setup? If it's greater than say 9.5:1, there might be issues with detonation/pinging, which might lead one to consider a dual plug setup.

Click to expand...

The asymetrical cam just means the lobes for intake and exhaust are not identical.

Yeah, as far as the carbs go, they dont keep them on hand, the bings are larger than your average carbs so they make the openings custom to match the application. I doubt Bing CVs were very desirable for high performance applications since this company has been around so making a replacement couldnt have been too high on the priiority list vs. Mikunis or Keihin.

As far as compression ratio, it increased it to 9.5:1 so not a huge change there once you factory in the valve overlap from the cam causing the dynamic compression to be even less of an increase...unless I'm understanding the information incorrectly, which is always a possiblity

20774 said:

Being slide carbs, I very much doubt that the Lectron carbs have diaphragms. In slides, the throttle cable is directly connected to the slide whereas in CV carbs, the cable controls the opening closing of a butterfly...which brings into play vacuum above the diaphragm.

Click to expand...

No kidding Kurt...…..thanks