850 Dyno Run

I don't think he's got a problem for low down power, just in excitement of dyno run he flubbed up a bit, just like yo've likely done missing a shift or over or under powering race take offs. Small ID headers, big valves, one .5# richer slide, notched spray tubes and hi-ish float level and standard cam with hi-ish CR and low gearing make for better off idle response. Could add a squeeze bulb near one grip to give a spriz as throttle snapped or spend for pumper flat slide carbs or Comstock fuel injection or boost too, but don't spray NOX down low or KaBooM.

Whitworth Ranch said:

Shrapnel, I am running #3 slides already and normal response on the road is fine. Just way too much way too low on the dyno.

This was the kind of snap open that EFI (which is almost exclusively what I dyno) doesn't mind at all.

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Ah yes, awaiting Jim Comstocks EFI for Commandos.

Hi Ken, thanks for the Ignitech update.
Will contact them and see what sort of feed back I get and will post once I sort what I really need.
Regards Mike

Dances with Shrapnel said:

Whitworth Ranch said:

Shrapnel, I am running #3 slides already and normal response on the road is fine. Just way too much way too low on the dyno.

This was the kind of snap open that EFI (which is almost exclusively what I dyno) doesn't mind at all.

Click to expand...

Ah yes, awaiting Jim Comstocks EFI for Commandos.

Click to expand...

It's available thru CNW. You just have to ship your bike to CO, and have about $3,500.00.

Brooking 850 said:

Hi Ken, thanks for the Ignitech update.
Will contact them and see what sort of feed back I get and will post once I sort what I really need.
Regards Mike

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I have one on my RD400, see another-ignition-consider-t4979.html?hilit=%20consider For sure on my next project I will use one and it will most likely be crank mounted.

Jean

Thanks Jean will follow it up
REgards Mike

Bogging the engine with a immediate snap to WOT is perfectly normal, as the carb was designed. The sudden removal of vacuum at the discharge nozzle stops fuel from being drawn up from the bowl. The resultant lean mixture causes the bog. Changing jetting, slide cutaway, etc. to an otherwise good running crab will not solve it. This shortcoming of design was THE reason CV carbs were introduced (at a greater cost and complexity). When the butterfly is snapped open, the vacuum increases only as much as the engine's current ability to create vacuum (RPM) and thus the slide comes up (relatively) slowly, enough to keep pace with the engine.
Accelerator pumps whole purpose is to mask this lean spot, a band-aid.
In the automotive side, the Holley carb with vacuum secondaries is an example of a way to get around opening up too much venturi too quick.
As the OP has already stated, proper operation makes it a non issue, roll into the throttle as quickly as you know it can take it. :idea:

Hehehe, CV's don't exactly bog on snaps to WOT like a plain carb but boy howdy CV's-SU's sure feel like it compared to a plain ole carb opened up a bit slower-smoother than a snap to WOT. Of course some CV's also have accelerator pumps to accelerate off idle or to fill in their slow fuel response of snapping to WOT when way over idle rpm - instead of feeling like a bogger while waiting on the response, compared to a plain Jane Carb. I guess we need to see the dyno charts with lessor peaks but better practiced throttle hand down low. Btw most dyno's can't really measure below 2000 rpm to believe in any way.

I had a 406 cid towing V8 built for pure torque, hi CR, bigger valves, jet boat cam in twisted wedge Al heads out Tri-Y headers with X-over tube near their ends then into big OD long tail pipe into motor home size turbo Corvair muffler, that made 449.8 lb ft torque at 5600 rpm, but would pull your teeth out right off idle with 423 lbft right off idle on a none accelerator pump 900 cfm Preditor carb, but did even better with double pumper 750 Holley with the 4 barrels wired together, that I still had to be a bit smooth or would bog. Then put on a 2 brl Holley 650 TBI fuel injection and was amazed at the response across the whole rpm range, to the point I hunted down Porsche's and Ferrari's and Boss Mustangs etc in Houston Frwy traffic in my 4800 lb cargo van. With my modified suspension it could out corner them below the ton into and out of clover leafs on/off ramps, much to their horror. The 373 posi-tract rear end helped too. Below 70 mph pulling a 3 ton jet boat and trailer with 6 passengers and cargo it could spin the tires if I snapped to WOT in lower gears. USed it out here in Ozarks durring period of my first and locally famous factory Combat to find the Combat didn't stand a change against it after 60 mph 2nd gear nor in the handling department on or off road. I called it Ole Blue but everyone else called it the ...

"Bogging the engine with a immediate snap to WOT is perfectly normal, as the carb was designed. "

+1 As noted, you need fuel injection, an accelerator pump (properly set up) or a CV carb to avoid it. I have never found it to be of any concern at all in normal on-road operation.

Agree with all that a little hesitation is really a non-issue but also agree with Rohan that this should not happen.

The questions are: what is happening and can it be tuned out?

The “event” starts at 2,400 rpm, and starts recovery at 2,500 rpm and plays “catch up” to around 2,800 rpm where it is back on trend.

Keep in mind that the throttle was whacked open at 2,000 rpm so I suspect a lean condition would have manifested itself earlier. An exhaust gas analyzer would be telling.

Maybe surplus fuel pooling in the intake tract finally made its way to the combustion chamber. Maybe a weak spark.

No need for alarm, I am just curious about this.

"No need for alarm, I am just curious about this."

Just trying to help splain it so as to satisfy the curiosity :wink:


"The questions are: what is happening and can it be tuned out?" No, it can't be tuned out, again, it's the design of the carburettor, (and every other non-CV/non-pump carb) It can be masked by way over richening the adjacent areas, but that would be extrememly poor tradeoff.

Opinions vary.

Note that the OP stated "Premier carbs and Tri-Spark" so I wonder what "Premier carbs" are and what the set up was/is?

In general - How about bad gas. How about a cold motor. I've seen bad gas or even different gas causing stumbles. How about a winter fuel blend being used in summer or a summer blend being used in winter. Have a look at what's happening with the dyno pull as the engine is pulling fine for a bit and then stumbles which suggests the stumble occured sometime after the throttle was whacked open. How about fouled plugs or the motor wanting a little more plug heat range. How about carbueration and relative atmospheric density during the dyno pull. I pissed away a good part of this last race season chasing an electrical problem that ended up being carbeuration and unusually low atmospheric density.

Yes, curiosity. So if I come across as a little persistent on this matter it is because I have seen and owned Nortons that also respond crisp and clean when you "whack open" the throttle both on a dyno and on the road.

Any tricks of the trade you can offer on how to get nil bog on idle rpm snap opens. My P!! was very touchy on street tire not to just sit in place and smoke not even trying to kick up heels, but even it would bog an instant if snapped to WOT from its 600 rpm idle but if smoother throttled once past 100O if was so eager to take WOT i leaned lots of wild lesions on water skiing a non lean-able cycle.

What matters The Most to me by far on off idle response is ability to climb loose steeps and can give very high throttle opening w/o bogging nor spinning out, just chugs right up till can ease off at top. This is a hard state of tune to reach, if it coughs once its dead, if it spits once its a banshee out of control. If I'm in a hurry I am in lower gear spun up anyway so never in bog risk to snap WOT in glee.

DYNO .

Should try 100 Octane Race Fuel . Instead of the lolly water they pass of as petrol these days . Might smooth out a way .

Egad , youre peak is at 6.000 Not the 5.800 quoted by Norton . :shock:

The flat spot at 2,500 could be the Mufflers . We Need a Comparo Run , With the Mufflers Removed . If you would .

15.000 miles . She'll be just run in nicely , if you havnt been flogging it .

Or a squirt of Acetone,- if you doubt it, add a small % [~5] to you bbq starter fuel - you`ll see an improvement in volatility..

Removing the mufflers can change the jetting requirement enough to burn a piston if the motor is on the limit of leaness. I only ever use methanol in my racer, but if it is jetted for max power, it is as critical as petrol. Amal Carbs use the same diameter needles as Mikunis. I use Mikuni petrol needles because they usually have three tapers where amal stuff tends to be crude . The critical tuning point always occurs at three-quarters throttle - (metering on the needle jet and needle). I run my carbs as lean as possible right down the needle until the motor coughs when you use the bike around the twisty stuff. If you lower the needle one notch, the motor will always cough, and it always coughs when cold - the chokes are needed until it gets quite warm. I always use the same procedure - lean it off until it coughs, then richen it one fraction. It is currently running rich on the main jets , but I haven't bothered to lean it off, it is still fast (and safe ).
Note : - You don't tune two strokes that way !
I don't know what you mean about snapping the throttle open. I use a TZ350 twist grip, but I don't usually ride that way. I've usually got my motor doing well over 3000 rpm up to 7000rpm, and the quick action grip is there to help smooth gear changes.

Dances with Shrapnel said:

Opinions vary.

Note that the OP stated "Premier carbs and Tri-Spark" so I wonder what "Premier carbs" are and what the set up was/is?

In general - How about bad gas. How about a cold motor. I've seen bad gas or even different gas causing stumbles. How about a winter fuel blend being used in summer or a summer blend being used in winter. Have a look at what's happening with the dyno pull as the engine is pulling fine for a bit and then stumbles which suggests the stumble occured sometime after the throttle was whacked open. How about fouled plugs or the motor wanting a little more plug heat range. How about carbueration and relative atmospheric density during the dyno pull. I pissed away a good part of this last race season chasing an electrical problem that ended up being carbeuration and unusually low atmospheric density.

Yes, curiosity. So if I come across as a little persistent on this matter it is because I have seen and owned Nortons that also respond crisp and clean when you "whack open" the throttle both on a dyno and on the road.

Click to expand...

Amal Premier. I thought these were widely enough known here that these did not need an introduction.

There's nothing strange going on with the dip. I turned the throttle too quickly on non-pumper/non-CV carbs with round slides, which just don't respond as well on rapid transitional throttle as other types of carburetors but do respond quite adequately with just a hair of restraint, as indicated below. The carb orifice got too large, too fast to provide signal to the jet, end of story.

Here was a clean run off the bottom. I didn't pick this one to post first because its peak wasn't as high, but is a quite normal run-to-run statistical variation in peak number. You'll note that the good bottom end was run 4, which immediately followed 3 with the dip, because I learned my lesson.

I've intentionally tuned my 850 motor to deliver torque, but I wouldn't know what a good figure is for it, I always work on the way the bike performs when ridden. I'd really like to quantify it. I'm pulling much higher overall gearing than I would normally expect from that size motor. I bought the 6 speed box so that I could get it mobile during clutch starts, and to make it more versatile elsewhere on tight circuits. I found that when using the previous close four speed box, that it came up through he gears very quickly, but first gear was far too high during a clutch start. I had to cook the clutch to get it going. I've yet to try the 6 speeder - in theory, it should be superb.

Still more fun with No mufflers tho . . Be intrested if there IS a differance , theory being loss recoverable . Not hearing any Snortons with a fair crack
to the pipesthese days. wondering if its the fuel composition , or the pattern mufflers .

Or - too much crack pipe? Maybe time for an audiologicackle exam Matt? Dose of Acetone perhaps? L.O.L.