For you with bike carb knowledge

General engine tech -- Drag Racing to Circle Track

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Truckedup
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Re: For you with bike carb knowledge

Post by Truckedup »

The two float bowl vents on each carb are located middle on each side with hoses that terminate near the bottom of the float bowl...They are exactly the same as my race bikes but it is possible the air is disturbing the float level...Or the air filter messing with the jetting air bleeds
I'll make some runs tomorrow with no air filters....if no change I'll redirect the vent hoses....
To simplify....at 3000 rpm roll open the throttle in third gear, the A/F meter drops from 14 to 12 and stays there until the throttle is backed off...Do it in 4th,the meter drops down to 10 and pretty much stays there...Same for 5th.... The beginning road speed in 3rd is about 40 mph, about 50 in 4th...
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Re: For you with bike carb knowledge

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Jetting changes with load. Engine Brake dyno's dont shown this when your testing at a given rpm/sec. Your spread is large which means your tune needs work. 12.0 is too fat, and 10.5 is something else.. Lean the bike down 3 jets straight away and tune it in high gear. Once you get high gear close 12.5-13.2 you can see what the other gears are doing. 123 dont worry too much because things happen very quickly.
V8 are the same a car jetted on the money for the 1/8th (which the majority of the work is done in 1st & 2nd) will be too fat at the quarter, (where the chugs the last 1/8th in 3rd) .
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Re: For you with bike carb knowledge

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Mummert wrote: Tue Jun 11, 2019 6:17 pm Jetting changes with load. Engine Brake dyno's dont shown this when your testing at a given rpm/sec. Your spread is large which means your tune needs work. 12.0 is too fat, and 10.5 is something else.. Lean the bike down 3 jets straight away and tune it in high gear. Once you get high gear close 12.5-13.2 you can see what the other gears are doing. 123 dont worry too much because things happen very quickly.
V8 are the same a car jetted on the money for the 1/8th (which the majority of the work is done in 1st & 2nd) will be too fat at the quarter, (where the chugs the last 1/8th in 3rd) .
...From expereience I know you have to consider this bike engine is 1930's technology.. no more than adequate air cooling with deep hemi chambers... The engine"s survival at full throttle needs a fatter mixture to cool things down so to speak...If the gauge is accurate, 12 to one is the leaner side of ideal at WOT...
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Re: For you with bike carb knowledge

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Truckedup wrote: Tue Jun 11, 2019 7:22 pm
Mummert wrote: Tue Jun 11, 2019 6:17 pm Jetting changes with load. Engine Brake dyno's dont shown this when your testing at a given rpm/sec. Your spread is large which means your tune needs work. 12.0 is too fat, and 10.5 is something else.. Lean the bike down 3 jets straight away and tune it in high gear. Once you get high gear close 12.5-13.2 you can see what the other gears are doing. 123 dont worry too much because things happen very quickly.
V8 are the same a car jetted on the money for the 1/8th (which the majority of the work is done in 1st & 2nd) will be too fat at the quarter, (where the chugs the last 1/8th in 3rd) .
...From expereience I know you have to consider this bike engine is 1930's technology.. no more than adequate air cooling with deep hemi chambers... The engine"s survival at full throttle needs a fatter mixture to cool things down so to speak...If the gauge is accurate, 12 to one is the leaner side of ideal at WOT...
My dad used to always say that air cooled engines are really fuel cooled.
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Re: For you with bike carb knowledge

Post by PackardV8 »

Truckedup wrote: Tue Jun 11, 2019 7:22 pm ...From expereience I know you have to consider this bike engine is 1930's technology.. no more than adequate air cooling with deep hemi chambers... The engine"s survival at full throttle needs a fatter mixture to cool things down so to speak...If the gauge is accurate, 12 to one is the leaner side of ideal at WOT...
Is this your race bike which spends its life at WOT? If so, that is the only consideration. Just how long do we expect it will be at WOT in fourth and fifth gears and how often? BTDTNA
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Re: For you with bike carb knowledge

Post by Mummert »

If the bike is at speed and has any kind of air going by the heads it will stay cool enough. 325F-350F CHT is fairly common. I have a fair bit of experience with air cooled engines. Air and Oil and Timing are going to be your main methods of cooling the engine. Running the motor slightly fat is one thing, 10.5 a/f in high gear is a mile off. Do your tuning in high gear to establish your main jet. Shoot for 12.0 if you wish. Trying to use fuel to cool the engine will lead to ring and cylinder wall wear, alot of oil changes and make a bitch to start when its hot.
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Re: For you with bike carb knowledge

Post by 408swinger »

Could it be your timing curve ? Maybe more advance in that situation.
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Re: For you with bike carb knowledge

Post by Circlotron »

Brian P wrote: Tue Jun 11, 2019 4:49 pm Open carbs with no airbox (pod filters = same thing) ... It is fairly likely that air circulation around and past the bike is affecting the vacuum signal in the float bowls and/or disrupting airflow into the air filters themselves. It's not exactly clean undisturbed airflow ...

Where are the float bowl vents connected to?
Yep, this is exactly what I’m thinking too.
Higher road speed and float bowl not pressure compensated.
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Re: For you with bike carb knowledge

Post by Dan Timberlake »

I only ever saw one dirt bike with the bowl vents inside the air filter from the factory, where car carbs have had them for four score years or so.
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Re: For you with bike carb knowledge

Post by Brian P »

The individual "pod" filter is not stock on any bike built in the last few decades (I will grant that I know nothing of the 1960s-era British bikes). The OEM setup would have the carbs drawing from an airbox that stabilizes the airflow patterns, and the float bowl vents would be to atmospheric pressure and the jetting set up to rely on the additional pressure signal through the airbox. It was pretty common for the float bowl vents to be connected to hoses that went to some strategic place on the bike that avoids exposure to the wind. On Kawasaki bikes with ram air, they went into the ram-air ducts (and if they came disconnected, or if the bike was serviced or modified by someone who didn't know what they were doing, they ran like crap).
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Re: For you with bike carb knowledge

Post by MadBill »

<EDIT: Written before seeing above post>
Generalizing several air-related comments above, I think variations in speed-related pressure differences or buffeting are the only causes that fit the symptoms.
o It could be the vents are experiencing a higher pressure relative to the inlet bells as the speed increases, thus creating a greater pressure drop across what ever passes for the venturis.
o Conversely, the inlet bells may be in an area that experiences a lower pressure at speed.
o It's also distantly possible that high speed buffeting is causing some flow reversal through the carbs, which would cause richness as the carb blindly meters fuel into the airstream regardless of which way the air is flowing. (i.e., a mild case of 'Megaphonitis")

Checks/fixes could include:
o Use a dual port -5"-0-5" H2O Magnehelic gauge or the like https://www.google.com/search?q=magnehe ... e&ie=UTF-8 , one leg plumbed to the carb vent location, the other to the carb bell to diagnose any speed-related pressure delta.
o A homemade U tube manometer would be a possible substitute.
o Assuming the AFR in the lower gears is fairly constant through the rev range at WOT >4500 RPM but is say 11:1 @ 5,000 in 3rd, richening to 10:1 @ 7,000, if it's a Δp issue, shifting to 4th (~ 5000 RPM?) will also show 10:1., thus confirming the fact.
o The obvious fix would then be relocate the vents to the air bells, but since you would now have the required instrument, why not fashion a static tube, mounted well out in the free airstream, compare the inlet air pressure to it and ensure it is at least at the same pressure if not higher? (Theoretically, a potential 7" H2O greater @ 120 MPH, good for a free 1.7%/1.0 HP...)

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Re: For you with bike carb knowledge

Post by dorset »

weird science.

i run a very similar 1972 750 triumph with 34mm VM mikunis, and when i put an AEM wideband UEGO on it, i need 11.5 on the gauge to max out.

i have the O2 sensor about ten inches down from the exhaust valve.

i run the machine on an airstrip and time it with a stop watch over about 800 feet at WFO throttle. 11.5 is faster than 12-anything. i assume i'm blowing unburned fuel out the pipe to need 11.5, but that's the jetting the motor wants.
i didn't mean to blow it up
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Re: For you with bike carb knowledge

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Brian P wrote: Tue Jun 11, 2019 10:39 pm The individual "pod" filter is not stock on any bike built in the last few decades (I will grant that I know nothing of the 1960s-era British bikes). The OEM setup would have the carbs drawing from an airbox that stabilizes the airflow patterns, and the float bowl vents would be to atmospheric pressure and the jetting set up to rely on the additional pressure signal through the airbox. It was pretty common for the float bowl vents to be connected to hoses that went to some strategic place on the bike that avoids exposure to the wind. On Kawasaki bikes with ram air, they went into the ram-air ducts (and if they came disconnected, or if the bike was serviced or modified by someone who didn't know what they were doing, they ran like crap).
The Mikuni TM flat slides on this bike are a general application performance carb...They are almost always run with no air filters or Pod filters....On my 96 Ducati 900M, I removed the stock air box and used large pod filters so I could have easy access to the carbs for jetting changes...Took a few tries to get the carb vents in a dead air location so the airstream at speed had no effect.
Originally this bike had Amal Carburetors with an air box...Something Triumph began using in 1971 till they folded in 1983 still using an engine designed in 1937..before that, individual filters were used in the usual vintage bike fashion..These bike when tuned nicely are not fast like modern sport bikes, maybe mid to low 13's in the 1/4, but they are entertining for oddballs like Dorset and me...It was Dorset who loaned me the A/F meter.Dorset also sneaked by and grabbed my land speed racing record.... :x
There are no huge information base for tuning these engines.The men who tuned them back when, like Jerry Branch and Axtell, have taken the info with then when they died..
I appreciate all the comments because the bits and pieces can help.
I'm going rding today without the filters to see what happens....Keep in mind as it is,once the engine is warmed up, it has excellent performance for what it is from idle to wide open..The engine responds instantly and linear to changes in the the throttle position..I just find the over rich situation in the higer gear puzzling..And I'm sure it does reduce power some when it occurs...
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Re: For you with bike carb knowledge

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Truckedup wrote: Wed Jun 12, 2019 7:14 am There are no huge information base for tuning these engines.The men who tuned them back when, like Jerry Branch and Axtell, have taken the info with then when they died..
^^^this is why i don't keep any secrets. those bastards were masters at what they did, but they kept everything so hidden from each other that we're all playing catch-up re-discovering stuff they took for granted when dinosaurs roamed the earth.
I appreciate all the comments because the bits and pieces can help.
I'm going rding today without the filters to see what happens....Keep in mind as it is,once the engine is warmed up, it has excellent performance for what it is from idle to wide open..The engine responds instantly and linear to changes in the the throttle position..I just find the over rich situation in the higer gear puzzling..And I'm sure it does reduce power some when it occurs...
could it be a foaming issue in the float bowls? i know you're fussy about float level but i keep remembering tom and his turbo BSA that went odd at a certain rpm, and when he put a sight tube alongside the float bowl he discovered that the mixture change occurred right when the fuel in the sight tube turned to foam.
i didn't mean to blow it up
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Re: For you with bike carb knowledge

Post by Truckedup »

Dorset,Anything is possible..Well, I fixed it until I broke it...I hand profiled the needles using a drill and tiny file...Ohh,a bit better..so I filed it more until it too rich....I ordered several more needles and main jets..Be a few days ,so I can mees up something else..
Dorset also knows some of the few guy I know who should have a mind full of facts about these old turds..I soon found out out they will tell you only one more fact than they think you know...Screw em, without their help we both built bikes faster than theirs.... :D
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