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Intake reversion question. #5429696 05/15/20 08:29 AM
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IMSA_Racing_Fan Offline OP
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Notice: This is about old motorcycle tech, i.e. carburetors. Newer bikes with EFI ECM improved heads etc are more efficient and powerful so that is now already addressed 🙂

I was watching this last night and it got the noggin jogging.

First of all his solution of restricting the airbox and results make sense as reversion makes a mess of the vacuum signal, but two things:
The cost of two pieces of foam (2 MM Jet Kit) is $80??? Comments are disabled.
Choking air as a fix bothers me.
https://sites.google.com/site/shoodabenengineering/products-and-pricing
In the past on air cooled KZs/CBs/FJs I've had good tuning results with the foam pod filters for cruisers that have a chrome plate at the end, even better than the pricier K&Ns that are tapered with smaller end plates. Could the plates have an unintended effect on reversion by acting as a air reflector against the air impulses coming back through the carbs while free flowing under full vacuum as intended?
https://ibb.co/nDRzk7b


I'm drinking a beer in the morning so forgive me if the question is stupid or worded incorrectly.

Last edited by IMSA_Racing_Fan; 05/15/20 08:40 AM.
Re: Intake reversion question. [Re: IMSA_Racing_Fan] #5429734 05/15/20 09:17 AM
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Cujet Offline
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Tuning "old school" carbs was always full of compromises. Stand off from reversion could be managed a few ways. As air went back out through the carb, it drew some fuel with it, often enrichening the mixture to the point of massive power loss. And leaving a "cloud" of fuel standing just outside the carb. On single cylinder engines, it was often best to accept the power loss of more mild cams to achieve acceptable mid range torque and ease of tuning.

All sorts of tricks were used to make things work on multi cylinder engines. A common one used by Kawasaki was to tie float bowl vents together with large tubing and hook to the airbox. Yes, the carbs have a float bowl vent built in, but those were subject to ram air issues. Relieving that ram pressure led to far easier tuning.

But the idea of restricting the inlet to a high performance engine's airbox with foam, is in my opinion, a crutch for improper tuning. Yes, we can make carburetor equipped engines run wonderfully. But in the end, fuel injection is the right answer.


People who count on their fingers should maintain a discreet silence.
Re: Intake reversion question. [Re: IMSA_Racing_Fan] #5429885 05/15/20 12:57 PM
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Bonz Offline
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I have a lot more ideas on this but the reality is if drivability for 95% of what you are doing is improved then it's questionable to me if you search for that extra 5% up top.

The video would not load, it just kept spiraling like it was trying.

Re: Intake reversion question. [Re: Cujet] #5430216 05/15/20 08:23 PM
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Silk Offline
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Originally Posted by Cujet
Stand off from reversion could be managed a few ways. As air went back out through the carb, it drew some fuel with it, often enrichening the mixture to the point of massive power loss. And leaving a "cloud" of fuel standing just outside the carb. On single cylinder engines, it was often best to accept the power loss of more mild cams to achieve acceptable mid range torque and ease of tuning.


When I were a lad, I spent a lot of time playing around with my BSA singles...not a lot of knowledge, but a lot of tinkering and learning. I used to use Gold Star cams, some of the most radical timing you'd see - 65 85 80 55 - that's 120 degrees of overlap. So yeah, lots of fog riding below the cam...my right leg would smell of petrol. I couldn't use a long ram tube, it would run rich at lower revs with all that fuel accumulating in the tube. Get intake and exhaust right and it would get some good top end.

The BMW Airhead is a good case of intake tuning - they are being cut up at an increasing rate these days for the cafe craze, and the fist thing to go is the airbox, and pod filters put on. I doubt if these things are ridden much, they are all for the look.... you can tune for some good top end, but rideability goes out the window. A look inside the uncool airbox will show the tuned length intake runners.


1987 BMW R65 - Penrite VTwin 20-50
2005 Nissan Expert - 5W-30 Castrol Edge
1996 Volvo T5 - Penrite HPR15 - 15W-60. Ryco syntec filter.
Re: Intake reversion question. [Re: IMSA_Racing_Fan] #5430218 05/15/20 08:24 PM
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ZeeOSix Offline
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At least the mod is quickly reversible. You can find foam easy for way less than "$80".

Are these bikes too lean in stock form? There shouldn't be any reason to try and "jet by restricting airflow" with a stock air box unless a full exhaust is causing it to run lean. Of course the right way is to re-jet the carbs (ie, new slow jet, move slide needle and main jet). Sometimes you can get buy on idle and low throttle by adjusting the idle air screw if it has one, then move the slide needle up a notch and go slightly bigger on the main jet.

Re: Intake reversion question. [Re: ZeeOSix] #5430345 05/16/20 12:46 AM
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Bonz Offline
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Modern CV carburetor on a 4-cycle road bike won't have an idle air screw, it's a pilot screw that controls fuel. Air screws are on the back side of the carb by the air intake, fuel screws are on the front side of the carbs.

Most carbureted bikes from the 80s and newer were beginning to be jetted lean from the factory, especially in the RPM range emission tests were done back in the day.

Dang video still won't load for me.

Re: Intake reversion question. [Re: IMSA_Racing_Fan] #5435379 05/21/20 07:47 PM
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CCI Offline
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Sometimes reversion is particularly bad because the choice of air cleaner, carb, cam, and exhaust just don't work together. If you have your heart set on that cam, a different air cleaner can do a lot.

Another way to address reversion and still optimize the intake is by paying attention to exhaust length and using cones at the exhaust port. Both work great, especially on Harleys.

When you get into the serious cams this is can be a noticeable problem but it is easily fixed. A slight and short restriction in the exhaust port (shaper cone) catches a lot of the reversion.

https://cv-performance.com/exhaust-torque-cones

One of the other mistakes is using exhaust pipe that is too large in diameter or too short on the mistaken idea that the larger pipe will flow better, it won't. We're looking for a balance here, velocity is essential. If we're running 1-3/4" drag pipes or a collector we can bolt up a shortened set, slide a piece of 2" over that, tach the bike up to wherever the reversion is giving us the most trouble, and slide the 2" back and forth like a trombone. You will hear it when the reversion is cancelled, there's no mistaking it. Measure that, then unbolt the temporary short pipes, cut the new ones to size, done.

Re: Intake reversion question. [Re: CCI] #5435434 05/21/20 08:51 PM
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Bonz Offline
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Years ago as I was modifying my ZRX 1200 a little bit at a time, it was a strong freaking bike with a stock engine after I adapted the CVK40mm carbs from a ZX12/ZZR1200 on to it. I was running a Holeshot full system.

Added ZZR 1200 cams and had a flat spot the size of Texas from 2750 to 3200 RPM. It couldn't pull its self out of the hole by its bootstraps for anything in 3rd, 4th or 5th gear. Talked to Dale Walker at Holeshot Performance and he said his header was set up and designed with stock cams and carbs. He thought the cams in unison with the bigger carbs were probably the culprit, as I had no issues with the carbs before the cams. I tuned those carbs and played with them for almost a year and it just wouldn't go away.

Had a buddy with a spare Muzzy exhaust system and I gave that a try. It is a 4-2-1design vs the 4-1 Holeshot system. The Muzzy system eliminated the flat spot and the bike was back to being the beast it always had been. I have since added high-compression pistons and it is as strong or stronger than a ZZR 1200 in a bike that weighs about a hundred pounds less than said ZZR with the few weight saving mods thrown in for good measure on top of the lighter Muzzy exhaust system.

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