Carb balance

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Brickboy
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Joined: 04 Jan 2020 16:15
Location: Bath UK

Carb balance

Post by Brickboy »

Had a go at mine yesterday.

I have a popping misfire on the front cylinder at tickover and a definite misfire when opening the throttle sharply at 4000rpm. If I gently accelerate I can get through it but it feels like I'm only on one cyl at those revs.

Initially the front cyl draws more vacuum at tickover. Turning the throttle stop out increases vacuum. On the rear cyl that action decreases vacuum! Hmm. Sundry fiddling with mixture screws and a quick carb strip and clean/air gun blast (no obvious blockages and all clean inside) has resulted in an even tickover at about the right speed and no misfire at tickover.

I'm looking forward to the test ride in the hope that I'm making progress!!

Simon
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72degrees
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Re: Carb balance

Post by 72degrees »

How are you doing the balancing - vacuum gauges or a u-tube manometer?
A Morgan Carbtune is a boon for this job. I have an ancient mercury filled one. Get the idle right and the columns align perfectly. Then just very gently open the throttle. If the columns dip in unison leave it alone - if not work on the cable adjusters of one or both carbs until they do. Get it spot on and you will also hear the difference. Naturally, jetting and carb condition need to be OK in the first place, valve clearances correct, good sparks, no air leaks. I do mine occasionally and definitely after any fiddling that might disturb the cables.
George 350
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Re: Carb balance

Post by George 350 »

In addition to balance, many people (me included) have found the need to reduce the pilot jets down from a '50' to a '42' or '43'. If you don't, then the flat spot at 4000-4500rpm can be really bad.
George
350 sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019
Brickboy
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Location: Bath UK

Re: Carb balance

Post by Brickboy »

Thanks all, that's very helpful. I'd like to try smaller jets - is there a recommendation for a supplier? I've not had to work on Dellorto carbs before, so I'm not sue where to source them.

Cheers Simon
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72degrees
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Re: Carb balance

Post by 72degrees »

Brickboy wrote:Thanks all, that's very helpful. I'd like to try smaller jets - is there a recommendation for a supplier? I've not had to work on Dellorto carbs before, so I'm not sue where to source them.

Cheers Simon
https://www.dellorto.co.uk/product-cate ... hbt-parts/

You may find them cheaper elsewhere.
Habilis
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Joined: 17 Mar 2013 15:16
Location: Cumbria
Location: North Cumbria

Re: Carb balance

Post by Habilis »

https://www.kmtproducts.co.uk/

I have used Kmt jets for several bikes
Quick post and sizing seems consistent
Price means a range of sizes can be bought
Brickboy
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Location: Bath UK

Re: Carb balance

Post by Brickboy »

Cheers guys, ordered and looking forward to losing the flat spot
Sicxtyone
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Location: Worthing
Location: Worthing West Sussex

Re: Carb balance

Post by Sicxtyone »

Carburettor tinkering! I've just reduced the pilot jets to 45's, from the original 50's. What I have found is a much cleaner running engine, but I have gained what I would only describe as a slight hesitation at you guessed it 4K rpm. Certainly not a flat spot and when accelerating isn't an issue.
Graham.
simonnorthroad
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Re: Carb balance

Post by simonnorthroad »

And my 507 engine with 28mm carbs spits back at 4000rpm on a trailing throttle which is a sign of weakness yet was way rich at this point when stuck on a Dyno! I've dropped the needles but still not cured it...
simonnorthroad
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Re: Carb balance

Post by simonnorthroad »

And my 507 engine with 28mm carbs spits back at 4000rpm on a trailing throttle which is a sign of weakness yet was way rich at this point when stuck on a Dyno! I've dropped the needles but still not cured it...
George 350
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Location: Northampton

Re: Carb balance

Post by George 350 »

Hi Simon,
Its a shame that Dell Orto don't offer more than the one slide. The symptoms suggest that the slide cutaway is too small. The Dell Orto way seems to be to adjust part throttle mixture with the needle jets (also called emulsion tubes). How this goes regarding the various numbers and letters I don't know, but I'm sure someone on here will be able to advise you which way to go, but basically you need less fuel going in.
If it were mine, and I had tried everything else I'd probably end up spending loads on a pair of slides and machining say 0.75mm from the cutaways and see how it goes, but that is really because I've only had to first principle set up Amal and Mikunis on my bikes in the past and both those carbs are/were available with different slides.
Regards, George.
George
350 sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019
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72degrees
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Re: Carb balance

Post by 72degrees »

George 350 wrote:Hi Simon,
Its a shame that Dell Orto don't offer more than the one slide. The symptoms suggest that the slide cutaway is too small. The Dell Orto way seems to be to adjust part throttle mixture with the needle jets (also called emulsion tubes). How this goes regarding the various numbers and letters I don't know, but I'm sure someone on here will be able to advise you which way to go, but basically you need less fuel going in.
If it were mine, and I had tried everything else I'd probably end up spending loads on a pair of slides and machining say 0.75mm from the cutaways and see how it goes, but that is really because I've only had to first principle set up Amal and Mikunis on my bikes in the past and both those carbs are/were available with different slides.
Regards, George.
I experimented with different slides on the PHBH28s on my 375 some years ago. Unfortunately, at the price they are 'bracketing' the problem wasn't an option and going 'leaner' didn't seem to help in my experience. The thing that made the most difference was needle height, but I've never got them perfectly set up. There may be a sweet spot needle/atomiser combination but I suspect it would need dyno time to nail it. I was going to take mine to a place not far away that has a dyno and a chap who apparently creates his own needle profiles - but this is for scooters. The principles are the same I suppose. Perhaps I will get round to it eventually, but the current setup on the road legal 2C/350 is good enough not to warrant expensive dyno time. One observation is that air filter arrangements can have an enormous effect, and to some degree exhaust system. The other main difference between my two otherwise very similar machines is cam - then that manifests itself more in main jet territory.
EVguru
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Re: Carb balance

Post by EVguru »

Dellorto do offer different slide cutaways, it's just that the availability of sizes for VHB and VHBZ carbs is now quite limited. I had to change the 40 slide on my 125H to a 50 to get rid of the off-idle stumble, changes to the idle jet just wouldn't sort it like they would have on a 350. The slide number is the height of the cutaway in 10ths of a mm.

Details of the emulsion tube hole patterns can be found online, but other than their basic size (260, 264, etc.), which sets home much fuel can bypass the needle, they're a transient device. Holes towards the top let in air, whilst holes towards the bottom let in fuel. The chamber in which they sit will supply additional fuel for a limited time. On 2 stroke applications they often have no holes at all.

You may need a different needle profile. Some needles have more than one taper along their length.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
norbert
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Re: Carb balance

Post by norbert »

There are diferent slides for the 28 VHBH. The ones mounted in the 501 Morinis have 40-slides. I´m using 50.

For many years I was very pleased with the following configuration in Rocinante (K-model with orignal 501 R from a Camel, original 350 airfilterbox with or without filterelemets, Lafranconi doblefoldsilencers for a 350 opened a bit to fit the 500 tubes): slide 50 instead of 40, needle X18 4th notch, atomizer 268T instead of 264BC, idle jet 50, choke 50.
The only thing it did not like was going contstantly at 5000 revs, all the rest was very fine.

When I finished Dulcinea in 2017 I tried the same allthough there are the following diffences: pot filters (first K+N replicas that I had to protect from rain and now Polinis), a selfmade 2-2 escape with a bit wider inner diameter tubes, the motor is a bit tuned (lighter pistons, lighter connecting rods, lighter valve train, lighter clutch, lighter primary), Probably someone sold me a L5 cam that was suposed to be a L5-2 :cry: . The carb configuration like in Rocinante did not work! I´m stll looking for the correct one, although I´ve made 19000km since may 2017 including trips to spain and Italy.

Last autumn I swapped the complete carbs. When I configured the carbs from Dulcinea with exact the same parts that have been in the ones of Rocinante and put them in Roci the bike was far from the performance that it had before. So the result ist that I now have two bikes that don´t work as they should. Roci now has real problems from 5000 to 6000 revs that have not been there before.

Lat week I changed the position of the needle in Dulcinea from 4th to second notch and noticed that I did not change the idle jet in Rocinante back to the original 50 (in Dulcinea they had 45). Waiting for better wether to try if anything has changed :roll:
Brickboy
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Joined: 04 Jan 2020 16:15
Location: Bath UK

Part solution...

Post by Brickboy »

Thanks guys - all good info.

During a frustrating session this week, I spent time pondering those weird vacuum gauge readings- front pot increases vacuum as throttle stop opened, rear pot decreases. Bike runs very badly, feels over-rich.

So, I swapped the gauge tubes over, and the problem moved. My gauges are knackered!

Retuned from static by ear and things are much better. A set of carbtune gauges are on their way.

Next up is fork oil (if I can find out volume and grade) and more investigation of the wildly oscillating speedo needle.

It's all fun in the Brickboy garage, I can tell you!!
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