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Hi, We have just put 600 miles on my friends rebuild DBD34 clubmans road bike, however we running 2 x 2446's cams as per scrambler set-up, but with the standard head and GP carp.

This set-up (with STD box) apparently is popular in the North of England where there are plenty of hills and bends and top end is not the objective. I have failed to find definite information for the best carb set-up, even if not using a GP?

Any thoughts?

We have set the timing accurately to 39 deg BTDC FA, on the basis we can pull it back a bit on the lever if required for better performance. We are going to try and find the best ignition timing and then fit a BTH mag with a fixed timing setting.


Cheers

Mike.



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I am running a BTH set at 36 degree on my CB34 Scrambler, I am running inlet 65-2454 and exaust 65-2446 and 9/1 compression and a 34 mm Dellorto carb. The head is a DBD type with 36mm inlet. This setup works very well for me.


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Is the current setup, with the GP, producing any odd results - flat spots, spitting, hot, hard starting?

The spec you describe is not one that there would have been 'factory' definitive settings for, as the GP wasn't standard on a scrambler, and neither was the big port head. That's not to say the combination won't work well, just that you won't be able to read the numbers out of a book.

The scrambles cams are still pretty wild, so I'd have thought that starting with what would be stock for the GP (assuming it's a routine GS size) would be a good place to start, and probably need little to get it right from there.

Mind you, if it was me, I'd put a Mikuni on it and be done with it.

If it ain't broke, don't fix it!

Last edited by Kerry W; 09/06/14 4:05 am.

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"and then fit a BTH mag with a fixed timing setting" ........ why , I run a BB34 with 2446 cams on the road and it goes very well , im running concertric carb , belt primary , no problems starting and it ticks over quite well , junk the GP they are a pain

Last edited by craig; 09/06/14 4:19 am.
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Originally Posted by 68triton


fixed timing setting.




How will you start it?


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Old or new (electronic) BT-H?

The old BT-H mags were reckoned to be a leap ahead of Lucas for quality.

The new electronic BT-H's are very good - I have 2 on different bikes. They are available with fixed timing to special order, though the standard modern BT-H unit has a built-in automatic retard function for starting. I'd go this way (well, actually, I have, twice) - I'd assume the fixed timing option would be for use with an old-style automatic timing device.


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I don't totally understand why none of us had problems kick starting the GS racers for years with the mags at fixed max advance and with a 1 1/2" GP carb. A friend advised he thought the weaker spark from even a comp mag at kick over vs the modern electronic sparklers was the reason. hmmm

BTW, I sure don't understand any of you are fitting 46/46 cams on a road Goldie. The 42/46 set will give you that upper end Gold Star Power with no problems going slow. hmmm Perhaps when I'm in my '80s I'll change my mind. heh


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Around here there was a batch of DBD scramblers that came fitted with a GP carb. Unfortunately I do not know how they were set up. The strange thing was the lack of an air filter of any kind. It was best to stay out in front of the pack.

This photo shows one of those machines as it came from the factory.

Gordo

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Originally Posted by Kerry W
The old BT-H mags were reckoned to be a leap ahead of Lucas for quality.
I know that's the folklore, but I just don't see it. The internal differences are pretty minimal.
Originally Posted by dave - NV
I don't totally understand why none of us had problems kick starting the GS racers for years with the mags at fixed max advance and with a 1 1/2" GP carb.
Could it be because an 18-year old was kick starting it back then while someone 50-60 years older than that is kick starting it now?

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Hi All,
Quote
The old BT-H mags were reckoned to be a leap ahead of Lucas for quality.
I know that's the folklore, but I just don't see it. The internal differences are pretty minimal.


MM will know that I'm a fan of the "real" BTH magneto's
I cannot remember if I asked him this question before,
Why are BTH mags fitted to all the aero engines that are on display in Duxford Museum ?
Not a Lucas to be seen !!!!

The modernBT-H things need a higher kickover speed to fire the engine over an original mag
This causes problems with some (older or weaker) owners

John

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I have had no problems with Lucas . BTH , etc mags once rebuilt (must be rewound) did Lucas never fit a mag (two) to a areo engine ? maybe they where at the cheaper end of the market ie bikes

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Originally Posted by chaterlea25
Why are BTH mags fitted to all the aero engines that are on display in Duxford Museum ?
Not a Lucas to be seen !!!!
John,

There are quite a few plausible, non-technical reasons one company's product might have such a monopoly. Once a company gets a military contract it often guarantees them repeat contracts because of the military's strong push for standardization. If the pickup for a BTH magneto breaks it helps a lot for the supply sergeant not to have to sort through pickups for Lucas, Sims, CAV, etc. to find the right ones to send out to the field to get a Spitfire back in the air without delay. To only a slightly lesser extent the same was/is true for commercial airlines. BOAC, TWA, PanAm, etc. also would have standardized to the extent possible. Because of this, bribes were not unheard of in the aerospace industry.

Even absent bribes to get their foot in the door, sales reps were/are successful to the extent they establish personal relationships with the buyers and the people who write the technical requirements. It was/is quite common to find a technically irrelevant requirement built into requests for quotes, whose sole purpose was/is to eliminate all but the favored vendor from being able to compete. If I was friendly with the BTH sales rep (bribed by him with cash or with extravagant gifts for my wife and me; went to public school with his brother; wanted him to get my lazy brother-in-law a job at his company; etc.) and wanted to be sure BTH got my company's magneto business I might specify a magneto with direct access to the magnet via a removable cover. While that sounds like a technical specification that might be important, all it actually does is eliminate Lucas from competition. As I wrote in the first paragraph, once that first military contract is let the odds of a continuing exclusive relationship existing are very high.

Anyway, because of such factors the exclusive use of BTH magnetos on military aircraft can't be taken as evidence of any actual technical superiority of BTH for even that specific application let alone for their rotating armature magnetos on motorcycles.

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BAE Systems do this all the time, most of there equipment is rubish and very expensive but the military here keep on useing them at a massive cost to the UK tax payer

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sounds about right , the military where still buying triumph TRWs into the 60s , I think they must be very long term contracts , so if you tool up they will buy X amount for the next 40 years , no one wastes money like the military

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Originally Posted by chaterlea25


MM will know that I'm a fan of the "real" BTH magneto's
I cannot remember if I asked him this question before,
Why are BTH mags fitted to all the aero engines that are on display in Duxford Museum ?
Not a Lucas to be seen !!!!


I hope you don't believe they put any motorcycle magneto on aeroplanes!


Quote
The modernBT-H things need a higher kickover speed to fire the engine over an original mag

John


Not good news for customers.


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Hi Kerry,

The standard set-up seems to be running quite well, but i do not have any experience to know better or make improvements.

On occasions with the engine hot the throttle response can be a bit erratic around the 2-3k mark up pick up, but then clears. This seems to be less with 99 grade fuel over 97.

For example i done a 70 mile non stop 1 way run yesterday with no issues, but after fuelling up and my return journey i had the erratic itrottle response.

I am running N4C plug and the colour is spot on, i am suspecting some whiskering / faltering of the of the plug.

Tappet clearance checked.

Thanks for everybody's replies.

Regards

mIke.





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Hi TT,

The new mag will be an electronic BTH, with auto advance / retard, however once you set the advanced timing point with the pinion it is set, unlike a tradition lever and mag where you can play with the best advance position.

Mike.


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Do the ignition change first - the results of that can affect any possible carb changes.

I see what you mean about the auto advance on the new BT-H; in other words, you can't set it with a little too much advance and pull it back to see where the motor actually likes the ignition. A purist (MM jump in here) would tell you that doing that was a bit of a fallacy with a conventional mag anyway.

When looking for indications of carb setup and whether you might be able to make improvements, you really need to note throttle position, rather than RPM. I put a bit of tape with marks (1/4, 1/2, 3/4, full) on the throttle housing and a white (typing correction fluid) mark on the twist grip so that if there are symptoms to note I check to see what opening amount I have - that will then give me an idea which carb circuits I need to consider jetting changes in.

Plug colour, for really good jetting isn't what is shown in your average Haynes Manual. Sooty black or spotty white indicate problems, but a chocolate brown is very safe. Reading plugs is a bit of an art, as it can be seen whether the ignition timing is OK, whether the plug heat range is correct, amongst other things. For main jetting, unless a proper plug chop is done after a long run at full throttle and high load, a proper reading cannot be done.

Sorts of symptoms you might look for, suggesting that all is not what it might be in the jetting department, can include erratic throttle response, surging at a fixed (partial) throttle opening or lots of rpm but a feeling of 'strain' (no real 'go'). Perhaps spitting as you open the throttle or sooty black smoke from the exhaust as the throttle is opened.


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Incidentally, neither of my BT-H electronic ignitions need any more kicking effort to generate a higher rotational speed to get a spark than the best magnetos I have used (usually an SR1 Lucas rotating magnet version).

My GS with the BT-H could not be an easier starter, regardless of what ignition was fitted. With the BT-H as you kick you can feel the weight come of the kickstart lever as the motor fires before the full stroke of the kick..

Last edited by Kerry W; 09/07/14 9:10 am.

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Hi Kerry,

I am using a Thor spark system for the ignition, so once i have determined the best ignition timing i can lock it in place. Using a timing disc and the dead stop method will measure the degrees required and apply this to the new B-TH electronic mag. I have cheap access to a dyno owned by a company who specialise in race bikes for the vintage racing etc.

The throttle position is more important than the RPM, so my problem is mostly in the 1/4 to 1/2 range when trying to maintain a lower speed.

I have good understanding of reading plugs and normally confirmed by some colleagues to get a consensus.

No smoke of any kind, no spitting etc.

Another run today:

Todays run of 100 miles went ok in the main, but i did have some erratic behaviour at the lower speed and throttle opening (after fuelling the tank to the brim), this then cleared as i opened the throttle up to get home, Goldie went really well!
When i came to a rest i noticed that the remote chamber was flooding slightly. I THINK I NEED TO INVESTIGATE THIS NEXT.

I spoke to an old fella today at a meet, he was telling me that he experienced the same effects when using a GP with a Velo Thruxton, the problem being a worn needle jet, this being quite common as it will wear very much quicker than the needle with modern fuels (no lead).

Regards

mike.



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Hi Mike,

Ok - my (incorrect assumption) was that you had an ignition that was considered less effective than the planned BT-H, and so I concluded that changing the ignition first would be a better plan, However, if the current ignition is doing the job, none of that should be a factor.

When you wrote that the bike ran erratically after filling the tank (without reading further at that point), my suspicions increased that the float chamber was not sitting off fuel supply from the tank when the float chamber was full -effectively 'tickling' the motor all the time. The extra weight of the fuel with a full tank wouldn't help here. Is it a remote float chamber?

Without doubt, resolving whether the float chamber is flooding the engine is the next step. Checking the flood height is correct would be a good idea while you have eyes in that area.

The wear on the needle jet is an issue. Replacement needle jets are cheap and easy to fit and I have seen bikes transformed in the past where a new jet has been fitted. Maybe change the needle at the same time.

Standing by.

Kerry


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Hi Kerry, The float bowl is remote and it was the double fuel banjo nut which had come a little loose and hence there was a fuel leak on the entry to the carb.

Tightened big nut and checked the bowl height ok.

Taking my friends bike another ride yesterday revealed that the erratic running was still there on occasions and this time the float bowl was flooding. Tapping on the float bowl cured this.

Set off and a similar problem still existed, but no leaking fuel this time, float body dry. So changed the plug N4C to N3C (all i had), what a difference! N4c was duff, but there was marked difference using the N3c. With the Goldie now running sweet, pulling strong from under 2k (not recommending) to 6k in all gears etc.

Often on British bikes there is more than 1 problem occurring simultaneously, mixed in with some Red Herrings!

I have received today a new needle, needle jet and Neoprene tipped float needle with a Webber conversion.

I will see how it goes in the next few days.

Thank you and others for your help.

Regards

mike.







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When I finally get my Pearson crank I am going to put my bike back together with the scrambles cam also, along with the 1 1/2" GP carb. I didn't have any problems with the bike starting or running properly with the clubmans cams and the GP. The only problem I had was with poor gas mileage, which apparently is normal. I think the Scrambles cam set up may improve the mileage. It may make a better road bike with these cams. Also apparently the horsepower difference is only about 3 HP at 7000 RPM. But I don't often rev my engine to 7000 RPM anyway. I like to ride pretty fast, but the acceleration out of the corners is probably more important to me. I am changing my bike over also from an SCT2 gearbox to an STD one. The extra torque of the scrambles cams may help the bike to pull well after a shift, when the RPM of the engine is a bit lower because of the bigger difference between the ratios.

According to Phil Pearson the best cams to use with the STD gearbox are the Touring cams. So if I still don't like the bike with the Scrambler cams I will change to the Touring ones later. I think a smaller carburetor would help to, but I don't see the point in changing that unless the inlet tract was also smaller, so I will stick with the GP for now.

Please when you finally figure out the right carb settings, publish them here as that will be a great starting point for me.

Nigel

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Big vote for me for the touring cams - work well with the STD box and very good gas mileage. (on a DB 350 years ago, on a Monobloc carb, it used LESS THAN HALF the fuel my DBD does now, on the 1/5" GP with club mans cams...)


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Howdy Mike,
My float bowl needle has a pronounced ridge at the sealing point, and I was wondering if anyone makes a Viton tipped needle. Where did you get your parts?
I don't want to have the same trouble as you, but haven't found a web site that advertises float bowl parts.

Jerry Roy

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