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Jubilee running badly

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Re: Jubilee running badly

Posted by Dan Field at September 25. 2017

A compression tester would be the easiest method for sure, sadly I've lost the adaptors for my gauge!

The reason I think the gasket (s) is blowing is that I can't feel the spitting I can hear behind the carb, but I think I can feel it inbetween the pipes, at the front, I cant remember what checking I did when I put it together, my intention was to do as little as possible until I could establish exactly what needed doing. I prob should have faced them on a glass plate. I have a spare set of heads and will take the opportunity of regrinding the valves, fitting new springs and Uli's new tappet adjuster lock bolts!

Taking the heads off is a pretty quick job (hopefully!)

Re: Jubilee running badly

Posted by Ulrich Hoffmann at September 25. 2017

Hello Dan!

Shold be quicker than removing a Commando head with those pesky bolts from underneath...

Spray some brake cleaner around the head so you can localize the leak.

My tappet adjuster lock bolts will make a huge difference, haha!

Re: Jubilee running badly

Posted by Alan Osborn at October 04. 2017

Previously john_holmes wrote:

Is your bike 12V, if so is the boyer feeding two 6V coils in series or two 12V coils in parallel. The 6V in series is the best option, the 12V one is marginal.

You can't run 2X12V coils on a Boyer (or RITA) ignition in parallel unless you use two steering diodes! 2X6V is the best option. A twin output coil has one good output and one mediocre output. So again it is 2X6V and the original Wipac is often the best option.

Re: Jubilee running badly

Posted by john_evans1 at October 04. 2017

Dan

not sure where I saw this info but I recall reading that mono block carbs run very different settings to later concentric??

might be worth enquiringly with your supplier.

Re: Jubilee running badly

Posted by Dan Field at October 04. 2017

Thanks Al, I've got one 12v coil with twin output. I have reground the valves and replaced the head gasket, it no longer splits and starts with the gentlest of kicks, and ticks over beautifully however although it's running much better it still won't pick up from quick throttle openings unless I choke the carb. So having checked the timing, replaced the 12v coil, and reground the valves. I'm coming back to thinking it's the carb.  It's a new concentric and I think I may have a duff one. I think I have enough bits to build up a monoblock to try that. I might also have a pair of 6v coils somewhere, just have to find them!

Re: Jubilee running badly

Posted by graham_squires at October 05. 2017

Hi Dan Any chance it could be contamination in the fuel tank?. My fuel tap filter was clogged ,the bike ticked over ok but the partially blocked filter starved the carb cause the fuel couldnt keep up with demand when on a run. When I got round to sealing the tank all was revealed.  I know its a long shot,but  it`s a possibility.

GRAHAM

Re: Jubilee running badly

Posted by Dan Field at October 05. 2017

Sadly not the tank, I cleaned it out with phosphoric acid and filled a new tap, the flow is excellent.

Re: Jubilee running badly

Posted by Dan Field at October 08. 2017

Ok I think I have a result! Having replaced the coils, checked the timing, reground valves, replaced head gasket, rejetted the carb, an a whole host of other stuff I still have a bike that dies when you crack open the throttle although it starts and ticks over beautifully.

today I took the manifold off again, and looking carefully found it had been worked on

 

Anyway i fitted a spare ( the one on the right) but it made no difference, so then I fitted a monoblock, built from a box of bits but with the right jets. It leaked like a sieve but the bike ran well and accelerated properly. So although my tinkering has improved it, it seems it was the sodding new concentric that was causing the problem, but I don't know why, the pilot circuit was clear, and the size of the main jet or needle position  made no difference.

So my next decision is whether to buy a new monoblock, or if they are as badly manufactured?

Re: Jubilee running badly

Posted by michael_sullivan at October 09. 2017

Dan:

Have you checked the jet needle for the proper taper?

It sounds to me that the mixture is still going lean when the throttle is opened.

Mike

Re: Jubilee running badly

Posted by patrick_mullen at October 09. 2017

Hi Dan  The manifold on the left (your original) looks like a Navigator one, the one on the right looks standard Jubilee.  The difference is pretty small though-the Jubilee carb is officially 25/32" and the Navigator is 7/8th. That is a difference of only 3/32" or a couple of mm. I was told many times that the traditional way to 'tune' the Jubilee was to use the Navigator manifold and 7/8 carb.  This should improve the top end at the expense of idling - the opposite of what you were getting.

I think you should clean up all the joints on the old monoblock - particularly the mounting flange-, get it to stop leaking, then do a proper trial run just to confirm that it is now noticeably improved. I have seen quite a few reports from people saying that the new carbs often contain manufacturing swarf  so it doesn't say much for their standard of quality control. Maybe just a rebuild on the old monoblock will do the job.

Patrick

Re: Jubilee running badly

Posted by patrick_mullen at October 09. 2017

A friend of mine a few years ago bought a pair of brand new standard HS4 SUs for his MGB.  (SU is the same company as AMAL I believe).  These idled perfectly -better then the old carbs- but were no improvement at all in ordinary running despite all our attention.  So I am a bit dubious as to the benefits of buying a new carb. It seemed a lot of money for little purpose (unless your engine idled most of the time).

 

Re: Jubilee running badly

Posted by Dan Field at October 09. 2017

Hi Patrick

The manifold is definately a jubilee one, the casting numbers are the same as my spare and the Navigator one I have is a different number. Interestingly the monoblock I have is in fact a 375/34, from a C15 which is 7/8th and might explain the widened manifold! However it's not in good shape and I had to rob bits from my Navigator engine to make it work! The leaks were petrol from just about everywhere on the feed side so I just filled it and switched the tap off! I was just looking at a rebuild kit, and might do that just so I can ride it before the weather sets in. Looks like about £40 so may do it providing the screw cap at the top holds!

The slide is worn but doesnt look terminal.

Re: Jubilee running badly

Posted by Dan Field at October 10. 2017

I canabalised my navi monoblock and built up a better one for the jubilee ... no more petrol leaks, however the threads for the cap are toast and I couldn't get a good seal at the top, plus the slide is knackered, although it picked up when the throttle was opened wide tick over was lumpy and the engine never was generally running way to rich, it smoked and you could smell the poorly burnt fuel. Interestingly tick over improved when I leant the bike over!

Conclusion? ....  The monoblock is knackered!  The concentric has something wrong with it (Amal's quality control issues?).

Solution?  A new monoblock for the best part of £200 or a mikuni at nearly half that. No one has one listed for a Jubilee but They do a VM 22 for a Triumph cub, which has similar jetting to the jubilee. That would be equivalent to a 7/8th. There is also a VM 20 but haven't found any comparable yet .... still looking... update, Allens do a 20mm and a 22mm for the cub, to replace the 25/32ths  375 carb, main jet will need increasing other than that it's a good match. Decisions decisions!

Re: Jubilee running badly

Posted by Terry Grimwood at October 10. 2017

Don't know if this will help but when I rebuilt my 375 Monobloc it peed petrol everywhere, until I discovered that the seating washer  under the banjo that is shown in all the drawings should be left out as it prevents the needle from closing off the fuel supply.

Terry

Re: Jubilee running badly

Posted by Terry Grimwood at October 10. 2017

Previously Terry Grimwood wrote:

Don't know if this will help but when I rebuilt my 375 Monobloc it peed petrol everywhere, until I discovered that the seating washer  under the banjo that is shown in all the drawings should be left out as it prevents the needle from closing off the fuel supply.

Terry

The overhaul kit from Burlen includes the washer and shows it in the drawing, which is what caught me out - naturally I fitted it. It was only when I looked at photos of my carb taken prior to dismantling that I realised there hadn't been one in the first place.

Of course this might not be the problem with your Monobloc but if it is it could save you discarding a perfectly satisfactory carb.

Terry

Re: Jubilee running badly

Posted by Dan Field at October 10. 2017

Thanks Terry, I think the petrol level needed some attention but the main issue is the thread on the top of the mixing chamber (or lack of it! ) and the wear of the tube and slide.......I think the bike had some farm use before I rescued it!  It even had a wooden primary chain tensioner -nicely made mind you!

Re: Jubilee running badly

Posted by patrick_mullen at October 10. 2017

Dan,

Those slide fixing rings seem to vary in size by design so if you have any choice in the matter try swopping over for another fixing ring.  I was told by an 'expert' at an autojumble that Amal actually made the fixing rings in undersizes to compensate for wear in the thread of the carb body.

I can vouch that when I tried different fixings from autojumbles there were noticeable differences i.e some were very tight on the thread, others could be so loose as to be useless.  Maybe easier said than done unless you have a box full of assorted carb bits. You used to get these 375 bits at the autojumbles for a few pence

Patrick

Re: Jubilee running badly

Posted by Ian Easton at October 11. 2017

Getting back to your plugs, I believe the insulator colour will indicate the heat range and the outer ring colour the mixture. It you have a porcelain white insulator and a black outer ring then the plug may be too hot.

Re: Jubilee running badly

Posted by Dan Field at October 11. 2017

Ian I think you are right, but it's not the plugs causing the problem. I'll sort them out when I've got it running better.

Re: Jubilee running badly

Posted by david_evans at October 11. 2017

While it won't help your situation Dan, I have had two branch members replace Commando carbs with new ones (not premiers) and never get them running right to the extent they gave up and went down the mikuni line. The source of the carbs was probably the issue. ie not the genuine article. They looked ok though and I couldn't see anything wrong with them.

Re: Jubilee running badly

Posted by Dan Field at October 11. 2017

I've just looked at everything again and am kicking myself. My concentric came from Surrey cycles so they should be genuine and they certainly gave me the impression that they knew what carb I needed, but now I've spotted the oversize manifold I can see that the carb is way to big, it is in fact 600/301 24mm. Standard the jubilee is 25/32ths or 20mm.  So now I'd don't know if it's a duff carb or a carb that too big. I suspect the latter. I'll speak to them tomorrow the trouble is I bought it well over a year ago.

Re: Jubilee running badly

Posted by Dan Field at October 13. 2017

Well I've bitten the bullet and bought a 25/32nds 375 monoblock. It Should arrive next week, they are a bit like rocking horse pooh at the moment.   I'll keep the concentric and see if it works on my M50 box of bits project! I think it's too big even for the navi.

Fingers crossed

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