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Series ‘A’ Twin Carburetion


greg brillus

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Just wondering if those with experience on the twins have had any issues with the front carby suffering from fuel vaporization after a run and engine up to temperature. Just fine tuning this new twin of Rodney's and it runs quite badly if you start it after the engine has been run and sat for say 15 Minuit's where it heat soaks the front carb quite a lot, and on re-start it struggles to run on the front cylinder until you ride it and the temp comes down again. I have the mixtures set quite rich and the float level is correct.......It is running new 289 carb's. Cheers.............Greg.
 

billirwinnz

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Hi Greg

I've not had the problem but I haven't ridden in hot weather. Perhaps just the warmer temperatures in Brisbane are the cause. I guess there isn't room for any sort of insulation or heat shield.

Cheers Bill
 

davidd

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Greg,

I know that Scott Dell, who bought an A-Twin from me, had very serious issues with overheating. While riding the bike in England, it had a mild melt down. The crank cases would eventually become heat soaked and take on the same temperature of the heads. He had to install an oil cooler to keep it working. I think he was eventually able to mount an RGM remote oil filter under the swing arm pivot and it would allow enough oil cooling to provide decent riding.

David
 

greg brillus

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Thanks for the replies ......... It's a tricky one, I'm thinking that perhaps it is rich causing the plugs (NGK B5 or 6 ES) that may be fouling and causing the rough running. I might experiment around some more. It is worth noting that the originals ran 276 carbs that have quite a different air flow versus the 289's which are much bigger, and also on the originals that the front carb ran a number 3 slide. That would indicate to me that they were trying to help get more fuel delivery, perhaps due to the horizontal carb body that relies more in drawing the fuel from the float versus the vertical type where the fuel sits more upward on the mixing chamber. Just for a bit of info .............. I gave the bike a run the other day of around 15 km's, came home and parked the bike up for 10 minutes, I then checked each carby for temperature with my digital temp gun and the rear float was about 40 degrees Celcius and the front one was about 75 to 80 degrees .......... so you can see the front suffers from heat soak big time versus the rear one.
 
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Robert Watson

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I run a pair of new 1 1/8ths on my A twin, and have had it in some pretty hot places. It does get hot but nothing like what you seem to be experiencing. If I have had to sit and idle for a bit in the heat it can get a bit of fluttering just when back in gear and rolling until some cool fuel gets to the carb. I think Dan has had some issues with his front carb as well but those are home made 1 1/4 type 289's. He has alloy heads and I have the old iron ones. I also have club post war 7.3:1 in there which probably run close to 8.5 or 9 in those original heads! I certainly can't recall having a problem with it not starting in the heat. It is on a Lucas mag with NGK 6's ...
 

greg brillus

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Thanks Robert, I'd be interested to know what number slides you are running, but don't disassemble them to find out. I made a few changes this afternoon and it seems to be better, including a slide with smaller cutaway closer to a number 3, the actual cutaway at just over 0.200 of an inch. And changed to Champion plugs, seem to look much cleaner after a run around the block.
 

roy the mechanic

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You may find a tufnel block between the head and carb adaptor will help. Also another plug ,change to bp6es, both of my twins run real good on them.
 

vin998

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Roy. Their discussing pre war twin - series A which has the stub which the carb mounts onto cast as a part of the cylinder head so no room for a tufnel block due to no separate adaptor. It would help on series B,C or D though.
 

Robert Watson

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Even if there was a flange to insert a tufnel block there is no room. I have to take of the little bell mouth just the get the carb mounted and then wiggle the bell mouth past the one rear intake valve spring and screw it back on!

The new carbs came with #4 cutaways which didn't work well on the front carb. Changed to a 3 as per Richardson. I was happy I could change the slide without removing the petrol/oil tank and all that goes with it. Nightmare indeed!
 

greg brillus

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Yes i think it needs the smaller cutaway on the front carb to help draw in fuel from the float bowl. I remember I did try a number 3 slide on this one but it seemed too rich at the time. The slide in there now is slightly larger than a 3 so I will run with that today and see how the bike performs. It starts up ok, just when it is hot it struggles to run on both until you get some airflow back through to cool it down. It might be possible to fit a simple sheet metal baffle to keep the heat getting to the bowl from the front cylinder muff/head but I'll try this first to see if it is better.
 

billirwinnz

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Yes i think it needs the smaller cutaway on the front carb to help draw in fuel from the float bowl. I remember I did try a number 3 slide on this one but it seemed too rich at the time. The slide in there now is slightly larger than a 3 so I will run with that today and see how the bike performs. It starts up ok, just when it is hot it struggles to run on both until you get some airflow back through to cool it down. It might be possible to fit a simple sheet metal baffle to keep the heat getting to the bowl from the front cylinder muff/head but I'll try this first to see if it is better.
Hi Greg

I’m enjoying my first ride on my new bike. I’m having exactly the same problem as Rodney after a brief stop. The weather has been cool and my bike has the insulated float bowl.

My thoughts are that most of the heat would be coming by conduction via the stub and the insulation could even hinder cooling. I believe that the fuel is vaporising and creating pressure that is holding the float valve closed. I’m going to try more breathing capacity in the cap. I’ll also try the tickler after a brief fuel stop to allow fresh fuel in.
The bike is running sweetly after a bit of carb fiddling although I still haven’t found the settings for first kick starting.

It’s a credit to Neal and Rodney’s thought and skills.

Cheers Bill
 

greg brillus

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Hi there Bill, I found on Rodney's bike that to start the engine when hot, and had been standing enough time to heat soak the carb's, that I simply closed the choke lever for the front carb only. This would create enough suction to help draw the fresh fuel in and run ok on both cylinders, just for a few seconds and then back the lever off once it happily ran on both. Neal's bike seems to be better in this regard but his has the original brass type float bowls which are quite hard to find. I don't believe the bowl would have a ventilation problem on account of the tickler would breathe enough. The float level on the front carb is quite strange, as it is correct when at the base of the main banjo where it bolts to the main carb body. This would to most folk appear to be too low, as this is not level with the main jet. However, if it "Is level" with the main jet the carb would continuously flood into the downward tract and into the front cylinder. This is why I feel the front carb uses near enough to a number "3" slide (which it is supposed to be on the original bikes using 276 carb's) this on any other post war Vincent engine would in fact be too rich. But it is the size of this "smallish" cutaway that assists in drawing fuel into the lower jet block area of the carby. Neal did tell me that it is occasionally spitting back on the front carb when cold, so you could use the choke a small amount until the engine warms a little, or richen the idle mixture by winding the thumbwheel clockwise a small amount and this should help. Glad you like the bike, they really are a work of art, and beautiful to ride.......Take care and cheers for now............ Greg.
 

billirwinnz

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Hi there Bill, I found on Rodney's bike that to start the engine when hot, and had been standing enough time to heat soak the carb's, that I simply closed the choke lever for the front carb only. This would create enough suction to help draw the fresh fuel in and run ok on both cylinders, just for a few seconds and then back the lever off once it happily ran on both. Neal's bike seems to be better in this regard but his has the original brass type float bowls which are quite hard to find. I don't believe the bowl would have a ventilation problem on account of the tickler would breathe enough. The float level on the front carb is quite strange, as it is correct when at the base of the main banjo where it bolts to the main carb body. This would to most folk appear to be too low, as this is not level with the main jet. However, if it "Is level" with the main jet the carb would continuously flood into the downward tract and into the front cylinder. This is why I feel the front carb uses near enough to a number "3" slide (which it is supposed to be on the original bikes using 276 carb's) this on any other post war Vincent engine would in fact be too rich. But it is the size of this "smallish" cutaway that assists in drawing fuel into the lower jet block area of the carby. Neal did tell me that it is occasionally spitting back on the front carb when cold, so you could use the choke a small amount until the engine warms a little, or richen the idle mixture by winding the thumbwheel clockwise a small amount and this should help. Glad you like the bike, they really are a work of art, and beautiful to ride.......Take care and cheers for now............ Greg.
Thanks Greg

I richened up the front air screw and re-synched the carbs and the bike is running sweetly. As well as any twin I’ve ridden.

I wonder what the difference is between the original float bowls and the new ones.

Cheers Bill
 

Robert Watson

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Not to stray from the subject but I always wonder about "so you could use the choke a small amount until the engine warms a little". It seems to me that if they tend to spit back a little it is a fairly small throttle openings, and say you gave it as much as 1/3 of the choke, the choke slide is not low enough to effect anything below about 1/2 throttle. Just asking?
 

greg brillus

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Yes Robert you are correct.......fundamentally if the engine starts without flooding too much or using the choke slides and the engine seems to run ok with no spitting, that is a good sign that the engine is really too rich. I know because my bike has been like that pretty much since I first got it back in 2007. So on a cold engine it is probably fine if it does in fact spit back a little until the engine warms up. I do use the chokes to start mine but I do not tickle the carbs, as doing both will definitely flood the engine and will not start. Once it fires I pull the levers back a small amount and ride the bike a couple of miles and then pull the levers back fully. On the "A" twins, because the front carb gets so hot after a run and you sit the bike for a while, as Bill mentioned, the fuel definitely tends to peculate on the front bowl and the carb struggles to give any proper mixture to the front cylinder. However, if you close the front carb choke lever, this causes such a restriction to the air flow, that it draws in fresh fuel and the engine will run happily on both pots. The early original float bowls were made of brass and had a separate angular delivery tube to the banjo that mounts it to the carb body, whereas the later bowls (probably made post war) are from MAZAC and the angular tube is part of the casting. It is possible that this might contribute to the heat transfer........I am not sure of the heat conductivity difference between the two, but the brass ones do seem to be a little better. The choke "Trick" I used on Rodney's twin seemed an easy enough way around the problem, I just thought that insulating the bowl from the heat coming off the front head/barrel might help...........
 

Robert Watson

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I was more commenting on what you said was what Neal was doing. I honestly don't recall what I do if the engine has only been shut off for a brief time, or if it has been siting idling and is hot, but it always seems to start reasonable easily. If not with a kick or two when hot I recall I tickle the rear and give it a boot and all is fine. Tickling the front with gloves on is a challenge!!
 

greg brillus

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Hi there Bill, yes the new carb's on the new bikes have all had that mod done. Rodney brought his bike down Yesterday and it was running quite rough and only on the rear cylinder this on a hot engine. Before he had shut the engine off, I quickly reached under the tank and depressed the tickler. The engine then ran on both cylinders quite happily again..........So I raised the float level twice and now the engine runs fine and on both cylinders. I made up a visual aid (tool) using an old lower banjo bolt and drilled a 5/16" hole in it's center, this enabled me to rig up another 289 carby horizontally in the bench vice. I then attached the float bowl using the modified banjo bolt and filled the bowl with fuel, then you can see the actual level of fuel. If the tickler is depressed the level rises quickly and the fuel dribbles through the hole in the carby jet block where the needle jet would normally screw in. I would say I have raised the level by 4 to 5 mm in height. It is enough that the tickler plunger needs a few mm shaved off it's length or else it will hold the bowl down and cause the bowl to flood continuously. I don't believe the original brass bowls have as low a fuel level as the later mazac bowls, but this really depends on the needle/float combination used..........As there is little to no information about this horizontal set up of these carb's, it has to be done by trial and error. Once the engine rev's come up, I feel the air/fuel flow through the carby is enough to overcome this low speed running issue, thank goodness, as otherwise it could have done some damage. So hopefully this will be sorted out and the engines will be more the happier for it. Cheers............ Greg.
 

billirwinnz

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Hi Greg

It sounds like you have sorted the problem! Unlike the rear carbs the float level has only a few mm between too much and too little. I found that holding the tickler down for a count of 6 seemed to overcome the hot start problem. I had also rotated the float to raise the level a little which cured the off idle spitting back. I assume that you will pass this information on to Mat and Neal.

Cheers Bill
 

bmetcalf

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I need to sort it, but I can slow the too-fast idle of my C Rap with 289 carbs by leaning the bike over.
 

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