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Series ‘A’ Twin Carburetion



greg brillus

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VOC Member
#1
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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VOC Member
#2
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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VOC Member
#3
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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#4
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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VOC Member
#5
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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VOC Member
#6
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.
 

vin998

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VOC Member
#8
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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VOC Member
#9
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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VOC Member
#10
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.
 


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