Starting Vincent Life with a Backfire!

[KennyNUT]

Hi, I am brand new to this club and have bought my first Vincent, a 1950 Comet.

It's in very nice condition indeed, has matching numbers, has just been registered with the VOC club etc and runs very sweetly and quietly, perhaps, due to it having been owned by a mechanic of 60 years prior to my recent uptake as its latest custodian. Anyway, to the point!

It is fitted with an Amal 289 and it loves to backfire upon trying to kick start it, seemingly laughing at me as it dribbles fuel all over the crankcase shortly after turning on the fuel taps. Actually, it loves to dribble fuel through the hole in the base screw thread of the carb, itself. This happens regardless of whether it's on the side stand or the rear stand. I have tickled it until it dribbles fuel out of the top hole in the float bowl and on separate occasions with very short tickling of only a few seconds without the characteristic dribble from the top hole. Fuel however, still dribbles out the base hole. Backfiring occurs on most occasions when kicking it over (when it does not catch to then start running).

So, how do I avoid this fuel leakage, the backfire and what damage could it do?

I only took delivery on Saturday, so am rather inexperienced with it, though it starts OK after just two or three kicks of following this procedure: open both fuel taps, switch on ignition, choke, on just about 1/4 way, tickle carb until fuel dribbles out top hole of bowl, gently turn over until TDC, pull on exhaust valve lever until past TDC, push down gently on the kick start lever, then kick the engine over whilst releasing the exhaust valve and pull on the throttle just a little. This is when it can back fire with some ferocity! It runs well when it catches, warming up nicely in conjunction with reducing the choke and pulls away in first gear cleanly and the throttle modulates nicely without any hesitation.

So, some advice would be wonderfuel

 

[mercurycrest]

Try this: open fuel tap, tickle carb until fuel dribbles out top hole of bowl, gently turn over until TDC, pull on exhaust valve lever until just past TDC, release the exhaust valve and pull on the throttle just a little. Let K/S return to top position allowing full stroke, switch on ignition, kick down with a full stroke.
Cheers, John

 

[greg brillus]

It sounds like the excess fuel is igniting in the exhaust pipe causing the large backfires. Seems as though your fuel level in the bowl could be a bit on the high side. With the bike on it's rear stand and fuel taps turned on, it should not drip fuel from anywhere, until you tickle it, which will raise the level. I can leave my Rapide on it's side stand (modern jap type mounted on left side foot rest plate) which admittedly keeps bike more upright than the original prop stands, with fuel taps both on...and it will not drip one drop of fuel all day long.....anywhere..........Cheers....Greg.

 

[chankly bore]

You should be able to get a local member to have a squint at the carburettor, failing that, turn off both fuel taps and disconnect the bottom float feed and then the float chamber itself. Check the float for leaks by immersing in to warm water.Cut a small groove across the bottom of the float needle such as will fit a small screwdriver. Use this to lap in the needle and seat using Brasso or toothpaste.New parts are available through the Club spares and other places.Minor adjustment of the float level can be made by twisting the carburettor on the inlet stub. You can observe the float level by taking off the chamber top, but the locknut is a perilous design which doesn't stand overtightening.The Rider's Handbook has a good diagram showing where the float level should be.At very last resort you can buy a brand new Type 289 or VM28 Mikuni. The risk of fire is a consideration, so please fix this problem for your own peace of mind.Fuel level should be below the pilot orifice and never issue from it even with the machine on the propstand.

 

[Williegunn]

As somewhat of a novice, who has trouble getting my comet to start, and has spent time reading the books. Why do you open both petrol taps? I thought the left hand one was used normally and the right hand one worked as a fuel reserve.

 

[Howard]

As somewhat of a novice, who has trouble getting my comet to start, and has spent time reading the books. Why do you open both petrol taps? I thought the left hand one was used normally and the right hand one worked as a fuel reserve.

I think you're right, but don't forget - right looking up at the tank, isn't the same as right looking down from the seat, as I discovered when my Egli went onto reserve, and there wasn't one!!!

I'm sure the comments above are right, but check the float needle too, early ones don't like the new petrol.

H

 

[nkt267]

Just to confuse things even further..just like a good VOC member should,,If I tickle the carb on my Comet at all,it will not start..Mine likes to have the fuel turned on and then just kicked..
I have the carb fuel level about right and find that,at a standstill by leaning the bike to the left it weakens and by leaning to the right it richens..John

 

[KennyNUT]

Firstly, thanks all for your thoughts. A couple of points perhaps not made clear by me is that the backfire is the fuel igniting back through the carb as it shot expanding gases past my left leg, not out the exhaust. Of course, now I try to start it from standing on the kick start side and not astride the bike. Today, after playing with the kids with it being Xmas day and all, I went back to starting the bike no or very slight carb tickling. It seems the choke position was quite crucial and I had it start several times including a couple of times without backfire. It still backfired more than it didn't!

Where to start.... I will try without tickling and also switching ignition on only before kicking. As far as taking the float bowl out and also checking it's position, I will see how I get on with the simple approaches first.

Also, I was advised to open both taps in a stepwise written starting sequence. With some ignorance, I admit I was not aware of the right hand tap being the reserve tap. Is it possible that this was overfilling the float bowl/carb causing excess fuel flow?

I believe the bike was running fine before I received it and what i am doing is just me. It has just passed its MOT (i asked for ths)and had been run 30 miles before I received it, so I reckon it should be ok.

I will check the float level if all of the above makes no difference. As a last resort how much is a modern day 289 or mikuni? Which is better and how should they be set up?

Thanks again.

 

[Bazlerker]

All bikes are different and each requires a slightly different drill...I never touch the choke levers on my machine when starting, mainly because they are in a box in the basement..they havent been on the bike since 1986.. 2 or 3 deptessions of the tickler, bring the pistons past TDC, and swing the kickstarter in a gentlemanly fashion whilst dropping the exhaust valve lifter just before the bottom of the kickstarters stroke does it 2nd or 3rd kick every time for my machine..

 

[clevtrev]

With a Comet you need to kick it really hard to make sure that when it fires there`s enough speed in the crank to keep it going, just in case your ignition is not at full retard when it fires, that`s when it backfires.