Comet Carb'

[Martyn Goodwin]

could be right about getting the carb' tuned, it started spitting after abbout 15 miles yesterday and then gave up the ghost requiring recovery vehicle.

I think i'm being distracted by the valves going out of adjustment just because I found the inlet slightly slack yesterday and that this is the cause of my woe's. As you suggest I'll strip down the carb' to ensure all is clean and that the pilot jet is not blocked.

If your valves are going out of adjustment that fast I would be checking the compression. If its down I suspect you will fine that there is a carbon build-up on the valve stem that is holding the valve very slightly open, thus causing the hard/impossible starting and the slack tappet. I had this happen with my Comet.
good luck
Martyn

 

[chankly bore]

And if this IS your problem, go to a low-ash oil "suitable for older engines" i.e. pre- paralytic perverter!

 

[davidd]

Another common reason for the push rods to go out of adjustment is if you have added new components without matching them well. The push rods like to be well matched to their appropriate sockets. If they are not, you will find yourself adjusting the valves at very short intervals. You can stone them by hand or use a drill press and paste, but if you do not, there may be a high pressure area that will cause the metal to move, and when it starts moving, it does not stop:



I have used aluminum push rods for a long time. In th U.S. Smith Brothers makes them, but they do not do international shipping. I have used full aluminum push rods and aluminum with steel ends and have not seen a difference in wear.

David

 

[Martyn Goodwin]

PEI specified stainless steel pushrods for a reason. They are relatively soft when made and thus in initial use their ends wear to conform with the sockets in the cam follower and the adjuster BUT as they wear in the metal also work hardens - ensuring a long life and stable tappet adjustment. I guess that if you wanted you could use an alloy shaft with stainless steel ends and realise the same benefits. I wonder if silver steel would work also, given that heat causes it to harden? Martyn

 

[davidd]

The original B push rods were silver steel, I believe. I have not been doing well on the killer B questions lately. The push rods in the photo above were stock SS. They were both brand new as were the followers and adjusters. I did not take the time to match them and one failed.

All aluminum works fine as long as it is the correct material. The 7075 rods that were used in the bike in the sixties bent immediately. The next set were 2024 and they lasted several seasons with no issues. With the aluminum, it is very important to match the cups. Having said all this, I have run the stock push rods to 8000 rpm with no valve bounce, so I cannot see any reason to use anything but stock push rods on a street bike. Even a very hot one. Why fool with aluminum? One of the advantages of the aluminum push rods to the racer is that you can make them up on the lathe quickly and choose your length, which can be a convenience.

David

 

[cinquecento]

wel the base of the carb' body won't move and I'm reluctant to apply any more leaverage, this was having drenched area in hot water. I would comment that looking through the removed bottom banjo fitting there appears to be slight trace of rubbery red hermatite!! maybe one of the PO's had probs with the base coming loose.

Pushrod end profile is interesting point, all I checked on replacement of both was the length but I would think I would have noticed such a significant diffrence.

Anyhow, yesterday having checked p'rod adjustment and points she fired up. But again after running for a short while and then turning off the motor would not even give a hint of firing. I changed the plug for a colder one as I thought the motor seemed to run hot and the B6 plug had a very curious slight yellow tinge.

Most alarming was that the pilot jet was about 3.5 turns out. I'm suprised it would start, withoutchoke, and run for as long as it did on that setting. I adjusted to the 'base' setting I use or all Amal pilot jets, 1.5 turns out and fitted with a B7 the motor fired up and rean. I put it through several cycles and at lest now it seems to start on demand.

Today starts gradually lengthier shake down runs and will be looking out for fouled plug, pushrods going out of adjusment and checking it starts on demand when hot. If that all checks out I think I'll take the wee beasty a little further afield. fingers crossed.

 

[lar1949]

My 289's weep fuel from the air hole only when they are flooded. Once the engine has started and the fuel level has normalized the leak disappears. If it leaks constantly I would suspect that the float level is too high.

 

[lar1949]

To remove the base off a 289 I have used a deep socket the outside diameter just somewhat smaller than the inside diameter of the throat of the carb. Placing that through the throat of the carb being careful that when you apply force to the base nut that the socket does not bear on the ends of the carb body. Holding the socket in place with a ratchet extension will allow plenty of force to be applied to the base nut. If an adhesive sealant was used on the carb base nut then more drastic measures might be needed. Many of the adhesive sealants will release by applying heat from a heat gun or torch. Carefully.

Larry

 

[nkt267]

I don't know if you have a magneto or coil ignition,but if you have a mag and the bike doesn't want to start for 30 mins(been there and got the T shirt) are you sure that your starting problems are not caused by a faulty condenser? John

 

[cinquecento]

OK, so the problm does, and again today, occur when the ( mag' ign' ) motors hot. But from expereince would a faulty condenser not show up while running and not just when trying to re start hot motor.

I'm looking at a nice plug that does not stink of juice while trying to start when hot. Therefore, as the manifold is d*mn hot which must transfer to the carb' body and although not OE but i'll try an insulting gasket to see if that effects a change as possibly the prob' is vapourisation of fuel.