Dominator 88 bogging down

Take out a spark plug and describe its condition.

Hi Rick,

   If your bike has been unused for 50 years, it might be that the auto advance mechanism is seized in the closed position so the engine is running retarded at higher revs. 

You can check the adv/retard unit is free  by removing the distributor cap and gently turning the rotor arm which should spring back under the action of the Adv/ret  springs. Not so easy as it sounds as the lower  cap spring clip is  innacessible  underneath and the  carbon brush in the cap is delicate. I use a screwdriver on the  cap clip  and leave the brush alone if its free to spring in and out. Sounds more likely you have a worn 106 needle jet or the needle is a notch too high in the slide. Needle jets have a short life and have been known to wear out in 1000 miles. Avoid E-Bay offerings.

It would also be a good idea to check if the carburettor is clean.  There are various tiny passages which can become blocked.  On my bike I had an off-idle stumble, which reduced but did not disappear when I enriched the idle mixture and raised the needle.  It turned out to be a compensator hole down-stream of the jet tube.  At different stages it passes air and also fuel.  I cleaned it carefully using a tiny piece of wire, tweezers and a magnifying glass.

Paul

Can't agree that the needle jet could be worn unless it is a concentric carb. It should still be a mono-block, which do not wear a fraction as much. Also if it was running rich (worn jets/needle wrong position) then would actually accelerate quite well, until it fouled the plugs. The idea that the ADV/RTD is stuck is favourite and an easy check. Of course it could turn out to be a magneto!

 We don't know anything about the carb you have ,or how old it,and its parts are or whether they are correct . It is worth doing the simple easy stuff first , but if nothing obvious turns up then you are going to have to read up and learn the hard way!.   Its always usefull to  do one thing at a time and check progress. Sometimes you can pinpoint the issue to carburation  or  ignition   ,if so this helps enormously as it halves the  things to work on and reduces the chances of making it worse by  fiddling with things  that are fine.  With a bike as old as this  there can be  corrosion  in any connection  which can weaken ignition  to a point where it  gives up under load. 

I think you need to go one step at a time. If it was my bike I would suspect the auto advance being stuck in the retarded position as that best matches your described symptom . If you are unsure how to check it ask for help from someone that is familiar with how it works as its difficult doing it based on written advice, Youtube is generally a good source for a visual aid. Once that has been checked and if the problem persists go on to the next most likely cause. If mechanical is new to you I would suggest not to start dismantling the carb but to get help from someone that is experienced. Good luck with it. 

Most unlikely  to be the main jet as you  don't get much use of it at the throttle opening you are using .   Could easily be restricted  fuel supply ,  Filter in tank, filter on carb, air breather in tank cap. Water in  main jet well. Phase seperation of EO5 fuel.!!

You have opened the air slide..haven't you ?

I was just about to say run it up to the revs it starts to fumble then close the choke a little so see what happens.  IF it improves you are running lean if, it dies you are rich. If nothing  changes check the choke slide is moving.  If its starting and running your primary circuits sound   good.  When you transition onto slide and needle you falter then one or the other is giving you a false mixture.

Tight cable for choke OFF!

Cheers

Jon

Another good option on this one. I have had this syndrome on other set ups/carbs time and again you rev it to clear it but it never does. Selected us of an air line has been successful. Other wise you have to strip the carb-again. Too much use of a water hose near a running engine can cause this especially under the cars bonnet. 

Not knowing what an air slide is  could be the problem .   Commonly called the choke, Normally controlled by a handlebar mounted lever.  If left partially closed  will give the described symptoms. Its a bloke thing not to read the instructions or users handbook.  I do it all the time, would be a nice easy fix if thats all thats wrong.

Not sure if we are talking at cross purposes here but I find the terms air valve and air slide to very confusing and have always called the lump of cylindrical metal in the carb - that causes the revs to rise and and fall - the throttle or throttle slide.  The other lump of rectangular metal - that causes the mixture to go rich - the choke or choke slide.

So, is it the raising of the choke slide that is causing the revs to rise or the raising of the throttle slide?

Amal use term "air slide" (for the 'choke') and "throttle slide" (for the bit that makes it go faster).

If tickover "rises rapidly when air slide is lifted",  note that tickover speed is only relevant when engine is warm and air slide is up. I don't think I ever leave it down for more than a couple of seconds...some people remove them (and seal the hole on top).

I'm not sure why you are concerned about how it runs when cold.  All internal combustion engines run best when warmed up. I open the air slide as soon as it fires up. I suppose if I want it  to tickover from cold while I put my riding gear on, open the gate etc  then it might be necessary to use the slide but that's all. The only question I asked was whether you are running with it closed? If so, it will be far too rich and will bog down. If you do open it as soon as the bike starts then your problem is elsewhere.(You might need it at the first traffic light stop when very cold...but that's for you to find out)

Sound like some basics need to be set.   There should be a little slack in the throttle cable when the throttle is closed . To start setting up the main slide (controlled by the throttle) needs to be all the way down with the throttle closed ,and not held up by the idle speed screw . Put your finger in the intake and check that the idle speed screw is far enough down to be not moving the slide, Then raise the screw till you feel it move the slide .now screw it in a whole turn .start the engine ,use the throttle a little if needed , Adjust the screw to give an idle. With the engine warmed up well you can follow the idle tuning instructions. The choke slide needs to be fully up . There is also a possibility that the choke slide is somehow picking up the throttle slide as its lifted.

Rick,

if the bike hasn't  been on the road since 1970, are we talking about a barn-find type scenario?

If so, I would advise you not to worry too much about carburation until you have stripped and cleaned out the sludge trap in the crankshaft. I have found these sometimes to be completely full and if the sludge has dried out during hibernation it can break up and block the big-end oil ways, especially if a modern detergent type oil is put in. Old 'straight' oils produced a lot more sludge than modern oils  and I have seen two 'time capsule' type bikes, a 99 and a BSA A7SS with their engines completely wrecked a few hundred miles after coming out of storage, so, personally, I wouldn't  risk it unless I knew it was definitely clean.

The ECU is not fitted to these bikes, the carb is tuned for best running on a warm engine only, the choke is there for the purpose of covering up the mixture being wrong for a cold engine and the rider is the ECU and has to control the choke manually combined with tickling of the carb.

The choke in ALL bikes and cars from this era is a necessary item. When cold the petrol engine needs a rich mixture to initially, start that is why we 'tickle' it and 'choke' it. Once started and beginning to run the 'choke' can be reduced or removed completely. Any one who finds he can start without choke is running an engine richer than needed. David and Robert above have it right.

hi Rick

Just a thought, I had a similar problem with my AJS a while ago after a long lay up and I assumed it was a carb problem, but after extensive work on the carb I switched to an electrical fault, it behaved very similar to yours, but wouldn't rev when under power.

Eventualy found the problem was the condenser!.......I replaced it and all problems solved. Is yours mag or coil?

Regards John O

Hi Rick

The condenser is very easy to test, and change If required, keep me ( and others) informed of your progress 

Regards John O

Thankyou Rick for updating us.  It can be a thankless task  trying  (often in vain!)  to diagnose  a fault from  a few lines of  script.  Its also  very easy to forget just   how clueless  WE  were when we first  got our fingers dirty  on these  old  mangles . 

The new fuels are different from old ones which the Amal carburettors were set up for.  What jet and needle position have you settled on?

Paul

In the mid '80s, I bought a BSA Super Rocket which had sat idle since at least the early '70s.  I took a big risk, fitted a Commando oil filter plate and ran high detergent diesel oils.  The bottom end stayed very happy.  I guess I was lucky.  Actually, the bottom end was the only part of the bike I didn't dismantle and rebuild.  What a pile of trouble it was, and then solid and reliable once I sorted it.  Actually, I regret selling it - but it did fund my current long term project, a total basket case Rapide.

I can't remember, but have you checked valve clearances? If they are too tight they can close up when hot and one or other valve might not close.  Result is that it might well carry on running very badly (although it's unlikely to re-start until it cools a bit).