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The impossible question.

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The impossible question.

Posted by vinny_tray at October 07. 2018

Hello all.

As the title suggests I know it’s almost impossible to diagnose issues without seeing or hearing the bike but I am now at my wits end.

Please bare with me I will try and give a brief history.

The bike is a 1957 Dominator 99 running a magneto and dynamo.

The problem is the bike will start hot or cold with one kick but as soon as you try to increase the revs the bike will not rev and serverly back fires.

I have just completed a full restoration but cannot get the bike to run correctly.

When the top end of the engine was stripped for in spection we found damaged piston due to a broken ring.

The bottom end look to be in good order so a rebore and new pistons were fitted and the head cleaned and valves relapped. Rocker clearance has been set at 6 / 8 thou respectively.

The cam and chain were left untouched and as the bike ran fine when owned by my Father prior to the restoration.

The cam followers and lifters were removed for the rebore but that’s it.

I have checked the links between the marks on the cam chain and I believe it to be correct at 10 links between the marks on the sprockets.

I have repeatedly checked the ignition timing using a buzz box to determine the points opening and have set the timing at 32 degrees BTDC using a degree wheel when fully advanced.

I have also fitted a new Amal 928/300 premiere with a 180 main a 106 needle  and RJ17 pilot. The carb has a number 3 throttle slide.

The bike runs an open gauze air filter and straight through reverse cone Megas.

Spark plugs are new NGK B6E S. New HT leads and champion plug caps.

I do not know if the mag has ever been rebuilt in its life time but as it fires and runs  I’ve been advised they either work or don’t.

I have tried running the bike on a mix of av gas and super unleaded also straight unleaded with no improvement.

Any advice would be greatly appreciated.

 

cheers in advance

 

Vinny.

Attachments

Re: The impossible question.

Posted by robert_tuck at October 07. 2018

Intermitent ignition sounds the most likely. Check the earth brush in the mag ,check that the champion caps are NON resistor. If the mag is a bit weak it wont like them. That popping and banging is fuel being ignited in the exhaust. Try closing the points gap on the mag to 10 thou.Check you have the leads the right way round.

Re: The impossible question.

Posted by vinny_tray at October 07. 2018

Previously robert_tuck wrote:

Intermitent ignition sounds the most likely. Check the earth brush in the mag ,check that the champion caps are NON resistor. If the mag is a bit weak it wont like them. That popping and banging is fuel being ignited in the exhaust. Try closing the points gap on the mag to 10 thou.Check you have the leads the right way round.

 

Phi Robert.

Thanks for your speedy response.

I will close the points tomorrow but a quick question would the bike start and run with the leads crossed.

I naturally assumed the timing would be be so far out it wouldn’t start.

Re: The impossible question.

Posted by Dan Field at October 07. 2018

That sounds very similar to how my lightweight was running after I bought a new concentric (I was sold one that was too large) fitting a new monoblock sorted it for me!  Do you have another carb you could try to rule out carb problems? Have you tried altering the needle clip position to see if that has any effect?

Dan

Re: The impossible question.

Posted by vinny_tray at October 07. 2018

Previously Dan Field wrote:

That sounds very similar to how my lightweight was running after I bought a new concentric (I was sold one that was too large) fitting a new monoblock sorted it for me!  Do you have another carb you could try to rule out carb problems? Have you tried altering the needle clip position to see if that has any effect?

Dan

Hi Dan

Yes I went down the carb route.

The new Amal I purchased was a direct replacement for the one fitted to the bike previously. Just the better version.

l made the purchase to try and cure the problem.

I do have the old one which had the needle in the middle position.

So I have just copied the setting to the new one.

Your thinking being running to rich does make me think the old pistons were a higher compression could that make that bigger difference?

 

Re: The impossible question.

Posted by philip_hannam at October 07. 2018

Hi Vinny.....When you first get your bike running,  it will only be using the pilot jet circuit. As soon as you open the throttle the second stage of the carb begins to take over........this will be the needle jet section. Which I would say from your description of the engine response is set far too weak.

928 carbs are a little too big for a 99. So the engine is alreay getting too much air to start with. A 626 would have been much closer in size to the original 376 Monobloc used on these engines. The main jet of the Concentric sounds too small as well. I would suggest using at least 220 with a cartridge filter and 240 with an open gauze K&N cone type.

I would suggest also checking the fuel flow to and through the carb.

Re: The impossible question.

Posted by Barry Carson at October 07. 2018

had a similar problem when unknown to me the block the main jet screws into had come loose and dropped down into the float bowl. i only had tick over and the only way the bike would pull off was if i mauled the engine and pulled off in second gear using full throttle

Re: The impossible question.

Posted by robert_tuck at October 08. 2018

If the leads were crossed it would not idle,so its not that.  I still wonder about a weak spark, close up the plug gaps to about 10 thou,If that helps it will be an indication. Mag condensor?,

Re: The impossible question.

Posted by Gordon Johnston at October 08. 2018

Looking at the video clip, it seems that when you open the throttle you are getting a weak cut (to use piston aero engine terminology). Check you are getting a decent fuel flow from the petrol tank. First thing is to up the main jet a few sizes. Next, raise the needle a notch. Either or both should help improve things. If it still spits when opening the throttle, try a smaller cutaway.

If none of the above help, look to the ignition side. What's your plug gap?

Were it mine, I would fit a more appropriate carb, a 1 1/16" monobloc preferably.

That's not a fibreglass tank is it?

Re: The impossible question.

Posted by vinny_tray at October 08. 2018

Previously robert_tuck wrote:

If the leads were crossed it would not idle,so its not that.  I still wonder about a weak spark, close up the plug gaps to about 10 thou,If that helps it will be an indication. Mag condensor?,

Thanks I’ll close the points and plugs tonight.

I couldn’t confirm if I had bought resistor plug caps so ordered new ones last night.

Re: The impossible question.

Posted by philip_baker at October 08. 2018

I've had similar problems with Commandos. One thing that I didn't think to check until the end of a similar experience was the fuel taps and filters. If the petrol tank  has been sitting around with ethanol fuel and general crud in it then it's worth removing the taps and cleaning the filters and bores. They can seem to be delivering fuel but the actual flow rate is not enough to keep the engine running (float bowl filled) above tickover speed. Are the petrol taps ethanol proof? If not, the rubber seals will swell up and restrict fuel flow.

Re: The impossible question.

Posted by vinny_tray at October 08. 2018

Previously Gordon Johnston wrote:

Looking at the video clip, it seems that when you open the throttle you are getting a weak cut (to use piston aero engine terminology). Check you are getting a decent fuel flow from the petrol tank. First thing is to up the main jet a few sizes. Next, raise the needle a notch. Either or both should help improve things. If it still spits when opening the throttle, try a smaller cutaway.

If none of the above help, look to the ignition side. What's your plug gap?

Were it mine, I would fit a more appropriate carb, a 1 1/16" monobloc preferably.

That's not a fibreglass tank is it?

Hi Gordon

I can confirm a good fuel flow.

The plug gap is set at 20 thou

Unfortunately the tank is fibreglass which came with the bike.

I do plan on changing it when funds prevails.

Whats your thinking on the tank?

Re: The impossible question.

Posted by robert_tuck at October 08. 2018

Take the carb off and check the by pass drilling is clear, this is the supply that takes over after the pilot and before the needle jet. Your pipes are showing a weak mixture.

Re: The impossible question.

Posted by vinny_tray at October 08. 2018

Previously Barry Carson wrote:

had a similar problem when unknown to me the block the main jet screws into had come loose and dropped down into the float bowl. i only had tick over and the only way the bike would pull off was if i mauled the engine and pulled off in second gear using full throttle

Thanks Barry.

Yes your description is spot on.

Re: The impossible question.

Posted by David Cooper at October 08. 2018

My 88ss has a pair of 928 for historical reasons. They might not be the optimum size but it runs fine. So I dont believe over size to be the issue.

And if you had the same issue with the previous carb it sounds like something else.

Are both float bowls full and clean? If there is fuel blockage in tap or tank it should at least run fine fot a minute or two. So if the carbs are about right then that pretty much leaves ignition.

Good luck!

Re: The impossible question.

Posted by ian_soady at October 08. 2018

It's the slide cutaway that takes over from the pilot not the needle jet (although they all overlap to some extent). My ES2 was spitting like mad till I changed the number 4 slide (theoretically correct) for a 3.5. It transformed the carburation.

Does it spit back through the carb (which would indicate weakness in that range) or fire in the exhaust, which may point to intermittent sparks as said.

I find that it's always worth fitting new HT leads and plug caps (non-resistor as said) which are cheap enough from the likes of the Green Spark Plug Co and eliminate at least one potential fault.

You can check flow rate through the tap(s) by directing the pipe into a large jug or similar and seeing how much flows in say 30 seconds. If it's more than half a pint or so it should be fine.

Re: The impossible question.

Posted by robert_tuck at October 08. 2018

Not quite correct Ian. The bypass drilling takes over before the slide. Its the hole behind the pilot and the engine side of the slide. As the slide is lifted it supplies fuel for the transition.

Re: The impossible question.

Posted by ian_soady at October 08. 2018

True, but it's not the needle jet. And we folk with monoblocs don't have bypass drilling (as far as I know...)

Re: The impossible question.

Posted by paul_standeven at October 08. 2018

Note that the standard carburettor for a 99 is a 1 1/16" Monobloc, but the cylinder head is the same as a Model 88, with 15/16" inlet tracts, so the larger carb won't make much difference.

I had a problem with my Model 88 when I first got it running, it would stumble on opening the throttle from idle..  A richer slide in the Monobloc reduced the issue, but did not entirely eliminate it.  The problem turned out to be the idle by-pass compensator hole between the spray tube and head.  Clearing it cured the problem.

But the carburettor here is new, so that is unlikely to be the issue  Bear it in mind that the needle jet on the old carburettor is likely to be worn, so setting a new one to the same spec may mean a lean needle mixture.  If the problem is carburetion, it will be the transition from idle jet to cutaway.  If the idle mixture is good, and the compensator hole is not the issue, try a richer slide

I use NGK BP7ES (they are low mileage but late '80s), which is a little harder, but this small difference is not likely to be causing this problem.  Have you checked the retarded timing of the ignition?  If the ATD is worn, it may be too retarded.  If the ATD springs are worn, it will advance too soon and give problems

Be patient and keep checking

Paul

Re: The impossible question.

Posted by philip_hannam at October 08. 2018

The attachment, with this posting, might be of some help to owners tuning a concentric carb.

Of interest.....the same article also points out that not all Concentric carbs had needle jets with acceleration (bypass) bleed holes.

Attachments

Re: The impossible question.

Posted by paul_standeven at October 08. 2018

I just watched the video.  The yellow flame from the exhaust accompanying the off-idle snatch, and discoloured pipes at the bend indicate lean mixture.  So check the compensator hole, and if that is not the issue, try a richer slide

There's still flame by the time the exhaust gases exit the 'silencers', so I think the exhaust pipes and silencers are too short, but I suppose that's a style decision.

Paul

Re: The impossible question.

Posted by robert_tuck at October 08. 2018

The monoblock does have a by pass, It draws fuel from the same well as the pilot but at a later point in the action, I have found it to be blocked . I have also had to drill it a bit bigger on ocasion. Thats a bit scary as you can't reverse the process. The Concentric does have a secondary by pass port ,in the groove below the slide. Seems to be back to front compared to a monoblock. If the secondary is blocked it will idle fine, but not pick up.

Re: The impossible question.

Posted by anna jeannette Dixon at October 08. 2018

hello now have watched your video clip  the  things  that are not right  with this bike one  weak mixture  bluing on the exhaust pipes tell me  there is weak mixture  try putting the choke on a then rev the the engine  you need bigger jet if you have a filter fitted  as phil say  a 180 main is more for a model 88,  try a 200 first if no better try a 220 main  if to rich drop the needle jet a few notches and try champion N9BCY there better igniting ethanol fuels yours anna j>

Re: The impossible question.

Posted by vinny_tray at October 08. 2018

Thankyou to all that have posted above it is really appreciated.

It would seem the majority believe it’s a fuelling issue.

i now have some avenues to go down.

Plan is to reduce plug and points gaps and change plug tops .

Regarding the carb I will fit the old one back on or

take the needle and body including main jet from the old carb and fit that in the new body as I know this worked well on the bike in 1983 when it was last used every day.

The only difference being the bike now has stock pistons instead of high comp. Please comment if you think the pistons could make a difference.

I have taken on board that there could be an issue with the petrol tank as it is fibreglass and at least 20 yeas old so I plan to rig up a remote tank and feed the bike that way to eliminate that issue.

I have tried using the choke at small increments but unfortunately this did not improve the situation.

which puts us down the ignition route but one thing at a time.

Thanks again to all for your time I will keep you posted.

 

Re: The impossible question.

Posted by David Cooper at October 09. 2018

When I changed comp ratio it had no noticeable effect on settings. I have a 928 on s 500. I played with slides. 2.5 was all.wrong, 3 was much better and 3.5 further improved. But even with 2.5 it did not misbehave like yours.

My bet is fuel starvation or ignition.

Re: The impossible question.

Posted by vinny_tray at October 09. 2018

Previously David Cooper wrote:

When I changed comp ratio it had no noticeable effect on settings. I have a 928 on s 500. I played with slides. 2.5 was all.wrong, 3 was much better and 3.5 further improved. But even with 2.5 it did not misbehave like yours.

My bet is fuel starvation or ignition.

Thanks David.

From all of the above advice I think the size of the carb will work.

As it has previously before the rebuild.

Jet changes are obviously needed.

But as you mentioned ignition may be the issue.

I will go down the fuel supply route first then I think I’m looking at a Mag issue.

Re: The impossible question.

Posted by vinny_tray at October 09. 2018

Previously robert_tuck wrote:

The monoblock does have a by pass, It draws fuel from the same well as the pilot but at a later point in the action, I have found it to be blocked . I have also had to drill it a bit bigger on ocasion. Thats a bit scary as you can't reverse the process. The Concentric does have a secondary by pass port ,in the groove below the slide. Seems to be back to front compared to a monoblock. If the secondary is blocked it will idle fine, but not pick up.

As the carb is new I would hope I would not have the issues you mention with blocked jets or passage ways.

Saying that it has been mentioned that the fibreglass tank could be an issue.

Not 100% sure why yet but maybe new type of fuel is breaking down the resins and then causing a blockages . Can anyone clarify the possibility of this.

Re: The impossible question.

Posted by robert_tuck at October 10. 2018

Anything resin based  is suspect under the attack of ethanol  contaminated fuel. The resins dissolve  and are burnt in the engine  coating the plug insulator with a hard irremovable shell that carbons up and shorts out the spark ,symptons are difficult starting and violent popping and banging. Tank liners are a  problem. Almost impossible to save the plugs,unless you find a powerfull solvent  pure ethanol??,or an old sandblasting plug cleaning machine. New carbs have been found blocked with swarf,anything is possible with old bikes

Re: The impossible question.

Posted by robert_tuck at October 10. 2018

If you have a sound steel tank and  ride the bike regularly replacing old fuel with new  ,have extra good fuel pipes and don't spill the fuel over that expensive paint job.then you would think we are all doom mongers talking rubbish. Later,rather than sooner you will wish you had listened.

Re: The impossible question.

Posted by Gordon Johnston at October 10. 2018

The present ethanol-enriched petrol will quietly corrode carb jets. However, GRP resin dissolves quite readily and can (will) coat carb internals with gunge.

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