Missing / losing power

[Taffius]

Hi all,

 

After sorting my rear earths out (and let fuel run low), I have taken my Spitfire 1500 out and I am losing power when opening the throttle.

 

It seems when I accelerate hard it splutters and jerks. Now I initially suspected low fuel so I put 5 litres in but its the same.

 

Now I am thinking fuel system crud, needles blocked, plugs, carbs, etc but do not know where to start. I crept home after only half a mile of running it. Its fine idling but when blipping the throttle there is a stutter from carb(s) which then manifests to the stutter when driving. Trying to pull off a junction is a nightmare!

 

How some have ideas when best to start please. Meant to be going for MoT week tomorrow but now might not make it!

 

 

[Guest]

Hello.

 

Do you have an in-line fuel filter close to the carb ?? If not I suggest you fit one asap and you can then ensure that any muck coming up from the tank is 99% held there. You will soon see if there is muck.

 

Secondly, this could be an air leak making the mixture very lean when opening the throttle at 2/3 open or full.

 

Finally, you may already have muck within the carb(s) so a quick strip down and rinse out will help to clear that.

 

Because you have said you ran your fuel low, I think it's a muck problem rather than anything more sinister.

 

Try the above and see how you get on.

 

Good luck.

 

Richard.

[Guest]

Sorry I meant to say, you will also need to clean the fuel chambers (where the float is housed) on each carb as these may be sticking due to muck within them. It's a bit of a Spring clean job really.

 

Regards.

 

Richard.  

[Pete Lewis]

I would check for debris in the fuel, pump, and remove the float chamber tops and check there is no particles in the back of the float needle.

hold a jar under the fuel pipe and crank it till a few squirts make its all clear

 

use engine oil in the dashpots, this is the aid to acceleration thin oil makes flat spots .

 

a bit unusual but we had a car with a serious running problem off idle, on removing the air piston one of the needles had dropped out, I dont expect that to happen.

 

Re check the points gap, check the dizzy spindle has no side play (wear) wich affects point gap.

make sure the HT lead in the coil has no green growing in the connection

 

you can blow the fuel line back to the tank and while in the carb float chamber , gef a straw and blow the jet supply port inthe bottom, wear googles. If you have a bike pump use that

 

if the cars stood for ages fuel wax up in the small pipe float chamber to the base of the jet

 

do check when you return the choke that the jets fully return back up against the agjusting nut under the carb..

 

pete

[AidanT]

With respect to the dash pots how do you know how much to put in? Can they be over filled??

 

Thx

[Guest]

Hello Aidan.

 

Yes they can be filled too much and also with the incorrect oil.

 

If you remove the damper from the piston pot and look inside; you will see a shoulder. The oil should be filled to this level, any higher will be squirted out wherever the carb decides and will also affect the movement of the damper within the pot - basically it will have too much resistance to operate correctly.

 

With regard to oil, you should use 20/50; this has the correct thickness (viscosity) and is the oil recommended by classic car manufacturers, not just Triumph.

 

Some people use ATF (automatic transmission fluid) but this is too thin; although some race track participants believe that the damper and piston is more effective with that style of driving and those requirements. Perhaps that is correct but I cannot comment upon that.  

 

Hope the above assists ??

 

Best wishes.

 

Richard.

[dougbgt6]

I frequently overfill my dashpots, the excess gets squirted out when the piston is replaced. There is a difference without the oil but nothing noticeable when "over filled". 

 

I would put the fuel filter before the fuel pump. The fuel pump has it's own filter which is more difficult to clean so it makes sense to put the additional filter before the pump.

 

In the past I've used nylon strimmer cord to dyno rod my fuel lines, works very well.

 

From what you say I would suspect an air leak, manifold gasket or split hose?

[Mjit]

One easy way to test for an air leak is to pull on the choke when you have the issue.  An air leak turns the mixture weak and the choke richens.  If the engine runs better when you pull the choke it's an air leak, if not it's something else.

 

Check the breather hoses between the carbs and the rocker cover.  Had similar problem where car would run normally till I mashed the pedal.  Turned out one of the breasher hoses had a 50% split right on one of the hose clamps (there ironically to try and stop air leaks).  Normally it sat closed but the extra vibration when pushing it would open the split, letting a load more air in - only to close up again when the RPM dropped.

[Pete Lewis]

the idea of the damper is to restrict sudden rising of the air piston, contrary to many thoughts holding the piston from rising when you open the taps  makes the air stream increase under the air piston and thissucks more fuel out of the jet, gives you the enrichment you need to acceclerate  same effect as the accelerator pump does on a down draft carb

 

so  with thin oil the piston rises too quickly and you get a weak mixture on opening the throttles, that gives you a the hic up flat spot you dont want 

 

some use 3 in 1 oil, heck knows what viscosity that has when hot , if it runs better on cats pee then something is very wrong with the mixture setting  and youre making SU and car makers design   to be ........unworthy....   well plonker came to mind  

[jim biggie]

Hi - Check out my story "losing power" posted last week. I reckon that if you clean the metal fuel lines, change all of the rubber fuel lines, remove and clean the fuel tank and change the air filters that may well do the trick.

 

Best of luck!

[AidanT]

So what are the symptoms if the oil is low? I must admit I have not checked this on a while

 

Aidan

[Pete Lewis]

If the dashpot level is low you ulitmately loose the damping effect, and may get some lost acceleration .

 

if you overfill it just overflows and descend to the holes in the base of the air piston and get burnt

And may be some extra blue smoke till its gone

 

some have a small breather hole in the top cover , it can escape there also

 

Strombergs tend to displace oil more than SU, if you have CDSE with a top mixture adjuster there is a

0 ring in the needle fitting , all held down with a sprag washer.

 

Its a when you check tyre pressures and coolant levels top up the screenwash do the dashpots at the same time sort of job.

 

pete

[Taffius]

Great info from all and very much appreciated, lots of checks, cleaning and bits to do then!

 

Many thanks, will keep all updated.

[Taffius]

It will hardly go now and the smoke from the exhaust is brown when it chokes up. I get backfires too. Cannot find any air leaks in fuel system.

Took a video of it running, will try to post.

Thanks

[Pete Lewis]

Back fires are often ignition related to stray Ht timing generally due to points , condenser and rotor

problems make sure the condenser is tight and its not oversized fouling on the points cam spindle.

check the small cotton covered wire that earths the moving plate is sound.

 

with cap off hold the coil HT centre lead a few mm above the rotor, get it cranked over , if spark jumps to the rotor is failed

 

check the brush in the cap and the rotor actually make contact.

 

 

double check the jets do return after using the choke

and try to get a compression test done, with all plugs out and throttles open

 

pete

[Taffius]

Thanks again Pete. With other stuff going on I had to cancel MoT today because I have not got time to address issue. Determined to make County Day, must try harder!

 

Will keep all updated, failed to get an MP4 uploaded, too large I suspect.

 

Thanks all.

[dougbgt6]

Hmm, I notice no one has mentioned splits in the rubber carburettor diaphragms? Worth a check. 

[Pete Lewis]

Doug  youve been on the coffee again...its a spitfire    with dreaded SUs   Ha !!!  

 

well many love them..

Pete

[JohnD]

Hesitation on throttle up canbe due to a rich as well as a weak mixture. The dreaded Waxstat thermostatic autochokes can do this. Do you have them fitted?

John

[dougbgt6]

[Taffius]

Hesitation on throttle up canbe due to a rich as well as a weak mixture. The dreaded Waxstat thermostatic autochokes can do this. Do you have them fitted?

John

 

Not sure I have auto chokes as I have a manual choke lever inside car. Don't know if the carbs themselves have thermostatic auto chokes...

[Taffius]

http://forum.tssc.org.uk/index.php?/gallery/image/1087-spitfire-edited/

 

I have tried to put a edited (free software so please excuse watermark) video of my issue. Its large but might be easier to see and hear issue. Worth a try I thought.

[Taffius]

I have had a friend over from work and it looks like fuel pump however concerned after market replacements might not be the correct route to go down after reading over pressure issues on forum.

Any advice on standard pumps welcome. He has cleaned carbs, timing looks ok, dizzy ok, etc.

[dougbgt6]

I got a replacement pump from Rimmers, been installed for 3 years, no problems with it. Maybe I've just been lucky? The original pump wasn't the only problem with my fuel system, the pipes were partially blocked with sludge and the rubber sections no longer rubber.

[Colin Lindsay]

wear googles. 

 

Just the thing for google eyes....