Engine Woes

[Dannypwills]

Hi all

I have a MK3 GT6 with HS4s which I recently posted about, and could do with a bit of advice.

When I posted, the car was constantly running rich and fouling the plugs, and so took it to see Dale at Moordale Motors who balanced and tuned the carbs, replaced the needles and reset the timing which, initially, saw it running very well. However, within a few weeks the car was back to running too rich/fouling plugs, and so I leaned it out a bit until it was running nicely.

However: (i) I am now experiencing a loss of power/misfire under load, particularly in 3/4th gear and not very noticeable in 1st/2nd or when pulling away; and (ii) having to start the car with the choke fully open, even when the engine is up to temp. 

All that suggests (in my basic knowledge) that I may be too lean, but grateful for any thoughts/whether this could be anything else (i.e. a timing issue etc.). I've had a look with a Colourtune, and if anything I am on the very border with running too rich, so that has confused me too!

Just to add, the HS4s I originally had on had an opening for the vacuum advance, but the right hand carb body cracked when I was trying to get the jet out to refurb them, so i had to replace with an eBay job which doesn't have anywhere to plug the advance into. Mike @ Moordale suggested that wasn't an issue and he preferred not to attach the advance in any event, but adding here in case relevant. 

Hoping I can resolve as it spoiled what was otherwise a great drive from North London to West Sussex down the A3 at the weekend!

Thanks in advance. 

Danny

 

[Andy Moss]

Send carbs away for recon.  Before you do - check advance curve throughout rev range.  Best thing to do is advance it until it pings the retard it a bit. Could try with a colour tune or check for air leaks (spray carb cleaner at spindles - may hear engine note change). I’ve had some luck with carb recon before, but also chased my tail for ages.  Recommend you get a recon pair. Doubt you’ll look back. 
Andrew Turner did my carbs (01476 860555), great job.

Low power could be caused by ignition timing. Not sure what your setup is, but expect it’s a Lucas unit.  I might have box of these somewhere. They can be repaired by H&H near Birmingham. 
Normally I advocate doing everything yourself, but having a professional rebuild some of these items with 50 years of ware can be very worthwhile. 

last this - have you done a compression test. Worth a check before you go to unnecessary expense. 

[Pete Lewis]

there is very little you cant replace on HS carbs  unless the spindle bore is worn , there is no reason you need to keep messing once set they stay as such for ever 

any carb make needing persistent messing is not right 

if the hoses have been meddled then there is a good chance some of the dreaded rubber slivers are messing with the supply and jamming the float needle valve up

thes can caus richness due to flooding fuel levels and weakness due to blockage 

i would remove the top covers pump some fuel into ajar you might catch them 

look behind the back of the needle valve for trapped small bits of rubber 

these little sods bread like rabbits and seem very linked to nasty fake fuel hose from overseas 

barricade is much tougher and these dont get sliced off when you insert the metal pipework 

i allways use the Carb exchange in leighton buzzard  small crew  no bullshine 

if you want some pointers and no myths take a trip up to me in sunny luton 

might cost some nice bicuits 

Pete

 

 

[KevinR]

Is there any reason that HS4s have been used, the standard fit for the GT6 is Strombergs.

To get HS4s to fit under the bonnet requires using much shorted dashpots - possibly from a Dolly Sprint AFAICR.

 

[Nigel Clark]

10 minutes ago, KevinR said:

Is there any reason that HS4s have been used, the standard fit for the GT6 is Strombergs.

To get HS4s to fit under the bonnet requires using much shorted dashpots - possibly from a Dolly Sprint AFAICR.

 

Correct, thought the short dashpot SU is the HS6. With choke diameter of 1.75", these carbs are too large for a standard 2 litre engine.

Nigel

[Colin Lindsay]

From what you've posted - and I'm far from an expert on carbs! - you reckon they're too lean; then enrich them again and see what happens. If that's the only thing that's been adjusted since Dale did the work, the unadjust it again. It takes very little adjustment to alter the mixture, so enrich gradually and see how performance improves. I'd maybe try different plugs too and see if that helps the fouling. My own Herald tells me the mixture is perfect, going by the colour of the plugs, but it starts without the choke from cold; however I go by how it drives and that keeps me happy. 

[chrishawley]

I suppose we could put the current symptoms into the category of 'misfiring under load' - about which it would be possible to write pages!

I'd suggest a timing issue would be low down the list of possibilities: Markedly over-advanced timing would pink very evidently ; markedly retarded timing would give low power but no particular tendency to misfire.

While not ignoring other possibilities on the ignition side (point, condensor, distributor cap etc) poor fueling seems a strong candidiate. Pete has described the rubber shards problem. But I'd expand that to include accumulated detritus anywhere from the tank forward. The GT6 has it's outlet at the base of the tank (unlike the Spitfire which pulls the fuel by syphon) so crud has only one way to go - along the fuel lines.

A worthwhile test might be as follows: Disconnect fuel line from carb. Disconnect +ve to coil. Turn engine on starter just one or two strokes. If unobstructed there should immediately be a forcible ejection of fuel - about a desert spoonful on each stroke. Jam jar useful. If I were considering a major recon on the carbs I'd check this anyway.

Not suggesting that that will solve your problem but for £0 it excludes a possible factor.

 

[Pete Lewis]

apart from SU have more variety of needles why do people think the SU swap has any valid reason 

I have an avid hate of SU and my 2000 mk2 has two of them,  but they are no trouble and never need twiddling 

i did have probems of the plastic float needle va;ve platform was worn ridged and that jammed the valve   refettling wont work 

new floats and its been fine .

take SU and Stroms apart and lay out  the  works  then you can see which is engineered and one is a meccano kit of odds and sods 

I'll leave you to decide 

both can do a good  job , both have their own problems 

but  ongoing messing is not normal ...ever 

there are just far too many myths around carbs and makes 

Pete

[Dannypwills]

Thanks all, and I may have to take you up on this Pete, as I am only down the road in North London. 

4 hours ago, Pete Lewis said:

if you want some pointers and no myths take a trip up to me in sunny luton 

The HS4s were on the car when I got it, with the PO having taken off a set of CD150s. I asked Dale whether I should get those refurbed and refitted, but he thought keeping the HS4s was the better thing to do. I am also not sure where the manifold has come from so would rather avoid having to start messing about with anything more than i need to!

I will try and and adjust to run a more rich mixture and see where we go from there, and have also ordered a compression tester to see if there is an issue there. I'm running a relatively new set of Bosch WR78s after fouling up too many sets of NGKs, and they seem to running better, and so hopefully it is just a case of having gone too lean with the fuel mix.

And then perhaps if that doesn't work I need to think about getting the carbs overhauled!  

[Pete Lewis]

my view you dont want any plugs with a R resistive suffix   not withour low HT   

 

[daverclasper]

4 hours ago, Pete Lewis said:

both can do a good  job , both have their own problems

Out of general interest. What are any possible problems on basic/earlier CD 150 Stroms, please Pete  

[johny]

Very little in my experience. Diaphragm punctures (some last longer than others), eventually spindle wear, needle sticking because jet needs centering, jet o-rings leaking when disturbed because theyve age hardened and thats about it...

[Iain T]

4 hours ago, Dannypwills said:

I'm running a relatively new set of Bosch WR78s after fouling up too many sets of NGKs

I bought W78 which are non resistive but obsolete. I think F78 which are available have the same gap but a smaller hex. 

Iain 

[Stratton Jimmer]

I'm with Uncle Pete on this one. My GT6 has HS4 and since I balanced and set the mix with a colourtune they have needed no adjustments at all in several thousand miles. Meanwhile, my Sixfire had HS6 and ran a lot better after I changed them for a pair of Strombergs. Since then, I have had several issues with the dreaded rubber slivers blocking the float needles. Ultimately, I replaced all of the rubber pipe with Gates Barricade, drained the tank (filtered the fuel and used it in the wife's Jazz) and stripped the carbs again. Rubber slivers everywhere! New fuel filters added, one before and one after the fuel pump. I'm hoping this will have resolved the issue.

I can't remember which needles are fitted in the HS4s but could take a look and let you know if you think it would help. Let me know... Incidentally, I have cheap (Ebay) pancake air filters on the GT6. They seem to do the job very well.

[Josef]

7 hours ago, chrishawley said:

unlike the Spitfire which pulls the fuel by syphon

Well, the squaretails do, the roundtails also have the fuel pipe straight out of the bottom of the tank. (Not trying to be overly pedantic, but thought it might be an important distinction for someone reading this thread in the future  )

[Pete Lewis]

1 hour ago, daverclasper said:

problems on basic/earlier CD 150 Stroms, please Pete  

Jonny has covered the most areas 

[daverclasper]

Thanks Guys

[NonMember]

2 hours ago, Josef said:

Not trying to be overly pedantic

No worries - I composed almost the same reply earlier on but decided not to send it as I've been accused of "point scoring" in the past, but we all know that you're not so inclined.

[Dave the tram]

Danny

Don’t know if you’ve cured this yet, but I had very similar symptoms to yours over many years. Mainly a loss of power quickly followed by hunting or coughing back through the carbs. This tended to be only when on full throttle in 3rd or 4th, especially uphill. For years I never dare overtake. Went though everything, ignition, coil, fuel pump, jets, new breather pipes, even refurbished carbs and fitted electronic ignition. It started and ran great, except for this issue which kept recurring. I’m convinced that I cured it after realising that the washers on the rocker cover, which was a bit warped, were not sealing and believe it needs some vacuum, not just for the auto advance under these conditions, but through the breathers etc. I’ve never heard anyone else report this and I think there was some scepticism on the forum, but the car has performed faultlessly for months now.
 

Because you only become convinced that it’s cured after many miles in varied conditions, I never got round to recreating the fault and was loath to mess with a perfectly running car. It’s off the road now for a respray otherwise I’d have tried it now. Be interested in others opinions.

cheers

Dave