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gt6 rough running engine after rebuild


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Hi All,

I'm looking for any advice after hitting a brick wall following the rebuild of my gt6 mk3 engine, and tearing whats left of my hair out!.  a previous failed piston has resulted in some surgery to get it back up and running again, and roll on what feels a lifetime I'm still struggling to get it back in healthy state, and don't really have/know anyone around the area to help out.  Pretty much everything has been renewed or reconditioned to date, and general symptoms is poor idle, rough running and low vacuum.  The car is sort of drivable although not pushing in much at the moment whilst rings bed in properly.  something isn't right though  
A quick run down of what been done so far:

block rebored to +.030.  Crank ground .020 under.  new camshaft, pistons, tappets.  carbs rebuilt (cd150se), Distributor reconditioned.

general observations are bit of a misfire/ random "putting" from exhaust, low vacuum reading.  no problems starting it, plugs look good, not fouling and doesn't seem to be overfueling.  
Oil pressure is a bit high at the moment with fresh build/tight clearances.  Engine is what I would call "rattly" under load.  timing is set at 7 degrees atm.  compression hot and cold is 150-160 across the board
new plugs, leads, cap and rotor on the ignition side.  has electronic ignition

These videos below hopefully gives an idea!  it does sound a bit better on the vids than in person, just to me something isn't right

Vacuum test

exhaust note @ 1500rpm

idle and exhaust note

Engine rev

Any advice or suggestions greatly appreciated, the lack of 2 years proper use is giving withdrawl symptoms

thanks so much


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How does the mis-fire behave - Does it get worse with higher revs or load..?
Fuel flow...? Carbs adjusted..?
Is cam a tooth or two out..?
Is the spark good from dizzy to all plugs..?
Free air flow..?

When I rebuilt my engine, I pretty much had the same stuff done - pistons/rings/crank/cam/dizzy/carbs(SU's)/all elec side etc... My mistake was the camshaft - I fitted a uprated fast road one, but in my haste putting things back together, I neglected the need for it to be positioned differently and clearances etc... Needless to say it ran awful to begin with, but once run through all the scenarios etc.. and realised, adjusted it, and it ran sweet 😎

Stick with it, these things end up being something pretty simple that we over look - Keep the updates coming, someone here, if not you will get it 🙂

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Hi Steve,

thanks for the feedback and encouragement!  I've spent the past couple of days faffing around with it, not really gotten very far in fact worse i think to the point it's thrown back in the garage for another day    .

Anyways, as timing keeps cropping up Ive revisited the cam setup.  As this is a standard cam fitted I used the Lift on overlap method as best i could without having to take too much apart.  I was getting a repeated 6 degrees too advanced showing on the pulley.  hmmm! So after removing the timing cover, the timing marks showed perfect alignment at tdc against the cam sprocket.  but lift on overlap check was then out.  I wasn't too sure what was best to do at this point, but as the book goes with equal lift on overlap method when no timing marks exist, i set it as that and re-scribed a new timing line.

after getting everything back together , and reset ignition timing back to 7degs...there was absolutely no change in rough idle, vacuum around 15 hg.  I had a drive out and  sounded quite rattly still.  felt a bit sluggish too

I've been through all the ignition again, and replaced coil just to be sure.  Spark is good from the dizzy, and i've used an inline tester.  pulling each wire there is a noticeable drop in idle so it's a very random miss.  not really noticeable when driving either.

after checking everything over with the carbs again it ended really playing up and refusing to idle properly.  I just cannot find any leaks, or at least sparying anything that makes any difference to the idle.  there a very noticeable stumble at the initial point you rev it.  it spat through the carbs a little, and i could see fuel pooling on the carb bridges (float height is 18mm) At that point i gave up!

im left thinking is it carbs....it's the only piece in the puzzle not been changed (apart form being rebuilt) and whether to bite the bullet and get another set.  

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Have you tried moving the ignition timing whilst the engine is running, by loosening and turning the distributor?  You'll see/hear straight away if this makes a difference and whether it needs more advance or retard.  I'd guess you might need more static advance than 7 deg; mine runs at about 12 or 14 or so, although I'll admit it's a different engine to yours.  It's worth a try though?

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Have you got a clean fuel filter before the carbs?  I just went through a similar scenario and it was the fuel filter causing the problems.

In the ens I had fuel occasionally spilling from the carbs,  then eventual starvation at the front carb.  cleaning the bowls sorted it.  I had to replace one of the fuel flow valves as I just could not get it cleared.  Also check the float heights are 17mm and that the float tab seats as squarely as possible

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Thanks for the input . I used to run around 10degs before rebuild , but set a little lower to start with and start dialling it in accordingly. I tried 8, 10 then 12 but had no difference in the idle quality - of course ,reset idle speed , carbs etc with each jump. Still couldn't get vacuum above 15 and noticeable miss + hesitation on rev . Reluctant to venture out too far on the roads !

Yes, I've got one of those glass inline filters before the fuel pump . I haven't tried removing it to test but I'll give that a go , thanks

I'm away from it all for a few days to collect my thoughts.   My plan when back is to revisit the carbs and any leaks , up timing a bit , try the filter remove too. I'm wondering I've I'm getting a lean misfire at idle ,  which is causing detonation on load when driving (rattle) . Even tho the plugs don't seem to indicate it.  


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Thats the problem when one does to much in one go,

lots of foot re treading needed.

as you said, stand back, give it not to much thought but try and solve it like it was someone elses car/problem,  it gives a clearer mind.

dont clutch at straws trying the not so obvious.

one thing I would want to know is this low intake pressure!!!!!

it says engine timing to me

can you identify the cam as being the correct one  


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Hi All

Quick update having returned to tackle this a somewhat.  First thing was to up the ign timing a bit since having readjusted the cam timing last week.  Have upped timing to 10deg btdc (from 7), idle set at 850rpm  and there is a slight improvement now in idle and doesn't miss/hesitate as much like it did before with a throttle snap - but fettled with idle speed, balance and mixture again so maybe a combination.  when cold it's still a rough and lumpy. improvement in drivability / low end. Still low + unsteady vacuum and have been around all the inlet manifold again to check for leaks and made sure all bolts are tight and seated correctly.  I did about a 30mile run tonight, got back and idle was back rough again and wanted to stall.  I'm still considering a different set of carbs just to tick that off the list.

Next I went to the fuel pump / filter.  Removed the glass filter I had but no difference.  Although i did a pressure test on the fuel pump and noticed that on each pump the pressure would drop 1psi and  wasn't constant, not sure if thats normal?.  I had a spare fuel pump to compare with and that held constant pressure with each pump so I stuck that on just to be sure.

Anyways, as i'm getting a bit more drivabilty, I'd focus on the rattle which i really dont think sounds right or healthy and don't want to do any damage!  I had this somewhat before the rebuild, but i put the previous knock/rattle down to piston issue.  but now it's even worse.   mounted a camera inside the engine bay to get a clearer sound - i can't detect when it's running at idle where it's coming from, as it's mostly under load.  it does quieten down when warmed up, and i really hope it's not piston slap

Heres the audio:

1min in is drive off and the 'that aint healthy' noise

...first bit is from cold and around the village, later on is open road / longer straights.  I hope it's something simple as im totally out of ideas.  thanks again

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I don't think it's pinking, it's more mechanical unless its so lean . But retarding ign just makes it run worse . And the rattle is still there

In some ways I'm hoping it's valve train related . Or something stupid / easy fix without ripping everything apart again . Just frustrating . Everything else has been done we can think of. A keen ear will know I'm sure and hopi g for some next diagnosis options.

Tonight I'm going g to try a makeshift extended stethoscope so I can try and pinpoint when driving . It does seem upper block area

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The rattle sounds to me like either a little end or two or piston slap.  Piston slap should reduce, even disappear as the engine warms, the little end rattle not so much.  Doesn't sound to me as though it changes all that much.  Were the little end bushes changed as part of the rebuild?

Other than that it sounds reasonably sweet to me.


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      I had a listen...the only thing i thought is ...could it be the timing chain rattling?....but that does not explain it running badly i suppose ,unless the timing sprockets  are out when you re built the  engine?
      Cheers S

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Thanks for the feedback. firing order is good to go, but i'm not pushing it just yet with running in so bit gingerly on the pedal and not worsen the rattle.  I was afraid of piston slap too.  i can't believe that with a rebore , pistons etc?  Little end bushes were done also yes, although there was no difference after .  Looking back at my records piston skirt to bore clearance was 0.0025" from the builders, and there was a 0.016" difference between bottom and top of piston width.  no pin offset however.  99.9% confident cam timing is spot on too

So this evening i made myself a makeshift stethoscope to attach it to specific parts of the block and listen from inside the car.  I tried manifold side, timing cover and then rocker cover first and wasn't anything that stood out.  Next i tried coil side just below the head.  Definitely something more audible there, and obvious louder rattle when under load.  Thats a far as I have got as it's gotten too hot.  is there anything specific i can look for to check around pushrod  or rocker related in that area that might cause this?  

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Ok so in a feat of excitement, i whipped off the rocker cover and had a good nosy.  Something that I haven't noticed before, but 3 of the pushrods are right up against the edge of the head - on exhaust valves no. 4, 8 and 12.  As attached - i wouldn't have thought that being right surely?  I'm concerned about them being bent now...

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Im hoping more so the source of the rattle.  only the 3 rods mentioned were making contact with the head, but everything looked centred with the valve stem.
The rocker assembly was one of the few things not touched, as there was little wear on the shaft and lobes all good just a good cleanup.  Thats why i can't figure out whats changed in that respect. couldn't ever imagine it being something like that but partly relieved and hopeful i've found something!  checking against the manual diagram is looks built correctly with the spacers in the right place. confirmed that a few of the pushrods are now bent tho   something for the shopping list.  I don't know what to do about the alignment of the assembly as the same thing is going to happen.  

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