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GT6: Rough revving + excessive fuel smell + reduced power + stalls at idle without choke


avivalasvegas
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What started as an occasional gradual stall at idle (without choke) and a slight petrol smell has now worsened to the above symptoms. The car was rolling road tuned <1000 miles ago and ran beautifully. Today, the motor wouldn't stop shaking through the rev range and it felt like the car was running on fewer cylinders. 

Would be good to know what the most likely culprits could be so that I can troubleshoot? 

 

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Yep, what Dave said. Combined symptoms sound like carb problems, so air filters off and start the engine. Any sign of leakage from carbs? If yes check reservoir inlet valves for crud and/or floats sticking. In short the dread rubber slivers! :o

Doug

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+1 for blocked float needle valve so its either blocked and causing low fuel levels or the opposite and its allowing excess and flooding 

two things both  give erratic  low power problems 

if its been messed with the rubber slivers are a right pain if hoses have been disturbed

check the air filter gaskets are not obstructing any carb   front face porting 

Pete

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At Moordale Motors as I type this. It became clear the carbs are not flooding but the symptoms above continue with terrible fuel consumption & black smoke added to the list. 

A quick compression test here yielded:

  1. 142
  2. 140
  3. 135
  4. 140
  5. 135
  6. 137

So compression is good with <5% variance between cylinders (despite the oily plugs). New plugs = all 6 cylinders now firing.


Investigations continue to determine why Plugs 3 & 4 are so oily and especially no. 4 blew oil out when it came out. 

 

390E60DB-EDBE-433C-9A66-6D92AB1CD4CD.jpeg

5F9B7718-5563-42D1-8AB4-AA05EC9365ED.jpeg

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looks to be mixture rich  

has the rocker cover breather got a gauze inside , these get blocked and needs some metal bending to release the gauze for a good clean

and its SU and Pancakes a............ good recipe for guess work 

bin the ngk plugs and start again with some w78 bosch 

check the choke is fully returning the jets , a common SU problem

Pete

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11 minutes ago, Pete Lewis said:

looks to be mixture rich  

has the rocker cover breather got a gauze inside , these get blocked and needs some metal bending to release the gauze for a good clean

and its SU and Pancakes a............ good recipe for guess work 

bin the ngk plugs and start again with some w78 bosch 

check the choke is fully returning the jets , a common SU problem

Pete

The GT6 was rolling road tuned in March. That said, Dale agrees and leaned the mixture by half a turn. 

The choke is definitely in need of rebuilding but it does appear to fully return the jets. Will check more closely now.

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6 hours ago, avivalasvegas said:

At Moordale Motors as I type this. It became clear the carbs are not flooding but the symptoms above continue with terrible fuel consumption & black smoke added to the list. 

A quick compression test here yielded:

  1. 142
  2. 140
  3. 135
  4. 140
  5. 135
  6. 137

So compression is good with <5% variance between cylinders (despite the oily plugs). New plugs = all 6 cylinders now firing.


Investigations continue to determine why Plugs 3 & 4 are so oily and especially no. 4 blew oil out when it came out. 

 

390E60DB-EDBE-433C-9A66-6D92AB1CD4CD.jpeg

5F9B7718-5563-42D1-8AB4-AA05EC9365ED.jpeg

Your compression readings are good. That suggests the first two rings in each piston are doing their job. The lowest (third ring, often called the oil control ring) on each piston prevents oil from climbing up the cylinders and entering the combustion chambers.

My Scimitar engine had a problem with oil consumption but ran well with decent compression. When I stripped it down, the oil control rings were stuck in the pistons and lost their springiness, so ineffective, allowing oil to get above the pistons.

Of course you don't want to strip the engine simply to check the rings on one or two cylinders. Sometimes, pouring diesel into the cylinders via the plug holes and leaving several days to soak will free stuck piston rings.

Could be worth a try before driving deeper into the engine.

One other thought... If you have access to a boroscope, try taking a look inside the cylinders to see if there's any visible scoring in the affected cylinders. There are cheap laptop compatible boroscopes on eBay.

Nigel

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10 hours ago, Pete Lewis said:

bin the ngk plugs and start again with some w78 bosch 

W78 do not appear available anymore?

4 hours ago, Nigel Clark said:

My Scimitar engine had a problem with oil consumption but ran well with decent compression.

The GT6 now consumes hardly any oil at all, which is most perplexing. Coolant is clear so the head gasket would also appear to be intact. I am currently mid-way through my Auto-RX engine oil treatment. This is the most effective method I have come across to renew motor seals safely, if a bit slowly, without the need for a rebuild, should that be the issue. 

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6 hours ago, avivalasvegas said:

Sadly only 2 plugs available on there (and anywhere online)

Very true, I hope its only a shortage and Bosch haven't discontinued them. 

Do you have an idea as to what the problem was? 

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1 hour ago, Nick Jones said:

The original problem sounds like running rich/drowning at idle due to leaking float valves and or excessive fuel pressure. Strombergs are prone to this.

Nothing to suggest anything mechanically wrong with the engine.

Nick

This car has twin SU HS6. The point about flooding at idle still applies, so worth checking float chambers for float height adjustment and condition of needle valves.

Nigel

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2 hours ago, Nigel Clark said:

This car has twin SU HS6. The point about flooding at idle still applies, so worth checking float chambers for float height adjustment and condition of needle valves.

First check was the carbs. No flooding detected. New needle valves were very recently installed. 

It is our current belief that increased crankcase pressure, caused by the incorrect carb setup when I purchased the car, combined with the rich idle mixture, has caused the plugs to oil up. I did a 100 mile trip to Canterbury and back today, often hitting 70mph, and the car performed flawlessly. If the plugs drown in oil again, then I'd be worried. 

3 hours ago, Nick Jones said:

Many thanks! I'm not sure what's so special about those plugs vs the regular WR78 but I trust the wisdom of this crowd and have ordered a replacement set. 

 

 

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R is the plug has a suppression/ resistance built in   with our small HT of 22kv  this can cause misfires due to the HT having a fight  on modern cars with very much higher HT its not 

a problem  Ive been involved with a number of cars in trouble and yep they had R in the plug suffix   it may be down to other things like the ceramic contamination but 

a change to straight   plugs works wonders 

Pete

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3 hours ago, Pete Lewis said:

R is the plug has a suppression/ resistance built in   with our small HT of 22kv  this can cause misfires due to the HT having a fight  on modern cars with very much higher HT its not 

a problem  Ive been involved with a number of cars in trouble and yep they had R in the plug suffix   it may be down to other things like the ceramic contamination but 

a change to straight   plugs works wonders 

Pete

Sadly, that would spell trouble as the W78 are discontinued. I've managed to assemble a set by purchasing from multiple sellers but I fear another alternative best plug will need to be found. 

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35 minutes ago, avivalasvegas said:

I fear another alternative best plug will need to be found

True there are a few posts on the forum just search spark plugs. Everyone one has an opinion what works for them! 

Iain 

 

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18 minutes ago, Iain T said:

True there are a few posts on the forum just search spark plugs. Everyone one has an opinion what works for them! 

Iain 

 

Yes, I read quite a few. The consensus appeared to be that the W78 was generally noticeably superior over the NGK's I am currently running. 

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