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Loss of power on 2 and 3 cyls 
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Joined: Mon Apr 15, 2013 9:20 pm
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Post Loss of power on 2 and 3 cyls
I’m posting for a friend who is temporariy without Internet.

The car is a 1380 A series GTM, with a freshly built engine. It has a 2nd hand modified head, new 649 cam, new MED 1.5 roller rockers, new lightened duplex, 1 ¾ HIF carb, std GTM 3-branch, std points ignition, std mechanical fuel pump, std duplex.

The car starts instantly, but doesn’t sound / feel right. Remove No 1 plug lead and the engine almost stops. Remove 2 or 3 and no drop of rpm, remove 4 and slight drop of revs. On the road, it feels as though it’s holding back, but drop it down a few cogs and it flies to 6000+rpm, but still doesn't feel right.

So
Starts and stops instantly
No coolant loss
No smoke
No oil loss or contamination
Not chuffing through breather
Checked compressions – 185 to 190 over all 4
Checked cylinder leakage – held on all 4
Checked valve clearances - ok
Checked ignition timing – ok
Checked mixture – ok
Checked spark – powerful and you could weld with it on all 4
Swapped plugs, points, cap, leads, rotor etc with another car – no difference and worked fine on other car.
Removed exhaust manifold and silencer and checked for blockage – none
Fitted new manifold gasket – no difference

We’ve spent days trying to get it to run properly and run out of ideas, any suggestions / pointers / theories no matter how wild would be welcomed. Thanks


Mon Apr 15, 2013 10:16 pm
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Joined: Wed May 09, 2012 7:27 pm
Posts: 33
Location: Hampshire
GTM: Libra
Post Re: Loss of power on 2 and 3 cyls
A few questions answered would help to diagnose your problem possibly.

Who modified the head ,was it from a well know tuning company.?
Who supplied the 649 cam was this a regrind or BL original second hand and ,have you checked the valve timing and lift of this cam for each cylinder ?
What SU needle are you using what spring colour ?
Who supplied the advance curve for the distributor or is it from a standard car what is the number stamped on the casting, have you checked your maximum advance what is it ???
Have you checked the jack shaft for wear and the ignition scatter?
I am confused how you reason it is loosing power on 2 and 3 cylinders?
Post me a PM and I will try to advise further.


Tue Apr 16, 2013 9:18 pm
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Joined: Tue Oct 30, 2007 8:44 am
Posts: 658
Location: Chester le Street
Post Re: Loss of power on 2 and 3 cyls
It's not unknown for the head gasket to leak between the fire rings on cylinders 2 and 3.

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Tue Apr 16, 2013 10:20 pm
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Joined: Wed May 09, 2012 7:27 pm
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Location: Hampshire
GTM: Libra
Post Re: Loss of power on 2 and 3 cyls
I have experienced fire ring failure on high mileage tuned engines and through lack of coolant due to hose failure during a track test.
The cylinder leakage test results will normally identify the problem.
With a recently built engine checks on the head flatness and block deck condition should have been made before assembly. I have replaced head gaskets on some dubious heads in the past and the engines have surprisingly been reliable when their owner have not been able to afford a re surfacing.This only proves the standard head gasket is quite accommodating in my opinion.

Piltv40


Tue Apr 16, 2013 11:44 pm
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Post Re: Loss of power on 2 and 3 cyls
Thanks for the advice and suggestions. Head was vaccum checked and decked both sides before fitting with the BK460 head gasket. Cam timing set and checked with DTI - all ok. No play in jack shaft bearings. Dizy fine.

Reason we know there's no power on 2 & 3 - removing the plug lead makes no drop in RPM.

Remove No 1 plug lead and engine almost stops - so its the cylinder that's doing all the work.

Remove No 4 plug lead and engine RPM drops - so this cylinder is doing some work.

Looked at it last night and the light came on :idea:

:!: :!: You can get very badly burnt doing this, on only do it on a stone cold engine :!: :!:

Started up and quickly felt the GTM 3-branch manifold tracts.

No 1 was hot, as was No 4, but the center pairing of 2 and 3 virtually stone cold.

Then looked and realised that the length of the tracts compared to the problem.

No 1 tract is the longest, and thats the cylinder thats doing all the work.

No 4 is the next longest, and thats also working - but not at full capacity.

Center tract is not only the shortest, but it also has to carry the exhaust pressure of 2 cylinders, and they are the 2 that are not working.

Identical engine works fine in a mini that is fitted with an LCB.

So, we needed to reduce the exhast gas, and not having a spare LCB, we removed the roller rockers and refitted standard rockers.

Fired it up, and hey presto, now sounds like it should, all cylinders performing ok, center exhaust tract gets hot.

So, the problem lies in the standard GTM 3-branch manifold, as it cannot flow sufficient exhaust gas.

Now going to modify and fit an LCB manifold, then re-fit the roller rockers, and I'll let you know how well it works.


Wed Apr 17, 2013 8:40 am
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Location: cheshire
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Post Re: Loss of power on 2 and 3 cyls
Have you checked you are getting spark on 2&3 this can be done by removing plug from cylinder putting in ht lead and resting on head start it up and see if it sparks. May need points adjusting inside dizzy or faulty dizzy cap but i would think more than likely points laters alex


Mon Apr 22, 2013 10:53 pm
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Post Re: Loss of power on 2 and 3 cyls
Thanks, already checked the spark.

As stated, it's so good you could weld with it.

Problem solved, as it is the original skinny GTM 3-branch manifold that is restricting the gas flow from 2 & 3.

The GTM 3-branch is probabbly ok with a small engine, but no good for big bore motors with wild cams.

Due to fit a decent 2-piece big-bore LCB soon, hopefully mated to twin inlet and twin outlet custom silencer.


Tue Apr 23, 2013 5:49 pm
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Joined: Tue Oct 30, 2007 8:44 am
Posts: 658
Location: Chester le Street
Post Re: Loss of power on 2 and 3 cyls
That's interesting.

The manifold may be restrictive on a tuned engine but I would have thought that it would still flow enough gas to let the engine idle without any problems.

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Wed Apr 24, 2013 7:38 pm
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Post Re: Loss of power on 2 and 3 cyls
We've now got it running ok on the standard GTM 3-branch.

It doesn't pull like it should, and you can tell it's restricted.

Engine and box coming out next week (going for it at Wiscome this weekend :twisted: ), then going to change diff and fit propper big-bore LCB etc.


Thu Apr 25, 2013 6:40 pm
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Post Re: Loss of power on 2 and 3 cyls
Engine is back in.

Now have a 4.3 diff and a 2" LCB.

Will be out at Wiscombe Hillclimb this weekend to sort out any niggles.


Mon Jun 17, 2013 1:56 pm
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