R Cubed

Mike you need to rule out a few things first.

1 Did it run and drive fine before you turned it to 12 volts ?

If so then you can rule out engine valve adjustment, distrobutor and points and fuel ect.

So why not put it back to 6 volts and try it on the road.

then you have a true starting point.

My offer of all the equipment off my 270 which was 12 volts you can borrow to prove a point one way or the other !!!!

Although everyone has a welth of experience with sugestions it does not help that you have so many things which people are suggesting could be wrong, you need a base to work from, then go through one thing at a time.

Good luck.

out of interest mike you didnt let jack near it at w&p did you

I dont think the issue is to do with no fuel I think Jack has learnt his lesson.

Mike this has just come to me what type of plugs are you using in the GMC, see my motorcycle thread !!!

Richard,

I see your theory but in reality, the Lucas specified coil for a 4.2 straight six Jag is the same one as fitted to a Reliant Robin, Land Rover 2a or 3, and a good number of other British and European vehicles, largest engine listed with same coil being a Rolls B81 straight 8 in a Dennis. There is no matter whether it is a 4 or 8 cyl and how large the engine is.

 

cheers Richard

Oh well got that wrong too, however it worked without any problems right up untill I put the Cummins 6BT in it.....

Oopps that might have let the cat out of the bag :wow:

Superior plug then but I have my suspisions now having lots of problems with strimmers and chainsaws and mowers not starting or dieing wonce running, fit a different make plug no more problems, and yes the NGK's that were fitted were the correct plugs as stated by the manufacturers....

Beware out there if you have an NGK plug which is newish and you are having starting issues or misfires after 10 or so miles try the plug first.

Thanks Richard,

 

The alternator exciter feed does go via a warning light - I forgot to mention that! Alternator has a healthy 13.8V at the battery.

 

As stated the coil is 12V standard ('standard' meaning non-ballasted). During the course of the search for the problem it has a had 5 coils including 3 brand new ones.

 

Thank you for your offer of parts. I'll look at the distributor tomorrow and go from there.

 

- MG

Just a thought how did select the "standard coil" I decided to go for a coil which was off an engine as close to the spec as mine so the 4.2 6 pot jag ws the choice the thinking behind this was that the time duration for the coil to re charge before the distributor triggers the coil to fire down the next lead would be the same as they are both 6 cylinders and the cc was just an extra bonus as it should give a simiar size spark.

Let us know how it goes, all the 270 bits are in the garage now as I have the other beasty in the GMC, only GMC I know which returns about 18 to the gallon :cool2:

Have any of the bike owners had any issues with NGK plugs failing or causing misfires ect ?

As Nick has pointed out Mike is using a conventional 12v coil & needs no ballast resistor.

 

The primary of this coil measured 3 ohms which seems right for a standard 12v coil. Using my coil testing machine it was able to demonstrate that it had a healthy output across a pointed spark gap of 12mm equivalent to about 8Kv.

All well and good I bow to your knowledge, but it is still playing up though !! there is not much to replace however in this situation when you are desparatly swapping things back and forth you can easily start fitting other bits which might be problematic and not be the actual cause for the fault.

Thats why I have offered all the bits on my 270 as I know that was fully sorted and used for probably 8 years with no problems at all.

Could anybody who has M8 or M20 tech manual scan some pics for me ?

I am after internal external pics of axles, torque rod lengths gearbox ratios, transfer box ratio and diff ratios ?

Thanks

Hi folks,

 

I'm being plagued by an ignition problem that is eluding all of us and probably has a simple fix. I hope someone may be able to offer some thoughts?

 

The GMC was converted from 6V to 12V by the installation of a 75A alternator and replacing the coil with a 12V one. The alternator is wired with the main charge wire to the stud on the starter and the exciter wire to the ignition switch. Initially all other components were un-changed. At the same time the truck was also changed from +ve to -ve earth and the coil was reversed to reflect this. The coil is standard type (as was the 6V one). The coil is wired as ignition switch to '+', '-' to distributor.

 

The truck ran ok on 6V & -ve earth so is believed to be mechanically ok.

 

On its first trip out it managed approx. 150 miles before breaking down with an intermittent cutting-out lasting maybe half a second before it would recover. This would get worse and worse until it became un-driveable. After leaving it for around 10mins to cool down it would go another 10 miles or so before starting to fail again. The frequency became worse and worse until the truck eventually had to be towed in. The truck will usually re-start and tick over but dies when you attempt to pull away.

 

Over the course of problem-solving we have changed in numerous coils, points, caps, rotor arms and condensers. We also tried running a live wire from the battery straight to the coil. Clive Elliot generously gave his time at W&PR last week to go through the whole stock of spare ignition components to weed out anything faulty (in the end this was only one of the spare condensers and a wrong-spec coil.). We fitted the best set of components and Clive was then kind enough to check them in-situ and was as satisfied as he could be that they were ok as presented. Understandably he couldn't comment on their performance under load or when hot but said these were standard tests for these components and should be conclusive. Unfortunately the truck failed after only 5 miles on the way home from W&PR and over the course of several stops every ignition component was changed for new items purchased at the show from Rex Ward. Nothing made any difference and it was again towed home.

 

The most important symptom is that the points get very hot and are losing material across the contacts. Normally this would be attributed to a failed condenser but as per the above tests all components appear to be fine.

 

My friends and I have experience of numerous other vehicles with 12V alternators and standard coil ignition systems which give no trouble at all so what on earth is going on?

 

My only thought is that I have missed something in the conversion from +ve to -ve earth, but what would that be?

 

Thanks in advance for any ideas!

 

- MG

Just had another thought I could bring up all my bits off my old 270 engine as I know they all work together and then you will have a base to work from for testing at least ?

Hi Mike, the small wire from the alternator should go to an ignition light ( charge light ) not sure how the alternator would react with a direct supply ! I did this on my GMC all I did was fit a new alternator and a coil off a straight 6 4.2 Jag car, closest match to the 270 cc wise and number of cylinders, did nothing else with the distributor at all.

With ref to coils it is very important that if you use a coil that should have a ballast and feed it direct you can get some very funny symptoms I speak from experiance on a 3.0 Ltr v6 transit a friend had !!!!!!

A ballasted coil runs through the resistor ( ballast ) normally all the time the only time the resistor is bypassed is when cranking the starter the idea being the heavy load of the starter dropps the volts by quite alot and so by bypassing the resistor the coils gets a much closer to opperating voltage than with out it so giving a good spark to get the engine started.

Try getting a 6v battery and feed just the old 6v coil and see what happens should run fine if so its the new coil you are using !!

Good luck

Had a similar problem after fitting new plugs leads and rotor arm on our Sierra, turned out to be the new NGK plugs which I had fitted, there was a fault with them and gave a very naff spark, fitted the old Motorcraft plugs back in and got a nice fat blue spark again.

In a separate incident a friend of mine had re built a Triumph 5SW and he could only get 10 miles or so before it would start missing or if stoped would not start again, he also had a NGK plug in it, after I told him of my problems he changed it to a Champion and has not had a problem since.

Check the plugs.

You put a Cummins in it..

Maaaaayyyyy bbbeeeee.

But which one, sensable or silly..!!!!

Headlights?

Whats wrong with the headlights no change there!! no very cold....

Oops just noticed this pic was taken before I refitted the head lights so you are right but still wrong. down lower say under the fuel tank.

2nd photo seems to show a hump in canvas at rear.

Yes but no, thats been there for years and years, wrong end......

not clear in photos but can't see exhaust where I would expect to on a 352.

Ah getting warmer, one obvious thing in the right hand side pic, and one very subtle thing in the left hand side pic...

May still not give the game away though :cool2:

Does not sound good, either you have got something jamming a valve open or the rubber diaphragm has a split in it this could allow fuel into the engine sump depending how the pump is fitted to the engine.

You need to investigate soon and not use it the fuel will effect the performance of the oil.

Of course a 6BT is 360ci and only 100cc larger than a 354 ..... might be nearer the mark.

Arrrrrr may be... have a look at the "has anyone fitted a non standard engine to their mv" just above this thread.

Well here we go then I have been up to something over the past 18 months.. Can you spot the differences in the pics from an ordinary GMC.

They are subtle you might need to know your GMC's. :whistle:

that is what it sounded like

Your hearing must be off some what may be by a few cubes :cool2:

RS Components...

You could do, it's just easier if you use a crane!

Fair point

Probably health and safety these days.

Yep the power of the internet, large companies have departments set aside to monitor social media sites just for this instant as a bad word or un happy customer can do much damage these days.

Good to hear its resolved.

Like 354 cubic inch ........ for example ?

............. nearer 360 cu in. :cool2: :cool2: :cool2: :wow:

 

The 6B or 6BT will go into a Jimmy without too much hassle.

 

Phil.

Hmmmmm Cummins 6BT in a Jimmy without too much hassle. I dont think so.

Depends what you term as hassle....

Stay tuned theres more.

Glad everything went well for you Rosie, in my experience you can't beat a well looked after GMC for reliability. Looking forward to seeing your photos on here.

Especially when its been Cubed......

Would do a good job as a leaf blower too.