Spluttering

[470HW]

Hi

I've just finished rebuilding my engine and road tested it earlier in the week. The engine ran well in the workshop. Some slight lumpiness at the higher rev range but given its a new engine and was confident that the tuning was correct, 10 degrees BTDC ignition, newly rebuilt carbs balanced and correct mixture, valve timing correct progressed to a road test.

Road test went well initially. Did about 2 miles in a range of driving conditions. Lumpiness was present but otherwise ran well. Put the car on the road and after around 6 - 10 miles the problem got progressively worse. Started loosing power after 3.5k revs with accompanied coughing and spluttering. This is the same if the engine is under load or standing on the drive. I wasn't confident in the distributor or the ignition and found some intermittent shorting accross the coil terminals which I sorted by fitting a new distributor and insulating the coil lead however the problem persits.

I'm changing tack now and suspect fueling may be the issue. I note that there has always been an additional fuel filter inline before the fuel pump. I've checked the fuel tank that seems clear and think I'll remove this filter completely and give it another try (the pre carb filter in glass bowl is new). Does this sound like I may be on the right track. I could do with some re-assurance / second opinions as I'm worrying about head gasket problems etc.

Ian

[piman]

Hello Ian,

it is similar to problems with debris in the fuel tank, although I note you say its 'seems clear'.
It is unfortunately not an easy thing to check although the fuel pump would tend to race if the inlet pipe in the tank is getting blocked?
One check is to run the car until it starts misbehaving, switch off and check how much fuel is in the float chambers, if low then it is a fuel supply problem.
As always there can be more than one problem at the same time which confuses things.
The mention of intermittent shorting of the coil terminals is unusual?
Is the new distributor the correct specification?

Alec

[470HW]

Thanks for the response Alec. I checked the fuel supply as you suggested and the float chambers were full. I've been on it all day and re-checked everything. I checked the torque on my head nuts which tightened a little but weren't excessively loose.

The engine seems to run fine up to 2.5k revs and then won't rev much over that. If I hold it there and very slowly open the throttle I can get it up to 3k revs but it starts to missfire / backfire through the exhaust. It seemed on that basis that its an ignition timing problem so I've re strobed it and all is correct. I checked the leads and firing order which was also correct. I checked the vaccum advance pipe which was leaking a bit but a clip sorted it out and still no change. I checked the distributor base plate which is moving easily enough. Besides, this problem started with the old distributor and I fitted a new one with no change in behaviour.

I took the plugs out and it was running extremely rich (sooty black plugs) although it doesn't smell of petrol and I can't see any visible smoke at the exhaust. I reset the mixture on the carbs following the manual (jets flush then 3.5 turns then adjust by pushing up on the pin and adjusting). Would an over rich mixture cause this problem? I took the auxilliary carburettor solenoid out and inspected the seating thinking it may be letting extra fuel into the intake but can't see anything wrong. It seems odd that it seals with a brass disc sitting on a brass seating. Is that correct. The solenoid lifts the disc so when its not live it seals via its own wieght and a light spring. Is this going to allow extra fuel into the intake? Am I missing something here?

I'm going to take the rocker covers off again and recheck the valve timing but I was confident that I'd got that right (correct tension on the chain and perfect alingment with the tool at tdc on both cams) and can't see how it would have slipped out especially since I used new fine adjustment sprokets when I rebuilt it and they where a good tight fit. Would valve timing cause this problem?

Not sure what I'm expecting off of the forum here. I suppose I'm thinking out loud to a certain extent. I've obviously gotten something wrong and will just have to keep going back over everything until I work it out. Any help much appreciated.

Ian

[piman]

Hello Ian,

yes a rich mixture can give spluttering.

I'm rather hazy with regard to the auxilliary carburettor but seem to remember that the brass disc is an air intake and that the fuel is metered by the needle, which is adjustable. However if your carburettor adjustment doesn't improve things it's worth further checks in that area. Do you have a manual with which to refer regarding it's adjustment?

The plugs will show only the last state of the engine in general terms. If it is still spluttering on a road test, that is the best time to switch off and check the plugs. The best way to check the ignition timing is by road test, with the aim to be just retarded off pinking but this can only be done once you are happy with the mixture. If you use your strobe and rev the engine above three thousand or so and look for an advance of around thirty degrees which checks if the mechanical advance is working.

I would leave the valve timing as it is not so likely to be the problem, unless all else fails

Alec.

[470HW]

Thanks once again for your advice Alec. There is a good explanation of the auxilliary carb in the manual and I used it to set it up initially so the mixture is just a little rich. Thanks for explaining the air shut off though, that was helpfull.

I've gotten to the bottom of this, this morning after proceeding with the valve timing check. Timing was spot on but valve clearances on number 6 where non existent at tdc. Rang the engineering shop who re-built the head and they're sending somebody out. Could very much use some advice now. I checked the clearances when it came back from the engineer initially after refiting the head and they were correct as the engineer pointed out. However they aren't now. This is extremely worrying since .004 is a big gap to have disapeared. Is this something you might expect on a newly re-built cylinder head? Could this be down to 'bedding in'. Seems unlikely to me and that a bigger problem is the root cause but what exactly I don't know. Valve seats? Also a compression test shows low, inconsistent compression accross all six cylinders 100 - 125. Not the 150 - 175 I would have expected so it seems the clearances are out accross all six cylinders.

Any help, advice thoughts greatfully received.

Ian

[piman]

Hello Ian,

jsut checking what you mean by TDC when checking no 6 cylinder?
Normally doing the valve clearances engine position is irrelevant and each valve is checked when vertical to the guide bucket. Compression pressure is more a balance of each cylinders readings relative to each other, rather than a specific pressure (My Triumph has about 150 psi with a 10:1 compression but a lively cam which tends to drop the pressure at crank speed), although I wouldn't expect to see a 25 Psi difference on a newly built engine.
If there is no clearance on no 6 inlet that is unusual and will need checking to see why?
Not much more to add at the present but see what the engineer comes up with?

Alec

[piman]

Hello Ian,

any progress, I would be interested in developments?

Alec

[pegasus352857]

This may be relevant or not, but I had a problem with my 2.4 when I drove it home from storage (previous owner had same problem and two garages didn't fix it!). Solexes rather than SUs of course. It was running fine, but a pain in the arse to start, and fell flat on its face and backfired through the carburettors at 1500-2000rpm at the slightest whiff of acceleration. It was also running rich, float bowls and filter bowl both full.
Worth checking yours hasn't had the same done to it as mine; additional paper filter shoved into the bowl but the original mesh screen left in the housing, and full of crud! I ran the whole filter assembly through the ultrasonic cleaner and ditched the paper filter, has been fine since. I can also see if there is sediment in the filter bowl as per the original design intent, as the feed from the tank went into the inside of the paper filter so you had no idea what it was like... So far I have out about 100 miles on it and the bowl is pretty clear. It was a wonder it was doing more than idle at all until now as the paper filter crammed full of crud despite the paperwork showing two garages had allegedly flushed the tank and fuel system within six months about 12 months before I bought it...

[470HW]

Hi

Updating for those who are interested and as a lasting and hopefully useful record to those who follow. Thanks Alec, I had indeed missunderstood how to measure the valve clearances and was totally wrong in concluding the valve clearances where incorrect. The engineering company sent out an experienced mechanic who confirmed they were correct. He spent some time setting up the carbs. Incedently the carbs (SUs) have been rebuilt and there is no problem with filters etc. He did note that the packing and gaskets on the inlet have been fitted at 90 degrees to where they should have been. This may be a possible cause but he was also certain that the missfire was down to the ignition module. I had fitted an Accuspark distributor and ignitor. I've since replaced it with a Pertronix from SNG Barratt and refitted it to the originial distributor. Unfortunately the engine wouldn't start at all after that. I ran the tests supplied buy Pertronix and found the module failed the Positive Earth test indicating a faulty transistor or Hall Cell and returned it. I didn't find any issues with the supply voltage or earth but have re-wired the ignition circuit in preparation for the new module that SNG are sending out today.

This will be the fourth electronic ignitor I've fitted now. The first broke down completely so it was obvious. The second I believe is responsible for the fault as reported in this thread ie a slight missfire under acceleration and unwillingness to progress past 3.5k revs. I have a couple of theories here. I believe the first and probably the second module could have broken down through overheating. I fitted a new oil pump/distributor drive when I rebuilt the engine and had to get the one that was supplied machined down by the engineering company as it didn't quite fit. It was still a little tight when it came back so it may have generated heat above the tollerance of the module although the supplier doesn't seem to think this would be a problem. I'm reasonably confident that having done a 1000 miles now on running in oil that things have settled down. Another theory is that the positive earth ignitors are extremely volatile in terms of voltage fluctuations. I've tested the volatges at rest and during cranking and found them to be well within the manufacturers thresholds but can't be sure if there are fluctations under load hence the re-wire. The third was either defective as supplied (unlikely) or ......... well, I'm not sure other than the supply voltage theory. I'm confident with the earth. I ground all earth points back to bare metal when I detailed the engine bay, coated the metal with zinc paint and tested resistance/continuity.

Its also possible I have more than one fault here. I'm going to continue until I have confidence in the ignition or revert to points and continue that way. I'll post the results when I've concluded but does anybody else have experience with electronic ignitors on positive earth vehicles. I'd be interested to see if they're generally susceptable to problems in this application.

Thanks

Ian

[470HW]

Got the replacemtent ignitor from SNG Barratt today. Fitted it and the engine started, ran for about 6 seconds and stopped. No spark at the plugs. Fitted points and condenser (new). Still no spark at the plugs. Tested the coil, 3 ohms resistance across the positive and negative terminals. 10k resistance accross the positive terminal and ht output. Cranked the engine holding the lead from the coil output close to earth and no spark. Put an analogue multimeter accross the coil terminals and cranked the engine and saw correct voltage fluctuation as expected.

I'm thinking I have a broken down coil even though its showing the right resistence. Tried another coil although I'm not sure if its any good but got the same results. I'm really stumped here. Could really use some help.

Ian

[piman]

Hello Ian,

peculiar, especially as it did start.

You don't mention, but have you tried another high tension lead for your test, as your measurement of the coil seems correct and that there is a fluctuating voltage showing on the primary?

Alec

[470HW]

Hi Alec, thanks for getting back. No, I haven't tried a new lead. I tested continuity and resistence of the existing lead and since it seemed alright I didn't. I'll get hold of a lead and give it a try. I ordered a new coil this morning as it was only £20, will give that a go too. Thanks once again. Ian