13/60 - Engine Not Running Properly After Head Gasket Replacement

[cbjroms]

My 13/60 was running quite sweetly last year but the middle cyclinders were a bit low on compression and I had concerns about the head gasket.

So I had the head off at the end of last year, had it checked out by a local macine shop, all valves were reseated and I put it back together before Chrsitmas.

Last weekend I checked the valve clearnces and set the timing but couldnt get it to fire. So today I have relaced plug leads/distributor cap (it is electronic ignition), checked the plug gaps and eventually managed to get it running on full choke. As the engine warms I can push the choke in until the knob is about 10mm proud of the dash at which point the engine is revving quite hard and sounds very rough. Pushing the choke in any further the engine just dies.

I have tried turning the distributor to check that the timing is right and also pulled the fuel hose off the carb to check for consistent fuel flow. All looks good.

Thing is that the engine needed very little choke at all to start and warm-up before I did the head gasket. Now it will only run (even when warmed-up) on part choke and it sounds very rough. I have checked each plug lead and they are receiving a good, strong spark and so I cant see that there is a problem with the coil. The carb is a CD150 which was fully reurbished last year and performed fine last summer.

What is the likey cause of this?

 

 

[glang]

It would be interesting to take the compression readings again just to ensure that everything is good now... 

[Guppy916]

looking at Rimmer's site I see there is more than one choice of head gasket ? have you got the correct one, Could the timing be 180 deg out, have you tried pulling one lead off at a time to check that they are all firing, 

 

[RobPearce]

I doubt it would run at all with timing 180 out. Head gasket on a 13/60 should be the flat type, so fitting "the wrong one" means recessed block type, which would run but spew coolant everywhere.

Two plug leads swapped could do it. You'd be running on two cylinders.

[ferny]

Have you adjusted the carb or just bolted it on? Sounds like you've done everything except set it up to the match the engine.

[cbjroms]

Thnaks for the comments above. I will check compressions today.

I have swapped the plug leads round a few times and checked that I have 1,3,4,2 10 or 15 times!

@Peter mentions setting-up the carb. I had it fully tuned before stripping the head and could see when the head was off that the mixture was about right. BUT, having replaced the head my assumption was that there was no need to retune. Wondering whether this is what I have missed?

[glang]

I wouldnt have thought the carb would be far wrong if it was running well previously but as its been removed there is a risk that you now have an air leak somewhere. This would produce a weak mixture and need more choke to compensate, much as you describe... 

[glang]

The other thing is that part of the internal mechanism of the carb could have been disturbed during its removal. With the airfilter off and engine stopped you should be able to see the air piston inside the carb mouth. It should be fully down but can be lifted with a finger, although there will be quite a lot of resistance, then when released will drop back down until it hits the bottom...  

[cbjroms]

Just checked compressions: 140/130/130/130

Then I removed air filter and the air piston looks to be in correct place:

 

When I was doing the compression test (throttle fully open, choke disconnected) I noticed a distinct lack of petrol fumes. Also the coil was buzzing.

Whilst I know that fuel is coming out of the hose supplying the carb, I wonder whether (@tight - part of the internal mechanism of the carb could have been disturbed during its removal. ) some sediment in the carb has been dislodged and block something. How could I check this?

[glang]

The piston looks good and can be raised and returns smoothly? I wouldnt expect a lot of fuel smell when doing the test because if all sparkplugs are out the engine sucks through those not the carb plus with the throttle fully open any air that does pass through wont produce enough pressure drop to suck fuel from the jet.

I dont like the coil buzzing - never heard that but if the sparks are good then thats not the problem.

Sediment in the reservoir at the bottom of the carb could jam the needle valve open or a stuck float would have the same effect but then you normally see excess fuel running out of the carb.

When you do get it running can you see the air piston move up and down as you rev the engine? Also from the photo your mixture adjuster (knob on underside of carb) looks to be set pretty weak but you say this cant have changed since it was running well? 

[RobPearce]

Quoted from glang-

 Also from the photo your mixture adjuster (knob on underside of carb) looks to be set pretty weak but you say this cant have changed since it was running well? 

Unless, of course, there was previously some rubber slivers or similar jamming the float valve open, and the mixture has been leaned way off to compensate, and removing the carb has dislodged them so the float level is now correct.

[cbjroms]

So, I took the carb off the engine, removed the bowl, cleaned-out a bit of grot, checked the valve and reassembled.

The engine now runs and no longer needs massive amounts of choke to keep it running. In fact, once warm it runs (just about) without choke.

But, the engine still sounds very rough and I am convinced that it is not running sweetly on all cyclinders. So I have replaced distributor cap, coil, plug leads and plugs but this has not made any difference and it sounds like there is something worng with the timing. Removing a plug lead (from whichever plug) makes no appreciable difference (ie bad or good) to the rough running. Moving the distributor either way affects the revs but not the roughness and (with the air filter removed) I frequently get a pop out of the carb when starting.

I have gone back to basics and checked the leads and it is definitely 1,3,4,2 (anticlockwise) with 1 being the front cylinder.

The only 2 things I have yet to re-check are the valve clearances and mixture (I increased and reduced richness to see if it improved the rough running but it made no difference). Could incorrect valve clearnces cause rough running? What other checks should I be doing?

[RobPearce]

It all depends on what you consider "rough". If the plug lead test made "no difference" then you probably don't have a persistent misfire on one cylinder, but it's likely your definition of "rough" is unusual. Pulling one lead should absolutely make the engine much rougher on any cylinder that's working properly.

Valve clearances can cause rough running, especially if one of them is way out, but again we'd need to know what you actually mean by "rough".

[cbjroms]

Thanks for your response @ Rob Pearce. I get your point completely, rough can mean all sorts of things.

I decided to go back over everything again as frustration was getting the better of me. Having used the Coloutune to get the mixture right and checked the valve clearnces again the engine is now running fine with a bit of accelerator.

I have narrowed the problem down to the way the engine runs at idle. Whether cold with a bit of choke, or warm, the revs go up and down at idle without any change of accelerator or choke. At it lowest revs the engine is just about stalling and either dies or revs start increasing to a very fast idle and then the cycle continues. So the only way to keep the engine running is to wind the idle screw right in so that idles at high revs.

So my thoughts are either a fuel flow problem (so the bowl is never getting full) but then it doesnt need much fuel at idle. Alternatively an air leak (as I have checked that the butterfly valve movement is crisp rather than floppy). The carb is tight to the manifold (via gasket and spacer),the air filter is off and the vacuum pipe is in place between carb and distributor. Looking inside the manifold I cant see any cracks around the bolt that holds the accelerator cable bracket in place but I did snap the exhaust manifold tab when I replaced the head gasket. 

 

 

[RobPearce]

If your remaining problem is unstable idle, and you've set the mixture with a colourtune, I think it may be that you're just set a bit lean. These old engines don't much like lean running and are way more sensitive at idle than elsewhere. So you may just need to give it one flat richer.

It's also well worth checking your dashpot oil - top it up with 20/50 engine oil, do NOT use anything thinner than that. This would mostly affect acceleration but it does help a bit at idle too.

You may also get some benefit from retarding the ignition a bit. Again, if the ignition is "optimal" then the idle air flow is too low to be stable - retarding and opening the idle screw a bit should help. (On late US market Spitfires they fitted a vacuum retard capsule to make the idle ignition really poor so that the air flow was enough for good fuel mixture control).

[Pete Lewis]

double check the diaphragm is not failed , or one made of thick rubber it should be a thin gossamer  flimsy .

make sure the location lugs are locating so the two holes in the  base of the air piston 

looking down are engine side not offset 

as you have had the carb on off check behind the float needle for the rubber slivers Rob mentions  these are formed when you push hose onto metal pipes and are crafty little sods 

at blocking the back of the float needle 

Pete

[cbjroms]

The problem appears to have been solved.

Convinced the problem was an air leak (just because symptoms seemed a close match) I did some testing with a water spray and found that there seemed to be a leak along the lower edge of of the intake manifold. Then I looked at the accelerator bracket which fits into the underside of the intake manaifold and, only then, spotted the problem.

I mentioned above that I had managed to snap the tab off the exhaust manifold when I took the head off. Obvioulsy the stud for the accelerator bracket goes through the tab (the tab snapped-off across this hole) into the threaded hole in the air intake and then a nut goes on top.  With the half the tab on the exhaust manifold missing, I had just tightened the nut to make the accclerator bracket tight. But I had not noticed that the nut was now jammed against the hole on the edge of the tab. So, rather than the stud being tight to the side of the hole, it was the nut in my case and this was preventing the lower edge of the inlet manifold fitting tight against its gasket.

Removing the inlet manifold and griding-off the tab has resulted in a sweet and even idle.