runs rough after head gasket replaced

F

fordmerc7

Member
Car runs rough after I replaced head gasket. Starts right away. Very rough at idle; Needs lots of spark advance and some throttle to run a bit better.

Head gasket replaced because oil leaking around it - gasket had delaminated in that area. Head was flat, no defects on inspection ! Car ran pretty well even then - better than now. I have checked timing many times. What did I do wrong? Or what should I do again? Or what else should I look for?
 
I did regap the points after replacing the head gasket. I can do it again.
The issue of distributor shaft rubbing on the gasket is something did not consider since the shaft seemed to be free before I put the head back on, and I really hope there is another solution forthcoming since I really don't want to start all over again! Thanks.
 
Did you pull the manifolds off? Check the gaskets and torque on them. Also check that the carburetor is snugged up to the intake manifold.
 
A friend replaced a head gasket and he had accidentally bumped a valve, It was not realized until he had a similar problem.
 
Neither manifold nor carb were touched since it ran perfectly. I lack an adapter for the compression test so I’ll look for (or create) one to do it; I lack equipment for the leak-down test so I will defer that for this moment. The valve issue is ?relevant: I noted that one of the exhaust valves (3) would rotate easily by hand, although it had no horizontal or vertical play. Is this a bad sign?
 
I think the compression check is in order, it'll let you know if some valve damage occurred and on which cylinder.

If the same distributor was used then nothing really should have to be done, just stick it in and go. But, its understandable to play with it since the engine doesn't run as it should.
 
If you have the equipment to fabricate an adapter for the compression test, you can probably fabricate one for the leakdown as well. All you need is something that mates an air hose to the 7/8" spark plug threads.
 
If you are going to fabricate a leakdown tester, it is best to get the correct restrictor to have somewhat correct results.

John
 
alexiskai said:
What would be an appropriate restrictor? 90 psi?
Click to expand...

Take a look at aviation type leak down tester
There should be a 0.040 dia restrictor between the gauges

Standard pressure is 100 psi on high gauge and leakage is read on low pressure gauge
90 psi on low gauge is 10% leakage
 
For a normal auto engine just a simple valve in the line to restrict the air pressure works fine. Turn it down to about 50# and start listening.
 
johnneilson said:
Take a look at aviation type leak down tester
There should be a 0.040 dia restrictor between the gauges

Standard pressure is 100 psi on high gauge and leakage is read on low pressure gauge
90 psi on low gauge is 10% leakage
Click to expand...
Actually for airplane engines we use 80 psi on the high side. The low side pressure desired is subjective depending on where you are losing pressure. 70 psi or greater on the low is what you want to see. This is done with the engine hot.The important issue for me when doing a leak down test on an airplane engine, which are required annually, is where the leakage is coming from. It’s normal to have some leakage from the crankcase vent, blowby past the rings. Leakages out the exhaust and intake are a clear indication of valve leakage. You listen at exhaust and intake for these. If I find valve leakage I then borescope the cylinder and take a look at the valves. Ideally you do not want any valve leakage, especially from the exhaust.
 
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GPierce said:
It’s normal to have some leakage from the crankcase vent, blowby past the rings. Leakages out the exhaust and intake are a clear indication of valve leakage. You listen at exhaust and intake for these. If I find valve leakage I then borescope the cylinder and take a look at the valves. Ideally you do not want any valve leakage, especially from the exhaust.
Click to expand...

Right, this is basically where I was coming from. With the kind of leakdown tests we're doing, getting a psi reading on the leakage is much less important than listening for leakage vs non-leakage. So all you need is a hose adapter and reasonable psi.
 
I started with just the air hose connection in the beginning and quickly found that I wanted to be able to quantify the amount of leakage. This is where the proper tool comes in handy.
While out racing, I test between runs to look for damage, a cylinder going down on a 4 banger is critical, too much to loss is not worth running until fixed.
Once back in the shop, the head comes off and is tested separately from the block assy. (overhead valve) This allows for ring condition to be tested independently from the head. Conversely, the head gets tested for valve seal before assembly back on the motor.

John
 
Not sure you need to go down the leak down / compression testing route quite yet.

If it was running better than it is now before the head gasket replacement there was something that was disturbed as part of the process. It is possible that you bent a valve but if you were reasonably careful that shouldn't have happened.

Did you by chance have a hard time getting the head off and used putty knives, screw drivers or pry bars to work it loose. If so the chance of a bent valve goes up.

Have you retorqued the head several times making sure that it is remaining at 55 lbs.? Takes several times and several heat cycles to get a head gasket to settle in for the long run.

Did you first try running the car without re-setting the timing? Re-setting the timing is not necessary after removing the distributor if you don't loosen and move the rotor.

Are you confident that you are going through all the necessary steps in setting the timing correctly? Spark adjustment lever moving fully from left to right in the opening of the distributor body? Raising spark lever all the way to the top before setting the timing?

Take a look at the moveable wire under the plate in the distributor to make sure it is in good condition.

Check the wire connections in the junction box on the fire wall to make sure they are all tight.

Pull the carb off and blow out the jets with compressed air. Maybe just maybe a chunk of crud got in there just by chance at the same time you were working on the head and causing a fuel problem not related to the head gasket replacement.

Just some ideas to try.
 
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