Tunebug engine diagnosis, thoughts anyone?

[AN5Sprite]

So... When I bought the Tunebug from Drew I purchased it knowing it had some "tuning issues". The issue as described was a high speed miss. Specifically at 3k it just began to run poorly and didn't want to rev. Sure enough, I experienced the same thing and began hunting for vacuum leaks. I found two possible leaks. Fixed them. No improvement. Ok....I go over the ignition sytem, carbs, valve adjustment, fuel filter, etc. I made a few adjustments, with no improvement on the problem. I'm confident everything is right. No vacuum leaks, idles fine, exhaust note seems fine to me, the plugs couldn't look any better. I hooked up a vacuum gauge to the intake. The reading is lower than it should be and the needle flickers fast. I'm not totally sure what that tells me other than it's a problem that's effecting one cylinder. Moving on to a compression test. I get 175psi on cylinders 2, 3 and 4 but only 100psi on 1. Here's the thing; there's no white foam under the valve cover, no water in the oil, no oil on the water. No over-heating. Low compression in #1 and no ability to rev under a load...

My questions:

1. Is there a way I might get out of this without pulling the head? I can't see one, but if there's a way I'd like to hear about it. Before I pull the head I want to know I have no other choice.
2. Is it possible to have a head gasket fail and not have over-heating, contaminated fluids etc? (...is it just beginning to fail, and the other symptoms come later if left alone?)
3. What's your guess as to what I find? Head gasket failure or something worse?

 

[Billm]

The first thing that I would do is check the valve gap settings. That is a special engine made by Hap so I don't know what the gap should be but I will bet that the gaps on #1 are not the same as on the rest of the engine.
If this is the case then set the gaps of #1 to be the same as those on the others and try running it again.
BillM

 

[bug_sixty]

have you checked the coil output?

 

[mightymidget]

do a dry and wet compression check. most likely not a head gasket failure. if valve train is set proper then you may have a weak intake valve seating issue. have you tried the dollar bill test at the tailpipe? shadetree but may help.

 

[nomad]

I agree with Bill, most likely closed up valve clearances. Barney Gaylord fo "MGA GuRu" tells how to check for cylinder leakage and to determine where it is coming from. Recently did it on my MGA and unfortunatly found that my valves were leaking. It invlolves making up a fitting to allow 20psi air pressure into the head then listening where the air is leaking.

Kurt

 

[Gerard]

Do a leak down test to narrow down the source (top, bottom, intake or exhaust)

 

[AN5Sprite]

The first thing that I would do is check the valve gap settings. That is a special engine made by Hap so I don't know what the gap should be but I will bet that the gaps on #1 are not the same as on the rest of the engine.
If this is the case then set the gaps of #1 to be the same as those on the others and try running it again.
BillM

Checked not once, not twice but thrice. setting is correct....Double checked the specs from APT and the tech sheets from Hap for the specific camshaft. .016"in/.018"ex engine warm....I didn't see any broken springs. I did see something that I wonder about though and I just went and reconfirmed this:
The rocker arm on the #1 exhaust valve is free to float fore and aft on the shaft about 1/16" as if a spacer/shim is missing? It's not free enough to fall off of the valve. I don't know. This is my first 1275 but I don't recall seeing that on the 948 and 1098 heads I've worked with. With those I recall a special washer that's not here.

 

[AN5Sprite]

Do a leak down test to narrow down the source (top, bottom, intake or exhaust)

I figured that was the next step, but I have to buy or build a leak down tester and possibly a better compressor...How much pressure do you need to perform an accurate test?

 

[AN5Sprite]

do a dry and wet compression check. most likely not a head gasket failure. if valve train is set proper then you may have a weak intake valve seating issue. have you tried the dollar bill test at the tailpipe? shadetree but may help.

I have tried that with a piece of paper-to see if it gets pulled in right? it doesn't.

 

[Jim_Gruber]

We need pics of the #1 Valve train to tell you if something is missing.

 

[Westfield_XI]

Do a leak down test to narrow down the source (top, bottom, intake or exhaust)

I figured that was the next step, but I have to buy or build a leak down tester and possibly a better compressor...How much pressure do you need to perform an accurate test?

There are two sorts of leak down test and tester. The simplest is just a fitting to allow you to put shop air into each cylinder individually. By listening for the sound of hissing air you can tell which components are failing. Noise from the carb is a leaky intake valve, from the exhaust pipe a leaky exhaust valve and from the breather or oil cap bad rings.

The second type has two air pressure gauges on a block with a restrictor between them. Regulated air pressure (usually 100 psi) is shown on the first gauge and then passes through the restrictor to the second gauge which is connected to the cylinder being tested and shows it's pressure. The two pressures are then compared to indicate engine health. There will always be some difference between the two even in a new racing engine, typically no more than 97%. In fact when I was consulting for American Honda's Superbike program they would replace any engine with more than 97% leak down. This type is also used in aviation and can be purchased from aircraft tool suppliers like Aviation Spruce.

In this case all you need do is put some air into the cylinder and listen for the leaks, so see if you can hollow out an old spark plug and braze an air fitting on to it so you can hook up your hose.

Some warnings!!! Make sure the fitting is fully screwed into the cylinder BEFORE you put pressure to it and put the car in neutral. If you put air into a cylinder, the piston will move and cause the car to lurch if it is in gear.

 

[drooartz]

Bummed that you're still having this issue, Steve. I had really hoped it was down to my inexperience in sorting things out. Vacuum leak would have been my next thing to check, but you've done that.

I'm starting to think you may need to pull the head and see if there is a valve sticking in #1. I had bought some extra gaskets when I was chasing what I thought was an exhaust leak -- there may be another head gasket in with them. Can't remember what was included in that set.

 

[Guest]

In't this a fairly fresh engine? I wonder if a cam lobe is going? Have you rotated the engine and watched the rockers?

 

[nomad]

Do a leak down test to narrow down the source (top, bottom, intake or exhaust)

I figured that was the next step, but I have to buy or build a leak down tester and possibly a better compressor...How much pressure do you need to perform an accurate test?

There are two sorts of leak down test and tester. The simplest is just a fitting to allow you to put shop air into each cylinder individually. By listening for the sound of hissing air you can tell which components are failing. Noise from the carb is a leaky intake valve, from the exhaust pipe a leaky exhaust valve and from the breather or oil cap bad rings.

The second type has two air pressure gauges on a block with a restrictor between them. Regulated air pressure (usually 100 psi) is shown on the first gauge and then passes through the restrictor to the second gauge which is connected to the cylinder being tested and shows it's pressure. The two pressures are then compared to indicate engine health. There will always be some difference between the two even in a new racing engine, typically no more than 97%. In fact when I was consulting for American Honda's Superbike program they would replace any engine with more than 97% leak down. This type is also used in aviation and can be purchased from aircraft tool suppliers like Aviation Spruce.

In this case all you need do is put some air into the cylinder and listen for the leaks, so see if you can hollow out an old spark plug and braze an air fitting on to it so you can hook up your hose.

Some warnings!!! Make sure the fitting is fully screwed into the cylinder BEFORE you put pressure to it and put the car in neutral. If you put air into a cylinder, the piston will move and cause the car to lurch if it is in gear.

Agree, I used a air chuck fitting brazed into a spark plug housing with the porcelain broken out. 20psi thru a regulator and the leakage was immediately evident.

Kurt.

 

[AN5Sprite]

In't this a fairly fresh engine? I wonder if a cam lobe is going? Have you rotated the engine and watched the rockers?

If I recall what Drew said he covered about 2k miles before the problem emerged. Since he's here he can confirm that...I have watched the rockers in action and I didn't see anything unusual. (can't say for sure that I would if there was though...)

I'll do the 20psi check tomorrow and report back. Thanks to all for that tip.

Thanks Drew, there is a spare head gasket in pile.

 

[drooartz]

That's the right number of miles. Other than re-torqueing the head at 500 miles and doing the basic tuning (valves, carb, timing, etc) I never got into the engine at all.

 

[Darwin]

Sounds like a bit of carbon on the valve seat. Or in the valve guide. You could try the old trick of squirting water in the carbs to free it up but that can be danerous as it could cause even more damage. May have to tear down the head to fix it.

 

[racingenglishcars]

IMHO, Many times testing an engine at zero speed can make it difficult to diagnose problems that occur at higher speeds.

It could be that a valve is sticking and shows up as a missfire at higher engine speeds, but the low compression reading could also be a poorly sealing valve or rings, that otherwise have nothing to do with a high speed miss. The ignition system is the culprit.

Most likely the problem is not rings as they won't loose compression at higher speeds and will only give a low compression reading at cranking speed. A slightly burned or mis-fitting valve will also only give low compression reading at cranking speed which will not affect combustion at higher speed

If it's the valve, you could possibly hear the rocker arm clacking when the valve floats, but not always. Neither would a sticking valve necessarily give a consistently low compression reading. Try measuring compression both cold and hot.

I'll take a chance and make a guess. Slightly sticking inlet valve that allows combustion gas into the inlet manifold at higher speeds, causing partial combustion of fuel that would otherwise reach cylinder #2. This would cause a power reduction of about half.

Just my 2kr

 

[Hap Waldrop]

So... When I bought the Tunebug from Drew I purchased it knowing it had some "tuning issues". The issue as described was a high speed miss. Specifically at 3k it just began to run poorly and didn't want to rev. Sure enough, I experienced the same thing and began hunting for vacuum leaks. I found two possible leaks. Fixed them. No improvement. Ok....I go over the ignition sytem, carbs, valve adjustment, fuel filter, etc. I made a few adjustments, with no improvement on the problem. I'm confident everything is right. No vacuum leaks, idles fine, exhaust note seems fine to me, the plugs couldn't look any better. I hooked up a vacuum gauge to the intake. The reading is lower than it should be and the needle flickers fast. I'm not totally sure what that tells me other than it's a problem that's effecting one cylinder. Moving on to a compression test. I get 175psi on cylinders 2, 3 and 4 but only 100psi on 1. Here's the thing; there's no white foam under the valve cover, no water in the oil, no oil on the water. No over-heating. Low compression in #1 and no ability to rev under a load...

My questions:

1. Is there a way I might get out of this without pulling the head? I can't see one, but if there's a way I'd like to hear about it. Before I pull the head I want to know I have no other choice.
2. Is it possible to have a head gasket fail and not have over-heating, contaminated fluids etc? (...is it just beginning to fail, and the other symptoms come later if left alone?)
3. What's your guess as to what I find? Head gasket failure or something worse?

I'm late on the scene here, wnet and raced my MGB for the first time this past weekend, so not checked here in a week or so. I recommend a leak down test as well, because it will tell if it's a valve or rings, and if a valve which one, so narrows it down. Yes head gasket can fail without overheating, I did it this weekend in my race car, the car ran wonderful on Saturday in the feature race, so much so, I won the race :banana: , but in Sunday's race starting on the outside overall pole, with the entire field behind me, as we approached the green flag, it missed and would not pick up and run, all my test showed I've blew a gasket between #3 and #4 clyinders, my guess is I was running a tick too much advance timing, I do not have any water leakage into the block, confirmed by a pressure check, no oil on the plugs and all the gauges looked wonderful when it happened. Answer to number #3 will be found with a leak down test and a pressure test.

Let me know what you find, I know you didn't buy the engine from me, Drew did, and it's been acouple of years, but it's still my engine and relatively fresh, so, I'll help you get it fixed anyway I can, and sort thru the problem, feel free to call me.

 

[AN5Sprite]

I went up to Napa and got the adapter I needed to hook the air hose up to the spark plug hole. I made sure both valves were closed, put the air to it and...

All the air comes right back out of the intake manifold. So there it is. It's the Intake valve. bummer.

On the bright side, I just read Hap's reply. Thank you Hap, I appreciate that. I'll call you once I get the head off.