MGB Engine Motor Compression Test Step by Step Guide

I'm really trying to push my KZbin channel forward for 2020, if you like the videos it would help me immensely if you would be kind enough to subscribe to my channel
A step by step guide to checking the compression on an MGB. It also covers the difference between checking with the throttle open and closed. I tested the compression with the engine cold and made a note of the readings, at an average of 205psi that is what I would expect from this high compression engine. A figure of around 160psi would be good for an unmodified B Series, it's also important to check the difference between each of the cylinders as mis-matched readings often means there is a problem.
Any questions are always welcome
The Gunson Hi Gauge can be found here www.opieoils.co.uk/p-90101-gu...

[@scottrobinson4022]

Just got my first MBG (73), and your videos are immensely helpful. I’ve subscribed, so please keep posting!

[@davidrussellwilks]

Many thanks Scott, delighted to hear that you are finding the videos useful!

[@cavanhare6970]

Holy hell! That is a fantastic looking MG!

[@jett3332]

David, as the owner of a 63 MGB you give me hope and inspiration.

[@wyatt92563]

Hello from across the pond 😃🤙🏻. I really like your commentary and camera angels. I have so many questions about working my ‘79 MGB. Your videos really help. I’m going to search your vids. I’m wanting to know how to adjust the timing and adjust and service my Zenith Stromberg carb. Thx again 🙂

[@davidrussellwilks]

Really glad to hear that you are enjoying the videos, sadly I don't have any covering the later cars. If I can help at all with any questions don't hesitate to ask.

[@jett3332]

That MGB YOU HAVE IS JUST SI COOL EVERY INCH OF IT.

[@davidrussellwilks]

Many thanks, glad you are enjoying the videos

[@wyatt92563]

My SMOG test is coming up in March ‘20. So I want educate myself on all the emissions ‘junk’ on my B as well. I want to test the air pump and check all of the vacuum hoses. I’m going to refurbish the charcoal canisters as well. Take care 🙂🤙🏻

[@davidrussellwilks]

Many thanks for your message HoBeau, I've not had much experience with the smog equipment fitted to these cars, I think it was for USA markets only. Hope all goes well with the rebuild, do let me know how it goes.

[@douglasgaither1346]

David, did you modify the washer pump to function as a starter button, or is that simply an aftermarket starter button that looks just like the washer pump? Love your content by the way.

[@davidrussellwilks]

Thanks for your message, the switch is a push button Lucas type www.merlinmotorsport.co.uk/p/lucas-push-button-ignition-switch-ts-spb106. It does look correct for the period which is why I chose it.

[@mikepaton3614]

Just done a compression test today on "72" MGB GT, 1.8L, twin SU. showing with warm engine, 125 to 122 throttle closed, with throttle open 135 to 124 back two cylinders a bit lower. Just got car last week so doing pre service compression. Plugs looked a bit sooted. Its an H engine so high compression according to tag. Is the compression numbers a bit low or standard for a 50 year old unit. Thanks again for the videos, keep them coming.

[@davidrussellwilks]

The numbers between cylinders are fairly close, and this is a good thing. I imagine a fresh engine would have around 150-160psi so you certainly are not a long way off.

[@mikepaton3614]

@@davidrussellwilks thanks for the reply, just done compression again this morning after a run and engine at full temp showing just over 70c on gauge and all plugs out this time, front to back throttle fully open 135, 136, 134, 135. Doing oil & filter change Tommorrow and the clutch slave cylinder i never got a chance to do last week. Cheers.

[@michaelmenard8913]

fun to watch again, but I see no fan? what do you use to move air thru radiator?

[@davidrussellwilks]

I don't use the car on the road so usually having no fan isn't a problem. The only time I get an issue is if I am waiting in the assembly area etc. I think once the car is on circuit having a fan just gets in the way of the radiator and it runs cooler with out it.

[@michaelmenard8913]

@@davidrussellwilks what are you running for plugs currently? And Ds3 Shooter has a GT in your class I believe.

[@Cell1000bc]

What did you disconnect on you ignition? The lead from the coil to the distributor at the coil end? And what other wire?

[@davidrussellwilks]

I have electronic ignition so disconnected the positive wire going from the coil to the module. The Pertronix modules can fail if left on for a long period without the engine running.

[@DavidUKesb]

How do you disconnect the fuel pump and is this essential?

[@davidrussellwilks]

Thanks for your message David, in my car it's fairly easy to disconnect the fuel pump as it's in the boot rather than under the car. I usually do this a matter of course but it certainly isn't essential, especially with the SU carbs. I had already drained the fuel from the car too and I wouldn't want the pump to run dry.

[@DavidUKesb]

@@davidrussellwilks Many thanks for reply David. I was interested as plan to do a compression test on my MGB GT and Triumph Stag soon. The MGB does have SU carbs, whilst the Stag has Strombergs.

[@davidrussellwilks]

@@DavidUKesb Thanks David, hope all goes well with the tests, I think you'll be fine with the fuel pump still connected on both cars. It's well worth removing all the plugs so it's easier to spin over. I found nothing conclusive about whether it's better to run the test hot or cold. With a Weber an open throttle does make a difference I think less so with SU's. I also found with my car 10 cranks gave a more steady reading than 4 or 5.

[@DavidUKesb]

@@davidrussellwilks Many thanks for your advice.

[@nevergiveupever1069]

I thought just taking the coil LT lead of to stop the car from firing up is enough. Why disconnect the fuel pump and why does the pressure increase with the throttle open?

[@davidrussellwilks]

I disconnect the electronic ignition module as the Pertronix units don't like being left on if the engine is not running. I also disconnect the fuel pump to prevent flooding the engine with fuel with the throttle open. With the throttle closed you may not get a correct reading as it won't be allowing air into the cylinder.

[@nevergiveupever1069]

@@davidrussellwilks hi thanks for that. I had assumed you disconnected the fuel pump to stop it from flooding. My dry readings are terrible so next step is to do some wet readings which i guess will increase the readings,however I'm not sure it will tell me if its the rings or valves without me having to take of the head. I have ordered some piston seal additive to see it that works. Have you used piston seal additive before and did it work?

[@davidrussellwilks]

You can try a leakdown test to try and find out where the pressure is being lost, I do find they are a bit more difficult than the compression test. I've never put an additive in the race engine, if there's been a fault in the past I've had the head or engine rebuilt.