998 Mini Compression Test Results
Discussion
So...
Been having some running issues with the old Mini Magoo for a while
(1981 - 998cc, single HS4 on water heated manifold (not connected), pancake K&N (freshly cleaned and oiled), red non waxstat jet. Minispares RC40 exhaust system, points ignition (red sliding) and Lucas 101 coil - both recently replaced in the last 4-6 months)
She's been starting beautifully from cold, a little lumpy sometimes to start with but generally ok-ish although definately under powered!. Problem has been when she's hot - not wanting to idle and being a bugger to start when warm?
On the L2B run I noticed a 'small' crack in the LCB 'Y' pipe (across the top of the 'Y') which I think was heating the fuel in the float bowl?
I've since welded that and resealed the rest of the system for good measure - but sadly the issues persist!
Tonight I have checked for sparks, there are plenty and bright and blue etc. I have observed the fuel filling the glass filter nicely and checked the operation of the float valve. The dashpot moves freely and has oil - it's either 20/50 engine oil or 3in1? can't remember what I put in last time.
As a precaution I have compression tested her showing around 120psi +/-5psi across all four cylinders! Internally she is completely standard, showing no signs of scoring etc at 115,000 miles with a copper head gasket installed about a year ago.
Ultimately my questions are:
1: Is 120psi enough for a standard 998 of this ilk? (8.3/1 comp???)
2: Could my running issues be timing/distributor related coupled with the fact that the manifold crack may have been present when I last replaced the points, condenser and coil. I adjusted the timing at the same time. (set at around 8 degrees from memory)
Thanks in advance!
Ben M
Been having some running issues with the old Mini Magoo for a while
(1981 - 998cc, single HS4 on water heated manifold (not connected), pancake K&N (freshly cleaned and oiled), red non waxstat jet. Minispares RC40 exhaust system, points ignition (red sliding) and Lucas 101 coil - both recently replaced in the last 4-6 months)
She's been starting beautifully from cold, a little lumpy sometimes to start with but generally ok-ish although definately under powered!. Problem has been when she's hot - not wanting to idle and being a bugger to start when warm?
On the L2B run I noticed a 'small' crack in the LCB 'Y' pipe (across the top of the 'Y') which I think was heating the fuel in the float bowl?
I've since welded that and resealed the rest of the system for good measure - but sadly the issues persist!
Tonight I have checked for sparks, there are plenty and bright and blue etc. I have observed the fuel filling the glass filter nicely and checked the operation of the float valve. The dashpot moves freely and has oil - it's either 20/50 engine oil or 3in1? can't remember what I put in last time.
As a precaution I have compression tested her showing around 120psi +/-5psi across all four cylinders! Internally she is completely standard, showing no signs of scoring etc at 115,000 miles with a copper head gasket installed about a year ago.
Ultimately my questions are:
1: Is 120psi enough for a standard 998 of this ilk? (8.3/1 comp???)
2: Could my running issues be timing/distributor related coupled with the fact that the manifold crack may have been present when I last replaced the points, condenser and coil. I adjusted the timing at the same time. (set at around 8 degrees from memory)
Thanks in advance!
Ben M
Cooperman said:
120 psi per cylinder does seem a bit low.
When you did the compression test were all the other plugs out and the throttle fully open whilst you cranked it over for a minimum of 5 seconds.
Ah ok, no plugs were out one at a time and throttle fully closed. Mmm should have googled before hand (muppet!) - will try again tomorrow! lolWhen you did the compression test were all the other plugs out and the throttle fully open whilst you cranked it over for a minimum of 5 seconds.
GTRMikie said:
davepoth said:
Have you tried a new condensor?
Worth a try. I've had brand new condensers that have been cr*p!Tested her again tonight, plugs out, full throttle - 125psi! Although it does sound like my compression tester is leaking!? I've calculated a turn over RPM of around 300rpm - the needle rises to 100psi then flicks to 125 in time with the rpm. I'm attempting to reseal the tester tonight for another go in the morning?!
Also is 180psi really what I sould be seeing? There's not a great deal of info on this that I can easily find - (I've been told a race spec 1500 Haybusa runs around 220psi, a Ford Eco Boost around 80psi and a calculation I found last night gives a max potential psi of 166???) Confused dot com!
Ben Magoo said:
I'm not fitting another condenser ever! Next time it fails she's having electronic ignition! I'm very confident the sparks are good.
Tested her again tonight, plugs out, full throttle - 125psi! Although it does sound like my compression tester is leaking!? I've calculated a turn over RPM of around 300rpm - the needle rises to 100psi then flicks to 125 in time with the rpm. I'm attempting to reseal the tester tonight for another go in the morning?!
Also is 180psi really what I sould be seeing? There's not a great deal of info on this that I can easily find - (I've been told a race spec 1500 Haybusa runs around 220psi, a Ford Eco Boost around 80psi and a calculation I found last night gives a max potential psi of 166???) Confused dot com!
The "compression testing" (which is actualy a cylinder peak pressure test) is not realy able to give stabile measurements, which can be compared to "factory standards". We usualy use cheap gauges which read erraticaly, the measured amount is dependant on the camshaft, cranking speed, oil type, temperature etc.Tested her again tonight, plugs out, full throttle - 125psi! Although it does sound like my compression tester is leaking!? I've calculated a turn over RPM of around 300rpm - the needle rises to 100psi then flicks to 125 in time with the rpm. I'm attempting to reseal the tester tonight for another go in the morning?!
Also is 180psi really what I sould be seeing? There's not a great deal of info on this that I can easily find - (I've been told a race spec 1500 Haybusa runs around 220psi, a Ford Eco Boost around 80psi and a calculation I found last night gives a max potential psi of 166???) Confused dot com!
The compression test is only good to compare the cylinders to each other. They should more-or less read the same if ok.
A much better way to test an engine is to complete a leak-down test. This will reveal the problems better.
Yep got tired of trying to be clever so I took her to bits for the sake of 30mins to get the head off.
Here's how she looked as the head came off:
27062012460 by Ben Magee Creative Arts, on Flickr
Down bore #3:
27062012456 by Ben Magee Creative Arts, on Flickr
How the head looked:
27062012461 by Ben Magee Creative Arts, on Flickr
Is it possible to tell if it's an unleaded head? I have it in my mind that the head is not the original - the casting says it's a CAM4810?:
27062012467 by Ben Magee Creative Arts, on Flickr
Oh and this is what I got off the magnet?:
27062012453 by Ben Magee Creative Arts, on Flickr
Got a strait edge along block and head, couldn't slide a 0.05mm (0.002") feeler between the rule and the head so I'm pretty confident she's flat!
???
Here's how she looked as the head came off:
27062012460 by Ben Magee Creative Arts, on Flickr
Down bore #3:
27062012456 by Ben Magee Creative Arts, on Flickr
How the head looked:
27062012461 by Ben Magee Creative Arts, on Flickr
Is it possible to tell if it's an unleaded head? I have it in my mind that the head is not the original - the casting says it's a CAM4810?:
27062012467 by Ben Magee Creative Arts, on Flickr
Oh and this is what I got off the magnet?:
27062012453 by Ben Magee Creative Arts, on Flickr
Got a strait edge along block and head, couldn't slide a 0.05mm (0.002") feeler between the rule and the head so I'm pretty confident she's flat!
???
Edited by Ben Magoo on Wednesday 27th June 22:48
Edited by Ben Magoo on Wednesday 27th June 22:50
id say that 99% that is an unleaded head, you can just see the ring of the seat around the exhaust valve - its the inner ring clse to the valve you have to look for, the wider ring is just a machining mark in the head.
if you take the valve out you will know for sure as you will see the seat in the throat
if you take the valve out you will know for sure as you will see the seat in the throat
Spent a good 3-4 hours over a few nights stripping the valves out etc. All seems ok - gone from this:
_MG_0476 by Ben Magee Creative Arts, on Flickr
With some cleaning and lapping to this:
_MG_0481 by Ben Magee Creative Arts, on Flickr
With an overall look of:
_MG_0485 by Ben Magee Creative Arts, on Flickr
Generally everything looks nice and consistent, the head and block are flat, valve gear appears fine?
Once back together I'm thinking timing issues with specific attention to vacume advance maybe???
Also what are peoples experiences with worn carb jets? My red, none waxstat is about 8 years old and has done over 20k?
_MG_0476 by Ben Magee Creative Arts, on Flickr
With some cleaning and lapping to this:
_MG_0481 by Ben Magee Creative Arts, on Flickr
With an overall look of:
_MG_0485 by Ben Magee Creative Arts, on Flickr
Generally everything looks nice and consistent, the head and block are flat, valve gear appears fine?
Once back together I'm thinking timing issues with specific attention to vacume advance maybe???
Also what are peoples experiences with worn carb jets? My red, none waxstat is about 8 years old and has done over 20k?
Lost my rag and burnt it out! (well, I wanted to tonight!)
Got it all back together - no go.
An exercise in futility (and for the dake of £3.99) I fitted a new condenser tonight.
No go.
At which point I spat my dummy out packed up and decided to play a game I could win! (The washing up)
Going to calm down and start again at the begining tomorrow - battery's on charge ready for some churning Tuesday evening.
She's being a complete f
king bh at the moment and I don't know why! FRUSTRATED DOT COM! 
Got it all back together - no go.
An exercise in futility (and for the dake of £3.99) I fitted a new condenser tonight.
No go.
At which point I spat my dummy out packed up and decided to play a game I could win! (The washing up)
Going to calm down and start again at the begining tomorrow - battery's on charge ready for some churning Tuesday evening.
She's being a complete f
king bh at the moment and I don't know why! FRUSTRATED DOT COM! What am I missing?
Air filter is clean and freshly oiled, carb is clean and free flowing, fuel's getting through.
The head gasket is fresh, the valves and clear and lapped.
The plugs are sparking the points are clean and set, the timing hasn't moved.
The exhaust is fully sealed and good to go.
She just ain't firing? I get the odd cough but she don't want to fking go!?
What the fking hell and I missing???
Air filter is clean and freshly oiled, carb is clean and free flowing, fuel's getting through.
The head gasket is fresh, the valves and clear and lapped.
The plugs are sparking the points are clean and set, the timing hasn't moved.
The exhaust is fully sealed and good to go.
She just ain't firing? I get the odd cough but she don't want to f
king go!?What the f
king hell and I missing???Gassing Station | Classic Minis | Top of Page | What's New | My Stuff