Spridget Tech Talk (MG Midget & Austin-Healey Sprite)
For the first time since I bought her three years ago (okay, granted, I’ve started working on her a month ago) my 1965 MG Midget is now idling. Still a bit rough, though; the mixture is way off. I’ll figure out how to crack that nut next, but right now I’m just happy that those dual carbs are balanced and doing their job.
Then again, no. Ends up that it was idling just because I had the RPMs so high that the engine had to choice but to run. A friend of mine came by and checked the compression. The verdict? 25. Yeah, should be around 150, so I'm going to remove the cylinder head and hopefully find that the valves are bad. Otherwise it's the cylinders, which is a whole new proposition. Maybe I'll have an excuse to swap out the 1098 for a 1275 after all!
Squirting a small amount of oil into the cylinders through the spark plug openings and rechecking the compression can give some indication of whether it's rings or valves. If the wet compression is the same, it points to valves. If the compression comes up it points to rings. Good luck!
Thanks! The compression came up the same, so maybe it's the valves. We'll see soon enough. I'm going to tear it apart this weekend and see what the damage is to my engine and consequently to my wallet.
it has been my experience with my 72 roundarch that what i have thought is the problem is almost always something else. an inability to idle at a low rpm or inability to correctly set the mixture may be the result of something much simpler and less expensive.
in my case, i replaced the worn throttle shafts on my twin hs carbs, as they were incredibly worn and allowing air to leak in.
less than 50 to fix, idles great. always fix the CHEAPEST thing first...
I hadn't thought of that, Joe. I'll have to check out the carbs.
If the compression is 25psi I don't think messing with the carbs will bring you results. With compression that low you may want to make sure you don't have problems that cause more damage with running it.
Sounds to me like the head will need to come off regardless of the actual issue. It does sound like very sticky valves or worse on the head... but it could be nasty rings as well (or both).
Feel that bore with the fingernail (or better yet borrow a bore gauge) for a noticeable ridge.
I took apart a "B" motor that ran but not well and had pretty poor compression... had a fractured #3 bearing cap and nearly every single ring was broken... a couple pistons had little "cups" in their sides form teh ring bits working around in there for who knows how long.
Yikes.
Sorry... hope it's only the valves.
Compression is 25 psi on all 4 cylinders? It's a miracle it will run at all...are you sure you did the test right?
I've had engines in the 60 to 80 psi range that quit running with no chance of getting them started again...worn slap out!
Take it apart & see what you find, but I think I'd be looking around for a good used 1275...
Had a friend of my dad's (while tuning his Sunbeam) relate a story about a TR4 that just would not stop consuming oil and didn't have much guts.
After trying this and adjusting that they finally took apart the motor...
no rings.
at all.
Ran "OK" but yowza!
One of those Union motors!
Wow, you guys really know how to cheer a guy up! :) I'll start breaking the head off this weekend and see what is waiting for me in the dark recesses of this thing. My wife is actually hoping the engine is bad so that I have to get the 1275. She really wants to see more power under the hood before I start driving the thing around. (I know, you all wish you're wives thought like that! :) )
Good news! I tore the head off and found a big chip in one of the valves--the one that was giving me 25 psi. The cylinders and pistons look good, so I'm going to get the head cleaned up, deal with the valves, and I should (cross my fingers) be good to go!
Great!
I was thinking all 4 cylinders were in the 25 psi range, that should be VERY fixeable (tu)!
