Mother of all engine problems
Discussion
I have a highly tuned Austin 7 that is in a single seat race car. The engine overheated and required a rebuild with new pistons, valves and valve springs. After the rebuild the engine would cutout, backfire, pop and spit around 4000 rpms and continue up to the max rpm. Before the rebuild 7500-8000 rpms was not a problem. Trying to correct the problem we have put heavier valve springs in as it acted like it had valve float, but that didn't help. Then a new coil, new distributor, new spark plugs, new plug wires, and a rebuilt carburator have all been tried. We are stumped!
Edited by geodavky on Thursday 13th April 14:20
Probably more up Peter Burgess's territory.
But exactly why was the engine rebuilt, and what was changed during that rebuild ?
What are mixtures like just prior to the problem ? and in general ? Has the new engine ever been tuned properly ?
Is the fuel system good ?
Does it do this at all loads/throttle openings but just the specific rpm range ?
But exactly why was the engine rebuilt, and what was changed during that rebuild ?
What are mixtures like just prior to the problem ? and in general ? Has the new engine ever been tuned properly ?
Is the fuel system good ?
Does it do this at all loads/throttle openings but just the specific rpm range ?
If everything is perfect maybe that means the cam is a tooth out? See how symmetrical the timing figures are at TDC and. with the head off I suppose, check when no1 in and no 1 ex valve are open about the same amount....you can make an allowance for this if cam not symmetrical.
We did have one 7 special with the wrong distributor in, they rotate the opposite way to most lucas distributors so it was retarding the timing off idle! was head scratching stuff for a while, static timing was about 20 then it got less with revs! We had to lock the distributor out and settle for 26ish max! It only takes a few degrees out to make the engine missfire!
Peter
We did have one 7 special with the wrong distributor in, they rotate the opposite way to most lucas distributors so it was retarding the timing off idle! was head scratching stuff for a while, static timing was about 20 then it got less with revs! We had to lock the distributor out and settle for 26ish max! It only takes a few degrees out to make the engine missfire!
Peter
geodavky said:
I have a highly tuned Austin 7 that is in a single seat race car. The engine overheated and required a rebuild with new pistons, valves and valve springs. After the rebuild the engine would cutout, backfire, pop and spit around 4000 rpms and continue up to the max rpm. Before the rebuild 7500-8000 rpms was not a problem. Trying to correct the problem we have put heavier valve springs in as it acted like it had valve float, but that didn't help. Then a new coil, new distributor, new spark plugs, new plug wires, and a rebuilt carburator have all been tried. We are stumped!
Given that this is a highly tuned race engine I'd like to assume there are competent professionals involved somewhere along the line including hopefully a dyno specialist to set everything up properly but you don't give any indication of that or that any actual diagnostics have been done other than swapping parts. Nothing to say you've ever done such basics as a compression test, ignition advance curve test, had the fueling A/F ratio properly measured on the rollers or even done anything as simple as check the fuel pump output. Just swapped a few bits and given up. Not very encouraging.Well firstly the engine doesn't behave even the tiniest weeniest bit like it has a valve spring problem so I don't know which bright spark diagnosed that. Not the engine builder I hope. Weak springs and valve float would make the engine lose power at a certain rpm and just refuse to rev any higher. It wouldn't make it backfire, pop, spit but continue to rev and least of all how could valve springs ever make the engine cutout? It would just run normally again as soon as the revs dropped below the critical point.
Nor could it possibly be some minor cam timing error, even a tooth out, which could certainly mess up the torque curve but again could never cause backfiring and cutting out or running fine up to one very specific rpm and not fine above that.
It's clearly fuel or ignition and 30 seconds on a rolling road dyno would tell you which but if you're stuck guessing at it then go through everything properly again. Fuel pump output, electrical connections etc.
You must have seen different race cams and ignition than us Dave, we have found, cam timing and ignition critical to avoid missfires on out and out race cams, very little leeway for error the 'hairier' the engine, one tooth out on race cams has shown as horrendous missfires, as you say, owner should get it tuned to find / rectify fault.
Peter
Peter
I have encountered a similar issue when the wrong distributor cap was used, the poles were in a different position and as the centre of the distributor moved round with centrifugal advance the rotor arm would approach the wrong pole and cause cross firing, this had similar symptoms to the ones you describe.
Dave
Dave
If you need Forum help I'd suggest you try the Austin Seven Forum or, if you're a member, the VSCC Forum.
That way you'll get help from people with the right experience, which you can't count on here!
I know a thing or two about pre-war engineering but have managed not to have had an A7 so can't offer specific help but, as suggested before, wrong camshaft timing is a possibility.
What's a freeze plug, by the way?
That way you'll get help from people with the right experience, which you can't count on here!
I know a thing or two about pre-war engineering but have managed not to have had an A7 so can't offer specific help but, as suggested before, wrong camshaft timing is a possibility.
What's a freeze plug, by the way?
Edited by Allan L on Friday 14th April 15:55
Allan L said:
If you need Forum help I'd suggest you try the Austin Seven Forum or, if you're a member, the VSCC Forum.
That way you'll get help from people with the right experience, which you can't count on here!
I know a thing or two about pre-war engineering but have managed not to have had an A7 so can't offer specific help but, as suggested before, wrong camshaft timing is a possibility.
What's a freeze plug, by the way?
It's American for core plugThat way you'll get help from people with the right experience, which you can't count on here!
I know a thing or two about pre-war engineering but have managed not to have had an A7 so can't offer specific help but, as suggested before, wrong camshaft timing is a possibility.
What's a freeze plug, by the way?
Edited by Allan L on Friday 14th April 15:55
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