Low power, gutless engine

Post by **rachel_and_ian** » Thu Aug 20, 2009 9:03 am

Hi,
JUst got the engine running after it had sat for a few years. It goes fine through 1st and 2nd but is fairly gutless in 3rd and I can't get it past 40 mph in 4th. Just trying to work out possible causes? It idles well enough and revs well at idle.

These are the things tried so far:

New distributor cap, plugs, rotor arm, points, coil.
Cleaned carb and adjusted the mixture.
Compression test- 123, 120, 113, 120.
Cleaned out fuel pump filter
Ran without air filter to rule this out.
Vacuum advance not working currently but I don't think this would cause it?

Any ideas? Obviously compression is low but I would still think it could go above 40 on a flat road. Just trying to decide whether to continue with this engine or pull it out and buy another?

Post by **bmcecosse** » Thu Aug 20, 2009 9:13 am

The compressions are horribly low - the piston rings will be 'stuck' after lying so long. You could try pouring a little diesel fuel into each cylinder and leaving it to soak down into the rings for a few days. Turn it over by hand - and add a litle more fuel each day. To do the job properly of course - remove head and sump and push the pistons out. May also be worth lifting the head and grinding in the valves - the 3rd reading is particularly low! Simple thing to check - if the carb piston free to ris and does it fall back with a clunk? Vacuum advance will make a slight difference. What engine size is it ??

Post by **rachel_and_ian** » Thu Aug 20, 2009 9:51 am

It's a 948 engine.
The carb piston rises and falls as it should, clunks back down fine.

Post by **bmcecosse** » Thu Aug 20, 2009 10:11 am

Well - as above really, try the diesel or pop the pistons out. I would do the head/valves anyway. You seem to have covered most of the other possibilities. A 948 is not going to be the fastest anyway - but should top 40 mph!

Post by **PSL184** » Thu Aug 20, 2009 10:13 am

You could try a half gallon of diesel in a full tank of petrol to give the whole system a clean out. This process is not agreed by all but I have been doing it for years without any detrimental effects - you do get a bit of smoke though !!!

Post by **bmcecosse** » Thu Aug 20, 2009 10:22 am

Good idea - but maybe after the initial diesel soaking ?

Post by **ssnjimb** » Thu Aug 20, 2009 11:01 am

Give the engine a good service.

Clean out the float chamber in the carb.

Static time the distributor with a bulb.

Clean the plugs and gap .16 i think.

Set the tappets to .12 cold

Also set the points to .16

Do as suggested above from bmc and psl

James

Post by **rachel_and_ian** » Thu Aug 20, 2009 11:51 am

Points and plugs are new and set correct. Timing was done with strobe. I'll clean the float chamber and soak the pistons in diesel for a few days and report back. Thanks for the help!

Post by **bmcecosse** » Thu Aug 20, 2009 12:14 pm

Bon chance!

Post by **rayofleamington** » Thu Aug 20, 2009 4:15 pm

My guess would be timing.
120psi is not far from normal for your engine. It may improve after a few hundred miles but it's not far from average already.

Points and plugs are new and set correct. Timing was done with strobe.

try doing the timing again, but static (use a bulb tester and check the points open at about 6 or 7 degrees BTDC) by rotating engine slowly with starting handle.
Minor dizzy has advance weights and at idle the vac advance will also change the timing - therefore I've found it quicker easier and more reliable to do static timing.
Also check the dizzy shaft will advance and spring back (turn the rotor arm by hand).

Vac advance is only for fuel economy so unless you're getting a serious air leak, it won't affect accelleration.

Post by **bmcecosse** » Thu Aug 20, 2009 6:08 pm

Lack of vacuum advance will unfortunately affect all the performance - the idling timing would normally be optimised with the vacuum connected - by twisting the dizzy back and forth to get best idling. Setting static timing is just pure guesswork otherwise! With no vacuum unit - very high basic advance will be applied to get good idling - and this will cause pinking/poor running all through the range. If that advance is not applied then poor idling/poor 'pick-up' and mediocre running will be the result - the mechanaical advance curve built into the dizzy is designed to be used WITH vacuum advance - an engine with no vac is very 'lifeless' - been there/done that! But - it should still beat 40 mph if in otherwise decent condition! As Ray suggsts though it definitely is worth checking the mechanical advance is operational.
The ~ 120 psi compression is desperately low for a 948 - it should be 150/160 psi for good running - even on that engine.