8 replies to this topic

[Slinky1313]

hey all, basically my friend and i started doing a few bits and bobs on the mini today like set the tappets and timing etc. and when we took it for a test run it suddenly started popping and jerking like crazy as soon as i reved it. now the theory we have at the moment(and i hope its this simple) is that the carb i have(hif44 with a BFZ needle) was already set up for a 1300 engine (atleast the seller said so), not my 1275. so this combined with the fact that the tappets and timing have been out, but somehow managed to work, just not well, now weve set the timing and tappets correctly its not running well anymore, so would a needle and possible spring change (if i can figure out which one i have in it already) rectify this? does it sound like a fueling problem?. or the other idea we came up with is as i had the head skimmed to compensate for some bowing that resulted in constant blowing of head gaskets, (they said they skimmed it by 12 thou, 12 thou of what i have no idea) would the rods be higher up in relation to the head and thus the tappets need a different clearance? so say instead of 0.30mm we were using, would it need something else? any help or ideas would be greatly appreciated as its just stumping me. Thanks all

Will

[miniobsessed]

No matter how much comes off the head (12 thou by the way is 12 thousandths of an inch) you need to set your tappet clearances the same. This clearance isn't for the valves but for the tappets living in the block (which we can't measure directly so we measure at the valves).

If it was running fine before you adjusted the timing I'd start by re-checking your timing

[TimmyG]

If you skimmed the head then just refitted it then the clearances would have closed up, but as part of the job of re-fitting the head is to re-set the valve clearances this should never happen, and no it doesn't mean the clearance needs to be different, you just adjust them cback out to standard clearance to compensate for the skim. AS above really plus re-check your tappet clearances again as if they are to tight it can cause this kind of runnign problem.

[dklawson]

Since the carb has been changed and came off another engine, make sure it is 100% in order.
You should never assume that a carb off one engine will be pre-adjusted to run right on another engine. This would be particularly true if the carb has been sitting around "off" the engine for a while as you don't know what someone may have done to the adjustments "just to learn" what each screw does.

Also make sure the general condition of the carb is OK before trying to adjust it.
The piston must be 100% free to rise and fall.
The damper must have oil in it.

Go through a complete tune up.
Start with adjusting the valves clearance on a cold engine.
Move on to setting the points gap, then the static timing.
Make sure the engine will start.
Then move on to the "pre-tune-up", basic carb settings. (The settings you make on a rebuilt carb just to get the engine started).
Then start the engine and when it is at temperature, set the mixture, idle, and fast idle.

[Ethel]

regarding the valve clearances,

"thou" is the usual abbreviation for thousandths of an inch. Skimming 12 thou off the head will make the engine shorter, relative to the pushrods and the lever ratio of the rocker gear will magnify it a bit. Since the valve clearance should be 12 thou yours would be less than zero without any adjustment.

[chlblnz]

Having had the head skimmed, the compression ratio has been increased. You may need to advance the ignition slightly otherwise the air/fuel mixture may incur detonation (mixture combusts due to compression rather than spark plug).

[Ethel]

Having had the head skimmed, the compression ratio has been increased. You may need to advance the ignition slightly otherwise the air/fuel mixture may incur detonation (mixture combusts due to compression rather than spark plug).

Higher compression ratios require less advance, if anything, because the fuel/air molecules are closer together and react (burn) faster. If you advance the ignition then it has longer to burn before tdc which means more pressure at tdc and so even more heat to get rid of to avoid "dieseling" the next cylinder full.

[chlblnz]

Having had the head skimmed, the compression ratio has been increased. You may need to advance the ignition slightly otherwise the air/fuel mixture may incur detonation (mixture combusts due to compression rather than spark plug).

Higher compression ratios require less advance, if anything, because the fuel/air molecules are closer together and react (burn) faster. If you advance the ignition then it has longer to burn before tdc which means more pressure at tdc and so even more heat to get rid of to avoid "dieseling" the next cylinder full.

Yes, but it is a controlled burn. If you allow the fuel to detonate from the compression ratio being too high then you risk putting a hole in the piston crown as there is more than one flame front in the cylinder.

I only meant a very small amount of advance. Detonation will increase the cylinder pressure far more than a degree of spark advance

[Ethel]

If it's close to detonation it can still detonate even if the plug has started ignition - that's pinking.