Midget engine rebore

[philthehill]

Amgrave
I cannot get the picture to landscape no matter how I try.

Les
Attached is a picture of the 1275cc gudgen pin removal tool I have.
Not made by me but purchased off 'e' bay as a one off ready made and ready to use item.
Phil[frame][/frame]

[bmcecosse]

My home made removal tool is not dissimilar - but obviously rather more crude! I just wanted the S rods off the pistons. I would never dare to try to fit the rods to a new set of expensive pistons..

[les]

Thanks Phil, looks pretty comprehensive!

[olderisbetter]

Today i went through all the basic things just incase something simple was causing the lumpy running issue, i put on an accuspark red coil and some triple fire spark plugs, locked the dizzy and changed the leads and cap, checked the carbs for free movement and oil, it starts and runs but the revs are irratic it can idle ok about 1500 rpm but then it goes up and down bye 500 rpm both ways, crank case pipe to carbs are ok and it revs free, i think the low compression on no.4 is going to be the reason but i was hoping to find something simpl e..
if i can host a video somewhere i will post it showing the running.

[shoebone]

Keeping it simple .. check to make sure the inlet manifold is not leaking

[IslipMinor]

What is the timing set to? With higher CR and longer duration cam and with NO vacuum connected, it ideally should be a maximum of around 30° BTDC with unleaded fuel, and a static of up to 10° BTDC, i.e. 10° of distributor advance.

Depending on the type of vacuum connection you may get some more advance at idle, which will help the idle quality. The standard Minor distributor gives the best part of 15° mechanical distributor advance, so cannot be advanced enough either at lower engine speeds or idle for an engine with a 'longer' cam, which need more low speed advance (but NOT than ~10° BTDC static or there can be problems when cranking).

[olderisbetter]

I set the timing randomly to get it running and then moved it about till it idled lower and it revved smoother, i did not use a timing gun as i lent it out with my compression testing kit to a family memeber never to be seen again, i have ordered a new one.

[bmcecosse]

Just to point out - 15 degrees of dizzy advance is 30 degrees of ignition advance, I've never been able to get an engine with anything like 10 degrees of static advance to start in cold winter mornings - it kicks hard against the starter. The ideal for me has been only 2 to 3 degrees static advance -then the vacuum kicks in slightly to give a decent idle. And when working hard the vacuum of course is ineffective - so the engine runs at the preset 30 +3 total advance. On some I have trimmed the mechanical stop to allow a bit more mechanical advance. The figure is stamped on the mechanical advance stop inside the dizzy. But I doubt any of this is your problem here - look more (as suggested above) for a hefty leak of intake air either at the manifold or perhaps at a servo if you have one ? I assume you have tested and proved the vacuum advance is working? And err -don't waste good money on a timing light - what will you set it to??? The fuel today is so different, the original 'settings' are not relevant any more.

[IslipMinor]

Our 1380 has had around 10° BTDC static advance ever since we have had it 18 years ago, and spins over on the starter extremely well in all conditions. I agree that much more is not good, but in our experience 10° BTDC is fine.

The above ignition settings are using current unleaded fuel and DO need a timing light to get it right! Roy, we will disagree on this one.

[bmcecosse]

I merely relate my experience. So with 30 degrees of mechanical advance - you have 40 degrees of advance when running hard - which is a LOT, especially with high compressions. I'm surprised .....

[olderisbetter]

I have no servo and have looked for pipes off or missing or holes with a plug missing, i am going to get a timing light anyway as there cheap enough, to have around, am i right in thinking low compression on no.4 would give and lumpy engine and hunting engine.

[olderisbetter]

Not sure if this link will work but it just shows the engine running .
https://vid.me/NCwp

[IslipMinor]

Roy,

The point I am making is that with 15° mechanical or 30° crank advance, a standard distributor is NOT suitable for a modified engine! That is why I suggested a distributor with 10° (20° crank) advance to give a TOTAL of 30° crank advance, which is in the right area for a modified engine AND unleaded fuel. This is the situation that the OP has.

Olderisbetter,

Interesting video - when the throttle is blipped there is no sign of a misfire, even though one cylinder is down on compression a bit. The idle is higher than it should need to be with the cam you have, have you tried reducing the speed to 1,000/1,100 rpm and rechecking the mixture? Also what advance do you have on idle, with and without vacuum connected, and do you know what the maximum mechanical advance is on your distributor?

[olderisbetter]

It can idle around 1100 but when it drops it almosts stalls so i turned it up a bit, and i have no way of checking the mixture as i am unsure on how to do it, is it using CO meter ? and the advance and timing are just guess work, i did chack the vacuum works but i have a box of dizzy bits and will swap the dizzy and see if the fault is still there.

[Chipper]

Use the lifting pin on the underside of the twin carbs - it lifts the carb piston by a few mm. Engine should briefly rise in revs a little then drop back:

"using the lift pin on the underside of the dashpot
mounting flange, press up until you feel contact with the piston and then
lift slightly (specs are 1/32 of an inch – good luck). If the engine RPM
increases when the pin is lifted, and remains elevated, your mixture is rich
– raise the mixture adjusting nut. If the engine RPM drops when the pin
is lifted your mixture is too lean, lower the mixture adjusting nut. If
the engine RPM initially raises as the pin is lifted, and then settles back
to the original RPM or very slightly above, your mixture is right."

Also use a length of garden hose to listen to the sucking of the air through each carb - if balanced, they should sound the same.

And check the spark plug tip colour - should be a nice tan colour, if the mixture is about right.

[olderisbetter]

Chipper,ok that seems easy enough i can do that in the morning, thanks.

[bmcecosse]

The lowest mech advance dizzy I have seen had 13 degrees on the stop (=26) and came from a low compression Mini 848 van engine. But I think you are running a mapped ignition now Richard? Obviously better if set up nicely on a RR. Most of us don't have that facility. Vintage cars had a timing retard lever on the steering wheel for good reason...to avoid broken wrists when swinging the starter!

[IslipMinor]

Roy,

The current mapped ignition is irrelevant, the previous distributor was an Aldon 'Yellow' and that seemed to have around 10° distributor mechanical, and that is what I had the 10° BTDC static with for the last 18 years! Just carried it over to the NODIZ unit a few weeks ago.

Don't forget that the OP has a modified engine, so we are not talking standard advance curves here.

[bmcecosse]

Indeed - and for my modified engines I filed the stop to increase the mech advance - but with barely any static advance, and they seemed to go well enough at the time! Certainly beating the other 998 engines in my class- and often the fancy 'Downton' 999 engines too - much to the chagrin of the owners - one of whom had the cheek to protest my engine and have the head lifted to check the capacity. Cost him his £5 - and a new gasket set I asked him to buy for me.......

[IslipMinor]

Roy,

I'm sure you understand that a road modified engine has other requirements other than out and out power, which is what you were looking for in a competition modified engine? Again I'm sure that you understand that the idle on a modified engine is often greatly improved with more ignition advance, but not too much or starting can be a problem?

For a road modified engine, to be able to get a reasonable combination of idle quality on an A-Series and good outright power, using 10° BTDC static plus around 10° of distributor advance (20° crank), i.e. 30° BTDC in total is very good starting point.