Engine Overheating

[Cjhols]

Hi all,
I have just rebuilt a Rover 3.5 (everything standard) and have fitted a QFT (Holley) 450 cfm into a 1979 MGB. Holley recommend using 6.5 psi pump on these. At the moment it is running at around 6 degrees advance, 1200 rpm and all the vacuum hoses are blocked. Initially, the headers were getting red hot and now after fiddling with the advance the headers are ok but the engine starts to boil (I turn it off immediately at this point). I can't seem to get the RPM any lower. There is also sometimes a little pop from the exhaust when I turn it off. Anyway, as it is still using the original MG fuel pump ( I think these run at 2-3 psi) I was wondering if this could be causing the issues above ?? Before the engine is started the sight glass on the side of the carbie is half full of fuel but when it is idling it is at the very bottom of the window.
Thanks in advance,

[Ian Anderson]

Glowing headers is not a good idea.

They could be caused by incorrect timing, too much fuel or too little fuel.

The sight glass when running being low does not sound right and would point towards it may be running lean because it does not have fuel, but the pop in the exhaust tends to point towards over feeling….enough unbury fuel getting through to the exhaust to get ignighted when turned off as it manages to get some air into the exhaust.

Overall I would thus say the carb is overfilling as 2 things point to this….low sight glass and pop.

So I would look back at getting the carb checked and set.

Just my 2c

Ian

[Cjhols]

Will do. So not the pump? Thanks

[ChrisJC]

The fuel pump is only there to fill the float bowl. It has no bearing on the quantity of fuel delivered to the engine.

Unless of course the fuel pressure is so high that it overcomes the valve in the float bowl and floods the carb and engine. But if that were the case I would have thought that it would run as rough as a bag of spanners (e.g. like having the choke right out)

Chris.

[Cjhols]

Hi, just a bit of an update and still looking for advice ( which this novice really appreciates). Today I hooked up a vacuum gauge to see what vacuum it was reading. Initially, it was set at 14 degrees advance with 16 Hg of vacuum with a dwell of 39 degrees ( at this point the gauge indicates late ignition timing). I then altered the advance (turning the distributor) to 19 degrees which gave me a vacuum reading of 18 Hg and a dwell of 45 degrees and 1200 rpm. I can continue to add advance which increases the Vacuum but also increases the rpm. The issue I'm having is that I can not atler the rpm using the idle speed screw as it has 'bottomed out' on the stopper. I haven't altered the idle mixture screws yet and was wondering if this is my next step?
Thanks
Peter

PS the engine and headers are no longer overheating.

[stevieturbo]

Typically you set base timing with the vac advance disconnected.

Dwell should not change with you rotating the dizzy, as this is simply a measure of the points closed, vs open time ( relative ), ie actual coil charging time. Assuming still on points.

30-40 deg is huge advance at idle for any car

What is the problem, or what are you measuring for ? or trying to achieve ?

[Cjhols]

Hi yeah I don't think I read the dwell correctly. I was trying to tune the carb per the Holley instructions but even though I can alter / improve the vacuum readings I can not get the revs down as the curb idle screw is bottomed out ie; turned all the way out. I was wondering whether the idle mixture screws will fix this?
Thanks

[Mc Tool]

Some of them Holliey's have an idle circuit in the secondary barrels , to stop fuel going off if you dont open the secondaries for a while ......( cant imagine it myself ) . If the secondaries are open to far in the idle position adjusting the idle speed screw ( on the primaries ) right out wont help because with the primaries closed completely the car is now idling on the secondary idle air and fuel circuits ( further evidence of their existence ). The secondary idle position is adjustable but Im not sure about your particular carb , you may have to remove it and adjust from the underside . Have a google and you may find a vid showing how . There will be a figure on how far the secondaries should be open , some use a feeler gauge , some use a small drill bit as a gauge .

[SuperV8]

I found my crank pointer was quite far out when I was trying to check my timing.
I would first double check your crank pointer is accurate?

You can do this by making a simple piston stop tool from an old spark plug. Cut off the porcelain and replace with a cap head bolt.
Fit this in cylinder 1 and turn the engine over by hand until the piston touches the tool. Mark the pully where the pointer is.
Now turn the engine over backwards until it stops and mark the pulley. The centre of these two marks is your zero - so check this lines up with the degree marks on the pully.

Mine didn't, so I had to re-mark the zero and plus and minus 10 deg with some marker paint on my crank pulley.

If this is out - then all your timing setting will be out to.

[Mc Tool]

One thing to bear in mind re piston stoppers .... make sure the piston is at TDC on the comp/power stroke .... not the ex stroke as the valves may not clear the piston stopper and bend or jam the valve . Some motors it will be ok others not so .